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A table of early changes to the detector high-voltage is available, but an automated record (more complete and current) is in the daily state-of-health summaries under the heading "HV Power Supply". Early changes to the attenuator operation algorithm and the event decimation strategies are also available, however much more complete information on these states is available on the RHESSI Browser -- select the "Uncorrected" option under Quicklook Plots.
June 9, 05:00 through June 24, 22:00 During this period RHESSI was pointing off the Sun for Crab Nebula observations. This includes the re-pointing periods on either end. If you are interested in analyzing Crab or solar data in this interval, please contact the instrument team.
October 18, 09:00 UT RHESSI was again put into drift mode to view the x-ray emission of the quiet Sun "out of the corner of its eye". This mode began at the time noted above. RHESSI was commanded back to the Sun at 06:10 UT on October 28, and arrived shortly after 08:00 UT on that day. The Sun was very quiet during this time, but anyone wishing to analyze data within that interval should contact the instrument team.
July 19, 06:00 UT At this time, RHESSI was put in a new mode that let it drift up to 1 degree away from the Sun and slewed back, repeatedly. The purpose of this mode was to study the x-ray emission of the quiet Sun (the Sun was extremely quiet beginning at this time). RHESSI returned to normal operations at around 04:00 UT on July 26. If you wish to analyze any flares during this week, please contact the instrument team.
June 3, 03:04 UT
At this time, we began off-pointing the satellite to visit the x-ray pulsar 3A 0535+262, currently in its first major outburst since 1994. We intend to spend a few days there before going to the Crab Nebula as we usually do each summer. We are extremely fortunate that the brightest known transient in this class is both near the Crab and going into outburst while the Sun is also in the neighborhood. Users should not attempt to analyze solar data from the offpointing period without help from the instrument team.
December 9, 08:00UT to present
From this time onward, events from Detector 8 are shut off during times when RHESSI's aft antenna is transmitting. The data are unusable due to noise in these brief intervals, so they are eliminated to save memory.
June 8 to June 25
During this period, we took RHESSI off the Sun to point at the Crab Nebula -- we believe the exposure was very successful and should have a good science return, the finest imaging ever done of a cosmic source in the hard x-ray/soft gamma-ray range. RHESSI data should not be used for any solar science during the period from 22:00 UT on 8 June to 22:00 UT on 25 June without special consultation with the instrument team.
During this period, pointing was offset toward a fictitious target near the Sun in rehearsal for the Crab nebula campaign coming up in June. Solar data should not be used between approx 23:00 UT on April 16 and 03:00 on April 18. Data within this interval, particularly near the edges, may be usable for some purposes, but potential users should communicate with the instrument team. No flares above a GOES level of approximately B4 were missed due to the offpointing.
Between the cryocooler valve opening operation (see previous entry below) and this date, the rear segment in detector G3 displayed significant noise in the fast channel when the spacecraft was in sunlight, presumably due to the leakage of a small amount of Earth albedo light into the cryostat. This did not affect the energy resolution, but caused significant deadtime. This deadtime problem was fixed on this date by raising the energy threshold of the fast lower-level discriminator slightly. In the interval, there had been no flares which reached high enough energies to make the rear segments useful, so this should not be an issue for solar analysis. The use of G3 may cause problems for non-solar projects in this period.
The long-term decrease in cryocooler efficiency noted since the beginning of the mission seems to have been halted, as hoped, by the cryocooler valve operation.
At this time, in hopes of improving the cryocooler performance, we fired the actuator which opens a valve in the rear of the spectrometer, venting it to space. Initially, this caused a certain amount of transient instability in the detectors (high-voltage dropouts or arcs), but detector high voltages were lowered across the board (see the detector high voltage page) and all detectors stabilized with no loss of performance.
An exciting couple of weeks, with several X-class flares showing gamma-ray line emission -- the most active period since launch. Unavoidably, parts of each flare were missed due to eclipses and SAA transits, but overall the coverage was very good. There were three spontaneous resets of the spacecraft CPU during this period, which resulted in some losses of data and pointing, but the major flares were unaffected by the data losses and at worst mildy affected by the pointing issues (i.e. good aspect solutions will become available but they may currently be of inferior quality). The cause of the resets is still under investigation, but they may be related to increased particle activity. During this period RHESSI has been seeing large fluxes of electron bremsstrahlung in the spacecraft and gamma-ray lines from the Earth's atmosphere due to solar and magnetospheric energetic particles. When analyzing the high-energy spectra of these flares, please consult with the RHESSI instrument team on background subtraction (contact David Smith: dsmith (at) scipp.ucsc.edu). A timeline of the X-class flares in this period, their RHESSI coverage, and the timing of the CPU resets and data losses has been prepared by Martin Fivian: http://sprg.ssl.berkeley.edu/~mfivian/xserie/timeline.html
Two operational experiments were done on the spectrometer today. The cryocooler input power was lowered by approximately 4W to calibrate the relation between cryocooler power and coldplate temperature. The power will be raised back to its nominal value in a few days and then a higher-power excursion will be executed. The other experiment involved detector G2, which has been operated in an unsegmented mode with significantly degraded performance since early in the mission. We tried two different operating voltages just barely above the segmentation threshold, but neither setting produced stable, well-behaved segmentation. We are considering other strategies for improving the performance of this detector. The period of (semi-) successful segmentation ran from 19:36 UT to 22:57 UT.
