Steepening of magnetosonic waves in the inner coma of comet 67P/Churyumov-Gerasimenko

Abstract

Abstract. We present a statistical survey of large amplitude, asymmetric plasma, and magnetic field enhancements at comet 67P/Churyumov-Gerasimenko from December 2014 to June 2016. The aim is to provide a general overview of these structures' properties over the mission duration. At comets, nonlinear wave evolution plays an integral part in the development of turbulence and in particular facilitates the transfer of energy and momentum. As the first mission of its kind, the ESA Rosetta mission was able to study the plasma properties of the inner coma for a prolonged time and during different stages of activity. This enables us to study the temporal evolution of steepened waves and their characteristics. In total, we identified ~70000 events in the magnetic field data by means of machine learning. We observe that the occurrence of wave events is linked to the activity of the comet, where events are primarily observed at high outgassing rates. No clear indications of a relationship between the occurrence rate and solar wind conditions were found. The waves are found to propagate predominantly perpendicular to the background magnetic field, which indicates their compressive nature. Characteristics like amplitude, skewness, and width of the waves were extracted by fitting a skew normal distribution to the magnetic field magnitude of individual events. With increasing massloading the average amplitude of steepened waves decreases while the skewness increases. Using a modified 1D MHD model it was possible to show that such solitary structures can be described by the combination of nonlinear, dispersive, and dissipative effects. By combining the model with observations of amplitude, width, and skewness we obtain an estimate of the effective plasma viscosity in the comet-solar wind interaction region. At 67P/Churyumov-Gerasimenko steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.

Ion‐Ion Cross‐Field Instability of Lower Hybrid Waves in the Inner Coma of Comet 67P

“…They are often asymmetric, with much longer rise times than decay times, and may be related to similar structures observed near the diamagnetic cavity (e.g., Goetz, Koenders, Hansen, et al., 2016; Hajra et al., 2018; Henri et al., 2017; Odelstad et al., 2020) and/or steepened fast magnetosonic waves discussed by Ostaszewski et al. (2021). Also shown in Figure 1b are a number of angular quantities (to be read off the right‐hand y ‐axis): cone angle θ B (red line) and clock angle ϕ B (green line) of the magnetic field (to be defined below), the angle α EB (blue line) between B and the probe separation vector d (cf.…”

Revisiting Mirror Modes in the Plasma Environment of Comet 67P/Churyumov-Gerasimenko

“…After this selection, we obtain a list of 32026 mirror mode candidates throughout the mission between November 1st 2014 and February 29th 2016. A closer inspection reveals that many of the mirror mode candidates found by the magnetic field only method are compressional magnetic field structures, also known as steepened waves (Ostaszewski et al, 2020). This is expected because compressional structures also have a large ∆B/B and may satisfy the angle criteria as well.…”

Solar Wind Protons in the Diamagnetic Cavity at Comet 67P/Churyumov‐Gerasimenko

“…The quieter plasma and magnetic field that was also observed after E1, can be observed again, this time from the start of the interval up to about 8:30 and then again at the very end of the interval. This also corresponds to the lower plasma densities in the quiet region compared to the interval directly after E5 where the density is more variable and higher, and the magnetic field is characterized by steepened wave structures (Ostaszewski et al., 2020 ). At 8:20, there is a change in the solar wind spectra, which go from broad and low energy to higher energies and more focused.…”

“…In the second spectrum their flux is low, while there is clear evidence of a heavy ion population in the third spectrum. The magnetic field measurements (bottom panel) show the characteristic signatures of steepened waves (Ostaszewski et al., 2020 ) with often sharp, large amplitude increases and gradual decreases. Two intervals were identified as diamagnetic cavity, but only one contains proton signatures.…”