RHESSI has resumed operations: Difference between revisions

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|title = RHESSI has resumed operations
|title = RHESSI has resumed operations
|number = 276
|number = 276
|first_author = Brian Dennis,  
|first_author = Albert Shih,  
|second_author =  Säm Krucker and Albert Shih
|second_author =  Säm Krucker and Brian Dennis
|publish_date = 20 June 2016
|publish_date = 20 June 2016
|next_nugget = TBD
|next_nugget = Spotlessness
|previous_nugget = [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Non-thermal_recombination_in_solar_flares_and_microflares Non-thermal free-bound emission]
|previous_nugget = [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Non-thermal_recombination_in_solar_flares_and_microflares Non-thermal free-bound emission]
}}
}}


=== Introduction ===
== Introduction ==


As described in  
As described in  
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/RHESSI%27s_5th_Anneal Nugget 269],
[http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/RHESSI%27s_5th_Anneal Nugget 269],
RHESSI began its 5th anneal cycle on February 23.  
RHESSI began its 5th anneal cycle on February 23, 2016.  
The annealing process involves raising RHESSI's detectors to an elevated
The annealing process involves raising RHESSI's detectors to an elevated
temperature, during which the crystal structure can realign itself and
temperature close to 100 degrees C for about 10 days, during which the effects of accumulated radiation damage are significantly reduced.
erase dislocations produced by high-energy particle interactions.
The data flow was turned off during the annealing operation but was restored starting on April 29, 2016. Since that time, each of the nine detectors have been turned on and their performance tested.
The data flow ceased during the annealing operation, but again the instrument
was restored to health, and routine observations have resumed in a slightly
modified operational scheme.  
Figure 1 shows images of SOL2016-05-24T10:20 (C1.3), illustrating RHESSI's
essential capability for hard X-ray imaging spectroscopy.  


[[File:276f1ab.png|600px|thumb|center| Figure 1: Left, Images in two spectral
As a result of this evaluation, we have implemented a new default mode of operation.  Starting on May 18 at ~1900 UT, only detectors 3 and 8 are kept on to allow the lowest operating temperature to be maintained while preserving RHESSI's core capability of X-ray imaging spectroscopy above 3–6 keV.
bands of SOL2016-05-24T10:20 (C1.3); right, the light curves and  
Figure 1 shows images of the C1.3 event, SOL2016-05-24T10:20 that illustrate this capability. 
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=1 RHESSI spectrogram].
 
[[File:276f1ab.png|600px|thumb|center| Figure 1: Left, RHESSI X-ray images in two spectral
bands of SOL2016-05-24T10:20 (C1.3). Right, GOES and RHESSI light curves and  
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=1 RHESSI spectrogram of the same flare].
]]
]]


Another example in the quicklook
Another example confirms RHESSI's sensitivity to an even weaker event: the B3 flare, SOL2016-06-09T13:25, shown in the quicklook
[http://sprg.ssl.berkeley.edu/~tohban/browser/ Browser]
[http://sprg.ssl.berkeley.edu/~tohban/browser/?show=grth1+qlpcr+qli02+rms4a&date=20160609&time=132455&bar=1 Browser].
service confirms RHESSI's sensitivity to a weaker event:  
[http://sprg.ssl.berkeley.edu/~tohban/browser/?show=grth1+qlpcr+qli02+rms4a&date=20160609&time=132455&bar=1 SOL2016-06-09 (B3.0)].
This is a nice microflare with  
This is a nice microflare with  
[http://solarphysics.livingreviews.org/open?pubNo=lrsp-2006-2&page=articlesu11.html type III]  
[http://solarphysics.livingreviews.org/open?pubNo=lrsp-2006-2&page=articlesu11.html type III]  
radio emission.
radio emission.


=== Functionality ===
== Functionality ==


As the cryocooler ages, the operating temperatures of the detectors have
The anneal successfully restored the operating performance of the nine detectors to the level that was anticipated. Four of the nine detectors retained their ability to
continued to increase very slowly.
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=27 segment], which allows them to have low-energy thresholds of ~3 keV and an X-ray spectral resolution of ~1.5 keV FWHM.
Figure 2 shows this via a routine engineering plot, showing a key temperature
The other five detectors are currently unable to segment, and thus have low-energy thresholds of >~15 keV and broader spectral resolution.  These five unsegmented detectors have all had issues segmenting in the past, and experience has shown that some may regain segmentation at some future date.  The following table shows the angular resolution and the segmentation status for the nine detectors.
value over the full life of RHESSI thus far.  
The temperature increase presumably results from the normal ageing of the  
cryocooler, and we expect it to continue indefinitely into the future.


