The al'tude of Saturn’s aurora: implica'ons for the auroral electron energy flux and spectrum J.‐C. Gérard , B. Bonfond , J. Gus2n , D. Grodent LPAP, Université de Liège, Belgium January 29, 2001
1. Color ra2o method The FUV color ra2o I(1230‐1300 Å)/I(1550‐1620 Å) is an indicator of the amount of absorp2on of auroral emission by hydrocarbons. It may thus be used to determine the characteris2c energy of auroral precipita2on in giant planet atmospheres. Example of electron energy derived from FUV spectra of the oval emission STIS image and slit posi2on 14
observations
best synthetic spectrum
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unabsorbed synthetic spectrum
Rayleighs/Å
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2
0
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Wavelength (Å)
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Auroral electron mean energies (in the noon sector) were found to be typically about 10 keV, that is an order of magnitude less than in the Jovian aurora (Gérard et al., 2000) 1700
‐  UVS spectra taken on board Voyager indicate that almost all spectra show no FUV absorp2on by hydrocarbons, implying that the bulk of the emission is produced above the homopause ‐  HST/STIS spectra taken near the central meridian (12:00 LT) indicate that the H2 emission is weakly absorbed, leading to an es2mate of E ≈ 10 keV ‐  Recent analysis of Cassini/UVIS spectra confirms the Voyager results: absorp2on by hydrocarbon is generally no observed (Gus2n et al., private comm.) ‐ FUSE observa2ons of Saturn’s EUV aurora set the al2tude of the emerging emission to a level of H2 = 3‐6 x 1019 cm‐2, corresponding to 0.1 μbar (Gus2n et al, 2009) ‐  UVS spectra taken on board Voyager indicate that almost all spectra show no FUV absorp2on by hydrocarbons, implying that the bulk of the emission is produced above the homopause ‐  HST/STIS spectra taken near the central meridian (12:00 LT) indicate that the H2 emission is weakly absorbed, leading to an es2mate of E ≈ 10 keV ‐  Recent analysis of Cassini/UVIS spectra confirms the Voyager results: absorp2on by hydrocarbon is generally no observed (Gus2n et al., private comm.) ‐ FUSE observa2ons of Saturn’s EUV aurora set the al2tude of the emerging emission to a level of H2 = 3‐6 x 1019 cm‐2, corresponding to 0.1 μbar (Gus2n et al, 2009) We have obtained new observa2ons of the loca2on of the aurora above the limb which have been obtained during the 2007‐08 HST/Cassini campaign. 2. Direct auroral limb imaging Zoom on southern high la2tudes The STIS or ACS field of views are wide enough to image the full planet + rings. Therefore the posi2on of the planetary center may be determined with a precision ≈1 pixel About 600 ACS images obtained in 2007‐08 show auroral emission at the south limb Gérard et al., GRL, L02202, doi:10.1029/2008GL036554, 2009
The mean peak al2tude, determined from the full ACS dataset is 1100 +‐ 350 km pressure level of emission layer (FUSE, self absorp2on), T = 400 K Taur (H3+) Moses et al., 2000 (equat.) Homopause ? suggested modified T profile (auroral lat.) The modified temperature profile is warmer at pressures less than 100 μbars, so that the 0.1 μbar level pressure level is reached at a higher al2tude, in agreement with the HST auroral images modified T profile uncertainty on pressure level Moses et al. (equat.) 1‐sigma uncertainty on al2tude of aurora from HST images Monte Carlo simula2ons Moses et al., 2001 ? Homopause < excita2on threshold A characteris2c energy of about 15 keV calculated with a Monte Carlo electron transport code appears to provide the best fit to the observed luminosity profile if the modified atmosphere model is used. Kappa
κ=2.33
kT=755 eV
Maxwellian
kT=6350 eV
Mono-energetic
kT=19.5 keV
Kappa
κ=2.33
kT=755 eV
Maxwellian
kT=6350 eV
 HST ultraviolet images of Saturn’s auroral limb clearly show a gap Moses et al., 2001 ? between the bright limb and the auroral emission layer  The al2tude of the auroral UV emission can be accurately determined, and the plate scale of the detector has been checked using the rings as a absolute reference. The al2tude the 1‐bar level coincides with the planetary limb observed outside the auroral region  Analysis of radial light curves indicates that the auroral peaks lies about 1100 ± 200 km above the 1‐bar level. The energy may occasionally be higher, causing FUV absorp2on by CH4   This result may only be reconciled with FUV the spectra if the Moses et al., 2001 ? temperature profile is different above the 100‐microbar level from that determined from Voyager low‐la2tude stellar occulta2ons (as used in Moses et al., 2000)   It implies that the auroral thermosphere is warmer at the 0.1 microbar level than at low la2tude   Monte Carlo simula2ons indicate that the primary auroral electrons have energies ≈ 15 keV (with the modified atmospheric model).   None of the classical auroral electron energy distribu2on fits really well the observed ver2cal emission profile. An addi2onal low‐energy component appears necessary   The proposed modified temperature profile could be checked Moses et al., 2001 ? against profiles deduced from high‐la2tude stellar occulta2on observa2ons. If they do not exist, such observa'ons should be made in the future.   It would be very interes'ng to compare results deduced from the FUV images with those which may be collected in the visible and infrared by Cassini cameras.