Tianwen-1 and MAVEN Observations of the Response of Mars to an Interplanetary Coronal Mass Ejection

Chi et al. (2023b) presented a detailed study of the evolution of two ICME events, which were detected by BepiColombo in the solar wind, and by Tianwen-1 and MAVEN at Mars in 2021 December. They studied the evolution of the ICME from the Sun to Mars, but they did not investigate the planetary effects of these space weather events. Therefore, this work focuses on the response of the Martian ionosphere to the ICMEs. One strong ICME event is of interest in this study. The ICME has a high-speed solar wind over 500 km s⁻¹, which was observed at Mars by Tianwen-1 and MAVEN from 2021 December 8 to December 13.

Figure 2(a) shows the relative position of Mars, Earth, BepiColombo, and the STEREO-A spacecraft with the ambient solar wind conditions in the ecliptic plane at 18:25 UT on 2021 December 6 estimated by the Heliospheric Upwind eXtrapolation time (HUXt) model (Owens et al. 2020; Barnard & Owens 2022). The BepiColombo spacecraft was almost aligned with Mars in the longitude of the HEEQ coordinate when the CME erupted, which can be used as the upstream solar wind monitor of Mars.

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Figure 2(b) shows the orbits of Tianwen-1 (red line) and MAVEN (blue line) in a cylindrical coordinate system for the time interval from 00:00 UT on 2021 December 8 to 00:00 UT on 2021 December 13, color-coded by distance from the Mars surface. As shown in panel (b), MAVEN's periapsis is on the nightside and Tianwen-1's apoapsis is on the dayside. The "x" symbols mark the MAVEN and Tianwen-1 periapsis locations. The MAVEN and Tianwen-1 periapsis solar zenith angles change slowly and systematically during successive orbits. The solar zenith angle of MAVEN periapsis is ∼124° with a periapsis altitude of ∼182 km. The solar zenith angle of Tianwen-1 periapsis is ∼81° with a periapsis altitude of ∼268 km. During 2021 November and December, Tianwen-1 apoapsis was far above the southern pole of Mars in the solar wind, spending about 50%–75% of its duration out of the magnetosheath (Wang et al. 2023). Thus, Tianwen-1/MOMAG primarily measured the IMF of the upstream solar wind of Mars. The LPW/NGIMS/MAG instruments on board MAVEN detected the response of the Martian atmosphere on the nightside.

The ICME was first observed at 0.67 au upstream of Mars by BepiColombo spacecraft. Figure 2(c) shows the magnetic-field observations from the BepiColombo/MPO-MAG in the Radial-Tangential-Normal coordinate. A forward shock was observed on December 6 at 15:07 UT, which was driven by the following ICME. The black-vertical line represents the arrival time of the shock. The purple-shaded region indicates the interval of an ICME ejecta with an enhanced and smooth changing magnetic field.

Both Tianwen-1 and MAVEN have detected the ICME at Mars. Figure 2(d) shows the in situ observations at Mars from Tianwen-1/MOMAG, MAVEN/MAG, and MAVEN/SWIA instruments. A possible forward ICME-driven shock was observed from the enhanced plasma density in solar wind at Mars on December 8 at 19:20 UT. The ejecta of the ICME arrived on December 10 at 00:00 UT and lasted for approximately 1.6 days.

The solar wind and Martian atmosphere have been continuously monitored by MAVEN since 2014 (Halekas et al. 2017; Lee et al. 2017). Figure 3 shows the in situ Martian observations from MAVEN LPW, NGIMS, and MAG instruments at altitudes between 185–500 km on the nightside of Mars (X_MSO < 0) during 2021 December 6–15. The ionosphere was much more disturbed after December 15, due to a subsequent Stream Interaction Region (SIR) that arrived at Mars on 2021 December 16 at 06:00 UT (Chi et al. 2023a), after which the disturbances of the Martian ionosphere with the influence of the ICME are difficult to distinguish from the influence of the SIR event. To compare the response of the ionosphere to the ICME during 2021 December 10–15, the profiles from December 6–9 were used as the background. Please note that the MAVEN profiles are not vertical. Therefore, we only utilized the nighttime (X_MSO < 0, SZA > 90) measurements of MAVEN between 185–500 km from consecutive orbits during 2021 December 6–15. The variation of solar zenith angle for these orbits at the same altitude is not very significant, making it possible to compare the MAVEN measurements along slanted orbit segments. The density profiles of the nightside ionosphere may vary from each orbit. To investigate the variations in the Martian nighttime ionosphere and its response to the ICME, a daily mean profile was calculated by taking the mean of multiple orbits.

