Diamagnetic depression observations at Saturn’s magnetospheric cusp by the Cassini spacecraft

The magnetospheric cusp is a region where shocked solar wind plasma can enter a planetary magnetosphere, after magnetic reconnection has occurred at the dayside magnetopause or in the lobes. The dense plasma that enters the high‐latitude magnetosphere creates diamagnetic effects whereby a depression is observed in the magnetic field. We present observations of the cusp events at Saturn’s magnetosphere where these diamagnetic depressions are found. The data are subtracted from a magnetic field model, and the calculated magnetic pressure deficits are compared to the particle pressures. A high plasma pressure layer in the magnetosphere adjacent to the cusp is discovered to also depress the magnetic field, outside of the cusp. This layer is observed to contain energetic He++ (up to ∼100 keV) from the solar wind as well as heavy water group ions (W+) originating from the moon Enceladus. We also find a modest correlation of diamagnetic depression strength to solar wind dynamic pressure and velocity; however, unlike at Earth, there is no correlation found with He++ counts.Key PointsDiamagnetic depressions are found in the cusp and are observed to continue into the adjacent magnetosphereA heated plasma layer of mixed composition is found to depress the adjacent magnetospheric fieldDiamagnetic depression strength is correlated to solar wind dynamic pressure and velocity but not to the observed He++ counts, like at EarthPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/1/jgra53517_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/2/jgra53517-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137687/3/jgra53517.pd

Source region and growth analysis of narrowband Z-mode emission at Saturn

Intense Z-mode emission is observed in the lower density region near the inner edge of the Enceladus torus at Saturn, where these waves may resonate with MeV electrons. The source mechanism of this emission, which is narrow-banded and most intense near 5 kHz, is not well understood. We survey the Cassini Radio and Plasma Wave Science data to isolate several probable source regions near the inner edge of the Enceladus density torus. Electron phase space distributions are obtained from the Cassini Electron Spectrometer, part of the Cassini Plasma Spectrometer investigation. We perform a plasma wave growth analysis to conclude that an electron temperature anisotropy and possibly a weak loss cone can drive the Z mode as observed. Electrostatic electron acoustic waves and perhaps weak beam modes are also found to be unstable coincident with the Z mode. Quasi-steady conditions near the Enceladus density torus may result in the observations of narrowband Z-mode emission at Saturn

Flux transfer event observation at Saturn's dayside magnetopause by the Cassini spacecraft

We present the first observation of a flux rope at Saturn's dayside magnetopause. This is an important result because it shows that the Saturnian magnetopause is conducive to multiple X-line reconnection and flux rope generation. Minimum variance analysis shows that the magnetic signature is consistent with a flux rope. The magnetic observations were well fitted to a constant-α force-free flux rope model. The radius and magnetic flux content of the rope are estimated to be 4600–8300 km and 0.2–0.8 MWb, respectively. Cassini also observed five traveling compression regions (remote signatures of flux ropes), in the adjacent magnetosphere. The magnetic flux content is compared to other estimates of flux opening via reconnection at Saturn

How does the Sun influence the magnetospheres of Jupiter and Saturn?

Spacecraft have visited Jupiter and Saturn at all phases of the solar cycle and thus we have a wealth of data with which to explore both upstream parameters and magnetospheric response. In this paper we review upstream parameters including interplanetary magnetic field strength and direction, solar wind dynamic pressure, plasma beta and Mach number. We consider the impact of changing solar wind on dayside coupling via reconnection. We also comment on how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and thus estimate the solar cycle effects on internally driven magnetospheric dynamics. Finally we place our results in the context of the now complete set of data from the Cassini mission at Saturn and the current data streaming in from Juno at Jupiter, outlining future avenues for research

A simple spacecraft – vector intersection methodology and applications

Observations with spacecraft-mounted instruments are usually limited by their field-of-view and are often affected by the spacecraft's shadow or wake. Their extent though can be derived from the spacecraft's geometry. In this work we present a robust method for calculating the field-of-view as well as the extent of a spacecraft shadow and wake from readily available spacecraft CAD models. We demonstrate these principles on Cassini, where we give examples of vector-spacecraft intersection for the Cassini Langmuir Probe, as well the field-of-view of the Langmuir Probe and the Cassini Plasma Spectrometer

