Abstract
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 3-D MHD simulations to fully assess this scenario.
Highlights
Magnetic bald patches are sections of a polarity inversion line (PIL) where the field lines are concave up rather than concave down as is the usual case for a loop geometry (aPIL is any location on the photosphere where the normal flux changes sign)
Combinations of latitude and dipole strengths that lie below this curve can support a bald patch topology, while those above will produce a null point above the surface
The shape of this curve is very similar to the one for the magnetic flux which indicates that the mean radial magnetic field inside the nested polarities does not change substantially as a function of latitude
Summary
Magnetic bald patches are sections of a polarity inversion line (PIL) where the field lines are concave up rather than concave down as is the usual case for a loop geometry (aPIL is any location on the photosphere where the normal flux changes sign). A common flux distribution that leads to bald patch formation is that of a small opposite polarity region embedded in a large scale unipolar background field. Such a nested polarity will appear if, for example, a bipolar active region emerges in a unipolar hemisphere, or if an ephemeral region emerges in a coronal hole or if a small magnetic carpet bipole emerges in the unipolar network. We expect this polarity distribution to be a generic feature of the solar photosphere
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