1. INTRODUCTION 2. THE DATA

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1 THE ASTROPHYSICAL JOURNAL, 513:961È968, 1999 March 10 ( The American Astronomical Society. All rights reserved. Printed in U.S.A. EXTENSION OF THE POLAR CORONAL HOLE BOUNDARY INTO INTERPLANETARY SPACE RICHARD WOO Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS , Pasadena, CA SHADIA RIFAI HABBAL Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA AND RUSSELL A. HOWARD AND CLARENCE M. KORENDYKE Hulburt Center for Space Research, Code 7660, Naval Research Laboratory, Washington, DC Received 1998 May 18; accepted 1998 October 13 ABSTRACT White-light measurements made by the SOHO LASCO C2 and C3 coronagraphs and the Mk III Mauna Loa K-coronameter, ranging from 1.15 to 30 R, have been combined with Kitt Peak daily He I 1083 nm coronal hole maps, and full Sun Y ohkoh soft _ X-ray images, to show that the boundaries of polar coronal holes, as determined by measurements of path-integrated density, extend approximately radially into interplanetary space. These results are in contrast to the long-standing view that the boundaries of polar coronal holes diverge signiðcantly beyond radial, evolving around the edges of streamers. The combined observations also show that the corona is dominated by raylike structures as small as a few degrees in angular size with respect to Sun center, originating from both coronal holes and the quiet Sun. This analysis provides further support for results originally derived from radio occultation measurements, namely, that the coronal density projects itself almost radially from the Sun into the outer corona, implying that open Ðeld lines abound in the quiet Sun from which the fast wind can also originate. Subject heading: Sun: corona 1. INTRODUCTION Knowledge of the distribution of electron density in the solar corona is not only fundamental for understanding the structure of the corona itself (e.g., Saito 1965; Newkirk 1967; Koutchmy 1977); it also provides important clues for the origin and evolution of the solar wind (Munro & Jackson 1977; Lallement, Holzer, & Munro 1986; Woo & Habbal 1997). The primary sources of information on electron density in the solar corona have been white-light (e.g., van de Hulst 1958; Koutchmy 1977; Guhathakurta & Fisher 1995) and radio occultation measurements (e.g., Bird & Edenhofer 1990). The former are made during total solar eclipses and with man-made white-light coronagraphs, while the latter are made when spacecraft radio signals or natural radio sources are occulted by the solar corona. The radio measurements of ranging and Doppler observe pathintegrated electron density and changes in path-integrated density, respectively. Although ranging and white-light both measure path-integrated density, their weighting functions are slightly di erent. By probing the coronal density as a function of latitude starting from the pole, ranging measurements of the outer corona near 20 R showed evidence for a slight increase in density at approximately _ 30 from the polar axis (Woo & Habbal 1997). When compared with the corresponding Mk III white-light coronagraph image of the Sun, this increase was found to occur where the line of sight intercepted the radial extension into the corona of the polar coronal hole boundary at the solar surface. In addition, the observed Doppler scintillation (reñecting density Ñuctuations) was found to remain low, and hence to still resemble that of a fast wind beyond this boundary. Woo & Habbal (1997) concluded that the boundary of the polar coronal hole extends almost radially from the solar surface, and that the 961 fast wind originates also from the quiet Sun (i.e., the solar surface other than coronal holes and active regions). Another result revealed by radio occultation measurements made at 20È30 R has been the predominance of _ raylike structures (Woo & Habbal 1997), also reported in eclipse observations closer to the Sun in the intermediate corona (see review by Koutchmy & Bocchialini 1998). Typically a few degrees in angular size with respect to Sun center, the brightest of these rays are the streamer stalks which occupy only a small volume in interplanetary space (Woo et al. 1995a, 1995b; Woo & Habbal 1997). The remaining raylike structuresèoften referred to as plumes or polar rays in polar coronal holesèare not limited to polar regions, but also originate from low-latitude coronal holes (Woo 1996b) as well as from within the boundaries of coronal streamers as seen projected o the limb in the plane of the sky (Woo & Habbal 1997). So far, these novel results were derived from one set of radio occultation measurements. The goal of this study is to take advantage of the unprecedented capabilities of the Large Angle Spectrograph Coronagraph (LASCO) on board the Solar Heliospheric Observatory (SOHO) (Brueckner et al. 1995), together with a complement of space- and ground-based observations, to explore the extension of the boundaries of polar coronal holes from the solar surface into interplanetary space, and the origin of the pervading raylike structures in the corona between 1 and 30 R. _ 2. THE DATA Examples of the combination of measurements used in this study are displayed in Figures 1È3. These consist of Kitt Peak coronal hole maps derived from He I 1083 nm fulldisk spectroheliographs, Y ohkoh soft X-ray full disk images

