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Boundaries between space plasmas occur in numerous contexts and scales, from astrophysical jets to planetary magnetospheres. Mass and momentum transport across boundaries poses a fundamental problem in magnetospheric physics. Kelvin-Helmholtz instability (KHI) is a promising mechanism to facilitate transport. Although previous studies have suggested KHI occurrence in various space plasmas, theory predicts that compressibility prevents KHI excitation at boundaries with large density gradients because of previously considered boundary structures where density varies with velocity. Based on the observations of a large density gradient boundary by MAVEN at Mars, where we can observe an extreme case, in this study, we show that it is the entropy, instead of the previously considered density, that varies with the velocity in the real velocity-sheared boundary. The entropy-based boundary structure places the velocity shear in a lower-density region than the traditional density-based structure and weakens the compressibility effect. This new boundary structure thus enables KHI excitation even at large density gradient boundaries, such as at the ionopause of unmagnetized planets and the plasmapause of magnetized planets. The result suggests the ubiquitous occurrence of KHI in the plasma universe and emphasizes its important role in planetary cold plasma escape from unmagnetized planets. [ABSTRACT FROM AUTHOR]

The presence of plasma waves upstream from the Martian bow shock, with frequencies near the local proton cyclotron frequency in the spacecraft frame, constitutes, in principle, an indirect signature for the existence of planetary protons from the ionization of Martian exospheric hydrogen. In this study, we determine the "proton cyclotron wave” (PCW) occurrence rate between October 2014 and February 2020, based on Magnetometer and Solar Wind Ion Analyzer measurements from the Mars Atmosphere and Volatile EvolutioN mission. We characterize its dependence on several wave and solar wind (SW) properties, and solar longitude ranges. We confirm a previously reported long‐term trend with more PCWs near perihelion, likely associated with changes in exospheric hydrogen density. Furthermore, we report for the first time a decrease in median PCW amplitude for each consecutive Martian perihelion. Such variability cannot be attributed to differences in the distribution of SW conditions. This trend could be associated with changes in solar inputs, foreshock effects, and asymmetries due to the SW convective electric field influencing newborn protons. In addition, we observe PCWs more frequently for low to intermediate interplanetary magnetic field (IMF) cone angles, slower SW speeds, and higher SW proton densities. The IMF cone angle preference likely results from the trade‐off between associated linear wave growth rates, wave saturation energies, and pick‐up proton densities. Moreover, the dependencies on SW speed and density indicate the importance of the characteristic SW transit timescale and the charge exchange process coupling SW protons with the hydrogen exosphere.Key Points: We confirm an annual periodicity of proton cyclotron wave occurrence rate upstream from the Martian bow shock between October 2014 and February 2020We report a decrease in median wave amplitude for each consecutive Martian perihelion based on available sampling of the Martian upstream regionPCWs occur more frequently near perihelion for low to intermediate interplanetary magnetic field cone angles, slower solar wind speeds and higher solar wind densities [ABSTRACT FROM AUTHOR]

Magnetic reconnection is often suggested to be implicated in a variety of dynamic phenomena observed in the complex magnetosphere of Mars. We present a first global survey of ion jets within current sheets, which are thought of as a primary product of magnetic reconnection, with Mars Atmosphere and Volatile EvolutioN (MAVEN) data obtained on the dayside and nightside in the Martian magnetosphere. We develop a fully automated algorithm to efficiently and reliably identify ion flow velocity deviations during current sheet crossings recorded by MAVEN. The obtained statistical results show that (i) sunward and anti‐sunward ion jets embedded within current sheets are commonly and widely observed in the dayside and nightside magnetosphere of Mars, (ii) measured magnetic field profiles and inferred magnetic topology during the jet crossings are generally consistent with those expected for reconnecting current sheets, (iii) the jet occurrence is apparently independent of upstream driver conditions, and (iv) most of the identified current sheets are thin with a roughly ion scale half‐thickness and are embedded in low beta plasma, suggesting that they are seemingly capable of reconnection based on the known onset conditions. The widespread distributions and common detection of ion jets, as well as the ubiquitous presence of thin, low‐beta current sheets, imply that magnetic reconnection could operate frequently in many locations around Mars, thereby playing an important role in the dynamics of the Martian magnetosphere for most, if not all, of the solar wind conditions in the present epoch. Key Points: Both sunward and anti‐sunward ion jets embedded within current sheets are widely and commonly observed in the near Mars spaceMagnetic field configuration and topology around the jets are generally consistent with those expected from magnetic reconnectionThe jet occurrence appears independent of upstream drivers, possibly resulting from ubiquitous formation of low‐beta, thin current sheets [ABSTRACT FROM AUTHOR]

The observations made by the Mars Atmosphere and Volatile EvolutioN spacecraft in the topside (≥200 km) ionosphere of Mars show that this region is very responsive to the variations of the external (solar extreme ultraviolet flux, solar wind, and interplanetary magnetic field [IMF]) and internal (the crustal magnetic field) drivers. With the growth of the solar irradiance the ionosphere broadens while with increase of the solar wind dynamic pressure it shrinks. As a result, the upper ionospheric boundary at solar zenith angles of 60–70° can move from ∼400 to ∼1,200 km. Similar trends are observed at the nightside ionosphere. At Pdyn ≥ 1–2 nPa the nightside ionosphere becomes very fragmented and depleted. On the other hand, the ion density in the nightside ionosphere significantly (up to a factor of 10) increases with the rise of the solar extreme ultraviolet flux. Large‐amplitude motions of the topside ionosphere also occur with variations of the value of the cross‐flow component of the IMF. The upper dayside ionosphere at altitudes of more than 300–400 km is sensitive also to the direction of the cross‐flow component of the IMF or, correspondingly, to the direction of the motional electric field in the solar wind. The ionosphere becomes very asymmetrical with respect to the Vsw×BIMF direction and the asymmetry strongly enhances at the nightside. The topside ionosphere above the areas with strong crustal magnetic field in the dayside southern hemisphere is significantly denser and expands to higher altitudes as compared to the ionosphere above the northern nonmagnetized lowlands. The crustal magnetic field also protects the nightside ionosphere from being filled by plasma transported from the dayside. The draping IMF penetrates deeply into the ionosphere and actively influences its structure. Weak fields and, correspondingly, weak magnetic field forces only slightly affect the ionosphere. With increase of the induced magnetic field strength the transport motions driven by the magnetic field pressure and field tensions seem to be intensified and we observe that the local ion densities at the dayside considerably decrease. A different trend is observed at the nightside. The ion density in the nightside ionosphere above the northern lowlands is higher than in the southern hemisphere indicating that plasma transport from the dayside is the main source of the nightside ionosphere. Nonstop variations in the solar wind, the IMF and the solar irradiance together with planetary rotation of the crustal magnetic field sources lead to a continuous expansion/shrinking and reconfiguration of the topside ionosphere of Mars. Key Points: Upper ionosphere of Mars is very sensitive to the solar wind variationsUpper ionosphere of Mars is very sensitive to the variations of the interplanetary magnetic fieldUpper ionosphere of Mars is very sensitive to the planetary crustal field [ABSTRACT FROM AUTHOR]

We investigate Martian ultralow frequency (ULF) electromagnetic waves generated by local plasma instabilities below the Martian bow shock. Recent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown that ULF waves generated upstream of the Martian bow shock can propagate down to the upper ionosphere, possibly facilitating heavy ion escape from Mars by heating the ionospheric plasma. In contrast to the upstream waves oscillating near the upstream proton cyclotron frequency, we identify narrow band ULF magnetic field fluctuations with frequencies near the local proton cyclotron frequency (fcp(local)) from MAVEN data. In addition to expected proton cyclotron waves locally generated in the magnetosheath, we newly identify compressional narrow band emissions near fcp(local) (and its harmonics for some cases) in the dayside upper ionosphere and in the nightside magnetotail. The dayside waves are preferentially observed for high solar extreme ultraviolet (EUV) conditions and are often associated with ring/shell‐like, hot protons of magnetosheath origin in the presence of cold, dense ionospheric protons. The nightside waves exhibit distinct preference for high‐solar‐EUV, strong‐solar‐wind conditions, under which both warm and cold protons are enhanced. The observed properties of these compressional waves are generally consistent with a proton Bernstein mode instability driven by a positive perpendicular slope in proton velocity distribution functions. The excited waves can cause perpendicular heating of thermal protons, thereby transferring energy from precipitating hot protons to cold ionospheric protons. Key Points: MAVEN observes narrow band ULF waves near the local proton cyclotron frequency below the Martian bow shockCompressional narrow band emissions are newly identified in the dayside upper ionosphere and in the nightside magnetotailThe wave‐particle interaction facilitates energy transfer from hot protons of magnetosheath origin to cold ionospheric protons [ABSTRACT FROM AUTHOR]

