Photometric Observations of Interstellar
Hydrogen by HDAC:
In-flight Calibration and First Results
Horst Uwe Keller1, Yuri Skorov1, Olga Katushkina2, Vladislav Izmodenov2,
Ralf Reulke3, and Karl-Heinz Glassmeier1
1
Institut für Geophysik und extraterrestrische Physik, Universität Braunschweig, Germany.
2Lomonosov Moscow State University, Russian Federation.
3 Institut für Informatik Computer Vision, Humboldt-Universität zu Berlin, Germany.
Fig. 1. The Cassini hydrogen and
deuterium absorption cell (HDAC)
instrument
Fig. 2. Saturn position from 2001 to 2017
relative to the upwind direction of interstellar
hydrogen (arrow).
Abstract
HDAC is part of the ultraviolet imaging spectrometer (UVIS) onboard the Cassini spacecraft. The instrument scans the Lyman-α
emission lines of hydrogen and deuterium atoms. In the photometer mode only the CEM detector is used to register the signals within
a 3 degree field of view. HDAC has been switched on in photometer mode most of the time producing a unique continuous data set for
more than a decade. An analysis of the Lyman-α background data serves two purposes: determination of the parameters of the
interstellar/interplanetary hydrogen and determination of the properties of the solar wind. The exhaustive pre-flight laboratory
calibrations included evaluation of the absolute sensitivity of the instrument; evaluation of the instrument spectral sensitivity; evaluation
of the off-axis response. During the mission these characteristics may change over time due to continuous time degradation of
electronics and/or abrupt events. For example, three dramatic sensitivity breakdowns were observed in 2001. Thus the only chance to
determine the current sensitivity of HDAC is to make in-flight comprehensive evaluation, e.g. measuring known fluxes from stars or
other bodies. We systematically analyzed photometric observations of the star SPICA in order to perform in-flight calibrations. All three
aspects listed above were explored. We found that the instrument is still in good condition. The current sensitivity of 12
count/s/Rayleigh is still sufficient to provide good signal to noise data. Off-axis responsivity is non-uniform and visibly differs from preflight determinations. At the same time the shape of the spatial sensitivity response is constant and can be used for all observations.
The calibrated data are compared with sophisticated theoretical models describing the spatial distribution of interstellar/interplanetary
hydrogen. First results will be reported.
HDAC
The Cassini hydrogen and deuterium absorption cell (HDAC) instrument which was built at the Max-Planck-Institut für Aeronomie
consists of a Channel Electron Multiplier (CEM) photo detector equipped with 3 absorption cell filters: a hydrogen cell, a deuterium cell,
and an oxygen cell. The cells are separated by MgF2 windows (Fig. 1). The oxygen cell was designed to be a static broadband filter
from 1150 to 1800 Å but was unfortunately vented prior to flight due to contamination caused by water vapor accumulating in the O2
cell. The hydrogen and deuterium cells function as adjustable absorption filters. When the absorption cells are not switched on HDAC
functions as a sensitive photometer and is used to observe the Lyman-α emission of the interstellar/interplanetary hydrogen atoms.
HDAC is part of the Cassini ultraviolet imaging spectrometer (UVIS) built by the university of Colorado (Esposito et al. 2004).
