Jupiter's X-ray and EUV auroras monitored by Chandra, XXM-Newton, and Hisaki satellite

Jupiter's X-ray auroral emission in the polar cap region results from particles which have undergone strong field-aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X-ray emission are essential to uncover the driving mechanism for the high-energy acceleration. The magnetospheric location of the source field line where the X-ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long-term monitoring of the X-rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long-term multiwavelength monitoring of Jupiter's X-ray and EUV auroral emissions was made by the Chandra X-ray Observatory, XMM-Newton, and Hisaki satellite. We find that the X-ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X-ray auroral region was found to be open to the interplanetary space. The other half of the X-ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin-Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high-energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators

Energetic Particle Increases Associated with Stream Interaction Regions

The Parker Solar Probe was launched on 2018 August 12 and completed its second orbit on 2019 June 19 with perihelion of 35.7 solar radii. During this time, the Energetic Particle Instrument-Hi (EPI-Hi, one of the two energetic particle instruments comprising the Integrated Science Investigation of the Sun, IS⊙IS) measured seven proton intensity increases associated with stream interaction regions (SIRs), two of which appear to be occurring in the same region corotating with the Sun. The events are relatively weak, with observed proton spectra extending to only a few MeV and lasting for a few days. The proton spectra are best characterized by power laws with indices ranging from −4.3 to −6.5, generally softer than events associated with SIRs observed at 1 au and beyond. Helium spectra were also obtained with similar indices, allowing He/H abundance ratios to be calculated for each event. We find values of 0.016–0.031, which are consistent with ratios obtained previously for corotating interaction region events with fast solar wind ≤ 600 km s⁻¹. Using the observed solar wind data combined with solar wind simulations, we study the solar wind structures associated with these events and identify additional spacecraft near 1 au appropriately positioned to observe the same structures after some corotation. Examination of the energetic particle observations from these spacecraft yields two events that may correspond to the energetic particle increases seen by EPI-Hi earlier

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

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

Saturn's dayside ultraviolet auroras:Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field

We examine a unique data set from seven Hubble Space Telescope (HST) "visits" that imaged Saturn's northern dayside ultraviolet emissions exhibiting usual circumpolar "auroral oval" morphologies, during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturn's bow shock over intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near similar to 15 degrees colatitude, together with intermittent patchy forms at similar to 10 degrees colatitude and poleward thereof, located between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras are found to relate to the (suitably lagged and averaged) IMF B-z, being present during all four visits with positive B-z and absent during all three visits with negative B-z. The most continuous such forms occur in the case of strongest positive B-z. These results suggest that the postnoon forms are associated with reconnection and open flux production at Saturn's magnetopause, related to the similarly interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these HST images. One of the intervals with negative IMF B-z however exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of significant IMF dependence in the morphology of Saturn's dayside auroras

Response of Jupiter's auroras to conditions in the interplanetary medium as measured by the Hubble Space Telescope and Juno

We present the first comparison of Jupiter's auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiter's auroras to the interplanetary conditions. We show that for ∼1-3 days following compression region onset the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought

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

Jupiter's X-ray Emission 2007 Part 2:Comparisons with UV and Radio Emissions and In-Situ Solar Wind Measurements

We compare Chandra and XMM‐Newton X‐ray observations of Jupiter during 2007 with a rich multi‐instrument dataset including: upstream in‐situ solar wind measurements from the New Horizons spacecraft, radio emissions from the Nançay Decametric Array and Wind/Waves, and UV observations from the Hubble Space Telescope. New Horizons data revealed two corotating interaction regions (CIRs) impacted Jupiter during these observations. Non‐Io decametric bursts and UV emissions brightened together and varied in phase with the CIRs. We characterise 3 types of X‐ray aurorae: hard X‐ray bremsstrahlung main emission, pulsed/flared soft X‐ray emissions and a newly identified dim flickering (varying on short‐timescales, but quasi‐continuously present) aurora. For most observations, the X‐ray aurorae were dominated by pulsed/flaring emissions, with ion spectral lines that were best fit by Iogenic plasma. However, the brightest X‐ray aurora was coincident with a magnetosphere expansion. For this observation, the aurorae were produced by both flickering emission and erratic pulses/flares. Auroral spectral models for this observation required the addition of solar wind ions to attain good fits, suggesting solar wind entry into the outer magnetosphere or directly into the pole for this particularly bright observation. X‐ray bremsstrahlung from high energy electrons was only bright for one observation, which was during a forward shock. This bremsstrahlung was spatially coincident with bright UV main emission (power> 1TW) and X‐ray ion spectral line dusk emission, suggesting closening of upward and downward current systems during the shock. Otherwise, the bremsstrahlung was dim and UV main emission power was also lower(<700 GW), suggesting their power scaled together