Yaxue Dong

Disappearing solar wind events (DSWEs), intervals when the solar wind proton density <1 , represent one extreme of the Mars solar wind interaction. Using data from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission from February 2016 to May 2024, 154 orbits across 90 unique days were identified. During these events, the solar wind kinetic energy flux had a median value of ∼0.08 and never exceeded 0.37 , whereas normal times have a median value twice as large (∼0.16 ) and maximum values exceeding 8 . In contrast, electromagnetic energy fluxes and solar ionizing irradiance remained near normal levels. Observations of and density and fluxes from MAVEN's SupraThermal and Thermal Ion Composition (STATIC) instrument were analyzed. During DSWEs, ion density and ion flux distributions are significantly restructured, consistent with expansion of Mars' ionosphere and induced magnetosphere. Ion escape rates are significantly reduced during DSWEs compared to normal solar wind conditions, with falling to ∼75% and to ∼88% of normal values (9.45 and 1.30 ions/s). DSWE ion fluxes binned by solar wind kinetic and solar ionizing irradiance followed the same empirical relations as under normal conditions, whereas DSWE ion fluxes were depleted relative to the background trend for solar wind electromagnetic energy. Thus, the observed decrease in ion escape is likely attributable to the reduced solar wind kinetic energy flux. These results suggest that even under extreme solar wind density reductions, Mars' ion escape remains mostly governed by the same background drivers as during normal times.

The Martian ionosphere hosts complex current systems driven both by solar wind interactions and neutral wind dynamo processes. While the global patterns of these current systems have been statistically examined, their detailed structures and orientations remain poorly constrained. Using magnetic field observations from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft during the Deep Dip 2 campaign, we resolve the horizontal components of dayside ionospheric currents over regions without strong Crustal Magnetic Fields (CMF). The currents are most prominent between 130 and 150 km altitude, with peak magnitudes of ∼1 μA/m ² , and are oriented counterclockwise from, and roughly perpendicular to, the draped Interplanetary Magnetic Field in the horizontal plane when viewed from above. Theoretically calculated neutral wind‐driven dynamo currents show significant discrepancies from the derived currents, likely reflecting uncertainties in neutral winds and conductivities. In contrast, simulated magnetohydrodynamic (MHD) ionospheric currents, which are purely solar wind‐induced in the model, share similar features with the derived currents. We conclude that, for the four cases examined here, which were sampled in near‐subsolar regions with weak CMF, the derived ionospheric currents are more consistent with a solar wind‐induced origin, whereas neutral wind‐driven dynamo currents may play only a minor role under these conditions.

Magnetotail current sheet (CS) flapping is a universal plasma phenomenon observed at multiple planets, yet its triggering mechanisms remain poorly understood outside of Earth. At Mars, single‐spacecraft observations have also reported tail flapping, but the processes responsible for its onset have never been identified. In this study, we investigate the potential correlation between magnetic reconnection and magnetotail flapping using multipoint measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN) and Tianwen‐1 (TW‐1) missions. We analyze an example event in which MAVEN observed a reconnection‐associated CS crossing in the near tail while TW‐1 simultaneously detected CS flapping further downtail. A statistical survey of joint observations from November 2021 to February 2024 identifies that about two‐thirds of TW‐1 flapping events coincide with reconnection signatures observed by MAVEN. Multiple magnetic flux ropes were also detected before or during flapping intervals, similar to previous observations at Earth, suggesting that reconnection‐generated magnetic flux ropes may propagate tailward and drive plasma instabilities that trigger the tail flapping at Mars. These results provide the first multipoint evidence of a potential statistical correlation between magnetic reconnection and magnetotail flapping at Mars, enabling us to explore the potential triggering mechanism of magnetotail flapping. Our findings also offer new insights into Martian magnetotail dynamics and broaden the comparative understanding of this fundamental plasma process across planetary environments.

At Mars, the MAVEN spacecraft has made observations of Hot Flow Anomalies (HFAs) in the foreshock. Due to the bow shock's proximity to the planet, it is theorized that HFAs contribute to atmospheric escape at Mars through the excavation of ionospheric ions. A case study investigates one HFA observation, with parameters suggesting a novel mechanism for planetary ion extraction. The event is further characterized by elevated number densities of and ions observed prior to the current sheet crossing. Statistical study is conducted on a set of 91 events, facilitating the estimation of HFA frequency at Mars to be 1/day. The estimated ion escape of the event is approximately 9 of the typical ion escape rate under nominal conditions at Mars. Calculation of further events reveals escape rates between 1 and 9 of the nominal conditions. This represents a modest contribution to the overall escape, highlighting a potentially underexplored pathway.

