RHESSI/GOES Observations of the Non-flaring Sun from 2002 to J. McTiernan SSL/UCB

ABSTRACT : In this work the temperature and emission measure for solar X-ray emission are calculated at times for which there are no solar flares. Approximately 8700 time intervals during the RHESSI mission, from launch in Feb 2002 until August 2006, have been analyzed, using RHESSI and GOES data. High temperature emission, in the temperature range above 5 MK, is typically present during active times and is attributed to solar active regions. From comparisons of the RHESSI and GOES temperature measurements, it is seen that RHESSI temperatures are typically higher than GOES, with smaller emission measure for RHESSI, but with values for the two instruments that are not necessarily well correlated. In particular, for many time intervals from 2003 to 2006, the GOES temperature is unexpectedly higher than the RHESSI temperature. This is probably due to the effect of background particles. This work was supported by NASA contract NAS and NASA grant NNX08AJ18G

1) COUNT RATES: Early in the RHESSI mission it was noticed that RHESSI was observing solar emission even when there are no flares present (Ref.1). The top panel shows the RHESSI 3 to 6 keV count rate for one orbit. There is a jump at the day-night boundary. The black dashed lines show an interval which was chosen for a temperature measurement. For each orbit, an interval of between 1 and 5 minutes was chosen based on the following criteria: No flares or particle events, no attenuators, no data gaps, and at least 5 minutes from the SAA. The intervals chosen have the minimum (daylight) count rate for the orbit, subject to a flatness test. The dispersion of the count rate for an interval is required to be less than 1.25 times the dispersion expected from a Poisson distribution. This insures that there are no microflares in the intervals. A total of 8747 time intervals were analyzed, from 14-Feb-2002 to 2-Aug Of these intervals, 6961 had enough counts above the background level in the 3 to 10 keV energy range for an isothermal spectrum to be fitted, resulting in temperature and emission measure values. We also obtained temperature and emission measure for each interval from GOES data. The RHESSI spectra were fit using the SSW XRAY package which (via chianti_kev.pro) uses version 5.2 of the Chianti database (Ref. 2). The GOES data were analyzed using results of White et al. (Ref. 3). Coronal abundances were assumed. The red histogram in each count rate plot is the expected background level, based on the spacecraft position.

2) BACKGROUND SUBTRACTION: This plot shows the background count rate in the 3 to 6 keV energy band for the RHESSI detectors used in the calculation. The non-solar background is mostly a function of the position of the spacecraft; here it is plotted versus the longitude of the ascending node of the orbit and the orbital phase. The path of the spacecraft for an orbit would be a vertical line on this plot. These data were obtained by totaling the count rates in the 5 minute periods before and after daylight for each orbit in the mission. For the low-latitude regions where most of the T measurements were taken, the uncertainty in the background is approximately 1/2 the background rate. The spectra for the fits were accumulated in 1/3 keV energy channels. To be included in the fit, the background-subtracted count rate for a channel must be greater than 3 times its uncertainty.

3) T AND EM: Here we have temperature (in MK) and emission measure (in cm -3 ) for RHESSI and GOES for (approximately) the first month of the mission. The RHESSI temperature is between 6 and 11 MK, and the GOES temperature is between 3 and 7 MK. The GOES EM is typically a factor of 50 to 100 times the RHESSI EM. This is consistent with a differential emission measure that decreases with increasing temperature. One point to note is that the GOES measurements do not include background subtraction. When the sun is less active, and the flux in the GOES channels reaches background level, the GOES temperature measurement becomes problematic, as we shall see.

4) AVERAGE T AND EM: The top plot shows 28-day averages of the RHESSI (black) and GOES (red) temperature. The average RHESSI temperature is higher than for GOES. The middle plot shows the emission measure. There is not much temperature variation with the solar cycle, but the emission measure decreases. These averages do not include intervals for which there was not enough emission for temperature measurements. The bottom plot shows the relative fraction of intervals for which there were no good measurements for RHESSI. (The red line is the normalized average sunspot number). As activity decreases, the likelihood for good T measurements decreases.

