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Observations of Coronal Holes Giuliana de Toma SHINE Workshop 2006

Observations of Coronal Holes Giuliana de Toma SHINE Workshop 2006. Talk Summary l definition of coronal holes l properties of coronal holes l observations at different l l identification & comparison with models . DEFINITION OF CORONAL HOLES

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Observations of Coronal Holes Giuliana de Toma SHINE Workshop 2006

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  1. Observations of Coronal Holes Giuliana de Toma SHINE Workshop 2006

  2. Talk Summary l definition of coronal holes l properties of coronal holes l observations at different l l identification & comparison with models

  3. DEFINITION OF CORONAL HOLES based on coronal models:Foot-points of the magnetic field lines open into the heliosphere, i.e., a source of solar wind based on observations: Regions of low coronal emission, with one predominant magnetic polarity no direct measurements to establish which magnetic field lines are open or closed lindirect evidence that the observed coronal holes are mostly open field regions in the corona l not obvious that all open magnetic field lines are rooted in the observed coronal holes (physical properties of the slow solar wind suggest that at least part of it originates from previously closed field regions that have temporarily opened)

  4. OBSERVATIONS OF CORONAL HOLES “Koronalocher” were first identified as regions of low coronal emission in the FeXIV at 530nm (Waldmeier, 1956) and later in eclipse and white-light coronal observations. In the ’70s were recognized in X-ray and in EUV images as dark regions and in HeI 1083nm images as bright regions. Historically, the best determinations of coronal holes were based on drawings made by careful and experienced observers: l Karen Harvey and Frank Recely coronal hole maps l Pat McIntosh synoptic charts Recent efforts to develop automated schemes to identify coronal holes using an objective set of criteriato produce digital coronal hole maps l NSO coronal hole maps (Carl Henney) l CISM coronal hole synoptic maps (de Toma et al.)

  5. WHY IS IT DIFFICULT TO IDENTIFY CORONAL HOLES? Coronal Holes have: llow T and r relative to the background quiet corona but contrast is not sufficient to uniquely define coronal holes llow magnetic flux <B> one predominant magnetic polarity but coronal holes cannot be identified using magnetograms alone

  6. CORONAL HOLES: STATISTICS (from observations) based on K. Harvey’s survey for 1995-2002, includes 732 coronal holes lnumber of coronal holes on the Sun usually varies from 2 to 15 l average size : 5% (polar) and 0.3% (non-polar) of the solar surface l total fraction of the solar area occupied by coronal holes: ~ 14 - 18% at solar minimum < 5% at solar maximum l fraction of the total solar magnetic flux |B| in coronal hole ~ 20 - 40% near solar minimum < 5% at solar maximum l average net magnetic flux <B> : ~ 6.0 Mx/cm2 for polar coronal holes ~ 12.5 Mx/cm2 for low latitudes coronal holes (also 99% of coronal hole regions < 40 Mx/cm2) l degree of unipolarity (B /|B|) : average value = 94%

  7. 3/94 9/95 3/97 9/98 3/00 9/01 CORONAL HOLES AS SOURCES OF THE SOLAR WIND AT THE EARTH J. Luhmann et al., 2002 October 1997 January 1999

  8. EUV Observations lHigh contrast l Good coverage l Bright coronal regions intercepting the line-of-sight can mask coronal hole boundaries l Cooler or/and less dense regions also appear dark (e.g. filament channels can be easily confused with narrow coronal holes)

  9. Helium Observations • lCoronal holes are not • obscured by overlying • regions • l Narrower filament • channels • l Lower contrast • l Difficult to distinguish • small coronal holes • from unipolar • quiet Sun regions • l Data gaps in ground • based observations, • but long data set

  10. CORONAL LINES AT HIGH TEMPERATURE A simple intensity criteria (i.e. fixed threshold) at one wavelength is not enough !

  11. BRIGHT CORONAL LOOPS December 1, 1998 AR loops & loops connecting ARs November 21, 2002 loops connecting ARs

  12. AND MORE……….. February 5, 2001 polar coronal hole at different l note filaments too!

  13. NARROW CORONAL HOLES? October 29, 2001 Very narrow coronal holes are best visible in high T lines, like FeXIV or X-ray. Predicted by coronal models, often missed in He lines. Narrow channels (Hudson, 2002; Arge et al., 2003) Open-field canals connecting coronal holes (Antiochos, 2003)

  14. CORONAL HOLE IDENTIFICATION Work done as part of the CISM validation of coronal models Our goal: To compare observations and models Regions that have magnetic properties of the observed coronal holes and appear as coronal holes at multiple l are likely coronal holes and are expected to be “magnetically open” lCombine observations at several l and magnetograms l Regions that are not coronal holes are first identified - regions of high magnetic flux (> 150 Mx/cm2) - regions adjacent to polarity inversion lines (filaments channels) - plages and filaments (Ha, HeI) l Apply a set of stringent intensity criteria to identify coronal holes as dark regions in HeII 30.4nm, FeIX,X 17.1nm, FeXII 19.5nm and FeXV 28.4nm and bright regions in the FeIX,X/FeXII ratio and HeI 1083nm. A pixel is labeled a coronal hole pixel if it is identified as a coronal hole in at least two lines.

  15. HOW WELL ARE WE DOING? lMaps of K. Harvey and F. Recely made at NSO l Digital coronal holes maps generated by our automated scheme advantages : objective, fast, high confidence disadvantages : subset of coronal holes

  16. Near Solar Minimum: Carrington Rotation 1922 – May 1997 Observed Holes : computer code NSO drawings WSA :87%86% SAIC:89%88%

  17. Observed Holes : computer code NSO drawings WSA :50%41% SAIC:36%32% Solar Maximum: Carrington Rotation 1968 – October 2000 (before polar reversal, after polar coronal holes disappeared)

  18. Observed Holes : computer code NSO drawings WSA :54%55% SAIC:47%44% Solar Maximum: Carrington Rotation 1970 – December 2000 (before polar reversal, after polar coronal holes disappeared)

  19. Solar Maximum: Carrington Rotation 1986 – February 2002 IMPORTANCE OF MAGNETIC BOUNDARY CONDITIONS AT THE PHOTOSPHERE IN MODEL RESULTS !!!

  20. DO WE CORRECTLY IDENTIFY ALL CORONAL HOLES? Likely no, especially at maximum Carrington Rotation 1968 – October 2000 Total flux in coronal holes= 370 Mx (~4.7% of total flux) D = - 60 Mx b = from 6.7 to 4.8 Carrington Rotation 1970 - December 2000 Total flux in coronal holes= 306 Mx (~3.9% of total flux) D = - 83Mx b = from 1.6 to – 1.7

  21. FINAL REMARKS l Observationally we can not determine which magnetic field lines are “open”, we rely instead on intensity and average magnetic properties l Narrow and small coronal holes are difficult to identify in observations, yet they can play an important role as sources of the solar wind l Solar min : good agreement between model and observations Solar max : models capture the large scale distribution of holes, but larger discrepancies in size and shape Polar reversal : problems in both models and observations lAre we missing coronal holes in observations? If so, why? l How can we improve the detection of coronal holes? lWhat is the best way to compare models and observations?

  22. Identification of non coronal hole pixels Examples of masks for Carrington Rotation 1986 (February 2002)

  23. line ratio is very useful to separate coronal holes from other dark regions at solar maximum, but not always reliable at solar minimum.

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