5.1 Radiation

Figure 41View Image illustrates the time variation of emissions observed in various wavelengths during a flare. At the beginning of a flare, the rapid increase of hard X-ray (HXR) (> 30 keV) and microwave emissions is observed, which is called burst or elementary bursts (e.g., de Jager and Sakai, 1991). This event occurs recurrently during the period of several seconds. While a burst is occurring, high-energy particles are usually generated. The total duration of a series of bursts is about a few minutes, which forms the impulsive phase of a flare. During this phase the most violent energy release occurs. After that, emissions from a flaring site gradually decrease in about ten minutes, which is called the gradual phase of a flare. In some events, the total duration of a series of bursts might be longer than 10 min. HXR and microwave emissions are strong only during the impulsive phase, while soft X-ray (SXR) (< 10 keV) and Hα emissions continue to increase after the impulsive phase and become dominant during the gradual phase.
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Figure 41: Typical time variation of emissions observed in various wavelengths during a flare (from Kane, 1974).

Space observations such as SMM (1980 – 1989), Hinotori (1981 – 1989), and Yohkoh (1991 – 2001) have provided the detailed information on the structure and evolution of a flare, which enabled us to identify the source region of emissions observed during impulsive and/or gradual phases. Figure 42View Image illustrates the configuration of magnetic field observed during these two phases, in addition to the source region of emissions observed in various wavelengths.

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Figure 42: Magnetic configurations in the impulsive (left panel) and gradual (right panel) phases of a flare. The source region of emissions observed in various wavelengths are also presented. These two kinds of configurations are applied to different types of flares such as impulsive flares and LDE flares (adapted from Magara et al., 1996Jump To The Next Citation Point). An observational example of these flares are displayed at the top left (Masuda, 1994Jump To The Next Citation Point) and top right (Tsuneta et al., 1992aJump To The Next Citation Point), respectively.

An HXR source is formed at the chromospheric footpoint of a loop observed in soft X-rays (SXR loop), which is called HXR footpoint source. High-energy electrons generated by magnetic reconnection in the corona are supposed to stream downward along an SXR loop, heating chromospheric plasma to form an HXR footpoint source there. These high-energy electrons also produce microwave emissions intermittently via gyro-synchrotron radiation while streaming downward along an SXR loop. Another type of HXR sources is formed above the top of an SXR loop, which is called HXR loop-top source (Masuda et al., 1994). The HXR loop-top source may be formed by a downward high-speed flow (jet) which has been produced by magnetic reconnection and collides with the top of an SXR loop. The collapsing trap effect may be occurring during this phase near the top of the SXR loop (Somov and Kosugi, 1997; Karlický and Kosugi, 2004; Veronig et al., 2006). Soft X-ray emissions start to increase gradually during the preflare phase of a flare, suggesting that plasma heating already occurs before the onset of a flare. During the impulsive phase, the intensity of soft X-ray emissions increases rapidly and the time derivative of the soft X-ray intensity rise corresponds to the time variation of hard X-ray emissions, which is known as Neupert effect (Neupert, 1968). The main contribution to producing these soft X-ray emissions comes from a loop filled with hot plasma whose temperature is about T ≥ 107 K. This plasma originally comes from the chromosphere via evaporation driven by the thermal conduction (and high-energy electrons in part) emanating from a super-hot region formed in the corona (around the region where magnetic reconnection occurs). The thermal conduction also continuously heats the evaporated plasma and tries to keep its coronal temperature (eventually the evaporated plasma reduces its temperature by radiative cooling and forms Hα loops).

During the impulsive phase, bright kernels are observed in Hα at the footpoints of an SXR loop. This also indicates the heating of chromospheric plasma by thermal conduction and high-energy electrons. It has often been argued that explosive evaporation in the impulsive phase is primarily due to electron beams, causing Neupert effect, whereas the gentle evaporation in the gradual phase is due to conduction (e.g., Veronig et al., 2010, and references therein). When a group of SXR loops appear almost simultaneously and form an arcade, the Hα kernels are observed as two ribbons distributed along the polarity inversion line (called Hα ribbons, see Figure 1View Image). In an extremely energetic case, high-energy electrons can penetrate the chromosphere and heat the photosphere, causing the enhancement of white-light emissions (white-light flare).

Hα emissions are also observed during the gradual phase of a flare. In this case the main contribution comes from a loop filled with cool plasma with T ∼ 104 K (Hα loop). An Hα loop starts to appear when an SXR loop experiences sufficient cooling via radiation and now is observed in Hα. As magnetic reconnection proceeds in the corona, a newly reconnected field line successively piles up on a preexisting SXR loop, so the apparent height of an observed SXR loop increases with time. In accordance with this, the distance between the two Hα ribbons observed at both footpoints of an Hα loop also increases with time (see Figure 1View Image). The postflare loops are seen on the disc in emission in Hα only if they are dense enough (say 12 −3 n > 10 cm; Švestka, 1976) which happens only in very powerful flares, so this is a rather rare phenomenon.

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