K. E. Simmons

Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Atomic hydrogen Lyman alpha radiation (121.6 nm) has been measured in emission from the atmosphere of Ganymede with the Galileo ultraviolet spectrometer. An exospheric model with the following parameters has been fit to the observational data: atomic hydrogen density directly above the surface (radius 2634 km) equal to 1.5 × 104 atoms cm−3 scale height 2634 km, exospheric temperature 450 K. A model calculation of the photodissociation of water vapor from surface ice at 146 K is used to obtain the photodissociation rate necessary to supply the hydrogen atoms that are escaping from the exosphere of Ganymede. The calculated escape flux of atomic hydrogen is 7 × 108 atoms/cm² sec. Two alternate but speculative sources of the atomic hydrogen escaping from Ganymede are photodesorption of water ice by ultraviolet photons in the wavelength range 120.5–186.0 nm and sputtering of water ice by Jupiters magnetospheric ion plasma.

The Galileo and Pioneer Venus ultraviolet spectrometer experiments - Solar Lyman-alpha latitude variation at solar maximum from interplanetary Lyman-alpha observations

Solar Ly-alpha latitude variation at solar maximum is examined on the basis of interplanetary Ly-alpha observations made during the Galileo and Pioneer Venus UV spectrometer experiments. A comparison is made of the latitude variation of the interplanetary (IP) Ly-alpha signal in 1986 at solar minimum from Pioneer Venus and in 1990 at solar maximum from Galileo. The Galileo EUV spectrometer shows that a large enhancement of the IP Ly-alpha emission occurred over the intervening four years near the solar equator. An IP Ly-alpha model is developed which considers the latitude variation of the solar Ly-alpha flux. The model fit to the data shows a 25-percent decrease of the full disk solar Ly-alpha flux from solar equator to solar pole in 1990. A detailed study of the Galileo IP Ly-alpha observations on day-of-year 190, 193, 197, and 200 in 1990 reveals that large variations occur in response to the 27-d solar variation. Analysis of these data shows that a maximum variation of 20 percent can be expected in the IP Ly-alpha upwind intensity over this 27-d period.

Latitude variations in interplanetary Lyman-α data from the Galileo EUVS modeled with solar He 1083 nm images

Observations of interplanetary Lyman-α obtained by the Galileo Extreme Ultraviolet Spectrometer (EUVS) experiment near solar maximum in 1990 show a distinct migration in the upwind brightness maximum from 36±5°N to 6±5°N ecliptic latitude over a period of several months. Our earlier models based on solar Lyman-α flux proxies appropriate for use in the ecliptic plane ( Pryor et al., 1992; Ajello et al., 1994) capture the solar rotation effects present in the data, but do not reproduce this latitude migration. We introduce a new type of model that uses He 1083 nm images to produce a proxy for Lyman-α as a function of both solar longitude and latitude. This model reproduces the observed latitude migration of the Lyman-α brightness maximum.

Galileo Ultraviolet Spectrometer Experiment: Initial Venus and Interplanetary Cruise Results

The Galileo Extreme Ultraviolet Spectrometer obtained a spectrum of Venus atmospheric emissions in the 55.0- to 125.0-nanometer (nm) wavelength region. Emissions of helium (58.4 nm), ionized atomic oxygen (83.4 nm), and atomic hydrogen (121.6 nm), as well as a blended spectral feature of atomic hydrogen (Lyman-β) and atomic oxygen (102.5 nm), were observed at 3.5-nm resolution. During the Galileo spacecraft cruise from Venus to Earth, Lyman-α emission from solar system atomic hydrogen (121.6 nm) was measured. The dominant source of the Lyman-α emission is atomic hydrogen from the interstellar medium. A model of Galileo observations at solar maximum indicates a decrease in the solar Lyman-α flux near the solar poles. A strong day-to-day variation also occurs with the 27-day periodicity of the rotation of the sun

Direct observations of the comet Shoemaker-Levy 9 fragment G impact by Galileo UVS

The Galileo Ultraviolet Spectrometer (UVS) team has detected the Shoemaker-Levy 9 fragment G impact on Jupiter in data recently played back from the spacecraft tape recorder. A 20% brightening of the disc-integrated signal of Jupiter was detected at 292 nm during a swath across Jupiter that lasted 1.6 sec and was centered at 1994-July 18 (day 199)/07:33:31 UT (all times in this paper are corrected to be the time of the event as seen from Earth). The emission brightness, when combined with simultaneous Photopolarimeter Radiometer (PPR) measurements at 945 nm, is consistent with thermal radiation at a temperature of 7800 (+500, −600) K emitted over an area of 40 (+60, −25) km². No excess signal was seen during swaths 5 1/3 sec before and after the detection swath.