Oscillation of Saturn's southern auroral oval

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently highly axisymmetric. In addition, the period of the Saturn kilometric radiation has been shown to vary over time. In this paper we present results from the recent Hubble Space Telescope observations of Saturn's southern ultraviolet auroral emission. We show that the center of the auroral oval oscillates with period 10.76 h +/- 0.15 h for both January 2007 and February 2008, i.e., close to the periods determined for oscillations in other magnetospheric phenomena. The motion of the oval center is described for 2007 by an ellipse with semimajor axis similar to 1.4 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.93, and center offset from the spin axis by similar to 1.8 degrees toward similar to 04 h LT. For 2008 the oscillation is consistent with an ellipse with semimajor axis similar to 2.2 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.99, and a center offset from the spin axis by similar to 2.2 degrees toward similar to 03 h LT. The motion of the auroral oval is thus highly elliptical in both cases, and the major oscillation axis is oriented toward prenoon/premidnight. This result places an independent constraint on the magnitude of the planet's dipole tilt and may also indicate the presence of an external current system that imposes an asymmetry in the ionospheric field modulated close to the planetary period

Saturn's equinoctial auroras

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

A large ground-based observing campaign of the disintegrating planet K2-22b

We present 45 ground-based photometric observations of the K2-22 system collected between 2016 December and 2017 May, which we use to investigate the evolution of the transit of the disintegrating planet K2-22b. Last observed in early 2015, in these new observations we recover the transit at multiple epochs and measure a typical depth of <1.5%. We find that the distribution of our measured transit depths is comparable to the range of depths measured in observations from 2014 and 2015. These new observations also support ongoing variability in the K2-22b transit shape and time, although the overall shallowness of the transit makes a detailed analysis of these transit parameters difficult. We find no strong evidence of wavelength-dependent transit depths for epochs where we have simultaneous coverage at multiple wavelengths, although our stacked Las Cumbres Observatory data collected over days-to-months timescales are suggestive of a deeper transit at blue wavelengths. We encourage continued high-precision photometric and spectroscopic monitoring of this system in order to further constrain the evolution timescale and to aid comparative studies with the other few known disintegrating planets

Detection of Auroral Emissions from Callisto’s Magnetic Footprint at Jupiter

HST observations of Jupiter’s aurora in a large campaign reveal several cases where the main oval emission appeared at unusually low latitudes, making it possible to search for the first time for auroral emissions from the magnetic footprint of Callisto without the overlapping bright emissions from the main oval. Several cases have been found where point-source emissions have now been detected from locations consistent with Callisto’s magnetic footprint on Jupiter at a brightness of ten’s of kilo- Rayleighs. These observations confirm that there is an electrodynamic interaction between Callisto and Jupiter’s magnetospheric environment that is similar to those at Io, Europa, and Ganymede, which all have auroral footprints. The properties of the emissions and a comparison with other observations and theoretical expectations will be presented in this paper

The HST Auroral Campaign Observations of Jupiter and Saturn

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven primarily by internal processes, with the main energy source being the planetsâ rapid rotation. Limited evidence has suggested there might be some influence of the solar wind on Jupiter's aurorae, and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from one AU and measured near each planet. The data indicate a consistent brightening of both the auroral emissions and Saturn Kilometric Radiation (SKR) at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the situation is less clear, with increases in total auroral power seen near the arrival of solar wind forward shocks, while little increase has been observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn

Ten years of Hubble Space Telescope observations of the variation of the Jovian satellites' auroral footprint brightness

