Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 38, 2002)

Science Nugget: Sept. 20, 2002

Mysterious Anomalies in better detail

What is it?

The South Atlantic Anomaly is a great nuisance for satellites (only those in low Earth orbit, though) devoted to studying X-rays and gamma-rays from space - Yohkoh, for example, but not Chandra, which is in a deep elliptical orbit. The figure below, with text as ripped off from a ROSAT page, explains what it is:


It is, to be precise, a region of the Earth's magnetic field that is a little bit weaker, resulting in a superabundance of Van Allen Belt particles. This superabundance is the red blotch in the map above. Except for an orbit almost exactly equatorial, which is energetically expensive as a rule, it cannot be avoided and therefore is a terrible source of background problems for high-energy instruments, since these particles produce spurious signals in these detectors. If only the Earth's magnetic field were beautifully symmetric (sigh!) life would be easier. As it is, every satellite in low Earth orbit is doomed to fly through the SAA, as it's known familiarly, every day.

What is new?

We now have RHESSI, which is (joke) the first huge germanium spectrometer devoted to magnetospheric research. The image below, from S"am Krucker's RHESSI and WIND summary plots, shows how well it works:

sam's plot

This needs some explanation. It is a multi-panel display, and we are only interested in the next-to-the-bottom panel (click to enlarge). This is a "dynamic spectrum" display of the RHESSI counting rates. The vertical dotted lines show orbit night intervals; during the orbit days one can see a nice solar flare at about 02:25 UT, but also two dreadful background effects at about 01:00 and about 02:10. These are not SAA ("The Anomaly") but are anomalous counts nonetheless. They occur when RHESSI reaches the northernmost (or southernmost) point of its orbit, and are highly variable in time structure and intensity. The spectrum shows that it has a maximum in the tens of keV range, unlike the flare, which peaks at a much lower energy. This is one of the ways that we recognize the nuisance and ignore it if possible!


These anomalous counts are trapped Van Allen radiation. With RHESSI we can see it in much better detail than have previous high-energy astronomy missions (but presumably not better in some ways than previous magnetospheric missions!). Perhaps with this better detail we can learn how to anticipate the background problems that these particles cause, and to compensate for their presence.

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September 20, 2002

Hugh Hudson hhudson@ssl.berkeley.edu