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We investigate the seasonal variations of ion precipitation, utilizing observations from the Mars Atmosphere and Volatile Evolution mission spanning from 2014 January 4 to 2023 February 14. Our analysis reveals that a diminishing pattern characterizes the transition from Mars season L s 0°–180° to Mars season L s 180°–360°, manifesting as a reduction in precipitating ion fluxes. Additionally, we discern a significant influence of the crustal magnetic field on the seasonal variations in precipitating ion fluxes. Intriguingly, within regions where the crustal magnetic field exhibits a strong quasi-horizontal orientation, opposite seasonal trends become evident. The underlying physical mechanism driving these seasonal variations in ion precipitation is probably attributed to the mass loading effect that may decelerate the solar wind and influence the magnetic pileup. A detailed investigation is further demanded in the future. [ABSTRACT FROM AUTHOR]

Partially ionized plasma is a common occurrence in astrophysical and space environments. The emergence and development of plasma instabilities are significantly impacted by the inelastic collisions between the ions and neutrals in the partially ionized plasma, such as the charge exchange. In this study, the effect of the ion–neutral collisions on the sheared E × B flow was experimentally investigated. In the weak collision range, the shear-driven plasma instability, such as Kelvin–Helmholtz instability, was excited by the velocity-sheared flow. However, increasing ion–neutral collisions resulted in a decrease in the magnitude of the sheared E × B flow due to charge exchange–induced drag forces. Consequently, the Kelvin–Helmholtz instability is suppressed, and the Rayleigh–Taylor instability is triggered. The underlying mechanism was elucidated through experimental findings and numerical analysis. The result of this study proposes that a transition between the two modes occurred with increasing ion–neutral collision strength. It could be applied to the study of the solar chromosphere and prominence and planetary ionospheres, where plasma is partially ionized and the sheared E × B flow is often encountered. [ABSTRACT FROM AUTHOR]

In this paper, we present a new tool to investigate the interaction of the solar wind with Venus with the approach of a global multifluid magnetohydrodynamics (MHD) model. The continuity, momentum, and energy equations for H+, O+, O 2 + , and CO 2 + are solved self-consistently together with Faraday's law. The photochemistry of ionospheric ions are considered as the source term in the density, momentum, and energy equations for each ion. We found that the simulated ionospheric density, temperature, and the bow shock location are consistent with previous observations and simulations for both the solar maximum and minimum. The simulated magnetic fields also agree well with the Venus Express observations. Meanwhile, the high-resolving power and low numerical diffusion makes the model capable of capturing the fine structures of the Venusian-induced magnetosphere, such as the Kelvin–Helmholtz instability and the nightside wake. The escape rates have also been estimated and the results are similar to previous estimations. The high-resolution model could be an efficient tool for the exploration of the fine structures of the Venusian space environment system, and also for the application to other unmagnetized planets. [ABSTRACT FROM AUTHOR]

Measurements of concentrations of neutral and ion species in the upper atmosphere of Mars by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile EvolutioN mission have served as model input and/or for comparison with model output in numerous earlier studies of the Martian dayside ionosphere. While many models reproduce the altitudinal density profiles of key ion species within a factor of a few, it has proven challenging to achieve a level of agreement within tens of percent for multiple ion species over a wide range of altitudes. We explore means to overcome this issue while keeping with a reduced chemical model and utilizing the assumptions of photochemical equilibrium and that the NGIMS data are devoid of any measurement errors. We entertain, for instance, the idea that the rate coefficient for the charge-transfer reaction between ${{\rm{CO}}}_{2}^{+}$ and O may vary with altitude as a result of a pressure-controlled internal energy distribution of the ${{\rm{CO}}}_{2}^{+}$ population.

Planetary magnetic fields can affect ionospheric plasma transport and coupling with the solar wind. In contrast to the terrestrial global magnetic field, there are only weaker and sporadic crustal magnetic fields on Mars. Many studies have indicated that Martian crustal fields can still modulate the topside ionosphere and its coupling with the solar wind. Nevertheless, it is unclear whether the crustal fields can affect the ionospheric main peak, which does not directly interact with the solar wind and is dominated by photochemical processes. In this study, the crustal field effect was identified from ionospheric measurements over unevenly distributed crustal fields. Both the intensity and configuration of the crustal fields were found to be capable of affecting the ionospheric main peak. The ionospheric peak electron density tends to decrease in the strong horizontal crustal field region. The results suggest that the crustal fields may affect the ionospheric main peak through modulating solar wind energetic particle precipitation; strong horizontal magnetic fields can partially prevent energetic particle precipitation and thus weaken impact ionization.

This study presents a synthesized analysis of in situ and ground-based observations to investigate electromagnetic coupling between Saturn and its rings. During the Cassini Grand Finale, the magnetometer detected gradients in the azimuthal magnetic field B _ϕ connected to Saturn’s B-ring on 17 out of 21 orbits. The B _ϕ gradients indicate that field-aligned currents are flowing into Saturn’s B-ring at ∼1.55–1.67 R _S in the ring plane, preferentially in the southern hemisphere. On average, these currents are magnetically conjugate with ground-based observations of nonsolar enhancements in ${{\rm{H}}}_{3}^{+}$ emissions from Saturn’s ionosphere and detected contemporaneously with ring-sourced, planetward electron beams and field-aligned charged dust grain inflow from the C- and B-rings into Saturn’s atmosphere. Collectively, these observations align with Voyager-era predictions of a phenomenon known as “ring rain,” where charged ring material generated inward of a nominal “critical radius” is drawn into Saturn’s upper atmosphere along the magnetic field. However, we show that the B-ring currents are not likely to be a direct signature of infalling field-aligned ring grains. Instead, we propose that the ring rain generation mechanism naturally results in a sharp gradient in the ionospheric Pedersen conductance at the ∼1.57–1.67 R _S boundary, which, combined with a v × B electric field in the ring ionosphere, could drive the observed B-ring currents. The Pedersen conductance in the high-conductance region of the southern ring ionosphere is constrained to ∼0.07–2 S and is observed to vary within this range on week-long timescales.

Mars's magnetosphere is a sensitive system, varying due to external and internal factors, such as solar wind conditions and crustal magnetic fields. A signature of this influence can be seen in the position of two boundaries; the bow shock and the induced magnetospheric boundary (IMB). The bow shock moves closer to Mars during times of high solar activity, and both the bow shock and IMB bulge away from Mars over crustal magnetic fields in the southern hemisphere. This study investigates whether large-scale atmospheric events at Mars have any signature in these two magnetic boundaries, by investigating the 2007 storm. The 2007 global storm lasted for several months and increased atmospheric temperatures and densities of both water vapor and carbon dioxide in the atmosphere, leading to an increase in atmospheric escape. Using Mars Express, we identified boundary locations before, during, and after the event, and compared these to modeled boundary locations and areographical locations on Mars. We find that, while it is unclear whether the bow shock position is impacted by the storm, the IMB location does change significantly, despite the orbital bias introduced by Mars Express. The terminator distance for the IMB peaks at longitudes 0°–40° and 310°–360°, leaving a depression around 180° longitude, where the boundary usually extends to higher altitudes due to the crustal magnetic fields. We suggest this may be due to the confinement of ionospheric plasma over crustal fields preventing mixing with the dust, creating a dip in ionospheric pressure here.

Exoplanets with short orbit periods reside very close to their host stars. They transition very rapidly between different sectors of the circumstellar space environment along their orbit, leading to large variations of the magnetic field in the vicinity of the planet on short timescales. This rapid change of the magnetic flux through the conducting and resistive layer of the planetary upper atmosphere may drive currents that dissipate in the form of Joule heating (JH). Here, we estimate the amount of JH dissipation in the upper atmosphere of Trappist-1e, and two hypothetical planets orbiting the Sun in close-in orbits. We find that the rapid orbital motion could drive a significant amount of atmospheric heating and could significantly affect the planetary atmosphere escape rate. Thus, the process should be accounted for when studying the long-term evolution of exoplanetary atmospheres.

We investigate the seasonal variations of ion precipitation, utilizing observations from the Mars Atmosphere and Volatile Evolution mission spanning from 2014 January 4 to 2023 February 14. Our analysis reveals that a diminishing pattern characterizes the transition from Mars season L _s 0°–180° to Mars season L _s 180°–360°, manifesting as a reduction in precipitating ion fluxes. Additionally, we discern a significant influence of the crustal magnetic field on the seasonal variations in precipitating ion fluxes. Intriguingly, within regions where the crustal magnetic field exhibits a strong quasi-horizontal orientation, opposite seasonal trends become evident. The underlying physical mechanism driving these seasonal variations in ion precipitation is probably attributed to the mass loading effect that may decelerate the solar wind and influence the magnetic pileup. A detailed investigation is further demanded in the future.