We present a physically motivated analytic framework for estimating solar wind mass-loading rates from ion sources using a simplified “particles in a box” (PIB) model. By applying conservation of energy and conservation of magnetic flux, we derive a general equation for the global pickup ion rate that explicitly accounts for both kinetic and electromagnetic (Poynting) energy fluxes supplied by the upstream wind and distributed to downstream particles and fields. Our analysis reveals that the global pickup rate may be fundamentally constrained by upstream wind parameters and the geometry of the interaction in a nontrivial manner. We further derive a theoretical upper limit to the maximum mass flux that a super-Alfvénic wind can transport. We call this limit the carrying capacity; it is an inherent property of the wind. We apply this framework to the Giotto flyby of Grigg–Skjellerup, and to unmagnetized and weakly magnetized terrestrial planets. The results suggest that the PIB framework may provide a useful big-picture approach for understanding and estimating stellar wind ion pickup in cometary and planetary environments inside and out of the solar system.

Magnetotail current sheet (CS) flapping is a universal plasma phenomenon observed at multiple planets, yet its triggering mechanisms remain poorly understood outside of Earth. At Mars, single-spacecraft observations have also reported tail flapping, but the processes responsible for its onset have never been identified. In this study, we investigate the potential link between magnetic reconnection and magnetotail flapping using multipoint measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN) and Tianwen-1 (TW-1) missions. We analyze an example event in which MAVEN observed a reconnection-associated current sheet crossing in the near tail while TW-1 simultaneously detected CS flapping further downtail. A statistical survey of joint observations from November 2021 to February 2024 identifies that about two-thirds of TW-1 flapping events coincide with reconnection signatures observed by MAVEN. Multiple magnetic flux ropes were also detected before or during flapping intervals, similar to previous observations at Earth, suggesting that reconnection-generated magnetic flux ropes may propagate tailward and drive plasma instabilities that trigger the tail flapping at Mars. These results provide the first multipoint evidence of a potential correlation between magnetic reconnection and magnetotail flapping at Mars, enabling us to constrain the triggering mechanism of magnetotail flapping. Our findings also offer new insights into Martian magnetotail dynamics and broaden the comparative understanding of this fundamental plasma process across planetary environments.

Introduction: Mars lacks an intrinsic global magnetic field, unlike Earth. Instead, the solar wind interacts directly with the upper atmosphere, creating an induced magnetic field that is much weaker compared to a global magnetic field. Localized strong crustal magnetic fields that rotate with the planet add to the complexity of the Martian plasma environment. Some interesting physical processes operate in this complex hybrid magnetosphere, including aurorae, reconnection, and ion escape. Multi-point measurements can help resolve spatial and temporal variability on small spatial scales and shorter timescales than a single spacecraft’s orbit. NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and ESA’s Mars Express (MEX) mission has been making simultaneous measurements for over a decade, occasionally approaching each other within a few hundred kilometers. These close conjunction events enable the direct comparison of plasma measurements and cross-calibration of the instruments on the two spacecraft, while exploring the spatial scales of plasma in five different plasma regions, shown in Figure 1.Figure 1: Five plasma regions near Mars in MSO coordinates, showing the bow shock and the magnetic pileup boundary (MPB).Science objectives: We present a comparative analysis of electron and ion observations from MAVEN and MEX during close conjunction events. Specifically, we compare electron energy distribution measured by MAVEN’s Solar Wind Electron Analyzer (SWEA) with MEX’s Electron Spectrometer (ELS) instruments, and ion (protons and heavy ions) energy distribution measured by MAVEN’s Suprathermal and Thermal Ion Composition (STATIC) and Solar Wind Ion Analyzer (SWIA) with MEX’s Ion Mass Analyzer (IMA) instruments. We have three science objectives: (1) To determine whether the two spacecraft sample the same plasma (electron and ion) population during close conjunctions, (2) To characterize the spatial scales over which plasma properties are coherent in different plasma regions of the Martian plasma environment, and (3) To identify possible sources of systematic differences between the measurements.Figure 2. (left panel) Electron energy spectrum measured by MAVEN/SWEA and MEX/ELS in the magnetosheath. (right panel) Double Maxwellian distribution function fitted to the measured distribution. Plasma population comparison during close conjunctions: Electron and ion distributions are compared during times when MAVEN and MEX are separated by up to 1000 km in some Martian plasma regions (magnetosheath, dayside and nightside at low altitude), and to 2000 km in the solar wind region. The separation of the spacecraft in each of these regimes is compared to the gyro radius and Debye length of each species in each region. We compare look directions of the instruments that overlap (within a small angular separation) and calculate the average energy distribution of electrons and ions over some time interval (see Figure 2. left panel). To determine the agreement between the measurements, we apply a chi-squared test to assess the differences in particle fluxes qualitatively and to reveal if the two spacecraft are probing the same plasma population. We also fit a Maxwellian to each distribution to determine plasma densities and temperatures and to assess the differences quantitatively (see Figure 2. right panel). Spatial scales in different plasma regions: We investigate plasma variability in five plasma regions near Mars: upstream solar wind, magnetosheath, low-altitude dayside, low-altitude nightside, and near crustal magnetic fields (see Figure 1.). Strong crustal magnetic field regions are identified using magnetic field strength measurements from MAVEN’s magnetometer (MAG) instrument. For each event, we analyze the full energy spectrum 10 minutes before and 10 minutes after closest approach, selecting times and separations when both spacecraft are likely in the same region based on the spectra. Differences between the two measurements from the two spacecraft are then evaluated as a function of separation. The chi-squared test is used again to quantify the level of agreement. This approach provides an estimate of the spatial scales over which the plasma changes in each region. Possible sources of systematic differences: We identify possible sources of systematic differences and mitigate them when possible. These include spacecraft potential that alters the measured spectra at lower energies if not corrected. Another is the percent overlap of the instrument look directions that can introduce discrepancies. Additionally, differences in instrument sensitivities and background noise can contribute to measurement disagreements. By identifying these sources and quantifying their influence, we aim to uncover the true differences in the observed plasma environment. The results of this multi-point Martian plasma study contribute to our understanding of solar wind interaction with weakly magnetized planets and characterize the spatial variability and timescales less than a single spacecraft orbit in different plasma regions. Moreover, these methods lay the foundation for future multi-spacecraft missions to Mars that study the Martian plasma environment.

