Physical properties of a coronal hole from a coronal diagnostic spectrometer, Mauna Loa Coronagraph, and LASCO observations during the Whole Sun Month

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A5, PAGES , MAY 1, 1999 Physical properties of a coronal hole from a coronal diagnostic spectrometer, Mauna Loa Coronagraph, and LASCO observations during the Whole Sun Month M. Guhathakurta,,2 A. Fludra, 3 S. E. Gibson, D. Biesecker, 4 and R. Fisher s Abstract. Until recently [Guhathakurta and Fisher, 1998], inference of electron density distribution in the solar corona was limited by the field of view of white-light coronagraphs (typically out to 6 Rs). Now, for the first time we have a series of whitelight coronagraphs (SOHO/LASCO) whose combined field of view extends from R s. Quantitative information on electron density distribution of coronal hole and coronal plumes/rays are estimated by using white-light, polarized brightness (pb) observations from the SOHO/LASCO/C2 and C3 and HAO/Mauna Loa Mark III coronagraphs from 1.15 to 8.0 R s. Morphological information on the boundary of the polar coronal hole and streamer interface is determined from the white-light observations in a manner similar to the Skylab polar coronal hole boundary estimate [Guhathakurta and Holzer, 1994]. The average coronal hole electron density in the regionol-l.15 R s is estimated from the density-sensitive EUV line ratios of Si IX 350/342 A observed by the SOHO/coronal diagnostic spectrometer (CDS). We combine these numbers with the estimate from whitelight (WL) observations to obtain a density profile from 1 to 8 R s for the plumes and the polar coronal hole. We find that white light and spectral analysis produce consistent density information. Extrapolated densities inferred from SOHO observations are compared to Ulysses in situ observations of density. Like the density inferred from the Spartan coronagraph, the current SOHO density profiles suggesthat the acceleration of the fast solar wind takes place very close to the Sun, within R s. The density information is used to put constraints on solar wind flow velocities and effective temperatures. Finally, these results are compared to the recent analysis of the Spartan white-light observations. 1. Introduction The importance of white-light density constraints in solar wind modeling has been recently discussed by Guhathakurta and Fisher [1998], Sittler and Guhathakurta [1999], and M. Guhathakurta and E. Sittler (manuscript in preparation, 1999). All solar wind modeling requires the knowledge of density and temperature at the base of the corona as boundary conditions. What is also important is the knowledge of the large-scale geometry of the coronal magnetic field. This large-scale geometry (the coronal hole boundary) together with the Ulysses measurements of the radial component of the magnetic field A large body of research has been devoted to modeling the (Br) can provide a complete description of the two- electron density in the corona [Guhathakurta, 1989; Bagenal dimensional (azimuthally symmetric) coronal magnetic field and Gibson, 1991]; Guhathakurta and Fisher, 1998, 1995; Fisher [Sittler and Guhathakurta, 1999; M. Guhathakurta and E. Sit- and Guhathakurta, 1995; Gibson and Bagenal, 1995] (see Gutler, manuscript in preparation, 1999]. The white-light polar- hathakurta et al. [1996] for older references). The primary ized brightness (pb) observations can provide both the density source of information concerning the distribution of electron [Guhathakurtand Fisher, 1998; Guhathakurta et al., 1996] and density is derived from electron scattering theory [van de Hulst, 1950; Billings, 1966] as applied to white-light observations. Using these inversion techniques, the electron density at any Catholic University of America, Washington, D.C. latitude can be calculated as a function of radial distance r 2Also at NASA Goddard Space Flight Center, Greenbelt, Maryland. 3Rutherford Appleton Laboratory, Space Science Department, from the Sun center. This assumption works very well inside a Chilton, Oxfordshire. polar coronal hole during the quiescent phase of the solar cycle 4School of Physics and Space Research, University of Birmingham, [Guhathakurta and Holzer, 1994]. However, daily white-light Birmingham, England. pb observations from Mark III coronagraph in Mauna Loa, SNASA Goddard Space Flight Center, Greenbelt, Maryland. Copyright 1999 by the American Geophysical Union. Paper number 1998JA /99/1998JA areal divergence [Guhathakurta and Holzer, 1994; Munro and Jackson, 1977] of flow/flux tubes. Indirect inference of hydrostatic temperature can also be obtained from the radial gradient of the electron density in the solar corona [Guhathakurta and Fisher, 1995, 1998] and such profiles match quite well the estimated "freeze-in" temperature in the coronal hole from the measured solar wind ionic charge states [Ko et al., 1997]. Using a known density distribution, geometry of flow tubes and particle flux measurements at 1 AU, we can infer the velocity of the solar wind near the Sun from the equation of conservation of mass flux. Hawaii, are typically available only from an inner radial distance of 1.15 R,. Density in the range R,, have been traditionally calculated from EUV emission [e.g., Mariska and Withbroe, 1975;

