HST/STIS Observations of Ganymede's Auroral Ovals at Eastern Elongation

We report on new Space Telescope Imaging Spectrograph (STIS) observations of Ganymede s auroral emissions obtained (to be obtained) during two visits with the Hubble Space Telescope (HST). The observations of the first visit, a five orbits, were obtained on November 19, 2010 and the second visit, also a five orbits, is scheduled for opposition in October/November 2011. We will present results of the full campaign, in case of a successful execution of the second visit. Our observations cover more than half a cycle of system III longitudes of Ganymede s positions within Jupiter s magnetosphere for each visit. We analyze the observations with respect to brightness and locations of Ganymede auroral ovals. Our goal is to set constrains on the interaction of Ganymede s mini-magnetosphere with Jupiter s magnetosphere, Ganymede s magnetic field and plasma environment, and if possible on Ganymede s neutral atmosphere

Gravity field determination of a Comet Nucleus: Rosetta at P/Wirtanen

One of the prime objectives of the Rosetta Radio Science Investigations (RSI) experiment is the determination of the mass, the bulk density and the low degree and order gravity of the nucleus of comet P/Wirtanen, the target object of the international Rosetta mission. The RSI experiment will use the spacecraft's radio carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) in order to measure slight changes of the orbit velocity via the classical Doppler effect induced by the gravity attraction of the comet nucleus. Based on an estimate of the background Doppler noise, it is expected that a mass determination (assuming a representative radius of 700 m and a bulk density of 500 kg/m^3) at an accuracy of 0.1% can be achieved if the spacecraft's orbit is iteratively reduced below 7 km altitude. The gravity field of degree and order two can be detected for reasonable tracking times below 5 km altitude. The major competing forces acting on the spacecraft are the radiation pressure and the gas mass flux from cometary activity. While the radiation pressure may be predicted, it is recommended to begin a gravity mapping campaign well before the onset of outgassing activity (>3.25 AU heliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order gravity field and will mask the contributions from the gravity field

Alternating North‐South Brightness Ratio of Ganymede's Auroral Ovals: Hubble Space Telescope Observations Around the Juno PJ34 Flyby

peer reviewedWe report results of Hubble Space Telescope observations from Ganymede's orbitally trailing side which were taken around the flyby of the Juno spacecraft on 7 June 2021. We find that Ganymede's northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter's magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede's aurora is the momentum of the Jovian plasma sheet north and south of Ganymede's magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant timevariability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1,356 Å emission making Ganymede a very poor auroral emitter

Orbital Observations of Dust Lofted by Daytime Convective Turbulence

Over the past several decades, orbital observations of lofted dust have revealed the importance of mineral aerosols as a climate forcing mechanism on both Earth and Mars. Increasingly detailed and diverse data sets have provided an ever-improving understanding of dust sources, transport pathways, and sinks on both planets, but the role of dust in modulating atmospheric processes is complex and not always well understood. We present a review of orbital observations of entrained dust on Earth and Mars, particularly that produced by the dust-laden structures produced by daytime convective turbulence called “dust devils”. On Earth, dust devils are thought to contribute only a small fraction of the atmospheric dust budget; accordingly, there are not yet any published accounts of their occurrence from orbit. In contrast, dust devils on Mars are thought to account for several tens of percent of the planet’s atmospheric dust budget; the literature regarding martian dust devils is quite rich. Because terrestrial dust devils may temporarily contribute significantly to local dust loading and lowered air quality, we suggest that martian dust devil studies may inform future studies of convectively-lofted dust on Earth

Mass and density determinations of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraft will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 and 24 July 2008, respectively. The objective of the Rosetta Radio Science Investigations (RSI) experiment at these flybys is a determination of the asteroid's mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closest approach. The spacecraft's flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the mass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20% . Unfortunately, a detectable trajectory perturbation seems to be hopeless for Otawara because of its small size and the large nominal flyby distance

Mass and density determination of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys.

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraft will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 and 24 July 2008, respectively. The objective of the Rosetta Radio Science Investigations (RSI) experiment at these flybys is a determination of the asteroid's mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closest approach. The spacecraft's flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the mass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20% . Unfortunately, a detectable trajectory perturbation seems to be hopeless for Otawara because of its small size and the large nominal flyby distance

Analysis of Cassini magnetic field observations over the poles of Rhea

We analyze Cassini magnetic field observations from the only two polar flybys of Saturn’s largest icy satellite Rhea (R2 on 02 March 2010 and R3 on 11 January 2011) which are scheduled between Saturn Orbit Insertion and the end of the mission in 2017. For the interpretation of these data, we apply estimations from simple analytical models as well as results from numerical hybrid simulations (kinetic ions, fluid electrons) of Rhea’s interaction with the incident magnetospheric plasma. In-situ observations of exospheric neutral gas and pick-up ions suggest Rhea to be embedded in a tenuous gas envelope. However, the interaction of this gas with the magnetospheric flow does not make any measurable contributions to the magnetic field perturbations detected above the poles of the moon. Instead, the field perturbations observed in these regions mainly arise from the absorption of magnetospheric particles with large field-aligned velocities, impinging on the north and south polar surface of Rhea. In addition to numerous interaction features known from preceding Cassini flybys of Saturn’s plasma-absorbing moons, the magnetic field data acquired above Rhea’s poles reveal perturbations of the flow-aligned field component, corresponding to a draping/Alfvén wing pattern. Based on our hybrid simulations, we suggest that these signatures arise from the finite extension of Rhea’s wakeside plasma void along the corotational flow direction, yielding a density gradient in corotation direction, and thereby generating a diamagnetic current from the Saturn-facing into the Saturn-averted hemisphere of the moon. This transverse current is responsible for generating a weak Alfvén wing pattern at Rhea which has been detected by the Cassini spacecraft during the R2 and R3 flybys. Due to the large gyroradii of the incident magnetospheric ions, this structure features a pronounced asymmetry with respect to the direction of the convective electric field. Results of our simulation, considering only plasma absorption on the moon, are in good agreement with Cassini magnetometer data from both flybys. At Saturn’s icy satellites Tethys and Dione, the low value of the magnetospheric plasma beta most likely prevents the formation of similar currents and measurable flow-aligned magnetic field distortions