Aikaterini Radioti

Jupiter's changing auroral location

[1] We examine the case of significant latitudinal shifts of the Jovian northern auroral emissions appearing in a data set spanning nine years of observations with the Hubble Space Telescope in the far ultraviolet. The extended data set makes it possible to compare the location of the main auroral emission with similar viewing geometries and satellite positions. The main auroral emission is assumed to originate from beyond the orbit of Ganymede (15 Jovian radii). At these distances, near corotation enforcement and transfer of momentum from Jupiter to the magnetospheric plasma is ensured by means of field aligned currents. The field aligned currents away from Jupiter are carried by downward energetic electrons loosing their energy to the polar atmosphere and giving rise to the main auroral emission. Analysis of the polar projected images shows that the latitudinal location of the main emission has changed by up to 3° over long periods of time. It also shows that the footprint of Ganymede follows a similar trend. We have used the VIP4 magnetic field model to map the emission down to the equatorial plane. This mapping suggests that internal variations of the current sheet parameters might be used as an alternative or complementary explanation to the changing solar wind conditions at Jupiter to explain the observed shift of auroral latitudes.

Auroral polar dawn spots: Signatures of internally driven reconnection processes at Jupiter's magnetotail

[1] We report the presence of polar spots located in the dawn auroral region, based on the HSTACS 2007 campaign. We study the location of these features in the equatorial plane as well as their time scales and periodicities, based on a comprehensive series of images taken between February 21 and June 11, 2007. It is shown that the majority of polar dawn spots magnetically map to the dawn sector. Additionally, they occur quasi-periodically every 2–3 days, a periodicity observed for the first time in auroral features. Because of theirmappedlocation andtheirperiodiccycle,weinterpretthe polar dawn spots as signatures of internally driven magnetic reconnection in the Jovian magnetotail. Citation: Radioti, A.,

Nightside reconnection at Jupiter: Auroral and magnetic field observations from 26 July 1998

[1] In this study we present ultraviolet and infrared auroral data from 26 July 1998, and we show the presence of transient auroral polar spots observed throughout the postdusk to predawn local time sector. The polar dawn spots, which are transient polar features observed in the dawn sector poleward of the main emission, were previously associated with the inward moving flow resulting from tail reconnection. In the present study we suggest that nightside spots, which are polar features observed close to the midnight sector, are related to inward moving flow, like the polar dawn spots. We base our conclusions on the near‐simultaneous set of Hubble Space Telescope (HST) and Galileo observations of 26 July 1998, during which HST observed a nightside spot magnetically mapped close to the location of an inward moving flow detected by Galileo on the same day. We derive the emitted power from magnetic field measurements along the observed plasma flow bubble, and we show that it matches the emitted power inferred from HST. Additionally, this study reports for the first time a bright polar spot in the infrared, which could be a possible signature of tail reconnection. The spot appears within an interval of 30 min from the ultraviolet, poleward of the main emission on the ionosphere and in the postdusk sector planetward of the tail reconnection x line on the equatorial plane. Finally, the present work demonstrates that ionospheric signatures of flow bursts released during tail reconnection are instantaneously detected over a wide local time sector.

Mapping the electron energy in Jupiter's aurora: Hubble spectral observations

Far ultraviolet spectral observations have been made with the Hubble Space Telescope in the time-tag mode using the Space Telescope Imaging Spectrograph (STIS) long slit. The telescope was slewed in such a way that the slit projection scanned from above the polar limb down to midlatitudes, allowing us to build up the first spectral maps of the FUV Jovian aurora. The shorter wavelengths are partly absorbed by the methane layer overlying part of the auroral emission layer. The long-wavelength intensity directly reflects the precipitated energy flux carried by the auroral electrons. Maps of the intensity ratio of the two spectral regions have been obtained by combining spectral emissions in two wavelength ranges. They show that the amount of absorption by methane varies significantly between the different components of the aurora and inside the main emission region. Some of the polar emissions are associated with the hardest precipitation, although the auroral regions of strong electron precipitation do not necessarily coincide with the highest electron energies. Outputs from an electron transport model are used to create maps of the distribution of the characteristic electron energies. Using model atmospheres adapted to auroral conditions, we conclude that electron energies range between a few tens to several hundred keV. Comparisons of derived energies are in general agreement with those calculated from magnetosphere-ionosphere coupling models, with values locally exceeding the standard model predictions. These results will provide useful input for three-dimensional modeling of the distribution of particle heat sources into the high-latitude Jovian upper atmosphere.