Properties of Jupiter’s auroral acceleration region inferred with HST-STIS spectral images

Jupiter’s dynamic auroral region is the signature of magnetosphere-ionosphere coupling. Precipitating auroral electrons are part of a current system which transports angular momentum from the planetary atmosphere to sub-corotating magnetospheric plasma. The magnitude of the currents and hence precipitating energy flux, are sensitive to the characteristics of the high-latitude magnetosphere, in particular the location of the auroral acceleration region (AAR) and the density and temperature of the high-latitude electron population. We use HST STIS observations of Jupiter’s aurora (Gustin et al. [2016]) to infer the location of the AAR and the properties of the precipitating auroral electrons. To do this, we determine the energy of the precipitating electrons and incident energy flux for the two distinct regions within the main aurora and within flare regions. The resulting relationships between energy flux and electron precipitation energy for the main auroral emission are then compared to the theoretical relationship derived by Lundin & Sandahl [1978], in order to derive the location of the AAR and the temperatures and densities of the electrons at the top of the AAR prior to acceleration. We find that that each emission region is best reproduced using a multiple auroral acceleration regions with different properties, rather than a single auroral acceleration region with a varying potential drop strength

Estimating the optical depth of Saturn's main rings using the Cassini Langmuir Probe

A Langmuir Probe (LP) measures currents from incident charged particles as a function of the applied bias voltage. While onboard a spacecraft the particles are either originated from the surrounding plasma, or emitted (e.g. through photoemission) from the spacecraft itself. The obtained current-voltage curve reflects the properties of the plasma in which the probe is immersed into, but also any photoemission due to illumination of the probe surface: As photoemission releases photoelectrons into space surrounding the probe, these can be recollected and measured as an additional plasma population. This complicates the estimation of the properties of the ambient plasma around the spacecraft. The photoemission current is sensitive to the extreme ultraviolet (UV) part of the spectrum, and it varies with the illumination from the Sun and the properties of the LP surface material, and any variation in the photoelectrons irradiance can be measured as a change in the current voltage curve. Cassini was eclipsed multiple times by Saturn and the main rings over its 14 yr mission. During each eclipse the LP recorded dramatic changes in the current-voltage curve, which were especially variable when Cassini was in shadow behind the main rings. We interpret these variations as the effect of spatial variations in the optical depth of the rings and hence use the observations to estimate the optical depth of Saturn's main rings. Our estimates are comparable with UV optical depth measurements from Cassini's remote sensing instruments

Estimating the optical depth of Saturn's main rings using the Cassini Langmuir Probe

A Langmuir probe (LP) measures currents from incident charged particles as a function of the applied bias voltage. While onboard a spacecraft the particles are either originated from the surrounding plasma, or emitted (for example, through photoemission) from the spacecraft itself. The obtained current-voltage curve reflects the properties of the plasma in which the probe is immersed into, but also any photoemission due to illumination of the probe surface: as photoemission releases photoelectrons into space surrounding the probe, these can be recollected and measured as an additional plasma population. This complicates the estimation of the properties of the ambient plasma around the spacecraft. The photoemission current is sensitive to the EUV part of the spectrum, and it varies with the illumination from the Sun and the properties of the LP surface material, and any variation in the photoelectrons irradiance can be measured as a change in the current voltage curve. Cassini was eclipsed multiple times by Saturn and the main rings over its 14-year mission. During each eclipse the LP recorded dramatic changes in the current-voltage curve, which were especially variable when Cassini was in shadow behind the main rings. We interpret these variations as the effect of spatial variations in the optical depth of the rings and hence use the observations to estimate the optical depth of Saturn's main rings. Our estimates are comparable with UV optical depth measurements from Cassini's remote sensing instruments