Introduction

Launched on February 5, 2002, RHESSI has now passed its fifth anniversary in space as a working observatory - through 27,437 orbits of the Earth as of the birthday.. Those familiar with RHESSI will recall that a last-minute accident greatly delayed the launch, and as a consequence we missed the great solar activity of 2001. Did this matter, in retrospect? This Nugget celebrates the launch by pointing out that no, contrary to what we confidently expected (a dead Sun by 2003), what we got was quite a different thing. Amazingly enough, there were X-class flares (four of them!) in December 2006, and we have not yet entered solar minimum. One of these was featured on the Astronomy Picture of the Day as a "tsunami".

First, though, let's congratulate ourselves on how well RHESSI works. Figure 1 below shows how RHESSI's inexpensive mechanical refrigerator has performed.

Figure 1: Efficiency of the Sun Power cryocooler over the life of RHESSI thus far. The yellow curve shows the cryooler efficiency (temperature differential per unit power), and the red curve shows the microphonics level. Both, after five years, look "nominal."

Weird patterns of activity

The main reason for our anxiety was that the Sun had essentially turned off, in terms of major flare activity, in 1992. The plots below illustrate this. Most forms of solar activity follow a somewhat sloppy 11-year cycle, which is actually like a rectified form of the 22-year Hale magnetic cycle of the Sun. The year 1991 was rich in X-class solar flares, as was 2001 - that was RHESSI's original quarry. But 1992 was a bad year and between then and 1996 there were exactly NO X-class flares. Thus the RHESSI team was concerned that maybe our years beyond 2002 would be duds, and we'd be left with no gamma-rays to study.

Figure 2: Joint variation of annual X-class flare numbers (Y-axis) with sunspot number (X-axis). The arrows show the direction of the decaying phase of Cycle 22 (blue), what we worried about - four years with only one X-class flare! But the decaying phase of Cycle 23 (red), starting in 2001, was entirely different, and better.

This kind of behavior has no real theoretical explanation. Solar activity results from the generation of magnetic fields in the solar dynamo, which varies inexplicably. Apparently there is something else inexplicable in terms of the association of X-class flaring with a given level of solar magnetism. This would be an abstruse point of astrophysical theory, except that these X-class flares may have dire consequences for the heliospheric environment ("space weather") and can restrict human activities. Thus there is a major effort under way to understand and predict these phenomena, either by hook or by crook.

The flares of December 2006

There were four X-class flares in December 2006, which we've already described in a science nugget on the last best active region of the cycle. The RHESSI data have now been analyzed more carefully and show clear evidence for gamma-ray emission, as shown in Figure 3 below, as well as spectacular CMEs and aurorae (the Figure shows an example from the Spaceweather aurora gallery).

Figure 3: (Left) The RHESSI spectrum shows the 0.511-MeV gamma-ray line produced by positron/electron annihilation. Click on the image for a fuller explanation. (Right) The aurora picture is from Reykjavik, Iceland (part of which is shown as the city lights below the bright aurora) and was taken by Gunnlaugur Juliusson. It was from Dec. 15, so from a later phase of the December activity.

Now we will show you our movie of the December 6 flare, as seen by RHESSI and TRACE (do not click on the thumbnail image unless you're ready for a 40-MB download). The movie shows a two-ribbon flare as seen in TRACE UV images, but RHESSI sees a much more concentrated emission. Note that the soft X-rays and hard X-rays are concentrated in quite different locations. We do not understand this at present.

Figure 4. Click on the image to the right for a movie showing TRACE and RHESSI data for the December 6, 2006, flare. The blue shows UV emission observed by TRACE, and the red and green, respectively, show RHESSI soft and hard X-ray sources. Note how each spectral band seems to be telling us something quite different.

Conclusions

RHESSI has suddenly entered middle age, namely 5 years, and the recent flare activity is a bit of a surprise to us. Solar activity has persisted relentlessly even though we keep expecting solar minimum to happen. The gamma-ray spectrum shown above establishes that RHESSI is still functioning even for high-resolution spectroscopy, something that we know to be degrading because of expected particle damage to the Ge detectors. At some point (stay tuned for a Nugget on this interesting subject) we plan to anneal the detectors and thus to correct this degradation. This is a procedure often carried out by the Integral spacecraft, which has similar Ge detectors. In the meanwhile we have plenty of data to work with in trying to sort out the high-energy aspects of solar flares and CMEs.

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Biographical note: Pascal Saint-Hilaire and Gerry Share are junior and senior RHESSI team members in Berkeley and NRL, respectively. The editor of the nuggets helped with this one but our format only permits two authors to be listed!