2 FIG. 1.ÈObservations on 1997 August 11. (a) Combined images of Y ohkoh soft X-ray and Mk III Mauna Loa K-coronameter pb. The yellow line on the disk traces the coronal hole boundary as deðned by the He I 1083 nm maps of the National Solar Observatory. (b) Combined images of SOHO LASCO C2 intensity and Mauna Loa pb. (c) Image of LASCO C3 intensity. (d) Image of LASCO C3 polarized intensity. (e) Azimuthal pb (top), C2 intensity (middle), and C3 intensity (bottom) proðles as a function of position angle (measured counterclockwise starting from 0 at the north pole). The straight yellow radial lines superimposed on the images (red lines on the proðles) of the corona indicate the radial extension of the polar coronal hole boundaries as deðned by the He I 1083 nm maps. Ledges from near these boundaries, and their low-latitude extent is indicated approximately by the dashed green lines on the proðles in (e).

3 POLAR CORONAL HOLE BOUNDARY 963 FIG. 2.ÈObservations on 1996 August 15, similar to Fig. 1 without a C3 pb image (Tsuneta et al. 1991), Mauna Loa Mk III polarized brightness (pb) measurements (Fisher et al. 1981) of the inner corona out to 2 R, and white-light images obtained with the LASCO C2 and _ C3 coronagraphs (Brueckner et al. 1995) extending from 2 to 30 R. Since the supporting pylon _ of the C2 and C3 external occulters is located in the southeast quadrant of the Ðeld of view (around position angle 130 ), and causes signiðcant vignetting there, we limit this investigation to the northern hemisphere, i.e., position angles 0 È90 and 270 È360 (where position angle is measured counterclockwise starting from 0 at the north pole). Each telescope in the C2 and C3 coronagraphs has its own 1024 ] 1024 pixel CCD camera, which records images with a two-pixel spatial resolution of 23@@ (C2) and 112@@ (C3), respectively, except near the inner edge of the Ðeld of view where vignetting by the external occulter degrades the resolution. For the purpose of this study, a background image was produced from a month of observations. For each day of observation, a given exposure of the total intensity of both the C2 and C3 coronagraphs

4 964 WOO ET AL Vol. 513 FIG. 3.ÈObservations of 1996 September 5, similar to Fig. 1 without a C3 pb image was corrected for cosmic-ray hits. An average image was then made from at least four exposures. To achieve a higher contrast for the faint structures, ratio ÏÏ images were produced by dividing the average by the background (see panels b and c in Figs. 1È3). The azimuthal proðles of the total intensities shown in Figures 1e, 2d, and 3d were computed from the corrected average images obtained by subtracting the background image from the average image. 3. CORONAL HOLE BOUNDARY The characteristic signature of the coronal hole boundary in the radio occultation measurements was manifested as a slight increase in density, or plateau, at a position angle matching the position angle of the coronal hole boundary in the plane of the sky as seen in white light (Woo & Habbal 1997). Starting near the radial extension of the coronal hole

5 No. 2, 1999 POLAR CORONAL HOLE BOUNDARY 965 FIG. 4.È(a) Same as Fig. 3a, except that the yellow lines follow the apparent boundary of the streamer. (b) Same as (a), except the radial yellow lines superimposed on the corona now indicate the radial extension of the coronal hole boundaries. (c) pb proðles from the Mk III Mauna Loa K-coronameter. Note that the pb scale is logarithmic. The red vertical lines indicate the location of the radial extension of the coronal hole boundary on the pb proðles, while the blue lines mark the locations of the coronal hole boundary corresponding to the yellow counter in (a). boundary at the Sun, this plateau extended toward lower latitudes. The increase in density across that boundary was about a factor of 2. In what follows the term ledge ÏÏ is used to refer to this boundary. The three examples given in Figures 1È3 illustrate how these ledges appear in the whitelight data, and how their detection depends on structures forming the boundaries of coronal holes. In the three representative examples of 1997 August 11 (Fig. 1), 1996 August 15 (Fig. 2), and 1996 September 5, the polar coronal holes, as projected on the solar disk and deðned by the He I 1083 nm maps, are traced in yellow on the corresponding soft X-ray images. The yellow straight lines in the corona trace the radial extensions of the He I coronal hole boundaries in the plane of the sky, starting from the boundaries at the limb and extending into the corona. Since the purpose of these lines is to guide the eye, they have been drawn only on those parts of the images where azimuthal variations cannot be discerned. These same boundaries are represented by the red vertical lines on the azimuthal proðles of the white-light intensities (Figs. 1e, 2d,3d). The coronal hole boundary in the outer corona is especially striking in Figure 1d, which shows the C3 image of polarized intensity with its contrast enhanced in order to