[[File:276f2.png|400px|thumb|center| Figure 1: Engineering overview of
{| class="wikitable" style="margin:auto;"
the "cold plate" temperature of the RHESSI cryocooler, covering many years
|-
of operation (degrees K).
! Detector || Angular Resolution || Segmentation?
The five large positive excursions are the times of the five anneal
|-
operations thus far carried out.
| 1 || 2.26 arcsec || yes
Much of the short-term fluctuation results from periodic orbital variations in the day/night duty cycle.
|-
| 2 || 3.92 arcsec || no
|-
| 3 || 6.79 arcsec || yes
|-
| 4 || 11.76 arcsec || no
|-
| 5 || 20.4 arcsec || no
|-
| 6 || 35.3 arcsec || no
|-
| 7 || 61.1 arcsec || no
|-
| 8 || 106 arcsec || yes
|-
| 9 || 183 arcsec || yes
|}
 
Our default mode of operation to date has been to have all nine detectors on, but each detector that is on contributes an additional amount of heat into the cryostat. 
As the cryocooler ages, the detector temperature has continued to increase slowly.
Figure 2 shows this via a plot of the detector "cold plate" temperature
over the full lifetime of RHESSI from launch in 2002 to the present. 
The gradual increase results from the loss in efficiency of the cryocooler that is expected to continue indefinitely.
The concern over the increasing detector temperature is two-fold: detector performance will deteriorate, and the cryocooler may exceed its operating limits.
 
[[File:276f2.png|400px|thumb|center| Figure 1: Plot covering the over-14-year lifetime of the mission showing the temperature in kelvin (K) of the "cold plate" on which the nine germanium detectors are mounted.
The five large positive excursions are at the times of the five anneal
operations to date.
Much of the short-term fluctuation results from periodic variations in the spacecraft's day/night duty cycle.
]]
]]


The temperatures are approaching the point where some detectors lose
To ensure that RHESSI will continue to perform its core function of hard X-ray imaging spectroscopy through the remainder of Cycle 24, our new default mode of operation is to operate only a subset of the detectors except during times of high solar activity or for special support activities. At the time of writing, only detectors 3 and 8 are on by default.
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=27 segmentation].
As shown in Figure 1, this still allows us to obtain hard X-ray images, time series, and spectra as before, but the images have less  
If this happens they will not function so well as gamma-ray spectrometers, because the
[https://en.wikipedia.org/wiki/Electronic_anticoincidence anticoincidence]
between independent front and rear segments will no longer
be possible, and the background counting rate will increase substantially.
Even without detector segmentation, though,  we expect that RHESSI will still
perform its core function of hard hard X-ray imaging spectroscopy.
 
To postpone the loss of segmentation, specifically to be able to observe
the last few gamma-ray flares of Cycle 24, we we now plan to operate RHESSI in a  
low-power mode.
Since the detector amplifiers dissipate some tiny amounts of heat in the cryocooler, turning some
of them off can decrease the rate of temperature increase.
Thus we now plan to  keep all but two of the detectors switched off except during times of  
high solar activity and for special support activities.
For example, at the time of writing, we are operating Detectors 3 and 8.
As shown in Figure 1, this still allows us to obtain hard X-ray images,  
time series, and spectra as before, but the images have less  
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=8 (u,v)-plane]
[http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=8 (u,v)-plane]
coverage.
coverage, limited to the range of angular resolutions at ~7 to ~106 arcsec.
Users of RHESSI observations from this point on should be aware that for most of the time with low and moderate solar activity only two detectors may be operating.
Users of RHESSI observations from this point on should be aware that for most of the time with low and moderate solar activity only two detectors may be operating.


In our new operating scheme, the full complement of detectors will be turned on at times of exceptionally
We look forward to turning on additional detectors as activity warrants or to support specific observing campaigns, such as coordinated observations with following instruments:
high activity, or to support specific observing campaigns.
* new microwave imaging by the [http://www.ovsa.njit.edu/ Expanded Owens Valley Solar Array (EOVSA)]
These might include the first solar observations with the  
* high-sensitivity X-ray imaging observations by [http://www.nustar.caltech.edu/ NuSTAR]
[http://www.ssalmon.uio.no ALMA]
* soft X-ray spectroscopy by the [http://lasp.colorado.edu/home/minxss/ Miniature X-ray Solar Spectrometer (MinXSS)] CubeSat, recently launched from the International Space Station (ISS)
radio telescope, possibly to be scheduled for a week or two at the end of 2016 or beginning of 2017.
* the first solar observations by the [http://www.ssalmon.uio.no Atacama Large Millimeter/submillimeter Array  (ALMA)] expected later this year or early next year
 


=== The future ===
== The future ==


RHESSI has returned successfully from its fifth annealing operation and we are now obtaining data in  
RHESSI has returned successfully from its fifth annealing operation. We are again obtaining data but with a reduced number of operating detectors.  While all detectors are operational with four fully segmented, we are limiting the number of detectors that are turned on at any one time in order to reduce the heat load on the cryocooler. In this way we expect to be able to maintain the detector temperature within the necessary operating range and retain the ability to turn on all detectors during any upcoming periods of high solar activity in Cycle 24. At present, we are operating detectors 3 and 8, which is sufficient for RHESSI's unique core mission of hard X-ray imaging spectroscopy.
a slightly different mode.
This involves using a reduced detector set to improve the temperature performance in the hopes of retaining
full functionality for X-class flare observations in any final major activity of Cycle 24.
At present we are operating detectors 3 and 8, for example, and find these to be sufficient for the core  
mission of hard X-ray imaging spectroscopy.

