Magnetic Nulls and Super-radial Expansion in the Solar Corona

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Monday, May 1, 2017

Magnetic fields in the sun's outer atmosphere, the corona, control both solar-wind acceleration and the dynamics of solar eruptions. We present the first clear observational evidence of coronal magnetic nulls in off-limb linearly polarized observations of pseudostreamers, taken by the Coronal Multichannel Polarimeter (CoMP) telescope.

Coronal pseudostreamer magnetic topology image
Coronal pseudostreamer magnetic topology and associated magnetic null produces a characteristic signature in linear-polarization magnitude. a–b) represent a prediction of this signature made by Rachmeler et al. (2014), and c) their manifestation in CoMP observations. a) Magnetic field lines are illustrated within a pseudostreamer, showing two closed-field loops surrounded by unipolar open field with the central magnetic null (i.e., where the magnetic field strength equals zero) indicated by an asterisk. b) Linear-polarization magnitude fraction (L/I = p (Q2 + U2)/I, where I, Q, U are Stokes vector components) is synthesized for this magnetic topology, resulting in a predicted polarization signature of pseudostreamer topology. Dark features outline three linear-polarization lobes and indicate van-Vleck angle crossings (see text); the magnetic null in (a) coincides with the intersection of these lobes. c) CoMP linear-polarization (L/I) observations of a South-pole pseudostreamer on Nov. 26, 2014 demonstrate for the first time that a magnetic null or nulls may be identified with coronal polarimetry. The solar photosphere is indicated by the yellow curves in (b) and (c).

These nulls represent regions where magnetic reconnection is likely to act as a catalyst for solar activity. CoMP linear-polarization observations also provide an independent, coronal proxy for magnetic expansion into the solar wind, a quantity often used to parameterize and predict the solar wind speed at Earth. We introduce a new method for explicitly calculating expansion factors from CoMP coronal linear-polarization observations, which does not require photospheric extrapolations. We conclude that linearly-polarized light is a powerful new diagnostic of critical coronal magnetic topologies and the expanding magnetic flux tubes that channel the solar wind.

Published online article at the Astrophysical Journal Letters, April 2017 (Arxiv link).