Comets

In one of your lectures you talked about comets, and helped your audience understand comets and their relation to solar winds. Let's talk a little about how you got interested in that problem.

I knew what everybody else knew about comets, that there was this remarkable, and as often happens in astronomy, accidental discovery that comets are actually a source of a very energetic phenomena, x-rays. It was accidental, in that there was a spacecraft carrying a telescope in orbit, and by chance a comet came into the field of view of the telescope. It was an x-ray telescope, and there was a signal, and so they discovered that comets are sources of x-rays. That's totally unexpected, it surprised everybody. They hadn't bothered to look, because there was no point in looking. It's also rather difficult to track a rapidly moving object like a comet. But in any case, they didn't look with their instruments, because the comets are cold objects, they're not sources of energetic phenomena. And yet, they are.

So I know a lot of atomic and molecular physics, and the processes that produce x-rays are essentially part of atomic and molecular physics. So I know all the processes. At least, I thought I knew all the processes that could produce x-rays. A number of people were working on this topic, not just me. Anyway, in thinking about it and making various suggestions -- how it could be happening? -- and finding that none of them worked when you worked out the consequences, then going through that list, there was one that did work, or that potentially could work. One couldn't dismiss it immediately. My research is, in a sense, rather straightforward. It was to take this suggested mechanism and work out its consequences in detail enough that it could be compared directly with observations. And it matched.

Since I never thought of this process before as a source of x-rays and astronomical environments, then the question became, "Well, where else might it be happening?" And it then became apparent it actually is contributing to what is called the diffuse, "soft" x-ray background. It was something that had we known for a long time, the existence of such a background, and it's clear that this mechanism that works in comets is actually going to contribute in a larger sense as well. So the topic sort of grew, and it grew from starting with this accidental discovery.

What were the reasons for rejecting the causal factors that were rejected, mainly the experimentation in the lab that led to the conclusion that certain explanations were not adequate?

It was the larger experiments or the totality of experiments, it's the basic knowledge that we have about the processes. One possibility was simply that x-rays on the sun itself could just be reflected by a material of the comet, and it would be like a mirror and we're just seeing the reflective side of the radiation. Well, we understand that well enough in quantitative terms to be able to show two things about it. One was that it would not be intense enough, it would be very weak. But also it would have the wrong spectrum --actually, a variation of energy of the x-rays that had been observed by the telescope would not be produced by this process.

So it was a question both of getting the spectrum right and getting the intensity right. All these other mechanisms that one might think of have failed.

What was the mechanism that seemed to work?

It's a rather complicated mechanism, but the sun, the solar wind, does have a small component of what are called heavy elements. They're fully ionized: they have no electrons, they're just bare nuclei. We know from laboratory experiments and also our theoretical knowledge of physics that when those nuclei collide with a neutral atom of the cometary atmosphere, they capture electrons into high states, which radiate x-rays. So there is the process.

It's the knowledge and understanding of atomic physics that points in that direction and proves the validity of the conclusion.

Yes. Not only is it a response for identifying the mechanism, but it validates it. So one does require, at least for this particular interpretation, an extensive knowledge of atomic and electrophysics.

Going back to the journal, an insight like this is reported in the journal and leads to, as you were suggesting earlier, all kinds of other research. In other words, it opens up new areas to pursue with broader implications for the study.

Absolutely right, yes.

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