Cusp observation at Saturn's high-latitude magnetosphere by the Cassini spacecraft

We report on the first analysis of magnetospheric cusp observations at Saturn by multiple in situ instruments onboard the Cassini spacecraft. Using this we infer the process of reconnection was occurring at Saturns magnetopause. This agrees with remote observations that showed the associated auroral signatures of reconnection. Cassini crossed the northern cusp around noon local time along a poleward trajectory. The spacecraft observed ion energy-latitude dispersions—a characteristic signature of the terrestrial cusp. This ion dispersion is “stepped,” which shows that the reconnection is pulsed. The ion energy-pitch angle dispersions suggest that the field-aligned distance from the cusp to the reconnection site varies between ∼27 and 51 RS. An intensification of lower frequencies of the Saturn kilometric radiation emissions suggests the prior arrival of a solar wind shock front, compressing the magnetosphere and providing more favorable conditions for magnetopause reconnection. Key Points We observe evidence for reconnection in the cusp plasma at Saturn We present evidence that the reconnection process can be pulsed at Saturn Saturns cusp shows similar characteristics to the terrestrial cusp

Jupiter's equatorward auroral features: possible signatures of magnetospheric injections

The present study investigates the characteristics of ultraviolet auroral features located equatorward of the main emission appearing in Hubble Space Telescope images of the northern and southern Jovian hemispheres obtained in 2000–2007. On average, one feature is observed every day, but several auroral structures are occasionally seen over a wide range of local times in the same image. Several properties of these features are analyzed, such as their location, emitted power, and lifetime. Additionally, we magnetically map the auroral features to the equatorial plane using the VIPAL model in order to compare their observed properties with those of magnetospheric injections detected by the Galileo spacecraft. The equatorward auroral features show up between the Io footpath and the main auroral emission, at all System III longitudes, in agreement with Galileo measurements. Moreover, we compare the magnetic flux associated with these features with estimates of the outgoing flux related to the radial transport of plasma in the Jovian magnetosphere, and we find that they could account for at least one third of this flux. This comparative study shows that the auroral features under study are most probably related to magnetospheric injections and thus sheds light on the processes involved in the magnetosphere-ionosphere dynamics.

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 Jupiters auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiters auroras to the interplanetary conditions. We show that for ∼1-3 days following compression region onset the planets 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 Jupiters magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought.

Magnetosphere-ionosphere mapping at Jupiter: Quantifying the effects of using different internal field models

The lack of global field models accurate beyond the inner magnetosphere (<30 RJ) makes it difficult to relate Jupiters polar auroral features to magnetospheric source regions. We recently developed a model that maps Jupiters equatorial magnetosphere to the ionosphere using a flux equivalence calculation that requires equal flux at the equatorial and ionospheric ends of flux tubes. This approach is more accurate than tracing field lines in a global field model but only if it is based on an accurate model of Jupiters internal field. At present there are three widely used internal field models—Voyager Io Pioneer 4 (VIP4), the Grodent Anomaly Model (GAM), and VIP Anomaly Longitude (VIPAL). The purpose of this study is to quantify how the choice of an internal field model affects the mapping of various auroral features using the flux equivalence calculation. We find that different internal field models can shift the ionospheric mapping of points in the equatorial plane by several degrees and shift the magnetospheric mapping to the equator by ~30 RJ radially and by less than 1 h in local time. These shifts are consistent with differences in how well each model maps the Ganymede footprint, underscoring the need for more accurate Jovian internal field models. We discuss differences in the mapping of specific auroral features and the size and location of the open/closed field line boundary. Understanding these differences is important for the continued analysis of Hubble Space Telescope images and in planning for Junos arrival at Jupiter in 2016.

Concurrent observations of ultraviolet aurora and energetic electron precipitation with Mars Express

The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures of CO Cameron and CO2+ doublet ultraviolet auroral emissions. This study has almost doubled the number of auroral detections based on SPICAM spectra. Auroral emissions are located in the vicinity of the statistical boundary between open and closed field lines. From a total of 113 nightside orbits with SPICAM pointing to the nadir in the region of residual magnetic field, only nine show confirmed auroral signatures, some with multiple detections along the orbital track, leading to a total of 16 detections. The mean energy of the electron energy spectra measured during concurrent ASPERA-3/ELS observations ranges from 150 to 280 eV. The ultraviolet aurora may be displaced poleward or equatorward of the region of enhanced downward electron energy flux by several tens of seconds and shows no proportionality with the electron flux at the spacecraft altitude. The absence of further UV auroral detection in regions located along crustal magnetic field structures where occasional aurora has been observed indicates that the Mars aurora is a time dependent feature. These results are consistent with the scenario of acceleration of electrons by transient parallel electric field along semi open magnetic field lines.

Saturn's elusive nightside polar arc

Nightside polar arcs are some of the most puzzling auroral emissions at Earth. They are features which extend from the nightside auroral oval into the open magnetic field line region (polar cap), and they represent optical signatures of magnetotail dynamics. Here we report the first observation of an arc at Saturn, which is attached at the nightside main oval and extends into the polar cap region, resembling a terrestrial transpolar arc. We show that Earth-like polar arcs can exceptionally occur in a fast rotational and internally influenced magnetosphere such as Saturns. Finally, we discuss the possibility that the polar arc at Saturn is related to tail reconnection and we address the role of solar wind in the magnetotail dynamics at Saturn.