Control of the topside Martian ionosphere by crustal magnetic fields

Abstract

Sounding (MARSIS) instrument onboard Mars Express of the thermal electron plasma density of the Martian ionosphere and investigate the extent to which it is influenced by the presence of Mars's remnant crustal magnetic fields. We use locally measured electron densities, derived when MARSIS is operating in active ionospheric sounding (AIS) mode, covering an altitude range from ∼300 km to ∼1200 km. We compare these measured densities to an empirical model of the dayside ionospheric plasma density in this diffusive transport-dominated regime. We show that small spatial-scale departures from the averaged values are strongly correlated with the pattern of the crustal fields. Persistently elevated densities are seen in regions of relatively stronger crustal fields across the whole altitude range. Comparing these results with measurements of the (scalar) magnetic field also obtained by MARSIS/AIS, we characterize the dayside strength of the draped magnetic fields in the same regions. Finally, we provide a revised empirical model of the plasma density in the Martian ionosphere, including parameterizations for both the crustal field-dominated and draping-dominated regimes.

Empirical model of the Martian dayside ionosphere: Effects of crustal magnetic fields and solar ionizing flux at higher altitudes

“…The electron densities at an altitude of 325 km are by a factor of about 2 larger in areas of strong crustal magnetic fields than in areas of weak crustal magnetic fields. This is consistent with the results obtained by Andrews et al [, ] who used local electron density measurements at altitudes higher than about 300 km. As for the effect of the magnetic field inclination, there is likely an interplay between the vertical and horizontal transport [ Andrews et al , ; Matta et al , ].…”

Ionospheric Electron Densities at Mars: Comparison of Mars Express Ionospheric Sounding and MAVEN Local Measurements

“…At higher altitudes, electron densities are below the lower detection threshold of the MARSIS radar sounding. Further, electron densities at altitudes higher than about 325 km generally follow a simple exponential decrease with the altitude (e.g., Duru et al, , ), which has been statistically well mapped using the MARSIS locally evaluated electron densities (Andrews et al, ; Němec et al, ). In the present study, LPW electron density measurements are important for the two main reasons: (i) Due to the lower perigee of MAVEN as compared to Mars Express, they allow us to fill the altitudinal gap between the MARSIS radar sounding and MARSIS locally evaluated electron densities, and (ii) at lower altitudes, one can compare LPW in situ measurements with electron density profiles obtained by the MARSIS radar sounding.…”

“…Following former statistical results on electron density profile shapes (Němec et al, ), they suggested to use an empirical shape composed of a smooth transition between two exponential dependencies with different scale heights, with the scale height H 2 at higher altitudes being larger than the scale height H 1 at lower altitudes (Gurnett et al, ; Morgan et al, ): where θ 1 =1/ H 1 , θ 2 =1/ H 2 , z 0 is a transition altitude corresponding to the altitude where the two asymptotic dependencies intersect, δ is a quantity proportional to the radius of curvature, and C is a constant. The value of θ 2 can be adopted from statistical models of electron density scale height at high altitudes (Andrews et al, ; Němec et al, ). For given values of z 0 and δ , the values of C and θ 1 are then unambiguously given by the conditions that (i) the electron density at the spacecraft altitude must be equal to the one actually measured, and (ii) the measured time delay at the lowest frequency available from the ionospheric sounding must be equal to the time delay calculated using the empirical electron density profile.…”

Mars: a small terrestrial planet

“…The lifetime of these features was about 10 days. They could be ice particles reflecting sunlight, or related to a solar wind event, however, until now, no clear explanation has been given to explain these observations, although it is interesting to note that this period was characterized by an active solar period, during which coronal mass ejections hit Mars (Andrews et al 2015).…”

“…Elevated ionospheric densities and localized spatial structures are seen above magnetic anomalies (Andrews et al 2014(Andrews et al , 2015Matta et al 2015;Ma et al 2014;Dubinin et al 2012;Safaeinili et al 2007), and can be explained to first order by a larger ionization due to higher energetic particle fluxes spiraling along the magnetic field lines (Lillis and Fang 2015).…”