Internally driven large-scale changes in the size of Saturn’s magnetosphere

Saturn’s magnetic field acts as an obstacle to solar wind flow, deflecting plasma around the planet and forming a cavity known as the magnetosphere. The magnetopause defines the boundary between the planetary and solar dominated regimes, and so is strongly influenced by the variable nature of pressure sources both outside and within. Following from Pilkington et al. (2014), crossings of the magnetopause are identified using 7 years of magnetic field and particle data from the Cassini spacecraft and providing unprecedented spatial coverage of the magnetopause boundary. These observations reveal a dynamical interaction where, in addition to the external influence of the solar wind dynamic pressure, internal drivers, and hot plasma dynamics in particular can take almost complete control of the system’s dayside shape and size, essentially defying the solar wind conditions. The magnetopause can move by up to 10–15 planetary radii at constant solar wind dynamic pressure, corresponding to relatively “plasma-loaded” or “plasma-depleted” states, defined in terms of the internal suprathermal plasma pressure

Local time variation in the large-scale structure of Saturn's magnetosphere

The large‐scale structure of Saturn's magnetosphere is determined by internal and external factors, including the rapid planetary rotation rate, significant internal hot and cold plasma sources, and varying solar wind pressure. Under certain conditions the dayside magnetospheric magnetic field changes from a dipolar to more disk‐like structure, due to global force balance being approximately maintained during the reconfiguration. However, it is still not fully understood which factors dominantly influence this behavior, and in particular how it varies with local time. We explore this in detail using a 2‐D force‐balance model of Saturn's magnetodisk to describe the magnetosphere at different local time sectors. For model inputs, we use recent observational results that suggest a significant local time asymmetry in the pressure of the hot (>3 keV) plasma population, and magnetopause location. We make calculations under different solar wind conditions, in order to investigate how these local time asymmetries influence magnetospheric structure for different system sizes. We find significant day/night asymmetries in the model magnetic field, consistent with recent empirical studies based on Cassini magnetometer observations. We also find dawn‐dusk asymmetries in equatorial current sheet thickness, with the varying hot plasma content and magnetodisk radius having comparable influence on overall structure, depending on external conditions. We also find significant variations in magnetic mapping between the ionosphere and equatorial disk, and ring current intensity, with substantial enhancements in the night and dusk sectors. These results have consequences for interpreting many magnetospheric phenomena that vary with local time, such as reconnection events and auroral observations

Cassini observations of ionospheric plasma in Saturn's magnetotail lobes

Studies of Saturn's magnetosphere with the Cassini mission have established the importance of Enceladus as the dominant mass source for Saturn's magnetosphere. It is well known that the ionosphere is an important mass source at Earth during periods of intense geomagnetic activity but lesser attention has been dedicated to study the ionospheric mass source at Saturn. In this paper we describe a case study of data from Saturn's magnetotail, when Cassini was located at ∼2200 hours Saturn local time at 36 RS from Saturn. During several entries into the magnetotail lobe, tailward-flowing cold electrons and a cold ion beam were observed directly adjacent to the plasma sheet and extending deeper into the lobe. The electrons and ions appear to be dispersed, dropping to lower energies with time. The composition of both the plasma sheet and lobe ions show very low fluxes (sometimes zero within measurement error) of water group ions. The magnetic field has a swept-forward configuration which is atypical for this region and the total magnetic field strength larger than expected at this distance from the planet. Ultraviolet auroral observations show a dawn brightening and upstream heliospheric models suggest that the magnetosphere is being compressed by a region of high solar wind ram pressure. We interpret this event as the observation of ionospheric outflow in Saturn's magnetotail. We estimate a number flux between 2.95±0.43×109 1.43±0.21×1010 cm-2 s-1, one or about two orders magnitude larger than suggested by steady state MHD models, with a mass source between 1.4×102 and 1.1×103 kg/s. After considering several configurations for the active atmospheric regions, we consider as most probable the main auroral oval, with associated mass source between 49.7±13.4 and 239.8±64.8 kg/s for an average auroral oval, and 10±4 and 49±23 kg/s for the specific auroral oval morphology found during this event. It is not clear how much of this mass is trapped within the magnetosphere and how much is lost to the solar wind

Modeling the compressibility of Saturn’s magnetosphere in response to internal and external influences

The location of a planetary magnetopause is principally determined by the balance between solar wind dynamic pressure DP and magnetic and plasma pressures inside the magnetopause boundary. Previous empirical studies assumed that Saturn's magnetopause standoff distance varies as math formula and measured a constant compressibility parameter α corresponding to behavior intermediate between a vacuum dipole appropriate for Earth (α≈6) and a more easily compressible case appropriate for Jupiter (α≈4). In this study we employ a 2-D force balance model of Saturn's magnetosphere to investigate magnetospheric compressibility in response to changes in DP and global hot plasma content. For hot plasma levels compatible with Saturn observations, we model the magnetosphere at a range of standoff distances and estimate the corresponding DP values by assuming pressure balance across the magnetopause boundary. We find that for “average” hot plasma levels, our estimates of α are not constant with DP but vary from ∼4.8 for high DP conditions, when the magnetosphere is compressed (≤25 RS), to ∼3.5 for low DP conditions. This corresponds to the magnetosphere becoming more easily compressible as it expands. We find that the global hot plasma content influences magnetospheric compressibility even at fixed DP, with α estimates ranging from ∼5.4 to ∼3.3 across the range of our parameterized hot plasma content. We suggest that this behavior is predominantly driven by reconfiguration of the magnetospheric magnetic field into a more disk-like structure under such conditions. In a broader context, the compressibility of the magnetopause reveals information about global stress balance in the magnetosphere

Mapping Saturn’s Night Side Plasma Sheet Using Cassini’s Proximal Orbits

Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements