High Temperatures in Active Regions

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Introduction

Since RHESSI was launched in February 2002, it has observed thousands of solar flares (more than 46,000 in the latest reprocessing of the RHESSI flare list). It was noticed in 2002 that RHESSI was observing solar emission even when there are no flares present (Ref. 1). The temperature required for this high energy emission is greater than 5 MK, a temperature range that is not often considered for solar active regions, mostly because the instruments used for T measurement of active regions, such as Yohkoh SXT, SOHO EIT or CDS, or didn't have much response to high T. Here we have measured the T for approximately 7000 time intervals from Feb 2002 through August 2006.

Background Subtraction and Interval Selection

Figure 1:This is the RHESSI 3 to 6 keV count rate for one orbit. The black dashed lines show an interval which was chosen for a temperature measurement. The red line is the expected background level.

Figure 1 shows the RHESSI count rate for a typical orbit for 8 April 2006. The plot clearly shows a jump at the day-night transitions. That is solar emission. Two things need to be accomplished before we can get a T measurement of the steady-state component that would be related to active regions:

1) Avoid microflares, SAA's, particle events, etc: So 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. For flatness, 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, there is one right after, though.

2) Find the background level: You cannot just assume that the background level can be given by the nighttime values before and after spacecraft day. The background level depends on the cosmic ray flux and local particle flux, and it varies with the position of the spacecraft. Figure 2 shows background data.

Figure 2:This is the RHESSI 3 to 6 keV background level, plotted versus the longitude of the ascending node of the orbit, and orbital phase. The longitude of the ascending node is the longitude at which the spacecraft passes over the equator, moving from south to north; the combination of this quantity and the orbital phase gives complete information about the latitude and longitude of the spacecraft. On orbit, a spacecraft follows a series of vertical lines on this plot, going up.

Here is how we calculate the background level: The background level was obtained in the following manner. The count rate has been accumulated in 20 second intervals during the 5 minute periods before and after daylight for each orbit in the mission. This resulted in approximately 500,000 spectra. Each of these spectra has an energy range from 3 to 300 keV. This energy range is split into 492 energy bands with 1/3 keV resolution from 3 to 100 keV and 1 keV resolution from 100 to 300 keV.

The spectra are then averaged over time, longitude of the ascending node, and orbital phase. The angular ranges are split into 10 degree bins. It takes a relatively long time for all of the possible angular combinations to be visited by the spacecraft; it turns out that a 56 day time interval is sufficient to ensure that there are measurements in each angular bin. For each of the 56-day time intervals, the individual spectra in each angular bin are averaged, resulting in a 36x36 array, similar to the image plotted in Fig.~\ref{fig:background}, for each of the 492 energy bands. We end up with an array of 492x18x36x36 for each 56-day interval; 492 energy bands, 18 detector segments, 36 bins of ascending node longitude, and 36 bins of orbital phase. Each of these arrays is stored in an IDL save file.

To calculate a background spectrum for a given time from this database, we restore the files accumulated for times that bracket the given time, and interpolate the spectrum over time and position. This can be done for any time or energy band for the whole RHESSI mission.

The uncertainty in the background is the dispersion obtained in each of the 36x36 angular bins during the averaging process, For the low-latitude regions where most of the temperature measurements were taken, the uncertainty in the background is approximately 1/2 the background rate. For the 3 to 6 keV energy band shown in Fig.~\ref{fig:count_rates}, this is approximately 1 count per second per detector. The uncertainty is higher for higher latitude regions (e.g., regions which are very dark in Fig.~\ref{fig:background}).