The Astrophysical Journal, 660:882 – 892, 2007 May 1
# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE SOLAR ECLIPSE OF 2006 AND THE ORIGIN OF RAYLIKE FEATURES IN THE WHITE-LIGHT CORONA
Y. -M. Wang,1 J. B. Biersteker,1,2 N. R. Sheeley, Jr.,1 S. Koutchmy,3 J. Mouette,3 and M. Druckmüller4
Received 2006 September 10; accepted 2007 January 5
ABSTRACT
Solar eclipse observations have long suggested that the white-light corona is permeated by long fine rays. By comparing photographs of the 2006 March 29 total eclipse with current-free extrapolations of photospheric field measurements and with images from the Solar and Heliospheric Observatory (SOHO), we deduce that the bulk of these
linear features fall into three categories: (1) polar and low-latitude plumes that overlie small magnetic bipoles inside
coronal holes, (2) helmet streamer rays that overlie large loop arcades and separate coronal holes of opposite polarity,
and (3) ‘‘pseudostreamer’’ rays that overlie twin loop arcades and separate coronal holes of the same polarity. The helmet streamer rays extend outward to form the plasma sheet component of the slow solar wind, while the plumes and
pseudostreamers contribute to the fast solar wind. In all three cases, the rays are formed by magnetic reconnection
between closed coronal loops and adjacent open field lines. Although seemingly ubiquitous when seen projected against
the sky plane, the rays are in fact rooted inside or along the boundaries of coronal holes.
Subject headingg
s: solar wind — Sun: corona — Sun: magnetic fields
1. INTRODUCTION
2. ECLIPSE IMAGES
White-light images of the solar corona recorded with high resolution and sensitivity typically show a mixture of rounded loops,
outward-pointing cusps, and narrow, linear features that extend
outward to great distances (see Sturrock & Smith 1968; Eddy
1973; Saito & Tandberg-Hanssen 1973; Koutchmy 1977; Loucif
& Koutchmy 1989; Koutchmy et al. 1992; Koutchmy &
Nikoghossian 2002; Woo 2005). Well-known examples of linear
structures include polar plumes and the long stalks above the
cusps of helmet streamers. In the outer corona beyond 2–3 R
from Sun center, the entire streamer belt can sometimes be resolved
into a collection of fine rays (see, e.g., Koutchmy et al. 1994, their
Fig. 1; Guhathakurta & Fisher 1995, their Fig. 1; Wang et al. 2000,
their Fig. 5). At greater distances, the existence of subarcsecond
filamentary structure within the interplanetary extensions of streamers has been inferred from Doppler scintillation measurements
( Woo et al. 1995; Woo 2006).
While it is natural to suppose that the linear white-light features trace out ‘‘open’’ magnetic field lines that extend from the
solar surface into the interplanetary medium, it is evident that
only a small fraction of the Sun’s open flux can be in the form of
bright rays; otherwise, they would fill the entire heliospheric volume, including interplume regions. In this paper, we discuss the
nature of the raylike structures seen during the 2006 March 29
eclipse. The photographic images are compared with potentialfield source-surface ( PFSS) extrapolations and with data from
the Extreme-Ultraviolet Imaging Telescope (EIT; Delaboudinière
et al. 1995) and the Large Angle and Spectrometric Coronagraph
( LASCO; Brueckner et al. 1995) on SOHO. These comparisons
allow us to identify three kinds of coronal rays, while at the same
time suggesting a common physical origin for them.
The total solar eclipse of 2006 March 29 occurred during the
late declining phase of solar cycle 23. At the time of the event,
the polar coronal holes were well developed, but sunspot activity
was still present at low latitudes.
Figure 1 shows an image of the white-light corona taken at
10:40 UT in Egypt by the Institut d’Astrophysique de Paris (IAP)
team; a radial neutral filter was used to offset the steep falloff of
the intensity with height. ( Here and in subsequent figures, solar
north is at top and solar east is to the left.) Although the mean
sunspot number for 2006 March was as low as RI ¼ 10:6, multiple streamers are observed over a wide range of latitudes at both
limbs. Long, relatively faint plumes fan out from both poles, with
diffuse background /foreground streamer material present in the
line of sight near their bases. An H prominence with surrounding cavity is centered under each of the two helmet streamers in
the northeast quadrant, while a long chain of small prominences
protrudes above the southeast limb; as is also evident from the
coronal field line plots of x 3, the corresponding filament channels
are aligned nearly perpendicular to the limb in the north and nearly
parallel to it in the south. A number of H macrospicules are
faintly visible in the south polar region. (Chromospheric features
at the west limb were occulted by the Moon’s disk at the time of
second contact when the photographic exposures were taken.)