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Figure 3(a) shows the electron density from the MAVEN/LPW instrument. The gray lines reveal the typical background ionospheric profiles during December 6–9 before the arrival of ICME. The atmosphere of Mars became remarkably variable after the arrival of the ICME ejecta on 2021 December 10. The ionospheric profiles during 2021 December 10–15 are plotted in time using blue to red colors. There was no ionopause before December 10. A sudden drop in the electron density of the topside ionosphere was observed on December 10 from 500–700 cm⁻² to less than 100 cm⁻². This signature is usually the formation of an ionopause, i.e., the uppermost side of the ionosphere. According to the daily electron density profiles, the height of the ionopause gradually decreased to 330–340 km on December 15, with an increase in electron density in the lowest region of the ionosphere below 200 km altitudes. The peak of electron density reached more than 10³ cm⁻² at an altitude of approximately 285 km altitude on December 15.

Another criterion of the ionopause used by Vogt et al. (2015) is that the ion density decreases significantly by at least a factor of 10 over an altitude range of 30 km. Figures 3(b) and (c) show the densities of the primary ${{\rm{O}}}_{2}^{+}$ and O⁺ ions in the Martian nightside ionosphere from the MAVEN NGIMS instrument. According to the ion density profiles, the ionopause had already formed at an altitude of approximately 440 km altitude during December 6–9. Then, the ionopause gradually descended after the ICME arrived at Mars on December 10, and was eventually lowered to 345 km on December 15. Figure 3(d) shows the electron temperature from the MAVEN LPW instrument. The processes of plasma transport dominate the diffusion region of the Martian ionosphere above ∼200 km altitude (Sánchez-Cano et al. 2020). An increase in temperature with increasing altitude was found. The variability in electron temperature is rather small from 200–350 km altitude, where it typically increases to 2000–3000 K. The electron temperature becomes variable above 350 km. In comparison to those before the arrival of the ICME during December 6–9, the electron temperature above 350 km became visibly perturbed during December 10–15. Figure 3(e) shows the magnetic field from the MAVEN MAG instrument. The magnetic field above 350 km was nearly constant before and after the ICME. The magnetic field is larger than 30 nT at low altitudes below ∼350 km. Though the daily variation of ionosphere was calculated to minimize the effects of crustal field contributions, the ionosphere below 350 km altitude could be influenced by regions over magnetic crustal anomalies with strong magnetic fields (>30 nT).

The ICME influenced the topside ionosphere of Mars. The ICME was detected near Mars at 00:00 UT on 2021 December 10, and then the ionopause occurred at 330–340 km from observations of electron density in the nighttime ionosphere. With the arrival of the ICME between December 10–15, it was observed that the height of the ionopause decreased until it reached 345 km and that the electron temperature near the ionopause increased to over 8000 K. The ionopause indicates the interface between the solar wind and the ionospheric plasma. Since the ionopause divides the upper atmosphere of Mars into two distinct regimes—a plasma diffusion region below it and a region of forced convection stripping by solar wind above it where wave heating and energy transport dominate—, the vertical structure of the nighttime ionosphere was changed (Nagy & Schunk 2009).

