A tiny white-light flare

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Revision as of 02:02, 7 December 2009

Introduction

We are accustomed to thinking of white-light flares - those detectable to the (protected) naked eye - as the most spectacularly energetic of all flares. That is how [Carrington] discovered flares in 1859, after all. It is generally true that a flare has to be energetic on the scale of Carrington's original one for naked-eye detection, but with sensitive instrumentation one can readily detect weaker events. This Nugget describes a flare at GOES class C6.2 - weak - that produced a clearly detectable signature in the [TRACE] white-light channel, as well as in one each of its UV and an EUV channels. This combination of observations lets us describe the broad-band continuum spectrum, or the [spectral energy distribution] as the astronomers quaintly refer to it.

The flare

The event was weak, but was from the same active region that later produced the Halloween flares, so it was noteworthy in that respect. Although it was weak in soft X-rays, the most common index defining the energy of a flare, it was relative bright in microwaves (360 SFU at 15.4 GHz, according to the [RSTN] network); one SFU is a unit of spectral energy flux: 10^-22 W/m²Hz. One will get an idea of how sensitive these radio observations are by thinking about this number in the context of, say, room illumination via a 100-W light bulb.

Figure 1:The TRACE white-light exposure at the impulsive phase of the C6.2 flare of October 20, 2003. The left panel shows X and Y cuts through the brightest point of the image (right), which is in a reversed color table.

Figure 1 shows a white-light image from [TRACE] at the peak of the flare. It is not very impressive, so the line plots at the left are there to give a feeling for how clear the detection really is. One sees a lot of noise in this difference image, but a clear black blob (reversed color table) with a significance of more than 10 [sigma]. The UV and EUV data (Figure 2) are much clearer, because the [solar photosphere] is so much fainter at those short wavelengths. Note that the time series (left panel) has a sharp peak; this is because the UV (and white light) emission is so concentrated at the onset of the flare. Also note that the image looks saturated at its brighter parts, the loop footpoints (the loops are faint but clearly recognizable). This is a common problem for photometry on such images, since their exposure times may not keep up with the rapid and highly variable changes at short wavelengths. For white light this is not such a problem since the maximum contrast (flare vs quiet photosphere) is normally fairly small. The white-light image (Figure 1) shows one of the footpoints; for unclear reasons both were not so easily detectable.

Figure 2: The 1600A observations of the flare. The time series at the left shows the UV flux integrated over the image area, and the image at the right shows what it looked like at the peak.

Spectral Energy Distribution

We can take these observations and plot a crude spectral energy distribution - three points, one each for white light (about 5000A), 1600A, and 171A.

Conclusions