Based on an improved model of the moon absorption of Jovian radiation belt particles, we investigate quantitatively and comprehensively the absorption probabilities and particle lifetimes due to encounters with four of the inner moons of Jupiter (Amalthea, Thebe, Io, and Europa) inside L < 10. Our results demonstrate that the resultant average lifetimes of energetic protons and electrons vary dramatically between ∼0.1 days and well above 1,000 days, showing a strong dependence on the particle equatorial pitch angle, kinetic energy and moon orbit. The average lifetimes of energetic protons and electrons against moon absorption are shortest for Io (i.e., ∼0.1–10 days) and longest for Thebe (i.e., up to thousands of days), with the lifetimes in between for Europa and Amalthea. Due to the diploe tilt angle absorption effect, the average lifetimes of energetic protons and electrons vary markedly below and above αeq ${\alpha }_{\mathrm{e}\mathrm{q}}$ = 67°. Overall, the average electron lifetimes exhibit weak pitch angle dependence, but the average proton lifetimes are strongly dependent on equatorial pitch angle. The average lifetimes of energetic protons decrease monotonically and substantially with the kinetic energy, but the average lifetimes of energetic electrons are roughly constant at energies <∼10 MeV, increase substantially around the Kepler velocities of the moons (∼10–50 MeV), and decrease quickly at even higher energies. Compared with the averaged electron lifetimes, the average proton lifetimes are longer at energies below a few MeV and shorter at energies above tens of MeV. Plain Language Summary: As a gaseous and giant planet, Jupiter has the largest magnetosphere in the solar system formed by the interaction between its internal magnetic field and the solar wind flows. Jovian radiation belts are regions of enhanced populations of energetic electrons and protons within its magnetosphere, with an intense and extreme radiation environment. An important feature is that various moons orbit around Jupiter within its magnetosphere, which creates chances for energetic particles to encounter moons, resulting in particle losses and modifying the spatial distributions of energetic particles and their energy spectra and pitch angle profiles. We improve an analytic model to evaluate particle lifetimes around the moon's orbits under the assumption that the moons are ideal insulators, though Europa has tenuous atmosphere and Io has ionosphere. We analyze particle absorption effects by encounters with some of the inner Jovian moons (Amalthea, Thebe, Io, and Europa) and give comprehensive analysis on energy and pitch angle dependences of energetic protons and electrons in Jovian radiation belts. Our improved quantifications of radiation belt particle loss due to moon absorption are suitable to be incorporated with other important physical processes to pursue understanding of the complex variability of the Jupiter's radiation belts. Key Points: The average lifetimes of energetic protons and electrons due to moon absorption vary dramatically from ∼0.1 to well above 1,000 daysConsidering the moons as insulators, we find that the absorption of energetic protons and electrons by Io is the most intenseAverage proton lifetimes are longer than electron lifetimes at energies below a few MeV but shorter at energies above tens of MeV [ABSTRACT FROM AUTHOR]

The ionospheric Alfvén resonator (IAR) is a structure formed by the rapid decrease in the plasma density above a planetary ionosphere. This results in a corresponding increase in the Alfvén speed that can provide partial reflection of Alfvén waves. At Earth, the IAR on auroral field lines is associated with the broadband acceleration of auroral particles, sometimes termed the Alfvénic aurora. This arises since phase mixing in the IAR reduces the perpendicular wavelength of the Alfvén waves, which enhances the parallel electric field due to electron inertia. This parallel electric field fluctuates at frequencies of 0.1–20.0 Hz, comparable to the electron transit time through the acceleration region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter suggests that a similar process can occur in the Jovian IAR. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions, indicating that the IAR resonant frequencies are in the same range as those at Earth. This model describes the evolution of the electric and magnetic fields in the low‐altitude region close to Jupiter that is sampled during Juno's perijove passes. In particular, the model relates measurement of magnetic fields below the ion cyclotron frequency from the MAG and Waves instruments on Juno and electric fields from Waves to the associated parallel electric fields that can accelerate auroral particles. Plain Language Summary: Just like at Earth, the polar regions of the planet Jupiter are circled by a luminous aurora (northern and southern lights) that can be seen from telescopes like the Hubble Space Telescope near Earth. The aurora on both planets is produced by electrons impacting the upper atmosphere, causing the atoms and molecules in this region to emit light. At Earth, these electrons are mainly produced by large voltages that cause all the electrons to be accelerated to nearly the same energy. However, recent observations from the Juno satellite at Jupiter shows that these electrons are mainly accelerated over a broad range of energies. This suggests that the voltages accelerating these electrons are fluctuating rapidly in time. Such fluctuations can be caused by the strong increase in the effective wave speed due to a rapid decrease in the number of electrons as the altitude is increased. We have developed a computer model to help understand these interactions. Key Points: Broadband acceleration of auroral particles at Jupiter, can be achieved by Alfvén waves propagating in the ionospheric Alfvén resonatorNumerical results indicate that electrons could be accelerated to the 10–100 keV range for observed levels of Alfvén wave activityThere is also an Alfvén resonator in the high‐Alfvén speed velocity region between the ionosphere and the dense plasma sheet [ABSTRACT FROM AUTHOR]

Ionization efficiency, the ratio between photoelectron impact ionization and primary photoionization, is an important quantity from the perspective of ionospheric energy balance and as a quantity to be parameterized for use in global simulations. We investigate ionization efficiency using MAVEN in situ measurements of solar extreme ultraviolet (EUV) flux, suprathermal electron flux, and neutral density. Its behavior with respect to pressure and solar zenith angle (SZA) is explained by ionization cross‐sections and photoabsorption, plus photoelectron transport near the terminator. We find similar ionization efficiencies between the species CO2, O, N2, and CO, with Argon ∼40% higher, explained largely by its higher ionization potential. Efficiency depends positively on solar activity, increasing by ∼50–100% (depending on season) from low to moderate solar EUV levels, before flattening at higher EUV levels. EUV spectral hardening appears to be responsible for only ∼15% of this increase, the remainder best explained by variability in neutral composition. We find a negligible dependence of ionization efficiency on the strength and direction of crustal magnetic fields. Efficiencies agree poorly with the theoretical model of Nicholson et al. (2009), https://doi.org/10.1111/j.1365-2966.2009.15463.x, possibly reflecting differences in model versus measured solar EUV spectrum and neutral densities. We fit the data to the empirical parameterization of Mendillo et al. (2011), https://doi.org/10.1029/2011ja016865 but find that a single fit cannot capture ionization efficiency behavior across all dayside SZA values. We believe that an empirical model of Mars ionization efficiency will require MAVEN data and validated transport modeling to reach to sufficiently high pressures to capture Mars' photochemical peaks M1 and M2. Plain Language Summary: Solar extreme ultraviolet (EUV) photons ionize neutral atoms and molecules in the upper atmosphere of Mars, forming a layer known as the ionosphere. Photoionization (PI) (i.e., direct ionization by photons) is augmented by further ionization caused by impact on neutral particles of the electrons freed by the PI. Ionization efficiency is the ratio of PI to this electron impact ionization. We investigate ionization efficiency using data from the MAVEN spacecraft and find that it varies between 0.1 and >1000, with its basic structure explained by wavelength‐dependent absorption of EUV radiation, as well as photoelectron transport from the day to night sides. This efficiency is similar for the primary gases CO2, O, N2, and CO but higher for the noble gas Argon. Efficiency can vary by up to a factor of 2 over the course of the Martian seasons and as the sun's ultraviolet output naturally varies. These patterns of variability will enable modelers to better simulate the global behavior of the Martian ionosphere, knowledge of which is important for understanding atmospheric escape and climate evolution and for communication and navigation for future human explorers of Mars. Key Points: Ionization efficiency dependence on solar zenith angle and pressure follows expectations, but no dependence on crustal fields is foundArgon is found to have ∼40% higher efficiency than the other primary atmospheric constituents, which are within 10% of each otherEfficiency varies by up to a factor of two with both season and solar activity, driven largely by variability in relative species abundance [ABSTRACT FROM AUTHOR]