We construct an analytical paradigm called ‘Particles in a Box’ to study the interaction between the solar wind and terrestrial ionospheres. Using energy flux conservation, we derive how wind deceleration drives magnetic pile-up and quantify the critical transition between kinetic-energy-dominated (super-Alfvénic) and magnetic-energy-dominated (sub-Alfvénic) regimes. This framework shows that pick-up ion escape rates in super-Alfvénic stellar winds, such as those experienced by the terrestrial planets in the solar system, can be computed solely from upstream parameters: stellar wind velocity, magnetic field strength, and the cross-sectional area of the interaction region. Hybrid modeling-informed cross-sectional areas are combined with the analytical equations to estimate heavy ion escape rates at Mercury, Venus, Earth, Mars, Titan, and Pluto. Comparisons with spacecraft observations show order-of-magnitude agreement, validating the paradigm’s utility for unmagnetized or weakly magnetized bodies. These results indicate that Particles in a Box is a promising approach for understanding ionospheric escape from weakly magnetized planets. While only solar system objects are directly considered in this study, the paradigm may be able to be extended to non-terrestrial exoplanets.

We present Mars Express (MEX) observations of heavy pickup ions (HPUI) at Mars and a new method to derive interplanetary magnetic field (IMF) properties. The MEX HPUI measurements were organized using the upstream IMF directions measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Since these HPUIs are mostly accelerated by the electric field in the upstream and magnetosheath regions, the IMF component perpendicular to the solar wind, both directions and magnitudes, can be derived from the HPUI flux directions, energies, and positions measured by MEX. The derived IMF properties were compared with the simultaneous IMF measurements by MAVEN, which showed good agreements with each other with the accuracy comparable to most previous IMF proxies based on MAVEN data. This new IMF proxy does not require any magnetic field data and hence can be applied to the long time period prior to the MAVEN mission.

Atmospheric ion escape plays a crucial role in the evolution of planetary climate and habitability. While Mars has been the focus of extensive in-situ spacecraft observations, our understanding of ion escape at Mars has been constrained by single-point spacecraft measurements, which fail to distinguish spatial and temporal variability. Observations from NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission and China’s Tianwen-1 mission provide complementary observations the Martian space environment and a unique opportunity to study the variability of ion escape. Here, we report that ion escape at Mars exhibits unexpected spatial-temporal variability under steady and weak external solar wind conditions. In the hemisphere where the solar wind electric field is directed toward the planet, a condition that usually hinders ion escape into space, we instead observe the transient appearance of escaping planetary ions with high energies and strong escape fluxes. This finding underscores that planetary ion escape can be unsteady and dynamic, even under stable external conditions.