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The YOHKOH Survey of Partially Occulted Flares in Hard X-Rays

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The YOHKOH Survey of Partially Occulted Flares in Hard X-Rays A&A 502, 665–678 (2009) Astronomy DOI: 10.1051/0004-6361/200911732 & c ESO 2009 Astrophysics The YOHKOH survey of partially occulted flares in hard X-rays M. Tomczak Astronomical Institute, University of Wrocław, ul. Kopernika 11, 51–622 Wrocław, Poland e-mail: [email protected] Received 27 January 2009 / Accepted 22 April 2009 ABSTRACT Context. Modern solar X-ray imagers do not completely resolve the problem of deriving detailed diagnostics of faint hard X-ray sources in the presence of stronger ones. This is the case for the impulsive phase of solar flares in which footpoint sources are usually stronger than loop-top ones. Aims. Flares that are partially occulted by the solar limb provide the most hopeful source of knowledge about hard X-ray loop-top sources. This work attempts to fill the gap between the published survey of partially occulted flares observed by RHESSI (Krucker & Lin 2008, ApJ, 673, 1181) and the extensive Yohkoh database. Methods. Among the 1286 flares in the Yohkoh Hard X-ray Telescope Flare Catalogue (Sato et al. 2006, Sol. Phys., 236, 351), for which the hard X-ray images were presented, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. Results. In most cases, we found that the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are cospatial to within 4 arcsec. For rest events, the components are separated, the non-thermal component clearly appearing to be situated higher. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. We can explain this difference as a consequence of intrinsically dissimilar conditions in coronal parts of flares, in comparison with the footpoints that usually dominate the hard X-ray emission of disk flares. At least two reasons for the difference should be taken into consideration: (1) stronger contamination of hard X-rays by emission from thermal plasma; and (2) different mechanisms in which non-thermal electrons radiate their energy. For events unbiased by the thermal component, the difference, Δγ = γ¯LT − γ¯ FP, equals 1.5. We found a lack of correlation between the altitude of flares and the hard X-ray power-law index γ. Conclusions. A schematic picture, in which a thin-target mechanism is responsible for the hard X-ray emission of loop-top sources and a thick-target mechanism for emission by footpoint sources, is modified by the presence of some coronal thick-target sources. Some of these sources exhibit evidence of magnetic trapping. For the characteristics of flares is conclusive the local magnetic configuration in which they occur. Key words. Sun: corona – Sun: flares – Sun: particle emission – Sun: X-rays, gamma rays 1. Introduction the thin-target model (Brown 1971; Lin 1974). In many cases, the coronal hard X-ray sources can be seen as the effect of mag- It has been commonly accepted that solar flares are caused by netic convergence (Tomczak & Ciborski 2007) or ultra-dense the reconnection of magnetic field lines in the corona. In this thick-target environment (Kosugi et al. 1994; Veronig & Brown process, energy originally stored in the magnetic field is redis- 2004). tributed into plasma heating, wave generation, and particle ac- celeration. However, details of the reconnection process as well Coronal hard X-ray sources can be easily distinguished when as general rules about energy partition are subjects of extensive a flare occurs behind the solar limb but close enough to record debate. Any progress strongly depends on carefully planned ob- emission from the higher part of the magnetic structure. In the servations and their interpretation. case of a partially occulted flare, the solar disk works like a Hard X-ray observations offer good insight into further eval- rough imager that stops the emission of usually brighter foot- uation of particles accelerated in the reconnection process. The point sources. Such a configuration had been used routinely for propagation of particles operates under the guidance of the mag- investigation of coronal hard X-ray sources before hard X-ray netic lines. Their bundles converge at the entrance into the lower imaging detectors began to operate (Zirin et al. 1969; Frost & part of the solar atmosphere, where the density of the ambient Dennis 1971; Hudson 1978; Hudson et al. 1982; Kane et al. plasma increases steeply. Electrons accelerated in the corona are 1982). stopped there, emitting intense hard X-radiation via electron- A first statistical attempt to analyze partially occulted flares ion bremsstrahlung. This mechanism, known as the thick-target was performed using data from the UCSD experiment onboard model (Brown 1971), works so efficiently