[1] During the past decade, FUV imaging of Jupiter's auroral region by the Hubble Space Telescope (HST) using two instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS), has provided detailed information on the electrodynamic interaction between Io's, Ganymede's, and Europa's atmospheres and plasma in Jupiter's magnetosphere. This interaction is responsible for the satellites' auroral footprints in Jupiter's atmosphere connected via magnetic flux tubes to the satellites' interaction regions. The observed brightness of each auroral footprint is considered to be one main observable quantity to characterize the interaction environment at the satellites. Previous observations of Io's magnetic footprints using HST STIS images showed that the footprint emission appears brightest when Io is centered in the plasma torus. With the much larger data set obtained from the 2007 HST campaigns, we find the same variation observed by Serio and Clarke (2008), but with significantly better statistics over a time period of 10 years. These results confirm that Io's footprint brightness varies mainly with the satellite's location in Jupiter's plasma torus over a long time scale. Additional observations of the downstream emissions and their variations were presented by Bonfond et al. (2007). In Ganymede's case, the relation between the footprint brightness and the satellite's position in Jupiter's magnetosphere shows some evidence for the same general trend, although the data are noisier than the data for Io. Ganymede's footprint brightness appears to be less consistent over time than Io's. The variation of Ganymede's footprints over short time periods was studied by Grodent et al. (2009). Europa's fainter footprint brightness makes it difficult to see any systematic trend

Case study of Ganymede’s footprint location shifts in respond with the volcanic eruptions at Io and the solar wind compression

We present here a study of the latitudinal variations of the location of Ganymede’s footprint (GFP) observed by the Hubble Space Telescope (HST) in 2007 and 2016. We assess the variation of GFP locations based on both internal and external factors, which are 1) fluctuations of the mass outflowrate (M) of the magnetospheric plasma originating from Io’s volcanic activity and 2) solar wind variations, respectively. The plasma density inside Jupiter’s magnetosphere increases due to the volcanic material on Io resulting in the stretching of magnetic field lines, which affects the equatorward shift of GFP. Meanwhile, the solar wind compression affects the decreased size of the magnetosphere, resulting in the poleward shift of GFP. Deviations of GFP location are assessed by comparison with the Ganymede mapped path by JRM33 (Connerney et al.,2022) and with the average path by Bonfond et al., 2017. We focus in particular on four epochs for which there are observations of the GFP with similar System III longitude: 1) DOY 054 and DOY 068 (February - March 2007), 2) DOY 132, DOY 154, and DOY 161 (May - June 2007), 3) DOY 178 and DOY 199 (June - July 2016), and 4) DOY 148 and DOY 155 (May - June 2016). We compare the observation with the magnetodisc model (Nichols et al., 2015) by considering the variation of the hot plasma parameter (Kh) andMto the field line mapping in Jupiter’s ionosphere for the magnetosphere size of 80 RJ and 50 RJ (during the compression). We found that the observational results are consistent with the positions mapped by the magnetodisc model. In addition, the modelled result shows that the compression of the magnetosphere could relate to the increase ofKh in Jupiter’s magnetosphere. The results show that the shifts of GFP in case 1 and case 3 could be affected by the external factor. We also found the slightly equatorward shift in case 4 which could be dominated by the internal factor. Additionally, we presented the special event in case 2 where GFP is located in a similar location, which could be affected by the internal and the external factors at the same time

Ganymede's Auroral Footprint Latitude: Comparison With Magnetodisc Model

peer reviewedVariations of Ganymede's auroral footprint locations are presented based on observations by the Hubble Space Telescope in 2007 and 2016. The poleward and equatorward shifts of Ganymede's footprint could be influenced by the mass outflow rate from Io and the solar wind compression, as the internal and external factors respectively. We compare our results with Ganymede's footprint mapping based on the magnetodisc model. The mapped footprint in Jupiter's ionosphere shifts equatorward with increased hot plasma parameter, Kh, which is associated with hot plasma pressure. We analyzed the effect of cold plasma number density (Nc), related to the mass outflow rate and connected to the material produced by Io. The results show that the magnetic footprint is shifted equatorward by 0.37° when the mass outflow rate is increased from 800–2,000 kg s−1. Iogenic plasma has a strong influence on the stretching of the magnetic field lines in Jupiter's middle magnetosphere, causing the equatorward shift of Ganymede's footprint. For external factors, Ganymede's footprint shifted poleward by 0.62° under the influence of solar wind compression while the mass outflow is kept constant at 1,000 kg s−1. We present similar locations of Ganymede's footprint based on the field lines mapped as a result of the compensation between an increase of Kh and the solar wind compression. Overall, the location of Ganymede's auroral footprint corresponds with the mass loading rate from Io and the solar wind dynamic pressure