# Presentation: 222020

## Saturn’s Auroras from Cassini UVIS

- By Dr. Wayne Pryor
- (Central Arizona College)
- for the UVIS team
- Presented at the Charm telecon
-  May 29, 2007

## Topics

- Introduction to Auroras
- Comparison of Earth, Jupiter, and Saturn
- Cassini Saturn Aurora Results
- Effects: Polar Haze at Jupiter, and Saturn
- Time-dependence of solar wind and the aurora
- Hubble comparisons
- Recent high-latitude observations: hints of auroral patterns that repeat in longitude

## Solar wind

- Solar wind is a flow of charged particles from the Sun
- Density ~5 particles/cm3 at Earth (TINY!!)
- Mostly **protons** and **electrons**
- Makes auroras when the particles hit planetary atmospheres

## The Earth has a dipolar magnetic field

**Notes:**
Intrinsic field, as though there were a bar magnet stuck inside the Earth (though it is actually due to 	currents in the core)
This is why a compass works for navigation -- we are just measuring the Earth’s own intrinsic 		magnetism
Our magnetic field protect us from Solar wind particles, and can interact with those particles in 		interesting ways as well

## Earth’s Magnetosphere and the Solar Wind

- When solar wind nears a magnetized body, it is slowed and deflected
- Region controlled by the Earth’s magnetic field is called the *magnetosphere*

## Currents flow along the magnetic field

## Field Aligned currents light up the sky- the Aurora

## The aurora as seen by the Space Shuttle (300 km altitude)

## Global Aurora

- UV (ultraviolet) image from the Dynamics Explorer satellite shows the northern auroral oval

## Other planets have auroras too!

**Notes:**
Planets need magnetic field and atmosphere to have aurorae

## Processes by planet

- Earth: solar-wind driven auroras
- Jupiter: auroras dominated by sulfur, oxygen plasma from the moon Io’s volcanoes.  Main aurora is due to “co-rotation breakdown”
- Saturn: mixture of solar wind driven and internal stuff from magnetospheric plasmas rich in water products from the rings and the moon Enceladus

## UVIS and VIMS are on the Cassini Orbiter

## Sample Saturn Spectral ImagesEUV                           FUV

## UVIS long-slit spectroscopy

- EUV channel 56.3-118. 2 nm
- FUV channel 111.5-191.3 nm
- 64 spatial x 1024 spectral pixels
- Spectral imaging is done
- by spacecraft slews
_**Saturn’s emissions**_**:**
- H Lyman- and H2 bands
- from auroras and dayglow.
_**Reflected sunlight spectrum**_**:**
- Rayleigh scattering in H2
- and acetylene absorption bands

## Saturn in ultraviolet (July 13, 2004): auroras!

- H Lyman-alpha
- H2 band emission

## Saturn H Lyman-

## Saturn Oxygen image 130 nm (peaks ~2 Rayleighs, cloud has 1012 grams of O)

## Saturn Auroral Spectrum

## June 21 (Day 172), 2005 03:30-14:30   “EUVFUV” from 35 Rs

- N-S-N UVIS scan
- Slit E-W
- Auroral oval imaged twice
- Images deconvolved
- Blue H2, H emission
- Orange reflected sunlight
- Aurora changes over ~1 hour
- Oval 70-75S

## Saturn Day 172, 2005

- Limb-fitting puts auroral oval in 70-75 S range

## ISS south pole methane, uv3 images

## Recent Cassini Camera S Polar Image

- Intense storm at the site of an unusual UV spectrum

## Jupiter’s UV-dark Polar Haze: persistent N-S asymmetry, like the auroral asymmetry: do the auroras make dark haze?

## Saturn also has UV-dark polar haze tied to the auroras

- Polar Haze from Voyager PPS

## Polar Haze Chemistry

- Several theoretical and laboratory studies suggest methane (CH4) chemistry after irradiation by the auroral electrons makes UV-dark hydrocarbon haze
- Benzene (C6H6) is an important step in this haze formation process
- We’ve looked for benzene in the UVIS data from 2005 day 172

## Polar Dark Spot (2005 day 172)

- 1738-1816      1816-1894     ratio 1st/2nd   1328-1426
-          wavelengths (A)

- Spot in 1st image is gone at longer wavelengths (2nd image)
- Localized small hydrocarbons?  (spectrum is noisy)
- Polar convergence & downwelling?

## UVIS polar benzene feature?Pantos et al., 1978: broad benzene feature peaks at cross-section of ~2.7x10-16 cm2 

- Model at left has C2H2 mixing ratio=10-6 off spot, 5x10-6 on spot, with C6H6 of 4x10-9 on spot

## Cassini CIRS sees enhanced benzene at 14.8 m at 80S (no 90S report yet)               column above 10 mb (cm-2)    10 mb mixing ratio37S                      1.1x1014                               0.98x10-1380S                      2.2x1014                               8.73x10-13

## Auroral Time-Dependence:50 day time-series moving away from Saturn near phase angle 90 with the spacecraft in the solar wind

## UVIS auroral time-series as Cassini recedes from Saturn...

-      ^ shock                         ^ shock

## Hubble Auroral Campaign Feb 17, 2005

- J. Clarke, J.-C. Gerard PI’s:
- Day-side S auroras by HST ACS in UV (5 orbits)
-  Night-side N auroras by Cassini VIMS and UVIS
- Cassini VIMS_003SA_THRCYLMAP001_UVIS_FOV
- Started 2005-048 T23:08:00 GMT, ran 8 h 22 m
- Cassini at ~800,000 km range
- Aurora was weak that day

## HST Campaign ACS Images: Feb 17, 2005

- 16:20-16:58

- 17:53-18:33

- 19:29-20:09

- 21:04-21:45

- 22:40-23:21

## HST Campaign UVIS Geometry VIMS_003SA_THRCYLMAP001

## HST campaign UVIS map projection

- Mostly UVIS slit was too far south, but...

