X-ray, EUV & WL emission heights observed by RHESSI & SDO

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Introduction

To get a complete picture of what is going on in a solar flare one has to look at different wavelengths of the electromagnetic spectrum. Hard X-rays tell us something about the flare accelerated electrons, their number and where they are stopped in the chromosphere while soft X-rays reveal information about the hot component of the flare heated plasma. In addition, observations in extreme ultraviolet give more detailed information about the temperature structure where different wavelengths give information about different temperatures. Finally, there is white light emission. First seen in the very first every observed flare, the famous Carrington event, it proves to be rather elusive and was long believed to only be present in very large flares. Although more recent observations suggest its presence also in smaller flares, the origin of the emission (in terms of height in the solar atmosphere) and the emission mechanism itself is still subject of debate. Combining RHESSI observations with SDO/AIA and SDO/HMI of source positions and applying a neat trick to find the chromospheric density and, most importantly, a reference position for the photosphere, we can now find not only the relative positions, but the relative heights of footpoint sources at different energies/wavelengths. The Sun collaborated nicely by providing a medium sized white light flare near the limb on February 24th 2011 ~ 07:31 UT, that allowed us to do this. Figure 1 shows an overview of the flare.

Mb epk nugget1.png Fig. 1: AIA 171 A image overlaid with RHESSI 6-12 (red) and 25-50 keV (blue) emission.


Finding the relative height of X-ray, EUV and WL sources

For this, we used the same trick that was used by Kontar et al. 2008, Kontar et al. 2010 , and Battaglia & Kontar 2011. In a classical thick target case, electrons with higher energies will penetrate deeper into the chromosphere. Therefore, the radial position of the observed X-ray footpoint is energy dependent, where the chromospheric density determines the position vs energy relation. Finding the radial positions in itself is no minor task but it is helped greatly by using visibilities (link to respecitve nugget). So, by assuming a function for the density, we can fit the observed position vs energy relation. This also gives a value for the radial position that corresponds to the photospheric height. Figure 2 illustrates this process in a simplified way. Mb epk cartoon.jpg Fig. 2: Cartoon that illustrates the method of finding the density and photospheric reference height using RHESSI source positions The important point to note is that with this method we find reference radial distance from the Sun center that corresponds to the photospheric height. We can now measure the distance from the Sun center for any source and subtract this reference distance to find the height above the photosphere. Since the pointing of both RHESSI and SDO are accurate to a few tenths of an arcsecond we can directly compare the source heights from RHESSI and SDO. The result of this is shown in Fig. 3.

Conclusions

As Figure 3 shows, the white light emission is clearly observed above the 30 - 100 keV emission, even taking all possible uncertainties regarding the relative positions into account. while the EUV emission appear even heigher, still. This places the white light emission in the upper chromosphere or transition region. The plasma in this region is partially or completely ionized which suggests free-bound and free-free continuum emission for the WL. Further, it is evident that the emission cannot have been cause by the high energy non-thermal electrons that caused the footpoint emission.

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