The microflare Height Distribution

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Number: 81
1st Author: Steven Christe
2nd Author: Hugh Hudson
Published: 28 January 2008
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RHESSI has been operating for over five years now and has observed its fair share of flares, including a myriad of microflares (see this previous nugget). In this nugget, we add to the microflare statistics discussion already begun and continued in two papers ([1] and [2]). RHESSI is unusually good at observing these small events due to its use of automatic shutters which allow high sensitivity. During intense solar activity these shutters close automatically, and during quieter times they open again for full sensitivity.

Microflare Positions

We have already discussed some initial results regarding the positions of these microflares. As shown in Figure 1 (see further discussion of this Figure in that earlier nugget) RHESSI finds microflares only inside magnetic active regions. These positions are as observed, meaning that has not been removed; hence the smeared active-region bands.

Figure 1: The observed position of every microflare observed by RHESSI between March 2001 and March 2006. The active latitudes are clearly visible but have some artifacts. Solar tilt has not been removed from these data. One obvious aspect of this image is that there are many many microflares near and at the limb. This is the topic of this Nugget. This is great news: the distribution of microflares at the limb can reveal the height distribution of the microflare sources.

Microflare Limb Distribution

In to get a proper limb distribution, one must be sure to take into account the apparent variation of the solar limb which changes by about +-1.7% due to the Earth's elliptical orbit. The following Figure 2 shows the the limb distribution as a function of the solar radius for 6-12 keV flare sources. This energy band is dominated by thermal emission with some nonthermal emission may also be mixed in. It is clear that the number of events should increase as one looks closer to the solar limb because the corona is optically thin to X-rays. The distribution above the limb clearly shows something more interesting. The peak in the distribution suggests a minimum height for the 6-12 keV sources. In order to test this hypothesis a simple model was developed wherein microflares can occur at any longitude and with latitudes similar to observed microflares. This last point may be important as we know that flares occur in the two distinct active region bands and not, say, at the poles. A distribution of heights was then assumed. In this case, the height distribution is assumed to be exponential with a scale height h0, and a minimum height of hmin. A Monte Carlo simulation is then performed where random locations and heights are generated which are then projected onto the solar disk. The results can be seen in the next figure. For comparison, a "flat" flare distribution (with height set to zero) was also simulated. The results can be seen below in Figure 2.

Figure 2:The height distribution of the observed microflares at 6-12 keV as a function of solar radii (red). Blue shows the results of a simulation for flat model microflares, ie ones with zero height, while the black curve represents a simulation of microflares with an exponential height distribution as defined in the text. We find that rather good fit for a minimum height of about 3000 km with a similar scale height. Of course, this fit is not perfect. The simulated distribution seems too sharply peaked but this is a reasonable first cut.


In this Nugget, we've just begun our exploration of microflares in 3D. With such great statistics, we can now get a handle on the height distribution of microflares. Stay tuned for more!

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