Study of the latitudinal dependence of H I Lyman and 0 VI. geometry of the outflow in the polar coronal holes

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A5, PAGES , MAY 1, 1999 Study of the latitudinal dependence of H I Lyman and 0 VI emission in the solar corona' Evidence for the superradial geometry of the outflow in the polar coronal holes Danuta Dobrzycka, Steven R. Cranmer, Alexander V. Panasyuk, Leonard Strachan, and John L. Kohl Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts Abstract. We study the latitudinal distribution of the H I Lyman c and O VI (103.2 nm and nm) line emission during the period of the Whole Sun Month campaign (August 10 to September 8, 1996) when the Sun was close to the minimum of its activity. The H I Lyman c and O VI line intensities appeared to be almost constant with latitude within the polar coronal holes and have abrupt increases toward the streamer region. We found that both north and south polar coronal holes had similar line intensities and lineof-sight velocities, as well as kinetic temperatures of H ø and The dependence of these parameters on latitude and radius is provided. We derived boundaries of the polar coronal holes based on the H I Lyman c and O VI line intensity distributions for several days during the Whole Sun Month campaign. We found that the polar coronal hole boundaries clearly have a superradial geometry with diverging factor fmax ranging from 6.0 to 7.5, and they are consistent with boundaries previously derived from the electron density distributions. We also found that, in general, they are not symmetric with respect to the heliographic poles, and their size and geometry change over periods of days. The H I Lyman c, O VI (103.2 nm), and the O VI (103.7 nm) line intensitie showed similar boundaries within the uncertainties of our data. We modeled the latitudinal distribution of the H I Lyman c and O VI (103.2 nm and nm) line intensities in the south polar coronal hole on August 17, 1996, assuming the coronal plasma outflow along either purely radial or nonradial flux tubes. A comparison of model predictions with the observed distributions shows evidence that the outflow velocity vectors follow nonradial intensity pattern. 1. Introduction The ultraviolet coronagraph spectrometer (UVCS) operating aboard the Solar and Heliospheric Observatory (SOHO) satellite is an advanced instrument that provides us with highquality ultraviolet spectra of the solar corona from 1.5 R s to 10 R s. It is designed to measure profiles and intensities of H I Lyman a (121.6 nm) and the O VI (103.2 nm and nm) doublet as well as emission lines of other ions [Kohl et al., 1995; Gardner et al., 1996]. However, a novel capability of UVCS is that it can also be used as an imaging instrument. Each day a standard synoptic sequence of observations is made to produce daily images of the corona by interpolating between slit positions. For the first time these sets of data enable us to study coronal line emission as a function of both latitude and radius up to several solar radii. The Whole Sun Month (WSM) campaign was held between August 10 and September 8, Its goal was to obtain different types of observations from several SOHO, ground-based and other space-based instruments to get a better understanding of the large- and small-scale structures typical for the solar atmosphere and corona and to study the characteristics of solar Also at Copernicus Astronomical Center, Warsaw, Poland. Copyright 1999 by the American Geophysical Union. Paper number 1998JA /99/1998JA plasma and the physical mechanisms responsible for coronal heating. UVCS/SOHO supported this campaign with specially designed observations and a synoptic sequence that was made each day of the campaign. During the WSM campaign the Sun was near the minimum of its activity cycle [e.g., Li, 1997]. These data are well suited for the analysis of the latitudinal dependence of the H I Lyman a and O VI line emission from the corona. This is because the latitudinal extent of the polar coronal holes is a strong function of solar cycle [see, e.g., Waldmeier, 1981; Bravo and Stewart, 1994; Ananthakrishnan et al., 1995; Dobrzycka et al., 1998], and at solar minimum they are large, stable structures surrounded by narrow streamers occupying the equatorial plane. The lineof-sight contamination by the streamers is thus significantly reduced compared to periods of enhanced solar activity [Dobrzycka et al., 1998], and we have a clearer picture of the plasma properties in the polar coronal holes. In the past, studies of the latitudinal dependence in the solar corona were based on white-light data observations [see, e.g., Munro and Jackson, 1977; Guhathakurta and Holzer, 1994; Kohl et al., 1998; Cranmer et al., 1999] which are equivalent to measurements of the electron density distribution as a function of latitude and radius [van de Hulst, 1950]. Here we present the first analysis of the latitudinal distribution of the H I Lyman a and O VI (103.2 nm and nm) emission in the solar corona. In section 2 we describe the observations and data reduction

