Abstract
Abstract. We present a statistical survey of large-amplitude, asymmetric plasma and magnetic field enhancements detected outside the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko from December 2014 to June 2016. Based on the concurrent observations of plasma and magnetic field enhancements, we interpret them to be magnetosonic waves. The aim is to provide a general overview of these waves' properties over the mission duration. 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 these waves and their characteristics. In total, we identified ∼ 70 000 steepened waves in the magnetic field data by means of machine learning. We observe that the occurrence of these steepened waves is linked to the activity of the comet, where steepened waves 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 compressional 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 steepened waves. With increasing mass loading, the average amplitude of the waves decreases, while the skewness increases. Using a modified 1D magnetohydrodynamic (MHD) model, we investigated if the waves can be described by the combination of nonlinear and dissipative effects. By combining the model with observations of amplitude, width and skewness, we obtain an estimate of the effective plasma diffusivity in the comet–solar wind interaction region and compare it with suitable reference values as a consistency check. At 67P/Churyumov–Gerasimenko, these steepened waves are of particular importance as they dominate the innermost interaction region for intermediate to high activity.
Highlights
The groundbreaking ESA Rosetta mission (Glassmeier et al, 2007a; Taylor et al, 2017; Glassmeier, 2017) was the first of its kind to orbit a comet and study its plasma environment over a prolonged time of about 2 years
We present a statistical survey of largeamplitude, asymmetric plasma and magnetic field enhancements detected outside the diamagnetic cavity at comet 67P/Churyumov–Gerasimenko from December 2014 to June 2016
The plasma pulses inside and outside of the diamagnetic cavity are similar in shape (Engelhardt et al, 2018; Hajra et al, 2018b), they must be of different nature because those detected outside the cavity have strong magnetic components
Summary
The groundbreaking ESA Rosetta mission (Glassmeier et al, 2007a; Taylor et al, 2017; Glassmeier, 2017) was the first of its kind to orbit a comet and study its plasma environment over a prolonged time of about 2 years. During both intervals, the outgassing rate was already high enough to facilitate the development of a diamagnetic cavity (Goetz et al, 2016a, b) and, with a high probability, a bow shock (Koenders et al, 2013, 2015). Of nonlinear waves near the bow shock region at 67P/CG is unconfirmed Such low-frequency waves were expected at lower outgassing rates but were never observed (Glassmeier, 2017). Since the characteristics of these waves did not fit a ring-beam instability, Richter et al (2015) suggested a cross-field current instability as the possible source mechanism Based on these observations, Meier et al (2016) proposed a modified ion-Weibel instability.
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