We report seasonal and dust‐related variations in neutral and ion species (CO2, O, and N2, and CO2+, O2+, O+, and N+, respectively) in the dayside Martian upper atmosphere between altitudes of ∼150 and ∼250 km observed by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft. The sinusoidal seasonal variations in CO2+ and O2+ densities are clearly identified, while that of O+ is less discernible. These observed variations in ion densities are well reproduced by a photochemical equilibrium model for CO2+ and O+ densities when we combine them with solar cycle variations. Furthermore, we find a decrease in O, O+, and O2+ densities in the whole altitude range at Ls = 342–346 in MY 33 during a regional dust event. The decrease in O density would lead to decreases in O+ and O2+ densities in the ionosphere through ion‐neutral reactions. Observed variations in ion and neutral species associated with the season and a regional dust storm are also confirmed in pressure coordinates. Observations show that the CO2+/O+ ratio at a given pressure level in the ionosphere varies by a factor of ∼3, which can modify the composition of ion outflow from the Martian atmosphere. Plain Language Summary: Early Mars had a dense CO2 atmosphere that could sustain a significant amount of liquid water, but present Mars only has a 7–9 mbar CO2 atmosphere. Many CO2 gases are considered to have been lost to space. One of the mechanisms for atmospheric escape on Mars is the acceleration of the ion species in the ionosphere by the interaction with the solar wind. Since ionospheric ions can be the source of escaping ions, variability in the number density in the ionosphere might affect the escape rates. The Neutral and Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft can provide a long‐term data set of number densities and composition in the ionosphere and thermosphere. We investigated the compositional change in number densities in the ionosphere and thermosphere because they are coupled through ion‐neutral reactions. Seasonal variations in neutral and ion densities are observed, and we find that the number densities of O, O+, and O2+ decrease when a regional dust storm occurs in the lower atmosphere. Key Points: Seasonal variations in CO2+ and O2+ in the ionosphere are significant, while those in O+ are less discernibleThe observed variations in CO2+ and O+ are well reproduced by a photochemical equilibrium modelDecreases in O, O+, and O2+ densities in the thermosphere and ionosphere are found during a regional dust storm [ABSTRACT FROM AUTHOR]

Different boundaries could be defined in a planetary ionosphere where the dominant process in function switches from one to the other. Identifying these boundaries and understanding their variations are hence crucial for disentangling the complexity of the ionosphere. Focusing on Mars, we perform a data-driven analysis of various boundaries and the associated time constants based on the multi-instrument measurements made by the Mars Atmosphere and Volatile Evolution mission during six campaigns that sample broadly different internal and external conditions. The boundaries we investigate include the photochemical equilibrium (PCE) boundary, the magnetic frozen boundary, the ion collision boundary, and the ion gyration boundary. Our analysis reveals systematic solar cycle and diurnal variations in that all boundaries tend to be elevated at enhanced solar activity and on the dayside and duskside of Mars. The variations with the magnetic environment are not observed for all boundaries except for the PCE boundary that exhibits an obvious elevation in strongly magnetized regions. Finally, our analysis suggests interesting species-dependent variations of different boundaries. In particularly, the PCE boundary shows the largest variability among all, with reduced boundary locations for all terminal species (NO ^+ , HCO ^+ , ${{\rm{O}}}_{2}^{+}$ , and H _3 O ^+ ) and one extra nonterminal species (CO _2 ^+ ) owing to different chemical properties rendered by different ions.

Partially ionized plasma is a common occurrence in astrophysical and space environments. The emergence and development of plasma instabilities are significantly impacted by the inelastic collisions between the ions and neutrals in the partially ionized plasma, such as the charge exchange. In this study, the effect of the ion–neutral collisions on the sheared E × B flow was experimentally investigated. In the weak collision range, the shear-driven plasma instability, such as Kelvin–Helmholtz instability, was excited by the velocity-sheared flow. However, increasing ion–neutral collisions resulted in a decrease in the magnitude of the sheared E × B flow due to charge exchange–induced drag forces. Consequently, the Kelvin–Helmholtz instability is suppressed, and the Rayleigh–Taylor instability is triggered. The underlying mechanism was elucidated through experimental findings and numerical analysis. The result of this study proposes that a transition between the two modes occurred with increasing ion–neutral collision strength. It could be applied to the study of the solar chromosphere and prominence and planetary ionospheres, where plasma is partially ionized and the sheared E × B flow is often encountered.

In the absence of an intrinsic magnetic field, the solar wind interacts directly with the Martian atmosphere, and the interplanetary magnetic field (IMF) drapes around the ionosphere. With the aid of multi-instrument measurements from Mars Atmosphere and Volatile Evolution (MAVEN), we investigate the altitudinal profiles of the draped magnetic field in the subsolar Martian ionosphere. Our analysis of 141 profiles found that they have a median profile with a nearly constant field strength of about 30 nT above 160 km. However, these profiles also show large variability, for example, with a standard deviation of 16 nT at 250 km. Timescale analysis found that magnetic field transport with downward ion plasma motion dominates the draping of IMF at high altitudes. At low altitudes, magnetic diffusion takes over due to increasing collisions. At even lower altitudes, approaching the ionospheric peak, the draped field is quickly damped owing to high conductivity. We used a 1D model simulation to confirm the above processes involved and further indicate that undulating magnetic fields with oscillation periods longer than tens of minutes can penetrate deeply into the ionosphere. This causes the large variability of draped field observed down to below 200 km. The diverse profiles of draped field observed by MAVEN are thus likely due to varying IMF conditions.

We present observations from Mars Atmosphere and Volatile EvolutioN of a reconnection exhaust, which was probably not ongoing, in the Martian magnetosheath. The reconnection jet with abundant heavy ions is directed outward to space. The magnetic field lines within the exhaust are found to be draped, suggesting that the reconnection exhaust was detached from the ionosphere and the heavy ions within it escaped from Mars. The dimensionless reconnection rate estimated from proton measurements is about 5.56%. The total oxygen loss rate via the reconnection exhaust is calculated to be 1.3 × 10 ^21 s ^−1 . This much lower ion loss rate than that estimated in active magnetic reconnection at the ionopause suggests that the reconnection associated with the reconnection exhaust is not ongoing, so most of the heavy ions have already escaped. Our results demonstrate that magnetic reconnection can directly contribute to the ion loss of Mars. Due to the participation of multiple heavy ions, the characteristics of magnetic reconnection could be very different.

The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express has been sampling the topside ionosphere of Mars since mid-2005. The analysis of the main reflection (nadir) of the ionosphere through the ionograms provided by the MARSIS instrument is typically performed manually due to the high noise level in the lower frequencies. This task, which involves pattern recognition, turns out to be unfeasible for the >2 million ionograms available at the European Planetary Science Archive. In the present contribution, we propose a modular architecture based on serverless computing (a paradigm that stands on the cloud) for optimal processing of these ionograms. In particular, we apply serverless computing to detect oblique echoes in the ionosphere, which are nonnadir reflections produced when MARSIS is sounding regions above or nearby crustal magnetic fields, where the ionosphere loses the spherical symmetry. Oblique echoes are typically observed at similar frequencies to the nadir reflections but at different times delays, sometimes even overlaying the nadir reflection. Oblique echoes are difficult to analyze with the standard technique due to their nonconstant and highly variable appearance, but they harbor essential information on the state of the ionosphere over magnetized regions. In this work we compare the proposed serverless architecture with two local alternatives while processing a representative data subset and finally provide a study by means of cost and performance.

The Cassini spacecraft’s Grand Finale flybys through Saturn’s ionosphere provided unprecedented insight into the composition and dynamics of the gas giant’s upper atmosphere and a novel and complex spacecraft–plasma interaction. In this article, we further study Cassini’s interaction with Saturn’s ionosphere using three-dimensional particle-in-cell simulations. We focus on how electrons and ions, emitted from spacecraft surfaces due to the high-velocity impact of atmospheric water molecules, could have affected the spacecraft potential and low-energy plasma measurements. The simulations show emitted electrons extend upstream along the magnetic field, and for sufficiently high emission rates, charge the spacecraft to positive potentials. The lack of accurate emission rates and characteristics, however, makes differentiation between the prominence of secondary electron emission and ionospheric charged dust populations, which induce similar charging effects, difficult for Cassini. These results provide further context for Cassini’s final measurements and highlight the need for future laboratory studies to support high-velocity flyby missions through planetary and cometary ionospheres.