Other striking features, to be discussed later, include the narrow dark lane that appears to extend outward from the activity
complex at P:A: 100 (where position angle P.A. is measured
counterclockwise from the north pole) and the ‘‘pseudostreamer’’
with jetlike spike at P:A: 240 .
Figure 2 shows a composite of the IAP eclipse picture and a nearsimultaneous He ii 30.4 nm image recorded by EIT at 10:40 UT.
The polar coronal holes appear as dark areas near the limb, with
the south (north) polar hole being tipped toward (away from) Earth
in March. Note the positional correspondence between the helium
and H prominences at the east limb, as well as between the helium and H macrospicules above the south polar hole (cf.
Georgakilas et al. 1999; Wang 1998a).
Figure 3 displays a white-light image of the March 29 eclipse
taken in Libya by M. Druckmüller and P. Aniol and further processed by S. Koutchmy using standard edge-enhancement software.
1
Code 7672, Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352; [email protected], [email protected]
.mil.
2
Harvard College, Cambridge, MA 02138; [email protected].
3
Institut d’Astrophysique de Paris, CNRS / UPMC, 75014 Paris, France;
[email protected], [email protected].
4
Faculty of Mechanical Engineering, Brno University of Technology, 61669
Brno, Czech Republic; [email protected].
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ORIGIN OF CORONAL RAYS
Fig. 1.—White-light corona at 10:40 UTon 2006 March 29, as recorded in Egypt
by the IAP team. The picture is a superposition of many short-exposure frames taken
using a 6 megapixel CCD camera with radial neutral filter attached to a telescope of
880 mm focal length. (Throughout this paper, images are oriented with solar north at
the top and solar east on the left.) The pink H chromosphere protrudes above the
east limb as a series of prominences and (in the south polar region) faint macrospicules. Coronal features of particular interest include (1) the long plumes and
dark interplume lanes projecting above the north and south poles; (2) the more diffuse, curved structures near their bases which appear to represent closed streamer
loops rooted just outside the polar coronal holes; (3) the prominence-enclosing
cavities underlying the pair of helmet streamers at the northeast limb; (4) the narrow
dark lane that overlies the bright coronal condensation (active region complex) at
the east limb; and (5) the low-lying, jetlike ‘‘pseudostreamers’’ at the southeast and
southwest limbs.
The whole corona appears to be permeated by fine raylike structures. As indicated by Figure 4, where this eclipse picture is cropped
and joined to a LASCO C2 unsharp-mask image with field of view
2.2–6 R , some of the streamer rays can be tracked out to great
distances. Also noteworthy is the extremely elongated appearance
of the helmet streamer at the northeast limb, which LASCO
running-difference images show to consist of a succession of
outward-moving loops.
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Fig. 3.—Edge-enhanced version of an image of the March 29 eclipse taken by
M. Druckmüller and P. Aniol in Libya. The picture represents a superposition of
51 short-exposure frames recorded with a 16 megapixel camera and a telescope
of 400 mm focal length. The entire white-light corona appears to be permeated
by narrow raylike features.
solar rotation). In the PFSS model, the corona is assumed to remain current-free (: < B ¼ 0) out to a spherical ‘‘source surface’’
r ¼ Rss , where the nonradial field components are set to zero, simulating the magnetohydrodynamic effect of the solar wind (Schatten
et al. 1969). At the inner boundary r ¼ R, the radial component Br
of the potential field is matched to the (nonpotential) photospheric
field, which is taken to be radially oriented at the depth where it is
measured (Wang & Sheeley 1992). All field lines that extend from
the photosphere to the source surface are defined to be ‘‘open’’; their
footpoint areas generally coincide with He i 1083.0 nm and soft
3. DERIVING THE CORONAL MAGNETIC FIELD
Let r denote heliocentric radius, L heliographic latitude, and
Carrington longitude (measured westward in the direction of the
Fig. 2.—Composite of the IAP eclipse picture and a near-simultaneous He ii
30.4 nm image recorded at 10:40 UT with EIT. The two images have been cropped
so that they overlap in the region 1.025 R < r < 1:1 R . The large darkish area
representing the south polar hole is tilted toward Earth at the time of the event, while
its northern counterpart is tilted away. Note the correspondence between the helium
and H features, including the tall macrospicule at the south pole and the long highlatitude filament that threads through the helmet streamer at the northeast limb.
