Identifying Active Kelvin–Helmholtz Vortices on Saturn's Magnetopause Boundary

Abstract

For ∼2,000 Cassini magnetopause encounters, we analyse plasma and magnetic fields. The boundary can be unstable to the Kelvin–Helmholtz instability (KHI), which can drive large‐scale flows identifiable in plasma measurements. Bulk flow reversed from the expected direction near the magnetopause can indicate vorticity associated with active KH, and events are found dominantly in the dawn–subsolar region. KHIs are also responsible for magnetic field fluctuations, and hybrid simulations indicate heating, and transport is significant in an actively growing vortex. Cassini observations are filtered for disturbed magnetic fields near the magnetopause, similar to the signatures from hybrid simulations, with significant fluctuation and current sheet crossings. We also find that these occur most frequently in the dawn–subsolar region. A turbulent heating rate density and mass diffusion coefficient are calculated for these disturbed events and compared with the hybrid simulation to test whether enhanced values for these quantities can identify active KH events.

Kelvin–Helmholtz‐Related Turbulent Heating at Saturn's Magnetopause Boundary

“…(2021) and Burkholder et al. (2020) show that heating rate densities ∼10 −15 W/m 3 occur roughly 5% of the time based on the Cassini magnetometer data. Integrating 10 −15 W/m 3 over 5% of the magnetodisc volume yields a power input ∼200 GW, consistent with empirical estimates by Bagenal and Delamere (2011).…”

“…10.1029/2020JA028479 2 of 11 (Burkholder et al, 2020). KH activity is expected in the prenoon sector where the fluctuations maximize.…”

On The Nature of Turbulent Heating and Radial Transport in Saturn's Magnetosphere

“…be associated with the growth and evolution of Kelvin-Helmholtz vortices on the magnetopause boundary and these vortices are independent of IMF orientation. Burkholder et al (2020) studied the heating rate density of the quiet and the disturbed events using the similar analysis at the magnetopause boundary. They showed that the heating rate density of the disturbed events is two orders of magnitude greater than the quiet events which is similar to our result.…”

“…This patchy network of disturbed events could be responsible for spot heating in the magnetosphere. Burkholder et al (2020) assumed that plasma parcels near the subsolar magnetopause boundary take ∼6 h before being swept down tail to exit the magnetosphere. Given q ∼ 10 −15 W m −3 for 6 h, the plasma temperature can change by 100's of electronvolt for average Saturn parameters.…”

Kronian Magnetospheric Reconnection Statistics Across Cassini’s Lifetime

“…Other patterns of detections exist in this dataset, namely the extremely high and low latitude detections of 2007, mid 2009, 2013, and 2014. Identifications at these latitudes are due to the highly angled orbital trajectory of Cassini at these times (see Figure 2), where it comes close to the magnetopause boundary and hence may identify Kelvin-Helmholtz (K-H) instabilities in magnetic field observations (Delamere et al, 2013b;Johnson et al, 2014;B. L. Burkholder et al, 2020).…”