Search

In the Earth's magnetosphere wave‐particle interaction is a major ion energization process, playing an important role for the atmospheric escape. A common type of ion heating is associated with low‐frequency broadband electric wave fields. For such waves the energy is not concentrated to a certain narrow frequency range and exhibits no peaks or dips in a power spectrum. If there are enough fluctuations close to the ion gyrofrequency the electric field may still come in resonance with gyrating ions and heat them perpendicular to the background magnetic field. We perform a proof‐of‐concept study to investigate if this heating mechanism may contibute significantly to the energization of planetary ions also in the induced magnetosphere of Venus. We assume Alfvénic fluctuations and estimate the electric field spectral density based on magnetic field observations. We find typical estimated electric spectral densities of a few (mV/m)2 ${(\text{mV/m})}^{2}$/Hz close to Venus. This corresponds to a heating rate of a few eV/s. We consider an available interaction time of ∼ ${\sim} $ 300 s and conclude that this mechanism could increase the energy of an oxygen ion by about a keV. Observed thermal energies are in the range 100–1,000 eV and thus, resonant wave heating may also be important at Venus. Key Points: Energization of O+ escaping from Venus by Alfvén waves to keV energies is plausibleThe estimated electric power spectral density at the typical oxygen gyrofrequency is in the range 0.5–10 (mV/m)2/Hz close to the planetAn estimated heating rate of ≈3 eV/s is enough to explain oxygen thermal energies up to a keV [ABSTRACT FROM AUTHOR]

At a low activity comet the plasma is distributed in an asymmetric way. The hybrid simulation code Amitis is used to look at the spatial evolution of ion velocity distribution functions (VDFs), from the upstream solar wind (SW) to within the comet magnetosphere where the SW is heavily mass‐loaded by the cometary plasma. We find that the spatial structures of the ions and fields form a highly asymmetric induced magnetosphere. The VDFs of SW and cometary ions vary drastically for different locations in the comet magnetosphere. The shape of the VDFs differ for different species. The SW protons show high anisotropies that occasionally resemble partial rings, in particular at small cometocentric distances. A second, decoupled, proton population is also found. Solar wind alpha particles show similar anisotropies, although less pronounced and at different spatial scales. The VDFs of cometary ions are mostly determined by the structure of the electric field. We perform supplementary dynamic particle backtracing to understand the flow patterns of SW ions that lead to these anisotropic distributions. This tracing is needed to understand the origin of cometary ions in a given part of the comet magnetosphere. The particle tracing also aids in interpreting observed VDFs and relating them to spatial features in the electric and magnetic fields of the comet environment. Key Points: Hybrid simulations with the Amitis code for a low activity comet show the formation of an asymmetric induced magnetosphereThe velocity distributions of solar wind protons form partial rings in the simulation as previously reported by observationsBacktracing the cometary ions in the tail shows that the shape of their velocity distributions is driven by electric field structures [ABSTRACT FROM AUTHOR]

Magnetosheath jets represent localized enhancements in dynamic pressure observed within the magnetosheath. These energetic entities, carrying excess energy and momentum, can impact the magnetopause and disrupt the magnetosphere. Therefore, they play a vital role in coupling the solar wind and terrestrial magnetosphere. However, our understanding of the morphology and formation of these complex, transient events remains incomplete over two decades after their initial observation. Previous studies have relied on oversimplified assumptions, considering jets as elongated cylinders with dimensions ranging from |$0.1\, R_{\rm E}$| to |$5\, R_{\rm E}$| (Earth radii). In this study, we present simulation results obtained from Amitis, a high-performance hybrid-kinetic plasma framework (particle ions and fluid electrons) running in parallel on graphics processing units (GPUs) for fast and more environmentally friendly computation compared to CPU-based models. Considering realistic scales, we present the first global, three-dimensional (3D in both configuration and velocity spaces) hybrid-kinetic simulation results of the interaction between solar wind plasma and the Earth. Our high-resolution kinetic simulations reveal the 3D structure of magnetosheath jets, showing that jets are far from being simple cylinders. Instead, they exhibit intricate and highly interconnected structures with dynamic 3D characteristics. As they move through the magnetosheath, they wrinkle, fold, merge, and split in complex ways before a subset reaches the magnetopause. [ABSTRACT FROM AUTHOR]

Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock. Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track. Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity. Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 1027–1031–s-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 102 km to 106 km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn. Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity. [ABSTRACT FROM AUTHOR]

Spatio‐temporal variations of ionospheric currents cause rapid magnetic field variations at ground level and Geomagnetically Induced Currents (GICs) that can be harmful for human infrastructure. The risk for large excursions in the magnetic field time derivative, "dB/dt spikes", is known to be high during geomagnetic storms and substorms. However, less is known about the occurrence of spikes during non‐stormy times. We use data from ground‐based globally covering magnetometers (SuperMAG database) from the years 1985–2021. We investigate the spike occurrence (|dB/dt| > 100 nT/min) as a function of magnetic local time (MLT), magnetic latitude (Mlat), and the solar cycle phases during non‐stormy times (−15 nT ≤ SYM‐H < 0). We sort our data into substorm (AL < 200 nT) intervals ("SUB") and less active intervals between consecutive substorms ("nonSUB"). We find that spikes commonly occur in both SUBs and nonSUBs during non‐stormy times (3–23 spikes/day), covering 18–12 MLT and 65°–80° Mlat. This also implies a risk for infrastructure damage during non‐stormy times, especially when several spikes occur nearby in space and time, possibly causing infrastructure weathering. We find that spikes are more common in the declining phase of the solar cycle, and that the occurrence of SUB spikes propagates from one midnight to one morning hotspot with ∼10 min in MLT for each minute in universal time (UTC). Finally, we discuss causes for the spikes in terms of spatio‐temporal variations of ionospheric currents. Plain Language Summary: Our active sun disturbs Earth's magnetosphere and causes current systems and space weather effects. For example, space weather driven currents in the upper ionized atmosphere (ionosphere) can cause large excursions ("spikes") in the magnetic field time derivative at the ground level, dB/dt. These in turn cause Geomagnetically Induced Currents (GICs) in the ground and in human infrastructure, potentially causing burnouts in powerlines and transformers. It is well‐known that dB/dt spikes are common during geomagnetically active time such as geomagnetic storms and geomagnetic substorms. However, much less is known about the spike occurrence during non‐stormy times. In this article we show that spikes also occur often during non‐stormy times. Spikes are most common in the declining phase of the 11‐year solar cycle. Moreover, we show that the spike occurrence propagate with time: in the beginning of a substorm, spikes typically occur in the pre‐midnight magnetic local time sector, but later in the substorm, the spike occurrence has propagated to the morning sector. Our results imply that human infrastructure is not safe even during non‐stormy times. Instead, it can be exposed to harmful weathering also during those times. Key Points: Potentially harmful dB/dt spikes are also common during non‐stormy times, for example, during weak SYM‐H and after substormsThe highest spike occurrence rate is in the declining phase of the solar cycleSubstorm spike occurrence exhibits propagation from midnight to morning sector: about 10 min propagation in MLT for each min in UT [ABSTRACT FROM AUTHOR]

No spacecraft visiting a comet has been equipped with instruments to directly measure the static electric field. However, the electric field can occasionally be estimated indirectly by observing its effects on the ion velocity distribution. We present such observations made by the Rosetta spacecraft on 19 April 2016, 35 km from the nucleus. At this time comet 67P was at a low outgassing rate and the plasma environment was relatively stable. The ion velocity distributions show the cometary ions on the first half of their gyration. We estimate the bulk drift velocity and the gyration speed from the distributions. By using the local measured magnetic field and assuming an E × B drift of the gyrocentre, we get an estimate for the average electric field driving this ion motion. We analyze a period of 13 hr, during which the plasma environment does not change drastically. We find that the average strength of the perpendicular electric field component is 0.21 mV/m. The direction of the electric field is mostly anti‐sunward. This is in agreement with previous results based on different methods. Plain Language Summary: Measuring the static electric field in space plasmas is difficult. Most spacecraft do not have dedicated instruments for it, and the Rosetta mission to comet 67P is no exception. But the electric field is one of the main governing factors behind the motion of newly born cometary ions. In this study, we use measurements of the cometary ions to estimate the average electric field close to the nucleus. The observations are made on the 19 April 2016 by the Ion Composition Analyzer, which measures the energy and travel direction of the different plasma species. The specific shape of the observed velocity distribution of cometary ions—a partial ring—indicates that the fields accelerating the observed cometary ions are relatively homogeneous. The spatial scale this applies to is approximately one gyroradius, which we estimated to be around 340 km. The resulting electric field is 0.21 mV/m, which is significantly smaller than the expected field in the upstream solar wind, far away from the nucleus. Key Points: Rosetta observations show partial ring distributions of cometary ions at comet 67P close to the nucleusFrom the velocity distributions the plasma bulk velocity and gyration speed are determinedWe estimate the perpendicular electric field component from the bulk velocity and find a mostly anti‐sunward field of 0.21 mV/m [ABSTRACT FROM AUTHOR]

