Magnetic reconnection
Ron Giovanelli is credited with the first publication invoking magnetic energy release as a potential mechanism for particle acceleration in solar flares.
[5] Giovanelli proposed in 1946 that solar flares stem from the energy obtained by charged particles influenced by induced electric fields within close proximity of sunspots.
James Dungey is credited with first use of the term “magnetic reconnection” in his 1950 PhD thesis, to explain the coupling of mass, energy and momentum from the solar wind into Earth's magnetosphere.
In the meantime, the first theoretical framework of magnetic reconnection was established by Peter Sweet and Eugene Parker at a conference in 1956.
[10] Magnetic reconnection is a breakdown of "ideal-magnetohydrodynamics" and so of "Alfvén's theorem" (also called the "frozen-in flux theorem") which applies to large-scale regions of a highly-conducting magnetoplasma, for which the Magnetic Reynolds Number is very large: this makes the convective term in the induction equation dominate in such regions.
The displacement current is neglected in both the Parker-Sweet and Petschek theoretical treatments of reconnection, discussed below, and in the derivation of ideal MHD and Alfvén's theorem which is applied in those theories everywhere outside the small diffusion region.
When this happens, the plasma is pulled out by Magnetic tension force acting on the reconfigured field lines and ejecting them along the current sheet.
The resulting drop in pressure pulls more plasma and magnetic flux into the central region, yielding a self-sustaining process.
The importance of Dungey's concept of a localized breakdown of ideal-MHD is that the outflow along the current sheet prevents the build-up in plasma pressure that would otherwise choke off the inflow.
However, this means that magnetic monopoles would exist, albeit for a very limited period, which would violate Maxwell's equation that the divergence of the field is zero.
[13] The reason is close to Dungey's original thoughts: at each time step of the numerical model the equations of ideal MHD are solved at each grid point of the simulation to evaluate the new field and plasma conditions.
This is often called "numerical resistivity" and the simulations have predictive value because the error propagates according to a diffusion equation.
One possible mechanism to explain the discrepancy is that the electromagnetic turbulence in the boundary layer is sufficiently strong to scatter electrons, raising the plasma's local resistivity.
Solar flares, the largest explosions in the Solar System, may involve the reconnection of large systems of magnetic flux on the Sun, releasing, in minutes, energy that has been stored in the magnetic field over a period of hours to days.
[14] These regions are known as quasi-separatrix layers (QSLs), and have been observed in theoretical configurations[15] and solar flares.
[16][17] The first theoretical framework of magnetic reconnection was established by Peter Sweet and Eugene Parker at a conference in 1956.
Numerical simulations of two-dimensional magnetic reconnection typically show agreement with this model.
[21][22] The fundamental reason that Petschek reconnection is faster than Parker-Sweet is that it broadens the outflow region and thereby removes some of the limitation caused by the build up in plasma pressure.
[23] The aspect ratio of the diffusion region is then of order unity and the maximum reconnection rate becomes
Theory and numerical simulations show that most of the actions of the shocks that were proposed by Petschek can be carried out by Alfvén waves and in particular rotational discontinuities (RDs).
In cases of asymmetric plasma densities on the two sides of the current sheet (as at Earth's dayside magnetopause) the Alfvén wave that propagates into the inflow on higher-density side (in the case of the magnetopause the denser magnetosheath) has a lower propagation speed and so the field rotation increasingly becomes at that RD as the field line propagates away from the reconnection site: hence the magnetopause current sheet becomes increasingly concentrated in the outer, slower, RD.
[24] In stochastic reconnection,[25] magnetic field has a small scale random component arising because of turbulence.
[28] Roughly speaking, in stochastic model, turbulence brings initially distant magnetic field lines to small separations where they can reconnect locally (Sweet-Parker type reconnection) and separate again due to turbulent super-linear diffusion (Richardson diffusion [29]).
However, the first direct observations of solar magnetic reconnection were gathered in 2012 (and released in 2013) by the High Resolution Coronal Imager.
[32] Magnetic reconnection events that occur in the Earth's magnetosphere (in the dayside magnetopause and in the magnetotail) were for many years inferred because they uniquely explained many aspects of the large-scale behaviour of the magnetosphere and its dependence on the orientation of the near-Earth Interplanetary magnetic field.
Cluster II is a four-spacecraft mission, with the four spacecraft arranged in a tetrahedron to separate the spatial and temporal changes as the suite flies through space.
The Magnetospheric Multiscale Mission, launched on 13 March 2015, improved the spatial and temporal resolution of the Cluster II results by having a tighter constellation of spacecraft.
[36] Dr. Vassilis Angelopoulos of the University of California, Los Angeles, who is the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger.".
For example, studies on the Large Plasma Device (LAPD) at UCLA have observed and mapped quasi-separatrix layers near the magnetic reconnection region of a two flux rope system,[38][39] while experiments on the Magnetic Reconnection Experiment (MRX) at the Princeton Plasma Physics Laboratory (PPPL) have confirmed many aspects of magnetic reconnection, including the Sweet–Parker model in regimes where the model is applicable.
The Kadomtsev model describes sawtooth oscillations as a consequence of magnetic reconnection due to displacement of the central region with safety factor