2 9792 DOBRZYCKA ET AL.: H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA x/ro Figure 1. Ultraviolet coronagraph spectrometer/solar and Heliospheric Observatory (UVCS/SOHO) slit position pattern used every day to produce daily images of the corona. The inner circle corresponds to the size of the solar disk, while the large concentricircles show the solar heights for which the complete latitudinal scans were obtained. The heights are 1.75 R s, 1.85 Rs, 2.0 Rs, 2.1 Rs, and 2.25 R s. (measured counterclockwise from the north heliographic pole). Observations are made starting above the west limb at six heliocentric heights in the corona. Then UVCS is rolled 45 ø in position angle for the next radial scan. The same procedure is repeated until 360 ø are covered. For the polar scans the heights are 1.5, 1.75, 2.0, and 2.25 Rs; for midlatitude scans they are 1.5, 1.75, 2.0, and 2.25 Rs; for equatorial scans they are 1.5, 1.7, 1.9, 2.2, 2.6, and 3.0 Rsu n. Figure 1 shows the typical pattern of synoptic observations. Images of the largescale corona can be made by interpolating between the slit positions. In particular, for each day of our observations we created complete latitudinal scans for specific heights in the corona: 1.75 Rs, 1.85 Rs, 2.0 Rs, 2.1 Rs, and 2.25 R s. Two of these heights, 1.75 R s and 2.25 Rs, were chosen so that the UVCS/SOHO observations could be compared with data acquired with other instruments within the WSM campaign [Strachan et al., 1997; Panasyuk et al., 1997]. The sample intervals of our latitudinal scans are 2.25 ø. For the data reduction we followed the standard techniques described by Kohlet al. [1997]. We used the UVCS Display and Analysis Software (DAS) for flat-field correction, wavelength and intensity calibration, and removal of image distortion. The uncertainties of the Lyman a and the O VI (103.2 nm and nm) line intensities due to radiometricalibration, photon counting statistics, and background subtraction are estiand estimate the observational uncertainties. In sections 3 and 4 we analyze the H I Lyman a and O VI line intensities and profiles as a function of latitude and radius. During the WSM campaign the large-scale structures in the solar corona were undergoing constant changes. On August 19, 1996, an extenmated to be 12-15% [Gardner et al., 1996]. Moreover, both H I Lyman a and O VI lines are affected by the contributions from the disk-scattered stray light, while the Lyman a profile contains an additional contribution from interplanetary scattered Lyman a. We estimate that for H I sion of the north polar coronal hole to low latitudes appeared Lyman a these corrections account for ---10% of the total line on the east limb. For the next few days it rotated onto the solar disk and assumed a shape similar to an "elephant's trunk." It extended down to southern latitudes where it connected to a region of an apparently strong magnetic field associated with intensity in the polar coronal hole regions and ---3% in the streameregions. For the O VI lines the stray-light correction accounts for ---4% in the coronal hole and only ---1% in the streamers of the total line intensities at the observed heights. an active region south of the equator [Bromaget al., 1997]. We also found that the above corrections give only 2-5% The "elephant's trunk" coronal hole eventually disappeared difference in the width of the observed profiles for both Lyman behind the west limb at the beginning of September In our analysis we mainly concentrate on the observations obtained on August 17, 1996, when the observable corona cona and O VI lines. Thus, in our analysis we ignore the stray light from the disk and interplanetary hydrogen contributions to the H I Lyman a and O VI lines. sisted of a large coronal hole at both poles and quiescent streamers in the equatorial regions. In section 5 we identify boundaries of the polar coronal holes based on the emission 3. H I Lyman a and the O VI Line Intensities as a Function of Latitude and Radius line intensity distribution, and we model the holes as being filled with flow tubes that have either a radial or superradial Figure 2 shows the latitudinal distribution of the H I Lyman expansion. In section 6 we model latitudinal distributions of the H I Lyman a and the O VI line intensities in the south a and O VI (103.2 nm) total intensities (integrated over the profile) from August 17, 1996, at three different heights: 1.75 polar coronal hole on August 17, 1996, assuming a derived R s, 2.0 Rs, and 2.25 R s. There are two obvious types of geometry, and investigate whether they show evidence of the outflow along purely radial or nonradial flux