We present an overview of solar flares and associated phenomena, drawing upon a wide range of observational data primarily from the RHESSI era. Following an introductory discussion and overview of the status of observational capabilities, the article is split into topical sections which deal with different areas of flare phenomena (footpoints and ribbons, coronal sources, relationship to coronal mass ejections) and their interconnections. We also discuss flare soft X-ray spectroscopy and the energetics of the process. The emphasis is to describe the observations from multiple points of view, while bearing in mind the models that link them to each other and to theory. The present theoretical and observational understanding of solar flares is far from complete, so we conclude with a brief discussion of models, and a list of missing but important observations.

We use RHESSI high-resolution imaging and spectroscopy observations from ~5 to 100 keV to characterize the hot thermal plasma during the 2002 July 23 X4.8 flare. These measurements of the steeply falling thermal X-ray continuum are well fit throughout the flare by two distinct isothermal components: a super-hot (T e > 30 MK) component that peaks at ~44 MK and a lower-altitude hot (T e ≲ 25 MK) component whose temperature and emission measure closely track those derived from GOES measurements. The two components appear to be spatially distinct, and their evolution suggests that the super-hot plasma originates in the corona, while the GOES plasma results from chromospheric evaporation. Throughout the flare, the measured fluxes and ratio of the Fe and Fe-Ni excitation line complexes at ~6.7 and ~8 keV show a close dependence on the super-hot continuum temperature. During the pre-impulsive phase, when the coronal thermal and non-thermal continua overlap both spectrally and spatially, we use this relationship to obtain limits on the thermal and non-thermal emission.

In this study we synthesize the results of four previous studies on the global energetics of solar flares and associated coronal mass ejections (CMEs), which include magnetic, thermal, nonthermal, and CME energies in 399 solar Mand X-class flare events observed during the first 3.5 yr of the Solar Dynamics Observatory (SDO) mission. Our findings are as follows. (1) The sum of the mean nonthermal energy of flare-accelerated particles (E nt ), the energy of direct heating (E dir ), and the energy in CMEs (E CME ), which are the primary energy dissipation processes in a flare, is found to have a ratio of ( ), compared with the dissipated magnetic free energy E mag , which confirms energy closure within the measurement uncertainties and corroborates the magnetic origin of flares and CMEs. (2) The energy partition of the dissipated magnetic free energy is: 0.51±0.17 in nonthermal energy of 6 keV electrons, 0.17±0.17 in nonthermal 1 MeV ions, 0.07±0.14 in CMEs, and 0.07±0.17 in direct heating. (3) The thermal energy is almost always less than the nonthermal energy, which is consistent with the thick-target model. (4) The bolometric luminosity in white-light flares is comparable to the thermal energy in soft X-rays (SXR). (5) Solar energetic particle events carry a fraction »0.03 of the CME energy, which is consistent with CME-driven shock acceleration. (6) The warm-target model predicts a lower limit of the low-energy cutoff at » e 6 keV c , based on the mean peak temperature of the differential emission measure of T e =8.6 MK during flares. This work represents the first statistical study that establishes energy closure in solar flare/CME events.

We present the second part of a project on the global energetics of solar flares and coronal mass ejections (CMEs) that includes about 400 M-and X-class flares observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during the first 3.5 years of its mission. In this Paper II we compute the differential emission measure (DEM) distribution functions and associated multi-thermal energies, using a spatially-synthesized Gaussian DEM forward-fitting method. The multi-thermal DEM function yields a significantly higher (by an average factor of ≈ 14), but more comprehensive (multi-)thermal energy than an isothermal energy estimate from the same AIA data. We find a statistical energy ratio of E th /E diss ≈ 2%−40% between the multi-thermal energy E th and the magnetically dissipated energy E diss , which is an order of magnitude higher than the estimates of Emslie et al. 2012. For the analyzed set of M and X-class flares we find the following physical parameter ranges: L = 10 8.2 − 10 9.7 cm for the length scale of the flare areas, T p = 10 5.7 − 10 7.4 K for the DEM peak temperature, T w = 10 6.8 − 10 7.6 K for the emission measure-weighted temperature, n p = 10 10.3 − 10 11.8 cm −3 for the average electron density, EM p = 10 47.3 −10 50.3 cm −3 for the DEM peak emission measure, and E th = 10 26.8 − 10 32.0 erg for the multi-thermal energies. The deduced multi-thermal energies are consistent with the RTV scaling law E th,RT V = 7.3 × 10 −10 T 3 p L 2 p , which predicts extremal values of E th,max ≈ 1.5 × 10 33 erg for the largest flare and E th,min ≈ 1 × 10 24 erg for the smallest coronal nanoflare. The size distributions of the spatial parameters exhibit powerlaw tails that are consistent with the predictions of the fractal-diffusive self-organized criticality model combined with the RTV scaling law.

We present results from the the first campaign of dedicated solar observations undertaken by the Nuclear Spectroscopic Telescope ARray (NuSTAR) hard X-ray telescope. Designed as an astrophysics mission, NuSTAR nonetheless has the capability of directly imaging the Sun at hard X-ray energies (>3 keV) with an increase in sensitivity of at least two magnitude compared to current non-focusing telescopes. In this paper we describe the scientific areas where NuSTAR will make major improvements on existing solar measurements. We report on the techniques used to observe the Sun with NuSTAR, their limitations and complications, and the procedures developed to optimize solar data quality derived from our experience with the initial solar observations. These first observations are briefly described, including the measurement of the Fe K-shell lines in a decaying X-class flare, hard X-ray emission from high in the solar corona, and full-disk hard X-ray images of the Sun.