6 966 WOO ET AL Vol. 513 highlight the subtle density change across the boundary. In the measurements of 1997 August 11 (Fig. 1), the narrowing of coronal streamers with increasing altitude in the Mk III images is reñected in the pb proðles above 1.15 R as an apparent shift of the coronal hole boundaries toward _ lower latitudes (Fig. 1e). This shift, however, is not evident in the higher altitude proðles of C2 and C3. On the contrary, the coronal hole boundary extends approximately radially with increasing heliocentric distance, and a prominent ledge characterized by an enhanced intensity (as indicated by the region bounded by the red and green dashed lines between 30 and 60 latitude on the northeast limb in Fig. 1e) emerges. The ledge, which is more readily detected in the azimuthal density proðles, is not always characterized by the same density enhancement. For example, the coronal hole boundary may mark the beginning of the steepening streamer, which then masks the ledge, such as on the northwest limb of Figure 1e. In this case, the ledge will eventually appear at larger heliocentric distances as the streamer narrows. Indeed, there are several factors that a ect the detection of the coronal hole boundary. The boundary is especially prominent when the density of the quiet Sun is high, as when active regions and/or coronal streamers are present at high latitudes, such as in the example of Figure 1. The measurements of 1996 August 15 (Fig. 2) illustrate that the coronal hole boundary is also evident when there is a highlatitude streamer but no active region, in which case the ledge is narrow (e.g., northwest limb around 50 È60 latitude), or when only a low-latitude streamer is present (e.g., northeast limb between 30 È70 latitude), resulting in a broad ledge (Fig. 2d). The northeast limb on 1996 September 5 (Fig. 3) is a case in which the radial extension of the coronal hole boundary in the outer corona was not observed. The east limb is relatively free of active regions and high-latitude streamers. The most likely reason for the absence of the boundary is the extension of the north polar coronal hole to low latitudes on the disk, as shown by the Y ohkoh soft X-ray measurements in Figure 3a. The latitudinal extent of the coronal hole boundary is another critical factor that a ects the detection of the signature of the hole boundary. Of the 20 LASCO proðles examined during the period of 1996 August 15 to 1996 September 8, the only four cases for which the coronal hole boundary was not detected involved low-latitude streamers. The coronal hole boundary has generally been thought to diverge signiðcantly with increasing radial distance following the boundaries of the adjacent streamers (Zirker 1977; Munro & Jackson 1977; Gosling et al. 1995). This is indeed the general impression one gets from white-light images of the inner corona as shown in the examples of Figures 1È3. A comparison of the boundary deðned by the streamers with the radial extension found in this study is shown in Figure 4, where yellow lines trace the apparent extension (Fig. 4a) versus the radial extension (Fig. 4b) of the coronal hole boundary. The locations of the corresponding two di erent boundaries are shown as blue and red lines respectively in the pb proðles of the Mk III coronagraph at altitudes 1.15È 1.95 R (Fig. 4c). (Note that the vertical scale is logarithmic in order _ to accommodate the wider range of pb values at the higher altitudes.) Closest to the Sun at 1.15 R, as _ pointed out earlier, pb rises by about a factor of 2 across the coronal hole-streamer boundary (see, for example, Figs. 1e and 2d). However, the highest pb values occur where the streamer narrows quickly with increasing heliocentric distance, and reach a level that is at least a factor of 10 higher than that in the coronal hole (compare, for example, panels for 1.15, 1.35, and 1.95 R in Fig. 4c). Unlike the pb values at the radial extension of _ the boundaries (red lines in Fig. 4c), those of the apparent coronal hole boundaries (at blue lines in Fig. 4c), are close to 1 order of magnitude higher than those in the