5) CORRELATION? Looking at average values, you might expect that the RHESSI and GOES measurements would show correlation, or, since GOES observes a lower energy range (above 1.5 keV ) than RHESSI (above 3 keV), at least the GOES T should be consistently less than the RHESSI T. This is true for the early part of the mission (top panel), but not for the full mission (bottom panel). Probably this is due to particles affecting the GOES measurements; we don't have a background subtraction method available for GOES to deal with this sort of problem. This erodes confidence in the GOES measurements, particularly when solar emission is weak.

6) PARTICLES’ EFFECT ON GOES DATA. This plot of GOES data is taken from the RHESSI Data Browser: (See Here at 1200UT on 2006-Apr-13, (the middle of the time range), is an example of an interval for which the GOES T is much higher than the RHESSI T. There is a large increase in the high energy GOES channel, presumably due to particles, with no corresponding increase in the low energy GOES channel (or in RHESSI) This results in a GOES T measurement of approximately 17 MK, twice the RHESSI temperature.

7) WHEN IS GOES OK? Here we have plotted in two ways the percentage of measurements for which the GOES T was greater than the RHESSI T, accumulated over 28 day periods. The top shows this quantity plotted versus time. The bottom shows it plotted versus GOES EM. We can use this as a tracer for when we might not want to trust the GOES measurements: Early mission = High GOES level = No GOES > RHESSI T. Later mission = Low GOES level = Many GOES > RHESSI T The EM above which all GOES T are less than RHESSI T is 3x10 48 cm -3, which is about C level. Below this level, it is not a good idea to trust non-background-subtracted GOES measurements..

7) TIME SINCE LAST FLARE: This is a plot of the RHESSI EM for each time interval versus the time since the most recent GOES flare. It might be expected that the high temperature emission is coming from post-flare loops. If this is true, then there should be some correlation between the high T emission measure and the amount of time elapsed since the last flare. Not much correlation here, but it is true that the brightest intervals tend to be less than a few hours after a flare. GOES has no position information, though, so it’s not clear if any of the flares are from the same active region(s) that are showing up in the RHESSI data. The red points are for the 405 time intervals for which a source could be located using the algorithm used for RHESSI flare-finding. These tend to have higher EM, and/or longer “flat” time intervals.

DISCUSSION: CAN WE ASSOCIATE THE HIGH-T EMISSION WITH FLARES? Of the 405 time intervals for which there are positions, 247 are less than 6 hours after a RHESSI flare that was presumably in the same active region (less than an arcminute away). These “could” be considered to be post-flare loops. The radiative cooling time (using CHIANTI) for a loop of 8 MK and density of 10 9 cm − 3 is about 200 minutes. The conductive cooling time (for a loop half-length of 10 5 km) is about 13 minutes. Using an approximation of total cooling time (Ref. 4) of: we find a cooling time of approximately two hours. Of the 405 intervals with good positions, it turns out that 190 are less than two hours after a nearby RHESSI flare. The trouble with this analysis is that there are gaps in the RHESSI flare record due to spacecraft night, etc… that may be hiding flares that may be responsible for heating. Thus the best that we can say is that a significant fraction of these time intervals can be associated with post-flare emission. Due to the presence of data gaps, we cannot yet rule out the possibility that all of the high T emission is post-flare emission. One more interesting point is that 31 of the time intervals are less than two hours before nearby flares. These might possibly be considered pre-flare brightening.

CONCLUSIONS: High temperature solar emission (> 5 MK) is usually present without flares, and is observed by RHESSI. The average RHESSI temperature ranges from 6 to 8 MK, with emission measures from to cm -3. Individual measurements may have higher T and EM. The average GOES temperature is 4 to 6 MK, consistently less than the RHESSI temperature, but the values are not well correlated. Probably this is due to the effect of background particles on GOES. For GOES EM less than 3x10 48 cm -3, (about C level), non-background subtracted GOES measurements are not reliable. The relative number of successful high T measurements decreases with solar activity. A significant fraction of the high-temperature intervals can be associated with post-flare loop emission. REFERENCES: 1)Benz & Grigis 2002, Solar Physics, 210, 431 2) Landi et al., 2006; ApJSS, 162, 261 3)White et al 2006, Solar Physics 227, 231 4)Cargill et al, 1994, Apj 439, 1034 PREPRINT: