FLARES HEAT THE UPPER CHROMOSPHERE OF THE SUN

Image: Anatomy of the Sun. Credit: NASA/Jenny Mottar

The standard model of solar flares explains many observational features of flares, assuming that they are driven by magnetic reconnection. Flares release approximately 10³⁰–10³³ erg into the plasma but it is not clear, however, how that energy is partitioned between in situ heating of the corona, particle acceleration, and wave generation, nor to what extent the observable features of a flare depend on the balance between different types of coronal energy transport.

In a recent paper (Reep & Russell, 2016 ApJL), the authors have developed a numerical model of flare heating due to the dissipation of Alfvénic waves propagating from the corona to the chromosphere.

A solar flare is a sudden flash of brightness observed near the Sun's surface and it ejects clouds of electrons, ions, and atoms through the corona of the sun into space. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior, and they are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona.

An Alfvén wave, instead, is a type of magnetohydrodynamic wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines. The wave propagates in the direction of the magnetic field. The motion of the ions and the perturbation of the magnetic field are in the same direction and transverse to the direction of propagation.

Image: Structure of the Sun. Credit: Wikimedia Common

The paper (Reep & Russell, 2016 ApJL) presents an investigation of the key parameters of these waves on the energy transport, heating, and subsequent dynamics. For sufficiently high frequencies and perpendicular wave numbers, the waves dissipate significantly in the upper chromosphere, strongly heating it to flare temperatures. This heating can then drive strong chromospheric evaporation, bringing hot and dense plasma to the corona. The authors derive three important conclusions: (1) Alfvenic waves, propagating from the corona to the chromosphere, are capable of heating the upper chromosphere and the corona, (2) the atmospheric response to heating due to the dissipation of Alfvenic waves can be strikingly similar to heating by an electron beam, and (3) this heating can produce explosive evaporation.

Reep & Russell, 2016 (ApJL) - Alfvénic wave heating of the upper chromosphere in flares (arXiv)

Structure of the sun --> (Nasa) (Wikipedia)
Solar flares --> (Nasa) (Wikipedia)
Alfvén waves --> (Wikipedia)

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