tubes. Finally, we summarize and discuss our results in section 7. large-scale structures present in the corona, bright streamers occupying the equatorial region and broad polar coronal holes which are not exactly centered on the heliographic poles. The North Pole was tilted toward Earth at that time, making the 2. Observations north coronal hole appear larger than the south one. The line intensities within the polar coronal holes remain roughly con- We used the UVCS/SOHO standard synoptic observations stant within _+30ø-40 ø around the poles, while in the streamer obtaine during the WSM campaign lasting from August 10 to region they vary by 15-20% for H I Lyman a and by as much September 8, Synoptic radial scans were taken daily with as 30-40% for O VI. The larger variations of the O VI (103.2 both the Lyman a and O VI spectrometer channels. The pri- nm) line intensities come from the known observational fact mary lines in each channel are H I Lyman a (121.6 nm) and O that equatorial streamers have a latitudinal distribution that is VI (103.2 nm and nm). The Lyman a and O VI channel more structured, and they typically show an intensity decrease spectrometer slits were 40 arc min long with slit widths of 14 in their centers when compared to their edges [Kohl et al., arc sec and 84 arc sec, respectively. In the standard synoptic 1997; Strachan et al., 1997]. This effect was studied by Noci et sequence the slits are scanned radially at eight position angles al. [1997a, b], and Raymond et al. [1997]. Raymond et al. [1997]

3 _ DOBRZYCKA ET AL.: H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA 9793 o _c 8. 6 T 4 > 2 0 ' Position An91e 4, 75Ro c Ro 2 o 20Ro % 1 o > 0 n- Position Angle ouu 2 25Ro Figure 2. (top) Relative H I Lyman a and (bottom) O VI (103.2 nm) intensities, plotted in arbitrary units, from August 17, 1996, as a function of position angle at three different heights: 1.75 R s, 2.0 Rs, and 2.25 R s. Marked points correspond to the identified boundaries of the polar coronal holes, 00(r), for that day. found that it is very likely caused by a factor of 3 depletion of the O elemental abundance in the core of the streamer relative to the O abundance along the edges. The deficit seems to be consistent with a gravitational settling of 0 5+ ions in the closed magnetic field line regions in the center of the streamer [Raymond et al., 1997]. Another characteristic feature is that the high-latitude active region streamers do not show the decrease of the O VI line intensities in the core [see, e.g., Strachan et al., 1997]. Further studies should determine what causes that dif- ference. Giordano et al. [1997] presented analysis of the H I Lyman a and the O VI (103.2 nm) emission in the north polar coronal hole observed with UVCS/SOHO on April 6-9, They focused on the small-scale structures in the coronal hole and found differences in the physical characteristics of plumes and interplume regions. Unfortunately, limited angular sampling of our latitudinal scans prevents a detailed analysis of these struc- tures. ffuu In Figure 2 we also plot points that are identified as the boundaries of the polar coronal holes at the given radii (see below). They correspond to the narrow transition regions at which the line intensity sharply increases from the low, coronal hole level to a much higher streamer level. We found that boundaries determined from the latitudinal distributions of H I Lyman a and O VI line intensities are very similar (see Table 1). To describe the radial dependence of the line intensities in polar coronal holes, we analyzed all the H I Lyman a and O VI (103.2 nm) intensities at latitudes +25 ø around the heliographic poles observed on August 17, Both north and south polar coronal holes appeared to have similar brightness within observational uncertainties, and the intensities across the coronal holes varied by less than 10-20% at the same radius. We fitted the radial intensity distributions using nonlinear least squares minimization and obtained Ih(Lyman a) = 2.76 X 1012(Rs/r) 8' x loll(rs/r) 8'62, (l) Ih(O VI nm)= 6.96 x 1014(gs/r) 33' x 10]l(Rs/r) ]]'77, (2) Ih(O VI nm)= 4.48 x lo 4(Rs/r) 37'ø x 101ø(gs/t') 10'64. (3) For the radial dependence of the intensities in the equatorial streamers we chose the west limb structure observed on August 17, 1996, which has the typical characteristics of a stable, quiescent streamer. We analyzed the H I Lyman a and O VI (103.2 nm and nm) intensities in the range of position angles PA ø. The range was chosen to include the edges and the core of the streamer. The nonlinear least squares best fits to the average bright- ness are Is(Lyman a) = 5.52 x lois(rs/r) 2s' x 1012(gs/t') 6'09, (4) Is(O VI nm)= 3.59 x lobs(rs/r)32'ø X 10 ø(rs/r) s'6, Table 1. Boundaries of the Polar Coronal Holes Northeast Limb Northwest Limb Southeast Limb Southwest Limb Line fmax O0(Rs), deg fmax Oo(Rs), deg fmax Oo(Rs), deg fmax Oo(Rs), deg August 17, 1996 H I Lyman a 6.5 O VI (103.2 nm) 6.5 O VI (103.7 nm) 6.7 August 19, 1996 H I Lyman a 6.8 O VI (103.2 nm) 6.7 O VI (103.7 nm) 6.7 August 20, 1996 H I Lyman a 7.5 O VI (103.2 nm) 7.0 O VI (103.7 nm) 7.0 August 24, 1996 H I Lyman a 6.8 O VI (103.2 nm) 6.8 O VI (103.7 nm)

