High resolution H-alpha photographs for studying chromospheric fine structure at active region polarity boundarie

Prata, S. W.

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HALE OBSERVATORIES
A N01'E ON CHROMOSPHERIC FINE STRUCTURE
AT ACTIVE REGION POLARITY BOUNDARIES.
By
Stephen W. Pra
BIG BEAR SOLAR OBSERVATORY, HALE OBSERVATORIES
CALIFORNIA INSTITUTE OF TECHNOLOGY
CARNEGIE INSTITUTE OF WASHINGTON
Received
H-a photographs show a variety of dark structures along
zero-longitudinal-field lines in active regions. At least
one form is apparent only in high-resolution photographs.
We wish to point out this form here. For comparison, we
will list first two better known forms.
1) Active region filament: These filaments lie along
lines of zero longitudinal field. The fine structure of
such a filament is parallel to the gross structure of the
filament. There is an appreciable component of magnetic
field along the filament. Such filaments are described,
for instance, by FOUKAL (1971). A few examples are labeled
"1" in Figure 1. Field and filament are parallel to the
magnetic boundary.
2) Arches: Figures 2 and 4 show active region arches
(labeled 112 11 ). In appearance and size they are similar to
AFS arches, such as discussed by BRUZEK (1967), ROBERTS
(1970), and WEART (1970). However, instead of overlying
bright plage (as AFS arches do) they connect plage regions.
Their similarity in appearance to AFS arches and the fact
that material often flows along the arches suggest that they,
too, lie along the magnetic field lines. In this case, both
field and arches are perpendicular to the boundary between
opposite polarity regions. These arches appear to have
lifetimes greater than the one hour characteristic of AFS
arches but in some cases less than 24 hours; this matter and
other properties of such arches merit further study. Arches
-2-
typically are of the order of 15,000 km long.
3) Field Transition Arches: On lower resolution photo-
graphs (Figure 3) these features (labeled 11 3 11 ) look like
rather crooked filaments separating bright plages of opposite
polarity. High resolution photographs, however, show that
the fine structure in these filaments is in fact cross fila-
ment and cross boundary (Figure 4). That is, the actual
structural form consists of a long series of parallel, closely
spaced dark threads joining regions of opposite polarity. As
in the preceding class, field and structure are perpendicular
to the magnetic boundary. However, these threads are much
smaller than the arches. They are of the order of 4000 km
or less in length. On the large scale, the feature can
last weeks. We do not yet know the time scale for the
individual threads, but it is clearly much shorter.
There are, of course, gradations between the various
classes. Figure 4, for example, shows features intermediate
in size and position between the last two classes. Then,
too, the angle between the filament structure and polarity
boundary can be between the two extreme angles of 0° (first
class) and 90° (the other two classes). For instance, near
the 11 3 1, in Figure 4, this angle is closer to 450.
We have noted the following points about where these
features occur:
1) The active region filaments lie on boundaries
between regions of weaker fields, while the cross-boundary
i
-3-
structures (the second and third classes above) occur at
boundaries between regions of stronger field (ZIRIN, 1970).
2) The arches often appear roughly radiating from
sunspots.
3) The field transition arches are found only where
opposite polarity regions of strong field are closely adja-
cent; bright plage can be found adjacent to both sides of
the feature.
4) Often a single boundary will have field transition
arches where the fields are strongest and regular filaments
where the fields are weaker; the transverse structure will
change to a shear structure.
The forms themselves are transmutable. Boundaries
spanned by parallel arches can become field transition fila-
ment regions, indicating that the width of the region of
horizontal field decreases. In some cases, field transition
filaments are eventually replaced by regular filaments; there
is one case on film where the reverse occurred. Changes such
as these are not the rule, but they are interesting. They
could be caused by motion of the opposite polarity regions
relative to each other or by the emergence of new flux in
the vicinity of the boundary.
These boundary regions will be investigated more closely
as more high-resolution movies are made. Information on
lifetimes, structure, and evolution of the boundary features
should give us greater knowledge of the structure,growth,
I.
-4-
and evolution of the magnetic field in active regions.
I wish to acknowledge an informative discussion with
Dr. Harold Zirin. The wor^ involved in this research was
supported by NSF Grant No. GA 24015 and NASA Contract No.
05-222-034.
REFERENCES
Bruzek, A.: 1967, Solar Phys. 2, 451.
Foukal, P.: 1971, submitted to Solar Lhhs.
Roberts, P. H.: 1970, "Velocity Fields in Magnetically Dis-
turbed Regions of the Ha Chromosphere," Doctoral Thesis,
California Institute of Technology.
Weart, S. R.: 1970, AE.J., 162, 987.
Zirin, H.: 1970, "The Manifold Structure of the Chromosphere
and Corona," paper prepared for presentation to the
Royal Society Discussion Meeting on Solar Studies.
-5-
FIGURE CAPTIONS
Figure 1 - Active region filaments (labeled °1' , ): filament
and magnetic field are both parallel to the
boundary between opposite polarities. (October 24,
1969). All photographs in this article are from
Big Bear Solar Observatory.
Figure 2 - Arches (labeled 112 11 ): These features are parallel
to the magnetic field and perpendicular to the
polarity boundary. (October 26, 1969).
Figure 3 - Field transition arches (labeled 11 3 11 ) under con-
ditions of low resolution look like a single
irregular active region filament. (September 9,
1970) .
Figure 4 - With higher resolution the true structure of the
field transition arches ( 11 3 11 ) becomes visible.
The features are parallel to the magnetic field
and connect regions of opposite polarity. Also
visible are some arches ( "2 11 ) . (September 9, 1970) .
IFigure 1
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Figure 2
Figure 3
Figure 4


A note on chromospheric fine structure at active region polarity boundaries

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