We present a study of the magnetic field in the Martian magnetosheath using data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and provide a proxy for the upstream interplanetary magnetic field (IMF) clock angles based on the sheath field directions. The magnetic field data from MAVEN taken in the sheath region are organized in the Mars Solar Electric field coordinate system and sorted by the IMF polarity. The sheath magnetic field distributions clearly illustrate the morphology of draped IMF around the planetary obstacle and show strong asymmetries between the quasi‐perpendicular and quasi‐parallel bow shock quadrants as well as a slight asymmetry between the +E and –E quadrants. We have also studied the effects of different drivers on the strengths and clock angles of the sheath magnetic field and find that the sheath field is strongly affected by upstream IMF and solar wind and that Martian crustal fields also have small contributions to the sheath magnetic field strength. Based on the morphology of the draped field in the sheath, we developed a simple IMF clock angle proxy as the clock angle of the orbital‐averaged sheath magnetic field, which works well under steady IMF conditions. This proxy has an error <44° for 75% of all instances. The uncertainty of the proxy depends on upstream conditions and spacecraft orbit geometry. We also provide estimates of the proxy uncertainty based on sheath magnetic field variability and proxy clock angle. Key Points: The magnetic field in the Martian magnetosheath shows asymmetric features with respect to the upstream electric field and IMF directionsThe sheath magnetic field is mainly controlled by IMF and solar wind conditions and slightly modified by Martian crustal fieldsThe sheath magnetic field can be used as an IMF clock angle proxy with an error <45 degrees for 75% of all instances [ABSTRACT FROM AUTHOR]

Abstract: Cold ion outflow is one of the candidate processes to cause significant atmospheric escape from Mars. We here report on the cold dense heavy ion outflow event observed in Martian‐induced magnetotail by Mars Atmosphere and Volatile EvolutioN (MAVEN) on 4 December 2014. In the outflow event, the cold dense heavy ion outflow was observed only in south dusk downward convection electric field (E) lobe. During the event, the strong crustal magnetic field was located on dayside of Mars. It suggests that combination of minimagnetosphere and downward E facilitates the cold dense heavy ion outflow. In order to estimate the CO2+ density, we adopted a fitting method of log‐normal distribution to the O2+ time of flight distributions to eliminate O2+ contamination in the CO2+ time of flight data. Contribution of O2+, O+, and CO2+ to the total heavy ions (~134 cm−3) is about 71%, 26%, and 3%, respectively. Average heavy ion flux was ~5.4 × 107 cm−2s−1, assuming the escape velocity at 2,000‐km altitude. [ABSTRACT FROM AUTHOR]

Abstract: The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magnetic field lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvénic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere. [ABSTRACT FROM AUTHOR]

Abstract: We perform a survey of 1‐Hz waves at Mars utilizing Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observations for a Martian year. We find that the 1‐Hz wave occurrence rate shows an apparent variation caused by masking of the waves by background turbulence during the times when the background turbulence levels are high. To correct for this turbulence masking, we select waves that occur in time intervals where the background turbulence levels are low. We find that the extreme ultraviolet flux does not affect the wave occurrence rate significantly, suggesting that the newly born pickup ions originating in the Mars's exosphere contribute minimally to the 1‐Hz wave generation. We find that the wave occurrence rates are higher for low Mach numbers and low beta values than for high Mach numbers and high beta values. Further, we find that a high percentage of 1‐Hz waves satisfy the group‐standing condition, which suggests that a high percentage of the waves seen as monochromatic waves in the spacecraft frame can be broadband waves in the solar wind frame that have group velocities nearly equal and opposite to the solar wind velocity. We infer that the wave occurrence rate trends with the Mach number and proton beta are a consequence of how the Mach numbers and beta values influence the wave generation and damping or how those parameters affect the group‐standing condition. Finally, we find that the 1‐Hz waves are equally likely to be found in both the quasi‐parallel and the quasi‐perpendicular foreshock regions. [ABSTRACT FROM AUTHOR]