Figure 4 shows the daily altitude profiles of MAVEN/LPW electron density in the Martian nightside ionosphere. The top panels show the background electron density during December 6–9. The N_e value remained less than 500 cm⁻² until a fast-forward shock of the ICME arrived at Mars on December 8 (Figure 4(d)), shown as a vertical line in Figure 2 detected by Tianwen-1 and MAVEN on December 8 at 19:20 UT. The ionopause cannot be identified as a result of a data gap in electron density at a higher altitude. The electron density was depleted above 475 km on December 9. The height of the ionopause is estimated to be in the range of 435–475 km.

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The middle six panels show the daily profiles of electron density in response to the ICME from December 10–15 compared with the background electron density from December 6–9. The electron density rapidly fell to depletion at a greater altitude, and the height of the ionopause showed a marked decrease. The height of the ionopause represents the lower side of the depleted electron density. In Figure 4(g), the peak of electron density was 800 cm⁻² at 345 km, while no measurements of electron density were obtained above 415 km on December 10. Figure 4(h) shows that the topside electron density at altitudes of 415–500 km was greatly depleted, and the height of the ionopause dramatically decreased to 415 km on December 11. During December 12–15, the ionopause gradually descended, and the electron density at lower altitudes between 250–320 km was enhanced. Finally, the height of the ionopause reached 345 km on December 15. The solar energetic particles associated with the ICME could be the origin of the ionization at low altitude. The high-energy electrons ionized the Martian atmosphere, forming a dense low ionospheric layer of ions and electrons on the Martian nightside (Sánchez-Cano et al. 2019; Krishnaprasad et al. 2021; Lester et al. 2022).

The lower six panels show the relative changes in the electron density (ΔN_e /N_e (%)) during and after the ICME interval. The topside electron density was completely depleted. The electron depletions extended from altitudes of above 415 km on December 11 to altitudes of above 345 km on December 15. The peak of the electron density at altitudes of 260–290 km was increased by more than 300%. According to the measurements of the Martian nightside ionosphere, the plasma in the Martian nightside ionosphere was depleted, which caused the ionopause to descend gradually in response to the ICME.

For this strong ICME, the solar wind dynamic pressure and foreshock pressure of the ICME should be expected to be high, and thus the solar wind are able to penetrate downward. The decrease in the height of ionopause with solar wind compression could potentially be related to pressure changes with their own accompanying magnetic pressure. Figure 5 shows the variations in the dynamic pressure of the upstearm solar wind and the ionospheric pressures with perturbations in the Martian upper atmosphere. The magnetic-field measurements from MAVEN/MAG and Tianwen-1/MOMAG, the solar wind measurements from MAVEN/SWIA, and the ionospheric plasma measurements from MAVEN/NGIMS and MAVEN/LPW were all included in the analysis. The top panel of Figure 5 shows the thermal pressure (${P}_{{\mathrm{th}}_{i}}$) and magnetic pressure (${P}_{{B}_{i}}$) changes in the ionosphere near the ionopause height that were averaged within ±20 km altitude. The magnetic pressures of the ionopause were investigated using multi-instrument measurements from MAVEN during its periapsis in the subsolar regions and Tianwen-1 during its periapsis in the antisolar regions.

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The red-shaded area represents the dynamic pressure of upstream solar wind (${P}_{{\mathrm{dyn}}_{\mathrm{sw}}}$) at Mars, which is obtained from the MAVEN/LPW key parameter in the MAVEN Science Data Center using:

$$\begin{eqnarray}&&{P}_{{\mathrm{dyn}}_{\mathrm{sw}}}={\rho }_{\mathrm{sw}}{u}_{\mathrm{sw}}^{2}{\cos }^{2}\chi \quad [\mathrm{dyn}\,{\mathrm{cm}}^{-2}]\end{eqnarray} \tag{ 1 }$$

where ρ_sw is the solar wind mass density, u_sw is the speed of solar wind, and χ is the solar zenith angle. To select undisturbed solar wind intervals and avoid contamination from measurements taken inside the bow shock, the dynamic pressure of upstream solar wind out of the bow shock was determined from only data points with the solar wind speed ∣u_sw∣ > 200 km s⁻¹, normalized magnetic-field fluctuations σ_B /∣B∣ < 0.15, altitude R > 500 km, and the ratio of proton scalar temperature to the solar wind speed $\sqrt{T}/| {u}_{\mathrm{sw}}| \lt 0.012$ (Halekas et al. 2017).

