Identifying Coronal Holes in Solar Disk Images

(and how to compare to models)

Working Group Report

Nick Arge Rachel Hock Carl Henney

Definitions (from our invited speakers)

Coronal holes: observable used as a proxy for open field regions– Dark regions in the EUV/X-ray (bright in He I 1083) which

correspond to regions of reduced density and temperature– Predominately unipolar– Sources of high speed solar wind (While true, perhaps not

the best observational criteria.)

Open field regions: regions of magnetic field that extend far into the solar wind

One of Many Motivations

• How can we validate solar wind models?

• Coronal holes/open field regions are heavily influenced by non-potential fields.

• We know that the magnetic fields in/near filaments and active regions are non-potential.

• Can we use observations of coronal holes to constraint/validate solar wind models?

• Invited talk by Larisza Krista– Overview of coronal hole observations – Range of historic/current detection methods

• Consensus: EUV images especially with AIA/STEREO + magnetograms (or ADAPT for farside??) give nice coronal hole maps.

• But there are MANY techniques. We need VALIDATION and estimates of the UNCERTAINTIES.– Not all dark regions are coronal holes (i.e. filaments)– Line-of-sight issues make some coronal holes less dark than others

Session 1: Observations and Methods

Session 1: Validation• Legitimate resistance to hand-drawn maps as "ground truth" as

they are subjective– Maybe maps shouldn't be binary (confidence levels)?

• Many observers/many times “fuzzy” coronal hole maps

– Still subject to human error but does provide something to quantitatively compare methods to

• If not hand-drawn maps, then what? USE OTHER DATA that is less ambiguous– Doppler maps – White light/limb synoptic maps (Nishu Karna) – Spectroscopic observations (Michael Hahn) – He I (less issues with the limb) – H-alpha for filaments

• Improving/validating the techniques for space weather and modeling may, at the same time, might lead to new understanding of coronal hole physics

• Invited talk by Jon Linker– Coronal holes and open magnetic field– Deficiencies in the observations that impact models

– Lack of polar field observations – Magnetograph calibration issues– No far side data ... can have flux imbalance when AR comes over

east limb– Total open magnetic flux: models that match coronal hole boundary

underestimate the flux; models that overestimate the coronal holes get the flux right!

– In models, closed magnetic fields near open flux regions can be dark!– Is this an artifact of evolving the model to a steady state?

Session 2: Modeling and Theory

• Spiro Antiochos started the discussion on the dynamics of the boundaries of open field regions– It is not an either (open) ... or (closed)!– How does this relate to the boundary of coronal holes?– What are the time- and length- scales?

– Larisza: from SOHO, observationally coronal holes boundaries vary: few hours/~30 Mm

– AIA observations are perfect for this!– Should spectroscopic observations be used as emissivity in the

EUV might not be enough?– Dynamics of boundaries should be observable both in coronal and

solar wind observations– Streamer stalk/HCS: currently observed in solar wind data– Plumes/jets/rays: observed in coronal images– Plasma sheet/pseudostreamers

Session 2: Modeling and Theory

Open ClosedInterface

EUV images (AIA & STEREO) + magnetograms are great for routine automated detection of

coronal holes.

Other observations need to be used to improve our understanding of how EUV coronal holes

relate to open magnetic field regions improve the utility of using EUV coronal holes in

validating models

Coronal holes and open magnetic field regions are dynamic and their boundaries may not be

the same!

What does this mean for static/steady state solar wind models?

There is a lot of work still to be done!

When (if funded): January 2014

Where: New Mexico State University (NMSU) Las Cruces, New Mexico

Objectives:1) Review and discussing the latest techniques for

identifying coronal holes in solar disk data 2) Ascertaining how modelers and other users can best

make use of coronal hole identifications.

A workshop to identify coronal holes & how best to compare them with models

Organizing Committee: James McAteer, Rachel Hock, Nick Arge, & Carl Henney

If interested, please send email to: nick.arge@kirtland.af.mil