4 9794 DOBRZYCKA ET AL.: H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA 0.6 _C -(D h o >, o R R I,, o loo 200 3oo Position Angle Figure 3. (top two plots) The 1/e half widths of the H I Lyman a and (bottom two plots) the O VI (103.2 nm) line as observed on August 17, The widths are plotted as a function of position angle at two heights' 1.75 R s and 2.25 Rs. Marked points show the boundaries of the polar coronal holes, 00(r), derived from the H I Lyman a and O VI (103.2 nm and nm, respectively) line intensity distributions. Is(O VI nm)= 1.42 x lobs(rs/r) 32'ø x 101ø(gs/r) 5'79. (6) The intensities (1)-(6) are given in photons - cm -2 sr Line-of-Sight Velocities and Kinetic Temperatures as Functions of Latitude and Radius We derived the 1/e half widths of the H I Lyman a and O VI (103.2 nm and nm) lines by fitting a Gaussian function to the observed profiles. We followed the standard technique, including the removal of instrument effects and backgrounds, that is described by Kohl et al. [1997]. The lines were not corrected for the stray light and interplanetary hydrogen contributions. The main uncertainties of the 1/e half widths are due to the instrument response, and we estimate the error bars to be nm for the H I Lyman a and nm for the O VI lines. The 1/e half widths, translated into Doppler velocity widths, provide direct measurements of the velocity distribution of particles along the line of sight [see, e.g., Cranmer et al., 1999; Kohl, Strachan et al., 1996]. Figure 3 presents the latitudinal dependence of the 1/e half widths of H I Lyman a and O VI (103.2 nm) profiles at 1.75 R s and 2.25 Rs observed on August 17, There is a significant difference in the O VI (103.2 nm) line widths in the polar coronal holes and the streamer region. In the streamers the widths are nm and nm, which correspond to -85 km s -1 and 100 km s -1, at 1.75 Rs and 2.25 Rs, respectively. In the polar coronal holes the line is much wider, nm and nm (200 km s -1 and 380 km s -1) at 1.75 Rs and 2.25 Rs, respectively. The 1/e half widths of H I Lyman a appear to be relatively constant with latitude although they show significant (10%) variations in the polar coronal holes. On average, they reach nm (185 km s -1) and nm (200 km s - ) at 1.75 R s and 2.25 Rs, respectively. Similar variations of the width of the ultraviolet emission lines between the polar coronal hole and streamer region were reported previously [e.g., Kohl et al., 1997; Antonucci et al., 1997; Strachan et al., 1997]. The 1/e Doppler half width of the observed profile can be expressed as kinetic temperature T k of the particles moving along the line of sight with the velocity where 2kTk V12/e -- --, (7) m

5 DOBRZYCKA ET AL.: H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA 9795 m Tk = T + (5v2). stant level at a given height as a function of latitude. There is a well-defined transition point at which the intensity sharply The symbols m, T, and 5v 2 are the ion mass, temperature, increases from the low, coronal hole level to much higher and squared nonthermal velocity, respectively, due to, for example, turbulence and/or wave motions. Since Tk contains an additional nonthermal term, it can only be considered as an upper limit for the ion temperature. The H I Lyman a profile streamer level. In our analysis we assume that this transition point is the boundary of the polar coronal holes, 0o(r). We locate the boundary by fitting the intensities in the polar coronal holes with a constant background. The latitude at which width corresponds to a hydrogen kinetic temperature of T there is a steep and significant intensity increase with respecto 2 x 106 K and 2.4 x 106 K at 1.75 R s and 2.25 R s, respec- that constant value we define as 0o(r). These points are identively. The widths of the O VI (103.2 nm) profiles correspond tified on Figure 2. to the kinetic temperature of the O s+ ions to be 7.0 x 106 K Since the polar coronal holes are structures presumably con- (9.7 x 106 K) in the streamer and 3.9 x 107 K (1.4 x l0 s K) taining open magnetic field lines while bordering streamers in the polar coronal holes at 1.75 R s (2.25 Rs). The above results are consistent with previous UVCS/SOHO measurements [Kohlet al., 1997, 1998; Cranmer et al., 1999]. The difference between the widths of the H I Lyman a and mainly with closed field lines, the boundaries of coronal holes can be associated with the last open magnetic field lines. However, our definition of 0o(r) can underestimate the true size of the polar coronal hole. This is because at the