Solar Wind Dynamic Pressure Upstream From Saturn: Estimation From Magnetosheath Properties and Comparison With SKR

Abstract

An analytical method is developed by which measurements made by the Cassini spacecraft in Saturn's magnetosheath can be used to infer the upstream solar wind parameters, specifically the solar wind speed (Vsw) and the dynamic pressure (Pd). The method is validated by comparing the results with other estimates of these parameters, including the mSWiM MHD model and magnetopause and bow shock models applied to observed boundary crossings. The comparisons suggest that the new inferred Vsw are on average ~40 km/s lower than the mSWiM values, and the dynamic pressure values are slightly lower as well. We find few of the lower Pd values predicted by mSWiM, probably because Cassini would have been inside the expanded magnetosphere under such conditions. Systematic temporal variations such as interplanetary shocks do seem to be captured well, with arrival times within several days of the MHD prediction. Compared to dynamic pressures estimated from boundary crossings with well‐known magnetopause and bow shock models, the magnetosheath‐inferred dynamic pressure tends to be somewhat lower, but within the uncertainties of the analytical derivation. Comparison of the inferred dynamic pressure with observed Saturn's kilometric radiation (SKR) activity reveals several episodes of very good temporal tracking between dynamic pressure and SKR intensity, with relatively short time delays (4–5 hr), suggesting rather direct driving. Such good tracking intervals occur almost exclusively on the dawnside of the magnetosphere, where the dominant SKR source is visible. When the tracking is good, the SKR fluxes vary roughly as the square of the dynamic pressure.

Saturn Anomalous Myriametric Radiation, a New Type of Saturn Radio Emission Revealed by Cassini

“…As reported in earlier studies (Bradley et al., 2020; Bunce et al., 2005; Cowley et al., 2005; Guo et al., 2018; Jackman et al., 2009; Mitchell et al., 2015; Reed et al., 2018; Thomsen et al., 2019), compressions of the Saturnian magnetosphere caused by interplanetary corotating interaction regions or Coronal Mass Ejections can trigger a series of magnetospheric responses including motion of the magnetospheric boundaries, dayside and nightside magnetic reconnection, magnetotail current sheet collapse, plasmoid release, hot plasma injection and the intensification of SKR. In the absence of an upstream probe at Saturn, studies have used magnetospheric boundary locations or propagated solar wind models (e.g., Tao et al., 2005; Zieger and Hansen., 2008) to infer the state of the upstream medium.…”

Density and Magnetic Field Asymmetric Kelvin‐Helmholtz Instability

“…The magnetosheath has a proton number density of about 0.1 cm 3 and the magnetopause has a water group (i.e., H 2 O + ) number density of about 0.01 cm 3 (Masters et al, 2010). The ion temperature is about 200 eV on both magnetospheric and magnetosheath side (Thomsen et al, 2010(Thomsen et al, , 2019. The magnetosheath bulk velocity is about 200 km s 1 (Burkholder et al, 2017) tailward, and magnetopause sub-corotational flow speed is about 100 km s 1 .…”

“…For Saturn, the solar wind velocity is about 400 km s 1 (Thomsen et al, 2019) and the magnetic field is about 0.5 nT (Jia et al, 2012) near 9.5 AU. The magnetosheath has a proton number density of about 0.1 cm 3 and the magnetopause has a water group (i.e., H 2 O + ) number density of about 0.01 cm 3 (Masters et al, 2010).…”

MSWIM2D: Two-dimensional Outer Heliosphere Solar Wind Modeling

“…These gaps must be filled through numerical modeling to extend the utility of a few observations to a much larger domain. Solar wind modeling has applications for the interpretation of spacecraft observations (e.g., Hundhausen & Gosling 1976;Du et al 2007;Elliott et al 2016), determination of planetary magnetosphere drivers (e.g., Tao et al 2005;Lamy et al 2017;Vogt et al 2019;Thomsen et al 2019), investigation of the effect of interplanetary shocks on the auroral activity of Jupiter and Saturn (e.g., Clarke et al 2009;Moore et al 2017), and study of the solar system interaction with the interstellar medium (e.g., Opher et al 2009Opher et al , 2015Pogorelov et al 2014;Izmodenov & Alexashov 2015). While some modeling efforts start at the Sun using coronagraph or magnetogram images (e.g., Prise et al 2015;Witasse et al 2017;Sachdeva et al 2021), this paper will focus on the propagation of in situ spacecraft measurements such as those made at 1 au into the outer heliosphere (OH), defined as the region outside of Earth's orbit but inside the solar system heliopause.…”

“…Similarly, tangential discontinuities are poorly represented, and the radial magnetic field component cannot be propagated in 1D due to the zero divergence of B (Zieger & Hansen 2008). Despite these limitations, the mSWiM model has significant utility to study solar wind conditions around a variety of solar system bodies, including Jupiter (e.g., Vogt et al 2019), Saturn (e.g., Thomsen et al 2019), and Uranus (e.g., Lamy et al 2017), as well as other objects (e.g., Zieger et al 2009;Edber et al 2016;Timar et al 2019) during periods of good alignment and quiet solar wind. Zieger & Hansen (2008) performs validation of the mSWiM model using available data from Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and Ulysses spacecraft from select intervals between 1973 and 2004, where L1 solar wind observations have good coverage.…”