Parallel HST-­‐UVIS observa2ons of Saturn ’s aurora (results from the combined HST-­‐Cassini campaign 2011) Jean-­‐Claude Gérard LPAP -­‐ Université de Liège based on a manuscript in collabora2on with J. Gus2n, D. Grodent, B. Bonfond, A. Radio2 W. Prayor J. Clarke J. Nichols submiVed for publica2on UVIS team mee2ng Braunschweig – June 2012 Previous observa2ons -­‐ UVS spectra taken on board Voyager indicate that almost all Saturn auroral spectra show no indica2on of 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 -­‐ 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 the 0.1-­‐ 0.2 μbar level -­‐ on-­‐going analysis of Cassini/UVIS spectra confirms the Voyager results: absorp2on by hydrocarbons is generally not observed (J. Gus2n) OUTLINE -­‐  remote sensing of primary electron mean energy -­‐  results of the HST-­‐Cassini 2011 campaign -­‐  comparison with limb imaging method -­‐  conclusions Auroral excita2on processes ep + H2  H2* (B, C, B’, D, …) + e These excita2on processes compete with ioniza2on: ep + H2  H2+ + ep + es The secondary electrons es can, in turn, collide and excite H2 levels: es + H2  H2* (B, C, B’, C ’, ...) + es 1a. Remote sensing of the energy of auroral electrons: the «color ra2o» method The FUV color ra2o may thus be used to determine the characteris2c energy of auroral precipita2on in giant planets’ atmospheres if the ver2cal distribu2on of hydrocarbons is known. Unabsorbed value: 1.1
The FUV color ratio (CR)
I (1550 ! 1620 Å )
I (1230 ! 1300 Å)
Example of electron energy derived from FUV spectra of the oval emission STIS image and slit posi2on 14
observations
best synthetic spectrum
12
unabsorbed synthetic spectrum
Rayleighs/Å
10
8
6
4
2
0
1100
1200
1300
1400
1500
Wavelength (Å)
1600
1700
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) 1b. Remote sensing of the energy of auroral electrons: the H2 self-­‐absorp2on approach FUSE EUV high-resolution spectroscopy
T(H2): 400K
H2 column:
3 - 6 x 1019 cm-2
Gus2n et al., 2009 • The «UV color ra2o » method is a powerful tool to probe the electron mean energy IF the distribu2on of HCs (mostly methane) is known •  EUV spectroscopy is very useful to determine the depth of the electron energy deposi2on (in terms of H2 column or pressure) , but so far without spa2al resolu2on 2. The HST-­‐Cassini 2011 campaign STISS 11 March 2011 !
!
UVIS !
!
11 March 2001 STIS (morning sector) !
UVIS (dusk sector) UVIS (late alernoon sector) !
!
STIS 11 April 2011 !
!
UVIS !
!
STIS spectra – 17 April 2011 Alernoon sector !
Morning sector !
Monte Carlo electron transport code !
Summary table: results of HST-­‐Cassini 2011 campaign 3. Comparison with direct imaging method Compila2on of remote sensing determina2ons of auroral electron energy Taur (H3+) pressure level of emission layer (FUSE, self absorp2on), T = 400 K Moses et al., 2000 (equat.) Homopause ? suggested modified T profile (auroral lat.) Gérard et al., 2009 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 4. Conclusions Moses et al., 2001 ? •  Monte Carlo simula2ons of observed FUV STIS and UVIS spectra indicate that the primary auroral electrons have energies of at most ≈10 keV, but frequently below 3 keV. •  No sytema2c dependence on local 2me is apparent •  The limb imaging observa2ons may only be reconciled with FUV spectra if the temperature profile is different above the 100-­‐
microbar level from that determined from Voyager low-­‐la2tude stellar occulta2ons •  This would imply that the thermosphere is warmer at the 0.1 microbar level than previously thought Carbon FUV lines observed with UVIS during the Cassini flyby of Venus B. Hubert et al. Icarus, in press - VENUS SWINGBY - 99-175
Second CASSINI
Venus UVIS
flyby by Cassini -­‐ June 24, 1999 1.0
N Pole
0.5
3h
45N
Venus radii
12h
6h
9h
0.0
Equator
-0.5
-1.0
Subsolar point
Cassini
Closest Approach (602 km )
Trajectory
UVIS lines-of-sight (60 sec spacing)
-1.5
1
0
Venus radii
-1
UVIS EUV-­‐FUV Venus disk spectrum Spectral resolu2on: 3.7 A FUV disk spectrum OI CO 4P This sec2on of the Venus spectrum is dominated by the OI 130.4 and 135.6 nm mul2plets and the CO 4th Posi2ve bands (+ C lines) Hubert et al., Icarus, 2010. Observed (doVed lines) and fiVed (dashed lines) UVIS spectrum obtained at the loca2on of the smallest emission angle (record 25) Contribu2ons of the fiVed CO-­‐4P bands (black), OI (red) and CI (green) mul2plets to the total fiVed spectrum The bright CI mul2plets at 156.1 and 165.7 nm are well separated from the surrounding CO-­‐4P bands. Zooms on panels a and b respec2vely, in the 2 wavelength intervals containing the brightest CI transi2ons. Intensi2es of the brightest CI and CII FUV mul2plets between ~125 and 180 nm along the Cassini-­‐UVIS track versus the solar zenith angle of the emixng layer. Cassini crossed the morning terminator (SZA~90°) and moved towards the planetary bright limb where highest intensi2es are recorded. Photochemical produc2on rates of the upper state of the CI 126.1 nm (a), CI 156.1 nm (b) and CI 165.7 nm (c) transi2ons, calculated for the observing condi2ons of UVIS record 25. The VTS3 CO2 and the reference C density profiles adopted in the model are shown in panel (d). Carbon density scaling factor es2mated along the Cassini track. CONCLUSIONS • A few carbon lines have been observed with UVIS in the Venus FUV dayglow with unprecedented spectral resolu2on • Their intensity varia2on across the sunlit disk follows the same distribu2on • The C density calculated by J. Fox appears too small by about an order of magnitude • The means that the O2 density es2mate needs to be revised downward accordingly Monte Carlo electron transport code: H2-­‐dominated atmosphere Al2tude distribu2on of Saturn’s aurora for various energies of the primary electrons The al2tude of the emission peak depends on the value of the electron mean energy (and energy distribu2on) since there is a range rela2onship between the electron energy and the H2 column crossed at the rest point