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Our satellite, Yohkoh, has the observation of solar flares as its highest priority. That's why it carries the pioneering hard X-ray imager HXT; this instrument focuses almost completely on non-thermal effects such as particle acceleration, a characteristic of solar flares. We mention this because most of the science nuggets don't deal with flares (see please the topic index for the first four years' worth of science nuggets).
So, when a colossal flare such as the X20 event of April 2, 2001 occurs (see NOAA's nice explanation for information on how flares are classified (what does X20 mean?), and much, much more) we have an obligation - more than that, a compulsion - to write a nugget about it.
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These standard NOAA plots show the solar X-ray flux (top); the flare is the major peak; solar particles from the flare (or its CME) reaching the Earth almost immediately (lower plot); and then within two days a magnetic storm on the Earth (red histogram), resulting from the collision of the flare ejecta (or CME) with the Earth's magnetosphere.
Coverage? - Well, we missed the maximum phase of the event, and we had a little gap during the rise phase, as shown in the plot below:
The lower plot gives a better and more discouraging view of the coverage, though; it is a linear plot, and one can see that most of the rise phase (the most interesting) was missed. Tant pis!
Interesting morphology? - Some SXT images:
The matrix of images above shows images from 21:29 UT, 21:36 UT, and 22:16 UT (compare with the plots above), ie from the preflare, earliest rise phase, and late phase. The remarkable thing here is the relatively small size of the late-phase arcade - compare please with the physical size of the Bastille Day flare (see our new toy, the topical index, subject "Bastille Day Flare" (not "Murphy") - although much less energetic, it was much larger in physical area. More on this anon.
Flare ejecta?
The data above aren't as clear on this point a movie would be, but in fact SXT does not see a flare ejection in this case. We attribute this to timing, since there was a coronal mass ejection, which invariably seems to involve flare ejecta too.
Hard X-rays? - There's a time profile in the coverage plot above, showing the HXT 23-53 keV channel. A nice event, but we missed the nicest part.
We can't learn much that is novel from this event, at least by nugget deadline, but there is one interesting thing to be obtained from the first whole-Sun image in our current archive, taken half a day after the X20 flare:
Its remnant, a bright arcade, can be seen on the upper right. In the meanwhile, yet another X-class flare occurred almost diametrically opposite - lower left. Thus we learn that distinct active regions can make major flares almost simultaneously. This fact has been debated. What's happened here is consistent with the basic idea that the emergence of magnetic flux from the photosphere, rather than the collapse of the coronal magnetic field, drives flares most directly.
That should be obvious from the above! We missed the fun part of this event, as often happens, so we need another one (preferably one jointly observed with HESSI). Huge events show us more of the things we like to study (Kahler's "Big Flare Syndrome").
April 6, 2001