The challenge of understanding white light flares

(News & views on the paper by Wang (2009), RAA, 2009, vol.9, 127-132)

Cheng Fang

(Department of Astronomy, Nanjing University; fangc@nju.edu.cn)

White light flares (WLFs) are defined as flares that have emission visible in the optical continuum (Fang et al. 2008). The first WLF was observed 150 years ago (Carrington  1859). However, up to now, there have only been less than 150 WLFs reported in the literature. Therefore, the rate of reported WLFs is about one per year! They are thought to be the rarest and most energetic flaring events. The occurrence rate of WLFs depends on the heliocentric angle of the flaring region, and whether WLFs can be detected depends on the observing wavelength (Hénoux et al. 1990Neidig et al. 1993Ding & Fang 1996) and probably the spatial resolution of the observations (see, e.g. Jess et al. 2008). In most cases, enhanced continuum emissions are detected at the Balmer and Paschen continuum, but recently, the enhanced emission at the continuum near the Ca II 8542 Å (Liu & Ding 2001) and even at 1.6μm has been reported (Xu et al. 20042006).

Recently, the space and ground-based observations with adaptive optics made possible the higher quality WLF observations. Using Yohkoh observations, Matthews et al. (2003) reported 28 WLFs in the G-band and found their strong association with hard X-ray emissions. Hudson et al. (2006) used on board instruments of The Transition Region and Coronal Explorer (TRACE) with a spatial resolution of 1 arcsec and detected white light emissions for events as weak as GOES C1.6. They guess that the white light continuum may occur in all flares. However, because TRACE WLFs have higher contrast than the traditional WLFs and sometimes there is saturation in the observations, it is unclear if the TRACE continuum is affected by UV emission. Fletcher et al. (2007) analyzed nine flares observed by TRACE in WL/UV. The subarcsecond resolution WLF observations with AO were carried out at the Dunn Telescope of NSO/SP (Xu et al. 20042006). The maximum intensity enhancements of two WLFs were 25% and 66%, compared to the quiet-Sun NIR continuum. Jess et al. (2008) used high resolution observations of the 1 meter Swedish Solar Telescope, and detected a white light emission peak at 300% above the quiescent flux for a C2.0 flare on 2007 August 24. This high emission peak is surprising. It needs to be further checked whether it is a common feature for high resolution WLF observations or just a special case.

Isobe et al. (2007) analyzed the G-band emission of the 2006 December 13 event observed by Hinode that had a spatial resolution of 0.2 arcsec, and found a corehalo structure, confirming what was reported by Xu et al. (2006). Jing et al. (2008) studied this event further, and found that the white light emissions correspond to a location with a strong magnetic reconnection rate. In particular, Wang  (2009) presented a study of 13 flares observed by Hinode, and found that there is a correlation between the GOES X-ray flux and flare emission in the G-band. The results are interesting, because it was the first WLF survey to make use of the very high spatial resolution data of Hinode. However, as the author pointed out, G-band observations are contaminated by the CH band emission, and the time resolution of Hinode is also limited, so that it is not clear if the white light emission cut-off and the peak values are the real ones.

The challenge of WLFs mainly comes from understanding their mechanism. How can one explain the heating and energy transport in WLFs in the low solar atmosphere, even around the temperature minimum region? There are several proposed mechanisms, including backwarming, nonthermal particle bombardment, magnetic reconnection, heating caused by Alfvén waves, chromospheric condensation, etc. It is probably necessary to distinguish two types of WLFs (Machado et al. 1986). Fang & Ding (1995) studied the different characteristics between them from the aspect of observations and atmospheric models. In the case of type I WLFs, there exists a good time correlation between the peak of hard X-rays or microwave radio burst and the maximum of continuum emission; there is a strong Balmer jump in the spectra; the Balmer lines, in particular the Hα line, are usually strong and broad. However, type II WLFs do not show the above features. These two types of WLFs show a difference in the energy release and transport processes (see, e.g. Fang & Ding 1995). For type I WLFs, the particle (mainly electron) bombardment and/or the following backwarming would be a plausible mechanism (Metcalf et al. 1990Fang & Ding 1995Ding et al. 1999Liu & Ding 2001Fletcher et al. 2007), but for type II WLFs, the possible mechanism is magnetic reconnection in the lower solar atmosphere (Fang & Ding 1995Chen et al. 2001). Further high resolution multi-wavelength observations, particularly spectral observations, are required to clarify these points.

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