Abstract
Mirror modes 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 mirror modes observed in the solar wind by Solar Orbiter for
heliocentric distances between 0.5 and 1 AU. Typically, mirror modes
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 mirror modes are measured, which have
been labeled previously as mirror mode storms. At present, only a
handful of existing studies have focused on mirror mode storms, meaning
that many open questions remain. In this study, Solar Orbiter has been
used to investigate several key aspects of mirror mode 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 mirror
mode storms often approach local ion scales and can no longer be treated
as quasi-MHD, 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.