4.10 Solar flares

Stereoscopic or tomographic 3D reconstructions of solar flares are extremely difficult due to the rapid evolution during the impulsive energy release phase. Even in the postflare phase, loop arcades are gradually expanding and subject to conductive and radiative cooling with constantly changing brightness in each soft X-ray and EUV temperature filter on time scales of minutes. The rapid dynamics excludes any solar rotation based method and allows at best only instantaneous stereoscopy. However, even instantaneous stereoscopy, e.g., with the STEREO/EUVI and SDO/AIA telescopes, are strongly hampered in the impulsive flare phase due to overexposure, pixel bleeding, and the appearance of diffraction patterns from the focal support structure in the telescope aperture. For these reasons, there exists not much stereoscopic literature about solar flares.

Some alternative 3D reconstruction techniques of solar flares have been invented in the pre-STEREO era, such as using Doppler-shift measurements of plasma upflows in flare loops to constrain the flare loop inclination angle (Nitta et al., 1999), or using the solar surface as a “Compton mirror” to view precipitating electrons from bottom up, a method called ‘stereoscopic electron spectroscopy’ (Kontar and Brown, 2006), or dual spacecraft measurements of hard X-rays with RHESSI and type III bursts with WIND in a partially disk-occulted flare (Krucker et al., 2008).

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Figure 45: 3D reconstruction of postflare loops of the 2007 Jun 3, 07:50 UT, flare with STEREO/A and B (left), and of the central flaring, non-eruptive filament in the 2007 Jun 10, 11:11 UT, flare (right). Note the difference in altitude for the eruptive flare with post-flare arcade (left) and the non-eruptive flare filament (right) (from Aschwanden et al., 2009bJump To The Next Citation Point).

A review and a flare catalog of 185 flare events observed with STEREO has been compiled for the first two years of the mission (Aschwanden et al., 2009bJump To The Next Citation Point), detected by GOES above the C1-level or with RHESSI at > 25 keV. CMEs were reported for a third of these events. This flare dataset contains: 79% events with impulsive EUV emission (coincident with hard X-rays), 73% show delayed EUV emission from postflare loops and arcades, 24% represent occulted flares, 17% exhibit EUV dimming, 5% show loop oscillations or propagating waves, and at least 3% show erupting filaments. The stereoscopic view allows us to triangulate a number of individual loops in the flare region, of which we show two examples in Figure 45View Image. The two examples characterize also two different flare categories: (i) eruptive flares that form a postflare arcade below the magnetic reconnection region after eruption, usually delayed in EUV with respect to soft X-rays due to the flare plasma cooling (Figure 45View Image, left); and (ii) confined flares that show an impulsive heating phase of a highly-sheared non-eruptive filament, while the overall magnetic configuration pretty much stays intact and unchanged during the entire flare. The altitude of the flaring non-eruptive filament was stereoscopically triangulated to h = 1.8 ± 2.4 Mm, which essentially corresponds to the upper bound of the chromosphere (Figure 45View Image, right). A number of flares appeared to be occulted by one STEREO spacecraft, but in full view by the other spacecraft, which produces two dissimilar EUV time profiles and helps to isolate the fainter coronal emission of a flare.

Other flare studies involving STEREO data include: Magnetic modeling with NLFFF models that localized quasi-circular flare ribbons at the separatrix between open and closed fields (Su et al., 2009); correlation of the height-time evolution of CMEs with flare hard X-ray emission (Chen and Kunkel, 2010); STEREO-constrained two-ribbon flare geometry and occultation height of RHESSI and radio observations Krucker et al. (2010); a STEREO-identified twisted helical penumbral filament before an M8.9 flare (Figure 46View Image) (Kumar et al., 2010Jump To The Next Citation Point); evidence of plasmoid-looptop interaction and magnetic inflows during a solar flare and CME eruptive event (Milligan et al., 2010); evidence for internal tether-cutting in a flare and CME event (Raftery et al., 2010); and a correlation between the CME acceleration profile and the energy release rate as measured in hard X-rays with RHESSI (Temmer et al., 2010). In most of these studies, STEREO data were used in a qualitative way to disentangle the 3D flare geometry, while quantitative 3D multi-wavelength modeling that could most suitably done with STEREO (and from triple view points in conjunction with AIA) is still lacking. Ultimately, 3D modeling of flare volumes is necessary to infer accurate values for total flare energies, to determine the 3D fractal dimensions, and to test spatio-temporal scaling laws and powerlaw occurrence frequency distributions in terms of self-organized criticality models (Aschwanden, 2011).

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Figure 46: STEREO/EUVI/A and B observations of a twisted helical filament with approximately two turns (red helical line) observed shortly before the M8.9 flare on 2007 Jun 4, 05:08 UT (from Kumar et al., 2010).

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