A Mechanism for Coronal Hole Jets

TL;DR

This paper investigates how magnetic topologies called bald patches in the solar corona transform into null point topologies when they open up, potentially explaining the dynamics of coronal hole jets observed by Hinode.

Contribution

It introduces an analytic model showing the transformation of bald patches into null points in coronal holes, linking magnetic topology changes to observed jet phenomena.

Findings

Bald patches are unlikely in open-field coronal hole regions.

Bald patches transform into null point topologies when the surrounding field opens.

The transition involves dynamic, non-ideal evolution processes.

Abstract

Bald patches are magnetic topologies in which the magnetic field is concave up over part of a photospheric polarity inversion line. A bald patch topology is believed to be the essential ingredient for filament channels and is often found in extrapolations of the observed photospheric field. Using an analytic source-surface model to calculate the magnetic topology of a small bipolar region embedded in a global magnetic dipole field, we demonstrate that although common in closed-field regions close to the solar equator, bald patches are unlikely to occur in the open-field topology of a coronal hole. Our results give rise to the following question: What happens to a bald patch topology when the surrounding field lines open up? This would be the case when a bald patch moves into a coronal hole, or when a coronal hole forms in an area that encompasses a bald patch. Our magnetostatic models show that, in this case, the bald patch topology almost invariably transforms into a null point topology with a spine and a fan. We argue that the time-dependent evolution of this scenario will be very dynamic since the change from a bald patch to null point topology cannot occur via a simple ideal evolution in the corona. We discuss the implications of these findings for recent Hinode XRT observations of coronal hole jets and give an outline of planned time-dependent 3D MHD simulations to fully assess this scenario.