Introduction

RHESSI has been operating for over 5 years now and has seen quite a number of flares: the official flare list (thanks to Jim McTiernan) has nearly 11,000 flare from March 2002 to March 2007. Of these, 7,839 are of the micro- variety, that is low C-Class to sub A-Class flares, with energies that are "micro" that of large flare. RHESSI is especially good at observing these wee events due to its automated shutters. During large flares these shutters are placed in front of the detectors protecting them from excess low energy photons but during quieter times they can be moved out, allowing the full sensitivity of RHESSI's detectors to observe these events.

The official flare list however looks for events in 12-25 keV, an energy range in which many microflares rarely get above background. So we looked closely at the shutter out times but at lower energies, 6-12 keV, and found 24,799 events.

Now that we have so many events there are many things we can do with them but to present a concise nugget we will be dealing with when and where these RHESSI microflares occur.

How often?

The first question to answer is how often do these events occur.

Figure 1: The microflaring rate over the 5 years of observations. The histogram has been corrected for the times RHESSI was not observing with the shutter out, hence the rate per live day. The red line and righthand axis indicate the number of sunspots (via NOAA). We can clearly see that the microflaring rate is closely tied to solar activity, with the number of microflares dropping as we approach solar minimum in early 2007.

The above histogram shows the microflaring rate, corrected for fraction of time RHESSI was not observing the Sun with the shutters out, which as we go from solar maximum to minimum we see a steady decrease in microflaring rate. In 2002 we could expect over 70 microflares per day, but by the time we get to solar minimum in 2007 we are getting under 10 microflares a day on average. Also shown in Figure 1 is the time profile of sunspot number over the same time period. Clearly the microflaring rate is tied to solar activity, which is unsurprising. Although over the same period the same is not true for the largest flares (see a previous nugget). The discrepancy in the above histogram for the first 6 months we believe is due to a combination of using a different strategy as to when the shutters came in and many more C,M and X-class flares hiding smaller events.

Where on the solar disk?

The next question is to ask where on the solar disk these events occur.

Figure 2: The longitude and latitude positions of all the microflares we trust the position RHESSI finds, about 10,000 events. All bar a few points can be easily associated with an active region. This is clear in the longitude plot as we see the trail of an active region with many microflares occurring as they move across the disk.

We find that for all the microflares we can trust the position information RHESSI obtains they are . These events are shown in Figure 2. In the longitude plot we can see the active regions traced out by their microflares as the move across the solar disk. There are a about 4 outliers which on closer inspection appear to occur during times in which we do not actually trust RHESSI's roll solution. So all the 6-12 keV events RHESSI observes are active region phenomena.

Imaging using visibilities

Of course with RHESSI we can do considerably more than just the times and positions of these events. One thing we can investigate is the image of the thermal emission, which we take to be 4-8 keV in these events. The resulting loop like structures are important as they give us an estimate of the emitting thermal volume and hence thermal energy.

Figure 3: (click above for the animated gif or a quicktime movie is available here) Images, using MEM_NJIT on the 4-8 keV visibilities, for the most prodigious microflaring active region AR10536. The scale changes from image to image, so those with the brighter backgrounds are smaller events. The solid circle indicates the size of the sunspot group and the time profile below the image shows the GOES 1-8A light curve for time +/- 12 hours of the microflare. Note that not all the flares as they are either from other active regions or occurred during periods when RHESSI was not observing with shutters out.

To make the imaging of so many flare practical we use visibilities (as detailed in a previous nugget) as they are quicker to use since you are working with a compact set of calibrated data, instead of the full RHESSI uncalibrated time profile. Shown in Figure 3 are the resulting 4-8 keV images using the MEM_NJIT algorithm ( Schmahl et al. 2007, Sol. Phys. 33), for 16 seconds about the peak time in 6-12 keV for each microflare we associated with active region AR10536.

This active region is particularly prodigious in producing microflares. We can see that some are occurring as repeated emission from the same loop structure. Others are occurring at different locations throughout the active region, either compact points or large loops reaching out from the active region, with one leg in the sunspot group.

A wealth of information

What we have shown here is just a brief overview of one aspect of these microflares showing that they are closely related to solar activity and are associated with active regions.

We have a wealth of information about these events with Forward Fitting shapes to the visibilities allowing us to investigate the spatial scales of the emission and fitting the spectrum of these events allows use to obtain the temperature, emission measure and parameters of the non-thermal power-law. The result is that we can investigate the distributions of both the thermal and non-thermal energies in these events and the implications this has for small flare and coronal heating. Something that will be the topic of a future nugget......