The offpointing to the Crab Nebula was successful. The Crab was acquired within 20' in three orbits of slewing on June 14. A small pointing correction was made on the morning of June 15 to compensate for drift. On the morning of June 16 RHESSI was commanded back to the Sun due to flare activity on the East limb. Although the Sun was always at least partially visible to RHESSI's grids, solar analysis during the following interval should not be attempted without the assistance of the instrument team: June 14, 19:00 UT to June 16, 22:00 UT.
Beginning at this time we began a second series of offpointing tests. Data between 05:00 UT on May 23 and 02:00 UT on May 24 should not be analyzed as solar data.
At this time we began a series of tests involving pointing the spacecraft up to three degrees from the Sun, as practice for n n the upcoming observation of the Crab in June. Off-pointing began at 11:00 and continued until approximately 06:00 UT on May 9. Data in this period should not be used for solar studies.
Due to a combination of factors, including temporary problems at the Berkeley Ground Station, the lack of rear decimation and front nightside shutoffs for the last few days, strong particle precipitation events, and a GOES class M2.5 flare, the onboard memory was judged to be too full at this time (around 68%). A period of very quiet solar data stretching from around 21:22 UT on December 18 to 19:30 UT on December 19 was deleted in order to keep memory free for increasing solar activity.
During the initial period of operation of flight software version 2.4, it was discovered that shutoff of the front segments at night and decimation of the rear segments at high magnetic latitude were no longer taking place (these functions are meant to keep the onboard memory clear for large flares). These functions were reinstated at this time by a patch to flight software version 2.4. Decimation of the rear segments is now only up to channel 512 (around 190 keV), instead of twice that, at the request of the science team.
Flight software version 2.4 was uploaded without interruption of science operations. This patch allows the spacecraft to detect very large particle precipitation events and shut off most counts during those rare periods, as well as providing other operational improvements. For the first four hours under 2.4 (04:30 to 08:30 UT) events from the front segments were unintentionally shut off.
A somewhat more severe regimen of front-segment decimation was implemented for times when the onboard memory is more than 30% full. The goal is to increase the fraction of time spent with both attenuators open.
The attenuator control algorithm was changed so that when the (thin, thick) attenuator is put in during a flare -- which happens at (8%, 10%) deadtime -- it won't be pulled out again until the deadtime drops below (1%, 2%). These latter values are lower than the previous values of (3%, 7%), so we expect to have fewer unnecessary shutter motions in large flares from now on.
The instrument computer was reset by an erroneous command at 10:25am PST, interrupting science operations. The spacecraft was unaffected, and the science instruments were turned back on and returned to their previous configurations between about midnight and 7:00am PST on August 16. All the germanium detectors settled down to their normal performance by about 3 hours later, after an initial period of degraded resolution.
The Berkeley Ground Station is now fully operational again, and the problem with the elevation drive was fixed in such a way that it is not expected to recur.
The X4.8 flare at the beginning of July 23 has produced the first clear signatures of gamma-ray lines in a flare observed by RHESSI. The 2.2 MeV line of neutron capture is extremely bright and showcases RHESSI's fine resolution (about 4 keV FWHM when summing the rear segments). Broadened lines from several other nuclei appear to be visible at lower levels.
Only the decay phase of the X flare of July 15 was seen by RHESSI due to Earth occultation. Data from roughly 09:00 to 21:15 on July 16 were skipped to make room in the onboard recorder for possible additional large flares from Region 30. Such data gaps will no longer be listed on this page. If RHESSI data are not available for a given date after about a week has passed, they were probably never recovered to ground. The operations team is working to minimize these losses.
Second X-flare, memory management: RHESSI saw its second X-class flare on July 3 at around 02:00 UT. There have also been several M flares in this period. Because of the high solar activity and the absence of coverage by the Berkeley Ground Station (under repair) two long intervals of data were deleted from the onboard memory which were known not to contain any major flares. These were roughly 22:45 July 3 to 04:20 July 4 and 16:30 July 5 to 03:00 July 6.
