This paper outlines the current understanding of solar flares, mainly focused on magnetohydrodynamic (MHD) processes responsible for producing a flare. Observations show that flares are one of the most explosive phenomena in the atmosphere of the Sun, releasing a huge amount of energy up to about 1032 erg on the timescale of hours. Flares involve the heating of plasma, mass ejection, and particle acceleration that generates high-energy particles. The key physical processes for producing a flare are: the emergence of magnetic field from the solar interior to the solar atmosphere (flux emergence), local enhancement of electric current in the corona (formation of a current sheet), and rapid dissipation of electric current (magnetic reconnection) that causes shock heating, mass ejection, and particle acceleration. The evolution toward the onset of a flare is rather quasi-static when free energy is accumulated in the form of coronal electric current (field-aligned current, more precisely), while the dissipation of coronal current proceeds rapidly, producing various dynamic events that affect lower atmospheres such as the chromosphere and photosphere. Flares manifest such rapid dissipation of coronal current, and their theoretical modeling has been developed in accordance with observations, in which numerical simulations proved to be a strong tool reproducing the time-dependent, nonlinear evolution of a flare. We review the models proposed to explain the physical mechanism of flares, giving an comprehensive explanation of the key processes mentioned above. We start with basic properties of flares, then go into the details of energy build-up, release and transport in flares where magnetic reconnection works as the central engine to produce a flare.

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Solar Flares: Magnetohydrodynamic Processes

We review the physical processes of particle acceleration, injection, propagation, trapping, and energy loss in solar flare conditions. An understanding of these basic physical processes is inexorable to interpret the detailed timing and spectral evolution of the radiative signatures caused by nonthermal particles in hard X-rays, gamma-rays, and radio wavelengths. In contrast to other more theoretically oriented reviews on particle acceleration processes, we aim here to capitalize on the numerous observations from recent spacecraft missions, such as from the Compton Gamma Ray Observatory (CGRO), the Yohkoh Hard X-Ray Telescope (HXT) and Soft X-Ray Telescope (SXT), and the Transition Region and Coronal Explorer (TRACE). High-precision energy-dependent time delay measurements from CGRO and spatial imaging with Yohkoh and TRACE provide invaluable observational constraints on the topology of the acceleration region, the reconstruction of magnetic reconnection processes, the resulting electromagnetic fields, and the kinematics of energized (nonthermal) particles.

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Particle acceleration and kinematics in solar flares – A Synthesis of Recent Observations and Theoretical Concepts (Invited Review)

Helioseismology requires nearly continuous observations of the oscillations of the solar surface for long periods of time in order to obtain precise measurements of the sun's normal modes of oscillation. The GONG project acquires velocity images from a network of six identical instruments distributed around the world. The GONG network began full operation in October 1995. It has achieved a duty cycle of 89 percent and reduced the magnitude of spectral artifacts by a factor of 280 in power, compared with single-site observations. The instrumental noise is less than the observed solar background.

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The Global Oscillation Network Group (GONG) Project

▪ Abstract Helioseismology has transformed our knowledge of the Sun's rotation. Earlier studies revealed the Sun's surface rotation, but now a detailed observational picture has been built up of the internal rotation of our nearest star. Unlike the predictions of stellar-evolution models, the radiative interior is found to rotate roughly uniformly. The rotation within the convection zone is also very different from prior expectations, which had been that the rotation rate would depend primarily on the distance from the rotation axis. Layers of rotational shear have been discovered at the base of the convection zone and in the subphotospheric layers. Studies of the time variation of rotation have uncovered zonal-flow bands, extending through a substantial fraction of the convection zone, which migrate over the course of the solar cycle, and there are hints of other temporal variations and of a jet-like structure. At the same time, building on earlier work with mean-field models, researchers have made great...

The Internal Rotation of the Sun

The formation of sunspots from the solar toroidal field. the earth's magnetism and magneto hydrodynamics

We study the subsurface meridional flow in the Sun as a function of latitude and depth in the period from 1994 to 2000, covering the solar minimum in 1996 and the maximum in 2000, with the technique of time-distance helioseismology. It is found that the velocity of meridional flow increased when solar activity decreased from 1994 to 1997. As solar activity increased from 1997 to 2000, a new component of meridional flow, centered at about 20° latitude, was created in each hemisphere. It moves away from its center. The new flow changes the observed meridional flow from poleward at solar minimum to equatorward at solar maximum at low latitudes. The velocity of the new component increases with depth at least down to a depth of 70,000 km.

Solar cycle variations of subsurface meridional flows in the sun

Scattering of p-Modes by Sunspots. II. Calculations of Phase Shifts from a Phenomenological Model

The Taiwan Oscillation Network (TON) is a ground-based network to measure solar intensity oscillations to study the internal structure of the Sun. K-line full-disk images of 1000 pixels diameter are taken at a rate of one image per minute. Such data would provide information onp-modes withl as high as 1000. The TON will consist of six identical telescope systems at proper longitudes around the world. Three telescope systems have been installed at Teide Observatory (Tenerife), Huairou Solar Observing Station (near Beijing), and Big Bear Solar Observatory (California). The telescopes at these three sites have been taking data simultaneously since October of 1994. Anl – v diagram derived from 512 images is included to show the quality of the data.

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Taiwan Oscillation Network

The increasing availability of high spatial resolution data of velocity and intensity variations on the Sun has stimulated the development of helioseismological techniques that probe the solar interior in localized regions. The techniques developed so far1,2,3,4 have yielded information on physical quantities (such as the flow velocity and magnetic field) below the surface, but are still far from providing a detailed picture of local subsurface inhomogeneities. Here we report the development and application of a new method for constructing three-dimensional solar images, utilizing acoustic noise (or stochastic P-mode oscillations) in the Sun. We treat a region of the solar surface as a phased array of acoustic sensors, which acts as a computational ‘lens’; acoustic waves ‘scattered’ by local inhomogeneities, such as sunspots, are collected and summed in phase, based on the knowledge of how (on average) they travel within the Sun. In this way, we are able to construct a three-dimensional image of a region of the solar interior.

Ambient acoustic imaging in helioseismology

The paper studies the vertical velocity of arch filaments in emerging flux regions by measuring their proper motion near the limb in H-alpha filtergrams and their radial velocity near the disk center in H-alpha spectrograms. Both show that arch filaments rise at a velocity between 10 and 15 km/s. The possible mechanisms producing this high velocity are discussed. None of them is satifactory, but it appears that the unloading of the arches by outflow must play a role. From the strength of H-alpha emission and D3 absorption, the temperature of arch filaments is estimated at 6000-15,000 K and the density at 10 to the 11th-12th/cu cm.

Dean-Yi Chou

Papers

Solar cycle variations of subsurface meridional flows in the sun

Scattering of p-Modes by Sunspots. II. Calculations of Phase Shifts from a Phenomenological Model

Taiwan Oscillation Network

Ambient acoustic imaging in helioseismology

The vertical structure of arch filament systems in solar emerging flux regions