Solar white light flares (WLFs) are a class of flares that can be observed in the optical continuum (Fang & Ding 1994), and seem to have no difference from ordinary flares except for the optical continuum. However, the energy emitted in the continuum is much higher than that in spectral lines, nearly 90% of the total emission energy. Therefore, WLFs are considered as an extreme case of flares. It is generally considered that the initial energy release in flares takes place in the corona. However, the continuum emission of WLFs comes mainly from the lower chromosphere and photosphere. It is difficult to interpret the phenomenon with the traditional theory of flare radiative transfer and atmospheric model (Neidig 1989).
86 WLFs had been observed from 1 September 1859 to 1992, according to the statistical results by Neidig & Cliver (1983a), Xuan et al. (1991), Sakurai et al. (1992), Neidig et al. (1993) and Schmieder et al. (1994). Later, 7 WLFs were identified from the data of solar flares (see Table 1 (click here)). It is possible that some WLFs have not yet been identified, and more will be found with new analysis. Machado et al. (1985), and Fang & Ding (1994) divided those WLFs into two classes according to the shape and the intensity of H lines in the Balmer series (see Table 2 (click here)). Most of the WLFs are in class I, and only 6 in class II. Physical mechanisms and origins may be different for these two types of WLFs (Fang & Ding 1994). Therefore, further study is necessary on properties and origins of WLFs.
The multi-band optical observations of the white light flare on 18 January 1989 with the Spectrograph SSHG of Yunnan Observatory (Xuan et al. 1991) are important and valuable in understanding the evolution of WLFs and their morphological characteristics.
|a spectral data, b filtergrams in multi-bands and c H filtergrams.|
|(1) Sotirovski et al. (1992), (2) Fang & Ding (1994), (3) Huang et al. (1991)|
|(4) Zhu & Shen (1992)|
|(5) Yunnan Observatory, Sighted in WL by naked eyes.|
|Class I||Class II|
|Shows significant temporal||No such relationships|
|maximum of WLF continuous|
|emissions and those of|
|hard X-ray and radio bursts|
|Strong Balmer jump in spectra||No Balmer jump shows|
|may be observed|
|Emissions of Balmer lines are||Emissions of Balmer lines|
|strong and wide||are weak and narrow|
|The center reversal of|
|H emission line is|