P. Heinzel1, L. Kleint2, J. Kašparová1, S. Krucker2
1. Astronomical Institute, Czech Academy of Sciences, 25165 Ondřejov, Czech Republic
2. University of Applied Sciences and Arts Northwestern Switzerland, Bahnhofstrasse 6, 5210 Windisch, Switzerland
Solar flares were discovered as white-light enhancements in active regions observed on the solar disc. The brightness enhancement of the so-called white-light flares seen on the disc is usually low in the visible range, typically below 10%, due to photospheric brightness itself. This makes it very difficult to disentangle between two assumed mechanisms: hydrogen recombination emission in the Paschen continuum (below 8203Å) and photospheric continuum enhancement due to H–.
On the other hand, off-limb observations of solar flares can provide important constraints on the height distribution of the flare emission. An off-limb white-light flare in the visible range has never been seen from the ground. Only recently, SDO/HMI has allowed such a detection, namely in the outermost wavelength channel of the Fe I 6173Å line, where continuum emission unaffected by the line itself is believed to be seen. We have investigated such rare observations of the off-limb continuum emission just above flare footpoints and found out that it is the hydrogen Paschen continuum that dominates the visible light emission at the chromospheric heights around or below 1000 km1.
Figure 1| Co-spatial white-light and hard X-ray flare footpoints seen above the solar limb for the 2012.07.19 flare. Note that all curves go to zero in the corona above the flare, due to the subtraction of the pre-flare emission. The intensity thus represents pure flare continuum emission. The three vertical dashed lines on the rightmost panel represent the zero height at τ5000 = 1, the HMI limb at height 350 km, and chromospheric height at 1000 km, respectively. The HMI limb is determined as the inflection point of the pre-flare limb darkening intensity, see long dashed profile in the right panel. It was assigned the height 350 km (with respect to zero atmospheric height at τ5000 = 1) and indicates the limb height at wavelength 6000Å.
An example of the HMI off-limb white-light emission is shown in Figure 1 for the M7.7 flare on 2012 July 19 together with the co-spatial RHESSI hard X-ray footpoint sources and the height distributions of the flare continuum emission (also see Ref 2). The height estimate of the peak emission is in the range of 800-1100 km above the τ5000 = 1 level.
Off-limb white-light emission does not contain the photosperic contribution as for the disc flares. Therefore, we have estimated the importance of several mechanisms: hydrogen Paschen continuum, Thomson scattering, and hydrogen free-free emission. The Paschen continuum emission arises as hydrogen recombination when hydrogen in the flaring atmosphere is highly ionized due to a strong temperature increase and due to colissions with non-thermal beam electrons and then protons capture thermal electrons and create neutral hydrogen atoms. The Thomson component comes from scattering of the diluted photospheric white-light radiation on flare-loop electrons and it is assumed to be coherent and isotropic. Ratio of the Paschen and Thomson continuum is a function of the temperature and linearly depends on the electron density in the flare loop. At electron densities 1013-1014 cm-3, relevant to stronger solar flares, the Paschen continuum dominates, as can be seen in Figure 2.
Figure 2| Theoretical ratio of the Paschen to Thomson continuum intensity at 6173Å, as a function of the electron density. The colors correspond to different temperatures: red – 6000 K, yellow – 10,000 K, green – 15,000 K, blue – 30,000 K, magenta – 50,000 K.
In the visible light and for longer wavelengths, hydrogen free-free emission becomes important, namely at higher temperatures around 20,000 K where this mechanism starts to dominate the Paschen continuum.
Using the FLARIX code3, radiation hydrodynamic simulation of the solar atmosphere heated by the electron beams confirms the analytical results (see Figure 3). The continuum emission above the limb is fully dominated by the Paschen continuum, while the Thomson scattering is about two orders of magnitude weaker. The hydrogen free-free contribution starts to prevail at about 900 km above the HMI limb. The off-limb white-light structure extends from about 450 to 950 km above the HMI limb, which is consistent with the HMI observations in Figure 1. Note that the HMI brightness is displayed in Figure 1 on a scale where zero is at the level of τ5000 = 1, and thus it goes to larger heights compared to Figure 3, where zero (the limb) is at photospheric height 350 km. The simulated continuum intensity could reach observed peak HMI values (Figure 1), provided that the actual geometrical thickness of the off-limb structure is larger than the nominal value 1000 km chosen in the simulation because the Paschen continuum emission is optically thin under these conditions.
Figure 3| Vertical variations of the line-of-sight total continuum intensity for flare-loop thickness 1000 km, from FLARIX simulation(red line). This is mostly dominated by the Paschen continuum (full black line), while the dotted-dashed line shows the hydrogen free-free contribution and the dashed line shows the Thomson scattering component. The blue line indicates the energy deposit rate due to the electron-beam precipitation. The zero height (the limb) is located at 350 km above the level of τ5000 = 1.
Finally, if off-limb Balmer continuum flare emission is detected by IRIS similar to that found on the disc by Heinzel & Kleint4, such an observation together with HMI continuum could provide a unique constraint on radiation mechanisms of white-light flares.
1. Heinzel, P. et al. 2017, ApJ, 847, 48
2. Krucker, S. et al. 2015, ApJ, 802, 19
3. Varady, M. et al. 2010, ITPS, 38, 2249
4. Heinzel, P. and Kleint, L. 2014, ApJL, 794, L23