# Statistical analysis of RHESSI microflares

As published in two major papers (Christe et al., 2008; Hannah et al,
2008) RHESSI has great sensitivity and can observe both hard and soft
X-rays from microflares in an unprecedented manner. These papers
described a sample of about 25,000 events observed over RHESSI's first
five years. These data confirmed the strong similarity of the
microflares with "real" flares orders of magnitude more energetic; they
showed that essentially all of them occur in active regions and that
they are too intermittent, both in space and time, to "explain" the
heating of the general corona. Nevertheless the energy represented in
these events is roughly comparable to that needed to support the general
corona or the non-flaring active-region loop structures. The Figure
below shows this sample distributed in heliographic coordinates (note
that this is approximate in that no B-angle corrections have been made
for this plot).

![](media/image1.png){width="3.3666666666666667in"
height="3.2784722222222222in"}

Figure caption: Heliographic coordinates of \~25,000 microflares
observed by RHESSI during its first five years of operation.

There are many open questions regarding the physics of microflares, as
described below, and the analysis will be doubly interesting with access
to the new sample that will arrive with the first high-latitude events
of the new solar cycle. The microflares tend to be smaller dimensionally
than more energetic events, although this remains to be quantified in
detail. As pointed out by e.g. Aschwanden et al. (2008), at some point
in spatial scale, the physics may change not only quantitatively, but in
major ways as well -- for example, a flare-like reconnection event in
the deep atmosphere, below the temperature minimum, will take place in a
different environment (for example, the Alfvén speed may be much lower).

The limb events are particularly interesting. A glance at the Figure
shows that many locations lie just above the limb (note that RHESSI's
high angular resolution -- precise at the sub-arc-second level for
centroiding, even for weak events. Thus a study analogous to that done
by Matsushita et al. (1992) for the much inferior HXT observations needs
urgently to be carried out. This and related studies can now be done in
the context of the STEREO data, which will greatly help in precise
understanding of the geometry (even in 2D, ie just for footpoint
identification) of limb events seen by RHESSI. Even without STEREO, ie
for the Cycle 23 sample already published, we will be able learn about
flaring loop altitudes and spatial scales. This will be particularly
interesting, now that we are sure of the identification of microflares
with active regions, because we can compare the flaring loops with
non-flaring loops. Note that Hudson et al. (2008) suggest that these are
usually physically distinct, ie that the pre-flare loops are not already
visible as hot active-region loops.

The use of RHESSI observations to study the departure from the thermally
relaxed state is particularly interesting, since the shutter system (Lin
et al. 2002) gives us access with great sensitivity and high time
resolution to the crucial 3-20 keV continuum spectrum, as well as to the
Fe feature resulting from K-shell transitions (e.g. Dennis et al. 2005).
The RHESSI observations of microflares have shown the general presence
of high temperatures (as revealed by the Fe feature) even in these weak
flare events.

We summarize our goals as follows:

• Analyze the data shown in the figure statistically for microflare
geometry.

• Systematically study microflares well-observed by both RHESSI and
*Hinode* to learn further about preflare physical conditions.

• Analyze in detail the set of limb events well-observed by RHESSI and
imaged on the disk by one of the STEREO spacecraft.

• Study physical conditions in the microflare thermalization stage
relaxation, making use of the Fe K-shell line feature well observed in
RHESSI time series.

• Begin the accumulation of a rise-phase sample of microflare events.

# References

Aschwanden, Stern, R.M., and Güdel, M., ApJ 672, 135L (2008).

Christe et al (2008) **microflares paper I**

Dennis, B. R., Phillips, K. J. H., Sylwester, J., Sylwester, B.,
Schwartz, R. A., and Tolbert, K. A. (2005) **COSPAR paper**

Hannah et al. (2008) **microflares paper II**

Hudson, H. S., Hannah, I. G., DeLuca, E. E., and Weber, M., in *Proc.
1^st^ Hinode Science meeting* (2008).

Lin, R. P. et al. (2002) **overview paper**

Matsushita, K., Masuda, S., Kosugi, T., Inda, M., and Yaji, K., Solar
Phys. 44, L89 (1992).

Table SAS-1: Phenomena observable with precise radius measurements

-   p-modes !

-   g-modes

-   r-modes ?

-   Granulation

-   Other convective motions

-   Sunspots !

-   Wilson depression

-   Faculae !

-   Active network !

-   Flares

-   Prominences

-   Coronal holes ?

-   Oblateness !

-   Oblateness time variations ?

-   Higher-order shape terms

-   Gravitational moments J2, J4...

-   Global temperature variation ?

-   Limb-darkening functions

-   Planetary tides

-   Dynamo signatures ?