edges of the O VI profiles in the polar coronal holes is clear evidence of a coronal hole the intensity may be enhanced because of projecstrong heating of O +s ions in the direction coinciding with the tion of much brighter adjacent streamers and active regions line of sight in the open magnetic field lines region. UVCS/ along the line of sight. This projection can be highly variable, SOHO observations provided preliminary evidence that this as the streamers and active regions are usually unstable, changphenomenon may extend up to the Mg +9 ions as well [Kohlet ing structures [see Wang et al., 1997]. al., 1997, 1999]. Many features of the observed preferential ion To derive the boundaries of the polar coronal holes, we heating can be explained by the presence of high-frequency utilized our complete latitudinal scans at 1.75 R s, 1.85 R s, MHD wave dissipation via resonance with ion-cyclotron mo- 2.0 Rs, 2.1 Rs, and 2.25 R s. For each height we identified tions about the coronal magnetic field lines [McKenzie et al., Oo(r =.75 ), Oo(r =.85 ), Oo(r = 2.0 ), Oo(r = 1995; Cranmer et al., 1997b, and references therein]. 2.1 Rs), and 0o(r = 2.25 Rs) on the west and the east limb. Since the broad O VI profiles are characteristic of the polar We find values of four parameters: Oo(Rs), fmax, r, and (rl coronal hole regions [Giordano et al., 1997], we compared with a nonlinear least squares fit to the 0o(r). In previous them with boundaries determined from the latitudinal distrianalyses of the polar coronal hole boundaries [Munro and bution of the line intensities (Figure 3). It is clear that the regions of large 1/e widths have similar boundary points. There is a good agreement especially in the south polar coronal hole. 5. Geometry and Boundaries of the Polar Coronal Holes As in previous analyses [e.g., Munro and Jackson, 1977; Koht et at., 1998; Cranmer et at., 1999], we model the coronal holes as consisting of flow tubes that have the general shape of the large-scale structure and that can deviate from a radial expansion. The total area A(r), which is a cross section of the large-scale coronal hole at radius r, can be described as A (r) = A (Rs)(r/Rs) 2 f(r). (8) We adopt the form of the superradial enhancement factor f(r) from Kopp and Holzer [1976]: f(r) = 1 + (fmax- 1) i + exp (9) 1 - exp [(Rs- ;- r)/rr ] r)/o'l]) ' where r and (r correspond to heliocentric distance and the width of the region where the flow tube deviates most rapidly from the radial expansion, respectively, fmax is the asymptotic (r - c) value of f(r). For flux tubes centered around the pole, the factor f(r) can be also expressed geometrically as 1 - cos Oo(r) f(r) = 1 - cos Oo(Rs)' (10) intensities are significantly reduced and remain at almost con- Jackson, 1977; Kohlet al., 1998; Cranmer et al., 1999] the parameters r and (r were found to have very narrow ranges of values: 1.3 Rs-l.5 R s and 0.5 Rs-0.6 R s, respectively. Moreover, we found that determination off max and Oo(Rs) is not sensitive to r and (r. Since precision of our fits improves with decreasing number of free parameters, we chose to fix the values ofr and (r and we assumed r = 1.5 R s and (r = 0.5 R s. Table 1 summarizes our results for different days during the WSM campaign and Plate 1 shows boundaries derived for August 17, 1996, overplotted on the composite image of extreme ultraviolet imaging telescope (EIT) at Fe XII (19.5 nm) line (solar disk) and UVCS in the O VI (103.2 nm) line (corona). Note a good agreement of the boundaries with the intensity distribution at all heights. Also, derived Oo(Rs) agrees very well with the disk observations. We compared our Oo(Rs) predictions for different days with coronal hole maps from the National Solar Observatory, Kitt Peak, Arizona, derived from the He I ( nm) observations. For most of the days we found a very good agreement within the expected uncertainties. However, the northeast boundary on August 20 and August 24 seems to be smaller, as seen by UVCS, because of a bright active region present behind the limb. Also, on August 17 the He I ( nm) data imply the boundary of the north polar coronal hole on the west limb Oo(Rs) 46 ø, which we cannot reconcile with our observation suggesting Oo(Rs) = 31 ø. The EIT image at Fe XII (19.5 nm) confirms our result. In general, we found that the polar coronal holes usually are not symmetric with respect to the poles: they have different where Oo(r) is the coronal hole boundary colatitude. Note that Oo(Rs) and are consistent with a different diverging geometry for pure radial expansion f(r) = 1. on the east and west limbs. Their size and geometry changes The polar coronal holes are clearly seen in our observations. constantly, although results in Table 1 show that the southwest They correspond to latitudes where the Lyman a and O VI line coronal hole boundary remained stable for at least 4-5 days.