- Red=aurora
- Green=no aurora
- Auroras ~75 N

## UVIS dark-side spectrum 

## EUV aurora data Feb 17, 2005 (black) Model w/o self-absorption (dashed brown)Model fit (solid brown) for H2 = 5x1020 cm-2 & T=500K

- Wavelength (Ångstroms)

- Relative
- Intensity

## Recent UVIS N Auroral Image: 2006 Day 303

- Emission on a complete oval
- Generally brighter pre-midnight
- Emission spot visible inside the oval after midnight
- Sub-spacecraft lat. 45 N
- Range 22.6 Rs

## 2007 day 96 high-latitude N auroras

- Sub-s/c lat=51o
- Range=21.6 Rs
- Phase=125.4o
- Nine ~2 hour images
- First image shown: reflected sunlight=red
- aurora=blue

## Polar projected, 5o grid in latsun at lower left, auroras 70-85N

## INCA-UVIS comparisons from Don Mitchell

## Cassini Revolution 42 KGS trajectory

- Equatorial view

- Meridional view

- During Revolution 42 Cassini moves from the dayside (day 90), through dusk to midnight between day 95 and day 98.  The spacecraft moves through dawn towards noon between day 98 and 103.  The spacecraft crosses the equator on day 98 –midnight (N to S), and from S to N on day 101 post-noon.

## Cassini Revolution 42 KGS trajectory

- The spacecraft trajectory is mapped using dipole plus ring current magnetic field model
- Dotted lines show ionospheric co-latitude in 5o multiples from the pole.
- Cassini probably crosses field lines connecting to the statistical location (in the south) of the auroral oval during day 98-99 near dawn, day 100 near noon, and day 97 near midnight.
**On** **day 96, the UVIS observations** the s/c is at highest latitudes (mapping to **~8-11o** in the ionosphere), at a local time near to **dusk.**  (Just inside the UVIS Oval!)

*Northern* ionosphere footprint of Cassini

## UVIS observations on 96

## UVIS interval

## Auroral Movie 2007-096 Results

- 2007-096: 9-frame movie in H2 band emission
- Shows multiple arcs at times, spiral stuff
- Auroras mostly 70-80 N
- Generally brighter on dawn side
- MAG comparisons don’t show much action: spacecraft field line passed *inside* the oval
- Intriguing comparisons with INCA energetic neutrals underway:
- INCA equatorial spirals spiral out (going clockwise)
- UVIS auroral spirals spiral in (going clockwise)
- Suggests mapping of patterns onto the ionosphere (larger L-shell goes to smaller co-latitude)
- Suggests oval shape primarily not solar wind controlled on that day
- Color ratios available-> electron energy estimates

## VIMS S25 H3+ image from Tom Momary/Kevin Baines (S25NorthPoleAuroraSumCube8053THRCLMAP002)

## Very Recent Good Stuff

- Auroral Movies Days 145-146 (May 25-26) 2007
- _UVIS_045SA_NAURMOV001_PRIME:_
- 2007-145T10:39:00 to 2007 -145T18:54:00.
- _UVIS_045SA_AURORA001_PRIME:_
- 2007-146T04:30:00 to 2007-146T09:30:00

## UVIS Summary Saturn Conclusions

- Saturn auroral oval has been imaged by UVIS (and VIMS)
- Auroral modeling with self-absorption in H2 and hydrocarbon absorption has been performed
- Variations have been seen in response to solar wind changes
- Small south polar dark spot (enhanced acetylene, and maybe benzene?) seen in reflected sunlight near 1750 ± 75 Å and not other wavelengths
- Now getting auroral views from over the poles-with implications for what is causing the auroras: internally or externally driven?
- (probably both)

## UVIS Auroral References

- Esposito, L. W., *et al*., The Cassini Ultraviolet Imaging Spectrograph Investigation.  *Space Sci. Reviews*, 115, 299-361, 2004.
- Esposito, L. W., *et al*., UVIS shows an active Saturnian system, *Science*, 307, 1251-1255, 2005.
- Ajello, J. M., *et al*., The Cassini Campaign Observations of the Jupiter Aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph,  *Icarus* 178, 327-345, 2005.
- Pryor, W. R., *et al*., Cassini UVIS Observations of Jupiter’s Auroral Variability, *Icarus *178, 312-326, 2005.
- Nichols, J. D., *et al*., Response of Jupiter’s UV auroras to interplanetary conditions as observed by the Hubble Space Telescope during the Cassini fly-by campaign, *J. Geophys. Res*. 112, A02203, doi:10.1029/2006JA012005, 2007.