Electric fields in the solar atmosphere: A review

Macroscopic electric fields in the solar atmosphere have received much less attention than magnetic fields, although they must play a role of comparable importance in plasma heating, and in charged particle acceleration and transport. We review various remote sensing techniques that have been developed, whose sensitivity is now 5–10 V cm ^–1 for measurement of the electric field component transverse to the line-of-sight. Our review of the processes most likely to produce observable fields in the solar atmosphere indicates that quasi-static, macroscopic values of E (the electric field component parallel to the magnetic vector) well above this detection threshold are predicted by the discharge model of flares, by models of return currents associated with flare particle beams, and by models of neutral sheets associated with two-ribbon flares and post-flare loops. In addition, both E and E components may be detectable in time dependent electric fields associated with MHD and plasma waves, and with plasma turbulence. The emission measures and time-scales associated with these electrified plasma volumes are as highly uncertain as our present understanding of the volumes, plasma conditions and processes involved in the liberation of flare energy. Observations of electric field vector intensities, orientations, time-behaviour and spatial distribution at the presently attained electric field sensitivity levels could provide new, direct information of great interest in the electrodynamics of solar magnetic structures.