Abstract: The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft arrived at Mars with the goal of determining the rates and mechanisms of atmospheric escape. Thermal hydrogen and hot oxygen escape are the two most important escape processes currently at work. Direct measurement of the escaping neutral hydrogen and oxygen atoms is impossible with current technology due to the low density and energy of escaping neutrals. However, when ionized and picked up by the solar wind, these escaping atoms can be detected by three particle detectors onboard MAVEN. By back‐tracing the trajectories of measured pickup ions, constraints can be placed on the density of neutrals at altitudes not accessible by other measurement methods. In this work, pickup H+ and O+ data from the Solar Energetic Particle (SEP), Solar Wind Ion Analyzer (SWIA), and SupraThermal and Thermal Ion Composition (STATIC) instruments are used to assess the variability of neutral H and O exospheres at Mars. From an analysis of 2.5 Earth years of MAVEN data, we show that a strong H escape seasonal dependence is observed by SWIA and STATIC with inferred H escape rates as low as 3 × 1025 s−1 near aphelion and as high as 4 × 1026 s−1 near perihelion. Hot O escape rates derived from SEP, SWIA, and STATIC data imply a much less variable hot O exosphere with escape rates fluctuating by a factor of 2 around a mean value of 9 × 1025 s−1. Both escape rates are in general agreement with the most recent theoretical, modeled, and observationally inferred rates. [ABSTRACT FROM AUTHOR]

Abstract: It has long been a goal of the heliophysics community to understand solar wind variability at heliocentric distances other than 1 AU, especially at ∼1.5 AU due to not only the steepening of solar wind stream interactions outside 1 AU but also the number of missions available there to measure it. In this study, we use 35 months of solar wind and interplanetary magnetic field (IMF) data taken at Mars by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft to conduct an autocorrelation analysis of the solar wind speed, density, and dynamic pressure, which is derived from the speed and density, as well as the IMF strength and orientation. We found that the solar wind speed is coherent, that is, has an autocorrelation coefficient above 1/e, over roughly 56 hr, while the density and pressure are coherent over smaller intervals of roughly 25 and 20 hr, respectively, and that the IMF strength is coherent over time intervals of approximately 20 hr, while the cone and clock angles are considerably less steady but still somewhat coherent up to time lags of roughly 16 hr. We also found that when the speed, density, pressure, or IMF strength is higher than average, the solar wind or IMF becomes uncorrelated more quickly, while when they are below average, it tends to be steadier. This analysis allows us to make estimates of the values of solar wind plasma and IMF parameters when they are not directly measured and provide an approximation of the error associated with that estimate. [ABSTRACT FROM AUTHOR]

Abstract: Mars Atmosphere and Volatile EvolutioN observations at Mars show clear signatures of the shocked solar wind interaction with the extended oxygen atmosphere and hot corona displayed in a lateral deflection of the magnetosheath flow in the direction opposite to the direction of the solar wind motional electric field. The value of the velocity deflection reaches ∼50 km/s. The occurrence of such deflection is caused by the “Lorentz‐type” force due to a differential streaming of the solar wind protons and oxygen ions originating from the extended oxygen corona. The value of the total deceleration of the magnetosheath flow due to mass loading is estimated as ∼40 km/s. [ABSTRACT FROM AUTHOR]