2 9802 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE Mariska et at., 1978; Fetdman et at., 1979], coronal forbidden 3. Analysis lines [e.g., Guhathakurta et al., 1993]. In this study the SOHO In Plates I and 2 we have presented apb image of the solar coronal diagnostic spectrometer (CDS) produces density procorona obtained from Mark III K-coronameter and LASCO files which significantly improve upon previous EUV analyses C2 telescope respectively, for August 17, The streamers due to its high spectral and spatial resolution, and its ability to in the LASCO C2 image have been blocked in order to enobserve very close to the solar limb [Harrison et al., 1995]. hance the view of the polar holes and the plumes. Plate 2 Finally, a direct and detailed comparison is provided of includes certain artifacts that are always present in C2pB data. SOHO/CDS coronal hole densities to those determined from The dark region in the southeast limb marked "A" is due to the shadow of the pylon holding the occulting disk, and hence this region cannot give us any information about the corona. There is another dark region on the north-east limb marked "B" white light. We use white-light pb data sets to obtain the boundary of the polar coronal hole and density model of the polar coronal hole and polar plumes (from 1.15 to 8 Rs) during the days of August 17-20, 1996, a period when coronal structure on the west limb was rather simple and relatively unchanging, the streamer was in the plane of the sky and the tilt of the magnetic axis was negligible. This polar coronal hole region is also the subject of a detailed study as its azimuthal symmetry enables two-dimensional (2-D) semiempirical MHD modeling of the corona and the interplanetary medium (M. Guhathakurta and E. Sittler, manuscript in preparation, 1999). The west limb equatorial streamer has been modeled by Gibson et al. [this issue], while a 2-D density model has been estimated by M. Guhathakurta and E. Sittler (manuscript in preparation, 1999) for this same period. The outline of this paper is to first provide a description of the white light observations in section 2, followed by inference of physical properties of the polar coronal hole region in section 3, followed by a summary of the results in section 4, and then conclusions in section Observations Since 1980, the imaging K-coronameter Mark III [Fisher et al., 1981] of the High Altitude Observatory at Mauna Loa Solar Observatory in Hawaii has measured the distribution of coronal polarized brightness in white light as a function of height and latitude around the limb of the Sun almost daily from a height of R s with a broad band pass of roughly 4000 and an effective wavelength around However, for quantitative information this range is typically R s. The LASCO C2 and C3 polarized brightness observations whose source is as yet unknown. As a result we were limited to only C2 west limb observations for studying the physical properties of the polar coronal hole region. Image from Mark III in Plate I shows the presence of a high-latitude structure (not polar plume) on the north western part of the coronal hole. Thus we chose only the southwestern polar coronal hole for this study. The observations of pb presented in Plate 3 show that the angular width of the polar coronal hole increases as a function of height and the variation of the width with height is presented in Plate 4. The radial variations in pb observations from the entire south polar coronal hole (west limb data only) for August 17-20, 1996, are plotted in Plate 5. Thus the data in Plate 5 includes position angle (PA) 180ø-215 ø (where PA 0 refers to the north pole of the Sun and the direction is counterclockwise) for the Mark III observations ( Rs, blue dots) and PA 180ø-250 ø for the LASCO C2 ( Rs), red dots) and C3 (5-8 Rs, green dots) data. Because of the weak signal in the coronal hole (see Plates 1 and 3), the data beyond 1.4 R s are not meaningful in the Mark III K-coronameter. Similarly, because of diffraction from the edge of the occulting disk and strong vignetting (see Plate 2), LASCO C2pB data are not reliable below 2.2 R s. The coronal hole signal in the outer field of view of LASCO C3 is very weak, and data beyond 8.0 R s are not reliable. LASCO C2 and C3 observations agree quite well in the overlap region ( Rs). However, a large gap in the data exists between 1.4 and 2.2 R s, which is perceived to be a critical region for coronal heating [Habbal et al., 1995] Geometry of a Polar Coronal Hole The boundary of a coronal hole is defined to be nearly are obtained as a daily sequence. The set consists of taking one identical to the edge of pb enhancements observed in whitepicture through each of the three linear polarizers oriented at light observations [Guhathakurta and Holzer, 1994]. This intervals of 60 ø. These