Mars Global Distribution of the External Magnetic Field and Its Variability: MAVEN Observation and MHD Prediction

Xiaohua Fang; Yingjuan Ma; Janet Luhmann; Yaxue Dong; Jasper Halekas; Shannon Curry

doi:10.1029/2023JA031588

Abstract We study the average global distribution of the external magnetic field at Mars, and its variability with the upstream solar wind dynamic pressure and interplanetary magnetic field as well as with the ambient crustal magnetic field strength. Our approach involves excluding the intrinsic planetary field from the total magnetic field by applying a crustal field model previously derived using low altitude measurements. The distribution of the average external field that remains is statistically analyzed using nearly 8 years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations and several global, time‐dependent magnetohydrodynamic simulations. Overall consistent results have been obtained from the data and model, which are complementary to each other and cross validate the findings. It is found that the external field is significantly enhanced from the upstream across the bow shock (BS) and further intensifies closer to the planet in the topside ionosphere. It peaks at ∼170 km altitude near the subsolar point, significantly decreasing with increasing solar zenith angle. There is a strong day‐night asymmetry in the external field, with a typical dayside intensity of ∼15–50 nT and a nightside intensity of ∼5–15 nT. Under high solar wind dynamic pressures and IMFs, the external field may be enhanced by a factor of ∼2 everywhere below the BS, on both the dayside and nightside. In addition, our model results suggest that strong crustal fields, which effectively withstand the penetration of the solar wind, reduce the external field at low altitudes. Plain Language Summary We use nearly 8 years of satellite observations and several global numerical simulations to analyze the average distribution of the external magnetic field induced in the solar wind‐Mars interaction. Our approach is to separate the intrinsic and external components both in field measurements from the MAVEN spacecraft and in simulation results from a global Mars‐solar wind interaction model. The intrinsic crustal magnetic field is rooted beneath the surface and is organized in the rotating, planet‐fixed reference frame. The external magnetic field is better described in a Sun‐Mars reference frame in light of the complex solar wind interaction with the Mars obstacle (combined magnetosphere and ionosphere). Mixing them together results in the appearance of complex magnetic field distributions and affects the understanding of physical processes. Our exclusion of the intrinsic component using a crustal field model enables us to focus on the distribution of the external magnetic field itself, which is relatively poorly understood. We investigate the variability of the external field distribution due to the changes of the upstream solar wind and magnetic field conditions and the ambient crustal field strength. Our work shows that the average external field distribution follows basic patterns despite complex variabilities. Key Points We use Mars Atmosphere and Volatile EvolutioN observations and magnetohydrodynamic simulations to investigate the external magnetic field environment after the crustal field is excluded The external field has a typical dayside (nightside) intensity of ∼15–50 nT (∼5–15 nT) and peaks at ∼170 km altitude at the subsolar point The external field intensity can be enhanced by a factor of ∼2 on a global scale below the bow shock during high solar activity

Mars Global Distribution of the External Magnetic Field and Its Variability: MAVEN Observation and MHD Prediction

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