The behavior of the Martian ionosphere during global dust storms has always been a popular topic. Based on many Mars Atmosphere and Volatile Evolution observations, a Martian ionospheric electron density model is constructed herein using a physical-grid deep neural network algorithm. The precision of the model is proved to be high, and the R ^2 value of the test set is 0.979. On this basis, the effects of global dust storms on the electron density and peak height of the Martian ionosphere are explored. The results show that the MY34 global dust storm significantly raised the peak height of the Martian electron layer, and its effects mainly ranged from 20°N to 55°N. The periodicity of the global daily average peak electron layer height following this dust storm was more regular than that before the dust storm.

Ionospheric chemistry plays an unexpectedly important role in the evolution of planetary habitability. This study is dedicated to a detailed modeling of the nightside Martian ionospheric structure and composition, a topic that has been poorly explored due to the absence of relevant measurements, but now becomes tractable owing to the unprecedented measurements made by the Mars Atmosphere and Volatile Evolution. Two-stream kinetic calculations and time-dependent fluid calculations are coupled to derive the nightside density profiles at 100–300 km for a large number of ion species, assuming solar wind electron precipitation as the only viable ionizing source in the ideal nonmagnetized atmosphere. Our calculations indicate the presence of a well-defined ionospheric peak at 146 km with a peak density of 8500 cm ^−3 , as driven by the strong atmospheric “absorption” of precipitating electrons at low altitudes. The distribution of nonterminal species is roughly under chemical equilibrium below 170 km, whereas for terminal species such as NO ^+ and HCO ^+ , diffusion is effective at essentially all altitudes, in direct contrast to the dayside behavior. In the more realistic magnetized atmosphere, the ionospheric peak seldom exists due to the patchiness of electron precipitation. In particular, our model results agree fairly well with the MAVEN measurements, especially in view of the coincidence between electron depletion and thermal plasma void seen along many MAVEN orbits. Compared to the dayside, the nightside ionospheric composition has a much higher proportion of NO ^+ and lower proportion of CO _2 ^+ , likely indicative of nightside enhancement of atmospheric O and N.

In this paper, we present a new tool to investigate the interaction of the solar wind with Venus with the approach of a global multifluid magnetohydrodynamics (MHD) model. The continuity, momentum, and energy equations for H ^+ , O ^+ , ${{\rm{O}}}_{2}^{+}$ , and ${\mathrm{CO}}_{2}^{+}$ are solved self-consistently together with Faraday’s law. The photochemistry of ionospheric ions are considered as the source term in the density, momentum, and energy equations for each ion. We found that the simulated ionospheric density, temperature, and the bow shock location are consistent with previous observations and simulations for both the solar maximum and minimum. The simulated magnetic fields also agree well with the Venus Express observations. Meanwhile, the high-resolving power and low numerical diffusion makes the model capable of capturing the fine structures of the Venusian-induced magnetosphere, such as the Kelvin–Helmholtz instability and the nightside wake. The escape rates have also been estimated and the results are similar to previous estimations. The high-resolution model could be an efficient tool for the exploration of the fine structures of the Venusian space environment system, and also for the application to other unmagnetized planets.

Over the last 20 years, a large number of instruments have provided plasma density measurements in Earth's topside ionosphere. To utilize all of the collected observations for empirical modeling, it is necessary to ensure that they do not exhibit systematic differences and are adjusted to the same reference frame. In this study, we compare satellite plasma density observations from Gravity Recovery and Climate Experiment (GRACE), Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), CHAllenging Minisatellite Payload (CHAMP), Swarm, and Communications/Navigation Outage Forecasting System (C/NOFS) missions. Electron densities retrieved from GRACE K‐Band Ranging (KBR) system, previously shown to be in excellent agreement with incoherent scatter radar (ISR) measurements, are used as a reference. We find that COSMIC radio occultation (RO) densities are highly consistent with GRACE‐KBR observations showing a mean relative difference of <2%, and therefore no calibration factors between them are necessary. We utilize the outstanding three‐dimensional coverage of the topside ionosphere by the COSMIC mission to perform conjunction analysis with in situ density observations from CHAMP, C/NOFS, and Swarm missions. CHAMP measurements are lower than COSMIC by ∼11%. Swarm densities are generally lower at daytime and higher at nighttime compared to COSMIC. C/NOFS ion densities agree well with COSMIC, with a relative bias of ∼7%. The resulting cross‐calibration factors, derived from the probability distribution functions, help to eliminate the systematic leveling differences between the data sets, and allow using these data jointly in a large number of ionospheric applications. Key Points: A systematic comparison of the plasma density data from CHAMP, C/NOFS, GRACE, COSMIC, and Swarm missions is performedElectron densities retrieved from COSMIC‐RO agree well with GRACE‐KBR observations showing a relative difference of less than 2%Intercalibration factors, allowing to eliminate the systematic offsets between the considered data sets, are presented [ABSTRACT FROM AUTHOR]

This paper reports the characteristics of the topside ionospheric O+ diffusive flux (ΦO+) during both geomagnetically quiet (0 ≤ Kp ≤ 2) and moderate (2 < Kp ≤ 4) times using incoherent scatter radar observations at Millstone Hill (42.6°N, 288.5°E) for solar minimum from 1970 to 2018. ΦO+ partially characterizes plasma mass exchange between the upper and lower part of the topside ionosphere through diffusion and sometimes serves as upper boundary conditions for ionosphere‐thermosphere models. The altitude where the flux sign changes (mainly during daytime) is termed the transition height and the time when the flux sign changes (mainly at dawn and dusk) is termed the transition time. At quiet times, the daytime transition height is ∼100 km above the F2 peak height (hmF2) in summer, and it is about 50 km above hmF2 in other seasons; the transition time is before 18 solar local time (SLT) in spring and winter, but after 18 SLT in summer and autumn. The daytime average upward ΦO+ above the transition height shows a significant seasonal variation with a minimum of 2.2×108cm−2s−1 in summer and a maximum of 3.7×108cm−2s−1 in autumn. Under geomagnetically moderate conditions, the transition height increases by ∼20 km in spring, winter, and autumn, but moves up by about 20–50 km in summer. The transition time occurs later by ∼1 hr in summer but ∼1 hr earlier in other seasons. The mean upward ΦO+ peaks in summer and minimizes in spring. Plain Language Summary: The topside ionospheric O+ diffusive flux (ΦO+) is a part of the total plasma flux that is a critical parameter to characterize the plasma exchange between the upper and lower part of the topside ionosphere, but has been poorly known due to lack of direct measurements. Using long‐term incoherent scatter radar (ISR) data observed at Millstone Hill for nearly five decades, this study investigates ΦO+ variations as a function of the altitude above the F2 peak height (hmF2), local time, and season in both geomagnetically quiet and moderate times, and further explores the physical mechanism causing these variations. During quiet times, the summer daytime transition height where ΦO+ changes from downward to upward along magnetic field lines occurs at ∼hmF2+100 km. However, it is usually near hmF2+50 km in other seasons. In addition, the transition time when ΦO+ changes from upward to downward at a fixed height is earlier than 18 SLT in spring and winter, but later than 18 SLT in summer and autumn. We also found that an increase in geomagnetic activity decreases the vertical gradient of plasma density during the daytime in all seasons, resulting in a ∼20 km increase in the transition height and an earlier transition time near dusk in spring, autumn, and winter, but a larger transition height change of about 20–50 km and a later transition time in summer. The change in the seasonal variations of the upward ΦO+ results from the competition between the variations of plasma diffusive velocity and density when the geomagnetic activity increases, as well as the changes in plasma vertical density profiles. Key Points: Topside ionospheric O+ diffusive flux (ΦO+) climatology at solar minimum is derived from incoherent scatter radar data at Millstone HillO+ diffusive flux has seasonal, local time, and altitude dependenceGeomagnetic activity changes the spatiotemporal variations of Φ(O+) by modulating the vertical gradient of the plasma density [ABSTRACT FROM AUTHOR]

