The RHESSI detector counting rates are made up of several components in addition
to the contributions from solar X-ray emissions. Consequently, the
interpretations of variations in the counting rates can sometimes be confusing.
This list is intended to describe the various effects that can result in
artifacts in the light curves of detector counting rates versus time. They
include effects caused by cosmic rays and trapped charged particles passing
through the detectors and surrounding material, instrumental effects, and
variations that are only seen during solar events. These descriptions are
intended to aid the user in differentiating between these various effects and in
identifying those variations that can be used to provide information on the
solar flare X-ray and gamma-ray emissions.
Artifacts from Charged Particles
Pseudo-sinusoidal variations with a period of about 45
minutes or half an orbit.
These variations result from the variations in the cosmic ray flux of charged
particles as the magnetic cutoff rigidity changes with geomagnetic latitude. The
cutoff rigidity is lowest at the highest north and south geomagnetic latitudes
reached by the spacecraft on each orbit. Thus, the detector background counting
rate is lowest over the equator and highest at high north and south latitudes.
The amplitude of this effect is about a factor of two, varying somewhat with
energy, and the spectrum is very hard.
Gradually varying increases in rate at intermediate southern
latitudes.
These are caused by high particle fluxes when the spacecraft gets close to the
South Atlantic Anomaly (SAA). The spectrum of these increases is very hard.
More rapid increases in counting rate at intermediate to high
north and south latitudes.
These sometimes occur at magnetic conjugate points in the spacecraft orbit that
are on the same magnetic shell. They are caused by precipitating electrons that
produce a very hard spectrum. They can last for as short as a minute but are
usually quite symmetric in time.
Instrumental Artifacts
Shutter In/Out
There are two thin disks of aluminum for each detector that can be moved between
each of the nine lower grids and the corresponding detectors to attenuate the
flux of lower energy photons. One of the disks is thinner than the other and it
attenuates the flux at energies below about 25 keV. The other thicker shutters
attenuate the flux to energies as high as 70 keV. The attenuators are generally
under onboard software control. During periods of low solar activity, both
shutters are moved out of the fields of view of all detectors. When the rate
increases during a flare and the dead time reaches a predetermined level like
15%, the thin shutters are moved into place in less than a second. If the rate
continues to increase to a second predetermined level, then the second thicker
set of shutters is moved into place, further attenuating the flux. Once in
place, the shutters cannot be removed for a fixed length of time of the order of
5 minutes. The shutters are removed in reverse order as the flare rate
decreases.
Data dropout.
This is manifest as periods as long as a second or more in which no counts are
recorded from a particular detector. This is believed to be caused by charged
particles passing through the detectors and producing large pulses that saturate
the electronics. The pulse height analysis is designed to provide the highest
possible energy resolution and hence shuts down when this happens. It does not
allow any further photon to be analyzed until it is fully recovered and can
perform the analysis with the required energy resolution. During this dead time,
no photons can be handled and so the detector is dead for that length of time. A
software correction is applied by default to allow for this dead time in
calculating the incident photon flux, but when short time intervals are used the
data dropouts will show up as zero fluxes for individual detectors.
Decimation
When the rate becomes too high or the onboard solid-state recorder (SSR) becomes
too full, the photon counts are 'decimated'. The front and rear detector
segments are decimated separately. For each, there is a decimation level -
the fraction of counts that will be removed, and a decimation energy - the
energy threshold below which counts will be removed. In practice, a given default
decimation level is applied to prevent the SSR from becoming too full and the
decimation is increased in steps if a flare occurs or the SSR begins to fill at
an unexpectedly high rate. Since decimation is a digital process (unlike the
rate reduction produced by the mechanical attenuators), the effect can
be accurately accounted for in the software and the user should not have to worry
about it. The only noticeable affect should be the reduction in the number of
counts recorded and the consequently larger statistical uncertainties in the
counting rates.
Sudden increases in detector 8 counting rate lasting for
several minutes.
These are caused by electrical interference from the aft antenna that is
situated very close to the detector 8 electronics.
Artifacts in Solar Flare Light Curves
Fast Modulation
This modulation is produced by the two grids above each detector as the source
appears to move in the rotating spacecraft coordinate system. It is the
modulation that allows images to be reconstructed. It can be clearly seen with a
moderate M-class flare in detectors 5 – 9 by plotting the counting rate with
0.1-s time bins.
Modulation in counting rate with half the spin period.
This arises during solar flares when the source is away from the spin axis and
is the result of the finite field of view through the grids above each detector.
All collimators except for those above detectors 7 and 8 have about a 1-degree
field of view or smaller in a direction perpendicular to the slits but much
greater than this parallel to the slits. Thus, as the source appears to move in
the rotating spacecraft coordinate system, the transmission through the grids is
modulated with twice the spin period. The phase of this modulation is different
for each detector and depends on the orientation of the slits. The amplitude of
the modulation depends on the offset of the source from the spin axis.
Modulation in counting rate with a period of ~75 s.
This can be particularly noticeable during the steady decay in counting rate after a large flare.
It is caused by the offset of the instrument imaging axis from the spacecraft spin axis.
The amplitude of this modulation
is largest for detector #5 light curves because of the relatively large offset of the axis of the bi-grid
subcollimator in front of this detector from the imaging axis. This whole issue
of recovering the true flare light curve from RHESSI count-rate measurements is
discussed in a paper by Zimovets et al. (2010).
Earth Albedo
This manifests itself during a big flare with a reasonably hard spectrum as a
modulation in the counting rate during a flare with a period equal to the spin
period. It results from the Compton scattering of the flare X-ray and gamma-ray
photons in the Earth’s atmosphere.
Day/Night Transitions
The Sun is readily detectable at the lowest energies covered by RHESSI down to 3
keV. A significant increase in rate is observed in the lower channels when the
spacecraft comes into sunlight each orbit. The magnitude of the increase depends
on the soft X-ray flux from all the active regions on the Sun at that particular
time. The spectrum of this flux is very soft corresponding to thermal emission
from plasma at a few million Kelvin.
Non-solar Events
Last updated 15 December, 2010 by Kim Tolbert, 301-286-3965. Content by Brian Dennis.