BACK TO THE SUN: RHESSI re-acquired the Sun (got within 0.4 degrees) around 17:10 UT on June 24. Some intervals of data from June 12-21 are missing due to various issues related to the Berkeley Ground Station being offline and the offpointing activity.
CRAB OPERATIONS: RHESSI is currently pointing away from the Sun and moving toward the Crab for a few days of science and calibration observations of the nebula and pulsar. Solar activity is very low. Two off-pointing exercises recently took place to support this activity, on June 8 and 13, during which solar coverage was interrupted (contact David Smith at the address below if you need more details). The actual offpointing sequence began on June 17 (11:26 UT) and will continue through approximately June 23.
RHESSI data were obtained during the partial eclipse of the Sun, and both scientific and calibration use may be made of the changes in count rate as the Moon eclipsed one or more x-ray emitting regions. A significant amount of data from the day or two before the eclipse had to be lost in order to make sure these data were recovered, due to the Berkeley Ground Station being offline. The Sun was in a state of low activity during this time; no major flares were lost.
Mechanical problems have taken the Berkeley Ground Station antenna temporarily offline, after it had successfully supported over 800 RHESSI passes. Repairs are expected within a few days. We are trying to acquire as much downlink as possible from other stations in the meantime, but some data loss should be expected.
During this orbit the aspect system was offline due to an error in commanding. It was restored without difficulty at the end of this period.
As of this time, to save memory, we are now decimating (i.e. throwing out some of) the events from the rear segments during periods of high background during the spacecraft night. This is in addition to our current practice of shutting off the front segments during spacecraft night. Solar observations should be unaffected. Details about decimation are on the decimation state page.
At this time, the spacecraft CPU reset. We believe that we understand the cause and that it can be easily prevented. The responses of the spacecraft and instrument were graceful, in fact the instrument never stopped taking data, but event time-stamps, which are taken from the spacecraft clock, are probably missing or erroneous shortly after this time. The clock was reset at approximately 11:15 UT (on the following pass), and other measures to restore normal operation were taken over the next few hours. During the recovery, the spacecraft was also commanded to spin back up to 15 RPM after having drifted down to about 14.3 RPM over the previous months.
Due to computer problems, we lost some data on the 26-28th of April, including a long stretch from roughly 17:00 UT on April 27 to 10:00 UT on April 28. The sun was extremely quiet during this period.
RHESSI observed its first X-class flare at this time, after accumulating approximately 40 M-class flares and numerous smaller events. The early parts of the flare were observed in daylight; the gradual decay lasted for several orbits so it includes periods of eclipse. There was so much data from the flare (a good thing) that some quiet-time data from times following it was deleted rather than recovered to make room in the onboard memory.
HESSI has been officially renamed the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) after Dr. Ramaty, who was a driving force behind its creation and design. Also as of this date, all RHESSI data are now available to the public; please visit the RHESSI Data Center.
The HESSI attenuators (aka shutters) are now operating in an automated mode, with both attenuators out unless a flare produces > 50 % deadtime in the detectors, in which case the thin attenuator is put in within seconds to reduce the count rate. From an M-class flare on March 12 HESSI gathered over 100,000,000 photons in about 6 minutes without having to trigger the attenuator to close.
March 7, 09:44 UT
A configuration error was fixed which had been suppressing photon event data from detector G2 since February 26, 18:30 UT. No other detectors were affected.
February 28, 18:55 UT
The HESSI particle detector (a small silicon detector that monitors the radiation environment) has two thresholds, one fixed and one adjustable. Until this time, both were set with fairly low threshold (less than or roughly to 50 keV) and so both counters registered cosmic rays, SAA protons, SAA electrons, and precipitating bands of radiation-belt electrons at high magnetic latitude. At the date and time shown we raised the adjustable threshold to roughly 650 keV, so that the associated counter now registers primarily protons, allowing us to distinguish between different components of the radiation environment.
For a brief period on this day, roughly 21:00-21:40, there are periodic gaps in the data which were introduced during the playback to the ground station; the spacecraft was in a poor orientation with respect to the ground antenna and a solar panel was periodically blocking the data stream as the spacecraft rotated. Normally we do not downlink data when in this orientation.
Final spin-up to 15 rpm and final spin balance were done today. At 18:33 UT, the integration time for the PMTRAS was changed from 4 ms (where it has been since turn-on) to 2 ms, a value more appropriate to 15 rpm.
