But there was a bigger one
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Nugget | |
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Number: | 139 |
1st Author: | Hugh Hudson |
2nd Author: | Lindsey Glesener |
Published: | 8 November 2010 |
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Previous Nugget: | SDO EVE Flare Observations |
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Contents |
Introduction
The Nuggets have been watching the current solar cycle as it begins (see Nuggets [A], [B], [C], [D], [E]); the amazing thing about these Nuggets is that they began in December 2008, when we already were baffled by the lengthy [sunspot] minimum period. It has taken almost two years now for us to reach a level of activity where we might have a major flare, as described below, but not yet.
It has been amazingly slow to set in, but we are beginning to get scientific results on flares from the new cycle. We discuss one of them in this Nugget, and another in a future Nugget.
The Spots Return!
Solar physicists interested in flares are eagerly awaiting the powerful ones of this cycle, since there is amazingly good new observational capability such as the powerful SDO satellite. Success here is measured in flares of [GOES X] category, none of which have yet occurred. We have had eight M-class flares (a factor of ten weaker) since SDO "first light", and we will discuss one of the more interesting ones in the next section. Meanwhile, Figure 1 shows that indeed the areas of [sunspots] are steadily increasing, as expected even if a couple of years late.
This spate of solar activity inspired the Max Millennium Chief Observer to declare a Major Flare Watch condition on November 5. Sure enough, the Sun cooperated and produced forthwith the most powerful flare yet this cycle, an M5.4 event, the very next day, November 6. In keeping with the smooth operation of this alert system, the Chief Observer ended the flare watch on November 7 and no important flares have happened since then.
But this Nugget announces to the world that the barn door in fact was closed too late. A more powerful flare probably occurred way back on November 3; it was listed as merely a C4.9 class, but at that time the active region (which later produced the Number 6 event, and several others) was well behind the west limb of the Sun. This means that its importance could not be easily recognized from front-side observations. We have STEREO, to be sure, but its instrumentation is not complete enough to characterize the magnitude of a flare.
A Powerful Occulted Flare
The STEREO images can locate a flare, if not learn much about it quantitatively. The flare of November 3 occurred at about W12, as viewed from the Behind spacecraft (this is the one heading west, lagging behind the Earth). From our perspective on RHESSI, in low Earth orbit, made it occulted by about 12 degrees; a tangent to the solar limb along our line of sight would pass 10-15,000 km above the site of the flare. This means that RHESSI can study purely coronal effects, without confusion from the [footpoint sources]. Moreover it is high enough to have occulted a large part of the soft X-ray source, making the GOES classification a lower limit. It is our guess from the RHESSI data that this flare was at least as powerful as the November 6 event. We show a couple of images in Figure 2.
Conclusions
RHESSI Nugget Date | 8 November 2010 + |
RHESSI Nugget First Author | Hugh Hudson + |
RHESSI Nugget Index | 139 + |
RHESSI Nugget Second Author | Lindsey Glesener + |