Variations of the auroral emission from Ios atmosphere

Variations of the auroral UV emission from Ios atmosphereLorenz Roth*J. Saur*, P.D. Feldman, D.F. Strobel, K.D. Retherford

*Institute of GeophysicsUniversity of Cologne, Germany

Talk about variation of the morphology of the UV aurora of the Jovian moon Io.I should better say the non-variation of the UV emission.1Observations of Ios auroral emission

Galileo SSI

visible 3 filters380 - 710 nm

Geissler et al., 1999Cassini ISS

near UV bluevisible red

Geissler et al.,2004

New HorizonsLORRI

visible - UV350 - 850 nm

Roth et al., 2011

HST / STIS

UV135.6 nm

Roesler et al., 1999Here are some of the previous aurora observations

Roessler (Januar 99): first resolved images of Ios aurora, installation of STIS camera on HASTGeissler 99 (Juni 99): visible -> in Eclipse! Spots close to the equator were associated with fixed geographic locations, did not vary with magnetic field.Geissler 04: emission at different wavelength in the visible range could be associated with emitting species and densities of these species have been derivedRoth et al: Huge Tvasthar plume radiating apart from equatorial emission features. Using an interaction model we could derive a column density in the plume for the first time that allows a pretty homogeneous plume emission2Generation of Ios auroraExcitation of Ios atmosphere by electron impact

Atmospheric properties:- column density of N (1-5)1016 cm-2 (sunlit) (Lellouch et al., 2007)- main constituent: SO2- various minor components: O, O2, S, S2, SO, Na, K, Cl, NaCl

Electrons in the plasma torus:- thermal electrons:kTe = 5 eVne = 1200-3600 cm-3- non-thermal electrons: kTe keV

The aurora as tool to find properties of:- atmospheric distribution, composition and density- plasma conditions- magnetic field configurationJust to remember what it seen in the aurora observations

Ios atmosphere is denser around the equator with column densities of ~10^16 cm^-2SO2 by far the main gas component in the atmosphere,But the atmosphere is pretty species-rich, we have O, S ....Sodium Potassium Chlorine, sodium Chloride

Exciting electrons probably are primarily the thermal electrons in the torus3HST / STIS observations of Ios UV aurora50 exposures during 26 orbits between1997 and 2001

2 exposures in eclipse

40 exposures containing OI 1356 emission- the clearest of emissions imaged

19 combined observation images(2 exposures during one orbit)

2 x better signal-to-noise ratio(after Retherford, 2002)

Overview of all observation campaigns, where Ios UV aurora has been observed by STISThe early images from 97 and 98 were mostly taken wenn Io was at western elongation, butAltogether the full orbit is fairly well covered by all observations campaigns.

OI 1356 is the clearest of all images emissions, Which means there are no other emission lines at wavelength too close to 1356, which could possibly distort the morphology or intensity.4Morphology of theOI 1356 emission

Morphology onlydepends on the: 1. observing geometry 2. tilt of the background field 3. position with respect to the torusIn this sketch we took all the 19 images and placed them at the position in Ios orbit, where the moon was during the observation.

The most images were taken, as I said at western elongation and they show the probably best known and most analyzed morphology with the two bright spot that by far exceed all other emission.If we follow now Io in its orbit around jupiter, we can see that the sub-jovian spot becomes brighter and more extended, whereas the anti-jovian faints and vanishes.The next images is the one taken during eclipse and shows a less distinct morphology. Then we see the sub-jovian spot wandering to the right limb and the anti-jovian re-appears on the left limb.The 3 images close to noon again show a stronger background on the disk, but the spots are still intentifiable.

-> all in all the emission profile appears to be constant in a way and the observed morphology might only depend on the 1. 2. 3.5Model of the 3D emission distributionGoal: Find a simple function that is able to describe the 3D emission profile around Io for all STIS observations with as few parameters as possible

Main aurora features:- Equatorial spots- background emission- limb glow (north - south differences)

Construction of 2D model images by integrating along line-of-sight

Comparison between model images and 18 combinedSTIS OI 1356 observations (Io in sunlight)So, we might assume that atmospheric variations due to e.g. plumes or changes in the plasma environment expect those, we just mentioned, DO NOT influence and aurora morhology.So, our idea was to find a analytical function that describes the 3D emission distribution.By integrating along the line-of-sight, we are then able to construct model images, which we will compare to all 19 observation images6

