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

ABSTRACT: Since RHESSI was launched in February 2002, it has observed thousands of solar flares. It has also observed solar emission above 3 keV when there are no flares present. The plasma temperature required for this high energy emission is greater than 5 MK, a temperature range that is not often considered for solar active regions. In this work we use RHESSI data for non-flaring times, along with GOES data, to determine the temperature and emission measure for the Sun in this high temperature range. The first step is to compare the temperatures and emission measures obtained from the two instruments using the isothermal approximation. The next step is to use the combined data from the two instruments to obtain a differential emission measure (DEM) for the 3 to 20 MK temperature range. The entire RHESSI mission, from launch until May 2006, is included. This research is supported by NASA contract NAS A

1) COUNT RATES: 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 8324 time intervals were analyzed, from 14-Feb to 31-May Of these intervals, 6578 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 (Landi et al., 2006; ApJSS, 162, 261). The GOES data were analyzed using results of White et al. (2006, Solar Physics 227, 231). 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. 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 6 MK. This is generally true, except for a set of intervals in April 2006, for which GOES temperatures of 15 to 17 MK were found. 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. The GOES and RHESSI T values are not well correlated – for the full sample the correlation coefficient between the two values of T is For emission measure it is The GOES measurements do not include background subtraction. When the sun is less active, and the flux in the GOES channels reach the digitization level, the GOES temperature measurement becomes problematic.

4) FULL MISSION T AND EM: The top plot shows monthly averages of the RHESSI (black) and GOES (red) temperature. The middle plot shows the emission measure. There is not much temperature variation with the solar cycle, but the emission measure decreases. The 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 dashed red line is the normalized average sunspot number. As activity decreases, the likelihood for good T measurements decreases.

5) THE RHESSI/GOES DEM: The top plot shows the calculated DEM for the temperature range from 3 to 25 MK for the first time interval. The DEM is calculated using a pixon method, similar to that used for RHESSI and Yohkoh HXT imaging (Metcalf et al, 1996, Apj ). The red stars show the values of the isothermal T and EM for the two instruments. The DEM surprisingly shows some emission greater than 10 MK, but with relatively large (50%) error bars. This is seen for almost all of the time intervals, but the large errors make it uncertain whether this is real emission or noise.

The middle plot is a comparison of the observed count rate spectrum with the data calculated from the DEM model. (The GOES fluxes have been normalized to look like a photon count. ) The line gives the observed data, the red diamonds are the calculated data. It turns out that if the RHESSI data between 3 and 5 keV are included, there is no good DEM solution. If a DEM fits the RHESSI data in this energy range, then the fit to the GOES data is bad. Conversely, if the GOES data is fit well, then the fit to the 3 to 5 keV RHESSI emission is bad. This is a situation that requires further study, but for now it is ignored, and the 3 to 5 keV RHESSI emission is not included in the fit. The bottom plot shows a comparison of the residuals for the fit with the observed data. The fit is excellent, with all but the last nonzero RHESSI data point being fit to within 6%. The uncertainty for the data ranges from 0.5% for the low energy GOES channel to 10% for the last nonzero RHESSI channel, and the fit data points are well within the uncertainty, except for the last one. The DEM calculation failed for approximately 1000 of the 6578 intervals for which there were good single T measurements. This is being investigated.

6) DEM vs. TIME: This plot shows the monthly average of the DEM integrated above 3, 5 and 10 MK for the RHESSI mission. There is a noticeable solar-cycle related decrease in the >3 and >5 MK range, and a smaller decrease in the >10 MK range. These averages do not include unsuccessful DEM calculations which become more prevalent with the decrease in activity, similar to the single temperature case.

7) WHAT’S THIS? This is a plot of the > 5MK DEM 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. Sophisticated statistical techniques will be necessary to pull any correlation out of this, but at least the highest values of emission measure tend to occur within 2 to 3 hours after a flare. The calculation only considered the previous day’s flares, so the points at 1440 minutes are lower limits.

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 6MK, consistently less than the RHESSI temperature, but the values are not well correlated. The relative number of successful high T measurements decreases with solar activity. The DEM can be calculated from combined RHESSI/GOES data, provided that the RHESSI data from 3 to 5 keV is ignored. The high temperature emission shows some variation with the solar cycle, but the variation is less noticeable at the highest (>10MK) temperatures.