Observations
The interplanetary Lyman-α signal varies between about 200 to 400 Rayleigh. HDAC has been switched on in photometer mode most
of time during the Cassini mission producing a unique continuous data set for more than a decade. An analysis of the Lyman-α
background data serves two purposes: i) determination of the parameters of the interstellar/interplanetary hydrogen, such as its
density, flow velocity, and temperature, and ii) determination of the properties of the solar wind, such as the latitudinal dependence of
its flux and speed, as well as the solar EUV emission. The long term HDAC observations from within the Saturnian system provide
unique views from different positions mostly from outside the hydrogen cavity around the sun (Fig. 2). The strongest contributions to
the Lyman- α signal in the upstream direction come from a distance of about 5 AU, while downstream the cavity extends to about 20
AU. The variation of solar-wind flux and speed with heliocentric latitude studied in detail by SOHO (Bertaux et al., 1997) leads to
deviation from the cylinder symmetry. The hydrogen density is not only influenced by solar Lyman-α radiation pressure and ionization
but also by charge exchange with the protons of the solar wind. It should be noted that the deviations from the isotropic (spherical solar
wind) model are maximum at a distance of about 10 AU (Bertaux et al.(1995), Izmodenov et al. (2009)), the heliocentric distance of
Saturn. Elaborate models predict the 3D brightness distribution of interplanetary Lyman-α, and for the long observational set of HDAC
numerous physical parameters and their variations with solar cycle can be derived. Such an analysis has already been successfully
carried out using observational data provided by the UV spectrometer onboard the Galileo spacecraft (Ajello et al., 1994), the Voyager
1/2 UV spectrometer (Quémerais et al., 2010), and the SWAN/SOHO H cell (Quémerais et al., 2008), and by the Nozomi spacecraft
(Nakagawa et al. 2009). Almost all these observations were made from within the cavity (near the earth). The HDAC observations map
the Lyman-α brightness consistently over several years from outside the cavity and therefore provide a unique check on the models
(Katushkina and Izmodenov, 2011, 2012)). The sensitivity of the photomultiplier decreased since insertion of Cassini into Saturn orbit.
The current value of 12 ct/R/s is still sufficient to provide data with good signal to noise (Fig. 3).
Calibration
The photometric sensitivity of HDAC has been calibrated in flight with regular observations of the star SPICA. The relative spatial
sensitivity varies across the circular 3 degree field of view (FOV) of HDAC as demonstrated by Figs. 4 and 5. This detail is only
important for the removal of star contributions since it has stayed constant. The final calibration is still ongoing. Therefore we only show
relative brightness variations.
Data
The complete data set of HDAC includes observations during the 7 year voyage of Cassini to Saturn. More interesting because unique
are the observations after orbit insertion of Cassini around Saturn in June 2004. HDAC is expected to continue operations to the end of
the Cassini mission 2017. When the Cassini spacecraft is sending data back to earth it rotates around the Saturn – Earth direction
and the FOV of HDAC rotates in the plane perpendicular to this direction. Figure 6 shows an example of the pointing directions of
HDAC (longitude and latitude) and the corresponding periodic raw signal (stars not removed). The signal to noise of the data can be
improved by adding the signals received during one data transmission block.
Fig. 4. HDAC off-axis response observed Fig. 5. Variations of off-axis response
from SPICA Raster Scan
in the central part of HDAC FOV
Fig. 3. HDAC Sensitivity between
2003 and 2010
Fig. 6. Example of data set and orientation
angles during sending data to earth.
Figs. 7-9. The HDAC observations of hydrogen in the photometric mode (stars). A preliminary comparison with the 3D theoretical
model (blue curve). 2 January 2011 – left, 27 February 2011 – center, 5 May 2011 – right panel. All variables are in dimensionless
units.
First Comparison with Model Calculations
In a fist attempt we compare HDAC observations with model calculations of the interplanetary background. Based on a recently developed
model for the interstellar H atom velocity distribution function in the heliosphere (Katushkina and Izmodenov, 2011a) the model takes into
account both global effects of H atom perturbations at the heliospheric boundaries and local (i.e. within 10–20 AU from the Sun) effects of
the solar ionization, charge exchange, solar gravitation and radiation pressure (Katushkin and Izmodenov, 2012). The observed brightness
was adjusted to the model before the rotation of the spacecraft started (staring in a constant direction) disregarding absolute calibration of
the HDAC counts. The variation of the brightness along the circle of rotation is well reproduced. The peak to peak variation flattens out in
the 5 months interval from January to May 2011 when the direction of the Saturn – Earth direction changed only by about 5 degree (see
Figs. 7 to 9).
Coming Activity
The data set needs careful screening to eliminate signals from UV stars. Then the data will be fully calibrated taking into account all the
observations (and calibration targets) and cross-calibrated with the Lyman- α channel of UVIS. This will improve the reliability of both
instruments. Once the data set is fully prepared we will compare the observations with models of the hydrogen flow in detail to constrain
the parameters of the hydrogen atoms and solar emission.
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