Auroral signatures of multiple magnetopause reconnection at Saturn

Auroral observations capture the ionospheric response to dynamics of the whole magnetosphere and may provide evidence of the significance of reconnection at Saturn. Bifurcations of the main dayside auroral emission have been related to reconnection at the magnetopause and their surface is suggested to represent the amount of newly opened flux. This work is the first presentation of multiple brightenings of these auroral features based on Cassini ultraviolet auroral observations. In analogy to the terrestrial case, we propose a process, in which a magnetic flux tube reconnects with other flux tubes at multiple sites. This scenario predicts the observed multiple brightenings, it is consistent with subcorotating auroral features which separate from the main emission, and it suggests north-south auroral asymmetries. We demonstrate that the conditions for multiple magnetopause reconnection can be satisfied at Saturn, like at Earth

X-rays Studies of the Solar System

X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolution for future instruments. These improvements will usher in a truly transformative era of Solar System science through the study of X-ray emission.Comment: White paper submitted to Astro2020, the Astronomy and Astrophysics Decadal Surve

Global response of the upper thermospheric winds to large ion drifts in the Jovian ovals

We use our fully coupled 3‐D Jupiter Thermosphere General Circulation Model (JTGCM) to quantify processes which are responsible for generating neutral winds in Jupiter’s oval thermosphere from 20 µbar to 10−4 nbar self‐consistently with the thermal structure and composition. The heat sources in the JTGCM that drive the global circulation of neutral flow are substantial Joule heating produced in the Jovian ovals by imposing high‐speed anticorotational ion drifts (~3.5 km s−1) and charged particle heating from auroral processes responsible for bright oval emissions. We find that the zonal flow of neutral winds in the auroral ovals of both hemispheres is primarily driven by competition between accelerations resulting from Coriolis forcing and ion drag processes near the ionospheric peak. However, above the ionospheric peak (<0.01 µbar), the acceleration of neutral flow due to pressure gradients is found to be the most effective parameter impacting zonal winds, competing mainly with acceleration due to advection with minor contributions from curvature and Coriolis forces in the southern oval, while in the northern oval it competes alone with considerable Coriolis forcing. The meridional flow of neutral winds in both ovals in the JTGCM is determined by competition between meridional accelerations due to Coriolis forcing and pressure gradients. We find that meridional flow in the lower thermosphere, near the peak of the auroral ionosphere, is poleward, with peak wind speeds of ~0.6 km s−1 and ~0.1 km s−1 in the southern and northern oval, respectively. The corresponding subsiding flow of neutral motion is ~5 m s−1 in the southern oval, while this flow is rising in the northern oval with reduced speed of ~2 m s−1. We also find that the strength of meridional flow in both auroral ovals is gradually weakened and turned equatorward near 0.08 µbar with wind speeds up to ~250 m s−1 (southern oval) and ~75 m s−1 (northern oval). The corresponding neutral motion in this region is upward, with wind speeds up to 4 m s−1 in both ovals.Key PointsUnderlying thermal and dynamical processes of the Jovian thermosphereForcing terms responsible for generating Jupiter’s neutral wind speedsSimulated neutral wind speed is compared with the measured wind speedPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146281/1/jgra52522_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146281/2/jgra52522.pd

Location and spatial shape of electron beams in Io's wake

The Galileo spacecraft observed energetic field-aligned electron beams very close to Io during several flybys. We apply a three-dimensional magnetohydrodynamic (MHD) model of the far-field Io-Jupiter interaction to simulate for the first time the location and spatial shape of field-aligned electron beams. Io continuously generates MHD waves by disturbing the Jovian magnetoplasm. Currents carried by Alfven waves propagate predominantly along the magnetic field lines. As the number of charge carriers decreases along the travel path, electrons are accelerated toward Jupiter. These energetic electrons precipitate into the Jovian ionosphere, visible as prominent Io footprint emission. Electrons are also accelerated toward Io and form the equatorial beams observed by the Galileo spacecraft. Unlike the beam formation, the position and spatial structure of these beams have not been addressed in detail before. We use a 3-D MHD model with initial conditions corresponding to the individual Galileo flyby and determine the spatial morphology of the beams in Io's orbital plane. Our results for the beam locations are in good agreement with the Galileo Energetic Particle Detector observations. We find that the ratio of the one-way travel time of the Alfven wave from Io to Jupiter and the convection time of the plasma past the obstacle controls the location of the beam. This leads to the conclusion that at other satellites with other plasma environments, the electrons might not be close to the satellite but can be shifted significantly downstream along its plasma wake. Thus, the future search for electron beams near a satellite should be further extended to the wake region

Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15-20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes

Enhanced C2_2H2_2 absorption within Jupiter's southern auroral oval from Juno UVS observations

Reflected sunlight observations from the Ultraviolet Spectrograph (UVS) on the Juno spacecraft were used to study the distribution of acetylene (C$_2$H$_2$) at Jupiter's south pole. We find that the shape of the C$_2$H$_2$ absorption feature varies significantly across the polar region, and this can be used to infer spatial variability in the C$_2$H$_2$ abundance. There is a localized region of enhanced C$_2$H$_2$ absorption which coincides with the location of Jupiter's southern polar aurora; the C$_2$H$_2$ abundance poleward of the auroral oval is a factor of 3 higher than adjacent quiescent, non-auroral longitudes. This builds on previous infrared studies which found enhanced C$_2$H$_2$ abundances within the northern auroral oval. This suggests that Jupiter's upper-atmosphere chemistry is being strongly influenced by the influx of charged auroral particles and demonstrates the necessity of developing ion-neutral photochemical models of Jupiter's polar regions.Comment: Accepted in JGR: Planet

Saturn's equinoctial auroras

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95061/1/grl26673.pd

Possible Transient Luminous Events observed in Jupiter's upper atmosphere

11 transient bright flashes were detected in Jupiter's atmosphere using the UVS instrument on the Juno spacecraft. These bright flashes are only observed in a single spin of the spacecraft and their brightness decays exponentially with time, with a duration of ~1.4 ms. The spectra are dominated by H2 Lyman band emission and based on the level of atmospheric absorption, we estimate a source altitude of 260 km above the 1-bar level. Based on these characteristics, we suggest that these are observations of Transient Luminous Events (TLEs) in Jupiter's upper atmosphere. In particular, we suggest that these are elves, sprites or sprite halos, three types of TLEs that occur in the Earth's upper atmosphere in response to tropospheric lightning strikes. This is supported by visible light imaging, which shows cloud features typical of lightning source regions at the locations of several of the bright flashes. TLEs have previously only been observed on Earth, although theoretical and experimental work has predicted that they should also be present on Jupiter.Comment: Accepted in JGR: Planets. 28 pages, 8 figure

Dynamic auroral storms on Saturn as observed by the Hubble Space Telescope

We present observations of significant dynamics within two UV auroral storms observed on Saturn using the Hubble Space Telescope in April/May 2013. Specifically, we discuss bursts of auroral emission observed at the poleward boundary of a solar wind-induced auroral storm, propagating at ∼330% rigid corotation from near ∼01 h LT toward ∼08 h LT. We suggest that these are indicative of ongoing, bursty reconnection of lobe flux in the magnetotail, providing strong evidence that Saturn’s auroral storms are caused by large-scale flux closure. We also discuss the later evolution of a similar storm and show that the emission maps to the trailing region of an energetic neutral atom enhancement. We thus identify the auroral form with the upward field-aligned continuity currents flowing into the associated partial ring current