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We report the first global, time-dependent simulation of the Mars upper atmospheric responses to a realistic solar flare event, an X8.2 eruption on 10 September 2017. The Mars Global Ionosphere-Thermosphere Model runs with realistically specified flare irradiance, giving results in reasonably good agreement with the Mars Atmosphere and Volatile EvolutioN spacecraft measurements. It is found that the ionized and neutral regimes of the upper atmosphere are significantly disturbed by the flare but react differently. The ionospheric electron density enhancement is concentrated below ~110-km altitude due to enhanced solar X-rays, closely following the time evolution of the flare. The neutral atmospheric perturbation increases with altitude and is important above ~150-km altitude, in association with atmospheric upwelling driven by solar extreme ultraviolet heating. It takes ~2.5 hr past the flare peak to reach the maximum disturbance and then additional ~10 hr to generally settle down to preflare levels.

A comprehensive modeling study has been conducted to investigate space weather effects at Mars during the 10 September 2017 solar flare, utilizing an integrated framework that combines the global magnetohydrodynamic (MHD) model and Mars Global Ionosphere‐Thermosphere Model (MGITM). This is the first time the thermosphere‐ionosphere‐magnetosphere system is self‐consistently simulated under realistic, time‐varying conditions. Our simulations align well with observations from the Mars Atmosphere and Volatile EvolutioN (MAVEN). Recognizing that complexities due to highly disturbed upstream conditions and rotating crustal fields obscure solar flare effects in orbit‐to‐orbit comparisons, we perform controlled simulations of nonflare and flare cases and exploit their contrast to quantify spatiotemporal variations in flare impact. Our results highlight pronounced and rapid dayside ionospheric perturbations, contrasting with weaker and delayed nightside responses. Notably, in the topside ionosphere, O2+ ${\mathrm{O}}_{2}^{+}$ and CO2+ ${\mathrm{O}}_{2}^{+}$ densities increase primarily on the dayside below ∼ ${\sim} $300 km altitude, peaking with an increase of 20%–30%. The O+ ${\mathrm{O}}^{+}$ density shows a more significant increase of up to ∼ ${\sim} $50%, extending into the magnetosphere and nightside via plasma transport, increasing its total loss rate by 14%. We observe distinct altitude‐dependent patterns in dayside electron density enhancements in percent, characterized by a weakening with altitude and a rapid decay below ∼ ${\sim} $150 km in line with the flare development, and a gradual intensification between ∼ ${\sim} $150–300 km due to plasma transport and flare‐induced atmospheric upwelling. Earlier Mars Global Surveyor observations were limited to the low‐altitude pattern due to atmospheric expansion and missed the higher altitude variations observed by MAVEN. Plain Language Summary: This study investigates the impact of a powerful X8.2‐class solar flare on 10 September 2017 at Mars. Using an integrated modeling framework that combines two state‐of‐the‐art global models, our simulations provide insights into distinct altitude‐dependent patterns of flare‐induced ionospheric density enhancements, pronounced day‐night asymmetry, and a moderate increase in ion escape. Key findings include rapid variations in ionospheric density enhancements at lower altitudes, with a gradual intensification at higher altitudes due to plasma transport and atmospheric upwelling. These findings contribute to a better understanding of how solar flares impact the space environment of unmagnetized or weakly‐magnetized planets like Mars. Key Points: The solar flare causes pronounced and rapid dayside ionospheric perturbations and weaker and delayed effects on the nightsideO2+ and CO2+ densities increase by 20%–30% above 150 km on the dayside, while O+ increases by up to 50% and over larger spacePercentage increase of dayside e− densities decreases with altitude and decays fast below ∼150 km, and rises and decays slowly above that [ABSTRACT FROM AUTHOR]

Understanding the interaction between Mars and the solar wind is crucial for comprehending the atmospheric evolution and climate change on Mars. To gain a comprehensive understanding of the Martian plasma environment, global numerical simulations are essential in addition to spacecraft observations. However, there are still discrepancies among different simulation models. This study investigates how these discrepancies stem from the considered physical processes and numerical implementations. We compare two global multispecies MHD models: the "Sun model" based on the BATS‐R‐US code and the "Sakata model" based on a newly developed multifluid model MAESTRO. By employing the same typical upstream conditions and the same neutral atmosphere for current Mars, along with similar numerical implementations such as inner boundary conditions, we obtain simulation results that exhibit unprecedented agreement between the two models. The dayside results are nearly identical, especially along the subsolar line, indicating the robustness of MHD models to predict dayside interaction under given upstream conditions and ionosphere assumptions. The escape rates of planetary ions are also in good agreement. However, discrepancies remain in the terminator and nightside regions. Detailed numerical implementations, including inner boundary conditions, magnetic field divergence control methods, and radial resolutions, are shown to influence certain aspects of the results greatly, such as magnetotail configuration and ion diffusion. Key Points: The same physical assumptions are employed for two different global multispecies MHD modelsThe results for the dayside interaction and planetary ion escape are in unprecedented good agreementDetails of the numerical implementations influence the results, especially on the nightside [ABSTRACT FROM AUTHOR]

