HRC Rates and High Solar Activity
Both science instruments (SIs) on Chandra are sensitive to the particle
radiation environment generated by high solar activity. On-board
monitoring of data from the EPHIN provides for an autonomous
safing of the SIs in the event of a severe solar flare or coronal
mass ejection. Ground monitoring of data from ACE and GOES allows
us to monitor for conditions under which we would desire to safe
the SIs but for which the autonomous safing is not likely to be
activated, possibly because EPHIN is not sensitive to the energy
of the particles that are a concern or the levels are persistently
just below the trip level. We have guidelines on the rates at
which we would consider stopping operations to safe ACIS but lack
such guidelines for HRC operations. Guidelines for HRC usage
should:
- Provide rate thresholds at which we would consider halting the
science mission in order to safe the HRC
- Provide rate thresholds at which we could resume the science
mission with HRC observations
One benefit of such guidelines might be that the time that would be
lost to ACIS observing due to high solar activity could be
recovered with HRC observations. The primary concern in developing
guidelines must be instrument safety.
HRC safing
The only known issue with operation of the HRC in a high radiation
environment is that the total rate in the MCPs may be so high as
to result in damage from "charge extraction". Charge extraction is
only an issue for MCPs at an operational HV level. We expect
the particle radiation to be distributed semi-uniformly over the
area of the MCPs and that each particle interaction will generate an
event. Laboratory measurements have shown that when
approx. 3×108 pC cm-2 of charge is extracted
from the MCPs, the modal gain will drop by approx. 10%. The charge
extracted per event is roughly 5 pC; so, we would damage the MCPs
once the particle fluence at the MCP reached approx.
6×107 cm-2. Budgeting at 10 solar events per
year over a ten year mission, we could allow a fluence at the MCPs
per event of 6×105 cm-2. We would have
to operate at an average sustained rate in excess of 1500 events
s-1 on the active detector for the 8 hours between
contacts to accumulate this exposure. Since this is far above the
telemetry saturation rate of 184 events s-1, it is
extremely likely that we will have stopped collecting useful
science data before we have reached this limit.
Translating the MCP rate limit to an external particle flux depends
on the spectrum of the particles and the shielding provided by the
observatory structures. Observations of the
quiescent background have shown a
correlation between the MCP total event rate and the EPHIN
Integral channel (electrons with E > 8.7 MeV, protons or ions
with E > 53 MeV/nucleon). Using the observed correlation, we
see a rate in the MCPs of 1500 events s-1 at an INT
channel flux of 1.9 particles s-1 cm-2
sr-1. It may be possible to use the higher-energy GOES
proton channels as a monitor during non-contact times.
There are times when the MCP rate flares above the quiescent rate,
however. This flaring background occurs during times when the
lower energy particle fluxes as measured by the EPHIN are also
flaring, although no detailed correlation with one of the EPHIN
channels is apparent. A comparison of the spatial distribution of
the HRC-S background during quiescent and flaring time periods of
an orbital-activation observation of Capella revealed a "shadow"
from the additional aluminum in the "T" over the central MCP
segment during the flares. Given these two observations, the
flaring background is most likely due to low-energy protons that
come through the HRMA to the focal plane, possibly the same
particles that lead to the CTI increase in ACIS. We have no
on-orbit data that can be used to determine what external
(e.g. ACE EPAM) low-energy proton flux measurement at which we
should safe the HRC.
Resuming observations
After an autonomous or ground commanded safing of the SIs we wish to
restart the science mission as soon as the radiation environment
permits. Since the particle flux given above at which we must safe
the HRC is so much higher than the level at which we expect to be
able to gather useful science data, we should use the latter as a
guide to resuming the science mission. On-orbit experience should
provide a starting point in determining when we might reasonably
resume HRC observations.
Figure 1 shows an example of HRC operation during a time of moderately
high solar activity; it shows the hourly averaged ACE EPAM P3 flux
and the HRC total and valid MCP and anti-coincidence shield
rates. The observation was performed a few days after the 2001
April 3 X20 flare which autonomously safed the SIs. At this time
radiation levels had dropped to a level at which is was deemed
safe to resume the science mission. During the HRC observation the
ACE EPAM P3 flux averaged 2.5×104 cm-2
s-1 sr-1 MeV-1, which is 1/2 the
average flux of our 2-hour fluence alert level for ACIS.
 |
| Figure 1: ACE EPAM P3 average hourly flux and HRC rates
as a function of time on 2001 Apr 6-7. The HRC-S/LETG
was used in a calibration observation of PKS2155-304
and the MCP HV remained on until preparing for
radiation zone entry.
|
For the HRC rates the dot density reflects the sampling rate
difference between when the HRC has the observing mode telemetry
allocation and the next-in-line allocation. The change in
telemetry format occurs shortly before the HRC is translated away
from the viewing position. The flaring of the background occurs
only while the HRC can view the sky. The dashed line drawn in the
valid rate plot is the telemetry saturation level. During the
observation there were several times when the valid rate exceeded
the telemetry saturation rate. It should be noted, however, that
the fraction of time and factor by which the telemetry saturation
rate was exceeded during this observation is not anomalously high
when compared other routine HRC observations. There is no reason
to rule-out HRC operations when the radiation environment is as
high as it was during this interval. One might even expect that we
could operate with an ACE EPAM P3 flux a few times higher but we
lack sufficient data to make this assessment.
Progress toward determining the bounds on an ACE EPAM P3 (or other)
flux at which we might be able to resume operations could be made
by performing real-time HRC turn-on operations during times of
elevated particle radiation. We would require that the RADMON
process be enabled and that the EPHIN rates be less than 1/3 of
their trip levels. Also a daily-load or dead-man SCS with
radiation zone entry commanding must be running before the MCP HV
would be raised to the operating level. It would appear that the
time window for the opportunity of performing such operations is
rather small.
Additional concerns
Resuming HRC operations during times of known high background
could have a serious impact on the quality of the science data
produced; the community would not be well-served by collecting
data from which the science goals could not be met. Careful
selection of targets would be required and presumably an approval
from the observer would be required as well. Any observation of
faint objects or diffuse sources would be compromised by the high
background, as would most grating observations and perhaps most
observations that require high precision timing.
The additional work-load in planning and reviewing products for the
resumption of the science mission with suitable HRC only targets
during the time of high solar activity may be a high price for
what to-date would appear to be an extremely limited return of
science time.
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memos page
Michael
Juda
mjuda@cfa.harvard.edu
Last modified: Mon May 21 13:59:34 EDT 2001