We present a series of models for the plasma properties along open magnetic flux tubes rooted in solar coronal holes, streamers, and active regions. These models represent the first self-consistent solutions that combine (1) chromospheric heating driven by an empirically guided acoustic wave spectrum; (2) coronal heating from Alfven waves that have been partially reflected, then damped by anisotropic turbulent cascade; and (3) solar wind acceleration from gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure. The only input parameters are the photospheric lower boundary conditions for the waves and the radial dependence of the background magnetic field along the flux tube. We have not included multifluid or collisionless effects (e.g., preferential ion heating), which are not yet fully understood. For a single choice for the photospheric wave properties, our models produce a realistic range of slow and fast solar wind conditions by varying only the coronal magnetic field. Specifically, a two-dimensional model of coronal holes and streamers at solar minimum reproduces the latitudinal bifurcation of slow and fast streams seen by Ulysses. The radial gradient of the Alfven speed affects where the waves are reflected and damped, and thus whether energy is deposited below or above the Parker critical point. As predicted by earlier studies, a larger coronal "expansion factor" gives rise to a slower and denser wind, higher temperature at the coronal base, less intense Alfven waves at 1 AU, and correlative trends for commonly measured ratios of ion charge states and FIP-sensitive abundances that are in general agreement with observations. These models offer supporting evidence for the idea that coronal heating and solar wind acceleration (in open magnetic flux tubes) can occur as a result of wave dissipation and turbulent cascade.

http://iopscience.iop.org/article/10.1086/518001/pdf

Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence

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)

Explosive energy releases in plasmas, such as in solar eruptions like flares and coronal mass ejections, are thought to be caused by magnetic reconnection in thin current sheets. Zhang et al. observed a magnetic flux rope during a solar eruption, highlighting its role in driving explosive energy releases.

Observation of an evolving magnetic flux rope before and during a solar eruption

Coronal jets represent important manifestations of ubiquitous solar transients, which may be the source of significant mass and energy input to the upper solar atmosphere and the solar wind. While the energy involved in a jet-like event is smaller than that of "nominal" solar flares and coronal mass ejections (CMEs), jets share many common properties with these phenomena, in particular, the explosive magnetically driven dynamics. Studies of jets could, therefore, provide critical insight for understanding the larger, more complex drivers of the solar activity. On the other side of the size-spectrum, the study of jets could also supply important clues on the physics of transients close or at the limit of the current spatial resolution such as spicules. Furthermore, jet phenomena may hint to basic process for heating the corona and accelerating the solar wind; consequently their study gives us the opportunity to attack a broad range of solar-heliospheric problems.

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Solar coronal jets: Observations, theory, and modeling

A new method for reconstructing force-free magnetic fields from their boundary values, based on minimizing the global departure of an initial field from a force-free and solenoidal state, is presented. The method is tested by application to a known nonlinear solution. We discuss the obstacles to be overcome in the application of this method to the solar case: the reconstruction of force-free fields in the corona from measurements of the vector magnetic field in the low atmosphere.

An optimization approach to reconstructing force-free fields from boundary data: I. Theoretical basis.

New X-ray observations of the north polar region taken from the X-ray Telescope (XRT) of the Hinode spacecraft are used to analyze several time sequences showing small loop brightenings with a long ray above. We focus on the formation of the jet and discuss scenarios to explain the main features of the events: the relationship with the expected surface magnetism, the rapid and sudden radial motion, and possibly the heating, based on the assumption that the jet occurs above a null point of the coronal magnetic field. We conclude that 2-D reconnection models should be complemented in order to explain the observational details of these events and suggest that alternative scenarios may exist.

