Observations and modelling of the Forced Reconnection in the Large-scale Solar Corona (adopted from Srivastava et al. 2019). SDO/AIA imaging observations on 3rd May 2012 at 14:10:08 UT display the formation of an X-point ( ′ a ′ – ′ b ′ ), which is subjected to a forced reconnection externally driven by a prominence (‘c’). The reconnection at the X-point and creation of inflowing and outflowing plasma channels are evident clearly along with the formation and thinning of current sheet (‘c’-‘d’).  The MHD model (‘e’) supports the occurrence of the forced reconnection. An important conclusion is emerged out from it that even if the resistivity (and magnetic diffusivity) at the X-point is small, then even the reconnection rate will be larger for the large amplitude external disturbance.

The Sun’s corona is maintained at a temperature in excess of a million degrees. The corona lies above the solar photosphere which is maintained at 6000 degrees. These regions are only a few hundred kilometers apart. A major scientific problem in solar physics is how and why is the corona so much hotter than the photosphere. Finding a solution to this outstanding problem provides cutting edge scientific information about the formation of the Sun’s magnetised atmosphere and its plasma structuring, heliosphere and associated magnetic field, space-weather and their triggering mechanisms, as well as their influences on the magnetosphere of various planets etc. This space weather can influence the Earth’s outer atmosphere, its satellite and communication systems, power systems etc. Moreover, the plasma processes causing the heating and eruptions may further provide clues about analogous physical processes in the laboratory, which help in controlling the confinement of diffusive plasma and its energetics. The source of coronal heating belong to the complex magnetic field, which is tightly woven within the fully ionised hot plasma gas there. Over the last few decades, numerous space and ground-based observatories have provided a variety of explanations for the origin of solar coronal heating, and the physical processes behind the eruption of large-scale transients (e.g., solar flares, coronal mass ejections, solar wind etc). Two basic physical mechanisms are hotty debated in an effort to explain the outstanding issues of the heating of the corona, and onset of solar eruptions.

The first is known as magnetic reconnection, which is related to the dissipation of turbulent magnetic energy. The second is energy provided by oscillating magnetic fields in the form of waves. Magnetic reconnection is defined as the self-organization and relaxation of complex and twisted magnetic fields leading to the liberation of stored magnetic energy in the corona. In the magnetized corona, the reconnection is one of the key physical processes to heat its atmosphere locally by releasing the stored magnetic energy in the form of heat, and also to generate various space weather candidates. The classical school of thought concerning magnetic reconnection is always challenged by some outstanding issues, e.g., the formation of current sheets and where they occur; appropriate reconnection rates and the extent of energy release; etc to understand exactly the role of reconnection in the evolution of various exotic plasma processes. Controlled or often termed “forced reconnection” is established theoretically, but it has not been observed at astrophysical or laboratory scales.

An international team including researches from five countries; Abhishek Kumar Srivstava, Sudheer K. Mishra, Dipankar Banerjee & B.N. Dwivedi from India; Petr Jelinek & Pradeep Kayshap from the Czech Republic; Hui Tian & Tanmoy Samanta from China; Vaibhav Pant from Belgium and Gerry Doyle from Armagh Observatory & Planetarium reports on a new discovery made using the observations from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory shows the first direct signature of the forced reconnection in the large-scale solar corona.

Prof. Doyle describes that “This discovery provides the first observational signature of forced magnetic reconnection in the Sun’s corona enabled by an external driver. It can help significantly in the energy release and localized coronal heating, as well as in triggering the solar eruptions. These first observational clues to forced reconnection have opened a path to further work via laboratory plasma experiments to constrain the behaviour of diffusive plasma, and to generate the energy”.


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