coronal hole. 4. RAYLIKE STRUCTURES The images and intensity proðles of the C2 and C3 coronagraphs shown in Figures 1È3 demonstrate the predominance of raylike structures in the extended corona as originally detected by radio occultation measurements at 20È30 R. The raylike structures are especially evident near the inner _ portions of the Ðelds of view of the coronagraphs, because of reduced sensitivity and dynamic range at larger heliocentric distances. In the Mk III coronagraph images, the rays appear only when image enhancement is applied. In comparison, the superior sensitivity and dynamic range of radio occultation measurements (ranging and Doppler) enables them to detect faint and low-contrast structures even at large distances from the Sun (Woo 1996a). As found in radio occultation measurements at 20È30 R _ (see Fig. 2 of Woo & Habbal 1997), raylike structures in the images of Figures 1È3 are equally present within and beyond the radial extension of the polar coronal hole boundary. The ones in the brighter corona originate from the quiet Sun and not the polar coronal hole; otherwise the structures within the radial extension of the polar coronal hole boundary would have been brighter. The rooting of raylike structures in both coronal hole and streamer regions near the Sun is also evident in the SOHO/UVCS images of the corona in Lya and O VI (Raymond et al. 1997). While the rays in the LASCO measurements are a few degrees in angular size, much smaller angular sizes were derived from radio occultation measurements (Woo et al. 1995a; Woo 1996c). The angular resolution of 0.5 for C2 and 1.1 for C3 as measured from Sun center, and corresponding to spatial resolutions of 23@@ for C2 at 2 R and _ 112@@ for C3 at 6 R, respectively, are not necessarily the _ limiting factor. The smaller scale structures that make up the larger plumelike structures are not readily evident in the LASCO images, most likely for the same reason that they are not in ranging measurements of path-integrated density (Woo & Habbal 1997). They are masked by the larger plumelike structures because the contrast of the smaller rays is only tenths or a few percent (Woo 1996a) compared with the ^ 10% for the plumelike structures. 5. DISCUSSION AND CONCLUSIONS The complement of observations covering distances from the solar surface to 30 R, in particular the ability of LASCO to image the corona _ beyond 4 R with unprecedented sensitivity and dynamic range, has _ made it possible to investigate the origin of the faint and low contrast structures that Ðll the corona. While streamers, characterized by enhancements in path-integrated density of factors of 10 or more relative to coronal holes, are the brightest features in the solar corona, their dominance in the corona has given a false impression of diverging coronal holes in white-light

7 No. 2, 1999 POLAR CORONAL HOLE BOUNDARY 967 pictures. This study shows that the boundary of the coronal hole, often characterized by a latitudinal increase of about a factor of 2 in path-integrated density from coronal hole to neighboring quiet Sun, extends into interplanetary space approximately radially. The boundary is not always evident. Detection depends on measurement sensitivity, the proximity of the coronal hole boundary to active regions and coronal streamers, and the latitudinal extent of the coronal hole boundaries on the solar disk. The apparent shift in coronal hole boundary in the Mk III proðles is a consequence of the boundary becoming increasingly weaker with heliocentric distance in comparison with the streamer, which remains relatively bright. For the C2 and C3 coronagraphs, detection of this boundary at large heliocentric distances is difficult for the same reason. While the pb values in coronal holes are conspicuously low, the azimuthal proðles of pb there show considerable variation. Enhancements in the pb values can often be traced back to denser structures at the solar surface, originating either from within the coronal hole or from its boundary. Soft X-ray images provide knowledge of the source of this contamination. For instance, the Mk III pb measurements on 1996 August 15 indicate that path-integrated density is not constant across the coronal hole at 1.15 R (see Fig. 2d) _ but has a minimum at 30 east of north. This is consistent with the Y ohkoh soft X-ray image in Figure 2a showing a corona free of foreground and background structures in that vicinity and background structures to the west leading to enhanced density there. In models of the solar wind, the divergence of Ñow tubes within coronal holes is a free parameter. In the work by Wang & Sheeley (1990), this divergence has been assumed to be smallest in the center of the coronal hole and to increase toward the coronal hole boundary. This smooth rate of change of divergence could not give