were then combined in a manner outboundary can be most accurately obtained by plotting pb oblined by Billings [1966], to provide pb estimates of the K servations inside the coronal hole for those days when the corona between Rs, in the wavelength streamer appears in the plane of the sky, as a function of range, with a spatial resolution of 12 arcsec for the C2 tele- latitude (Plate 3). We then define the polar coronal hole scope [Brueckner, 1995]. Similarly, the C3 telescope provides boundary to be that region where a sharp transition exists in pb estimates between R s with a 56 arcsec spatial reso- the pb observations in latitude as indicated by the stars in lution. Plate 4 [Guhathakurta et al., 1996]. The uncertainty in deter- Lacking an independent calibration for the LASCO C2 tele- mining the boundary (or the position angle of the star) of the scope, we determined a calibration for C2 by matching the data polar coronal hole is +2 ø. The geometry of the coronal hole to Mauna Loa Mark III K-coronameter observations. The derived in this manner is presented in Plate 4. For comparison Mark III data of pb versus heliocentric height were fit with a power law function in the interval of R s. The fit was extrapolated to the C2 FOV and C2 data were scaled to fit this extrapolated curve in the range R s. This was done for 4 days using observations from streamers only. The average value of the scale factor was used as a normalization for C2pB images. LASCO C3 pb was then calibrated with respect to C2 in the region of overlap. the Skylab polar hole geometry is also plotted with triangles. Within limits of uncertainty the two geometries are not significantly different. The cross-section area of this polar coronal hole as in the case of the Skylab coronal hole increases faster than it would in a spherically symmetric geometry when radial boundaries would appear as vertical lines in Plate 4. The departure of the coronal hole boundary from radial geometry is the greatest at

3 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE 9803 MLSO-MK5 Aug DOY 2,30 '" 18:03UT Corona 15 images -07LIT I:o 19:45UT North is straight up ScaIMg: --25D to 2800 Plate 1. Polarized brightness observations from Mark III K- coronameter for August 17, The white circle is at 1.0 R s Position angle from south pole to equator Plate 3. Daily pb values (August 17-20, 1996) inside the south polar coronal region above the west limb (PA 180ø-270 ø) have been plotted for seven different heights, 1.16, 1.2, 1.3, and 1.4 from Mark III coronagraph and 2.5, 3.0, and 4.0 from LASCO C2 coronagraph. The red stars indicate the boundary between the coronal hole and the streamer , I ' ' ' I ' ' ' I ' ' ' I I 6 I A A 2 A I A I LASCO C2 pb, Aug 17, 96 Plate 2. Polarized brightness observations from LASCO C2 telescope for August 17, A circular mask of 1.8 is placed over the occulting disk I... I... I... ß Position angle from the equator (west limb) Plate 4. Boundary of the south polar coronal hole during August 17-20, 1996 (stars) For reference the boundary of the north polar coronal hole obtained from Skylab is plotted with open triangles [Guhathakurta and Holzer, 1994].

4 , '; Projected radius in Ro Plate 5. Radial profiles of pb from HAO/MLSO observations (PA 180ø-215 ø, blue dots), SOHO/LASCO C2 observations (PA 180ø-250 ø, red dots), and SOHO/LASCO C3 observations (PA 180ø-250 ø, green dots) for August 17-20, The solid and dotted black lines are the fit to the background polar coronal hole and polar plumes/rays of the SOHO pb data. I... i.* Radial distance r in Ro Plate 6. Densities determined from the Si IX 342/350 line ratio for August 18, south polar hole (purple star) and north polar hole (August 24, blue diamond, August 31, pink plus, and September 7, green triangle). The blue vertical dots are the inferred electron densities from observed Mark III pb observations. The black solid line is the model inferred (equation (3)) south polar coronal hole density and the black dotted line is the plume/ray density (equation(4)) using Mark III observations only. The solid green line is the model inferred polar plume density using Mark III and CDS observations (equation (5)). The red line is the north polar coronal hole density (equation (6)), obtained in September 1995 from Spartan coronagraph [Guhathakurtand Fisher, 1998] Ne ½. '-'.. ' g 105 V, ' '. 't,,* 5.r 300 o # ß t* ' ' _... ß Radial distance r in units of Ro lo ø lo 1 lo loo Radial distance r in units of Ro Plate 7. (a) Inferred density for the south polar coronal hole from WL observations only is represented by the solid black line while for the polar plume it is a dotted black line. The dot-dash green line is an estimate of the polar plume density using CDS and WL observations. The dashed red line represents density estimate of the north polar coronal hole from Spartan WLC in September 1995 [Guhathakurtand Fisher, 1998]. Also plotted are the inferred velocities for the corresponding densities in the same color and line styles for a flow tube with an expansion factor of 8.4. The blue lines with the same line styles as the densities are the corresponding lower limits on flow velocity for purely radial geometry. (b) Same as in Plate 7a excepthe x axis range is from 1 to 215 R s. o