Abstract: Normal solar wind flows and intense solar transient events interact directly with the upper Martian atmosphere due to the absence of an intrinsic global planetary magnetic field. Since the launch of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, there are now new means to directly observe solar wind parameters at the planet's orbital location for limited time spans. Due to MAVEN's highly elliptical orbit, in situ measurements cannot be taken while MAVEN is inside Mars' magnetosheath. To model solar wind conditions during these atmospheric and magnetospheric passages, this research project utilized the solar wind forecasting capabilities of the WSA‐ENLIL+Cone model. The model was used to simulate solar wind parameters that included magnetic field magnitude, plasma particle density, dynamic pressure, proton temperature, and velocity during a four Carrington rotation‐long segment. An additional simulation that lasted 18 Carrington rotations was then conducted. The precision of each simulation was examined for intervals when MAVEN was in the upstream solar wind, that is, with no exospheric or magnetospheric phenomena altering in situ measurements. It was determined that generalized, extensive simulations have comparable prediction capabilities as shorter, more comprehensive simulations. Generally, this study aimed to quantify the loss of detail in long‐term simulations and to determine if extended simulations can provide accurate, continuous upstream solar wind conditions when there is a lack of in situ measurements. [ABSTRACT FROM AUTHOR]

We present multi-instrument observations of the effects of solar irradiance on the upper Martian ionosphere and escape fluxes based on the Mars Atmosphere and Volatile EvolutioN (MAVEN) data from November 2014 to February 2016. It is shown that fluxes of oxygen ions with E > 30 eV both inside and outside of the Martian magnetosphere are nonsensitive to EUV variations. In contrast, the fluxes of ions with lower energies extracted from the upper ionosphere increase with solar irradiance. Such an enhancement is nonlinear with the EUV variations and exhibits a growth by almost 1 order of magnitude when the EUV (0.1-50 nm) radiation increases to ≥0.1 W/m2 implying an enhancement of total ion losses of the low-energy component to ∼1.8·1025 s−1. The flow of cold ions in the near-Mars tail occurs very asymmetrical shifting in the direction opposite to the direction of the solar wind motional electric field. Fluxes of the low-energy ( E≤30 eV) ion component are also nonsensitive to the variations in solar wind dynamic pressure. [ABSTRACT FROM AUTHOR]

We present a novel method to determine solar wind proxies from sheath measurements at Mars. Specifically, we develop an artificial neural network (ANN) to simultaneously infer seven solar wind proxies: ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude and its vector components, using spacecraft measurements of ion moments, magnetic field magnitude, magnetic field components in the sheath, and the solar extreme ultraviolet flux. The ANN was trained and tested using 3 years of data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. When compared with MAVEN spacecraft's in situ measured values of the solar wind parameters, we find that the ANN proxies for the solar wind ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude have percentage differences of 50% or less for 84.4%, 99.9%, 86.8%, and 79.8% of the instances, respectively. For the cone angle and clock angle proxies, 69.1% and 53.3% of instances, respectively, have angle differences of 30∘ or less. Plain Language Summary: We introduce a new technique for determining solar wind parameter values upstream of Mars using spacecraft‐measured values of parameters in the plasma environment near Mars. This technique involves using an artificial neural network. Data in the solar wind and the plasma environment near Mars measured by the Mars Atmosphere and Volatile EvolutioN spacecraft, which has been orbiting Mars since 2014, were used to train an artificial neural network to simultaneously infer seven solar wind parameters: solar wind density, speed, temperature, and interplanetary magnetic field and its three components. Comparison of the neural network‐inferred values to the in situ measured values reveals that the artificial neural network can infer the solar wind density, speed, temperature, and the interplanetary magnetic field magnitude with high accuracies and the orientation of the magnetic field with moderate accuracies. Thus, this artificial neural network can be successfully used for inferring solar wind parameters at Mars. Since Mars lacks a dedicated solar wind monitor, unlike at Earth, this technique is useful for obtaining solar wind parameters during times when a Mars orbiter does not traverse through the solar wind upstream of Mars. Knowledge of the solar wind parameter values is essential for studying how the solar wind influences Mars' atmospheric escape. Key Points: An artificial neural network (ANN) is implemented to infer solar wind proxies from sheath measurements at MarsThe ANN can determine solar wind ion density, speed, temperature, and magnetic field magnitude with high accuraciesThe ANN can determine the magnetic field cone and the clock angles with moderate accuracies [ABSTRACT FROM AUTHOR]