Auroral signatures of injections in the magnetosphere of Saturn

Poster (2011, July 11)

Transient auroral features at Saturn: Signatures of energetic particle injections in the magnetosphere

in Journal of Geophysical Research. Space Physics (2009), 114

We report for the first time transient isolated auroral spots at Saturn's southern polar region, based on Hubble Space Telescope (HST) FUV images. The spots last several minutes and appear distinct from ... [more ▼]

We report for the first time transient isolated auroral spots at Saturn's southern polar region, based on Hubble Space Telescope (HST) FUV images. The spots last several minutes and appear distinct from the rest of the auroral emissions. We study two sets of HST and Cassini observations during which Cassini instrumentation detected signatures of energetic particle injections close to the region where, on the same day, HST observed transient auroral spots. On the basis of the simultaneous remote and in situ observations, we discuss the possibility that the transient features are associated with the dynamical processes taking place in the Kronian magnetosphere. Given the limitations in the available observations, we suggest the following possible explanations for the transient aurora. The injection region could directly be coupled to Saturn's ionosphere by pitch angle diffusion and electron scattering by whistler waves, or by the electric current flowing along the boundary of the injected cloud. The energy contained in the injection region indicates that electron scattering could account for the transient aurora process. [less ▲]

Coordinated measurements of auroral processes at Saturn from the Cassini spacecraft and HST

Conference (2008, December 01)

One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora-its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents ... [more ▼]

One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora-its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents, relationship with solar wind pressure and magnetic field, and its large scale mapping to the magnetosphere. By design, the Cassini orbital tour included high inclination and low periapsis orbits late in the prime mission specifically to address many of these topics. In this presentation, we will provide a snapshot of the current state of our investigation into the relationship between magnetospheric measurements of particles and fields, and the aurora. For in situ data, we will show measurements of upward traveling light ion conics (~30 keV to 200 keV), often accompanied by electron beams (<20 keV to ~1 MeV) and enhanced broadband noise (10 Hz to a few kHz), throughout the outer magnetosphere on field lines that nominally map from well into the polar cap (dipole L > 50) to well into the closed field region (dipole L < 10). Sometimes the particle phenomena and the broadband noise occur in pulses of roughly five-minute duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. Correlative observations of solar wind (Cassini) and aurora (HST) have established a strong relationship between solar wind pressure and auroral activity (brightness) (Crary et al., Nature, 2005; Clarke et al., JGR, 2008). A similar correspondence between bright auroral arcs and ring current ion acceleration will be shown here. So while some auroral forms seem to be associated with the open/closed field boundary (i.e. in the cusp-Bunce et al., JGR, 2008), we also demonstrate that under some magnetospheric conditions for which protons and oxygen ions are accelerated once per Saturn magnetosphere rotation at a preferred local time between midnight and dawn, simultaneous auroral observations by the HST reveal a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent energetic neutral atom enhancements coincide closely with bursts of Saturn kilometric radiation, again suggesting a linkage with high latitude auroral processes. Finally, we will show some intriguing results of auroral movie sequences from the Cassini UVIS instrument with corresponding ring current movies from the Magnetospheric Imaging Instrument Ion and Neutral Camera (MIMI/INCA). [less ▲]

Jupiter’s diffuse auroral emissions - Comparison of HST and Galileo data,

Conference (2008, April)

Discontinuity in Jupiter's main auroral oval

in Journal of Geophysical Research. Space Physics (2008), 113

On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few ... [more ▼]