The yellow circles represent the thermal pressure of the nightside ionopause (${P}_{{\mathrm{th}}_{i}}$) obtained by:

$$\begin{eqnarray}&&{P}_{{\mathrm{th}}_{i}}={N}_{e}{k}_{{\rm{B}}}({T}_{e}+{T}_{i})\quad [\mathrm{dyn}\,{\mathrm{cm}}^{-2}]\end{eqnarray} \tag{ 2 }$$

where N_e is the electron density from MAVEN/LPW, k_B is the Boltzmann constant, T_e is the electron temperature from MAVEN/LPW, and T_i is the ion temperature. The ion temperature T_i is not available, and thus T_i is assumed to be approximately equal to T_e at altitudes of the ionopause above 350 km (Mendillo et al. 2011).

The blue squares and diamonds represent the magnetic pressure of the nightside and dayside ionopause (${P}_{{B}_{i}}$) obtained by:

$$\begin{eqnarray}&&{P}_{{B}_{i}}=\displaystyle \frac{{B}^{2}}{2{\mu }_{0}}\quad [\mathrm{dyn}\,{\mathrm{cm}}^{-2}]\end{eqnarray} \tag{ 3 }$$

where B is the magnetic-field magnitude from MAVEN/MAG and Tianwen-1/MOMAG, and μ₀ is the magnetic permeability of a vacuum. The horizontal components of magnetic fields were used in calculating the magnetic pressure of ionopause. The direction of the magnetic field is crucial because the vertical field in the ionosphere does not make an effective pressure perpendicular to the ionopause boundary (Sánchez-Cano et al. 2020).

The Martian ionosphere is mostly in a magnetized state (Sánchez-Cano et al. 2020). The thermal pressure of the nightside ionopause was generally less than 0.1 nPa. The thermal pressure of the ionopause is less significant compared to the magnetic pressure. The red vertical-dashed line represents the arrival time of the shock at 19:20 UT on 2021 December 8, when the dynamic pressure of solar wind was larger than 2.0 nPa. Both the magnetic pressures of the nightside and dayside ionopause were enhanced to over 0.4 nP after the shock arrival. This indicates a transient compression at Mars, with a higher solar wind dynamic pressure driven by a fast-forward shock of the ICMEs. The gray color band between two black vertical-dashed lines represents the interval of the ICME. The magnetic pressure of the dayside ionopause returned to its previous level. The high dynamic pressure of solar wind is a very important factor that governs the response of Martian space environment to the ICME. The high solar wind dynamic pressure and foreshock pressure of this strong ICME are greater than the ionosphere pressure, allowing the solar wind to penetrate into the ionosphere and enhance the erosion of ionospheric plasma. Furthermore, the magnetic pressures on the dayside and the nightside were both simultaneously increased during the ICME passage. One reason for this is that it could be a result of the relatively close positions of MAVEN and Tianwen-1 periapses. In addition, the horizontal flow across the terminator and downward diffusion of plasma in the ionosphere could make a contribution.

It it worth noting that after the arrival of the ICME at Mars, fluctuations in the magnetic pressure of the ionopause can also be observed intermittently on the nightside but not on the dayside. The occasional disturbances in the ionospheric magnetic pressure are consistent with the previous observations that the topside Martian ionosphere becomes highly fragmented and composed of intermittent plasma with the effects of the high pressure of solar wind (Dubinin et al. 2009; Zhang et al. 2021a). The intensification of solar wind plasma and magnetic-field strength within an ICME can have a significant impact on the magnetopause and ionosphere of Mars. The size and location of the regions with open magnetic field lines can be significantly altered by the passage of a large ICME (Crider et al. 2005). The energetic electrons precipitate and ion outflow occurs due to the considerable weakness of the shielding effect of Mars' crustal fields (Lillis & Brain 2013).

