26 Jul 2022
 | 26 Jul 2022

Statistical distribution of mirror mode-like structures in the magnetosheaths of unmagnetised planets: 2. Venus as observed by the Venus Express spacecraft

Martin Volwerk, Cyril Simon Wedlund, David Mautner, Sebastian Rojas Mata, Gabriella Stenberg Wieser, Yoshifumi Futaana, Christian Mazelle, Diana Rojas-Castillo, Cesar Bertucci, and Magda Delva

Abstract. In this series of papers, we present statistical maps of mirror mode-like (MM) structures in the magnetosheaths of Mars and Venus and calculate the probability of detecting them in spacecraft data. We aim to study and compare them with the same tools and a similar payload at both planets. We consider their dependence on Extreme Ultraviolet (EUV) solar flux levels (high and low).

The detection of these structures is done through magnetic field-only criteria and ambiguous determinations are checked further. In line with many previous studies at Earth, this technique has the advantage of using one instrument (a magnetometer) with good time resolution facilitating comparisons between planetary and cometary environments.

Applied to the magnetometer data of the Venus Express (VEX) spacecraft from May 2006 to November 2014, we detect structures closely resembling MMs lasting in total more than 93,000s, corresponding to about 0.6 % of VEX's total time spent in the Venus's plasma environment. We calculate MM-like occurrences normalised to the spacecraft's residence time during the course of the mission. Detection probabilities are about 10 % at most for any given controlling parameter.

In general, MM-like structures appear in two main regions, one behind the shock, the other close to the induced magnetospheric boundary, as expected from theory. For solar maximum, the active region behind the bow shock is further inside the magneosheath, near the solar minimum bow shock location. The ratios of the observations during solar minimum and maximum are slightly dependent on the depth Δ B / B of the structures, deeper structures are more prevalent at solar maximum. A dependence on solar EUV (F10.7) flux is also present, where at higher F10.7 flux the events occur at higher values than the daily average value of the flux. Combining the plasma data from the Ion Mass Analyser with the magnetometer data shows that the instability criterion for MMs is reduced in the two main regions where the structures are measured, whereas it is still enhanced in the region in-between these two regions, implicating that the generation of MMs is transferring energy from the particles to the field.

This study is the second of two on the magnetosheaths of Mars and Venus, and a third paper comparing the results obtained at the two planets will follow.

Martin Volwerk et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-645', Anonymous Referee #1, 01 Aug 2022
  • RC2: 'Comment on egusphere-2022-645', Anonymous Referee #2, 18 Aug 2022

Martin Volwerk et al.

Martin Volwerk et al.


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Short summary
Freshly created ions in the solar wind start gyrating around the interplanetary magnetic field. When they cross the bow shock they get an extra kick and this increases the plasma pressure against the magnetic pressure. This leads to the creation of so-called mirror modes, regions where the magnetic field decreases in strength and the plasma density increases. These structures help in exploring how energy is transferred from the ions to the magnetic field and where around Venus this is happening.