X-ray and UVWL footpoints

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Number: 76
1st Author: Lyndsay Fletcher
2nd Author: Hugh Hudson
Published: 21 January 2008
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A "white-light flare," as originally observed in 1859 by Carrington,, revealed the participation of the photosphere itself in the flare process. The mere fact that flares can be observed this way (visually, with a small telescope) says that the energy of a solar flare can be highly concentrated. The fractional surface area of the Carrington flare, multiplied by the solar luminosity, gives a flare energy of order 1032 ergs, consistent with the total energy of a flare estimated in other ways. Figure~1 shows a breakthrough here, in that the total solar irradiance monitor on board the SORCE satellite has at last detected the excess emission of a solar flare bolometrically.

We argue in this Nugget that the RHESSI "footpoint" sources, the subject of many nuggets, can be closely identified with the photospheric/ chromospheric sources of the UV and white light emission. We could call these "UVWL footpoints" because the continuum radiation that dominates flare energy release overlaps the ultraviolet (UV) and white light (WL) spectral regions. Thus high-resolution images at these wavelengths, can serve as a good proxy for the hard X-ray sources and locate the primary energy release of a flare.

Figure 1: A breakthrough observation: SORCE observes the excess bolometric luminosity of a solar flare for the first time. It is not so clear from this figure that the emission, presumed to be largely in the UV, has both an impulsive and a gradual component. Note also the "large" fluctuations away from the flare. These result from convective flows and global oscillations. The white-light emission links the coronal energy storage, in our current view of flares, with its loss to excess radiation - the flare itself (see our previous Nugget). The earlier Nugget used TRACE and RHESSI observations to confirm the close timing relationship between this powerful continuum emission and the X-rays showing its origin.

The energetics

We can make a quantitative comparison of the energy derivable from the hard X-ray spectrum (RHESSI) with the energy inferred from the TRACE WL/UV excess emission. This latter dominates the flare luminosity. To deduce the energy radiated in WL/UV from the TRACE data, it is necessary to have a model of the emission spectrum, since we only have observations in two TRACE passbands. The curves in Figure 2 below show the spectral responses of the two TRACE passbands and the spectra according to three different models: a simple blackbody, a model featuring the Balmer continuum, and a full-fledged calculation, due to Joel Allred, that makes use of the physics of hydrodynamics and radiative transfer to compute a more realistic output spectrum from the flaring atmosphere. We are not yet able to use the Allred spectrum, but with the blackbody model and the Balmer model we are able to calculate an upper and a lower limit to the energy radiated in UV/WL. The question then is, what does this mean for the electrons that are believed to power that emission?

Figure 2: Left, spectral distributions of the two TRACE passbands (dash-dot curves) and three models of flare emission: BB is a blackbody, BP a model of Balmer and Paschen continua, and RH a more realistic "radiation hydrodynamics" model computed by J. Allred. Note the hydrogen line emission features in the RH model. Right, correlation between nonthermal electron power (estimated from the hard X-ray spectrum) and WL/UV continuum power, estimated from the spectral dependence described in the left panel. Using the well-understood theory of the collisional thick target, applied to the HXR sources coincident with the UV/WL foopoints, we have been able to deduce that accelerated electrons above 25 keV would have sufficient energy to power the white-light flare, and by extension, the entire flare luminosity. Now we can focus on how this happens, i.e. what is the physics that converts the excess energy of the coronal magnetic field, stored there by a current driven through it, into fast particles? If the source electrons come from the corona, which seems less likely now, they must be accelerated to a thousand times their thermal energy.

What do we know about the footpoints?

The physical conditions in the footpoint regions of flaring coronal magnetic loops clearly must be important in understanding the overall process, since that is where it focuses the bulk of the energy. The most elementary observable would be their scale (spatial and/or temporal). This would help to estimate the energy density. From hard X-rays, e.g. as seen in a previous Nugget, we get only limited spatial information. This is because RHESSI's finest collimator has only a 2.6 arc s resolution, and because of the way RHESSI works this fine detail may be obscured by larger-scale structures. Thus TRACE, Hinode, or any good ground-based telescope can do far better. Figure 3 (below) shows an example.

Figure 3: TRACE white-light image (note the big sunspot) from a white-light flare observed on July 24, 2004. The orange contours show RHESSI hard X-rays, and the blue contours show white light excesses.


Based on the study sketched above, we believe that the true spatial scales of the hard X-ray sources are not yet known, since TRACE does not resolve their intimate counterparts at other wavelengths with much better resolution. An instantaneous image of a single footpoint hard X-ray source might show structure at a scale of a few hundred km (areas much less than 1016 cm2). This means that flare heating is much more intense than most models heretofore have envisioned.

Biographical note: Lyndsay Fletcher and Hugh Hudson are RHESSI researchers at the University of Glasgow and the University of California, Berkeley, respectively.

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