The bottom panel of Figure 5 shows the column abundance density of electrons (blue), ${{\rm{O}}}_{2}^{+}$ (yellow), and O⁺ (red) integrated between 185–500 km, with the corresponding altitude of the ionopause as circles from density profiles for electrons, ${{\rm{O}}}_{2}^{+}$, and O⁺, respectively. From the profiles of ion and electron densities, the height of the ionopause dramatically decreased after the arrival of the ICME shock, from 445 km altitude on December 9 to 345 km altitude on December 15. The ionopause was lowered by the high dynamic pressure of solar winds. There is an apparent depletion of the observed nightside ionosphere at the time of the ICME passage. Ions escaped to space through the interaction of the ICME and Mars. Between 00:00 on December 10 and 20:00 on December 11, the ICME passed through the space environment surrounding Mars. A significant decrease in the column abundance of ionospheric plasma was observed by MAVEN. The column abundance of electrons above 180 km decreased by 34% from 1.3 × 10¹⁰ cm⁻² on December 10 to 8.5 × 10⁹ cm⁻² on December 11. The column abundance of ${{\rm{O}}}_{2}^{+}$ decreased by 61% from 2.1 × 10⁹ cm⁻² on December 10 to 8.1 × 10⁸ cm⁻² on December 11. The column abundance of O⁺ decreased by 73% from 2.1 × 10⁹ cm⁻² on December 10 to 5.7 × 10⁸ cm⁻² on December 11. Two days later, the column abundance densities of electrons, ${{\rm{O}}}_{2}^{+}$, and O⁺ returned to their undisturbed levels on December 13, indicating a perpetually evolving equilibrium state. The depleted column density of electrons ${P}_{{e}^{-}}$ is estimated to be ∼4.5 × 10⁹ cm⁻². Taking the width of the erosion channel d ∼ 400 km and the length of the erosion channel in the horizontal direction L ∼ π/2 · R, where R ∼ 4000 km, yields the total number of loss ions ∼1.3 × 10²⁶. Such an amount of ions can be lost in ∼5 minutes with a flow velocity V_esc ∼ 20 km s⁻¹ (Dubinin et al. 2009), and thus the corresponding ion loss rate is 4.3 × 10²³ s⁻¹. Ion escape is minimized in the subsolar regions and enhanced in the antisolar regions (Jakosky et al. 2018). Therefore, this is a conservative estimate given that the ionospheric plasma outside of intruded regions can be driven into a global convention and atmospheric circulation (Cui et al. 2015; Niu et al. 2020). Due to the limited coverage of observations, a direct estimation of the global loss rate of ions during ICME cannot be accurately determined. Brain et al. (2015) found that the outward flux of ions dominates the inward flux on the nightside of Mars and the outward rate of ions on the nightside is 2.5–3.0 times larger than that on the dayside. From the MAVEN measurements, Mars's ion escape was significant during the passage of the ICME, as indicated by a noticeable drop in the averaged column density of plasma in the nighttime ionosphere.

It is interesting to note that during the passage of some ICMEs at Venus, the disappearance of the Venusian ionosphere was also observed (Cravens et al. 1982). In contrast to ionospheric holes that form in localized regions, the ionospheric plasma was depleted throughout the nightside ionosphere. the disappearing ionosphere on the nightside of Venus is attributed to the high solar wind pressure cutting off the thermal pressure gradient that drives transterminator flow of ionosphere from the dayside. In this situation, a similar physical mechanism may apply to depletions of the observed nightside ionosphere at Mars. In addition, increasing solar wind dynamic pressure pushes the dayside ionopause lower in height. As a result, the nightside should thus be accessible to plasma at low altitudes, which is consistent with a rise in the nightside ionospheric electron density at lower altitudes in this study. Further investigations can provide insight with future multi-instrument data from Tianwen-1, MAVEN, and upcoming Mars exploration missions, which will provide supportive evidence and test the hypothesis.