Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs

Abstract

[1] We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturn's ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field-aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100-360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave-particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturn's unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturn's magnetopause.

Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra

“…Baron et al (1996) identified a positive correlation between Jupiter's spatially-unresolved IR auroral intensity and the solar wind dynamic pressure. Large-scale polar brightenings and multiple arc bifurcations have also been observed in Saturn's polar infrared emission, reflecting the solar wind interaction (Stallard et al 2008;Badman et al 2011aBadman et al , 2012a. Interestingly, the Io footprint was first observed in the infrared by Connerney et al (1993) and remains the only moon footprint to have been observed at wavelengths outside the ultraviolet.…”

“…The altitude profile of IR emission observed by Cassini shows the peak altitude lies at ∼ 1155 ± 25 km, almost the same as that of the UV, while the emission profile seems narrower in height than the UV profiles (Stallard et al 2012a). This is explained by the large contribution to UV emission at higher altitude by Lyman α. Ambiguity in the H 2 profile assumed in the model study leads to a broader estimate of IR emission across altitudes.…”

“…Using high-sensitivity Cassini instruments, the signatures of transient magnetopause reconnection events have been identified in the noon and post-noon sectors. These appear as bifurcations of the main auroral arc that have been observed to travel poleward while the end connected to the main oval sub-corotates (Radioti et al 2011a;Badman et al 2012a. Similar dusk sector features have been shown to be non-conjugate with their appearance in each hemisphere related to the direction of the IMF; specifically, they will be favoured in the northern hemisphere for B Y < 0 and in the southern hemisphere for B Y > 0 because of the different sense of the associated field aligned current patterns and the source plasma populations (Meredith et al 2013).…”

Planetary space weather: scientific aspects and future perspectives

“…In particular, each reconnection event is estimated to open $10% of the flux contained within the polar cap. Additionally, Cassini multi-instrument observations, including auroral UV and IR data, confirmed that the auroral arcs are related to bursty reconnections at Saturn involving upward field-aligned currents (Badman et al 2012) and suggested that these are efficient in transporting flux . A more recent study based on conjugated IMF and HST observations (Meredith et al 2014) provided evidence of significant IMF dependence on the morphology of Saturn's dayside auroras and the bifurcated arcs.…”

“…Future in-situ plasma and energetic neutral particle observations could provide significant feedback for the determination of the planet's space weather conditions and for determining if they indeed have a key role in this context. The planetary period modulation of the field-aligned currents at Uranus may lead to modulated auroral electron acceleration in regions of upward currents, as it happens in the case of Saturn (e.g., Nichols et al 2008Nichols et al , 2010aNichols et al , 2010bSouthwood & Kivelson 2009;Badman et al 2012;Hunt et al 2014); associated modulated radio emissions at kilometre wavelengths generated via the cyclotron maser instability could be present (Zarka 1998;Lamy et al 2010Lamy et al , 2011. Such observations can be essential for the identification of space weather phenomena at Uranus.…”