5 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE 9805 the radial distance R s, while beyond 2.5 R s it is essentially radial. The cross section of the polar coronal hole can be represented as A (s) = Ao(so) f(s)(s/so) 2, where the subscript refers to quantities evaluated near the solar surface and f(so) is equal to unity and s is the distance along the coronal hole boundary. We have used an analytic expression for f(s), which is provided by Kopp and Holzer [1976]. The function f(s) increases with height and attains an almost constant value of 8.4 beyond 2.5 R s. This formulation suggests that the nonradial divergence (increase in area when compared to pure radial geometry) of the polar coronal is Inference of Electron Density Density in the height range R s was inferred from density sensitive EUV emission lines observed by SOHO/CDS for 4 days 18 (south), 24 (north), and 31 (north) of August and September 7, 1996, over the north and south polar holes. CDS recorded lines of Si IX 342 and 350 from the same area. The Si IX 350/342, line ratio can be used as a density diagnostics for densities below 109 cm -3, down to 107 cm -3. The Si IX line intensities have been averaged over concentric arcs of a constant radius to produce an averaged radial dependence of intensities, and the electron density has been derived as a function of the radial distance above the limb (Plate 6), as described by Fludra et al. [1997, this issue]. The angular widths of these measurements were broad and hence encompassed both background polar coronal hole as well as plume structures. Thus the densities obtained from CDS data represent an average of plume and interplume density. The statistical error associated with these measurements varies from _+15-30% Polarization Brightness Estimate in the Polar Coronal Thus the total density variation of a factor of observed Hole in CDS data can be fully accounted for by statistical errors alone. No significantrend in density was observed between the The observed quantity pb is related to the electron density north and the south polar holes. through the line-of-sight integral given by The observed values of pb as a function of height in the coronal hole region, combined with the assumption that these (2) values vary with r only (Plate 7) [Guhathakurt and Holzer, pb(r, O, d))= ff Ne(l)G(l) dl, 1994], permit calculation of coronal hole electron density following the technique of Van de Hulst [1950] in a manner where Ne([ ) is the electron density measured in units of similar to that outlined by Guhathakurta et al. [1996]. Densities (cm-3), l is the distance along the line of sight, and G(l ) is a inferred from WL observations only are used to produce confactor combining parameters such as limb darkening coeffi- tinuous profiles in the height range R s in the south cient, Thompson scattering cross section, intensity of photopolar coronal hole region for both unstructured background spheric radiation, and a complicated function of r describing polar coronal hole and polar plumes/rays (black line in Plates the changing geometric relationship between the radius vector 6 and 7). In Plate 7 we have plotted these two profiles extrapfrom the Sun and the direction of the observer. As observed in olated to 1 R s. We used the CDS average density estimates in Plates 1 and 2, the polar plumes are an integral part of the conjunction with the WL polar plume estimates to obtain a polar coronal hole region and separating the two components continuous plume density profile from to 8.0 Rs (green (the dark unvarying background coronal hole and the plume/ line in Plates 6 and 7). For comparison, the polar coronal hole ray structures [Guhathakurt and Holzer, 1994; Guhathakurta density inferred from Spartan is plotted as a red dashed et al., 1996] is a difficult task. The effect of many overlapping line. The density from Spartan is within _+ 15% of the structures along the line of sight is evident in Plate 5 where for unstructured polar coronal hole density from SOHO, in the each position angle the pb signal varies by about a factor of range 1-3 R s, beyond which Spartan density is 30% lower Similar variations in the polar coronal hole data have The electron density distribution in terms of power law fit been observed in the analysis of Skylab and Spartan 201 corofor polar coronal hole (Nch) and polar plume (Npp) obtained nagraphic data during the quiescent phase of the previous and from WL observations, polar plume density obtained from current solar cycle [Guhathakurtand Holzer, 1994; Fisher and CDS and WL observations (Nppc), and Spartan coronal hole Guhathakurta, 1995; Guhathakurta et al., 1996]. The observa- (Nspch) are tional data presented here are subject to photometric error associated with measurements ofpb signals as well as temporal evolution. The 4 days of our study did not show any signif- Nch(r) = r r ) r2 X 105, (3) icant