6 9796 DOBRZYCKA ET AL.- H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA --4-.,! Plate 1. Boundaries of the polar coronal holes derived for August 17, 1996, overplotted on the composite image showing the solar disk and extended solar corona on that day. The composite image was taken by two instruments: EIT at Fe XII (19.5 nm) line in the inner region and UVCS at O VI (103.2 nm) in the outer region. Note a good agreement of the boundaries with observed geometry of the coronal holes at all heights. The boundaries determined from the latitudinal distribution of the H I Lyman c and the O VI lines are quite similar within data uncertainties. Derived fmax ranges from 6.0 to 7.5, and we estimate its uncertainty for less than 0.5. This value agrees with fn ax found from the electron density distribution analyses [Munro and Jackson, 1977; Kohlet al., 1998; Cranmer et al., 1999]. This agreement is not surprising as I(Lyman c 0 cr n e' For each boundary in Table 1 we computed the asymptotic colatitude Oo(r ) = arccos (1 - fmax (l -- COS Oo(Rs))) (11) of the polar coronal holes and found it changing from 60 ø to 82 ø with the most typical value of 66 ø. In 1995, while ascending in latitude from the south solar pole, Ulysses encountered an abrupt change from the region of the high solar wind to the low-speed streamer belt plasma around latitude -22 ø (or south polar colatitude 68ø). This agrees with typical extent of the polar coronal holes we found. Kyr61g et al. [1997] used data from the solar wind anisotropies (SWAN) instrument onboard SOHO to determine the latitudinal distribution of ionization of the interplanetary medium by the Sun. They found a clear distinction between two large-scale structures: polar coronal holes and the streamer region. The SWAN H I Lyman c data showed the ionization and the mass flux to be nearly flat for all solar latitudes except the narrow belt from -20 ø to 20 ø around the solar equator. Another result that confirms our prediction that the polar coronal holes have a superradial expansion comes from a recent analysis of the long time series data of the wind speed, density, and magnetic field strength obtained by Ulysses [Roberts and Goldstein, 1998]. Roberts and Goldstein [1998] found evidence that the wind flow near heliographic latitude -25 ø comes from a region poleward of -60 ø.

7 DOBRZYCKA ET AL.: H I LYMAN cr AND O VI EMISSION IN THE SOLAR CORONA Model of the Latitudinal Distribution of the H I Lyrnan ct and the O VI Emission To model the latitudinal distribution of the H I Lyman c and the O VI line intensities, we concentrate on the solar corona above the southwest limb as it was observed on August 17, It contained large-scale structures typical for the quiescent corona: a well-defined polar coronal hole and a narrow streamer in the equatorial region. As inputs to our code, we use empirical models for several plasma parameters above a polar coronal hole and as a function of latitude between the polar hole and the equatorial streamer belt. The numerical methods used to synthesize the radiative-transfer diagnostics have been described by Strachan et al. [1993], Kohlet al. [1998], and Cranmer et al. [1997a, 1999]. In order to compute a synthetic emission line from a model corona, its geometry must be specified for the line-of-sight integrations in the optically thin plasma, and its small-scale velocity distribution parameters (number density, outflow velocity, and most probable speeds) must also be specified as a function of position along the line of sight. In our computations we assume that the corona has an axisymmetric geometry, with the electron density above coronal holes independent of latitude. This assumption is well justified by the polarized brightness observations [Guhathakurta and Holzer, 1994; Cranmer et al., 1999; Guhathakurt and Biesecker, 1998], as well as by the constant mass 10 i i i PA- 180 ø (degrees) Figure 4. Latitudinal distribution of the H I Lyman c intensities at 2.0 R s on the southwest limb on August 17, The solid curve corresponds to the model latitudinal dependence with outflow along nonradial flux tubes, and the dotted curve corresponds to the latitudinal dependence predicted by the model with purely radial outflow. The hatched areas correspond to the model uncertainties associated with the errors of the polar hole boundary prediction. which we found consistent with our data. These model values flux in the polar coronal holes detected by SWAN [Kyr6lii et al., self-consistently reproduce observed UVCS/SOHO coronal 