Effect of the Crustal Magnetic Field on the Day-to-night Plasma Transport in the Martian Nightside Ionosphere

Abstract:

Day-to-night plasma transport and electron precipitation from the solar wind (SW) are two most likely sources for the Martian nightside ionosphere. Although Mars does not have a large-scale intrinsic magnetic field, the existence of the crustal magnetic field and the induced field associated with the SW/Mars interaction can affect the morphology and ion-dynamics of the Martian ionosphere considerably. As revealed from the magnetometer/electron reflectometer (MAG/ER), the most intense crustal magnetic fields at Mars are located in the Terra Sirenum region. Mini-magnetospheres can be formed by the crustal magnetic anomalies, which can shield the Martian ionosphere from the upstream solar wind flow. Strong horizontal magnetic field may favor the day-to-night plasma transport and hinder the electron precipitation and the vertical plasma diffusion. In the cusp-like regions where the magnetic field line is nearly vertical, the connection with interplanetary magnetic field can permit the precipitation of the SW and the energetic particles.

Here, we use the MARSIS subsurface total electron content (TEC) data to study the role of day-to-night plasma transport in the Martian nightside ionosphere. As an extended work of Cui et al. (2015), we will study the effect of different crustal magnetic components on the transport process in the Time in Darkness (TD) domain. The Bx component points to the local north, By points to the local east, and Bz points to the nadir. It is supposed that eastward By term will enhance the day-to-night plasma transport, and vice verse for the westward By term. Constraining the observations in the strong crustal magnetic region of the southern hemisphere, we find that TEC generally maintains a higher value for By > 100 nT compared with By < -100 nT as the ionosphere is turning into the night. The opposite is found for the Bz component, and it may indicate that large vertical B-field can make the plasma diffuse up or down according to the direction of Bz, which further slow down the day-to-night transport and lead to lower TEC in the night. We also find that the maximum TD value is much larger for the southern hemisphere than that for the northern hemisphere. This indicates that the crustal magnetic field mainly located in the southern hemisphere helps to maintain the nighttime ionosphere.