The Astrophysical Journal, 612:1171–1174, 2004 September 10 # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.
ROLE OF CLOSED MAGNETIC FIELDS IN SOLAR WIND FLOW Richard Woo Jet Propulsion Laboratory, California Institute of Technology, 1800 Oak Grove Drive, MS 238-725, Pasadena, CA 91109;
[email protected]
Shadia Rifai Habbal University of Wales, Aberystwyth SY23 3BZ, UK; and Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138;
[email protected]
and Uri Feldman ARTEP, Inc., 2922 Excelsior Spring Court, Ellicott City, MD 21042; E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375-5352;
[email protected] Receivved 2003 December 4; accepted 2004 May 18
ABSTRACT In this paper we demonstrate how closed magnetic fields appear to be playing a significant role in solar wind flow. Confinement or trapping of plasma is the physical process, while confinement duration, as characterized by the first ionization potential ( FIP) bias, is the attribute that divides the fast- and slow-wind regions. The trapped plasma is released along ubiquitous and predominantly radial open field lines, presumably by continual reconnection at the base of the corona, with evidence for this process coming from the appearance of the imprint of polar coronal holes, quiet Sun, and active regions in the outer corona and interplanetary space. When trapping is not long enough to enrich the elemental abundance ( FIP bias near 1), the coronal radial density gradient is steep, coronal temperature is low, and a fast wind flows in the overlying corona. However, the presence of closed fields still influences the flow of the fast wind, as revealed by the fact that flow speed is anticorrelated with, or characterized by, the density at the base of the corona. When trapping is long enough to enrich the abundance (FIP bias > 1), a slow wind flows, and FIP bias characterizes its properties. Enhanced FIP bias gives rise to a decreased coronal radial density gradient, as manifested by the extension of coronal streamers in white-light coronal images, increased coronal temperature, and decreased solar wind speed in the overlying corona. Subject headingg s: solar wind — Sun: abundances — Sun: corona — Sun: magnetic fields
of the corona at 1.15 R show that density increases from pole to equator by a factor of 2–3 ( Hansen et al. 1969). Such relative changes are found replicated in the outer corona at a fixed height (on a concentric spherical shell) in the high- but not low-latitude region ( Woo & Habbal 1999a, 1999b). The latitudinal density profile is replicated in the highlatitude region because the radial density gradient (density falloff with radial distance) is uniform or independent of latitude; it is not replicated in the low-latitude region because the radial density gradient there is reduced and nonuniform. For instance, although density is a factor of 2–3 higher in the streamer than in the polar coronal hole at 1.15 R, it is at least a factor of 10 higher in the streamer than in the polar coronal hole in the outer corona at 5.5 R (Woo & Habbal 1997). The extended streamers seen in white-light images of the corona are a manifestation of this significantly reduced density gradient. The radial density gradient, therefore, divides the highand low-latitude regions; it is steepest and uniform in the former and reduced and nonuniform in the latter. Flow speeds deduced from Doppler dimming measurements show that the high-latitude region comprises fast wind, while the low-latitude region includes the slow wind and the transition from slow to fast wind ( Habbal et al. 1997). Simultaneous density measurements also reveal that velocity and density are anticorrelated in the outer corona (Woo & Habbal 2000). Solar wind speed is hence another characteristic that separates the high- and low-latitude regions; it is fast in the former, and slow in the latter.
1. INTRODUCTION There is growing evidence from integrated solar and heliospheric observations that the solar wind expands approximately radially along open magnetic field lines emanating from all over the Sun ( Woo & Habbal 2003). While these observations have improved our understanding of the morphology of the density, velocity, and magnetic field in the solar corona, they have provided little insight into the physical process producing the variations of solar wind speed. In this paper we show how composition measurements at the Sun (Feldman 1998) and in the solar wind (Geiss et al. 1995; von Steiger et al. 2000) fill this gap. We briefly review in x 2 the evidence during solar minimum for a radially expanding solar wind based mainly on density and velocity measurements of the solar corona and interplanetary space. We then combine them with composition measurements in x 3 to illustrate the role that closed fields appear to be playing in solar wind flow and summarize the conclusions in x 4. 2. RADIALLY EXPANDING SOLAR WIND Density measurements of the inner and outer corona have shown that the corona is divided into two distinct latitudinal regions during solar minimum: one at high latitude overlying polar coronal holes and the quiet Sun and referred to as the high-latitude region, and the other at lower latitude overlying the remaining quiet Sun and active regions and termed the low-latitude region. White-light measurements near the base 1171
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Fig. 1.—(a) Latitude-longitude map of the contours of solar wind speed reproduced from Neugebauer et al. (1998) and based on Ulysses and Wind measurements during Carrington rotations (CRs) 1891–1895. Velocity contours are in increments of 50 km s1, varying from 350 to 700 km s1, with velocity increasing from the two ‘‘islands’’ of slowest speed to 700 km s1 at the boundary of the streamer belt. Wind speed is greater than 700 km s1 poleward of the streamer belt. (b) Synoptic map of Yohkoh soft X-ray for CR 1893. The dark polar regions represent the polar coronal holes, while the two bright regions on opposite sides of the Sun are the active regions. (c) Synoptic map of polarized brightness measured by the High Altitude Observatory Mauna Loa Mk III K-coronameter on the east limb at 1.74 R for CR 1893. Polarized brightness contours represent three levels of polarized brightness: 200, 100, and 50 ; 1010 B, with the highest levels of 200 ; 1010 B coinciding with the islands of peak density and decreasing to 50 ; 1010 B near the boundary of the streamer belt. (d) Latitude-longitude map of the contours of the Mg/O abundance ratio reproduced from Neugebauer et al. (1998) and based on Ulysses and Wind measurements during CRs 1891–1895. (e) Superposition of (a) and (b). ( f ) Superposition of (a) and (c). (g) Superposition of (b) and (d). (h) Superposition of (c) and (d).