1997]. The electron density over streamers is constrained to be hole intensities and line widths near solar minimum over the a Gaussian function of latitude [see, e.g., Guhathakurta and Holzer, 1994], which peaks at the equator at 15 times the corresponding value over the pole. heliographic poles. Finally, the resonant scattering also depends on the outflow velocity u of the solar wind, which we assume here to be The width of this Gaussian is a function of radius and was parallel to the superradially diverging field lines. These field originally identified by the latitude at which the density is 15% of its equatorial maximum value. We assumed that this hole/ streamer boundary is given by the boundary model we computed in section 5 (Table 1). The radial dependence of the electron density is given by the south coronal hole fit of Gibson et al. [this issue]. For the H I Lyman c or O VI (103.2 nm and nm) lines we must also specify the number densities of H ø atoms or 0 5+ ions, respectively. For neutral hydrogen we assume ionization equilibrium using the ionization and recombination rates delines are constrained to follow colatitudes proportional to the hole boundary at that radius; that is, at each colatitude the 4>(r) = Oo(r) x cb(rs)/oo(rs). Note that this is only a simple and approximate means of locating the superradially expanding flux tube boundaries within the coronal hole, and this should ideally be replaced with a computation of the true magnetohydrodynamic (MHD) configuration of the field lines. The magnitude of the outflow velocity, as a function of radius, is given by proton mass flux conservation in a steady state wind: scribed by Raymond et al. [1997]. This equilibrium depends on the electron temperature, which we adopt from the in situ np(r)u (r)r2f(r) = const. (12) charge-state ionization models of Ko et al. [1997]. For the We use in situ measurements to evaluate the constant, with ionized oxygen both the elemental abundance and the "frozen- npu = 2 x 108 cm -2 S -1 at r = 215 R s = 1 AU [Goldstein in" ionization balance are assumed to be constant in radius, et al., 1996], and we assume//p = 0.8 //e for a fully ionized with log o(nos+/np) = [Cranmer et al., 1999]. plasma with 10% helium. The resonant scattering for the H I Lyman c and O VI lines depends on the velocity distribution of the scattering atoms or ions, which we characterize as an anisotropic bi-maxwellian with independent most probable (l/e) speeds w ll and w 2_ that are parallel and perpendicular, respectively, to the superradi- Figure 4 shows the observed latitudinal distribution of the H I Lyman a line intensities at 2.0 R s on the southwest limb on August 17, 1996, normalized to the intensity at the South Pole. We overplotted it with the computed latitudinal dependence assuming the outflow along nonradial flux tubes and depenally diverging magnetic field lines and a parallel macroscopic dence predicted for purely radial outflow. In the model with outflow velocity u. The most probable speeds w ll and wñ correspond to independent "kinetic temperatures" in the two directions, but we retain the velocity units because these quantities may contain both microscopic random motions as well as any unresolved bulk fluctuations along the line of sight (e.g., transverse wave velocities). The most probable speeds are given here by the anisotropic models A1 (for H I) and B1 (for O VI) described by Kohlet al. [1998] and Cranmer et al. [1999], purely radial outflow the latitudinal distribution is constant across the hole and increases toward the equatorial streamer. The latitudinal dependence predicted by the model with outflow along nonradial flux tubes has a much faster increase as a function of colatitude. This is because the Doppler dimming effect [see, e.g., Withbroe et al., 1982; Strachan et al., 1993], which has an important role in the polar coronal holes, depends primarily on the radial component of the velocity. If the

