The solar wind protons inside the induced magnetosphere of Mars

Sammanfattning: Mars is an unmagnetized planet. Mars has no intrinsic magnetic field but has local magnetic anomalies in the crust. The solar wind, which is the plasma flowing from the Sun at supersonic speed, interacts with the magnetic fields of the currents induced in the conductive Martian ionosphere, deviates and slows down to subsonic speeds. A void in the solar wind is formed around the planet as an induced magnetosphere.At the boundary of the induced magnetosphere, the plasma composition changes from being dominated by the major ion in the solar wind (protons) to being dominated by heavy ions of planetary origin. Also, the interplanetary magnetic field, being carried by the solar wind, starts to pile up against the planet to form a magnetic barrier on the dayside, drapes around the planet, stretches due to mass loading, and forms a magnetotail.The gyroradius of a heated proton in the magnetosheath is large in comparison with the size of the induced magnetosphere. Therefore, a fraction of the proton population penetrates the induced magnetosphere boundary, enters the upper layer of the atmosphere (the ionosphere) and subsequently neutralizes at low altitudes. We have conducted a detailed study of an event, in which the magnetosheath protons penetrate the Martian induced magnetosphere boundary (IMB). The spatial extent of the proton precipitation region reached several thousands of kilometers along the orbit of the Mars Express spacecraft.The interaction of the precipitating protons with the Martian atmosphere was modeled using a direct simulation Monte Carlo method. The inclusion of a horizontal magnetic field in the model significantly increased the backscattering of protons compared to the case without a magnetic field. More than 50% of the incoming energy is reflected backwards for a magnetic field of strength 30 nT, compared to 4% in the case of no magnetic field. We have also used hybrid modeling to study the spatial pattern of the precipitation onto the Martian atmosphere both for solar wind protons and protons originating from the planetary atmosphere. The solar wind protons and the exospheric (planetary) protons contribute 60% and 40%, respectively, of the deposition of mass at the exobase for the given input parameters. The precipitating flux decreases substantially at the subsolar point, due to the backscattering of the incoming protons by the more intense piled-up magnetic field.