Cassini UVIS auroral update for Caltech 2014 January meeting Wayne Pryor, Central Arizona College

Figure from Jacques Gustin et al.(2014) on the left, a UVIS occultation at high latitudes and on the right, an occultation at medium latitudes. The first panel shows the observed transmission in the A spectral region, sensitive to absorption by Lyman and Werner H2 transitions, + best fit. The second panel shows the corresponding modeled temperature profile and the third shows the H2 density profile. The bottom line of all this is that the altitude of the temperature gradient is much lower in the high latitude case, and the exospheric temperature is higher. Since the altitude of the gradient is linked to the altitude of the hydrocarbon homopause, this is important for the determination of the energy of the precipitating electrons from CH4 absorption; The paper is submitted and I will probably show this at the UVIS meeting in June, but feel free to show this plot in your talk if you like!

Slides from Katerina Radioti follow:

Nightside bursty reconnection at Saturn ? 19:42 UT 21:49 UT 20:46 UT 23:56 UT 01:00 UT 22:53 UT DOY LT 18 LT 12 LT Radioti et al., in preparation

19:42 UT 21:49 UT Multiple reconnection onsets ? Arcs and spot-like structures Substorm at Earth (IMAGE) Nightside bursty reconnection at Saturn ? Radioti et al., in preparation

First observation of Theta Aurora at Saturn Conditions for the formation of an Earth-like transpolar arc at Saturn can be exceptionally met at Saturn (Radioti et al., submitted)

My DPS talk for 2013 Denver follows

Cassini UVIS Saturn Auroral Images from the May 2013 HST/Cassini Campaign Wayne Pryor, Alain Jouchoux, Larry Esposito, Frank Crary, Aikaterini Radioti, Denis Grodent, Jacques Gustin, Jean- Claude Gerard, William Kurth, Don Mitchell, Jonathan Nichols, Sarah Badman, Cassini Campaign Team, Cassini UVIS Team

Cassini Orbital Inclination Profile Inclination (degrees)  Date 2017 Cassini Obtained High-Latitude Auroral Data During the 2013 Joint Campaign with HST

HST ACS data going from top day :54 to bottom day :41. Saturn north aurora brightens and moves poleward near dawn, probably in response to passage of a denser solar wind feature (CIR). We will look at UVIS data from the same period starting an hour later at day :43

UVIS Polar Movie (2013 Day ) of Sunlit Northern Auroral Oval Six-frame movie runs day : :01 Sub-spacecraft latitude changes from N, range from Rs (spatial resolution ~400 km for 1 mrad pixels along UVIS long slit) Start End

Near simultaneous UVIS and HST images (J. Nichols) Raw UVIS Raw HST Polar UVIS Polar HST

2 nd bright “storm” in 2013 Day movie extends over 4 degrees latitude, more than 60 degrees longitude, lasts several hours

2008 day 129 data were obtained from farther away (16 Rs) They tied UVIS brightenings to rapid tail reconnection followed by the magnetic field slowly becoming more dipolar (dipolarization) Dipolarization leads to a substorm current wedge Currents produce auroras substorm current wedge picture at Earth Jackman et al., 2013 analyzed earlier UVIS 2008 Day 129 images 2008, DOY 129

Nightside bursty reconnection at Saturn ? 19:42 UT 21:49 UT 20:46 UT 23:56 UT 01:00 UT 22:53 UT DOY LT 18 LT 12 LT Radioti et al., in preparation

19:42 UT 21:49 UT Multiple reconnection onsets ? Arcs and spot-like structures Substorm at Earth (IMAGE) Nightside bursty reconnection at Saturn ? (K. Radioti)

Ring Current Movie Frames from CASSINI MIMI Inca (from Don Mitchell) May, May, 2013 Just before UVIS/HST storms During UVIS/HST auroral storms noon UVIS the UVIS bright midnight-dawn-noon sector was not in view for Inca; (x points sunward, y to dusk) but the ENA enhancement corresponds to weak dusk-midnight UVIS enhancement 23:56 UT

Cassini RPWS (from Bill Kurth) May brighter during May UVIS/HST events May 20  May 22 

2013 Day Summary UVIS imaged Saturn’s northern auroral oval from about 6 Rs. Higher spatial resolution look than the 2008 Day 129 case UVIS captured two brightening events also seen in HST images Storms (brightenings) may be due to reconnection events Storms migrate sub-corotationally in local time towards noon. (J. Nichols finds 44% of co-rotation for the 2 nd storm) Active period also seen in Cassini MIMI Inca ring current data and RPWS radio data.

Since DPS, we looked at satellite links again Specifically, for the UVIS movie from the DPS talk (and Katerina’s talk) there is a strong link with sub-Mimas longitude Jon Nichols independently found the auroral storm is moving at about the Mimas orbital rate from HST – Go to p of auroral_book_flat_cube_fuv_enceladus_mimas.pdf (2013 day )

What is this satellite link about? Not an obvious Mimas “footprint”- current along the Mimas-crossing Saturn field lines Enceladus footprints are rare, too Instead, we often see periodic bright disturbances on the main oval tied to the sub- Mimas longitude and sometimes the sub- Enceladus longitude I presented this at Santorini and at DPS 2012 and earlier UVIS meetings

What causes this? Working picture follows – Magnetospheric disturbances (reconnection events, Kelvin-Helmholtz instabilities) set up magnetospheric oscillations – Magnetospheric field lines “wrap” around satellite obstacles as they oscillate – This periodically launches Alfven waves towards the main oval – Disturbs/enhances the auroral emissions – Events persist because the injected plasma in the partial ring current tends to sub-corotate at angular speeds near the Mimas rate

Paper Plans for Icarus special issue (March submission) HST/UVIS auroral campaign papers Jon Nichols.. on HST auroral results Katerina Radioti.. on UVIS auroral results Other papers of interest: Ulyana Dyudina.. On ISS auroral results Don Mitchell.. On UVIS/Inca 1-hour auroral pulsations from images (again, Mimas-linked) – Go to auroral_book_cube_flat_fuv_enceladus_mimas.pdf page p – This will be his 2 nd paper on that day’s UVIS data Wayne Pryor.. On UVIS satellite-linked 1-hour auroral pulsations during the 2009 ISS “snake” movie

ISS Snake Image (Ulyana Dyudina)

Auroral Campaign Press Release Activity Jia-Rui Cook is editing draft text this week I’ve mentioned the Mimas connection in my latest inputs UVIS movie (and others) to be released

For the future “Mini-Mi-mas” observations: could we mini- scan the sub-Mimas part of the oval to see higher time resolution spatial changes and better catch the peak emissions, which may be ~1 hour apart? Interest modelers in Alfven wave simulations to see if the “working picture” makes any physical sense