Global distribution of coronal mass outputs and its relation to ... - Wiley

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. A6, 1238, doi:10.1029/2002JA009439, 2003

Global distribution of coronal mass outputs and its relation to solar magnetic field structures Fengsi Wei, Xueshang Feng, Hongchang Cai, and Qingjun Zhou Key Laboratory for Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing, China Received 12 April 2002; revised 11 March 2003; accepted 17 April 2003; published 14 June 2003.

[1] The basic characteristics of the coronal mass output near the Sun are analyzed with the

statistic and numerical methods by using observational data from K corona brightness, interplanetary scintillation, and photospheric magnetic field during the descending phases (1983) and the minimum (1984) of solar activity. The methods used here are based on the global distribution of the solar magnetic field on the source surface (at 2.5 solar radii (RS)). Our main results are the following: (1) There are certain regular persistent patterns in the global distributions of coronal mass outputs flux Fm (density r
speed V), which shows that the highest Fm in 1983 and 1984 display more regularly double peaks and single-peak wave-like patterns on the source surface (2.5 RS), respectively. The highest and the lowest Fm are associated with the coronal current sheet and the polar corona regions, respectively, and the other regions are associated with a moderate Fm. (2) The speed dependence of Fm is different for various magnetic structures. The dependence is nearly constant in the polar coronal region and monotonically rises in the current sheet regions both for the descending (1983) and the ascending (1976) phases. (3) The different frequency number distributions of Fm also correspond to different magnetic field structures,
m,p = 8.3
1011 particles/cm2(s) for the polar coronal region and F
m,c with average values F 11 2 = 17.7
10 particles/cm (s) for the coronal current sheet. (4) As a theoretical test, a preliminary numerical study of the global distribution near 2.5 RS for the Carrington rotation 1742 in 1983 has been made by solving a self-consistent MHD system based on the observations of K coronal brightness and the photospheric magnetic fields. The numerical results indicate that the global distributions of the coronal mass outputs on the source surface could be used to understand/predict the change of the interplanetary INDEX TERMS: 7509 Solar Physics, Astrophysics, and Astronomy: Corona; 7524 Solar conditions. Physics, Astrophysics, and Astronomy: Magnetic fields; 2134 Interplanetary Physics: Interplanetary magnetic fields; 2164 Interplanetary Physics: Solar wind plasma; 2169 Interplanetary Physics: Sources of the solar wind; KEYWORDS: coronal mass output flux, solar magnetic field, current sheet, global structure Citation: Wei, F., X. Feng, H. Cai, and Q. Zhou, Global distribution of coronal mass outputs and its relation to solar magnetic field structures, J. Geophys. Res., 108(A6), 1238, doi:10.1029/2002JA009439, 2003.

1. Introduction [2] The study for the coronal mass outputs and their relationship with the magnetic structures near the Sun is an important topic either in improving space weather prediction [Shea and Smart, 1998; Dryer, 1994; Wu et al., 1997; Guo and Wu, 1998] or in determining the initial boundary conditions of constructing three-dimensional (3-D) structures near the Sun and in the heliosphere for the understanding of the solar wind origins [Priest et al., 1998; Leer and Sandbek, 1991; Veselovsky, 1995]. At present, although direct observations near the Sun are not available due to the limitation of space measurements, some progress in this topic has still been made. The rapid developCopyright 2003 by the American Geophysical Union. 0148-0227/03/2002JA009439

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ment in much correlative knowledge is also very important to the future investigation of the coronal mass outputs. [3] The coronal mass output has been estimated from space measurements by some authors. Schwenn [1983] reported that the ratio of the mass flux Fm between the high- and low-speed solar wind is 1.83 from the observational data of Helios 1 and 2 and concluded that flux tubes carrying fast, low-density solar wind are compressed by some 15% in the cross section than those carrying slow, high-density plasma and that the mass flux density in highspeed streams are only half the value found in slow solar wind. Since there are no in situ measurements of solar wind parameters to the polar corona regions near the sun, one must use remote sensing observations to get empirical information about mass fluxes. Lallement et al. [1986] deduced from the observations of solar Lyman alpha coronal radiation Ly-a that the polar mass flux is lower than the