8 9798 DOBRZYCKA ET AL.: H I LYMAN a AND O VI EMISSION IN THE SOLAR CORONA outflow is purely radial, the reduction of the intensity due to Doppler dimming is constant across the polar coronal hole. However, if the outflow is superradial, the radial component of the velocity decreases toward the boundaries of the hole, the intensity reduction decreases, and the latitudinal distribution shows a slight increase. The high intensities in the streamer are the result of increased density and a lack of Doppler dimming in the slower equatorial wind. Figure 4 demonstrates that the observed latitudinal distribution of H I Lyman a intensity follows the model with outflow along nonradial flux tubes rather than the model with purely radial outflow. The same effect can be seen in the latitudinal distribution of the O VI line. Similar evidence for the superradial outflow in the polar coronal holes was found in the analysis of the UVCS/SOHO data obtained between November 1996 and April 1997 [Kohlet al., 1998; Cranmer et al., 1997a, 1999]. 7. Conclusions Bravo, S., and G. Stewart, Evolution of polar coronal holes and sunspots during cycles 21 and 22, Sol. Phys., 154, 377, We analyzed the UVCS/SOHO H I Lyman a and O VI Bromage, B. J. I., G. Del Zanna, C. DeForest, B. Thompson, and J. R. (103.2 nm and rim) observations during the period of the Clegg, An equatorial coronal hole at solar minimum, in Fifth SOHO workshop: The corona and solar wind near minimum activity, Eur. WSM campaign when the Sun was close to the minimum of its Space Agency Spec. Publ., ESA SP-404, 241, activity. We found the solar corona to be rather stable with Cranmer, S. R., et al., UVCS/SOHO empirical models of solar coronal only two types of large-scale structures present: extended polar holes, in Fifth SOHO workshop: The corona and solar wind near coronal holes and equatorial streamers. minimum activity, Eur. Space Agency Spec. Publ., ESA SP-404, 295, 1997a. The H I Lyman a and O VI line intensities appeared to be Cranmer, S. R., G. B. Field, G. Noci, and J. L. Kohl, The impact of almost constant, as a function of latitude at the same height, UVCS/SOHO observations on models of ion-cyclotron resonance within the polar coronal holes with an abrupt increase toward heating of the solar corona, in 31st ESLAB symposium: Correlated the streamer region. We found that both north and south polar phenomen at the Sun, in the hellosphere, and in geospace, Eur. Space coronal holes had similar line intensities and line-of-sight ve- Agency Spec. Publ., ESA SP-415, 89, 1997b. Cranmer, S. R., et al., An empirical model of a polar coronal hole at locity distributions, as well as kinetic temperatures for hydro- solar minimum, Astrophys. J., in press, gen and O s+ ions at corresponding locations. We also found Dobrzycka, D., L. Strachan, M.-P. Miralies, J. L. Kohl, L. D. Gardner, that the line-of-sight velocities and kinetic temperatures are P. L. Smith, S. R. Cranmer, M. Guhathakurta, and R. Fisher, in Cool consistent with previous measurements [Kohl et al., 1997; Cran- Stars, Stellar Systems, and The Sun, edited by R. A. Donahue and J. A. Bookbinder, 607 pp., Astron. Soc. of the Pac., San Francisco, mer et al., 1999]. Calif., We showed that the latitudinal distribution of H I Lyman a Gardner, L. D., et al., Stray light, radiometric and spectral characterand O VI (103.2 nm and nm) emission can be success- ization of UVCS/SOHO: Laboratory calibration and flight perforfully used to determine boundaries of the large-scale structures mance, in Ultraviolet Atmospheric and Space Remote Sensing: Methods and Instrumentation, edited by R. E. Huffmann and C. G. in the corona like polar coronal holes. We found that the polar Stergis, p. 2, Int. Soc. for Opt. Eng., Bellingham, Wash., coronal holes appear to have superradial geometry with a di- Gibson, S. E., A. Fludra, F. Bagenal, D. Biesecker, G. Del Zanna, and verging factor fm x ranging from 6.0 to 7.5. Our results agree B. Bromage, Solar minimum streamer densities and temperatures with previous fits to the boundaries of the polar coronal holes [Kopp and Holzer, 1976; Munro and Jackson, 1977; Cranmer et al., 1999]. We demonstrated that there is evidence that the boundaries derived from line intensities are consistent with those derived from the electron density distribution. We found that the polar coronal holes, in general, are not symmetric with respecto the heliographic poles, and their size and geometry change constantly. However, H I Lyman a, O VI (103.2 nm), and O VI (103.7 nm) line intensitie showed similar boundaries within data uncertainties. The changes are due to effect of solar rotation; we see another part of the polar coronal hole on the limb every day. However, they can also be caused by the time evolution of the boundaries. We modeled the latitudinal distribution of the H I Lyman a and the O VI line intensities in the polar coronal hole above the southwest limb for August 17, In our computations we assumed that for each radius, individual flux tubes follow colatitudes proportional to the hole boundary we derived for that day. 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