changes in the coronal hole region. Also, the pb signal in the coronal region was higher where plumes were visible and the variation ofpb as a function of latitude reflected this very Npp(r) = r }- r4.63 -}- r2 X 105, (4) well (see Plate 3). Instrumental error which is random is found to be around 15-25% for Mark III data in the height range R s. We do not have an instrumental error estimate for Nppc(r)- r }- r4.49 -}- r2 X 105, (5) LASCO observations yet. Thus the observations presented in while Plates 3 and 5 include contributions from polar plume structures as well as noise. We chose the lower boundary (solid line) of the range of pb bars at each height to represent an upper Nspch(r) = r }- r - -}- r2 X 105. (6) limit to the true background coronal hole and the elevated values (dotted line) to represent the relatively faint plumes/ Both the coronal hole and plume density profileshow a 1/r 2 rays. variation in the outer corona as was first inferred from obser- vations of Spartan coronagraph. Proton density measured by particles and fields experiments aboard the Ulysses spacecraft [Philip et al., 1995] from 1.8 to 4 AU for the period , in the high-latitude range of _+80 ø to +40 ø, for the north polar hole, was estimated to be in the range

6 9806 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE 0.3 cm -3 [Guhathakurta and Fisher, 1998] when extrapolated tively, based on Ulysses observations. Computed velocities are to 1 AU. When extrapolated to 1 AU the coronal density Nch is 2.99, Npp and Npp c are 4.49, and Nspch is 1.91 in units of plotted as a function of radial distance for ranges 1-5 R s and Rs, for the polar coronal hole and polar plumes. Plate 7 cm-3. Thus it appears that SOHO observations seem to over- shows that for all three density profiles solar wind reaches its estimate the plume density at 1 AU, and the coronal hole density is also on the upper limit of Ulysses density observations. This 50% difference between SOHO/LASCO and Spartan density estimates could be a direct consequence of larger uncertainty in LASCO pb observations which perhaps is asymptotic velocity within R s. The Spartan 201 polar coronal hole data which match Ulysses lower bound on density at 1 AU (1.9) gives rise to wind that is 790 km/s at 1 AU in agreement with Ulysses observations. The SOHO polar coronal density when extrapolated to 1 AU is 2.8 cm -3 and prothe result of both instrumental as well as calibration uncertain- duces wind with asymptotic velocity of 530 km/s which is sigties. Thus, for modeling the solar wind it might not be a good idea to extrapolate the SOHO density estimate to 1 AU but rather use the Ulysses density values at 1 AU. Although for the purpose of demonstrating the usefulness of accurate density inference in the corona we have used SOHO densities given by (3), (4), and (5) to estimate the solar wind flow velocity. The uncertainty in the coronal hole electron density follows directly from the uncertainty in the observed pb data plus that from modeling and is also a function of height in the corona. The uncertainty in the polar region from 1.15 to 1.4 R s is around 15-30%, from 2.4 to 4.0 R s is 15-40% and from 5.0 to 8.0 R s it is 20-60%. The uncertainty in the region from 1.4 to 2.4 is the greatest where there is no overlap between Mark III and LASCO C2 coronagraphs. These uncertainties do not reflect the uncertainty in LASCO calibration yet. Thus the relative gradient between Mark III, LASCO C2 and C3 might change relative to each other when such numbers become available. The authors, however, feel reassured that the LASCO polar coronal hole density gradient profile agrees quite well with the Spartan density profile. nificantly lower than the velocities observed by Ulysses in the polar coronal hole. There is no significant difference in the plume and polar coronal hole flow velocity at 1 AU since the effect of density variation between the two regions is canceled by the effect of mass flux variation (see equation (8)). In the corona, the three velocity profiles are the same up to 1.7 R s beyond which they diverge rapidly (see Plate 7a). Thus, for the same magnetic geometry and mass flux (1 AU), density plays a crucial role in estimating solar wind flow velocity both near the Sun as well as in the interplanetary medium. If instead of super-radial divergence we choose radial divergence then the expansion factor f drops out in (7) and provides a lower limit to the flow velocity in the inner corona for the same density and mass flux (see Plate 7a, blue lines). The asymptotic velocity does not change for different flow tube geometries because field lines eventually become radial in all geometries. Such model inference of velocity in future could be compared to other model estimates of proton velocity such as from SOHO/ UVCS and IPS observations taken during the whole Sun month Solar Wind Velocity Flow velocity in the coronal hole has not been directly measured. Indirect inferences of flow velocity have been obtained from Spartan 201 and SOHO UVCS measurements using spectral line intensities