May 21: pub!

It's 21 May, a very significant date. In my PhD thesis I analysed X-ray data from three solar flares, on 10 April, 21 May and 5 November 1980. The data had been collected by the Hard X-ray Imaging Spectrometer (HXIS) on the SMM satellite.

At that time we believed that hard (penetrating) X-rays from the Sun would come from the ends of magnetic loops (like those shown in the accompanying picture of the Sun in ultraviolet light), where they meet the dense solar atmosphere. HXIS showed us for the first time where on the Sun the hard X-rays were coming from. Sure enough these three flares displayed X-rays coming from pairs of points on the Sun, presumably the two end points of the loops involved: "footpoints". It would have been a big surprise if X-rays had come from above the surface - although in due course this too was seen. In Glasgow we weren't the first people to look at these footpoint observations but we tried to see what more they could tell us about the workings of flares.

The 21 May flare was one of the most intensively studied at that time. It was a big (X-class) flare, it did lots of different, interesting things and was inspected by several leading edge instruments, like HXIS. So many people wrote articles on aspects of this flare that it eventually played the starring role in its very own review article, where two famous solar astronomers summarised the various studies and drew them all together.

Many individual solar flares have been important in our developing understanding of these events: the first big flare seen in some new instrument, a flare that did something in a particularly simple way so that cause and effect seem clearer than in most cases, a flare that did something dramatic never seen before.

Over the decades there have been many such significant events. This was useful when I was young and frivolous. Today, for example, we could say, "it's the anniversary of the 21 May 1980 flare. We need to celebrate this - let's go to the pub!" In fact, if we scoured the solar physics literature we could probably find a solar flare to celebrate on most days of the calendar, especially now a couple of decades on: 13 January, 20 January, 23 February, 24 May, 3 June, 7 June, 14 July, 28 October....that's enough, you get the point, and I'm sorry so few of them have nice web resources and so many of the links are technical. Maybe that should be a wee job for somebody: "Flare of the day" blog. Anyway we never needed to wait very long to have an excuse for a wee pub visit, and if we really needed an excuse we could probably scour the literature and find somebody with their own wee solar observatory, lost and forgotten in the woods or clinging to some unvisited mountainside in some far-off and exotic land, who had observed a flare on that particular date.

Anyway, nowadays those pub visits are much rarer. We probably only used a famous solar flare as an excuse on a few occasions, to be honest - most of the time we didn't worry about excuses. But as I headed home on the bus this beautiful May day, I spotted lots of people sitting outside enjoying a beer or a glass of wine and I was glad to see that the X-class footpoint flare of 21 May 1980 is still celebrated vigorously.

Image: NASA Solar Dynamics Observatory AIA instrument ultraviolet image of solar loops from January 2012

"German satellite hurtling towards Earth"

ROSAT, a German satellite is indeed "hurtling towards Earth", as the Guardian and many other news outlets have been telling us in recent days. Well, "hurtling" isn't quite accurate, but it'll reach us sometime in the next few days as its orbit gradually decays (in fact, since I typed this it looks as though it's come down).

I was slightly surprised to learn it hadn't come down a long time ago. It was a pioneering X-ray astronomy space mission of the early 1990s. When the first X-ray detectors were thrown briefly up above the atmosphere on rockets, there were no great expectations. This was in 1962 and imaginations stretched only to a faint glow from the Sun's million-degree outer atmosphere, or possibly from the Moon's surface as energetic particles in the solar wind smashed into it. As the world knows, the results were much more exciting: bright, point sources of X-rays beyond the solar system, among the stars of the Milky Way; a cosmic "background", X-rays coming from all directions on the sky; X-radiation from the Crab Nebula. A new window had opened on the high-energy universe of black holes and neutron stars; nobody thought much about X-rays from the Moon for a long time.

ROSAT closed this particular historical circle by detecting, for the first time, X-rays from the Moon. The ROSAT image of the Moon, illuminated only on its sunward side, looks so familiar that you might need to look twice and think about it to realise just how amazing it is. It is lit up only on the sunward side because the Moon's surface is reflecting X-rays from the Sun (strictly, scattering those X-rays, and apparently also fluorescing).

ROSAT was used to make many important advances in X-ray astronomy; a brief list is here. I always had a soft spot for it because it closed the lunar loose end. None of this prevented its fiery doom, however, which no doubt is as it should be. None of us escapes his doom, watery, fiery or otherwise, no matter what sort of songs we've sung in the meantime.

None of the news reports let us forget it was a German space mission, rather than, say an "X-ray telescope", "orbiting X-ray laboratory" or some such border-less phrase. I wonder why? If it had actually fallen on somebody's head, would the German nation have been held to account? Would we have seen invoked the ghost of Wernher van Braun? I'm really not sure what's behind this.