Fig. 4.—Edge-enhanced Druckmüller-Aniol eclipse picture cropped at r ¼
2:2 R and joined to a LASCO C2 image recorded at 10:46 UT. An unsharp mask
has been applied to the LASCO white-light image by subtracting from it a smoothed
version of itself. Some of the streamer rays can be tracked outward to r 6 R .
Note also the extremely distended appearance of the helmet streamer at the northeast limb.
Fig. 5.—Latitude-longitude maps for CR 2041 (starting date 2006 March 14). Vertical dashed lines mark the locations of the east and west limbs on March 29 (central
meridian was at ¼ 164 ). (a) MWO photospheric field. Gray scale ranges from Br < 20 G (black) to Br > þ20 G (white). (b) Fe xv 28.4 nm intensities recorded
by EIT. Black (white) areas represent coronal holes (active regions). (c) Derived open field regions, color coded to indicate their flux-tube expansion factors or associated wind speeds v at 1 AU (see Wang et al. 1997a). Blue: v < 450 km s1; green: 450 km s1 < v < 550 km s1; yellow: 550 km s1 < v < 650 km s1;
white: 650 km s1 < v < 750 km s1; red: v > 750 km s1. Dark (light) gray background denotes Br < 0 (Br > 0). (d) Source-surface field at r ¼ Rss ¼ 2:3 R .
Black: Br (Rss ; L; ) < 0:15 G; dark gray: 0:15 G < Br (Rss ; L; ) < 0 G; light gray: 0 G < Br (Rss ; L; ) < þ0:15 G; white: Br (Rss ; L; ) > þ0:15 G.
ORIGIN OF CORONAL RAYS
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Fig. 6.—Coronal field line configuration superposed on the edge-enhanced eclipse image of Fig. 3. The coronal field was derived by applying a PFSS extrapolation with
Rss ¼ 2:3 R to the MWO photospheric map for CR 2041. Open field lines are coded blue (green) if directed outward (inward); closed field lines are orange if they extend
beyond r ¼ 1:5 R , red otherwise. Black, dark gray, light gray, and white denote areas of the photosphere where Br < 10 G, 10 G < Br < 0 G, 0 G < Br < þ10 G, and
Br > þ10 G, respectively.
X-ray coronal holes (Levine 1982; Wang et al. 1996; Neugebauer
et al. 1998, 2002; Liewer et al. 2004). In this study, we optimize our
fit to the streamer structures by setting Rss ¼ 2:3 R , slightly less
than the value of 2.5 R adopted in Wang et al. (1996). As demonstrated in Neugebauer et al. (1998), the results obtained with the
PFSS model agree well with numerical MHD calculations such as
those of Mikić et al. (1999) in the region R P r P Rss . However,
the model breaks down beyond r Rss , where nonradial flows
driven by transverse pressure gradients continue to redistribute
the open flux until it becomes independent of latitude and longitude,
with a current sheet forming where B suddenly reverses its direction. Also omitted from the model are the surface currents required
to maintain pressure balance at the boundaries between high- and
low-density regions (or flux tubes).
For the photospheric field, we employ the synoptic map for
Carrington rotation (CR) 2041 (starting date 2006 March 14) provided by the Mount Wilson Observatory (MWO). The magnetograph measurements were corrected for the saturation of the Fe i
525.0 nm line profile by multiplying the field strengths by the
latitude-dependent factor (4:5 2:5 sin2 L) (see Wang & Sheeley
1995a; Arge et al. 2002; Ulrich et al. 2002).
In Figure 5, we display the distributions of photospheric magnetic flux, Fe xv 28.4 nm intensities, open field regions, and
source-surface field for CR 2041. ( In the EIT synoptic observations, coronal holes appear as dark areas; no He i 1083.0 nm map
was available for this period.) The locations of the east and west
limbs on March 29 are marked by vertical dashed lines. The photospheric field is dominated by a small chain of new active regions
centered below the equator near 90. Just behind the east limb,
an equatorward extension of the south polar hole links up with the
eastern edge of this activity complex. Another highly sheared hole
lies embedded within the active region remnants at the west limb.