as well as line widths [Strachan et al., 1997] and from interplanetary scintillation (IPS) measurements as close as 10 R s [Grall et al., 1996; Breen et al., 1997]. Flow velocity can also be inferred from the knowledge of the electron density profile out to 1 AU and particle flux measurements at 1 AU. What is also required is the knowledge of the magnetic field geometry. The geometric areal expansiona(s) of a flow tube is defined to be the same expression as obtained for the polar coronal hole in section 3 with a nonradial divergence of 8.4. A steady, one-fluid description of a proton-electron wind can be described using equation of conservation of mass flux: N(s) t(s)4 (s) : X, (7) applied along the flow tube s. The velocity of a particle along the flow tube can be represented as (NeVe) ( 1A U) (NeVe) 4d( 1A U) 1A U 2 V(s) = N(s)A (s) = N(s) f(s)s 2 ' (8) 3.5. Scale Height Recent results from SOHO/UVCS [Strachan et al., 1997] suggest that the protons are much hotter than the electrons in the corona. In this context the effectiveness of a single-fluid solar wind model has been questioned. In this section we present calculations to show that from coronal density alone we can determine an effective temperature of the corona which can provide constraints on observed or modeled temperatures in the corona. In the case of a single-fluid solution we can follow the same general approach described by Guhathakurta et al. [1992], with the assumptions that the coronal gas is locally isothermal, is in radial hydrostatic equilibrium along the line of sight, and has a helium abundance of We express the density scale height parameter as ho = x 10-6 rs, (9) where T s is the scale height temperature. Numerically, one can determine T s from the radial gradient of the electron density. -1 Ts=5.95 x 106[dligR st d The range of proton flux at Earth ((NeVe)) in polar solar Since the electron density is analytically prescribed as a wind as measured by Ulysses at AU and when extrapo- function of r, the effective scale height temperature T s can be lated to 1 AU was in the range _+.3 x l0 s cm -2 s - derived without assuming isothermal condition. With density [Guhathalcurtand Fisher, 1998]. Using a known density dis- known as a function of r, total pressure P can be obtained as tribution and geometry of flow tubes and ignoring the effects of a function of r [see Gibson et al., this issue]. Once P is known, solar rotation, velocity of the solar wind can be inferred from T s can be directly computed (T s = P/Nk) and there is only a the above equations. We chose the mass flux to be 1.5 and % difference with temperature estimated by assuming lofor the polar coronal hole and polar plume flow tubes respec- cally isothermal conditions (equation(9)). Thus the original

7 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE ' ' ' ' ' ' I '... I... I... I... I... I Radial height Figure 1. Variation in coronal scale height temperature as a function of height. The solid and dotted black lines are the temperature Ts, for the polar coronal hole and polar plumes respectively, from SOHO observations. The dashed line is the temperature from Spartan coronal hole observations. titative information (electron density, width and geometry of structures) about the K corona, the F corona has to be subtracted from the total brightness data. However, in the absence of a reliable, independent, F coronal model the total brightness data cannot be used for quantitative analysis. Since the F corona is considered unpolarized at least up to an altitude of 5 Rs, the polarized brightness measurements are free of the F component. Thus, for estimating density it is extremely important that we observe polarized brightness as opposed to total brightness which includes F corona as well as stray light. However, beyond 5 R s the ratio F/K is large so that only a minute polarization of F component can cause significant errors in the determination ofpb, especially at the poles during solar minimum [Guhathakurta, 1989]. Also, LASCO C2 and C3 coronagraphs typically take only one sequence of pb observations, that is only onepb frame every 24 hours. We learned from the Skylab and Spartan 201 coronagraphs that to reliably model the coronal hole region where signal is very weak, we need to add many frames to get a meaningful signal to noise ratio for quantitative analysis [Guhathakurta and Holzer, 1994; Guhathakurta and Fisher, 1998]. Since we are using only one LASCO pb frame per day, the signal to noise in the C3 outer field of view (8-30 Rs) was too low to provide any meaningful contribution to the estimate of density. Thus, even though the field of view of the LASCO coronagraphs (C2 and C3) extend from 1.5 to 30 R s, in principle the only useful range in our coronal hole study is R s. Although LASCO calibration data are still not available, cross-calibrating LASCO C2 data to Mark III data produced a data set that showed reasonable agreement in the radial gradients of densities between SOHO/LASCO and Spartan 201 observations. In the inner corona (which is constrained