Science
Radiation Detection
The
star scanner is measuring ~5 MeV electrons in the Jovian environment. This is
known by:
1) A very good correlation
with real time data from the B1 and DC3 channels of Galileo’s Energetic Particles
Detector. These channels measure 1.5 to 10.5 MeV and >11 MeV electrons,
respectively.
2) Several analyses of the amount of the shielding around the Star Scanner’s photomultiplier tube and lenses suggest that ~ 5 MeV electrons are capable of penetrating there.
3) Qualitative comparison of the star scanner data with various instruments on the Pioneer and Voyager spacecraft.
The
star scanner is able to distinguish the following features:
Ø The enriched flux of
electrons trapped at the Jovian magnetic equator.
Ø The decreased flux of
electrons caused by sweeping from the moons Europa, Io, Amalthea and Thebe.
Ø The decreased flux of
electrons caused by absorption onto ring particles.
The figure below is an example of the sort of data
the star scanner can provide.
Star Scanner Data for the I27A Orbit. The count rate
is the star scanner’s native unit uncorrected for several small instrument biases.
C/A is the time of Closest Approach to Io. MEC is the time of the predicted
Magnetic Equator Crossing. RJ = Jovian Radii = 71,492 km.
The Star Scanner and
EPD
There
are two instruments that are capable of measuring high energy electrons in the Jovian
environment, the Star Scanner and The Energetic Particles
Detector (EPD). These two compliment each other quite well. For many
purposes, EPD is the superior instrument. It provides a faster sampling of the
data, is able determine the direction of the particle flux and was intended to
be a science instrument from the start. It covers a range far out beyond 14 RJ
where the star scanner sees only noise. However, the star scanner can take good
measurements inside 8 RJ where the EPD has often been turned off to protect its
detectors. The star scanner has an enormous benefit in that it is always on -
even during a spacecraft safing when all other instruments have turned off.
Since the two instruments are independent, each can provide the service of
confirming what the other is seeing.
Data
The
data sets on the PDS include time, twist angle, latest sighted star intensity,
latest radiation measurement, star scanner status word, clock angle of the star
scanner, spacecraft distance in from Jupiter, magnetic latitude, longitude and
L-shell distance, planetocentric latitude and longitude.
Data
generally come down at one sample per 400 seconds as determined by the overall
telemetry downlink rate. Higher sample rates of one per 80 seconds, 20 seconds
and 0.667 seconds are also seen at various times.
Data Limitations
Ø The Star Scanner was not
sending down radiation data in its telemetry stream until shortly before
Galileo’s entry into Jupiter’s orbit in December 1995. Thus, there is no data
from the Earth, Venus or asteroid fly-bys.
Ø With the exception of the
C22 orbit, the Star Scanner becomes insensitive to electrons outside of 14 RJ.
The best quality (low noise) data is inside of 10 RJ.
Ø The star scanner is
providing omni-directional measurements – no pitch angle information is
available in the data sets.
Ø Although a complete search
has not been done, many thousands of hours of data taken well away from Jupiter
have never shown evidence of a solar flare or magnetospheric effects such as
bow-shock crossings. This is not a surprise as the associated electrons are not
expected to have sufficient fluence and energy to be seen by the star scanner.
Calibration
The star scanner is calibrated at each orbit by observing stars of known intensities to obtain an internally valid calibration. Preliminary conversion of star scanner counts to omni-directional electron flux is available and more rigorous conversions are close to completion.
The star scanner power has held constant through-out the mission and the internal temperature has been held to 10.3 +\- 2.1 oC through-out operations at Jupiter. The major instrument biases are a gradual aging of the photomultiplier tube and a quick decline in sensitivity due to radiation damage taken at each orbit. Both effects can be largely corrected for.
Data Availability
Star
scanner data within 25 Jovian radii (RJ) of Jupiter is available to the public
through NASA’s Planetary Data
System at the Plasma Planetary Interactions node. Data may be used freely.
Published articles will contain an acknowledgment of NASA, JPL and the
Principal Investigator as the source of the data.
The
star scanner data inside 4 RJ taken during the recent A34 encounter are still
being reduced, but appear to roughly match previous measurements by the Pioneer
spacecraft [1,2]. The values reported [18]
are somewhere between that modeled by Divine and Garrett [3] and recent but indirect
and remote measurements by