Both of these narrow low-latitude holes have positive polarity,
matching that of the south polar hole. The PFSS-derived open
field regions, which are color coded to indicate the rate of fluxtube divergence or associated solar wind speed (see Wang et al.
1997a), show reasonable agreement with the Fe xv coronal holes.
The source-surface field is characterized by a four-sector polarity
structure at the equator, indicating that the large-scale field has a
significant nonaxisymmetric quadrupole component (l ¼ 2, jmj ¼
2). The source-surface neutral line, which separates open flux of
opposite polarity and coincides with the inner edge of the heliospheric current sheet (HCS), reaches its maximum northward excursions at 70 and 260 , corresponding roughly to the
locations of the east and west limbs on March 29. Thus, at the
time of the eclipse, the HCS is oriented perpendicular to the sky
plane and viewed edge-on at both limbs near latitude L þ30 .
These northward excursions of the HCS and coronal streamer belt
are clearly associated with the coronal holes at the two limbs,
whose positive-polarity open flux fans outward from near the equator to encounter the negative-polarity flux from the north polar
hole.
Figure 6 shows the calculated coronal field line configuration
for March 29, superposed on the edge-enhanced eclipse image of
Figure 3. Here, open field lines are coded blue if Br > 0 and green
if Br < 0; closed loops are orange if they extend above r ¼
1:5 R , red otherwise. Because of projection effects and the tendency for the open flux to diverge rapidly with height, the figure
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Vol. 660
Fig. 7.—Coronal field lines superposed on the IAP eclipse image of Fig. 1. In this case, only those open field lines that are rooted next to closed loops are plotted. We
propose that the white-light rays are formed by interchange reconnection along such interfaces, with the reconnection taking place near the tops of the loops. ( Because
the resolution of the MWO photospheric map is insufficient to show small ephemeral regions, the plot does not indicate the locations of coronal plumes.)
gives the impression that open field lines are ubiquitous and even
permeate closed field regions. As is apparent from Figure 5c,
however, the footpoints of these field lines are confined to the
limited areas of the photosphere that correspond to coronal holes.
In Figure 7, the coronal field is shown superposed on the IAP
eclipse image. Here, to highlight the calculated streamers and
their immediate surroundings, we have plotted only field lines
that are located within 45 of either limb, and only those open
field lines that are rooted next to closed loops. On the whole, a
reasonable correspondence is seen between the computed and
observed large-scale structures. However, an obvious discrepancy
occurs at the northeast limb, where the observations show two
separate helmet streamers, but the PFSS model yields a single
broad structure. The nonpotential nature of the north-northeast
streamer in the eclipse image is evident from its extremely distended shape. Indeed, LASCO running-difference images show
both streamers at the northeast limb undergoing continual outward
expansion before, during, and after the day of the eclipse, suggesting a gradual opening-up of the underlying loop arcades (cf.
Sheeley & Wang 2007; Wang & Sheeley 2006). A large streamer
blowout with cylindrical flux rope topology was also observed at
the northwest limb on March 27–28.
Z. Mikić, J. A. Linker, and their colleagues at Science Applications International Corporation used magnetograph data and a
numerical MHD model to predict the appearance of the corona
during the March 29 eclipse.5 The field line configuration that
they derived is similar to that obtained here, except that the closed
loops tend to be narrower and more pointed in the MHD simulation due to the outward-stretching effect of the gas pressure. As expected, their steady-state computations do not reproduce the greatly
distended shape of the helmet streamer observed at the northeast
limb.
An inspection of Figures 6 and 7 suggests that the linear features seen in white light represent open field lines along which
the electron density is relatively enhanced. As we discuss in the
following sections, the bulk of these raylike structures fall into
three categories, depending on whether the associated open field
lines are located at the boundaries between opposite-polarity coronal holes (and hence alongside helmet streamers), at the interfaces between like-polarity holes, or within the hole interiors.
4. RAYS ASSOCIATED WITH HELMET STREAMERS
To a first approximation, the white-light streamer belt beyond
r 2:5 R can be represented by a narrow plasma sheet centered
on the HCS (Wang et al. 2000; Liewer et al. 2001; Saez et al.
2005; Thernisien & Howard 2006). The brightest streamer structures (‘‘stalks’’) occur where this warped, tilted sheet is viewed
5
See http://shadow.adnc.net /corona /mar06eclipse.
