Laila Andersson

Electromagnetic wave activity in the collisional lower Martian ionosphere has been consistently observed by MAVEN in regions of plasma density gradients and moderate collisionality. This study presents a detailed analysis of a representative wave event and applies a two‐fluid theoretical model developed in a companion paper to interpret the underlying instability mechanism. The observed wave signatures, including compressional magnetic perturbations and near perpendicular propagation, are consistent with a gradient‐driven drift–Alfvén instability. Growth rates computed along the MAVEN trajectory, using observed plasma parameters and a parametric analysis of key free quantities such as drift speed and gradient scale length, show spatial and temporal consistency with the wave active region. A statistical survey of 22 additional events reveals similar conditions, including altitudes between 140 and 160 km, moderate electron collisionality, and density gradients that support the broader relevance of the mechanism. These findings provide observational support for a local electromagnetic instability in the collisional Martian ionosphere just past the dawn terminator, extending current understanding beyond electrostatic drift‐wave theories and offering a framework for identifying unstable regions along MAVEN’s orbit.

Reconnection observations involving large density gradients and multiple heavyion species are scarce, motivating the analysis of satellite observations at Mars. The analyzed event exhibits anti-parallel magnetic fields with a guide field, accompanied by a substantial density gradient of approximately one to two orders of magnitude involving multiple species: e−, H+, O+, O+2 , and CO+2. The event is examined in terms of the temporal and spatial scales of the heavy ions of an assumed reconnection event, with the expectation that the structure is maintained by electron currents. The distance between the shocked solar wind and the collision-dominated ionosphere–thermosphere is small compared with the characteristic lengths of the X-line and outflow region. The ionospheric ions are accelerated within the reconnection region but transition to predominantly outflow-directed motion outside the exhaust. Away from the X-line, a boundary layer forms populated by accelerated ionospheric ions. The observations suggest that multiple X-lines form within the same areographic region and collectively sustain the outflow region over an extended period. For each newly formed Xline, reconnection is proposed to advance to lower altitudes, locally eroding the Martian atmosphere. This event demonstrates that magnetic reconnection can efficiently exchange plasma between the solar wind and the Martian ionosphere.

Polar cap patches are regions of dense plasma in the polar cap that convect from noon to midnight, moving against the neutral gas background. The gradient drift instability (GDI) destabilizes the edge of these patches as they convect, yet the rate of this destabilization is not well known due to the scarcity of coincident neutral wind and plasma measurements. Polar cap patches detected by Swarm with conjunction measurements from GRACE‐FO are assessed for their GDI growth time with and without the neutral wind and for high and low collisionality regimes. This analysis shows significant differences in growth times with and without the neutral wind, and that the low collisionality growth times are different by a factor of 65%. These results demonstrate that coincident plasma and neutral wind measurements are essential for accurately determining how the GDI, and by extension polar cap patches, evolve.

At planets that possess strong dipole magnetic fields, the Zwan-Wolf effect acts to squeeze plasma along magnetic flux tubes, aiding in the deflection of the solar wind flow about the planet. While the effect has been most studied at Earth, candidate observations have also been made at the outer planets. Here we present observations of the Zwan-Wolf effect occurring at Mars, an unmagnetized planet that lacks a dipole magnetic field. Our analysis of observations made by NASA's Mars Atmosphere and Volatile EvolutioN spacecraft suggest that while the Zwan-Wolf effect is likely continuously active within the Martian ionosphere, it operates below detection thresholds of typical plasma analyzers most of the time. However, an interplanetary coronal mass ejection impact at Mars in December 2023 greatly enhanced the Zwan-Wolf effect within the ionosphere, allowing it to be observed, and highlighting the importance of space weather events for these unmagnetized planetary systems.

The state of Mars' induced magnetosphere is highly dependent on upstream solar wind conditions. Changes in dynamic pressure alter the locations of magnetospheric boundaries such as the bow shock and induced magnetosphere boundary, while the orientation of the interplanetary magnetic field (IMF) impacts the formation of the bow shock. We present Mars Atmosphere and Volatile EvolutioN observations from a period when the solar wind dynamic pressure was low and the IMF was nearly radial. The quasi‐radial IMF appeared to prevent the formation of the usual quasi‐perpendicular bow shock, instead producing a spatially extended quasi‐parallel shock in the observed region. The ambipolar potential in the magnetosheath/shock region enabled planetary heavy ions to leak through the shock and flow upstream. These ions were first accelerated by the solar wind motional electric field, which remained an important ion energization mechanism even though the solar wind flow and IMF were roughly aligned throughout most of the system. The ionosphere had expanded out to very high altitudes as the result of low dynamic pressure. The ionosphere was also unmagnetized, perhaps because of the formation of an ionopause‐like boundary. Finally, we compare two adjacent orbits and suggest that Mars' remanent crustal fields may fundamentally alter bow shock formation during radial IMF conditions. Studying unusual combinations of solar wind conditions like the case presented here is critical for understanding the fundamental physics driving induced magnetospheres.