February 24, 00:00 UT
Data will be recovered at both Berkeley and Wallops for the near future, and this is sufficient to bring down all photons without decimation. Several solar flares have been observed ranging from microflares up through mid-range M flares. In-flight calibration of the grids for imaging is in process using these events. At least one bright flash from the Soft Gamma Repeater SGR 1900+14 (an unusual neutron star) has been observed. The thin attenuator continues to be occasionally pulled out for brief periods to observe microflares.
February 23, 00:00 UT
Detector performance has been optimized at a stable coldplate temperature of about 72K. Spin rate is still 14 rpm. It will be brought up to 15 rpm and the final balancing done next week. Operations are going very well. Data at 4 Mbit/s have been received at Berkeley, Wallops and Weilheim stations.
February 17, 08:00 UT
This page has been down for a few days due to a computer outage that did not affect HESSI operations.
Detector temperature has been 69-70K; cryocooler power was lowered further today. Over the past few days, detector high voltages and electronic thresholds have been varied, and occasionally one detector has had either its event output or its high voltage turned off for diagnostic purposes. Decimation (accepting only a fraction of the events below a threshold energy) has also been turned on and off to control the rate of data flow into the onboard memory. A precise calendar of these operations will be posted soon. Most significantly, the thin attenuator disks were successfully removed from the field of view and put back in twice, briefly allowing the full area of the detectors to see the Sun down to 3 keV.
Spin rate is at 14 rpm and holding for aspect calibrations. The pointing and aspect systems are working well but the spacecraft is being allowed to "cone" slightly about the Solar axis for a few days in order to calibrate the whole surface of the Solar Aspect Sensor (SAS) CCDs. Flares can still be imaged in this mode.
February 13, 02:00 UT
Operational modes are being exercised; balancing and calibration of aspect systems continue. Detector temperature is 73K and the cryocooler has been backed off to lower power level for long-term operations.
February 12, 01:00 UT
Detector high voltages have been turned on to the point of detector segmentation, so that all 9 HESSI germanium detectors are now working spectrometers. There is no unexpected noise at the nominal energy thresholds (3 keV front segments, 20 keV rear segments). Detector temperature is 77 K and still falling.
February 10, 21:00 UT
Ramp to final spin rate (15 RPM) has begun. Detector temperature is 98K. Detectors will be turned on starting with the first Berkeley pass on Monday (early afternoon). Aspect calibration continues. No problems.
February 9, 18:00 UT
Balancing is ahead of schedule and going well. Detector temperature is 124K. No problems.
February 8, 18:00 UT
Spin rate is 12 RPM and holding for balance procedures, as planned. Detector temperature is 164K. No problems.
February 8, 07:30 UT
The backup roll aspect system (PMT-based) has been activated successfully. Spin rate is over 9 rpm. Aspect system calibrations are in progress.
February 8, 00:30 UT
Cover has been removed from the roll aspect system. Telemetry at Berkeley Ground Station is now at its full 4Mbits/s error-free.
February 7, 20:00 UT
Spin rate is 6.4 RPM and climbing normally. Cryocooler is at 90% of full power and detectors are cooling at the expected rate (currently at 200K). Successful detection of the solar disk by the solar aspect system.
February 7, 00:30 UT
Subsystem turnons continue on schedule and without any major anomalies. Accomplished so far today: turnon of detector interface electronics, turnon of charged particle detector, beginning of spin-up, cryocooler ramp to 3/4 power, turnon of solid state data recorder, turnon of instrument aspect system.
February 6, 07:00 UT
On the last three contacts of the day, the instrument computer was turned on, the detectors' cryocooler was turned on, and it was gradually ramped up to about 1/2 power, where it will remain as planned until the next series of passes.
February 6, 02:00 UT
The third ground contacts were successful, and HESSI is now being automatically run by its Absolute Time Sequence (onboard command store). For one thing, this means it now turns on and off its transmitters at the right time automatically.
February 6, 00:30 UT
The second ground contacts went well; we are within a few degrees of the Sun, as expected at this stage. Cryocooler power-on is scheduled to begin later this evening.
February 5, 22:40 UT
The first ground contact at Berkeley and followup contact at Wallops were perfect. Solar arrays are deployed, the pointing was within 10 degrees of the Sun and closing at a good rate, 100% of command attempts at both sites were successful, and the downlink signal strength was as good or better than expected. Power and temperature readings are all nominal. The spin (imparted by the rocket on separation) was roughly 1/2 RPM as expected.
The HESSI Pegasus launch went perfectly after a 1/2 hour delay caused by communication problems between the Cape and the aircraft. The first reported orbital parameters were 586.85 x 600.24 km at 38.023 degree inclination, well within specifications.
This page is maintained by David Smith, who can be reached at dsmith (at) ssl.berkeley.edu