Coronal holes as seen in soft X-rays

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Coronal holes as seen in soft X-rays

• H. S. Hudson (UCB and SPRC)

SOHO-11, Davos, March 13, 2002
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Coronal Holes (CH)

• Originally discovered via soft X-ray images (see
  also Waldmeiers koronale loecher).
• CH appear to show regions of open coronal field.
• Soft X-rays (Yohkoh SXT) have large contrast and
  may define CH boundaries sharply.
• He I 10830A gives a somewhat different view -
  broader but not so clear.
• This talk does not specifically review the solar
  cycle, since there is as yet no SXT literature.

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Waldmeier images from the past - not loecher, but
polar plumes?
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Topics

• Boundaries of equatorward-extension CHs
• Transient CH and dimming
• CH channels
• Enveloping CH
• Interpretation

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Large-scale issues

• Source surface representation of coronal field
• Variation of beta in the corona
• Coronal transients and Golds open flux problem -
  the need for magnetic reconnection, but where?

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Source-surface models and why I distrust them

• A fictitious concentric surface, typically at 2.5
  R, defines the open magnetic flux by serving as
  the outer boundary for a potential-field
  extrapolation.
• This model totally distorts the top of the
  corona.
• There is no reason to expect that the best-fit
  source surface would not vary with time.
• The potential-field representation of the coronal
  volume leaves no room for flares or CMEs!

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Example of a source-surface model, from
the Berkeley group
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Streamer
Filament
Slow wind
1
Quiet loops
0.1
Fast wind
Plasma beta
CH
0.01
AR loops
0.001
-4 -3
-2 -1 0
1 log(h/AU)
1.5 15 150 Mm
Height above photosphere (h)
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SXT observations of rigid CH
boundaries(Kahler Hudson, submitted to ApJ)

• Identify three types of boundary morphology in
  Yohkoh equatorial extensions.
• See no direct evidence for heating associated
  with reconnection as boundaries move.
• Find that CH areas evolve smoothly, independent
  of X-ray bright points and transient coronal
  holes.

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Loopy boundary
YCH1
Ragged boundary
YCH2
Smooth boundary
Categories of CH Boundaries (Kahler Hudson 2002)
YCH3
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Significance of loopy or ragged boundaries

• The closed corona just outside the CH contains
  small-scale loops in most cases.
• It is therefore hard to imagine reconnection on
  large (streamer) scales as the mechanism for
  boundary evolution
• Cartoons to be shown later

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Transient coronal holes
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Transient coronal holes and dimming

• SXT sees transient coronal holes associated
  both with flares and with quiet-Sun arcade
  events.
• There are other varieties of dimming events
  associated with CMEs (depletions also seen in
  white light).
• It is clear morphologically that such events
  alter the solar open flux, if CHs provide an
  accurate indicator of open field lines.

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Coronal hole channels
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Coronal hole channels

• Soft X-rays show CH channels well (these are
  probably Waldmeiers locher when on the limb).
• The channels may contain substantial flux but may
  not easily be detected in 10830 because they are
  too narrow.
• The source-surface models often find them
  successfully.

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Enveloping CH
Defines an enveloping CH boundary.
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Shows the CH boundary as it appears on a
magnetogram.
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No streamer forms over the enveloping CH
At the times of limb passage, the CH and AR
complex did \ not produce a streamr.
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Enveloping CHs

• A ring-shaped CH area may appear when a new
  active region emerges within a CH boundary
  (rosette or anemone).
• Such an enveloping CH may contain multiple
  active regions.
• In the example shown, an isolated streamer did
  not form over such an inclusion.
• However, it is known from radio type III bursts
  that active regions often contain open field
  lines, even prior to an eruptive flare.

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Variation of CH area

• CH boundaries change smoothly, except during
  eruptive events.
• TCH recovery pattern proceeds in a curious
  manner.
• CH areas can increase without magnetic
  reconnection, but to decrease CH area requires
  that two open field lines meet and reconnect,
  somewhere within the corona.
• At present we dont know where this reconnection
  takes place (but note the interesting LASCO
  observations of reversed flow).

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How does a CH change its area? - open-open
reconnection - problem of heat flux dropouts
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A better cartoon? - reconnection in low corona
- use of network field for wave of
reconnection
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Conclusions

• Yohkoh data show that CH boundaries evolve
  smoothly, not altered significantly by XBPs or
  TCHs.
• Yohkoh identifies most CH boundaries with
  small-scale loops.
• Yohkoh sees no physical evidence for reconnection
  at moving CH boundaries.
• We need to exploit the Yohkoh database and look
  forward to future exploration of the inner
  heliosphere, perhaps with Solar Orbiter. The top
  of the corona is a new frontier.

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Conclusions not achieved yet

• Cannot determine the properties of the open-field
  regulation
• Cannot locate the reconnection needed for this
  regulation.
• Cannot directly observe solar/heliospheric
  connectivities.