No. 1, 2007
ORIGIN OF CORONAL RAYS
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Fig. 8.—Field line configuration of the helmet streamer at the northwest limb (a) as seen from Earth and (b) rotated by 90 in longitude (so as to appear at central
meridian). For color key, see Fig. 6 legend. The PFSS model somewhat overestimates the equatorward extent of the observed streamer.
edge-on in the plane of the sky. In high-resolution images, the
outer streamer belt (and thus the plasma sheet) appears as a multitude of fine rays, along which small density inhomogeneities accelerate outward at speeds characteristic of the slow solar wind
(Sheeley et al. 1997). Running-difference sequences of LASCO
C2 images show such ‘‘blobs’’ being emitted continually from
the cusps of helmet streamers at r 3 R . In Wang et al. (1998a),
we suggested that both the blobs and the heliospheric plasma sheet
itself are produced by interchange reconnection between the helmet streamer loops and the open flux from the adjacent coronal
holes (see also Crooker et al. 2004). Footpoint shearing due to
differential rotation or supergranular convection causes the loop
tops to rise, triggering three-dimensional reconnection with the
overlying open flux. As a result, material is transferred from one
leg of a streamer loop to a neighboring open field line, whose outward extension becomes part of the plasma sheet. At the same time,
the lower segment of the open field line closes down by reconnecting with the other leg of the streamer loop.
Figure 8 shows the calculated topology of the northwest helmet streamer and the surrounding open flux. While the process
itself is not described by the PFSS model, we expect reconnection between the open field lines (of either polarity) and the outermost streamer loops to occur near the source-surface neutral line.
A white-light streamer ray forms after a given loop leg becomes
connected to an open field line (see Fig. 9). There is no net change
in the number of open or closed field lines, which are simply undergoing footpoint exchanges with each other (cf. Nash et al. 1988;
Fisk & Schwadron 2001; Crooker et al. 2002; Wang & Sheeley
1993, 2004; Lionello et al. 2005; Mackay & van Ballegooijen
2006).
A helmet streamer is here defined as a large-scale structure separating open flux (coronal holes) of opposite polarity; the three
flux systems (two open and one closed ) intersect at the sourcesurface neutral line, forming a Y-type configuration (see Sturrock
& Smith 1968). When the source-surface neutral line or the axis of
the underlying loop arcade is oriented more or less perpendicular
to the sky plane, we see the familiar dome-and-spike structure characterizing the streamers at the northwest and northeast limbs.
Fig. 9.—Formation of a streamer ray. A helmet streamer loop reconnects with a
neighboring (but noncoplanar) open field line of either polarity, effectively undergoing an exchange of footpoints with it. The reconnection takes place in the vicinity
of the helmet streamer cusp and injects material from the closed field region into the
heliospheric plasma sheet. A similar interchange process may give rise to pseudostreamer rays.
Fig. 10.—Field line configuration of the ‘‘face-on’’ helmet streamer at the east limb (a) as seen from Earth and (b) rotated by 90 in longitude. The underlying streamer
arcade (which in this case encloses three photospheric neutral lines and their associated active-region loop systems) is aligned almost parallel to the limb, rather than
perpendicular to it as in Fig. 8.
Fig. 11.—Field line topology of pseudostreamers, which separate coronal holes of the same polarity. Pseudostreamer at the southwest limb (a) as seen from Earth and (b)
rotated by 90 in longitude. Pseudostreamer at the southeast limb (c) as seen from Earth and (d ) rotated by 90 in longitude. ( In the eclipse images, both of these structures are
observed against a background of nearly face-on helmet streamers.) Note that the cusp ( X-point) of a pseudostreamer, unlike that of a helmet streamer, is located below the source
surface.
ORIGIN OF CORONAL RAYS
Fig. 12.—Composite of the IAP eclipse picture and an EIT Fe ix 17.1 nm image recorded 20 minutes later at 11:00 UT. The two images have been cropped
and joined at r ¼ 1:125 R , with no overlap. The smooth transition between the
EUVand white-light polar plumes indicates that they are indeed part of the same
structure. A small equatorial coronal hole is visible at the west limb (cf. Figs. 5b
and 5c). The narrow dark lane overlying the activity complex at the east limb is
also present in the uncropped 17.1 nm image (not displayed here).
