Solar Max Arrives
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Nugget | |
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Number: | 159 |
1st Author: | Hugh Hudson |
2nd Author: | Richard Schwartz |
Published: | 26 September 2011 |
Next Nugget: | TBD |
Previous Nugget: | Two-stage SEE Shows Reconnection] |
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Introduction: The Solar Cycle
The stately progression of solar cycles hit a major snag in between Cycle 23 and Cycle 24, as documented in previous RHESSI Science Nuggets (1, 2, 3, 4, 5, 6), although indeed signs of life had been there. Now there is a spectacular new sunspot group, NOAA 11302, which is highly active at the time of writing. Prior to this group's emergence, we had already had three X-class flares in September 2011 alone. Figure 1 shows where it fits into the generally anemic pattern of sunspot areas in Cycle 24; the histogram on the left compares the Cycle 24 areas with those of Cycle 23, with a vertical dashed line for the area reported for NOAA AR 1302 on 25 September 2011. The histogram on the left is a little subtle: of course there have been fewer spots thus far in Cycle 24, but the comparison of histograms shows a steeper distribution. Thus Cycle 24 has fewer big spots and/or they have not been lasting as long, consistent with the Livingtson-Penn effect.
The flares of September, 2011
Whatever the sunspots are doing, aficionados of solar flares are delighted with the major activity, summarized in Figure 2 from NOAA sources. The spike seen in soft X-rays (left panel here) is the major flare SOL2011-09-24 (X1.9).
We note that this month has seen not one, but three X-class events, in fact only one of them associated with the huge sunspot group illustrated in Figure 1. Sometimes many years pass between flares of this magnitude, so whatever is unusual about the sunspots of Cycle 24, it is not necessarily reducing the flare magnitudes. However we can speculate. Yes, the right-hand panel of Figure 2 shows the presence of solar energetic particles, but not from each event. In fact, these X-class flares have been strikingly impulsive, and that also is a bit of an anomaly, even though they certainly supported CMEs and the usual panoply of coronal radio emission such as type II radio bursts. Are Cycle 24 flares sytematically different from Cycle 23 flares in this respect?
From the RHESSI point of view, the answer is "NO," because we are seeing nice gamma-ray emissions from some of these flares. This link shows the RHESSI Browser view of SOL2011-09-24 (X1.9). This includes the Fermi response to the flare, and future Nuggets will report some astonishing solar discoveries being made with Fermi's large-area gamma-ray telescope at very high energies.
Conclusions
We are happy that solar activity has resumed. It may be, in fact, that this year is the year of the maximum of Cycle 24, which has started in an unusual manner as we've documented in earlier Nuggets. Nevertheless we are getting healthy flare activity and are busy analyzing the new and wonderful data that have become available.
RHESSI Nugget Date | 26 September 2011 + |
RHESSI Nugget First Author | Hugh Hudson + |
RHESSI Nugget Index | 159 + |
RHESSI Nugget Second Author | Richard Schwartz + |