We present partial ring distributions of solar wind protons observed by the Rosetta spacecraft at comet 67P/Churyumov‐Gerasimenko. The formation of ring distributions is usually associated with high activity comets, where the spatial scales are larger than multiple ion gyroradii. Our observations are made at a low‐activity comet at a heliocentric distance of 2.8 AU on 19 April 2016, and the partial rings occur at a spatial scale comparable to the ion gyroradius. We use a new visualization method to simultaneously show the angular distribution of median energy and differential flux. A fitting procedure extracts the bulk speed of the solar wind protons, separated into components parallel and perpendicular to the gyration plane, as well as the gyration velocity. The results are compared with models and put into context of the global comet environment. We find that the formation mechanism of these partial rings of solar wind protons is entirely different from the well‐known partial rings of cometary pickup ions at high‐activity comets. A density enhancement layer of solar wind protons around the comet is a focal point for proton trajectories originating from different regions of the upstream solar wind. If the spacecraft location coincides with this density enhancement layer, the different trajectories are observed as an energy‐angle dispersion and manifest as partial rings in velocity space. Plain Language Summary: Particles of solar origin, called the "solar wind," flow straight from the Sun in interplanetary space. When this solar wind meets an obstacle, such as a planet, it gets deflected around it. At comet 67P/Churyumov‐Gerasimenko, visited by the Rosetta spacecraft from 2014 to 2016, our instrument Rosetta Plasma Consortium (RPC)‐Ion Composition Analyzer (ICA) measured the main constituents of this solar wind: protons and alpha particles. When the comet is far away from the Sun, the solar wind protons are usually observed coming from the sunward direction with only slight deflection and constant velocities. On 19 April 2016, the main case for our study, we measure solar wind protons arriving in a wide range of directions. The velocity of these protons depends on how much they have been deflected. This creates partial ring distributions, which we visualize and quantify using a method specifically developed for this purpose. We show that these partial rings are a rare observation of a spatially confined region where solar wind protons from different regions of the solar wind are observed simultaneously. Key Points: Broad energy spectra in our observations are due to solar wind protons forming partial ring distributionsThe partial ring distributions form due to solar wind proton trajectories focusing at a density enhancement layerFrom the partial ring distributions we estimate the average upstream magnetic field direction and the average bulk plasma drift velocity [ABSTRACT FROM AUTHOR]

The presence of a large‐scale planetary magnetic field is thought to be a protective factor for atmospheres, preventing them from being blown off by the solar wind. We focus on one key aspect of atmospheric escape: how does a planetary magnetic fields affect the energy transfer from the Sun to the atmosphere? We estimate the solar wind energy currently dissipated in the Earth's atmosphere using empirical formulas derived from observations. We show that it is significantly higher than the energy dissipated in the atmosphere of a hypothetical unmagnetized Earth. Consequently, we conclude that the Earth's magnetic field enhances the solar energy dissipation in the Earth's atmosphere and that, contrary to the old paradigm, an intrinsic magnetic field does not necessarily reduces atmospheric loss. Key Points: The solar wind energy dissipated in the Earth's upper atmosphere is higher than it would be if the Earth were not magnetized [ABSTRACT FROM AUTHOR]

We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future. Keywords: Mars; Solar wind; Magnetospheres