Latest revision as of 13:05, 28 June 2016


Nugget
Number: 276
1st Author: Albert Shih,
2nd Author: Säm Krucker and Brian Dennis
Published: 20 June 2016
Next Nugget: Spotlessness
Previous Nugget: Non-thermal free-bound emission



Introduction

As described in Nugget 269, RHESSI began its 5th anneal cycle on February 23, 2016. The annealing process involves raising RHESSI's detectors to an elevated temperature close to 100 degrees C for about 10 days, during which the effects of accumulated radiation damage are significantly reduced. The data flow was turned off during the annealing operation but was restored starting on April 29, 2016. Since that time, each of the nine detectors have been turned on and their performance tested.

As a result of this evaluation, we have implemented a new default mode of operation. Starting on May 18 at ~1900 UT, only detectors 3 and 8 are kept on to allow the lowest operating temperature to be maintained while preserving RHESSI's core capability of X-ray imaging spectroscopy above 3–6 keV. Figure 1 shows images of the C1.3 event, SOL2016-05-24T10:20 that illustrate this capability.

Figure 1: Left, RHESSI X-ray images in two spectral bands of SOL2016-05-24T10:20 (C1.3). Right, GOES and RHESSI light curves and RHESSI spectrogram of the same flare.

Another example confirms RHESSI's sensitivity to an even weaker event: the B3 flare, SOL2016-06-09T13:25, shown in the quicklook Browser. This is a nice microflare with type III radio emission.

Functionality

The anneal successfully restored the operating performance of the nine detectors to the level that was anticipated. Four of the nine detectors retained their ability to segment, which allows them to have low-energy thresholds of ~3 keV and an X-ray spectral resolution of ~1.5 keV FWHM. The other five detectors are currently unable to segment, and thus have low-energy thresholds of >~15 keV and broader spectral resolution. These five unsegmented detectors have all had issues segmenting in the past, and experience has shown that some may regain segmentation at some future date. The following table shows the angular resolution and the segmentation status for the nine detectors.

Detector Angular Resolution Segmentation?
1 2.26 arcsec yes
2 3.92 arcsec no
3 6.79 arcsec yes
4 11.76 arcsec no
5 20.4 arcsec no
6 35.3 arcsec no
7 61.1 arcsec no
8 106 arcsec yes
9 183 arcsec yes

Our default mode of operation to date has been to have all nine detectors on, but each detector that is on contributes an additional amount of heat into the cryostat. As the cryocooler ages, the detector temperature has continued to increase slowly. Figure 2 shows this via a plot of the detector "cold plate" temperature over the full lifetime of RHESSI from launch in 2002 to the present. The gradual increase results from the loss in efficiency of the cryocooler that is expected to continue indefinitely. The concern over the increasing detector temperature is two-fold: detector performance will deteriorate, and the cryocooler may exceed its operating limits.

Figure 1: Plot covering the over-14-year lifetime of the mission showing the temperature in kelvin (K) of the "cold plate" on which the nine germanium detectors are mounted. The five large positive excursions are at the times of the five anneal operations to date. Much of the short-term fluctuation results from periodic variations in the spacecraft's day/night duty cycle.

To ensure that RHESSI will continue to perform its core function of hard X-ray imaging spectroscopy through the remainder of Cycle 24, our new default mode of operation is to operate only a subset of the detectors except during times of high solar activity or for special support activities. At the time of writing, only detectors 3 and 8 are on by default. As shown in Figure 1, this still allows us to obtain hard X-ray images, time series, and spectra as before, but the images have less (u,v)-plane coverage, limited to the range of angular resolutions at ~7 to ~106 arcsec. Users of RHESSI observations from this point on should be aware that for most of the time with low and moderate solar activity only two detectors may be operating.

We look forward to turning on additional detectors as activity warrants or to support specific observing campaigns, such as coordinated observations with following instruments:


The future

RHESSI has returned successfully from its fifth annealing operation. We are again obtaining data but with a reduced number of operating detectors. While all detectors are operational with four fully segmented, we are limiting the number of detectors that are turned on at any one time in order to reduce the heat load on the cryocooler. In this way we expect to be able to maintain the detector temperature within the necessary operating range and retain the ability to turn on all detectors during any upcoming periods of high solar activity in Cycle 24. At present, we are operating detectors 3 and 8, which is sufficient for RHESSI's unique core mission of hard X-ray imaging spectroscopy.

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