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WEI ET AL.: GLOBAL VARIATIONS OF SOLAR MASS AND MAGNETIC FIELD

equatorial average flux. In the study of a solar wind model, Withbroe [1988] took a mean value from Lallement et al. [1985] and Withbroe et al. [1985], which reads Npv = 2.3
108/cm2(s), as an empirical constraint of the partial flux for high-speed wind near the ecliptic plane at 1 AU and also deduced that Fm in the polar coronal region is about one half of Fm from the other regions at 1 AU. [4] Because of the development of interplanetary scintillation (IPS) technique [Hewish and Duffett-Smith, 1987; Asai et al., 1998; Kojima et al., 1998; Jackson et al., 1998] and the improvement of observation and computational methods of the photospheric magnetic fields [Hoeksema et al., 1983; Hoeksema, 1992; Zhao and Hoeksema, 1996], one is able to obtain large-scale structures of solar wind plasma and their evolution with various solar activities and to study the relationship among the coronal density, solar wind speed, and magnetic field [Kojima and Kakinuma, 1987; Kojima et al., 2001; Rickett and Coles, 1991; Wei and Dryer, 1991; Kojima et al., 1999; Ohmi et al., 2001]. For example, Rickett and Coles [1991] studied the evolution of solar wind structure over a solar cycle by comparing interplanetary scintillation measurements with coronal observations and indicated that these IPS observations allow the large structures to be studied over solar latitudes from 60N to 60S over more than a solar cycle. The global distribution of the solar wind speed on the source surface (at 10 solar radii (RS)) has been studied by many authors from IPS observations [Gapper et al., 1982; Kojima and Kakinuma, 1987; Rickett and Coles, 1991; Lu et al., 1997; Kojima et al., 1998]. Kojima et al. [2001] found that lowspeed stream is originated in a large expanding flux tube from the vicinity of an active region. [5] In order to understand the relation between the solar wind speed and coronal magnetic structures, Neugebauer et al. [1998] mapped the solar wind speed observed by Ulysses to the source surface at (2.5 RS) and compared it with the coronal hole structure. Their results show that the highest-speed wind comes from the polar coronal holes, with the wind originating deeper within the hole being faster than the wind coming from near the hole boundary. Wang et al. [1990] proposed an empirical correspondence between daily solar wind speed at 1 AU and the rate of magnetic flux tube expansion in corona using solar wind observations extending over a period of 22 years (1967 – 1988) together with the potential field approximation to extrapolate the observed photospheric field to a source surface at 2.5 RS, where the field lines are required to be radial [see Schatten et al., 1969]. Wang and Sheeley [1991] also adopted a steady state, one-fluid model to understand the correspondence between the solar wind speed and flux tube expansion and indicated that the proton flux at the Earth increases with the expansion rate of magnetic flux tube and the initial speed on the source surface near the Sun. Meanwhile, the north-south asymmetry in speed (typically 15– 20 km/s) between the Northern Hemisphere and the Southern Hemisphere was also reported from Ulysses data [Goldstein et al., 1996] and IPS measurement analysis [Kojima et al., 2001]. The events of coronal mass ejection responsible for interplanetary shock waves follow Gaussian distributions with respect to the angular distance from the coronal current sheet with the maximum frequency number near the current sheet [Wei et al., 1990], since

