# Magnetic nulls and super-radial expansion in the solar corona

## TL;DR

This paper provides the first observational evidence of coronal magnetic nulls in the solar corona using polarized light, introduces a new method to measure magnetic expansion, and highlights the diagnostic power of linear polarization for understanding solar wind and eruptions.

## Contribution

It presents the first clear detection of coronal magnetic nulls in off-limb observations and introduces a novel technique to calculate expansion factors directly from polarization data.

## Key findings

- Detection of magnetic nulls in the corona.
- A new method to compute expansion factors without photospheric extrapolation.
- Linear polarization as a diagnostic tool for coronal magnetic topology.

## Abstract

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. 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.