Background emission:

Emission on the flanks (equatorial spots):

Rotate the 3D profile into the B field coordinate system Tilt determined byBrightness proportional to distance to torus center ( )13 fit parameters (in red)6 areas: - off-disk emission / on-disk emission - anti-Jovian spot / sub-Jovian spot (9x9 pixels) - limb glow North / South + position of the equatorial spotsFind least squares for all 18 observations:

Aurora model

Here is the function, which we use to construct the model images. It constits of two parts.First a kind of global background emission, that peaks at the surface and exponentially decreases with height. Here we also implement a north south difference depending on Ios position in the torus during the observation.It is rather mathematical than a physical equations. Apart from the first term in the background emission, which reminds of a exponentialy decreasing isothermal atmosphere.There is no physical reason for e.g. using the hyperbolic tangent hereThe Second part describes the enhanced flank emission, it strenght and extension in all directions.

Since the spots follow the variations of the background field somehow we rotate our emission profile into the background field about a certain angle, which is lower than the angle between B and the polar axis.B field determined with VIP4 model, Connerny et al. 1998

We also implement that the total brightness is proportional to the distance of Io to the torus center und thus the density of the plasma.

The constructed function depends now on 13 parameters, which we fit to get the best agreement with all 18 images with one parameter set.Therefore we compared the emission intensity in 6 areas: .....The spots locations where set to the area of the 9X9 brightest pixels in both model and observation images.We also compared these automotatically determined locations.By finding the least squares, we got our best-fit parameters.7

Observations - Model 3 Examples.upper left images: limb glow: South bright, north lowupper right imeage: Background brighter when spot on_disk and not on the limb.Middle right image: both spots, disk fainters

8ResultsAurora model is able to reproduce the main features of the morphology observed over 5 years (1997 2001)

Tilt of aurora ~ 80% of tilt of background fieldSpots:- longitudinal extension: 22- spot center shifted downstream from sub-/ anti-jovian meridian: ~12 / ~4- Distance of center to surface: ~ 50 km- brightnesses and extensions similar!Eclipse observation:- faintest emission of all STIS observations- but: also faintest modeled emission- sub-Jovian spot appears smaller and further upstream

Most important: Our model is able to reproduce the important features of all STIS observations.Plots:Off_disk / on_ diskLeft / rightNorth limb / south limbThe data is taken over a period of almost 5 years and regarding the volcanic activity, which is supposed to be highly variable, it might be relatively surprising that there are no major variations in the aurora.We certainly cannot infer a non-variable atmosphere since the correlation of gas density and aurora brightness is highly non-linear, but we can constrain the atmospheric varations to a certrain degree.

The advantage of the model is furthermore , that it is possible to seperate aurora variations, which can be reduced to geometrical changes from the real changes in the emission.

For example : Eclipse: Faintest images of al stis images but also modelled intensity is the lowest of all.Funktioniert bis zu nem gewissen Grad!!!!Daten ber 5 Jahre reproduziert!!aurora-Antwort auf Atmosphre nicht-linear:Atmosphren variation nicht klarWarum modell funktionert ist spekulation

Mit Model ist es mglich die Beobachtungs- und die Umgebungsgeometrie aus den Daten rauszurechnen und so andere Variationen zu bestimmen

Off-disk emission strongest at Eastern /Western elongationNorth/south differences clearly follow the position in the torus (thermisches plasma stims!)

Keine Strahlung im Schweif notwendig

Tvashtar: (raus)- 6 STIS observations while Tvashtar was active- No statistically significant emission enhancement around Tvashtar in the UV emission!

Tvashtar Aktivitt von November 99 bis Februar 01 beobachtet

9SummaryWe examined 18 HST/STIS observations of the OI 1356 emission from Io`s atmosphere taken between 1997 and 2001

Assumption: Morphology of the OI 1356 aurora only depends on the:1. observation geometry2. tilt of the background field3. position of Io with respect to the torus

We established a phenomenological model forthe 3D emission distribution around Io thatreproduces the observed aurora morphology.Only weak variations of the3D emission profile between 1997 and 2001.

Comparisons of other (new) observations with the aurora model

(Roth et al., Simulation of Ios auroral emission, Icarus, available online)

A quick summary:

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