rise to the observed ledge or increase in path-integrated density beyond the radial boundary. Since the ledge appears in path-integration measurements, it could be argued that it represents structures (e.g., streamer stalks) far from the plane of the sky that are viewed in projection. However, the data taken here were taken around solar minimum when no streamers were observed to extend beyond 30 from the equator. Hence, such a projection could not have consistently produced a radially extended ledge, especially at high latitudes where it is most prominent. Also worth noting is that the azimuthal density proðles do not seem to change signiðcantly with increasing heliocentric distance. The northern polar coronal holes of 1996 August 15 (Fig. 2d) and 1996 September 5 (Fig. 3d), and the eastern portion of the northern polar coronal hole of 1997 August 11 (Fig. 1e), provide supporting examples. Thus, the coronal hole boundary does not appear to be an isolated feature of the corona that extends radially, but seems to be an integral part of the general coronal density proðle that is preserved during radial expansion. The almost radial expansion of the coronal density into interplanetary space thus implies that open Ðeld lines extend from the solar surface not only from coronal holes but also from a signiðcant fraction of the solar surface, namely, quiet regions. This is consistent with the coexistence of closed and open Ðeld lines observed in high spatial resolution eclipse pictures (November & Koutchmy 1996). The azimuthal density proðles imply that the pathintegrated density does not vary by more than a factor of 2 between coronal holes and the adjacent quiet regions. The emergence of open Ðeld lines from quiet region latitudes covering 30 È60 coincides with the latitudes where Ulysses has measured fast solar wind. Upon close inspection of the Ulysses in situ measurements, it is clear that there is a slow yet systematic decline in the velocity from higher to lower latitudes, which can be accounted for by the change in coronal density from coronal holes to quiet regions (McComas et al. 1998). Furthermore, Ulysses solar wind composition measurements show that, despite this slight systematic decline in velocity, the composition of the solar wind remains the same (Geiss et al. 1995), indicating that the wind from the quiet Sun and polar coronal hole is similar. Closer to the Sun, latitudinal proðles of solar wind velocity in the corona deduced from the SOHO/UVCS Doppler dimming measurements also indicate that the wind emerging from the quiet Sun is relatively fast (Habbal et al. 1997). That fast wind originates from quiet regions as well as from coronal holes Ðrst became evident from Doppler scintillation measurements, indicating that the density Ñuctuations associated with the ledge were similar to those with the polar coronal hole (Woo & Habbal 1997). Taken together, the results from the radio, ultraviolet, white-light, and in situ plasma measurements therefore strongly support this new view. In conclusion, the LASCO observations of the predominance of raylike structures in the corona, and of the radial extension of the coronal hole boundary, provide additional support for the earlier implications from radio occultation measurements. In contrast with the proðles of streamers which are characteristic of a feature that evolves with increasing radial distance, the proðles of coronal holes appear to be preserved during radial expansion. With the exception of the narrowing of streamers into stalks, the near radial extension of the coronal hole boundaries is apparently only one of the manifestations of the radial projection from the Sun into the outer corona of coronal density itself. These results reinforce the view that fast wind originates from quiet regions as well as from coronal holes. We thank J. W. Armstrong for many useful discussions, C. Copeland for producing the Ðgures, and J. Burkepile and H. Higgins of HAO for generously providing the Mauna Loa K-coronameter (operated by NCAR/HAO) data. The NSO/Kitt Peak He I 1083 nm data used here are produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/ SEL. We thank the Y ohkoh Data Archive Center for the Y ohkoh soft X-ray images. This paper describes research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Support for S. R. Habbal was provided by NASA grant NAG LASCO was constructed and is operated by an international consortium consisting of the Naval Research Laboratory (Washington, DC), the Department of Space Research at the University of Birmingham (Birmingham, United Kingdom), the Max-Planck-Institut fu r Aeronomie (Lindau, Germany) and the Laboratory for Space Astronomy (Marseilles, France). SOHO is a mission of international cooperation between ESA and NASA.

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