Coronal holes as seen in soft X-rays H. S. Hudson (UCB and SPRC) SOHO-11, Davos, March 13, 2002

Coronal Holes (CH) Originally discovered via soft X-ray images (see also Waldmeier’s “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.

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

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

Large-scale issues “Source surface” representation of coronal field Variation of beta in the corona Coronal transients and Gold’s open flux problem - the need for magnetic reconnection, but where?

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!

Example of a source-surface model, from the Berkeley group

log(h/AU) Mm Height above photosphere (h) Plasma beta AR loops CH Fast wind Slow wind Streamer Quiet loops Filament

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.

YCH1 YCH2 YCH3 Ragged boundary Smooth boundary Loopy boundary Categories of CH Boundaries (Kahler & Hudson 2002)

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

Transient coronal holes

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.

Coronal hole channels

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

Enveloping CH Defines an “enveloping” CH boundary.

Shows the CH boundary as it appears on a magnetogram.

No streamer forms over the enveloping CH At the times of limb passage, the CH and AR complex did \ not produce a streamr.

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.

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 don’t know where this reconnection takes place (but note the interesting LASCO observations of reversed flow).

How does a CH change its area? - open-open reconnection - problem of “heat flux dropouts”

A better cartoon? - reconnection in low corona - use of network field for “wave of reconnection”

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.

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.