Morphology of Ganymede's FUV auroral ovals

Abstract

We study the morphology of Ganymede's FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The observations cover the eastern and western elongation as well as various magnetic latitudes of Ganymede within the Jovian plasma sheet. We find both asymmetries in the spatial distribution of auroral brightness on the observed moon disk and temporal variation correlated to Ganymede's changing magnetic latitude. The total disk brightness is on average 1.42 ± 0.07 times brighter on the leading side (95.4 ± 2.1 R) than on the trailing side (67.2 ± 2.9 R). The brightness ratio of the sub‐Jovian hemisphere to the anti‐Jovian hemisphere is 1.81 ± 0.06 on the leading side and 1.41 ± 0.14 on the trailing side, respectively. Inside the Jovian current sheet, the brightness of the auroral ovals increases by a factor of 1.45 ± 0.02 on the leading side and decreases by a factor of 0.80 ± 0.02 on the trailing side. At the current sheet center, the auroral ovals shift 4.1° ± 0.7°latitude toward Ganymede's planetographic equator on the leading side and 2.9° ± 1.5° toward the poles on the trailing side. Both effects, the variation of brightness and the movement of the ovals are correlated to a stronger interaction of Jupiter's magnetospheric plasma with Ganymede's minimagnetosphere inside the current sheet. Finally, we calculate the latitudinal difference of the northern and southern ovals from Ganymede's magnetic equator. The result suggests a farther westward orientation of Ganymede's dipole magnetic moment at approximately 47° + 58°/−43°west longitude compared to previous estimates.

Hubble Space Telescope Observations of Variations in Ganymede's Oxygen Atmosphere and Aurora

“…Based on Figure , we noted that, with the exception of exposure LS4, our observations are dimmer than previous observations obtained at similar λ III . All brightness values were extracted using the same procedure, and the intensities we measured for the previous HST campaigns agree with previously published intensities (Feldman et al, ; Musacchio et al, ) within the statistical uncertainties (small differences from the published values may be due to slight variations in the data reduction process, particularly the solar subtraction and the choice of detector regions used to estimate the background noise).…”

“…We measured a mean 135.6 nm intensity (plus‐minus standard deviation) of 36 ± 4 R on the trailing hemisphere while Ganymede was within this region, compared to 41 ± 7 R outside of it. While observations are sparse and the standard deviation is relatively large, Musacchio et al () also note this difference in the behavior of the two hemispheres near the plasma sheet center in their analysis of the morphology of Ganymede's aurora as observed in previous HST/STIS campaigns. We used the TIMETAG data to split the trailing hemisphere COS exposure closest to λ III = 111° (TC4) into seven 200‐s portions to look for evidence of a systematic change in auroral intensity as Ganymede moved closer to the magnetic equator over the course of the observation.…”

“…Figure shows the brightness of Ganymede's 135.6‐nm emissions as a function of λ III . Intensities extracted from previous HST data sets (campaigns 7939, 8224, and 12244: see Feldman et al, ; McGrath et al, ; Musacchio et al, ; Saur et al, , for descriptions of the data sets) are also included as faded points to extend the range of longitudes covered and indicate how our data compare to previous observations. The shaded areas in Figure indicate the regions of densest Jovian plasma, at | θ m | < 5°.…”

“…Disk‐averaged 135.6‐ and 130.4‐nm intensities were calculated from the COS and STIS spectra by summing the signal over all pixels within the 2“ × 2“ extraction region centered at those wavelengths for STIS, and within the 2.5“ PSA for COS, and normalizing to the projected size of Ganymede's disk in pixels. Previous observations of Ganymede have shown that a significant fraction of the aurora can be located outside of the disk (Feldman et al, ; McGrath et al, ; Musacchio et al, ; Saur et al, ), and including counts from the wider instrument aperture rather than only those originating within the disk is therefore important. Ganymede's angular size during our observations was between 1.59 and 1.70“ The STIS‐derived disk averages therefore included counts out to 1.18–1.25 R G from Ganymede's center, where R G is the radius of Ganymede.…”

The Far‐UV Albedo of Europa From HST Observations

“…Separately, to determine both x ‐ and y ‐positions, we use the Lyman‐α PSF‐convolved 2‐D solar reflection image and convolve that with the observed Lyman‐α image. We then set the location of x ‐ and y ‐positions of Europa where the convolution is maximized, similar to the method described by Musacchio et al (). This technique produces a consistent y ‐location value with the Gaussian method.…”