New Publication: “Mirror Mode Storms Observed by Solar Orbiter” by Andrew Dimmock et al.

Mirror modes (MMs) are ubiquitous in space plasma and grow from pressure anisotropy. Together with other instabilities, they play a fundamental role in constraining the free energy contained in the plasma. This study focuses on MMs observed in the solar wind by Solar Orbiter (SolO) for heliocentric distances between 0.5 and 1 AU. Typically, MMs have timescales from several to tens of seconds and are considered quasi-MHD structures. In the solar wind, they also generally appear as isolated structures. However, in certain conditions, prolonged and bursty trains of higher frequency MMs are measured, which have been labeled previously as MM storms. At present, only a handful of existing studies have focused on MM storms, meaning that many open questions remain. In this study, SolO has been used to investigate several key aspects of MM storms: their dependence on heliocentric distance, association with local plasma properties, temporal/spatial scale, amplitude, and connections with larger-scale solar wind transients. The main results are that MM storms often approach local ion scales and can no longer be treated as quasi-magnetohydrodynamic, thus breaking the commonly used long-wavelength assumption. They are typically observed close to current sheets and downstream of interplanetary shocks. The events were observed during slow solar wind speeds and there was a tendency for higher occurrence closer to the Sun. The occurrence is low, so they do not play a fundamental role in regulating ambient solar wind but may play a larger role inside transients.

Mirror modes (MMs) observed on 19 July 2021. Plotted in panels (a and b) are |B| and Brtn, a wavelet spectrogram of B is shown in panel (c), and the ellipticity of the magnetic field is shown in panel (d). Panels (e–k) depict Ni, |Vi|, Ti, differential energy flux, βi, and RMM, respectively. Regions that are highlighted in yellow correspond to localized reductions in ellipticity and the manifestation of MM structures since they should have zero ellipticity.

Full Article:
Dimmock, A. P. (SHARP), Yordanova, E., Graham, D. B. (SHARP), Khotyaintsev, Y. V. (SHARP), Blanco-Cano, X., Kajdič, P., et al. (2022). Mirror mode storms observed by Solar Orbiter. Journal of Geophysical Research: Space Physics, 127, doi: 10.1029/2022JA030754

License: CC BY 4.0