On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few hundreds to a few tens of kiloRayleigh, forming a discontinuity in the oval. It is shown that the discontinuity is present in both hemispheres and confined in magnetic local time. Its equatorial source is located in the prenoon and early noon sector. The main auroral oval is associated with the ionosphere-magnetosphere coupling current system which is related to the breakdown of corotation in the middle magnetosphere. Necessary for the electron precipitation in the ionosphere and the formation of the main auroral oval is the presence of upward field-aligned currents, carried by downward moving electrons. Field-aligned currents inferred by Pioneer, Voyager and Galileo in situ observations in the near equatorial plane showed evidence of reduced or/and downward field-aligned currents in the prenoon and early afternoon sector, the location of the equatorial source of the discontinuity. Additionally, we estimate the precipitation energy flux in the ionosphere, for a typical reduced upward field-aligned current value at that region, which is found to be within the range of the observed brightness of the discontinuity. Field aligned current distributions in the ionosphere based on magnetohydrodynamic simulations of the interaction between the solar wind and the magnetosphere have predicted a region of downward currents implying a discontinuity at the main auroral oval emission, in very good agreement with the HST observations presented in this work. [less ▲]

Energetic ion composition during reconfiguration events in the Jovian magnetotail

in Journal of Geophysical Research. Space Physics (2007), 112(A6),

[1] On the basis of the first 15 orbits of the Galileo spacecraft the composition of the energetic ion population of the Jovian magnetosphere is studied on a global scale. Analyzing data from the ... [more ▼]

[1] On the basis of the first 15 orbits of the Galileo spacecraft the composition of the energetic ion population of the Jovian magnetosphere is studied on a global scale. Analyzing data from the energetic particles detector onboard Galileo, we study the relative ion abundance ratios of S/O, S/He, O/He and p/He at various energy/ nucleon values. Prominent enhancements of S/O, S/He, and O/He abundance ratios are observed in the predawn sector and are associated with substorm-like events in the Jovian magnetotail. During these reconfiguration events, frequent small-scale variations of the south-north component of the magnetic field are present. Acceleration by such magnetic field variations is examined as a possible mechanism for particle energization in that region. When the timescale of the magnetic field variation is comparable to the particle gyro period, the particle is accelerated by the induced electric field. It is shown that, during the Jovian substorm-like events, the heavier ions are more efficiently energized than the lighter leading to the observed ion abundances. Ion composition measurements in other parts of the magnetosphere have shown quasiperiodic 3-day modulations suggesting that the internally driven processes affect the ion composition in the whole magnetosphere. [less ▲]

Internal control of the magnetospheric processes at Jupiter and Saturn

Conference (2006)

Ion abundance ratios in the Jovian magnetosphere

in Journal of Geophysical Research - Space Physics (2005), 110

Galileo, as the first orbiting spacecraft around Jupiter, provides the opportunity to study globally the composition of the energetic ion population in the equatorial plane of the Jovian magnetosphere ... [more ▼]

Galileo, as the first orbiting spacecraft around Jupiter, provides the opportunity to study globally the composition of the energetic ion population in the equatorial plane of the Jovian magnetosphere. This enables us to derive the relative importance of the various sources and sinks of plasma and energetic particles in the largest magnetosphere of our solar system. In this paper we use data from the Energetic Particles Detector (EPD) on board Galileo and derive relative ion abundance ratio maps of S/O, S/He, O/He, and p/He. We extend the previous work in terms of global coverage, species, and energy range. In addition we compare them with previous results and especially with those derived on board Voyager 2. We found that the S/O abundance ratio is relatively constant throughout the magnetosphere decreasing slightly with radial distance. Within the error bars the S/O ratios could be reproduced. Only a minor energy dependence is observed for this ratio pointing to a common source for both ions. The S/He-, O/He-, and p/He-ratios decrease with increasing radial distance, furthering the notion that sulfur, oxygen, and protons originate mainly from a source within the Jovian system, in contrast with helium, which originates from the solar wind. A spectral kink observed at energies of several 100 keV/ nuc for all species and most pronounced for helium explains the observed energy dependence of the ratios relative to helium. Differences in the abundance ratios up to more than one order of magnitude between sequential orbits are evidence of large temporal variations taking place in the Jovian magnetosphere. These variations and the strong energy dependence can explain the differences between the results derived from Galileo EPD measurements and those from Voyager data. [less ▲]

Jovian ion energy spectra - Ion stochastic acceleration by Alfven waves

Conference (2005)

Plasma composition in the Jovian magnetosphere

Poster (2004, August 03)