At last, the EUV Spectrum
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obvious one that the EVE instrument can indeed make brilliant observations | obvious one that the EVE instrument can indeed make brilliant observations | ||
of the Sun as a star, and can characterize the EUV spectrum of the impulsive | of the Sun as a star, and can characterize the EUV spectrum of the impulsive | ||
- | phase. | + | phase of flares. |
- | The importance of this capability | + | The importance of this capability lies in the information we can glean about |
the fundamentally important energetic processes of a flare - its luminosity | the fundamentally important energetic processes of a flare - its luminosity | ||
and its eruption of mass, both into the corona and further out into the solar | and its eruption of mass, both into the corona and further out into the solar |
Revision as of 15:48, 7 February 2011
Nugget | |
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Number: | 145 |
1st Author: | Hugh Hudson |
2nd Author: | |
Published: | 7 February 2011 |
Next Nugget: | (21 February 2011) |
Previous Nugget: | Black and White Flares |
List all |
Contents |
Introduction
We can detect stellar flares easily, in spite of the competing brightness of the star. Until recently, though, we could not do that for solar flares: only through imaging, as in the original Carrington flare, did the excess emission from the flare actually become detectable. Partly this reflects the much greater surface brightness of the Sun, as compared with that of the usualy faint dMe flare stars, and partly no doubt it reflects the relative ease with which ground-based astronomers can do precise photometry on a point source. Now (at last!) we have clean observations of the all-important impulsive phase of a solar flare.
New "Sun-as-a-star" observations of solar visible and EUV emission have revolutionized our understanding of the energetics of solar flares. Figure 1 of an earlier Nugget shows the breakthrough observations (from the SORCE satellite) of the flare SOL2003-10-28, a GOES X17.2 "superflare." Viewing the Sun as a star (i.e., measuring the total solar irradiance or solar constant), the SORCE radiometer found a trifling 0.03% increase for this huge flare. No wonder it took a century and a half of technical development beyond Carrington to get to this point.
In this Nugget we describe a major step towards identifying the source of the SORCE radiation, which (as many suspected) has a close relationship to the white-light flare mechanism.
A well-observed gamma-ray flare
The newly-launched Solar Dynamics Observatory has a Sun-as-a-star EUV spectroscopy instrument, EVE, led by Tom Woods. For the first time we get uninterrupted spectra (at 0.1 nm resolution and 10 s sampling) of basically the entire EUV spectrum. The instrument has other features, including a set of very sensitive broad-band EUV radiometers. Figure 1 shows the passbands of these photometers, plus their responses to the flare we described in the previous Nugget: a flare exhibiting white-light continuum and gamma-ray emission.
This figure only gives a glimpse of the data, which included many details of individual spectral lines and their properties. Note that even solar flares have excellent observability in the EUV, because the photosphere has too low a temperature to emit much at these short wavelengths.
UV spectral synthesis
We would like to make a spectrum that shows just the impulsive phase of the flare. Figure 2 (left) shows when this phase happened for this event. We have used this as a guide and generated the broad-band spectrum in Figure 2 (right). We believe that this spectrum represents the first of its kind, just like the SORCE observation of the TSI signal described way back in Nugget No. 10.
The spectrum (Figure 2, right) shows a spectral energy distribution: we have plotted νfν (the same thing as λfλ). Such a plot has equal areas for equal energies per decade (or per octave), so that one can see at a glance which wavelengths dominate. In this case the EUV and X-ray bands do not dominate; instead we have narrowed the principal energy contribution to the near UV. EVE has data that cover this range as well (though not for this flare), and we look forward eagerly to analyzing them.
Conclusions
This single flare hardly lets us draw sweeping conclusions, except for the obvious one that the EVE instrument can indeed make brilliant observations of the Sun as a star, and can characterize the EUV spectrum of the impulsive phase of flares. The importance of this capability lies in the information we can glean about the fundamentally important energetic processes of a flare - its luminosity and its eruption of mass, both into the corona and further out into the solar wind as a coronal mass ejection.
References
RHESSI Nugget Date | 7 February 2011 + |
RHESSI Nugget First Author | Hugh Hudson + |
RHESSI Nugget Index | 145 + |