The neutral particle detector (NPD) on board Mars Express has observed energetic neutral atoms (ENAs) from a broad region on the dayside of the martian upper atmosphere. We show one such example for which the observation was conducted at an altitude of 570 km, just above the induced magnetosphere boundary (IMB). The time of flight spectra of these ENAs show that they had energies of 0.2-2 keV/amu, with an average energy of ~1.1 keV/amu. Both the spatial distribution and the energy of these ENAs are consistent with the backscattered ENAs, produced by an ENA albedo process. This is the first observation of backscattered ENAs from the martian upper atmosphere. The origin of these ENAs is considered to be the solar wind ENAs that are scattered back by collision processes in the martian upper atmosphere. The particle flux and energy flux of the backscattered ENAs are 0.9-1.3 x [10.sup.7] [cm.sup.-2] [s.sup.-1] and ~9.5 x [10.sup.9] eV [cm.sup.-2] [s.sup.-l], respectively. Keywords: Mars, atmosphere; Solar wind; Atmospheres, dynamics; Atmospheres, structure; Ionospheres

We have studied the interaction of fast solar wind hydrogen atoms with the martian atmosphere by a three-dimensional Monte Carlo simulation. These energetic neutral hydrogen atoms, H-ENAs, are formed upstream of the martian bow shock. Both H-ENAs scattered and non-scattered from the martian atmosphere/exosphere were studied. The colliding H-ENAs were found to scatter both to the dayside and nightside. On the dayside they contribute to the so-called H-ENA albedo. On the nightside the heated and scattered hydrogen atoms were found also in the martian wake. The density, the energy distribution function and the direction of the velocity of H-ENAs on the nightside are presented. The present study describes a novel 'ENA sounding' technique in which energetic neutral atoms are used to derive information of the properties of planetary exosphere and atmosphere in a similar manner as the solar wind photons are used to derive atmospheric densities by measuring the scattered UV light. A detailed study of the direction and energy of the scattered and non-scattered H-ENAs suggest that the ENA sounding is a method to study the interaction between the planetary atmosphere and the solar wind and to monitor the density, and likely also the magnetization, of the planetary upper atmosphere. Already present-day ENA instrument should be capable to detect the analyzed particle fluxes. Keywords: Mars, atmosphere

Measurements of energetic neutral atoms (ENA) generated in the magnetosheath at Mars are reported. These ENAs are the result of charge exchange collisions between solar wind protons and neutral oxygen and hydrogen in the exosphere of Mars. The peak of the observed ENA flux is 1.3 x [10.sup.11] [m.sup.-2] [sr.sup.-1] [s.sup.-1]. For the case studied here, i.e., the passage of Mars Express through the martian magnetosheath around 20:15 UT on 3 May 2004, the measurements agree with an analytical model of the ENA production at the planet. It is possible to find parameter values in the model such that the observed peak in the ENA count rate during the spacecraft passage through the magnetosheath is reproduced. Keywords: Mars; Solar wind; Magnetospheres

The Neutral Particle Detector (NPD), an Energetic Neutral Atom (ENA) sensor of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) on board Mars Express, detected intense fluxes of ENAs emitted from the subsolar region of Mars. The typical ENA fluxes are (4-7) x [10.sup.5] [cm.sup.-2] [sr.sup.-1] [s.sup.-1] in the energy range 0.3-3 keV. These ENAs are likely to be generated in the subsolar region of the martian exosphere. As the satellite moved away from Mars, the ENA flux decreased while the field of view of the NPD pointed toward the subsolar region. These decreases occurred very quickly with a time scale of a few tens of seconds in two thirds of the orbits. Such a behavior can be explained by the spacecraft crossing a spatially constrained ENA jet, i.e., a highly directional ENA emission from a compact region of the subsolar exosphere. This ENA jet is highly possible to be emitted conically from the subsolar region. Such directional ENAs can result from the anisotropic solar wind flow around the subsolar region, but this can not be explained in the frame of MHD models. Keywords: Mars, atmosphere; Solar wind; Magnetospheres