X-Ray Jet Dynamics in a Polar Coronal Hole Region

We present observations of a confined partial eruption of a filament on 2012 August 4, which restores its initial shape within 2 hr after eruption. From the Global Oscillation Network Group Hα observations, we find that the filament plasma turns into dynamic motion at around 11:20 UT from the middle part of the filament toward the northwest direction with an average speed of 105 km s–1. A little brightening underneath the filament possibly shows the signature of low-altitude reconnection below the filament eruptive part. In Solar Dynamics Observatory/Atmospheric Imaging Assembly 171 A images, we observe an activation of right-handed helically twisted magnetic flux rope that contains the filament material and confines it during its dynamical motion. The motion of cool filament plasma stops after traveling a distance of 215 Mm toward the northwest from the point of eruption. The plasma moves partly toward the right foot point of the flux rope, while most of the plasma returns after 12:20 UT toward the left foot point with an average speed of 60 km s–1 to reform the filament within the same stable magnetic structure. On the basis of the filament internal fine structure and its position relative to the photospheric magnetic fields, we find filament chirality to be sinistral, while the activated enveloping flux rope shows a clear right-handed twist. Thus, this dynamic event is an apparent example of one-to-one correspondence between the filament chirality (sinistral) and the enveloping flux rope helicity (positive). From the coronal magnetic field decay index, n, calculation near the flux rope axis, it is evident that the whole filament axis lies within the domain of stability (i.e., n < 1), which provides the filament stability despite strong disturbances at its eastern foot point.

/pdf/confined-partial-filament-eruption-and-its-reformation-w4suj3tsod.pdf

Confined partial filament eruption and its reformation within a stable magnetic flux rope

A saddle-like structure is clearly discerned in SOHO EIT images in the Fe wavebands in the active region NOAA 8113, from 6 to 10 December 1997, when the active region reached the solar limb. It appears after the emergence of a small patch of parasitic polarity near the footpoints of the loops forming the outer boundary of an arcade. There is a 3D null point in the center of the saddle. The height of the null is in good agreement with the relationship between the parasitic flux and the background magnetic field. In spite of the fact that a saddle-like or hyperbolic magnetic configuration is usually assumed as the necessary condition for magnetic field-line reconnection and magnetic energy release, no additional heating is observed inside the saddle structure. Moreover, interior parts of the saddle seem to be darker than the neighbouring loops of the arcade.

Observation of a 3d Magnetic Null Point in the Solar Corona

A filament eruption on 2010 October 21 observed from three different viewpoints by the Solar Terrestrial Relations Observatory and the Solar Dynamic Observatory is analyzed by also invoking data from the Solar and Heliospheric Observatory and the Kanzelhoehe Solar Observatory. The position of the filament just before the eruption at the central meridian not far from the center of the solar disk was favorable for photospheric magnetic field measurements in the area below the filament. Because of this, we were able to calculate with high precision the distribution of the coronal potential magnetic field near the filament. We found that the filament began to erupt when it approached the height in the corona where the magnetic field decay index was greater than 1. We also determined that during the initial stage of the eruption the filament moved along the magnetic neutral surface.

https://iopscience.iop.org/article/10.1088/0004-637X/773/1/10/pdf

A Filament Eruption on 2010 October 21 from Three Viewpoints

Recent EUV observations reveal that the `image' of the prominence overlaying coronal emission sometimes suddenly changes from absorption of EUV radiation to emission during the eruptive phase. This change reveals fast heating of the plasma within the prominence. We propose a kinetic mechanism of heating the fluid particles that transforms magnetic energy of the pre-eruptive magnetic configuration stored in the filament electric current into heat through collision processes of counteracting flows. The shape of the flux that the filament is made of should include upward concave segments to provide the counter flows within the erupting prominence. A typical twisted flux rope easily meets this requirement. Gas dynamic calculations are offered in addition to permit a quantitative evaluation of the relevant parameters and their time variations.

Boris Filippov

Papers

X-Ray Jet Dynamics in a Polar Coronal Hole Region

Confined partial filament eruption and its reformation within a stable magnetic flux rope

Observation of a 3d Magnetic Null Point in the Solar Corona

A Filament Eruption on 2010 October 21 from Three Viewpoints

About the prominence heating mechanisms during its eruptive phase