Structure and dynamics of the solar wind/ionosphere interface on Mars: MEX‐ASPERA‐3 and MEX‐MARSIS observations

Abstract

The measurements of the local plasma parameters of the ionospheric and solar wind plasmas and the magnetic field strength carried out by the ASPERA‐3 and MARSIS experiments onboard Mars Express (MEX) in the subsolar region of the induced Martian magnetosphere provide us with a first test of the pressure balance across the solar wind/ionosphere interface. The structure of this transition is very dynamic and is controlled by the solar wind. For a broad range of the solar wind dynamic pressures, the magnetic field in the boundary layer raises to the values just sufficient to balance the solar wind pressure. The magnetic field frozen into the electrons is transported across the magnetospheric boundary (MB) where solar wind terminates and the planetary plasma begins to prevail. The dense ionospheric plasma has a sharp outer boundary the position of which is usually a little closer to the planet than the MB. Although the number density reaches on this boundary ∼103 cm−3 the contribution of the ionospheric thermal pressure is rather small and the ionosphere is magnetized. There are also cases when the magnetic field almost does not vary across both boundaries.

Plasma boundary variability at Mars as observed by Mars Global Surveyor and Mars Express

“…The crustal fields a b have previously been shown to influence the boundaries by using MGS measurements and also MEX measurements on their own Brain et al, 2005;Fränz et al, 2006b;Edberg et al, 2008). However, no previous study has used such a large data set of crossings as we have in this study and we can now for the first time produce global maps of the radial distance of the boundaries.…”

“…However, we have analyzed the entire set of MEX/ELS and MEX/IMA data from 2004 until 2009 and have identified 5014 MPB crossings and 3277 BS crossings. The number of crossings during the overlapping mission time between MGS andMEX (February 2004-November 2006) decreases to 2500 and 1840, respectively, which is still twice as many as MGS observed from 1997 until 1999 when it was in an elliptic orbit similar to the orbit of MEX Trotignon et al, 2006;Edberg et al, 2008). It should be noted that we can only get values of the measured solar wind dynamic pressure when the BS is crossed, meaning that we do not know the upstream pressure for all of the 5014 MPB crossings.…”

“…However, due to the IMF direction and the crustal fields, the shapes can in fact be asymmetric and uneven. The crustal fields can, on average, cause the MPB to be ∼10% and the BS to be ∼20% farther out in the Southern Hemisphere than in the Northern Hemisphere (Edberg et al, 2008). The IMF direction can cause the BS to be 13% farther out in the hemisphere of locally upward electric field .…”

“…The shapes of both the BS and the MPB have been shown to be well represented by conic sections r=L/(1+ cos(θ)), where r and θ are polar coordinates with origin at X 0 referenced to the MSO x-axis and and L are the eccentricity and semi-latus rectum, respectively, of the conic section. At the subsolar point, where the boundaries are at their closest to the planet, the BS is on average located at an altitude of 0.58 R M (∼2000 km) and the MPB at an altitude of 0.33 R M (∼1100 km) (Edberg et al, 2008). The altitudes of the boundaries then increase on moving toward the nightside, such that on the day/night terminator the BS is on average at an altitude of 1.6 R M (∼5400 km) and the MPB at 0.45 R M (∼1500 km).…”

“…Factors that have been shown empirically to have an effect include the IMF direction Brain et al, 2005), the crustal magnetic fields Dubinin et al, 2006;Fränz et al, 2006b;Edberg et al, 2008) and the solar wind dynamic pressure (Crider et al, 2003;Brain et al, 2005). Dubinin et al (2008) also studied how the solar wind dynamic pressure balances the thermal pressure in the magnetosheath and the magnetic pressure inside of the MPB. When describing the shape of the boundaries as conic sections they are usually assumed to be rotationally symmetric around the x-axis.…”