The connection between the bow shock at Mercury and the interplanetary magnetic field

Abstract: As the solar wind reaches Mercury it interacts with the planet’s magnetic field slowing down, forming a bow shock in front of the planet and diverting the flow around it. Along with the solar wind comes the interplanetary magnetic field, an extension of the sun’s magnetic field. The interaction between the bow shock and the interplanetary magnetic field impacts the behaviour of the plasma both up- and downstream of the bow shock. An important factor is the angle between the normal to the bow shock surface and the interplanetary magnetic field, θBN. The angle can be divided into two categories: quasi-parallel for when θBN < 45° and quasi-perpendicular for θBN > 45°. It is expected for a quasi-parallel configuration to have stronger fluctuations in both the solar wind upstream of the bow shock and in the magnetosheath downstream caused by reflected particles backstreaming into the solar wind. Quasi-perpendicular configurations are expected to have less fluctuations in both regions due to fewer solar wind particles being reflected back. In this thesis this connection is investigated at the bow shock at Mercury using magnetic field data from the MESSENGER mission. By looking at the data when the spacecraft travels through the thin bow shock the local θBN angle can be calculated. The fluctuation level is then calculated as the standard deviation of the magnetic field in a 30 second period upstream and downstream of the crossing. The results found are unexpected as the correlation between θBN and the fluctuation levels are weaker and more uniformly distributed than expected compared to similar studies conducted at Earth using the Cluster satellites. This is most likely due to the smaller spatial scale of the Hermean system: the structures perpendicular to the interplanetary magnetic field of upstream activity, such as SLAMS, cover a greater proportion of the bow shock than at Earth allowing them to extend over into neighbouring regions of different θBN values, giving a more uniform distribution of the fluctuation levels.