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite on Mars Express have been analyzed to determine the mass composition of the escaping ion species at Mars. We have examined 77 different ion-beam events and we present the results in terms of flux ratios between the following ion species: C[O.sup.+.sub.2]/[O.sup.+] and [O.sup.+.sub.2]/[O.sup.+]. The following ratios averaged over all events and energies were identified: C[O.sup.+.sub.2]/[O.sup.+] = 0.2 and [O.sup.+.sub.2]/[O.sup.+] = 0.9. The values measured are significantly higher, by a factor of 10 for [O.sup.+.sub.2]]/[O.sup.+], than a contemporary modeled ratio for the maximum fluxes which the martian ionosphere can supply. The most abundant ion species was found to be [O.sup.+], followed by [O.sup.+.sub.2] and C[O.sup.+.sub.2]. We estimate the loss of C[O.sup.+.sub.2] to be 4.0 x [10.sup.24] s-1 (0.29 kg[s.sup.-1]) by using the previous measurements of Phobos-2 in our calculations. The dependence of the ion ratios in relation to their energy ranges we studied, 0.3-3.0 keV, indicated that no clear correlation was found. Keywords: Ionospheres; Mars, atmosphere

We have analysed ion escape at Mars by comparing ASPERA-3/Mars Express ion measurements and a 3-D quasi-neutral hybrid model. As Mars Express does not have a magnetometer onboard, the analysed IMA data are from an orbit when the IMF clock angle was possible to determine from the magnetic field measurements of Mars Global Surveyor. We found that fast escaping planetary ions were observed at the place which, according to the 3-D model, is anticipated to contain accelerated heavy ions originating from the martian ionosphere. The direction of the interplanetary magnetic field was found to affect noticeably which regions can be magnetically connected to Mars Express and to the overall 3-D Mars-solar wind interaction. Keywords: Mars, atmosphere; Ionospheres

Localized dynamic pressure pulses in the magnetosheath, or jets, have been a popular topic for discussion in recent decades. Studies show that they can propagate through the magnetosheath and impact the magnetopause, possibly showing up as geoeffective elements at ground level. However, questions still remain on how geoeffective they can be. Previous studies have been limited to case studies during few days and with only a handful of events. In this study we have found 65 cases of impacting jets using observations from the Multiscale Magnetospheric mission during 2015–2017. We examine their geoeffectiveness using ground‐based magnetometers (GMAGs). From our statistics we find that GMAGs observe responses as fluctuations in the geomagnetic field with amplitudes of 34 nT, frequencies of 1.9 mHz, and damping times of 370 s. Further, the parallel length and the maximum dynamic pressure of the jet dictate the amplitude of the observed GMAG response. Longer and higher pressure jets inducing larger amplitude responses in GMAG horizontal components. The median time required for the signal to be detected by GMAGs is 190 s. We also examine if jets can be harmful for human infrastructure and cannot exclude that such events could exist. Key Points: Ground‐based magnetometers observe damped oscillations resulting from impacting jetsThe median damping time and observed frequency is 370 s and 1.9 mHz respectivelyWe suggest that larger and more energetic jets might increase the amplitude of the ground‐based magnetic response [ABSTRACT FROM AUTHOR]

The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs -- Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ~75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical-like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the "freshly" rebounded BBFs from the near-Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear-induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs. [ABSTRACT FROM AUTHOR]

In Earth's magnetotail plasma sheet, the sunward-tailward Bx component of the magnetic field is often used to separate the region above and below the cross-tail current sheet. Using a threedimensional magneto-hydrodynamic simulation, we show that high-speed flows do not only affect the north-south magnetic field component (causing dipolarization fronts), but also the sunward-tailward component via the formation of a magnetic dent. This dent is such that, in the Northern Hemisphere, the magnetic field is tailward while in the Southern Hemisphere, it is earthward. This is opposite to the expected signatures where Bx > 0 (Bx < 0) above (below) the neutral sheet. Therefore, the direction of the magnetic field cannot always be used to identify in which hemisphere an in situ spacecraft is located. In addition, the cross-tail currents associated with the dent is different from the currents in a tail without a dent. From the simulation, we suggest that the observation of a dawnward current and a tailward magnetic tension force, possibly together with an increase in the plasma beta, may indicate the presence of a magnetic dent. To exemplify, we also present data of a high-speed flow observed by the Cluster mission, and we show that the changing sign of Bx is likely due to such a dent, and not to the spacecraft moving across the neutral sheet. [ABSTRACT FROM AUTHOR]

