White-light Flares

From RHESSI Wiki

Introduction

Solar flares happen frequently and spectacularly, as these nuggets testify. Human knowledge of this remarkable (and still unexplained) phenomenon is rather new. In 1859, Carrington, a typically English, and financially well-enough endowed, amateur astronomer observed a powerful solar flare by accident; he (and R. Hodgson, another observer) > knew they were onto something important. This first flare must have been extremely bright, because in 1859 there were no specialized filters and no X-rays either - Roentgen was still in the future. Carrington was circumspect in his description of the event ("...the brilliancy was fully equal to that of direct sun-light..."). The other observer, Hodgson, was much more effusive: he said that the flare was "..very brilliant... much brighter than the Sun's surface... most dazzling... compared to the dazzling brilliance of the bright star alpha Lyrae..."; either this was indeed an exceptional flare, since no other flare could be described this way, or else Hodgson was given to overstatement.

RHESSI and TRACE

Solar observations have come a long way since Carrington's original flare observation; we now have observatories in space. The sesquicentennial year (2009) of Carrington's flare is coming soon. By then we should be in the rising phase of the next solar cycle. In particular RHESSI gives us the best-ever hard X-ray data, and " TRACE gives us the first high-resolution optical and UV observations of these events.

It is known that a close association exists between white-light flares and X-rays (energetic X-ray flares are more likely to have white-light emission). This association could have been known in 1859 if only Roentgen had been more prompt in discovering X-rays.

The TRACE "white light" data differ from all previous observations, in that a special UV-sensitive coating on its CCD allows it to observe far into the ultraviolet. This part of the spectrum has never before had adequate photometric observation. The importance of this UV response is that TRACE images capture, for the first time, the bulk of the radiant luminosity of a solar flare. Another recent pioneering observation, the SORCE total-irradiance measurements, does something similar and was described in a previous RHESSI nugget. Figure 1 is an example of TRACE images of a white-light flare:

Figure 1: CME kinetic energy derived from LASCO images plotted against the energy of various components of the associated flares as indicated. The diagonal line is the line of equality between the CME and flare energies.

These images cover a time interval of 30 sec; the vertical scale is about two arc min and the emission appears as two highly warped "ribbon" structures. The extraordinary time-and-space variability ("intermittency") can be seen better by clicking on the image; the full view shows the individual 0.5 arc-sec TRACE pixels.

Is white light different, and where is the energy?

Below (Figure 2), we compare the RHESSI hard X-rays and TRACE white light in terms of time variations:

Figure 2: Red, RHESSI hard X-rays; blue, TRACE 1600 Â.

This log-scaled figure is a little deceptive, but it shows clearly the exact times of hard X-ray integrations (red) and white-light snapshots (blue). As has been seen many times before, the light curves have striking similarities (a common peak time, within the relatively high resolution of these data) but intriguing discrepancies (the light curves don't agree well either early or late, ie outside the "impulsive phase"). There has also been a single <a href="http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ApJ...406..306N&db_key=AST&data_type=HTML&format=&high=3f9e7b5a9f29431"> ground-based</a> white-light flare observation with really excellent time resolution, and with it Don Neidig found hints of interesting tiny timing differences.