Signs of helicity in solar prominences and related features

Abstract

The chirality of filaments and features in their environment are reviewed in preparation to discuss how some of the observational signatures of chirality have been differently interpreted as corresponding to either positive or negative helicity. Complete agreement exists among authors that the positive and negative signs of chirality of sigmoids and coronal arcades correspond respectively to positive and negative signs of helicity. However, to date there has not been a consensus on whether the observational definitions of right and left handed signatures of the chirality of filaments and filament channels correspond respectively to positive and negative signs of twist helicity or to the inverse. Three proposed observational tests are discussed on how to establish the correct associations but only Doppler velocity measurements during erupting prominences have been generally regarded as a definitive test. Doppler shifts in spectra and multi-wave length sets of filtergrams of filament eruptions recorded with at least three different instruments at different observatories have provided data showing the largely unexpected result that both signs of twist helicity exist in different parts of erupting prominences. Supporting evidence of both signs of helicity in some erupting prominences are additionally shown in 195A images from TRACE, 304A images from EIT/SOHO and vortical motions in the chromosphere observed at NSO/Sacramento Peak Observatory. This accumulation of evidence also reveals a general pattern of dual helicity associated with a rolling motion in the top and twist of opposite helicity in the legs of these erupting prominences. A force generating a rolling motion implies no introduction of twist unless applied non-uniformly to a non-rigid structure. If the non-rigid structure is part of a closed magnetic field system, like a prominence, and a force is applied at its top as inferred from the observations, both signs of twist helicity are necessarily produced. It is suggested that such an unknown force produces the observed “roll effect” in erupting prominences and that the source of this initiating force needs to be identified.