When the source-surface neutral line or the axis of the helmet
arcade is nearly parallel to the sky plane, the streamer appears
as a fan-shaped structure, like those observed near the equator
at both the east and west limbs. In the case of the ‘‘face-on’’ east
limb streamer, the central core is composed of active regions and
is bounded by opposite-polarity holes located behind and in front
of the limb (Fig. 10). The narrow dark lane that threads this
streamer in the eclipse images may be interpreted as a gap between
the bright streamer rays, analogous to the interplume lanes over
the poles. Such gaps are also frequently observed in Fe ix 17.1 nm
images, where they appear as dark interstices between the spiky or
sheetlike structures that represent the legs of streamer loops and
rays.
In the PFSS model, the closed portion of a helmet streamer
must extend out to the source surface; if the streamer cusp were
located at r < Rss , a current sheet would necessarily be present
between the cusp and the source surface, violating the : < B ¼
0 condition.
5. RAYS ASSOCIATED WITH PSEUDOSTREAMERS
At both the southeast and southwest limbs, the March 29 eclipse
images show structures that resemble low-lying, edge-on helmet
streamers. However, as may be seen from Figure 11, these domeand-spike features separate open field lines of the same polarity;
we will therefore call them ‘‘pseudostreamers.’’ In this type of
configuration, a pair of like-polarity holes (or two parts of a single extended hole, as is the case at the southeast limb) are divided
by a photospheric region of the opposite polarity; thus, two neutral lines are present under the streamer, and the closed field region
consists of a pair of loop arcades.
The cusp of a pseudostreamer corresponds to an X-type neutral point, where the two open and two closed flux systems come
into mutual contact. In this case, two types of interchange reconnection may occur. First, analogous to the process that gives rise
to helmet streamer rays (Fig. 9), a closed loop may undergo threedimensional reconnection with an open field line rooted next to it.
Second, the closed loop may reconnect with an open field line
rooted in the other coronal hole, resulting in the transfer of open
and closed flux in opposite directions across the X-point. The
latter process is similar to that which triggers jets when bipoles
emerge inside coronal holes (see Shibata et al. 1992; Wang et al.
889
2006); if it occurs in a quasisteady manner, such interchange reconnection may also provide the main heating source in polar
plumes (x 6). Koutchmy et al. (1994) have suggested that coronal
streamers in general are characterized by a magnetic quadrupole
topology, with the fine rays representing jets. However, we note
that the raylike features described here do not exhibit the impulsive characteristics of LASCO white-light jets, whose leading
edges are observed to propagate outward at speeds ranging from
400 to over 1000 km s1 ( Wang et al. 1998b; Wang & Sheeley
2002).
When the axis of the underlying double arcade is oriented parallel rather than perpendicular to the sky plane, the pseudostreamer
appears fan- rather than spike-like. A spectacular combination of a
face-on and edge-on pseudostreamer may be seen at the east limb
during the eclipse of 2001 June 21.6 This large structure consists of
a low-latitude activity complex bounded by three negative-polarity
coronal holes, one located in front of the limb, the other two situated just behind the limb on opposite sides of the equator. This
configuration gives rise to a fan-shaped structure pierced by a long,
narrow spike that extends radially outward from a point above the
east limb.
Empirical studies have shown that the rate of flux-tube expansion in the corona is inversely correlated with the solar wind speed
at 1 AU (Levine et al. 1977; Wang & Sheeley 1990; Arge & Pizzo
2000). The open flux near the boundary between coronal holes of
opposite polarity diverges rapidly with height, since Br ! 0 at the
source-surface neutral line; thus, the helmet streamer rays and
their plasma sheet extension are associated with very slow wind.
In contrast, the flux tubes at the boundary between holes of the
same polarity are characterized by low expansion factors, since
the outward-fanning field lines from the two holes ‘‘collide’’ with
each other and are diverted into a more radial direction (see
Sheeley & Wang 1991). Accordingly, pseudostreamers should be
sources of high-speed wind (cf. Fig. 5c).
6. CORONAL PLUMES
Figure 12 shows a composite of the IAP eclipse picture and an
EIT Fe ix image recorded 20 minutes later. It is apparent that the
white-light plumes above the polar regions coincide positionally
with the diffuse EUV structures inside the polar holes (see also
DeForest et al. 1997, their Fig. 10). The EUV plumes are more
prominent in Fe ix 17.1 nm, with a peak temperature of T 1:3 ; 106 K, than in higher temperature emission lines such as
Fe xv 28.4 nm. The ‘‘plume haze’’ occurs where unipolar flux
concentrations inside coronal holes are in contact with minoritypolarity flux, in the form of decaying ephemeral regions / EUV
bright points (see Wang et al. 1997b). In Figure 12, a bright point
is clearly visible under the EUV/white-light plume at the eastern
edge of the north polar hole (see also the corresponding He ii
30.4 nm image of Fig. 2).