We present a new application of a multispacecraft method which provides estimates of the full electric current density vector in the ionospheric low‐ and mid‐latitude F region. The method uses the three satellites of the Swarm constellation in configurations when they were close to each other. The current density is calculated inside triangular prisms defined by the satellite positions. The technique is inspired by similar approaches such as the curlometer. Here, we propose an alternative mathematical treatment, which involves the use of the well‐known curl‐B technique and a least squares minimization. The proposed formalism provides a rigorous way to propagate the errors through the problem and special care is taken to validate the algorithm and assess the relevance of the derived currents. Such techniques are powerful tools to study the F‐region ionosphere currents as they naturally exclude all the other contributions outside the considered volumes. The F region is known to host currents with a complex spatiotemporal variability. Besides the intrinsic interest of this system itself, understanding the dynamics of these currents is also critical for global geomagnetic field modeling as they are an important source of error in the models. In this paper, we present the full algorithm and test the method on both synthetic and real Swarm satellite data on February 15, 2014. Several sources of error are investigated. Additionally, the results reveal expected characteristics of F‐region interhemispheric field‐aligned currents. Some new characteristics of F‐region currents, potentially associated with dynamo and pressure currents, are also observed. Key Points: A new application of a multispacecraft method to derive the three components of the current density in the F‐region ionosphere is presentedAn assessment of the effect of biases and errors on the current density estimates is providedApplication of the method on February 15, 2014 reveals both expected and new characteristics of F‐region currents [ABSTRACT FROM AUTHOR]

The present study aims to investigate the influence of the zonal wind velocity on equatorial plasma bubble (EPB) occurrences over Southeast Asia. The observation of the EPB occurrence is obtained from the GPS Rate of TEC change index. Meanwhile, the zonal winds were measured using a Fabry‐Perot interferometer located at Kototabang and Chiang Mai stations, and the height of F layer was acquired using an ionosonde at Chumphon station near the magnetic equator. This is the first study to report the influence of zonal wind velocity variation on EPB occurrences with the presence and absence of EPB using GPS data in the Southeast Asian sector. The results illustrated that the average magnitude of zonal wind velocity during the presence of EPB (78 ± 23 m/s) was higher than that of its absence (68 ± 21 m/s). It was observed using long‐term data analyses which led to in‐depth analyses. The analysis of temporal variation of zonal wind variation demonstrated that the zonal winds during EPB were higher in the evening compared to midnight and postmidnight periods from medium to high solar activities. The dependence of zonal wind velocity on EPB over local time was obtained based on the analysis which utilized the data collected during equinox in high solar activity. Besides that, a positive correlation was obtained between the zonal wind velocity and EPB occurrences during pre‐reversal enhancement (PRE) corroborated the effects of zonal wind influence on PRE, and thus EPB occurrences. Key Points: The zonal wind velocity was continuously monitored using the Fabry‐Perot interferometer during medium to high solar activity levelsZonal wind velocity tends to be higher during equatorial plasma bubble (EPB) existenceThe range of zonal wind velocity during EPB is higher in the evening compared to midnight and postmidnight [ABSTRACT FROM AUTHOR]

Lightning induced perturbations of the lower ionosphere are investigated with very low frequency (VLF) remote sensing on a unique overlapping propagation path geometry. The signals from two VLF transmitters (at different frequencies) are observed at a single receiver after propagation through a common channel in the Earth‐ionosphere waveguide. This measurement diversity allows for greater certainty in quantification of perturbations to the ionospheric D region. Changes in amplitude and phase are modeled with the Long Wave Propagation Capability (LWPC) software package to quantify changes in reference height and steepness of the two parameter D region electron density model. Since the nighttime D region profile prior to the perturbation is found to strongly affect the resulting quantification, and is highly variable and generally unknown at nighttime, an error minimization method for identifying the most likely ionospheric disturbance independent of the ambient profile is used. Analysis of 12 large lightning perturbations resulting from discharges with peak currents greater than 160 kA shows that the ionospheric reference height can change by 2–8 km. We investigate both early/fast events (direct ionization and heating from lightning) and lightning‐induced electron precipitation (LEP) events, induced by lightning hundreds of kilometer away. LEP events increase D region electron density while early/fast events can lead to a increase or decrease in electron density. Multi‐point observations along a VLF propagation path are needed to further improve ionospheric perturbation quantification with VLF remote sensing. Plain Language Summary: The lowest part of the ionosphere is the D region (65–95 km altitude) and is difficult to measure directly since it is too high for aircraft and too low for spacecraft. Furthermore, it is a very weakly ionized plasma so most electromagnetic waves penetrate through it. Observing very low frequency (VLF) waves is one the few ways to diagnose these region. VLF waves reflect from the ionosphere but in a complicated way. The novelty of our work is that we use signals from two powerful VLF transmitters on an overlapping path. Since two signals propagate on the same path we are able to diagnose this path with more certainty. We use an error minimization technique that can diagnose a change in the D region independent of its previous state. We show that powerful thunderstorm lightning can change the D region height by several kilometers and that the effects of powerful lightning are as dramatic as sunrise/sunset effects. Key Points: Very low frequency (VLF) remote sensing of lightning perturbations using overlapping propagation pathsQuantification of changes in h′ and β independent of ambient ionospheric conditionsLarge lightning induced perturbations change VLF reflection height by 2–8 km [ABSTRACT FROM AUTHOR]

The Martian ionosphere is produced by a number of controlling processes, including solar extreme ultraviolet radiation (EUV) and X-ray ionization, impact ionization by precipitating electrons, and day-to-night transport. This study investigates the structural variability of the Martian ionosphere with the aid of the radio occultation (RO) experiments made on board the recent Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. On the dayside, the RO electron density profiles are described by the superposition of two Chapman models, representing the contributions from both the primary layer and the low-altitude secondary layer. The inferred subsolar peak electron densities and altitudes are 1.24×105 cm–3 and 127 km for the former, and 4.28×104 cm–3 and 97 km for the latter, respectively, in general agreement with previous results appropriate for the low solar activity conditions. Our results strengthen the role of solar EUV and X-ray ionization as the driving source of plasma on the dayside of Mars. Beyond the terminator, a systematic decline in ionospheric total electron content is revealed by the MAVEN RO measurements made from the terminator crossing up to a solar zenith angle of 120°. Such a trend is indicative of day-to-night plasma transport as an important source for the nightside Martian ionosphere.

Cassini observations over the past ten years have revealed that Titan possesses a chemically complex ionosphere. In this study, we investigate the relative contributions of different ion species to the total ion escape on Titan, by dividing all ion species probed by the Cassini Ion Neutral Mass Spectrometer (INMS) into six groups according to their mass-to-charge ratios (M/Z). For the three lightest ion groups, with characteristic M/Z of 22, 41, and 52 daltons , the observed scale heights tend to be lower than the scale heights predicted by assuming diffusive equilibrium; for the three heavier groups, observed and predicted scale heights are in general agreement, implying that most ion escape from Titan is by relatively light species, with M/Z < 60 daltons. A diffusion model is constructed to describe the density distribution of each ion group in regions where the effect of ionospheric chemistry could be neglected. The data model comparison predicts an optimal total ion escape rate of 3.1×1024 s–1, of which more than 99% is contributed by relatively light ions with M/Z < 32 daltons.

The term "sluggishness" was coined by E. V. Appleton in the 1950s to describe the time delay between peak irradiance at solar noon and the resulting peak in ionospheric electron density. Sluggishness can be understood as an inertial property of the ionosphere that manifests as a lag of the ionospheric response to a solar driver. As shown by Appleton, estimates of sluggishness can be used to study the chemistry of the lower ionosphere, of the D-region in particular. In this study, for the first time, we have examined ionospheric sluggishness in terms of the time delay between the peak irradiance during a solar flare and the resulting peak in ionospheric electron density using HF instruments. Estimates of the delay are obtained using HF observations from riometers and SuperDARN radars that are primarily sensitive to absorption in the D-region. Two new methods for measuring delay are introduced. Sluggishness is shown to be anti-correlated with peak solar X-ray flux and positively correlated with zenith angle and latitude. The choices of instrument, method, and reference solar waveband affect the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient in the D-region. The coefficient was found to vary by orders of magnitude with peak flare intensity. We argue that the variation in effective recombination coefficient with peak flare intensity is highly sensitive to changes in the negative and positive ion chemistry of the D-region, which is sensitive to the incoming solar X-ray and EUV radiation. [ABSTRACT FROM AUTHOR]

