There
are two solar wind spectrometers on-board the Genesis
spacecraft: the Genesis Ion Monitor (GIM) and the Genesis
Electron Monitor (GEM). The primary purpose of these
spectrometers is to enable the collection of appropriate
samples of the solar wind by the Genesis sample collectors.
This involves determining the type of solar wind that
is flowing past the spacecraft, adjusting high-voltages
in the Concentrator for the current conditions and deploying
the appropriate Collector Array for the type of solar
wind present, all in realtime. The secondary function
of the Monitors is to obtain high-quality solar wind
data that can be used for various scientific studies.
Interested parties are referred to the detailed instrument
description paper by Barraclough et al. (2003), listed
on the publications page.
The Genesis Ion Monitor (GIM)
GIM consists of a 120° spherical-section electrostatic
analyzer (ESA) followed by an array of eight channel
electron multipliers (CEMs) for energy and angle analysis
of incoming ions. The ESA is negatively biased by a
high-voltage power supply that steps across a number
of voltage levels to build up an energy spectrum of
the plasma population. The GIM is basically an E/q analyzer
but does have a mass analysis capability in this instance
due to the similar flow velocities of all ions in the
solar wind beam. The energy range of the instrument
is ~100 eV to 14 keV, with a resolution of 5.2%, but
only a small fraction of this range is used at any one
time. Onboard software tracks the solar wind flow speed
and autonomously adjusts the energy range that is scanned
to keep it centered on the beam.
GIM has a field-of-view (FOV) that is ~3.0° in azimuth
by ~26° in polar angle and one of the narrow edges
of the FOV is aligned such that it slightly overlaps
the spacecraft rotational axis. Given this configuration,
during one spin of the spacecraft GIM sweeps out a circle
on the sky that is ~24° in radius with the center
of the circle being coincident with the average solar
wind flow direction at 1 AU. During each spin, GIM steps
forty times across ten individual energy steps and this
process is repeated for four spins of the spacecraft
with the energy steps being varied for each spin. These
four spins comprise a complete data cycle and require
approximately 2.5 minutes to complete. Thus GIM acquires
a complete measurement of the ion distribution function
every 2.5 minutes. The data product consists of ion
counts for eight polar angle and forty azimuthal angles
and forty energy levels.
The Genesis Electron Monitor (GEM)
The GEM sensor head is almost an identical copy of the electron spectrometers that are currently flying on ACE
(SWEPAM-E) and on Ulysses (BAM-E) but the electronics
are of a new design. Basically, GEM consists of a 120°
ESA that is backed by an array of seven CEMs for energy-angle
analysis of incident plasma electrons. The energy range
of the instrument is 1 to 1400 eV, the energy resolution
is ~14%, and the FOV is ~12° in azimuth (this varies
somewhat with polar angle) by ~150° in polar angle.
The center of the FOV is centered along a normal to the
spacecraft spin axis and consequently the FOV sweeps out
approximately 94% of the sky during each spin. Data acquisition
of the GEM is synchronized with that of the GIM and also
takes four spins of the spacecraft to execute. A complete
data matrix for the GEM consists of electron counts for
seven polar angles and twenty four azimuthal angles and
twenty energy levels.
Both the GEM and GIM were in continuous operation since shortly after launch in August 2001 until August 4, 2004,
a month before sample re-entry. |