by Mark III observations) the coronal hole densities from SOHO/ LASCO and Spartan coronagraphs are in very good agreement but in the outer corona at 5.5 R s Spartan 201 densities are >30% lower. When extrapolated to I AU the difference is as large as 50%. The higher density from SOHO/LASCO pro- assumption of the corona being locally isothermal appears justified. In the multifluid context T s is simply/ (1 + 2a/l + 4a) P/Nek [see Gibson et al., this issue], where a is the helium abundance fraction while in the two-fluid context T s is simply ( Te + Tp)/ 2 [Guhathakurta and Fisher, 1998]. duces a 50% lower (compared to Spartan estimate) asymptotic The height variation of coronal hole density can be inter- velocity at 1 AU. A detailed cross-calibration check will be preted in terms of a local estimate of the scale height temperature. Temperature estimates for polar plumes/rays and polar coronal hole presented in Figure I show an increase in temperature from the coronal base to a peak temperature near heights between 1.8 and 2.4 R s and then a decrease in temperature with height. Within limits of uncertainty (since we are dealing with gradone between the Spartan 201 white-light coronagraph and the LASCO C2 coronagraph during the fifth flight of Spartan 201 mission in October Finally, from the near simultaneous observations from CDS, Mark III, and LASCO C2 and C3 we were able to infer a density profile starting at the base of the corona out to 8 R s. Density inferred from white light and spectral data were consistent. dients of density, the uncertainty is at least as large as that of the density itself or higher) all three profiles seem to agree with each other. One interpretation of the scale height temperature profiles of the coronal holes and polar rays as a function of 5. Conclusions In conclusion, we would like to stress the following main height is that there is a source of heat that is being deposited in the corona in the region R s. This idea was explored in detail by Habbal et al. [1995]. points: 1. Cross-calibrating SOHO LASCO pb observations to Mark III pb observations allowed us to infer electron densities in the south polar coronal hole and polar plumes/rays. The 4. Discussion SOHO polar coronal hole density profile show good agreement with Spartan observations in the range 1-2 R s but The LASCO C2 and C3 coronagraphs take observations then begin to diverge producing a 30% higher density at 5.5 R s. mostly of the total brightness of the white light corona which is a sum of the brightness from the K corona and that of the dust-scattered F corona. The F coronal brightness begins to dominate K coronal brightness by 2 R s. Thus C2 and C3 total brightness images look rather flat and unstructured due to the large contribution from the F corona. In order to obtain quan- The radial gradients of the three density profiles agreed quite well within limits of uncertainty. 2. Electron density was inferred close to the coronal base (1.008) by using density-sensitive EUV lines. Using these, with the white-light inference of density, we were able to obtain continuous density profiles from 1 to 8 R s in the polar coronal

8 9808 GUHATHAKURTA ET AL.: PHYSICAL PROPERTIES OF A CORONAL HOLE hole and coronal plumes/rays. Plume density was a factor of 1.4 _+ 0.3 higher than the coronal hole density. 3. The boundary of the polar coronal hole was determined from the white-light pb observations. Within limits of observational uncertainty the shape of the boundary of the polar coronal hole was not significantly different from the boundary of the Skylab coronal hole. The expansion factor for the SOHO polar coronal hole was estimated at SOHO observations confirm the Spartan observations that high-speed solar wind from the polar coronal hole region seem to be accelerated much closer to the Sun (10-15 Rs) than had been previously thought. 5. The simple mass conservation equation, together with the knowledge of density, mass flux and flow tube geometry allowed us to infer the flow velocity near the Sun and at 1 AU by assuming the flow tube geometry to have same profile as given by Kopp and Holzer [1976] with an expansion factor of 8.4. Lower limits on flow velocities were obtained in the corona by using a radially diverging flow tube. Acknowledgments. We appreciate the support of Alice Lecenski and Joan Burkpile for kindly providing the Mk-III K-coronameter data. Support for M. Guhathakurta was provided by the Solar Physics Research and Technology and SOHO Guest Investigator Program through NASA grants and , respectively, to Catholic University of America. Janet G. Luhmann thanks Barbara J. I. Bromage and Andy R. Breen for their assistance in evaluating this paper. References Grall, R. R., W. A. Coles, M. T. Klinglesmith, A. R. Breen, P. J. S. Williams, J. Markkanen, and R. Esser, Rapid acceleration of the polar solar wind, Nature, 379, 429, Guhathakurta, M., The large and small scale density structure in the solar corona, Ph.D. thesis, Univ. of Denver, Denver, Colo./Univ. of Colo., Boulder, Guhathakurta, M., G. Rottman, R. Fisher, F. Orrall, and