The open issues focused on in this chapter presumably will determine the nearest future as well as the most interesting perspectives of plasma astrophysics. Magnetic energy release due to reconnection in the very complex systems of magnetic flux tubes (for example, a cosmic MHD turbulent plasma) is one of these issues.

Magnetic Reconnection and Turbulence

[1] Recent global simulations of substorms show that before the onset of near-Earth reconnection the pressure equilibrium in the tail breaks down. This instability has no cross-tail variation and is thus not a ballooning mode, and it is also distinct from the tearing mode. Here, we analyze an Open Geospace General Circulation Model simulation run of the 23 March 2007 substorm and find the same instability. Because this mode has no significant cross-tail variation associated with it we call it the KY0 mode. Besides the KY0 mode we also find the classical ballooning mode in the simulation. It has a wavelength of ∼0.5 RE and is marginally, but sufficiently, resolved as shown by a higher-resolution control run. These results suggest a new scenario for the substorm expansion phase onset. During the growth phase magnetic flux is added to the lobes and the plasma sheet thins but remains in equilibrium. When force balance is no longer possible the KY0 instability grows and accelerates plasma tailward. The divergence of the resulting tailward flow reduces the normal magnetic field and thereby makes the current sheet tearing unstable. The tearing mode grows right out of the KY0 mode. The classical ballooning mode grows at the same time and is superimposed on the KY0 mode, but its role in initiating reconnection is still unclear. The growth time of the KY0 mode, ∼2 min, is both consistent with the notion of an explosive growth phase and with recent ground-based observation of the initial growth of auroral arcs before auroral breakup.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JA015876

Open Geospace General Circulation Model simulation of a substorm: Axial tail instability and ballooning mode preceding substorm onset

Electron and proton acceleration in reconnecting current sheets in electron-rich solar flares is considered. A significant three-dimensional magnetic field is assumed in the current sheet where the particles are accelerated by the DC electric field. The tearing instability of a pre-flare current sheet leads to the formation of multiple singular lines of magnetic field where the electric and magnetic fields are coaligned. Magnetized electrons are shown to be accelerated to a few tens of MeV before they leave the vicinity of a singular line. The acceleration time is estimated to be less than 10−3 s. By contrast, much heavier protons are unmagnetized and their energy gain is more modest. The model explains a high electron-to-proton ratio and the unusually intense gamma-ray continuum above 1 MeV observed in the electron-rich flares.

Particle Acceleration in Reconnecting Current Sheets in Impulsive Electron-Rich Solar Flares

http://iopscience.iop.org/article/10.3367/UFNe.2016.09.037923/pdf

Thin current sheets: from the work of Ginzburg and Syrovatskii to the present day

The physics of flares on the Sun is now ‘an etalon’ for contemporary astrophysics, in particular for gamma and X-ray astronomy. In contrast to the flares on other stars and to many analogous phenomena in the Universe, solar flares are accessible to a broad variety of observational methods to see and investigate the magnetic reconnectioneffect in high-temperature strongly-magnetized plasma of the corona as well as in low-temperature weakly-ionized plasma in the photosphere.

Evidence of Reconnection in Solar Flares

Magnetic reconnection provides an efficient conversion of the so-called ‘free magnetic energy’ to kinetic and thermal energies of cosmic plasmas, hard electromagnetic radiation, and accelerated particles. This phenomenon was found in laboratory and space, but it is especially well studied in the solar atmosphere where it manifests itself as flares and flare-like events. We review the works devoted to the tearing instability — the inalienable part of the reconnection process — in current sheets which have, inside of them, a transverse (perpendicular to the sheet ‘plain’) component of the magnetic field and a longitudinal (parallel to the electric current) component of the field. Such ‘non-neutral’ current sheets are well known as the energy sources for flare-like processes in the solar corona. In particular, quasi-steady high-temperature turbulent current sheets are the energy sources during the ‘main’ or ‘hot’ phase of solar flares. These sheets are stabilized with respect to the collisionless tearing instability by a small transverse component of magnetic fiel, normally existing in the reconnecting and reconnected magnetic fluxes. The collision tearing mode plays, however, an important and perhaps dominant role for non-neutral current sheets in solar flares. In the MHD approximation, the theory shows that the tearing instability can be completely stabilized by the transverse fieldB

 n
 if its value satisfies the conditionB

 n
 /B≫S
−3/4
B is the reconnecting component of the magnetic field just near the current sheet,S is the magnetic Reynolds number for the sheet. In this case, stable current sheets become sources of temporal spatial oscillations and usual MHD waves. The application of the theory to the solar atmosphere shows that the effect of the transverse field explains high stability of high-temperature turbulent current sheets in the solar corona. The stable current sheets can be sources of radiation in the radio band. If the sheet is destabilized (atB

 n
 /B≪S
−3/4) the compressibility of plasma leads to the arizing of the tearing instability in a long wave region, in which for an incompressible plasma the instability is absent. When a longitudinal magnetic field exists in the current sheet, the compressibility-induces instability can be dumped by the longitudinal field. These effects are significant in destabilization of reconnecting current sheets in solar flares: in particular, the instability with respect to disturbances comparable with the width of the sheet is determined by the effect of compressibility.

Tearing instability of reconnecting current sheets in space plasmas

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Tearing instability of reconnecting current sheets in space plasmas