Fig. 1: MEX MaRS observation of (a) the extended Mars dayside ionosphere on Day of Year (DoY) 102 (2019) and (b) the compressed dayside ionosphere on DoY 100 (2021). Grey crosses are the original data points, while the blue line indicates the smoothed data. The vertical grey line indicates the 1ˑσ noise level of the observation.The Mars Express (MEX) spacecraft has been orbiting Mars since December 2003. Its MaRS (Mars Radio Science) experiment uses spacecraft-Earth radio occultations to provide information about the ionospheric electron density and the pressure, density and temperature of the lower neutral atmosphere. However, due to geometrical constraints, these remote-sensing observations are never accompanied by simultaneous in-situ observations at the same position from MEX itself.Since the arrival of the MAVEN spacecraft in 2014, new opportunities have emerged to complement MEX MaRS data with in-situ observations. This study compares the coordinates of MEX MaRS dayside ionospheric occultation events (from 2014 through the end of 2022) with MAVEN's pericenter positions to identify instances where the two datasets are spatially and temporally comparable.The analysis shows that opportunities where MEX MaRS and MAVEN pericenter observation are comparable, are extremely rare. This work presents a detailed case study of two events where meaningful comparisons can be made under the condition that the planetary ionosphere remained sufficiently stable (Fig. 1).Combining MEX radio science observations with MAVEN's in-situ data enables more comprehensive scientific insights into Martian ionospheric dynamics—insights that are otherwise difficult to obtain. Future and proposed multi-spacecraft missions like EscaPADE and M-MATISSE will be instrumental in expanding our understanding of planetary environments through coordinated observations.

Mars’ nightside ionosphere is highly variable and poorly understood, with several internal, external, and local factors affecting multiple sources of ionospheric plasma, all combining to drive highly complex behavior. The three primary plasma sources are a) planetary rotation and cross-terminator ion winds transporting dayside plasma to the nightside, particularly at dusk, and impact ionization when magnetic topology connects nightside upper atmosphere to b) dayside suprathermal photoelectrons c) solar wind electrons (accelerated or not) precipitating from the magnetotail. These sources are impacted by different subsets of: season, local time, subsolar longitude, IMF direction (which determines tail draping configuration), solar wind pressure, solar EUV flux, and the local strength and geometry of crustal magnetic fields that combine with global draping patterns to guide electrons toward or away from certain areas. We utilize 130.4 nm auroral observations from the Emirates Mars Ultraviolet Spectrograph, simulations of terminator ion winds, and in situ measurements from MAVEN of ion and electron density and temperature and suprathermal electron precipitation, as well as upstream measurements and proxies of IMF clock angle and solar wind pressure. Analysis reveals some interesting features. First, in regions of weak or no crustal magnetic fields, surprisingly, pre-dawn (~4 am) plasma densities vary by up to a factor of 30 in response to solar EUV varying by only a factor of ~3. Second, 5R aurora occurrence rate in a given region can vary by up to a factor of 7 with IMF orientation, generally highest for southward IMF and lowest for westward IMF, with higher auroral occurrence post-dusk, decreasing toward and past midnight before increasing again towards dawn. A broad diversity of IMF and local time dependence of plasma density and aurora occurrence is observed across Mars’ crustal magnetic fields. The polarity of a given crustal field determines whether its field lines preferentially magnetically connect to dayside ionospheric or nightside magnetotail electron sources in the post-dusk or pre-dawn sectors. These results highlight the complex and dynamic nature of Mars’ magnetic topology in controlling auroral electron access. Future studies integrating two-dimensional ionospheric models with in situ electron measurements with synoptic and limb auroral imaging will further constrain the relative contributions of the many factors controlling Mars’ enigmatic nightside ionosphere.

Direct electric field measurements during certain ionosphere‐magnetosheath transitions on the dayside of Mars reveal a presence of localized (20 km thickness along vertical direction) strong (40 mV/m) electric field located at the solar wind stagnation point. This electric field is nearly collocated with the ion composition boundary where ionospheric oxygen ions are observed to be accelerated up to 1 keV, forming a layer of higher temperature plasma around the stagnation point. Simulations demonstrate that the observed localized electric field enhancement can create this hotter plasma layer population on either side of the boundary. This plasma layer can have an impact on the solar wind coupling with the planet and forms a reservoir for heavy ion escape.

Mars, being a small planet with a tenuous atmosphere, does not have a sharp boundary between regions dominated by solar wind plasma and planetary plasma. Instead, this transition is typically extended, allowing the interplanetary magnetic field (IMF) to penetrate into the Martian ionosphere. However, the depth of this penetration is not well understood. Using 6 years of MAVEN data, we statistically assess locations where a transition exists between the dominance of magnetic versus cold (1 eV), thermal plasma pressure to better understand the reach of the IMF. We identify the presence or absence of pressure transitions from 200 to 800 km altitude for each MAVEN orbit and find a clear transition in 55% of cases. The pressure transition locations are mapped in different coordinate systems that provide insight into the solar and planetary driving conditions that cause a detected transition region. Transitions are more likely to occur under weak‐to‐nominal solar wind conditions, away from strong crustal magnetic fields, near the terminator, on the dusk side of the planet compared to the dawn side, and in the negative solar wind motional electric field hemisphere. We speculate on possible causes for asymmetries that arise in the mapped locations of these pressure transitions and the effect that penetrated IMF may have on driving plasma dynamics in the Martian ionosphere.