0----- Attempting Submission 1 (Tue Jul 12 12:55:00 GMT 2011) -----Many previous observations of Saturn have been successfully obtained with these exposure parameters.Long term observations of Saturn's northern aurorasAuroral emissions are a vital tool in diagnosing the dynamics of planetary magnetospheres. While Saturn|*|s southern UV auroras have been observed with high-sensitivity cameras onboard the Hubble Space Telescope (HST), the northern auroras have only been observed at very oblique angles. Our understanding of Saturn|*|s auroral emissions is thus only half complete. However, Saturn has now passed equinox and is moving toward summer in the northern hemisphere, such that the northern auroras are now visible from Earth, and recent results from HST have indicated that Saturn|*|s northern auroras are not simply mirror images of the southern. The changing seasons are also expected to result in significant changes in magnetospheric phenomena related to the auroras. Observing these changes is a specific goal of the Cassini Solstice Mission (CSM) and, since joint HST-Cassini observations have repeatedly proved to be invaluable, CSM operations are currently being planned specifically with joint HST observations in mind. The observations proposed here will thus execute over Cycles 18-20, and will address the following science questions:
What is the morphology of Saturn|*|s northern auroras?
Do Saturn|*|s auroras change with the planet|*|s season?
How are the auroral emissions of different wavelengths related?
The importance of long term HST observations of Saturn|*|s northern auroras are highlighted by the fact that recent key discoveries would have been missed without the multiyear archive of observations of the planet|*|s southern auroras. The opportunity to obtain HST images while Cassini makes specifically-tailored supporting observations is an extremely valuable opportunity, and HST is the only instrument capable of providing sustained, high time resolution observations of Saturns auroral emission.These observations will be obtained with the Solar Blind Channel (SBC) of the ACS. The field of view of the ACS/SBC is large enough to encompass the whole disc of the planet plus a significant portion of the rings. The observations could, however, also be obtained with the STIS/FUV-MAMA instrument if the ACS is not functioning, but in practice we prefer to use the ACS, since the sensitivity using e.g. the F125LP filter is |*| 1.5 |*| 10|*|3 counts pixel|*|1 s|*|1 for 1 kiloRayleigh of H2 plus Lyman-|*| emission, versus |*| 0.5 |*| 10|*|3 counts pixel|*|1 s|*|1 for the STIS using the equivalent F25-SrF2 filter. Higher count rates are desirable since planetary rotation introduces |*|1|*|/100 s of blurring of features near the central meridian longitude. Previously measured count rates are |*|25,000 counts s|*|1, well below the limit of 200,000 counts s|*|1. We will use the F115LP filter during the Low Sky portion of the orbit and the F125LP filter outside of this in order to reduce the Ly-|*| background contamination from the geocorona. For each set of observations, we will also obtain a set of images using the F140LP filter in order to provide the background reflected sunlight for subtraction. We will obtain 100 s exposures, a length experience shows is sufficient to obtain a good signal-to-noise ratio. We request 30 orbits over 3 cycles divided as follows: 5 orbits in Cycle 18, 10 orbits in Cycle 19 and 15 orbits in Cycle 20. This increasing number of orbits per year reflects the increasing view of the northern auroral region as shown in Fig. 1. Observations will be obtained near opposition each year, with pre-opposition providing the best viewing angles due to the Earth|*|s orbital motion. In the past, Saturn observations have been successfully obtained in both 2-gyro and 3-gyro mode.SOLAR SYSTEM12falsefalsefalsefalse0.00.0SUPPORTEDSOLAR SYSTEMPLANETPLANET SATURNSTD=SATURNSEP OF SATURN RHEA FROM EARTH GT 10.0",
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