coronal mass ejections tend to occur near the magnetic neutral lines in the solar atmosphere [e.g., Hundhansen, 1988]. Recently, Fisk [2001] considered the processes that should result in motions of the magnetic field in the quasisteady polar coronal holes near solar minimum, where three assumptions used are (1) the magnetic field and the solar wind that flows along it from the polar coronal hole undergo a nonradial expansion, (2) the nonradial expansion is not centered on the rotation axis of the sun, and (3) the magnetic field is anchored in the differentially rotating photosphere. He also concluded that the interaction between the differential rotation of the photosphere and the nonradial expansion of the solar wind in the corona can lead to largescale, systematic motions of coronal magnetic fields and could influence the structure of the heliospheric magnetic field. However, to seek observational evidence in support of this model from magnetic field data in the heliosphere is a difficult task. In addition, Schulz [2001] claimed that the diffusion of field line foot points on the photosphere couples with latitude-dependent solar rotation to define an eigenvalue problem, such that coronal magnetic structures are found to rotate rigidly about the sun. All of these results mentioned above imply that the evolution (or motions) of the magnetic field structure on the photosphere could take an important role in controlling the magnetic structures in the corona and even in the heliosphere. [6] The numerical MHD model has also been used for understanding the global picture of the solar corona and inner heliosphere for the ‘‘Whole Sun Month.’’ Riley et al. [2001] [see also Linker et al., 1999] proposed an empirically driven global MHD model of the solar corona and inner heliosphere. They used the output of the coronal solution directly to provide the inner boundary condition of the heliospheric model. At the lower boundary in modeling the solar corona they specify the radial component of the magnetic field, Br, based on the observed line-of-sight measurements of the photospheric magnetic field and uniform, characteristic values for the plasma density and temperature. An initial estimate of the field and plasma parameters are found from a potential field model and a Parker transonic solar wind solution [Parker, 1963], respectively. Their results show that the simulations reproduce the overall large-scale features of the observations during the ‘‘Whole Sun Month’’ (August/September 1996), although the specified lower boundary conditions are very approximate. Liewer et al. [2001] analyzed the relation of streamers to the heliospheric current sheet for Carrington rotation 1935, using potential source surface (PSS) magnetic field models [Wang and Sheely, 1992] that assumes the field lines being radial at the source surface (R = 2.5 RS) and found that each streamer is shown as a radial ‘‘quill’’ starting at its location on the source surface and that all streamers lie near the warped heliospheric current sheet. They have further traced the magnetic field lines from the source surface to the photosphere using the appropriate PSS magnetic field model, where a potential magnetic field is computed between the photosphere and the source surface using the specific radial component at the photosphere and assuming the field is radial at the source surface (2.5 RS). Their result shows that some coronal streamers are the likely regions of enhanced density associated with outflow from active regions. This is consistent with the concept that the plasma

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quantitative low-boundary condition in numerical models of space weather events.

2. Statistical Research

Figure 1. Distribution of solar plasma number density (n) on the source surface at 2.5 solar radii (RS) during the Carrington rotations 1733– 1742 in 1983 (estimated from K coronal brightness in the same period). flows out along these newly opened field lines, creating the plasma sheet [Wang et al., 1997, 1998] and with the analysis of ‘‘blobs’’ of outflowing plasma seen in the Large-Angle Spectrometric Coronograph (LASCO) data by Wang et al. [1998]. [7] The observational analysis and theoretical studies mentioned above have been made mainly on the basis of the space measurements at 1 AU and/or far from the sun, and the numerical simulations by MHD models of the corona and the heliosphere have been performed under the radial component approximation of the magnetic field and the flow. These achievements are very important for understanding the coronal mass outputs and their relationship with the magnetic structures near the sun. However, the knowledge about the global distributions of the various coronal physical parameters on the source surface (at 2.5 RS) near the sun are not sufficient for the quantitative study. The purpose of the present paper is to explore the basic characteristics of the coronal material outputs for space weather. The main problems addressed here are whether there is any persistent pattern in the global distribution of the coronal mass outputs, Fm, near the sun, what are the speed dependence and frequency number distribution of Fm and how are they related with the various magnetic structures, and how to examine the capability of a more realistic, nonuniform global distribution of the coronal mass outputs on the source surface. As one knows, the task understanding these problem mentioned above undoubtedly will be very arduous. The main difficulties are that the global observations of velocity, temperature, and magnetic field in the solar atmosphere are very limited; the coronal magnetic field calculated from the photospheric field by the potential field method only represents a lowest approximation of the large-scale field configuration, because those complex dynamic effects occurring in the low corona have not been considered; the transfer processes of the mass and energy are not clear from the photosphere to the low corona, e.g., 2.5 RS, and so on. Because of these difficulties, we have to make a qualitative statistical analysis from the limited observations of the photosphere, corona, and interplanetary space to obtain some information concerning the global features of the coronal mass outputs on the source surface (2.5 RS). Finally, a numerical result is given to test the results obtained by statistic analysis and the capability of a MHD source surface model used for understanding the variation of the basic solar wind parameters near 1 AU, which would lead to a more

