; /emstar2/stewart/saturn_airglow_outline.txt
29nov00 11:00 mst
;
; outline of uvis requirements for saturn airglow observations
;
SCIENCE
The EUV and FUV airglow and aurora of Saturn are dominated by HI
Lyman-alpha and the Lyman and Werner bands of H2. A weak emission of
great interest is HeI 584A. The brightness of these emissions depend
on solar and auroral activity and also on homopause/thermospheric
dynamical conditions. The 'electrglow' phenomenon also depends on
magnetic and electric field conditions. In addition, airglow scale
heights are a proven indicator of thermospheric temperatures. The
airglow and aurora thus carry the imprint of many processes of great
scientific interest.
OBSERVATIONS
The goal is to observe the airglow and aurora in a systematic way,
exploring its spectral, kronographical, and temporal characteristics
and variations. Whole-disc observations can be made from relatively
large distances (20-50 Rs), at phase angles allowing full coverage of
the sunlit and dark disc and of ring-shadowed regions. High spatial
resolution is needed for limb-profile and some auroral studies, and
require observations from 5 Rs or less. Studies of the atmosphere's
response to solar activity and solar wind conditions calls for a
program of regular observations at intervals smaller than the sun's
25-day rotation period.
These goals can be accomplished using four basic observation types:
1) full-disc spectral images
2) N-S and E-W disc maps
3) 'Limb Skims' to observe airglow layers
4) full-rotation 'stares'
In all cases the EUV and FUV spectra are recorded at full spatial
resolution (1 mrad). Spectral binning is used to reduce data rates and
volumes.
ACTIVITIES DESCRIPTIONS
1) Full-disc Spectral Images
The UVIS fields of view are passed across Saturn's disc in a direction
perpendicular to their long axes. Spatial resolution in one dimension
is provided by the 60 spatial elements along the slits, and in the
other dimension by suitable integration periods. Good signal-to-noise
on the strong H and H2 emissions is obtained with a 60-sec integration
period. The apparent angular velocity of Saturn should be 1 mrad/min.
The number of passes across the disc depends on the angular size of
Saturn. From 40 Rs, Saturn subtends 50 mrad and one pass is sufficient.
From 20 Rs, two are required. The spatial resolution achieved at the
sub-spacecraft point are 2400 and 1200 km respectively.
2) Disc Spectral Maps
Disk Spectral Maps replace Full-Disc Spectral Images when the angular
size and angular velocity of Saturn make the former impracticable.
A single pass across Saturn's disc is made, again with an apparent
angular velocity of 3.5 mrad/min. At 10 Rs this will last 1 hours. The
map will be limb-to-limb, and at the sub-spacecraft point it will be
about 0.6 Rs wide with spatial resolution of 600 km.
3) Spectral Limb Skims
To adequately define the airglow limb profiles, measurements should
be made over a signal range of x100 with signal-to-noise everywhere
10 or greater, and with spatial resolution no greater than an airglow
scale height. The large slant ranges to the limb (> 2.5 Rs), the
small scale heights (about 150 km for the H2 emissions), and the large
spacecraft velocities (15-18 km/sec below 5 Rs) render these
requirements impossible to reach with the classic limb-profile
observation, in which the instrument slit is parallel to the limb and
is carried downward by vheicle motion or by platform slew.
Good observations are achievable with a 'limb-skim' technique. The
fov is carried along a line tangent to the limb at an altitude just
below the airglow peak; the slit is parallel to this line. The rate
of motion of the slit in altitude is now much less than the S/C orbital
velocity, and the dwell time within a given altitude interval is much
greater and produces a much larger signal-to-noise. The observing
requirements stated above can be met for the HI and H2 emissions.
The limb-skim will last 1-2 hours.
4) Stares
The longitudinal dependence of airglow and auroral emissions will be
studied using a stare mode. The range will be about 40 Rs, with the
UVIS slits laid along Saturn's rotation axis. Duration will be 11
hours, allowing for a full rotation.
RANGES, ORIENTATIONS, DATA RATES, DURATIONS, DATA VOLUMES, ETC
Obs. type
Range
bps
durn
dat vol
frequency
Image, 1-pass
40 Rs 5200 1 hr
19 Mb 2 per orbit
Image, 2-pass
20 Rs 5200 4 hrs 76 Mb 1 per orbit
Limb Skim
< 5 Rs 5200 1 hr
19 Mb 1 per orbit
Disc map
~ peri 5200 2 hrs 38 Mb 1 per orbit
Stare
any
1300 11 hrs 52 Mb 1 per 5 orbits
Total Data Return
182 Mb per orbit
Notes: xbin = 4 for all observations
ip = 1 min for all except Stare
ip = 4 min for Stare