Oft and Hard X-rays, Flares, and the Corona

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This Nugget further discusses the remarkable [GOES] database of solar soft X-ray fluxes, but at the opposite end of the activity spectrum from the previous Nugget. Here we are interested in the quiet Sun, and specifically in understanding the relationship between the diffuse hot corona seen outside active regions, and the soft X-ray emission from the quiet Sun.

In stellar astronomy the X-ray luminosity (LX), and its ratio to the total luminosity, plays a central role in defining a star's coronal structure. The GOES data provide a measure of the solar (LX), but for historical reasons this measurement is at a higher photon energy than for typical measurements of [stellar X-rays]. That can make comparisons a little tricky.

The solar X-ray corona

Soft X-ray images, such as the iconic one (Figure 1) from Yohkoh/SXT show the X-ray corona to have a very large image dynamic range. Even outside coronal holes, so-named because of their blackness in the original Skylab images, the diffuse global corona is much fainter than the active regions. That is why nonlinear scaling is invariably used to represent such images.

Figure 1: The classic full-resolution soft X-ray image from Yohkoh/SXT, from 8 May 1992. Non-linear image scale makes the faint features visible here, as well as the bright ones.

This image is a snapshot, and movies of the corona show that it has ceaseless activity. But not quite ceaseless, because on larger spatial scales the structures seem remarkably stable with time. The spatial structure of the coronal variability has considerable significance for our understanding of the underlying plasma physics. The best and most detailed views of this variability now come from the extreme ultraviolet via the Solar Dynamics Observatory; these views closely reflect the basic X-ray emission from the hot corona.

The X-ray variability of the Sun as a star

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