Simultaneous conjugate observations of small-scale structures in Saturn's dayside ultraviolet auroras: Implications for physical origins

Abstract

Small-scale features in Saturn's dayside UV auroras are examined using images obtained on 32 Hubble Space Telescope visits close to Saturn equinox when both northern and southern emissions were simultaneously observed, allowing their interhemispheric conjugacy to be investigated. Eastward-propagating patches in the dawn-to-noon sector were observed on ∼70% of visits, which when present were nearly always observed both north and south. The patches were generally not closely conjugate, however, but typically displaced in local time by ∼0.5-1 h, with maxima in one hemisphere falling near minima in the other. Averaged angular velocities were ∼80% of rigid corotation, larger than plasma angular velocities reported in the outer magnetosphere to which the emissions are likely conjugate. We suggest the patches are associated with field-aligned currents of eastward-propagating ULF waves, specifically second harmonic Alfvén resonances with typical azimuthal wave numbers m ≈ 20 and plasma rest frame periods ∼80 min, plausibly driven by drift-bounce resonance with hot magnetospheric water ions. Transient dusk sector emissions of ∼10-30 min duration were also observed on ∼40% of visits, and found to be strictly nonconjugate, with enhancements in one hemisphere, north or south, being unaccompanied by enhancements in the other. We suggest an association with open flux tubes, and discuss one scenario where hemispheric symmetry is broken on newly opened flux tubes via the interplanetary magnetic field Y component, plausibly consistent with nonconjugate events north and south, preferential postnoon occurrence, and time scales of a few tens of minutes, though the expected relationship with the Y component remains to be established. Key PointsConjugacy of Saturn's auroras is studied using unique near-equinox HST imagesDawnside patches are consistent with expectations for second harmonic ULF wavesDusk transients are strictly non-conjugate, suggesting an open flux tube origin ©2013. American Geophysical Union. All Rights Reserved.