A model analysis of the northern and southern hemispheric electron density profiles at Mars
Abstract
The Mars Express electron density (N _{e}) profiles near terminators have indicated more complicated structure of the Martian high-latitude ionosphere than previously thought. Some of these profiles over the northern hemisphere show wide and narrow shapes of the main N _{e} peaks while others over the southern hemisphere have shown anomalous characteristics of the topside plasma distribution. We use our 1-D chemical diffusive model coupled with the Mars - Global Ionosphere Thermosphere Model (M-GITM) to interpret both northern (67 ^{o}N, 235 ^{o}E and 66 ^{o}N, 341 ^{o}E) and southern (82 ^{o}S, 180 ^{o}E) hemispheric Ne profiles. Our model is a coupled finite difference primitive equation model which solves for plasma densities and vertical ion fluxes. The crustal magnetic field at the northern locations is mainly horizontal and does not allow plasma to move vertically. Thus, the primary plasma loss for the topside ionosphere at the northern locations is likely caused by diverging horizontal fluxes of ions, indicating that the dynamics of the upper ionosphere of Mars is controlled by the solar wind. The situation at the southern location is different where the field lines are nearly vertical and open to the access of solar wind plasma through magnetic reconnection with the interplanetary magnetic field. This can lead to the acceleration of electrons and ions during the daytime ionosphere. The downward accelerated electrons with energies >200 eV penetrate deep into the Martian upper ionosphere along vertical magnetic field lines and cause heating, excitation and ionization of the background atmosphere. The upward acceleration of ions resulting from energy input by precipitating electrons can lead to enhance ion escape rate and modify scale heights of the topside ionosphere. The primary source of ionization in the model is due to solar EUV radiation. An extra ionization source due to precipitating electrons of 0.25 keV, peaking near an altitude of 145 km is added in the model to interpret the measured ionospheric structure at the southern location. We find that an upward flux of O _{2} ^{+} of 1.8 x 10 ^{6} cm ^{-2} s ^{-1} is needed to explain the northern topside electron density profile at 66 ^{o}N while this value is increased by a factor of two at 67 ^{o}N. However, the behavior of the topside ionosphere at the southern location is interpreted by the vertical plasma transport simulated by vertical ion velocities, whose values can be interpreted as drift velocities along magnetic field lines. We find that the variation of the topside N _{e} scale heights is sensitive to magnitudes of upward and downward drifts with an imposed outward flux boundary condition at the top of the model. The model requires an upward flux of more than 10 ^{7} ions cm ^{-2} s ^{-1} for both O _{2} ^{+} and O ^{+}, and drift speeds of 200 m/s to interpret the measured topside ionospheric structure. The model results for both the northern and southern locations will be presented in comparison with the measured electron density profiles. This work is supported by MBRSC, Dubai, UAE.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2128M