The Halloween Flares and Large-Scale Correlations

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Introduction

The celebrated phenomena of the "Halloween Flares" of 2003 have reappeared, more weakly, in October 2003. In the waning phase of the previous sunspot maximum, a remarkable group of apparently independent sunspot groups appeared within a few days of one another. Something like this has just happened, or is just happening, in the declining phase of our current sunspot maximum ([Cycle 24]). For reasons unknown in detail, and probably unknowable any time soon, the organization of solar activity visible at the surface has large-scale correlations in space and time, meaning that flares bunch up in a highly non-random manner:

October flares

Year    Class M   Class X   MX regions

2003    37          7       5 
2013    25          2       6

From this little table one can see that the 2003 case had an even more extraordinary concentration of major flares in a small (but multiple) number of active regions.

Time scales and Correlations

In our conventional view of the sources of flare energy, we turn to the magnetic field in the corona. But it is the low corona, in active regions, that contains the bulk of the energy, and this appears above the photosphere only when flux emerges from the interior. Now we need to consider characteristic scales of space and time on the Sun. In the photosphere, the convective motions are organized into granulation and supergranulation, the latter also the source of the chromospheric network. The time/distance scales for these features are roughly 10 min/1000 km and 1 day/30,000 km respectively.

Now consider the occurrence pattern of the Halloween flares of 2003 and 2013, as illustrated in Figure 1. The major flares occur in each case on spatial and temporal scales quite inconsistent with what we see in the photosphere. Therefore it seems likely that some slowly-developing, large-scale structure in the deep interior - the legendary [solar dynamo] process - must be responsible.

Fig. 1: The active regions responsible for the major flares of October 2003 and 2013, respectively, obtained from SolarMonitor. In 2003 three regions independently produced X-class flares, and in 2013 one did; the other circle in the right shows the source of an M9.3 almost-X flare, again in an independent region.