2.1. Data and Analysis [8] In the present analysis the contours of K corona white light brightness at 1.7 RS are obtained from High Altitude Observatory (HAO), and solar photospheric magnetic field data are from Wilcox Solar Observatory (WSO). Solar wind speeds estimated by IPS measurements are taken from Japanese IPS observatory [Kakinuma and Kojima, 1983]. [9] It is known that the K coronal polarized brightness pb can be used to estimate coronal density. Here, following the mapping method similar to those of Hulst [1950] and Saito et al. [1977], the K coronal brightness is averaged over the 10 Carrington rotations 1733 – 1742 in 1983 and used to calculate the coronal density on the source surface (2.5 RS) according to relation between the brightness and the density obtained by Lu et al. [1995]. In the approach of the radial assumption [Suess et al., 1996; Sturrock et al., 1996] the average global density distribution at 1.7 RS, similar to that at 2.5 RS, is adopted. The result is given in Figure 1, where the various colors represent the different density intervals labeled at the right side. The highest and the lowest density values are 9
105 cm3 (coronal current sheet) and 1
105 cm3 (polar coronal hole), respectively. This range of density values is consistent with the result derived from broadband measurements of electron-scattered white light radiation 0.8
105 cm3 (polar coronal hole) and 7.8
105cm3 (quiet corona) by many authors [e.g., Withbroe, 1988]. [10] By using the method pioneered by Kojima and Kakinuma [1987], the original locations of the solar wind speed estimated by IPS observations are mapped back onto the source surface along an Archimedian spiral from the maximum point of the weighted function on the line of sight. Since IPS measurements are biased by a line-of-sight integration effect, the error in speed estimation could be in the range of 5 – 7%. In this approach the speed variation from the position of IPS observations to the source surface has been corrected by use of the relation between speed and distance obtained by Withbroe [1988] and the speed value is assigned to a mesh with 3.75
3.75 on the source surface. With this treatment of IPS data mentioned above, as an example, Figure 2 gives the average solar wind speed

Figure 2. Distribution of solar plasma speed (V) on the source surface at 2.5 RS during the Carrington rotations 1733– 1742 in 1983 (calculated from the mapping of interplanetary scintillation observational data in the same period).

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Figure 3. The superposition map of low-speed regions of 1983. The current sheet (heavy solid line) is for rotation 1739 (after Figure 3e given by Kojima and Kakinuma [1987]). distribution on the source surface during the same Carrington rotation as Figure 1. The width of the low-speed belt on the source surface, as shown in Figure 2, is basically similar to that in Figure 3 obtained by Kojima and Kakinuma [1987] on the source surface at 10 RS, but our result is slightly larger. One of the possible causes is that the highspeed regions at high latitudes expand in the way of ballistic radial flow to low latitudes from the solar surface to interplanetary space, which could reduce the width of low-speed belt. In addition, the width of the low-speed belt is also related with speed range selected. Here the region for the velocity 1500 (as shown in green, blue, and deep black regions in Figure 4) basically displays a wavelike structure on the source surface at (2.5 RS), in which two large peaks, a small peak, and a large trough variation are located in the intervals of 30 –50, 230– 300, 330, and 150– 210, respectively. This pattern is an enhanced,

Figure 4. The global distribution of the coronal mass flux output Fm on the source surface at 2.5 RS during the Carrington rotations 1733– 1742 in 1983.

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Figure 5. The white light solar corona at height 1.7 RS observed by Maura Loa MK III K Coronameter of High Altitude Observatory for the Carrington rotation 1739 in 1983. Here, 1, 2, 3, 4, and 5 indicate various brightness regions with PB < 1, 1 < PB  0, 0 < PB  1, 1 < PB  2, and >2, respectively. The polar corona boundaries is designated by the regions labeled with the number 3. The dotted lines show an average coronal current sheet in 1983. regular global distribution in the coronal mass outputs Fm on the source surface. Comparing Figure 4 with Figure 3, the pattern is similar to the current sheet configuration with a two peaks for the typical rotation 1739 during 1733 – 1742, 1983. In other words, the larger Fm mainly distributes along the coronal current sheet. It can also be seen from a large-scale configuration of the larger K corona brightness for the rotation 1739 in Figure 5, in which the solid circles show an average coronal current sheet in 1983. In addition, a south-north asymmetry in Fm with the southern part higher than the northern part is also displayed in Figure 4. Figure 6 shows an average asymmetry of Fm in latitude with the Southern Hemisphere (