R. Altrock, Coronal density and temperature structure from coordinated obser- vations associated with the total solar eclipse of 1991 March 18. Guhathakurta, M., and R. R. Fisher, Coronal streamers and fine scale structures of the low latitude corona, Geophys. Res. Lett., 22, 1851, Guhathakurta, M., and R. Fisher, Solar wind consequences of coronal density profile: Spartan coronagraph and Ulysses observations from 1.15 R s to 4 AU, Astrophys. J., 499, 215, Guhathakurta, M., and T. E. Holzer, Density structure inside a polar coronal hole, Astrophys. J., 426, L782, Guhathakurta, M., G. Rottman, R. Fisher, F. Orrall, and R. Altrock, Coronal density and temperature structure from coordinated observations associated with the total solar eclipse of 1998 March 18, Astrophys. J., 388, 633, Guhathakurta, M., T. E. Holzer, and R. M. MacQueen, The large scale density structure of the solar corona and the heliospheric current sheet, Astrophys. J., 458, 817, Habbal, S. R., R. Esser, M. Guhathakurta, and R. R. Fisher, Flow properties of the solar wind derived from a two-fluid model with constraints from white light and in situ interplanetary observations, Geophys. Res. Lett., 22, 1465, Harrison, R., et al., The coronal diagnostic spectrometer for the Solar and Heliospheric Observatory, Sol. Phys., 162, 233, Ko, Y-K., F. A. Lennard, J. Geiss, G. Gloeckler, A. B. Galvin, and M. Guhathakurta, An empirical study of the electron temperature and heavy ion velocities in the south polar coronal hole, Sol. Phys., 171, 345, Kohl, J., et al., UVCS/SOHO empirical determinations of anisotropic velocity distributions in the solar corona, Astrophys. J., 501, L127, Bagenal, F., and S. Gibson, Modeling the large scale structure of the Kopp, R., and T. Holzer, Dynamics of coronal hole regions, Steady solar corona, J. Geophys. Res., 96, 17663, polytropic flows with multiple critical points, Sol. Phys., 49, 43, Biesecker, D., S. E. Gibson, B. J. Thompson, D. Alexander, A. Fludra, Mariska, J. T., and G. L. Withbroe, Analysis of EUV limb-brightening N. Gopalswamy, and T. Hoeksema, The synoptic Sun during the first observations from ATM, 1, Model for the transition layer and the Whole Sun Month Campaign: August 8 to September 10, 1996, J. corona, Sol. Phys., 44, 55, Geophys. Res., this issue. Mariska, J. T., U. Feldman, and G. A. Doschek, Measurements of Billings, D. E., A Guide to Solar Corona, Academic, San Diego, Calif., extreme-ultraviolet emission-line profiles near the solar limb, Astro phys. J., 226, 698, Breen, A., et al., Ground and space-based studies of solar wind accel- Munro, R., and B. Jackson, Physical properties of a polar coronal hole eration, Eur. Space Agency Spec. Publ., SP-404, 223, from 2 to 5 R s, Astrophys. J., 213, 874, Brueckner, G. E., et al., Solar wind research with the Large Angle Phillips, J., et al., Ulysse solar wind plasma observations from pole to Spectroscopic Corongraph (LASCO) experiment onboard the Solar pole, Geophys. Res. Lett., 22, 3301, and Heliospheric Observatory (SOHO) satellite, Sol. Phys., 162, 357, Sittler, E. C., and M. Guhathakurta, Semi-empirical MHD model of the solar corona and the interplanetary medium, Astrophys. J., in Feldman, U., G. A. Doschek, and J. T. Mariska, On the structure of the press, solar transition zone and lower corona, Astrophys. J., 229, 369, Strachan, L., et al., Spectroscopic observations of the extended corona Fisher, R., and M. Guhathakurta, Physical properties of polar coronal during the SOHO Whole Sun Month, Eur. Space Agency Spec. Publ., rays and holes as observed with the Spartan coronagraph, SP-404, 691, Astrophys. J., 447, L139, Van de Hulst, H. C., The electron density of the solar corona, Bull. Fisher, R. R., R. H. Lee, R. M. MacQueen, and A. I. Poland, New Astron. Inst. Neth., 11, 135, Mauna Loa Coronograph systems, Appl. Opt., 20, 1094, Fludra, A., G. del Zanna, B. J. I. Bromage, and R. J. Thomas, EUV D. Biesecker, University of Birmingham, School of Physics and line intensities above the limb measured by CDS, Proceedings of the Space Research, Edgbaston, Birmingham, B15 2TT, U. K. Fifth SOHO WORKSHOP, Eur. Space Agency Spec. Publ., ESA SP- (doug@ aj cannøn'nascøm' nasa' gøv) 404, 385, R. Fisher, S. E. Gibson, and M. Guhathakurta, NASA Goddard Fludra, A., G. del Zanna, D. Alexander, and B. J. I. Bromage, Electron Space Flight Center, Code 682, Greenbelt, MD (lika@tristang. temperature and density of the lower solar corona, J. Geophys. Res., gsfc.nasa.gov; gibsøn@søhøps'gsfc'nasa'gøv; fisher@c682h.gsfc.nasa.gov) this issue. A. Fludra, Space Science Department, Rutherford Appleton Labo- Gibson, S. E., and F. Bagenal, The large-scale magnetic field and ratory, Chilton, Didcot, Oxfordshire OXll 0QX, U. K. (fludra@cds8. density distribution the solar minimum corona, J. Geophys. Res., 100, nascøm'nasa'gøv) 457, Gibson, S. E., et al., Solar minimum streamer densities and temperature using Whole Sun Month coordinated data sets, J. Geophys. Res., (Received April 22, 1998; revised September 23, 1998; this issue. accepted October 16, 1998.) 1997.