Abstract
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.
Highlights
Observations of Saturn’s aurorae at ultraviolet (UV) wavelengths suggested they formed a narrow (1–2◦ ) ‘main oval’ ring of emission circling the poles at ∼ 20◦ co-latitude [Broadfoot et al, 1981]
The location and rate of reconnection depend on the plasma beta parameter on both sides of the magnetopause, the interplanetary magnetic field (IMF) strength and orientation and the velocity of the solar wind [Swisdak et al, 2003; Jackman et al, 2004; Bunce et al, 2006]
The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause
Summary
Observations of Saturn’s aurorae at ultraviolet (UV) wavelengths suggested they formed a narrow (1–2◦ ) ‘main oval’ ring of emission circling the poles at ∼ 20◦ co-latitude [Broadfoot et al, 1981]. Interpretation of auroral images combined with modelling and in situ measurements have demonstrated that Saturn’s main oval emissions are generated by a field-aligned current system associated with the boundary between open and closed magnetic field lines [Cowley et al, 2004; Badman et al, 2006; Belenkaya et al, 2007; Bunce et al, 2008a]. The location and rate of reconnection depend on the plasma beta parameter on both sides of the magnetopause, the interplanetary magnetic field (IMF) strength and orientation and the velocity of the solar wind [Swisdak et al, 2003; Jackman et al, 2004; Bunce et al, 2006] These latter parameters have been shown to influence the large-scale behaviour of Saturn’s aurora [Badman et al, 2005; Cowley et al, 2005; Grodent et al, 2005]. The auroral morphology is described and related to the relevant in situ plasma and magnetic field measurements
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