Since their formation in the protosolar nebula some ∼4.5 billion years ago, comets are in storage in cold distant regions of the solar system, the Kuiper Belt/scattered disk or Oort Cloud. Therefore, they have been considered as mostly unaltered samples of the protosolar nebula. However, a significant dose of energy is deposited by galactic cosmic rays (GCRs) into the outermost tens of meters of cometary nuclei during their stay in the Oort Cloud or Kuiper Belt. We investigate the impact of energy deposition by GCRs on cometary nuclei. We use experimental results from laboratory experiments and the energy deposition by GCRs estimated by Gronoff et al. (2020), to discuss the depth down to which the cometary nucleus is altered by GCRs. We show that GCRs do not significantly change the isotopic composition of cometary material but modify the chemical composition and the ice structure in the outer layers of the nucleus, which cannot be considered as pristine solar nebula material. We discuss the effect of the collisional history of comets on the distribution of processed material inside the nucleus and its implication on the observation of comets. [ABSTRACT FROM AUTHOR]

Plasma structures with enhanced dynamic pressure, density, or speed are often observed in Earth's magnetosheath. We present a statistical study of these structures, known as jets and fast plasmoids, in the magnetosheath, downstream of both the quasi‐perpendicular and quasi‐parallel bow shocks. Using measurements from the four Magnetospheric Multiscale (MMS) spacecraft and OMNI solar wind data from 2015–2017, we present observations of jets during different upstream conditions and in the wide range of distances from the bow shock. Jets observed downstream of the quasi‐parallel bow shock are seen to propagate deeper and faster into the magnetosheath and on toward the magnetopause. We estimate the shape of the structures by treating the leading edge as a shock surface, and the result is that the jets are elongated in the direction of propagation but also that they expand more quickly in the perpendicular direction as they propagate through the magnetosheath. Plain Language Summary: The solar wind is a stream of charged particles continuously emitted from the upper atmosphere of the Sun. When it approaches Earth, it is slowed down and creates the bow shock. The region with high temperature and lower speed, downstream of the bow shock is called the magnetosheath. From time to time, plasma jets with speeds close to the solar wind speed are observed in this magnetosheath. They are thought to be formed at the bow shock, which is the boundary between the magnetosheath and the solar wind. In this article, we use data obtained by the four MMS spacecraft, while they passed through the magnetosheath, in a statistical study of the properties of the jets. We have found that they slow down as they move through the magnetosheath and that, in the beginning, they are elongated in the direction of their motion, but also that they expand to become rounder as they move along. Key Points: The jets grow larger and slower as they move away from the bow shockThe deceleration of jets and fast plasmoids in the quasi‐perpendicular magnetosheath is twice as fast as in the quasi‐parallel magnetosheathJets propagate deeper into the magnetosheath for smaller angles between the interplanetary magnetic field and the bow shock normal [ABSTRACT FROM AUTHOR]

We discuss a method to estimate the properties of a magnetospheric generator using a quasi‐electrostatic magnetosphere‐ionosphere coupling model and in situ or remote sensing observations of discrete quiet arcs. We first construct an ensemble of Vlasov equilibrium solutions for generator structures formed at magnetospheric plasma interfaces. For each generator solution, we compute the ionospheric electric potential from the current continuity equation. Thus, we estimate the field‐aligned potential drop that allows us to assess several properties of the discrete auroral arc, such as the field‐aligned potential difference, the field‐aligned current density, the flux of precipitating energy, and the height‐integrated Pedersen conductance. A minimization procedure based on comparing the numerical results with observations is defined and applied to find which solution of the current continuity equation and which generator model give auroral arc properties that best fit the observations. The procedure is validated in a case study with observations by DMSP and Cluster and can be generalized to other types of data. Key Points: We discuss a method to estimate the properties of the auroral generator from low‐altitude observations of auroraThe method is based on a magnetosphere‐ionosphere quasi‐stationary coupling model, a parametric description of a magnetospheric interface generator, and a minimization procedure that finds the generator model whose auroral effects best fit in situ observationsThe method can be generalized to other types of generators and observables [ABSTRACT FROM AUTHOR]