Hemispheric asymmetries of the Vertical Total Electron Content (VTEC) were observed during the first recovery phase of the geomagnetic storm on September 7-8, 2017. These asymmetries occurred at the mid latitudes at two different local times simultaneously: In the European-African sector (early morning), the storm time VTEC in the southern/northern hemisphere was higher/lower than the quiet time value, suggesting the southern/northern hemisphere entered the positive/negative phase (N-S+). In the East Asian-Australian sector (afternoon), the storm time VTEC change was positive in the northern hemisphere, but negative in the southern hemisphere (N+S-). The electron density profiles from digisondes demonstrated that the asymmetries appeared in the F region density as well. The plasma drifts data from digisondes, the column-integrated [O]/[N2] ratio from GUVI onboard the TIMED satellite, and the detrended VTEC were utilized to study the drivers of the asymmetries. Traveling Ionospheric Disturbance (TID) signatures were identified in the digisonde drift and detrended VTEC data before the appearance of the asymmetry. The magnitude of TIDs was larger in the hemisphere where the negative phase occurred later. The storm time [O]/[N2] ratio change was positive in Africa (S+) and negative in Europe (N-). However, the [O]/[N2] measurements were not available in the East Asian-Australian sector during the focused period. The hemispheric differences in the vertical drifts were also observed in both sectors. Therefore, the observed hemispheric asymmetries in both sectors are suggested to be due to the hemispheric asymmetries in the thermospheric composition change, vertical drift, and TID activity. [ABSTRACT FROM AUTHOR]

We report the impact of the 2018 Mars global dust storm on the ionosphere, in particular, its peak altitude during the onset and growth phases of the event. A one‐dimensional photochemical model is applied to estimate the ionospheric electron densities below 200 km using the measurements of in situ neutral densities and ionizing solar radiation by the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Our model results show that while the peak density of the ionosphere on the dawn side during the dust storm is hardly impacted, the altitude of the peak layer may be lifted by ∼10 km. The simulation results support the MAVEN radio occultation observations that there is a time delay in the response of the ionospheric peak altitude in the southern hemisphere (SH), compared with the northern hemisphere (NH) during the dust storm development. Although the 2018 global dust storm developed in the northern Martian mid‐latitudes on 1 June and developed into a global dust event on 17 June, our study suggests that the impact on the ionospheric peak altitude near the equatorial and southern mid‐latitudes became important only after 22 June. This suggests that there was a time‐lag of about 3 weeks in the response of the Martian upper atmosphere of the SH to the dust storm that originated in the NH. Plain Language Summary: Mars has the greatest dust storms in the solar system, which occur infrequently but are able to obscure the planet's surface and last for several months. Such Planet encircling dust events affect all layers of the Martian atmosphere, the most notable effect being the expansion of the neutral atmosphere due to intensified solar heating through atmospheric dust loading. As a result, the peak of the ionosphere, where the plasma density is maximized, is elevated to greater heights. Such a global storm event took place on Mars in June 2018 and lasted for 2–3 months. The spacecraft observations of the Martian ionospheric peak during this event were mostly confined to the northern hemisphere where the storm originated and only very limited observations are available in the southern hemisphere (SH). In the present study, we investigate how the peak of the ionosphere in the SH of the planet varied during the onset phase of the global storm in 2018. We found that the peak altitude increased by 7–10 km. In addition, we report that the response was not immediate, but instead took around 22–26 days for the enhancement to appear in the ionospheric peak altitude of the SH. Key Points: Ionospheric electron densities are calculated in the southern hemisphere (SH) during the onset and growth phases of the 2018 global dust stormThe dust storm caused an uplift of the ionospheric peak altitude by ∼10 km in the SHThe uplift of the ionosphere near equatorial and southern mid‐latitudes lagged 22–26 days from the storm's onset in the northern hemisphere [ABSTRACT FROM AUTHOR]

A bow shock is generated by the interaction of the solar wind with the planetary global dipole field (e.g., Earth), or with (mainly) the planetary ionosphere (e.g., Mars). The cross-shock potential has been well studied at Earth but not yet for Mars. We infer and approximate the peak in the frame invariant de Hoffmann-Teller shock potential profile at Mars (ϕ) with data from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. We find that ϕ and its ratio to the solar wind ram ion energy (Eram) vary similarly against solar zenith angle (SZA, a proxy for the angle between the solar wind flow and the shock normal) and magnetic latitudes. Our results also reveal no significant dependence of the shock potential on parameters such as the angle between the upstream interplanetary magnetic field (IMF) and the shock normal and plasma beta of upstream solar wind. There is a somewhat positive correlation with the magnetosonic Mach number and the magnetic amplification ratio across the shock. We also find a solar cycle effect on the shock location, closer to the planet near the solar minimum, as expected. Lastly, similarities and differences of cross-shock potentials at Mars and Earth are discussed. Characterizing electron energization and high-altitude ion loss at Mars is influenced by the bow shock and thereby the work here. [ABSTRACT FROM AUTHOR]

Studying how planetary ionospheres respond to changes in solar irradiance can inform our understanding of how planetary atmospheres have reacted to our changing Sun over the course of solar system history. The response of Mars's main ionospheric peak to changing solar irradiance has been well-studied. Less well understood are the altitudes above the main peak. We use radio occultation observations by Mars Global Surveyor and ground-based solar radio flux measurements to quantify the response of Mars's electron densities to changing solar irradiance at these altitudes. This analysis shows that the electron densities respond modestly at and just above the main peak, with the response increasing above or below the peak. We use a one-dimensional photochemical equilibrium model to reproduce the observed electron densities and investigate the influence of minor ions, changing electron temperatures, and changing neutral densities and composition on the resultant electron density. We find that the neutral atmosphere is primarily responsible for the observed behavior. We also make comparisons to Venus, which shows a more dramatic increase in its ionospheric response as a function of altitude. We show that while the ionospheric responses of Mars and Venus as a function of altitude are markedly different, recasting the analysis in terms of number of scale heights above the main peak brings the two planets into closer agreement, especially at the main peak. The agreement is worse at higher altitudes and this is attributed to Venus's larger proportion of atomic oxygen. [ABSTRACT FROM AUTHOR]

The Global-scale Observations of the Limb and Disk (GOLD) mission, launched in 2018, aims to investigate the low latitude ionosphere from a geostationary orbit at 47.5°W. It uses two identical spectrometers measuring the wavelength range from 134.0 to 163.0 nm. The configuration of the Earth's magnetic field shows that the largest offset between geographic and geomagnetic equators occurs in the longitude sectors sampled by GOLD. In an attempt to investigate the longitude dependence of the occurrence rate and time of onset of plasma bubbles, or plasma depletions, GOLD data were separated in three sectors: 65°-55°W, 50°-40°W, and 10°W--0°. Observations of the nighttime emissions in 135.6 nm on November 2018 and March 2019 show plasma depletions occurring very frequently at these longitudes. The growth rate of the Rayleigh-Taylor instability was computed at these longitudes under similar low solar activity conditions, assuming an empirical model of upward plasma drifts. The time and value of the maximum growth rates obtained cannot always explain the observations. On average, the observed occurrence rate of plasma depletions is high, with a maximum of 73% (observed during November 2018 at ~45°W). Most of the depletions observed in November at 45°W and 60°W occur within 1 h after sunset. When compared with the November 2018 observations, depletions in March 2019 occur at later times. [ABSTRACT FROM AUTHOR]

In this study, electron temperature (Te) observations from the Defense Meteorological Satellite Program F17 and F18 spacecraft during 2010--2017 are analyzed to explore the longitudinal differences of Te in the topside ionosphere on both sides of zero geomagnetic declination lines in the northern mid-latitude under geomagnetically weak activities (Kp ≤ 3). Te in the North American sector (45ºW--135ºW) has prominent longitudinal differences on sector scale. Te increases from the west side to east side in the morning, but vice versa in the afternoon. The relative value of longitudinal differences in Te peaks in May to August. The East Asian sector (45ºE--135ºE) of similar geomagnetic field configurations is selected for verification. There are similar local time and seasonal dependencies in the two sectors, while the intensity of longitudinal differences of Te weakens in the latter sector. The estimation of contributions to the longitudinal difference of Te from thermosphere winds provided by the empirical horizontal wind model (HWM14), peak height (hmF2) and peak density (NmF2) of the F2 layer indicates that the magnetic declination modulation effect of neutral winds at mid-latitudes changes electron density (Ne), which indirectly changes the longitudinal difference in Te. Furthermore, the zonal wind at different local times and the seasonal variation of meridional winds may be the main effect factors of dependencies in Te longitudinal differences. The sector longitudinal differences of both geomagnetic declination and horizontal winds are possibly major reasons causing the different intensity of this characteristic in the two sectors. [ABSTRACT FROM AUTHOR]