We examine the newly discovered phenomena of sinuous aurora on the nightside of Mars, using images of 130.4 and 135.6 nm oxygen emission measured by the Emirates Mars Mission EMUS ultraviolet spectrograph, and upstream measurements from the MAVEN and Mars Express spacecraft. They are detected in ∼3% of observations, totaling 73 clear detections. These emissions are narrow, elongated (1,000–6,000 km), cross Mars' UV terminator, and are oriented generally toward the anti‐solar point, clustering into north, south, east, and west‐oriented groups. Diverse morphologies are observed, though some spatial features, such as broad curves, may in some cases be due to temporal aliasing of aurora motion as each image is built up over 15–20 min. Sinuous aurora form away from Mars' strongest crustal magnetic fields and can be interrupted by moderate crustal fields. Sinuous aurora occurrence increases strongly with solar wind pressure, though brightness shows only a weak positive dependence on pressure. Interplanetary magnetic field (IMF) clock angle affects their occurrence and orientation: sinuous aurora show a broad range of orientations centered on the solar wind convection electric field (Econv) direction and forming in the +Econv hemisphere, although with moderate clockwise and counterclockwise average "twists" for westward and eastward IMF, respectively. From these features we infer a link between sinuous aurora and electron energization in Mars' magnetotail current sheet, where field geometry on the +Econv side of the sheet is more organized and symmetric. Determination of specific triggering conditions for sinuous aurora requires further investigation. Plain Language Summary: Sinuous aurora are narrow, extended patterns of UV emission caused by long, thin channels of energized electrons striking Mars' nightside upper atmosphere. We study images of these aurora taken by the Emirates Mars Mission EMUS instrument. 73 cases of sinuous auroras were found (∼3% occurrence rate) with lengths ranging from 1,000 to 6,000 km. These auroras usually cross Mars' day‐night boundary and extend in the direction opposite to the Sun. They tend to cluster into groups oriented toward the north, south, east, and west directions with a diverse array of shapes. Sinuous aurora generally form away from Mars' strongest crustal magnetic fields. They occur more frequently for higher solar wind pressure. Their orientations are affected by the interplanetary magnetic field (IMF), displaying a broad range of orientations centered on the direction of the electric field in the solar wind, and forming in the hemisphere to which this electric field points, although with moderate counterclockwise and clockwise average "twists" for eastward and westward IMF, respectively. From these features we infer a link between sinuous aurora and a sheet of current in Mars magnetic tail, wherein the aurora‐causing electrons may be energized before falling into the upper atmosphere to produce aurora. Key Points: These narrow emission features form away from strong crustal fields, oriented anti‐sunward, cross the terminator, are detected in 3% observationsOccurrence increases with solar wind pressure, certain interplanetary magnetic field orientations, and in the positive motional electric field hemisphereSinuous aurora may be related to magnetotail asymmetry and electron energization processes occurring in the tail current sheet [ABSTRACT FROM AUTHOR]

On 26–27 December 2022, Mars experienced an extremely low‐density solar wind stream, which was encountered first by Earth because of the radial alignment of the two planets (i.e., Mars opposition). During this event, two important properties of the ionospheric and magnetospheric states changed significantly in response to the low solar wind ram pressure, as inferred from the superthermal electron observations from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. The interface between the ionosphere and magnetosphere expanded to thousands of kilometers, outside of the nominal bow shock locations, coinciding with the expansion of the cold planetary ions. Meanwhile, the ambipolar electrostatic potential arising from the ionospheric electron pressure gradient increased from the nominal ∼ −0.7 to ∼ −2 V (relative to the lower ionosphere). This enhanced ambipolar potential likely facilitated the observed ionosphere expansion. Key Points: This study characterizes Mars's magnetospheric and ionospheric response to the disappearing solar wind event in December 2022During the event, open and closed field lines extend beyond the nominal bow shock location, just as the planetary cold ionsThe ionospheric ambipolar potential drop is enhanced from the nominal ∼ −0.7 to ∼ −2 V, likely facilitating the ionosphere expansion [ABSTRACT FROM AUTHOR]