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Register a new userSolar chromospheric flares: energy release, transport and radiation
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Lyndsay Fletcher
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This paper presents an overview of some recent observational and theoretical results on solar flares, with an emphasis on flare impulsive-phase chromospheric properties, including: electron diagnostics, optical and UV emission, and discoveries made by the Hinode mission, especially in the EUV. A brief perspective on future observations and theoretical requirements is also given
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We analyze a grid of radiative hydrodynamic simulations of solar flares to study the energy balance and response of the atmosphere to nonthermal electron beam heating. The appearance of chromospheric bubbles is one of the most notable features that we find in the simulations. These pockets of chromospheric plasma get trapped between the transition region and the lower atmosphere as it is superheated by the particle beam. The chromospheric bubbles are seen in the synthetic spectra, appearing as an additional component to Balmer line profiles with high Doppler velocities as high as 200 km/s. Their signatures are also visible in the wings of Ca II 8542A line profiles. These bubbles of chromospheric plasma are driven upward by a wave front that is induced by the shock of energy deposition, and require a specific heating rate and atmospheric location to manifest.
Determining the energy transport mechanisms in flares remains a central goal in solar flares physics that is still not adequately answered by the standard flare model. In particular, the relative roles of particles and/or waves as transport mechanisms, the contributions of low energy protons and ions to the overall flare budget, and the limits of low energy non-thermal electron distribution are questions that still cannot be adequately reconciled with current instrumentation. In this White Paper submitted in response to the call for inputs to the Next Generation Solar Physics Mission review process initiated by JAXA, NASA and ESA in 2016, we outline the open questions in this area and possible instrumentation that could provide the required observations to help answer these and other flare-related questions.
Solar flares are driven by the release of magnetic energy from reconnection events in the solar corona, whereafter energy is transported to the chromosphere, heating the plasma and causing the characteristic radiative losses. In the collisional thick-target model, electrons accelerated to energies exceeding 10 keV traverse the corona and impact the chromosphere, where they deposit their energy through collisions with the much denser plasma in the lower atmosphere. While there are undoubtedly high energy non-thermal electrons accelerated in flares, it is unclear whether these electron beams are the sole mechanism of energy transport, or whether they only dominate in certain phases of the flares evolution. Alfvenic waves are generated during the post-reconnection relaxation of magnetic field lines, so it is important to examine their role in energy transport.
We report observations of white-light ejecta in the low corona, for two X-class flares on the 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles coronal rain, appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 $pm$ 8 $mathrm{km s^{-1}}$. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments, and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution.
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 M and X-class flare events observed during the first 3.5 years of the Solar Dynamics Observatory (SDO) mission. Our findings are: (1) The sum of the mean nonthermal energy of flare-accelerated particles ($E_{mathrm{nt}}$), the energy of direct heating ($E_{mathrm{dir}}$), and the energy in coronal mass ejections ($E_{mathrm{CME}}$), which are the primary energy dissipation processes in a flare, is found to have a ratio of $(E_{mathrm{nt}}+E_{mathrm{dir}}+ E_{mathrm{CME}})/E_{mathrm{mag}} = 0.87 pm 0.18$, compared with the dissipated magnetic free energy $E_{mathrm{mag}}$, 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: $0.51pm0.17$ in nonthermal energy of $ge 6$ keV electrons, $0.17pm0.17$ in nonthermal $ge 1$ MeV ions, $0.07pm0.14$ in CMEs, and $0.07pm0.17$ 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 with the thermal energy in soft X-rays (SXR); (5) Solar Energetic Particle (SEP) events carry a fraction $approx 0.03$ of the CME energy, which is consistent with CME-driven shock acceleration; and (6) The warm-target model predicts a lower limit of the low-energy cutoff at $e_c approx 6$ keV, based on the mean differential emission measure (DEM) peak temperature of $T_e=8.6$ MK during flares. This work represents the first statistical study that establishes energy closure in solar flare/CME events.
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