Our knowledge of the physical properties of the topside ionosphere is still incomplete. A key point still not fully understood is how field-aligned currents are generated, evolve and dissipate in the ionosphere. Answering to this question is fundamental for a better understanding of the mechanisms regulating the coupling between magnetosphere and ionosphere and to shed light on the physical processes inherent to space weather events occurring in the Earth's ionosphere. In this framework a relevant role is played by the ionospheric conductivity. The purpose of this study is to analyze the main properties of the electrical conductivity parallel to the geomagnetic field from a climatological point of view. The statistical study of the electrical conductivity is proposed using four years of in-situ electron density and temperature measurements at 1 Hz acquired by the European Space Agency's Swarm A satellite. Variations due to seasonal effects are also investigated. Finally, starting from observations and comparing our results with those obtained using International Reference Ionosphere model, we give a first estimation of the conductivity mainly due to particle precipitation. [ABSTRACT FROM AUTHOR]

Energy and momentum can be transferred from the magnetosphere to the ionosphere through field-aligned currents (FACs), which show seasonal variations due to seasonal variation of the ionospheric conductance. Using Magnetospheric Multiscale Mission observations, we investigate the characteristics of kinetic-scale (close to local ion inertial length) FACs in the plasma sheet boundary layer (PSBL), such as occurrence rate, magnitude, and carriers. The results show that: (1) the occurrence rates of FACs exhibit a trident distribution; they are also larger in the northern hemisphere than in the southern hemisphere, especially for the earthward FACs. (2) Distribution patterns vs. plasma β are different for the beam-type and the flow-type FACs. Flow-type FACs are observed closer to the central plasma sheet (higher β), and their occurrence rate decreases linearly toward the tail lobe (lower β), while the beamtype FACs are mainly and equally distributed within 0.1 < β < 1.0. (3) The magnitude of FACs shows little dependence on β, and generally increases closer to the Earth. (4) Occurrence rate and magnitude of FACs both increase during active periods, consistent with ionospheric observations. (5) The main carriers for FACs in the PSBL are thermal electrons, while cold electrons also contribute occasionally, especially during active periods. Hemispheric asymmetry of occurrence rate and dependence on geomagnetic activity of the kinetic-scale FACs in the PSBL are consistent with FACs at ionospheric altitudes. This fact demonstrates that kinetic-scale FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to the magnetosphere in the nightside. [ABSTRACT FROM AUTHOR]

The present investigation evaluates the assimilation of synthetic data which has properties similar to actual Global-scale Observations of the Limb and Disk (GOLD) level-2 (L2) temperatures and other conventional lower atmospheric observations. The lower atmospheric and GOLD L2 temperature (Tdisk) are assimilated in the Whole Atmosphere Community Climate Model with thermosphereionosphere eXtension using Data Assimilation Research Testbed. It is found that inclusion of the GOLD Tdisk improves the forecast root mean square error (RMSE) and bias by 5% and 71%. When compared to lower atmosphere only assimilation, the improvements in RMSE and bias are 20% and 94%. An investigation of the global diurnal westward-propagating wavenumber 1 (DW1) and local diurnal tidal characteristics shows that inclusion of the GOLD temperatures improves the DW1 by about 8% and diurnal tide by more than 17%. Larger percentage improvements in the tides are seen in the lower thermosphere. Considerable improvements in the model state are also seen at times and locations where there are no GOLD observations available. These results and the background data assimilation procedure are presented here, which demonstrate that GOLD thermospheric temperature is an excellent data set that can be used for thermospheric assimilation studies and operational purposes. Plain Language Summary A perfect numerical model simulation of the Earth is the one that can reproduce the whole atmosphere-ionosphere-thermosphere (AIT) system at any point of time. With time, the numerical models are evolving and the simulation capabilities are enhanced with new understanding of the AIT system dynamics, but they are still far from perfect. On the other hand, if one can measure any parameter or state of the AIT system at any point of time then the numerical models will be of no use. In the absence of both the above highly ambitious extreme possibilities, both the state-of-theart model capabilities and AIT measurements can be combined in a data assimilation framework to study the dynamics and to better understand the AIT system. The present investigation evaluates GOLD mission level-2 disk temperatures and finds that they can significantly improve the thermospheric assimilation capability. [ABSTRACT FROM AUTHOR]

The increasing amount of the Global Navigation Satellite System (GNSS) ground receivers provides extremely abundant data resource to ionospheric community. In present paper, a hybrid data assimilation method is proposed to derive the global ionosphere maps (GIM) using the GNSS measurements. This method can balance the weights of background information from the International Reference Ionosphere model (IRI‐2016) and actual observations to reach a reasonable estimation, and its background error covariance (BEC) is a weighted linear combination of climatological static BEC and ensemble‐based flow‐dependent BEC. The results show that this method is capable of improving the background outputs significantly. To evaluate the quantitative contribution of background information on the estimated GIM, the background influence analysis is performed. The background information contributes less in the data‐rich areas and more in the data‐sparse areas. The mean background influence is up to 0.93 for the reconstructed GIM; that is, about 93% of the contribution is due to the background information, and the complementary 7% is the influence of the assimilated observations. Key Points: A hybrid data assimilation method is proposed to derive the GIM using the GNSS measurementsThe background influence analysis is performed to assess the quantitative contribution of background information on the estimated GIMThe sparse matrix technique is adopted to reduce the computational cost [ABSTRACT FROM AUTHOR]

This paper presents the response of the equatorial ionosphere to the noon time annular solar eclipse of 15 January 2010. The observation has been made using a Digisonde, Meteor Wind Radar, and Proton Precession Magnetometer over Trivandrum, (8.5oE; 77oN; dip lat 0.5oN), a geomagnetic dip equatorial station in India. It has been found that the E, F1, and F2 regions of the equatorial ionosphere respond to solar eclipse with different time delays, F1 being responding faster and the E and F2 regions slower. Though there have been studies on the delayed response of F2 region during the solar eclipses, the delayed response of E region is quite unexpected since this region is dominated by the recombination chemistry. The plausible reasons for this have been explored, and it is suggested that the downward diffusion of atomic oxygen plays a major role for the observed phenomenon. Key Points: This paper discusses about the altitudinally varying response of equatorial ionosphere to the annular solar eclipse of 15 January 2010It is intriguing to note that the E region electron densities were found to be responding to the solar flux changes later than those in F1 regionThe plausible reasons behind this are discussed in terms of prevailing neutral and electrodynamics and the diffusion chemistry [ABSTRACT FROM AUTHOR]

Ionospheric outflow is the flow of plasma initiated by a loss of equilibrium along a magnetic field line, which induces an ambipolar electric field due to the separation of electrons and ions in a gravitational field and other mass‐dependent sources. We have developed an ionospheric outflow model using the transport equations to determine the number of particles that flow into the outer magnetosphere of Jupiter. The model ranges from 1,400 km in altitude above the 1 bar level to 2.5 RJ along the magnetic field line and considers H+ and H3+ as the main ion constituents. Previously, only pressure gradients and gravitational forces were considered in modeling polar wind. However, at Jupiter we need to evaluate the effect of field‐aligned currents present in the auroral regions due to the breakdown of corotation in the magnetosphere, along with the centrifugal force exerted on the particles due to the fast planetary rotation rate. The total number flux from both hemispheres is found to be 1.3–1.8 × 1028 s−1 comparable in total number flux to the Io plasma source. The mass flux is lower due to the difference in ion species. This influx of protons from the ionosphere into the inner and middle magnetosphere needs to be included in future assessments of global flux tube dynamics and composition of the magnetosphere system. Key Points: An ionospheric outflow model is developed for use at Jupiter's auroral regionsThe model evaluates the effect of field‐aligned currents and centrifugal forcesA total number flux of 1.3 – 1.8 × 1028 s−1 is found, which is comparable to number flux from Io [ABSTRACT FROM AUTHOR]

Ionospheric outflow is driven by an ambipolar electric field induced due to the separation of electrons and ions in a gravitational field when equilibrium along a magnetic field line is lost. A model of ionospheric outflow at Saturn was developed using transport equations to estimate the number of charged particles that flow from the auroral regions into the magnetosphere. The model evaluates the outflow from 1,400 km in altitude above the 1 bar level, to 3 RS along the field line. The main ion constituents evaluated are R+ and R3+. We consider the centrifugal force exerted on the particles due to a fast rotation rate, along with the effects of field‐aligned currents present in the auroral regions. The total number flux from both auroral regions is found to be 5.5–13.0×1027 s−1, which relates to a total mass source of 5.5–17.7 kg s−1. These values are on average an order of magnitude higher than expected without the additional effects of centrifugal force and field‐aligned currents. We find the ionospheric outflow rate to be comparable to the lower estimates of the mass loading rate from Enceladus and are in agreement with recent Cassini observations. This additional mass flux into the magnetosphere can substantially affect the dynamics and composition of the inner and middle magnetosphere of Saturn. Key Points: An ionospheric outflow model is developed for use at Saturn's auroral regionsThe presence of field‐aligned currents and centrifugal forces enhances outflow by an order of magnitudePredicted total outflow flux rate of 5.5–13.0×1027 s−1 is comparable to flux calculated from Cassini data [ABSTRACT FROM AUTHOR]

