Coherent wave activity in Mercury's magnetosheath

Abstract

This study presents a statistical overview of coherent wave activity in Mercury's magnetosheath. Left‐handed electromagnetic ion cyclotron waves are commonly found behind the quasi‐perpendicular section of the bow shock, where they are present in ~50% of the spacecraft crossings of the magnetosheath. Their occurrence distribution maximizes within the magnetosheath, approximately halfway between the bow shock and the magnetopause, and the waves are generally strongly Doppler shifted up to frequencies above the local ion cyclotron frequency. Downstream of the quasi‐parallel shock, the magnetosheath often exhibits large‐amplitude pulsations with wave periods around 10 s and peak‐to‐peak amplitudes of up to 100 nT that dominate the magnetic field structure. These waves are circularly left‐hand polarized with wave vectors in the direction of the local shock normal. The data suggest that they have been generated upstream of the shock and transmitted into the downstream region. Their occurrence rates maximize at the near‐parallel shock, where they are present approximately 10% of the time, and where they also show their largest wave powers. Some evidence is also found of waves with a right‐handed polarization in the spacecraft frame. These consist of both whistler waves above the local ion cyclotron frequency and ion cyclotron waves propagating against the magnetosheath flow with Doppler shifts exceeding the intrinsic wave frequency, which results in a change in their apparent polarization. These waves are in minority compared to the left‐handed observations, which indicates a preference for ion cyclotron waves propagating in the direction of the plasma flow.

Magnetic Holes in the Solar Wind and Magnetosheath Near Mercury

“…Further evidence against local generation of magnetic holes in the magnetosheath is the fact that a search for waves in the Hermean magnetosheath by Sundberg et al. (2015) showed that ion‐cyclotron waves were very common, but did not identify any mirror mode structures. They interpreted this as an effect of a lower plasma β value for the Mercury magnetosheath than for other planets, since the solar wind β increases with heliocentric distance.…”

“…We apply the same criterion in the magnetosheath, σ B,norm < 0.05, both to increase the likelihood of identifying clean events, and to exclude the large-amplitude, embedded pulsations found behind the quasi-parallel bow shock (Sundberg et al, 2013(Sundberg et al, , 2015.…”

Solar wind magnetic holes can cross the bow shock and enter the magnetosheath

“…However, this is not necessarily so. Assume, e.g, that magnetic holes are long cylinders, as suggested by Sundberg et al (2015) for magnetospheric magnetic holes, and that the magnetic field strength as a function of distance from the minimum of the holes is given by a Gaussian expression. Then it is easily show that the times series of the magnetic field strength for any straight spacecraft path through the hole is given by a Gaussian, only the depth of the hole will be different.…”

Magnetometer in-flight offset accuracy for the BepiColombo spacecraft

“…Mercury's plasma environment is highly dynamic and home to a plethora of wave modes and fluctuations (Russell, 1989;Boardsen et al, 2009Boardsen et al, , 2012Sundberg et al, 2015). We separate the MESSENGER orbit segments into solar wind, magnetosheath, and magnetospheric parts, based on an extended boundary data set from Winslow et al (2013) || .…”