But why does a solar flare occur? What triggers the instability and suddenly ignites an explosion from magnetic fields that remain unperturbed for long intervals of time?
It might be triggered when magnetized coronal loops are pressed together, driven by motions beneath them, meeting to touch each other and merge (Fig. 7.10).
Magnetic fields have a direction associated with them, and if oppositely directed magnetic fields are pushed together, they can interact. When the merging magnetic fields are closed coronal loops, they will be broken open to release magnetic energy in the form of flare heating and particle acceleration. But the magnetic fields are not permanently broken, and they simply reconnect back to a closed state. For this reason, the technical name for this merging and coupling is magnetic reconnection.
In 1956, Peter A. Sweet (1921-2005), then at the University of London Observatory, realized that electric current would form between the merging, oppositely directed magnetic fields, ^e current flows in a flat, two-dimensional plane, shaped like a sheet on a well made bed, and is hence called a current sheet. And since the magnetic direction cancels into neutrality at their meeting place, the term neutral current sheet is also used, ^e American solar physicist Eugene Parker (1927- ) derived the detailed mathematics of magnetic reconnection during solar flares in 1963, and in the same year another American Harry E. Petscheck (1930-2005) showed how stored magnetic energy might be rapidly released during reconnection in the current sheet. Nowadays, the two modes of magnetic reconnection are called Sweet-Parker or slow reconnection and Petchek or fast reconnection.
So, we now think of these powerful outbursts as stemming from the interaction of coronal loops, ^ey are always moving about, like swaying seaweed or wind-blown grass, and existing coronal loops may often be brought into contact by these movements. Magnetic fields coiled up in the solar interior, where the Sun's magnetism is produced, can also bob into the corona to interact with pre-existing coronal loops. In either case, the coalescence leads to the rapid release of magnetic energy through magnetic reconnection.
^e explosive instability has been compared to an earthquake, with the moving roots or footpoints of a sheared magnetic loop resembling two tectonic plates. As the plates move in opposite directions along a fault line, they grind against each other and build up stress and energy. When the stress is pushed to the limit, the two plates cannot slide further, and the accumulated energy is released as an earthquake, ^at part of the fault line then lurches back to its original, equilibrium position, waiting for the next
FIG. 7.10 Magnetic connection A pair of oppositely directed, twisted coronal loops come in contact, releasing magnetic energy to power a solar flare. Arrows indicate the direction of the magnetic field lines and the sense of twist. Such magnetic encounters can occur when newly emerging magnetic
FIG. 7.10 Magnetic connection A pair of oppositely directed, twisted coronal loops come in contact, releasing magnetic energy to power a solar flare. Arrows indicate the direction of the magnetic field lines and the sense of twist. Such magnetic encounters can occur when
fields rise through the photosphere tojoin pre-existing ones in the corona, or when the twisted coronal loops are forced together by underlying motions. [Adapted from
Thomas Gold and Fred Hoyle, "Origin of Solar Flares",
Monthly Notices ofthe Royal Astronomical Society 120,
earthquake. In this analogy, the magnetic fields become stressed to the breaking point and similarly regain their composure after an explosive convulsion on the Sun, fusing together and becoming primed for the next outburst.
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