Photoelectrons are an extensively studied component of planetary ionospheres which have been frequently used as a diagnostic of ambient magnetic fields. We show in this study that they also provide information on atmospheric composition via the altitude variation of photoelectron intensity at specific energies. Such an idea is applied to Mars for which a large observational sample of photoelectrons is available from the Solar Wind Electron Analyzer measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN). Our analysis reveals a strong decline in photoelectron intensity from 160 to 200 km, but this trend is restricted to a narrow energy range of 10–15 eV. By employing analytical yield spectrum calculations, we derive the model variations based on different choices of the background atmosphere. We find that the presence of CO is crucial for reproducing the observed variations. This allows atmospheric CO densities to be estimated from photoelectron measurements, which are a factor of 3–7 lower than the published densities based on the MAVEN Neutral Gas and Ion Mass Spectrometer measurements and in better agreement with existing model results. In general, the usefulness of photoelectron intensity at a specific energy as a tracer of atmospheric composition relies critically on the species‐dependent inelastic electron impact cross section at this energy. Plain Language Summary: On each solar system body with a substantial atmosphere, the ionization of atmospheric neutrals by solar photons produces photoelectrons at a typical energy of several eV to several hundred eV. These electrons have caught extensive research interests for decades because their spatial distribution has been recognized as a good tracer of ambient magnetic fields. In this study, we propose that photoelectrons could also be used as a tracer of atmospheric composition. Such an idea is applied to Mars, showing that the change of photoelectron flux with altitude at 10–15 eV relies critically on the distribution of atmospheric CO. When applied to other planets, the combination of the atmospheric species and photoelectron energy involved may vary, depending on the detailed characteristics of inelastic collisions between electrons and neutrals. Our idea provides a potentially useful constraint on the abundance of key atmospheric species when it cannot be measured accurately by a mass spectrometer, a situation often encountered in modern planetary missions. Key Points: The altitude variation of photoelectronintensity at specific energies is a potential tracer of atmospheric compositionThe altitude variation of photoelectronintensity is controlled by the species‐dependent inelastic electron impact cross sectionFor Mars, a strong altitude variation of photoelectron intensity occurs at 10–15 eV and is sensitive to the CO abundance [ABSTRACT FROM AUTHOR]

Titan's magnetic environment is a dynamic and unique place. We detect 85 flux ropes during all the Cassini flybys of Titan from 2005–2017. Analysis describing the location of flux ropes in Titan's ionosphere as well as where Titan is in Saturn's magnetosphere shows that the flux ropes are more often found when Titan is in the noon sector of Saturn's magnetosphere. A secondary peak of occurrence is found in the postmidnight area, where it is expected that Saturn's magnetosphere is highly dynamic. We also find that the flux rope occurrence is correlated with the average electron density profile of Titan's ionosphere. A force‐free model is utilized to estimate the radii and core magnetic field strength of the flux ropes. We find a large range of radii from 150–500 km with a small number of very large flux ropes (500–1,000 km) and a range of core field strengths of 1–15 nT, again with some much larger values(20–40 nT). The model also shows that more flux ropes are right‐handed than left‐handed in twist; however, we are unable to determine if there is a physical reason or if this is due to an observer bias. Additionally, we evaluate the goodness of fit for the model in each instance and conclude that, on average, the flux ropes may be better represented by a non‐force‐free model. Key Points: Eighty‐five flux ropes are detected in Titan's ionosphere using all of Cassini's flybys of TitanFlux ropes are present when Titan is in dynamic areas of Saturn's magnetosphereFitting a force‐free flux rope model leads to the conclusion that on average they are unlikely to be force free [ABSTRACT FROM AUTHOR]

This paper investigates the latitudinal distribution of quiet and storm time irregularities over the African ionosphere in the month of March 2015. The average rate of change in total electron content (TEC) index (ROTIave) calculated from Global Positioning System data is used as a proxy for the irregularities. The result during the quiet condition shows that irregularities are mostly observed between 19:00 and 00:00 LT regardless of the hemisphere. Its appearance in the Southern Hemisphere is earlier, though with less severity mainly around the equatorward edge of the equatorial anomaly peak. The peak of its development occurred at 21:00 LT, as it is observed in all the latitudes except those outside the equatorial region. The strength is found to be most severe at ∼12° magnetic latitude (up to 1.10 TECU/min.) in the south. Beyond ∼18° magnetic latitude, it is rarely noticed in the south. However, it is not observed around this latitude in the north, indicating a major hemispheric asymmetry in terms of the latitudinal boundary between the Northern and Southern Hemispheres. Furthermore, its disappearance is found to be most rapid around the poleward verge of the equatorial anomaly region particularly in the Southern Hemisphere. In contrast to quiet time observations, irregularities were absent in all latitudes during the storm days considered. This can be attributed to the local time of the storm's onset and the possible imposition of westward electric fields in the dusk period. Key Points: The pattern of latitudinal distribution of irregularities is investigated in the African regionThe distribution shows hemispheric asymmetry in terms of latitudinal boundaryThe 17 March 2015 storm event inhibited the appearance of irregularities for the storm days considered [ABSTRACT FROM AUTHOR]

In this study, we present some new features of the enhancements of electron density in the ionospheric F region at low latitudes. The peak parameters of the ionosphere were recorded by Sanya (109.6°E, 18.3°N) ionosonde and electron density profiles analyzed were measured by the Qujing (103.8°E, 25.6°N) incoherent scatter radar. The peak electron density of the F2 layer (NmF2) is found to be enhanced in the afternoon and sunset hours. Common features in the cases demonstrate that the NmF2 increases at low latitudes are characterized by a descent of the peak height (hmF2). Meanwhile, the electron density at Qujing shows an increase at the bottom of the ionosphere and a depression at topside altitudes. It is consistent with those of the ionosonde measurements at Sanya, for both the daytime and nighttime enhancements. Cross correlation analyses reveal a lag of NmF2 to hmF2 in the cases. After taking time shifts some events of the enhancements in NmF2 are still associated with a downwelling hmF2, confirming the processes which drive a lower hmF2 and a convergence of plasma flux may possibly bring about a rise in NmF2. Key Points: Peak electron density of the F2 layer presents an increase characterized by a descent of the peak height at low latitudesNe at Qujing decreases (increases) at the topside (bottomside) of the ionosphere during the enhancement in NmF2 with a downwelling hmF2The lags between changes in hmF2 and NmF2 are applied to identify the descending hmF2 associating with increase of NmF2 [ABSTRACT FROM AUTHOR]

The ionosphere of Venus is a weakly ionized plasma layer embedded in the planet's upper atmosphere. Planetary ionospheres provide an excellent opportunity to study how our variable Sun affects the planets in our solar system. Because ionospheres are reservoirs from which atmospheric species can be lost to space, studying how ionospheres respond to changes in solar activity can help us understand how planetary atmospheres have evolved since their formation. While variations of the main and lower ionospheric peaks of Venus have been well studied, the behavior of the ionosphere above the altitude of the greatest electron density has not been fully constrained. To investigate the behavior of this region, we use electron density profiles obtained by the Venus Radio Science experiment aboard Venus Express. An increase in the response of the electron density to increasing solar irradiance with increasing altitude above the peak is readily apparent in these data. By using a one‐dimensional photochemical equilibrium model to investigate the factors that drive the variations of the ionosphere of Venus, we find that changes in the composition of the underlying neutral atmosphere are responsible for the observed increase in ionospheric response with altitude. Key Points: Electron densities in Venus's ionosphere respond more strongly to solar activity changes with increasing altitude above the ionospheric peakPhotochemical models are used to reproduce observed electron density profiles and assess causes of this behaviorLarge‐scale inflation of the neutral atmosphere with increasing solar activity causes the enhanced response with increasing altitude [ABSTRACT FROM AUTHOR]