Reconstruction of the magnetic connection from Mercury to the solar corona during enhancements in the solar proton fluxes at Mercury

Abstract

Aims. We study the magnetic connection between Mercury and the solar corona based on energetic proton events measured near Mercury by MESSENGER during 2011−2013 in order to identify the possible source of the accelerated particles on the solar surface. Methods. The transport of the magnetic field lines in the heliosphere was evaluated with a Monte Carlo code that gives a random displacement at each step of the integration along the Parker magnetic field model. The simulation was tailored to each specific event by using the magnetic fluctuation levels obtained at Mercury by MESSENGER and the values of the solar wind velocity measured at 1 AU by the Advanced Composition Explorer satellite. We selected seven case studies for which an increase in the proton fluxes of at least two orders of magnitude with respect to the background level was observed. For each selected case, we took the background magnetic field map (magnetogram) at the source surface of the solar wind (r = 2.5 R⊙) into account. By considering the relative position of Mercury and the Earth on the day on which the enhancement in the proton fluxes was observed by MESSENGER, we obtained the position of the active regions on the solar surface as seen by Mercury. Results. The footpoint of the Parker spiral passing Mercury was reconstructed for all of the selected events. By considering the values of the fluctuation levels of the interplanetary magnetic field recorded by MAG-MESSENGER two days before the event and the values of the fluctuation levels of the interplanetary magnetic field on the day on which the event was observed, we are also able to appreciate the effects on the solar wind magnetic field perturbations induced by the shock of the coronal mass ejection. This technique will also be useful for the interpretation of energetic particle observations by BepiColombo.