Abstract
AbstractOn July 13, 2016, the Hubble Space Telescope observed the onset of a dawn storm in Jupiter's northern ultraviolet aurora, while the NASA Juno spacecraft simultaneously traversed the dawnside outer magnetosphere. This represents the first concurrent auroral and in situ magnetospheric observations of the onset of a dawn storm at Jupiter. Mapping the auroral emission to the magnetosphere reveals the dawn storm corresponds to a source region at ∼60 Jupiter radii, and the eastward edge propagates toward local noon at speeds exceeding corotation. Particle observations from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic particle Detector Instrument (JEDI) reveal the presence of enhanced hot plasma density in the outer magnetosphere during this interval, and pitch angle distributions measured with JEDI reveal pronounced field‐aligned proton and heavy ion motion. Juno magnetometer (MAG) signatures reveal a reversal in the azimuthal magnetic field at the time of storm onset, suggesting acceleration of the hot plasma population above typical sub‐corotational speeds. JEDI also detects a region of energetic particles which persists throughout the day following the storm, a feature which is not observed during subsequent perijoves. We interpret this dawn storm as the result of reconnection at earlier local times, possibly associated with a disruption of the azimuthal magnetodisk current.
Highlights
Jupiter's ultraviolet auroras are dominated by the main auroral oval, an almost continuous narrow band or series of bands of emission encircling the magnetic poles of the planet (e.g., Clarke et al, 2004; Grodent et al, 2003)
From Waves, we primarily focus on the decametric (∼10–40 MHz) and hectometric (∼200 kHz to a few MHz) emission frequencies, and utilize electron densities derived from the low-frequency cutoff of continuum radiation by the method described by Barnhart et al (2009)
HST images obtained on July 13, 2016 showed the onset and early progression of the storm, with a 1° poleward expansion of the main emission corresponding to either a ∼10 RJ or ∼80 RJ outward expansion of the magnetospheric source region depending upon the mapping model used
Summary
Jupiter's ultraviolet auroras are dominated by the main auroral oval, an almost continuous narrow band or series of bands of emission encircling the magnetic poles of the planet (e.g., Clarke et al, 2004; Grodent et al, 2003). Dawn storms were first observed in Jupiter's northern far ultraviolet (FUV) aurora using the Faint Object Camera (FOC) and Wide Field Planetary Camera 2 (WFPC2) on HST (Ballester et al, 1996; Clarke et al, 1998; Gérard et al, 1994), and were characterized as bright enhancements of the dawnward arc of the main emission These enhancements expand poleward and eastward longitudinally over several tens of minutes, appear fixed near dawn for several hours before returning to typical auroral intensities. Some similarities exist in the interpretation of Saturnian auroral storms, which are observed to originate along the dawn flank and expand poleward of the main oval, and are thought to result from large-scale flux closure in the magnetotail (Clarke et al, 2005; Cowley et al, 2005; Nichols et al, 2014). Juno's particle instruments show increases in the high energy particle flux simultaneous to these observations, as well as a long-lived hot plasma feature throughout the day following the storm
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