The elevated densities in plumes, estimated to be 2–3 times
higher than the interplume background (Ahmad & Withbroe 1977;
Wilhelm et al. 1998; Cranmer et al. 1999), require strong heating to
be present near their bases (Wang 1994, 1998b). This base heating
gives rise to a local temperature maximum (with T 1 ; 106 K)
in the region r P1:05 R ; at greater heights, however, the dense
plume gas is cooler than the interplume material because of its
much larger radiative losses. The tendency for plumes to overlie
EUV bright points suggests that interchange reconnection between
small bipoles and the surrounding open flux is the source of the
6
See http://www.zam.fme.vutbr.cz /druck / Eclipse.
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Vol. 660
Fig. 13.—Field line configuration of a polar plume (a) as seen from Earth and (b) tilted to show the underlying mixed-polarity flux. In this example, the small pockets of
positive-polarity field near the north pole may represent noise, since the MWO photospheric synoptic map does not have sufficient spatial resolution and sensitivity to show
ephemeral regions close to the limb.
enhanced heating. In effect, coronal plumes represent small-scale
pseudostreamers (see Fig. 13).
While plumes are certainly present in nonpolar holes (Wang
& Sheeley 1995b; Del Zanna & Bromage 1999), they are difficult
to identify at low latitudes because of the bright active region and
streamer background. The linear structures seen outside the polar
holes in the March 29 eclipse pictures are a mixture of helmet
streamer rays, pseudostreamer rays, and plumes from the holes
located near the east and west limbs.
In addition to the polar plumes, a background of diffuse, curved
white-light features can be seen above the polar caps. We note in
particular the feather-shaped structure at P:A: 25 , which is
clearly rooted in the region of enhanced Fe ix 17.1 nm emission
just outside the north polar hole (Fig. 12) and which must therefore represent a collection of closed loops. The similar morphological appearance of the other low-lying structures above the
polar holes suggests that they are likewise foreground/background
streamer loops, even though the PFSS extrapolations (Figs. 6 and 7)
show little closed flux in the line of sight above the north pole. A
possible reason for this discrepancy is that the streamer loops bordering the north polar hole, like the elongated helmet streamer at
the northeast limb, are undergoing outward expansion and thus in
a highly nonpotential state.
7. SUMMARY AND DISCUSSION
In this study, we have proposed that the linear structures pervading the white-light corona are produced by reconnection between open field lines and adjacent closed loops. (We have ignored
the contribution of CME events, in the course of which such structures may be formed by the opening-up of loops without interchange reconnection.) Based on images taken during the 2006
March 29 eclipse, we have identified three types of raylike features, all of which originate inside or at the boundaries of coronal
holes:
1. Plumes are rooted inside polar and nonpolar holes, at locations where small bipoles (in the form of ephemeral regions or
EUV bright points) are in contact with unipolar flux concentrations.
They consist of open flux of the dominant polarity but contribute
only a minor fraction of the solar wind, most of which comes
from the dark interplume regions. X-point reconnection with the
small underlying bipoles provides the strong base heating that
maintains the high plume densities. The characteristic 1 day
No. 1, 2007
ORIGIN OF CORONAL RAYS
lifetime of a plume is determined by the decay timescale of an
ephemeral region in the supergranular flow field.
2. Pseudostreamer rays occur at the boundaries between holes
of the same polarity. The open field lines overlie a photospheric
region of the opposite polarity bounded by a pair of loop arcades—
the same basic quadrupole topology that gives rise to coronal
plumes. However, the closed field region now extends to greater
heights, with the X point located well above the photosphere but
below the source surface; a pseudostreamer is thus analogous to
a large-scale plume. When the underlying arcades have their axis
oriented perpendicular to the sky plane, the rays line up to form a
bright, narrow stalk, just as in a helmet streamer. Pseudostreamer
rays provide a very small contribution to the fast solar wind.
3. Helmet streamer rays occur at the boundaries between holes
of opposite polarity and extend outward to form the heliospheric
plasma sheet, which surrounds the HCS. They result from footpoint exchanges between the highest streamer loops and the adjacent open flux; the reconnection takes place near the source-surface
neutral line and transfers plasma from closed to open field lines.
