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:
  1. Provide rate thresholds at which we would consider halting the science mission in order to safe the HRC
  2. 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.

ACE EPAM P3 hourly average flux
and HRC rates
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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Michael Juda
Last modified: Mon May 21 13:59:34 EDT 2001