that footpoint hard the OSO-7 satellite (Roy & Datlowe 1975; McKenzie 1975). X-ray sources usually strongly dominate the spatial distribution In fifteen months of OSO-7 operation, from among 601 X-ray of flare emission in this energy range. bursts above 103 photon cm−2 s−1 keV−1 in energy channel ex- It is difficult to observe the particle acceleration without ef- tending over 5.1–6.6 keV, 54 bursts were unaccompanied by Hα fects introduced by the propagation of particles because in the flares. Since a cinema flare patrol was in progress during those less-dense corona, hard X-radiation is emitted less efficiently in bursts, they probably occurred behind the solar limb. Article published by EDP Sciences 666 M. Tomczak: The YOHKOH survey of partially occulted flares in hard X-rays From the group of partially occulted flares, McKenzie (1975) occulted flares observed by RHESSI. They detected 55 of these chose eight major soft X-ray events and found that all had sig- events between 2002 February and 2004 August. Two differ- nificant hard X-ray emission in the 30–44 keV range. Roy & ent components of coronal hard X-ray emission were detected Datlowe (1975) analyzed all available hard X-ray spectra of par- in 50 flares: (1) a thermal component with a gradual time pro- tially occulted flares. They found that 25 of 37 bursts had a non- file; and (2) a non-thermal (power-law spectra with indices thermal component. The average value of the spectral indices γ mostly between 4 and 7) component exhibiting more rapid at peak 20 keV flux for these 25 over-the-limb events was 4.6, time variations. Both components were usually cospatial within whereas for 59 center events (0◦ <θ<60◦) it was 3.8. ∼2 × 103 km with only a few exceptions. The main conclusion of the above mentioned papers from The interpretation of the extensive Yohkoh database would the pre-imaging era was that hard X-ray emission was not con- benefit from a survey of partially occulted flares, which this pa- centrated close to the solar surface but took place in extended per attempts to complete using the Hard X-ray Telescope Flare regions in the corona. Catalogue. The development and structure of this catalogue are Modern hard X-ray telescopes onboard the Yohkoh (Hard described in Sect. 2. The criteria of selection of flares enclosed in X-ray Telescope) and Reuven Ramaty High-Energy Solar the survey are given in Sect. 3. In Sect. 4, a statistical approach to Spectroscopic Imager (RHESSI) satellites have opened a new some characteristics describing partially occulted flares is given perspective in investigating coronal hard X-ray sources. Many and the results are compared to their counterparts obtained for important discoveries have been made (Krucker et al. 2008a) non-occulted flares from the catalogue. In Sect. 5, the results by including the most famous – the discovery of the presence are discussed and compared to the reports of other authors. In of above the loop-top sources in flares (Masuda et al. 1994). Sect. 6, short descriptions of some interesting groups of events However, in many cases the low dynamical range of a hard in the catalogue are given. Our main conclusions are summa- X-ray telescope creates problems for even a qualitative analysis rized in Sect. 7. of images, since detailed diagnostics of faint hard X-ray sources become difficult to measure in the presence of stronger ones. Alexander & Metcalf (1997) demonstrated that weak sources 2. Development and structure of the YOHKOH HXT can be suppressed during the image reconstruction and mimic Flare Catalogue the appearance of stronger neighbors. This is indeed true for the impulsive phase of a typical flare when we observe strong foot- The Japanese solar satellite Yohkoh operated during the years point and weak loop-top sources simultaneously. 1991–2001 providing a huge amount of excellent data. The Hard To avoid this complication, partially occulted flares have X-ray Telescope, HXT, (Kosugi et al. 1991) was a Fourier syn- been studied. From the Yohkoh database, one or a few examples thesis imager observing the whole Sun. It consisted of 64 inde- of behind-the-limb flares have previously been selected many pendent subcollimators measuring spatially modulated intensi- times. This specific configuration was usually used to derive ties in four energy bands (L: 14–23 keV, M1: 23–33 keV, M2: joint diagnostics from hard and soft X-ray images by includ- 33–53 keV, and H: 53–93 keV). During the flare, the intensities ing full-Sun soft X-ray spectra recorded by the Bragg Crystal were integrated, in each energy band, over 0.5 s. Some recon- Spectrometer. In this way, soft X-ray bright loop-top kernels struction routines (Maximum Entropy Method, Pixons) that al- (Khan et al. 1995; Mariska et al. 1996; Sterling et al.
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