Reproduced from Neugebauer et al. (1998) in Figure 1a is the latitude-longitude map of solar wind speed contours derived from Ulysses in situ measurements during solar minimum, showing the streamer belt containing the distribution of the slow wind and the transition from slow to fast wind. Note that there are no contours for the fast wind poleward of the streamer belt because this Ulysses map does not distinguish speeds over 700 km s1. The matching of relative density profiles of the inner and outer corona in the high-latitude region implies that the fast wind from the Sun expands radially into interplanetary space. This radial expansion is confirmed when the statistical properties of the longitudinal variations of density in the Ulysses fast wind poleward of the streamer belt are found to be similar to those at 1.15 R (Woo et al. 2000; Habbal & Woo 2001). Note that this observed fact contradicts the notion that the Ulysses fast wind poleward of the streamer belt in Figure 1a originated in and expanded superradially from polar coronal holes.
The low-latitude region corona also extends approximately radially, mapping into the streamer belt of Figure 1a. Shown in Figure 1b is the soft X-ray map of Yohkoh, revealing two active regions that dominated the corona on opposite sides of the Sun during the Ulysses measurements. When the Ulysses velocity and Yohkoh maps are superimposed in Figure 1e, the two ‘‘islands’’ of slowest wind are the unmistakable imprint of active regions in interplanetary space (Woo & Habbal 2003). Shown in Figure 1c is the map of polarized brightness ( pB) contours at 1.74 R, representing the coronal density distribution observed by the High Altitude Observatory Mauna Loa Mk III K-coronameter. Because of the anticorrelation between density and velocity, this pB map also characterizes the velocity distribution in the corona ( Woo & Habbal 2000). The superimposed Ulysses velocity and pB maps in Figure 1g reveal that the velocity distributions of the corona and solar wind are similar.
No. 2, 2004
CLOSED FIELDS AND SOLAR WIND FLOW
Taken together, the appearance of the imprint of active regions in the solar wind, the similarity of coronal and solar wind velocity distributions, and the radial extension of streamer belt boundaries from the Sun ( Woo & Habbal 2000) imply that the low-latitude region also extends radially along ubiquitous open field lines that permeate the inner corona, thus mapping approximately into the streamer belt in interplanetary space. The solar wind, therefore, expands radially from the entire Sun, carrying the imprint of polar coronal holes, quiet Sun, and active regions into interplanetary space and explaining why the directions of magnetic fields deduced from polarization measurements of the inner corona over 3 decades ago were found unexpectedly to be predominantly radial ( Eddy et al. 1973; Arnaud & Newkirk 1987; Habbal et al. 2001). We add composition measurements to this picture in x 3 and discuss their implications. 3. COMPOSITION MEASUREMENTS AND THEIR IMPLICATIONS Elemental abundance is often characterized in terms of the first ionization potential (FIP) bias, which is the factor by which low FIP ( 1), slow wind is produced, and confinement time or FIP bias characterizes its properties. Enhanced FIP bias decreases the coronal radial density gradient, as manifested by the extension of streamers in white-light pictures, increases coronal temperature, and decreases solar wind speed in the overlying corona. By themselves, measurements of elemental abundances and ion charge states by Ulysses could not have arrived at this scenario, since corresponding measurements along open field lines in the inner corona are not available. Instead, the results of this study were obtained by using the observational fact that the process of elemental abundance enrichment at the Sun is associated with plasma confinement time and by synthesizing Ulysses elemental abundance measurements during solar minimum with corresponding density and velocity measurements of both corona and interplanetary space. We do not know how the process of prolonged trapping takes place, but it must depend on the topology and magnetic strength of the closed structures and such factors as the sizes and temperatures of the loops. However, since abundance enrichment
reflects the process, solar wind speed is more directly connected to FIP bias than to the morphology of the closed loops (Schwadron et al. 1999; Gloeckler et al. 2003; Fisk 2003). It is for this same reason that the boundary between fast and slow wind identified by FIP bias measurements does not correspond to any visible feature in the soft X-ray map of Figure 1a. The shift in concept of coronal magnetic field topology and solar wind flow—from a corona divided into either open (coronal holes and fast wind) or closed field (slow wind) regions to a corona permeated by closed fields at its base and divided into regions of short (fast wind) or prolonged (slow wind) trapping—makes it easier to understand the solar cycle dependence of the relationship between solar wind speed and composition. Since closed-field regions capable of trapping plasma during solar minimum are limited to a narrow range of latitudes near the equator, and cover only a small percentage of the surface of the Sun, solar wind speeds and composition observed by Ulysses feature bimodal distributions (Zurbuchen et al. 2002). During solar maximum, polar coronal holes disappear, density at the base of the corona is enhanced, and closed field regions capable of trapping plasma for prolonged periods, such as active regions, spread to higher latitudes and cover a larger percentage of the solar surface. As a result, Ulysses observes a slower fast wind and a continuum in the distribution of solar wind speeds and composition (Zurbuchen et al. 2002). Composition of the steady fast wind during solar maximum is similar to that of the fast wind during solar minimum, not because it emanates from coronal holes ( McComas et al. 2002), but because, like the fast wind from the quiet Sun during solar minimum, it originates from coronal regions incapable of trapping plasma for prolonged periods. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration and funded through the internal Research and Technology Development program.
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