Unlike plumes and pseudostreamer rays, helmet streamer rays form
a continuous sheet encircling the Sun. They are associated with
minimum wind speeds and with very low He/H ratios (Borrini
et al. 1981); the latter property has been attributed to a reduction
of the Coulomb drag exerted by protons on particles in the region of rapid flux-tube expansion near the Sun (Bürgi 1992).
We interpret the dark linear features or narrow lanes often seen
in white-light and EUV coronal images as low-density regions between the bright streamer/pseudostreamer rays or plumes, analogous to the familiar interplume lanes above the polar holes. Such
gaps reflect the highly nonuniform nature of magnetic reconnection
and coronal heating.
An inspection of Figure 4 suggests that coronal plumes and
pseudostreamer rays fade more rapidly at great heights than helmet streamer rays. (Compare, for example, the narrow stalk of the
pseudostreamer at the southwest limb with that of the helmet
streamer at the northwest limb.) A possible explanation for this
behavior is as follows. Along a given flux tube, the density (and
hence the Thomson-scattered intensity) varies as
n / B=u;
ð1Þ
where u is the flow speed, and time-dependent effects are ignored.
Because it omits the isotropizing effect of the HCS (or, equivalently, of transverse pressure gradients in the solar wind), the PFSS
model does not correctly describe the variation of B beyond r Rss . In particular, the flux tubes in the vicinity of the HCS, having
undergone rapid expansion near the Sun, subsequently ‘‘reconverge’’ in solid angle; conversely, the more gradually expanding
flux tubes located far from the HCS continue to diverge superradially beyond r Rss and end up with the largest net expansion
factors (see Fig. 4 in Wang & Sheeley 1990). Thus, the field
891
strength B beyond the cusp of a helmet streamer declines less
rapidly than that along the outer coronal extension of a pseudostreamer or plume. At the same time, the velocity u increases less
steeply with r in the slow than in the fast solar wind, with the sonic
point being located respectively at r 5 R and r 2:5 R . According to equation (1), both of these effects will tend to make the
helmet streamer rays dominate the brightness of the outer corona.
Large, closed loops in the quiet corona and in decaying active
regions typically show a strong enrichment in elements of low
first-ionization potential (FIP) relative to their photospheric abundances (Sheeley 1995, 1996; Feldman 1998). Accordingly, we
might expect both helmet streamer and pseudostreamer rays, which
are formed from such coronal loops, to be enriched in low-FIP
elements such as magnesium. While high-speed wind is normally
characterized by photospheric abundances and slow wind by lowFIP enrichment (Geiss et al. 1995; von Steiger et al. 2000), Widing
& Feldman (1992) found that the Mg/Ne ratio was enhanced by a
factor of order 10 in a polar plume (cf. Wilhelm & Bodmer 1998,
who derived an enhancement of 2–3; and Del Zanna et al. 2003,
who found no significant FIP effect in plumes). Wang (1996) suggested that the Widing & Feldman result could be explained by
chromospheric evaporation following interchange reconnection
near the base of the plume.
It should be emphasized that the white-light rays discussed here
constitute only a small fraction of the Sun’s open flux, the bulk
of which threads the dark interplume regions of coronal holes.
In particular, most of the slow solar wind originates not by interchange reconnection with helmet streamer loops, but directly
from the rapidly diverging, open flux tubes inside the hole boundaries. The rate of flux-tube expansion determines the height at
which coronal heating takes place. If the field strength falls off rapidly with height, the heating occurs close to the coronal base,
driving a large mass flux but reducing the energy per proton, and
thus the asymptotic wind speed. Conversely, if the field diverges
slowly, the heating extends to greater heights (r 2 R ) and more
of the deposited energy goes into accelerating the wind plasma to
high speeds, rather than into overcoming the gravitational potential
near the solar surface and increasing the mass flux. In the case of
plumes, two sources of heating are required, one localized very
near the coronal base (and presumably associated with magnetic reconnection), the other extending outward toward the sonic
point.
We are indebted to N. B. Rich for constructing the Fe xv
synoptic map and to the SOHO LASCO and EIT teams for the
use of their observations; we also thank R. K. Ulrich for providing the MWO photospheric field data. J. B. B. was in the NRL
summer student program. This work was supported by NASA
and the Office of Naval Research.
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