Magnetic helicity and energy storage in the solar corona

Abstract:

The lower solar corona is a highly structured and dynamic region of the Sun’s atmosphere, dominated by magnetic fields. X-ray observations of the million-degree coronal plasma reveal that solar flares and coronal mass ejections both have their origins in this region. Studies have shown that these phenomena, which require 10^25 Joules, are powered by the conversion of free magnetic energy stored in field aligned electric currents into other forms of energy. The resistive process of magnetic reconnection is central to this energy conversion. Another important, and related, aspect of these current carrying magnetic fields is the quantity known as magnetic helicity. Magnetic helicity describes the topological structure of the magnetic field. That is, how individual magnetic flux tubes are twisted and distorted and how different flux tubes are linked or braided together. Since magnetic helicity is an approximately conserved quantity, even during resistive processes, it is thought that coronal mass ejections act as a ‘valve’ which removes magnetic helicity from the corona.   The energy required to power solar flares and coronal mass ejections cannot be supplied to the corona on the timescale of a dynamic event and so instead is thought to be built up and stored in the coronal magnetic field in the hours and day beforehand. The question then arises of which physical processes are involved in generating the field aligned currents and magnetic helicity in the corona.    This talk will review both observational and modelling approaches to understanding the injection of magnetic energy and helicity into the corona. From observations of magnetic flux emergence from the solar interior into the corona and ongoing photospheric motions to the resulting coronal structures and locations where the energy is stored. The latest work to redefine the state of the art methods to estimate the amount of magnetic helicity based on photospheric injection, photospheric magnetic connectivity and volume computations will also be explored.