Abstract

Abstract. We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.

Highlights

  • Ionization of atoms and molecules of cometary origin is the most important process for the interaction of strongly outgassing comets and the solar wind

  • We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism

  • A more detailed correlation analysis between the individual values of both quantities at the same radial distance reveals a linear Pearson correlation coefficient of r = 0.52. This correlation reflects the large spread of both wave energy and neutral gas density

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Summary

Introduction

Ionization of atoms and molecules of cometary origin is the most important process for the interaction of strongly outgassing comets and the solar wind. I. Richter et al.: Low-frequency waves at comet 67P/Churyumov-Gerasimenko vational findings in the cometary magnetosphere (Tsurutani and Smith, 1986; Yumoto et al, 1986; Neubauer et al, 1986; Glassmeier et al, 1989; Glassmeier and Neubauer, 1993; Volwerk et al, 2014). Rosetta’s journey (Glassmeier et al, 2007a) alongside comet 67P/Churyumov-Gerasimenko allows for electromagnetic waves to be studied at the beginning of cometary activity, at the birth of the cometary magnetosphere (Nilsson et al, 2015). It should be noted that, under these low-activity conditions, typical solar wind–cometary interaction regions like the bow shock and magnetic pileup region are not expected (e.g., Koenders et al, 2013; Rubin et al, 2014) and not observed (Nilsson et al, 2015). We shall report on wave observations at distances of 2.7–3.6 AU from the Sun and 10–1000 km from the comet

Mission and instrumentation
Observations
Discussion and possible wave source mechanism
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