• PDF / 1,232,972 Bytes
• 19 Pages / 439.37 x 666.142 pts Page_size
• 47 Downloads / 189 Views

DOWNLOAD

REPORT

---

Observations and Explanations of X-Ray Emissions in Flaring Loops

5.1 Overview The studies on solar X-ray emission can provide a direct imaging about flaring loops, which is commonly composed of thermal and NT components, and the HXR emission is closely associated with the acceleration of energetic electrons [1, 2]. The softhard-soft (SHS) pattern appears in spectral evolution of a considerable part of X-ray flares, which can be directly understood by the evolution of energetic electrons in the acceleration process [3–11], while the soft-hard-harder (SHH) pattern also appears in a part of X-ray flares, which is commonly explained by the mirroring effect on energetic electrons in flaring loops [12–14]. Most studies as mentioned above were limited in spatially unresolvable data observed in the full-disk, and a part of studies were comparatively made on HXR and MW data [15, 16]. Regarding the spatially resolvable data from earlier Yohkoh to recent RHESSI satellites, the X-ray brightness and spectral distributions in flaring loops have been performed [17–23]. According to the classical flare models, after the energetic electrons are accelerated in the reconnection site above the loop top (LT) and injected into flaring loops, they are separated into trapped and escaped parts due to the mirroring effect in flaring loops, the former one collides with high-density plasma in photosphere to emit bremsstrahlung, while it is thermalized fast, and the latter one moves forth and back between two reflected points inside the loop to emit MW, and it is finally thermalized. It is first noticed that two temporally and spatially coincident sources of HXR and MW frequently observed nearby the footpoints (FPs), which supports that both of them are contributed by the particle acceleration in the same flare, and consistent with the joint analysis of HXR and MW time profiles. With continuous enhancement of spatial resolution of relevant telescopes, it was first found by Yohkoh that there is possibly a stronger X-ray source nearby or above LT (called as Masuda flare) [17], which is considered to the evidence of particle acceleration due to magnetic reconnection above the LT, furthermore, RHESSI detected the coronal source that more close to the reconnection or acceleration site [24]. Meanwhile, a stronger MW emission also detected by NoRH nearby the LT, which may © Science Press, Beijing and Springer Nature Singapore Pte Ltd. 2018 G. Huang et al., Solar Flare Loops: Observations and Interpretations, https://doi.org/10.1007/978-981-10-2869-4_5

197

198

5 Observations and Explanations of X-Ray Emissions in Flaring Loops

be explained by the anisotropy of pitch-angle distribution of NT electrons [25]. It is noticed that the X-ray LT source is often observed at lower energies in comparison with that nearby the FPs, which means that the X-ray sources in LT and FPs are detected from different energies, while MW emission at a given frequency can be measured simultaneously in LT and FPs. In a study of five HXR flares observed by RHESSI [21], th