Author: Gerry Doyle (Armagh); Abhishek Srivastava, Sudheer Mishra, Bhola Dwivedi & Dipankar Banerjee (India), Petr Jelı́nek & Pradeep Kayshap (Czech Republic); Tanmoy Samanta & Hui Tian (China); Vaibhav Pant (Belgium)
Magnetic reconnection is a physical process that may yield various phenomena in astrophysical and laboratory plasma, e.g., energetic flares, geomagnetic substorms, tokamak disruptions, controlled fusion experiments, etc [1,2]. In the high temperature astrophysical plasmas of solar and stellar coronae, it is basically defined as the self-organization and relaxation of complex and twisted magnetic fields leading to the liberation of stored magnetic energy . In the magnetically dominated solar corona, magnetic reconnection is one of the key physical processes to heat its atmosphere, and also to generate various space weather candidates (e.g., flares, prominence eruption, and coronal mass ejections), which may influence the Earth’s outer atmosphere, its satellite and communication systems, power systems etc [3,4,5,6]. The variety of exact physical conditions of the magnetic reconnection region are still poorly known despite several novel discoveries both in theory and observations in the astrophysical and laboratory plasmas. Some known burning issues that need to be explored in a more deterministic manner are the formation of current sheets and their morphology, appropriate reconnection rate, establishment of natural diffusion regions and their physical properties, etc to understand exactly the role of reconnection in various exotic plasma processes.
Forced Reconnection is Unveiled in the Large-Scale Solar Corona
Using multi-wavelength observations of the solar corona from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) on 12 May 2012, we establish directly that forced reconnection at a considerably high rate occur locally in its magnetized plasma (Fig. 1; ). It was triggered in the corona when two oppositely directed magnetic field lines forming an X-point are perturbed by an external disturbance (prominence motion in the present case). This type of reconnection has only been reported in theory [8,9], and has never been directly observed in the Sun’s large-scale corona.
Figure 1 displays SDO/AIA imaging observations on 3rd May 2012 at 14:10:08 UT, which show the formation of a temporary X-point and a forced reconnection region. In the righthand panel (“a”) we have the composite image of AIA 171Å, 304Å showing the off-limb region. In the top middle panel (“b”) we have a zoomed view of the region of interest as shown by the dotted-black box in panel “a”. In the top left panel (“c”) we have the difference image map of AIA 171Å, which clearly display the formation of the temporary X-point and the set of the inflowing and outflowing plasma in the large-scale corona exhibiting the reconnection. This is forced reconnection which is driven externally by an impulse generated by a prominence.
The Differential Emission Measure (DEM) map for coronal temperature (shown in the bottom left panel “d”) display the formation of the current sheet and an instance of the ongoing forced reconnection. The modelling of current sheet initialized by the observed initial conditions of the forced reconnection (shown in the middle bottom panel “e”) exhibits an important result that the implementation of the external driver increases the rate of the reconnection even when the resistivity required for creating a normal diffusion region decreases. The detailed description of these first observational results are published in the Astrophysical Journal .
In conclusion, the dynamical corona may be episodically subjected to the rapid forced reconnection driven by external perturbations. It can help significantly in the energy release and in the evolution of the eruptive phenomena. These first observational clues to the forced reconnection can also be extended to the laboratory plasma, where it can be employed to constrain the behaviour of the diffusive plasma, and to generate the energy.
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