Yohkoh observations of the solar corona

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Adv. Space Res. Vol. 19, No. 12, pp. 38394848. 1997 0 1997 COSPAR. Published by Elsevier Science Ltd. All rights mewed Printed in Gmt Britain 0273-1177/97 $17.00 + 0.00 177(97)am35-9

YOHKOH OBSERVATIONS OF THE SOLAR CORONA J. L. Culhane Mullard Space Science Laboratory, University College London, Holmbury St. Mury, Dorking, Surrey RHS 6NT, U.K.

ABSTRACT The Yohkoh soft X-ray telescope obtains several images every 90 minutes. Data from the declining phase of

the solar cycle have been used to compare the X-ray signal with other indicators of activity and to study coronal heating. X-ray emission from a north polar coronal hole is found broadly consistent with results of previous EUV observations. In diffuse emission regions, temperature rises to around 2.2 MK and levels off in the height range 1.5 - 1.9 R,. Such emission underlies streamers and may be the source of the low-speed solar wind. X-ray signatures for Coronal Mass Ejection (CME) events which involve the detection of reduced X-ray intensities in the corona, have been developed with Yohkoh data. CME observations are described (r)1997COSPAR.Published by Usevier Science Ltd. 1. INTRODUCTION The Yohkoh Soft X-ray Telescope (SXT) operates in the photon energy range 0.25-4.0 keV. Pixel sizes used in the CCD detector can be selected at 2.5, 5.0 and 10.0 arc sec. While the core of the point spread function (PSF) is I 3 arc set, the effective angular resolution is closer to 5 arc set because of pixel sampling. Following spacecraft launch in August 1991 by Japan’s Institute for Space and Astronautical Science (ISAS), the telescope has been used to study a broad range of solar phenomena including solar flares, active regions, coronal jets and bright points together with the nature and evolution of the large-scale structure of the corona. While the magnetic fields in coronal active regions are generally thought to be closed other than perhaps during flares, fields in coronal holes are open and these regions are the sources of the high speed solar wind. In large scale structures such as streamers, the fields appear closed in the lower corona though individual closed loops are rarely resolved in the otherwise diffuse plasma. However in the high corona as viewed with coronagraphs, the underlying diffuse regions appear connected over a wide range of latitudes to streamers of helmet configuration. These may be the sources of the low speed solar wind though the question remains open. The corona also emits large quantities of matter sporadically. Known as Coronal Mass Ejections (CMEs), these events can lead to the release of 2 lo13 kg of plasma into interplanetary space. They appear to result in large scale rearrangement of the coronal magnetic tleld though this is not always the case. Much work is required to understand these events not least because of their magnetic and particle impacts on earth. In this brief review of Yohkoh SXT observations we will i) discuss the evolution of X-ray emission in 1x39

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the declining phase of the solar cycle, ii) describe the nature of the emission from a polar coronal hole, iii) present observations of diffuse coronal structures and iv) comment on X-ray studies of CMEs.

2. CORONAL X-RAY EMISSION IN THE DECLINING PHASE OF THE SOLAR CYCLE The changing magnitude of the X-ray output of the corona in the course of a solar cycle has been known for some time. The result of the progressive reduction in the number of active regions has been more directly illustrated with SXT. Two full-sun images taken 3.5 years apart in the declining phase are shown in Figure 1. While the plot indicates a drop of approaching two orders in intensity, the absence of active regions also leads to a dramatic change in the nature of the radiation source - it is less concentrated while large scale diffuse structures and coronal holes are more apparent particularly at high latitudes.

Fig. 1. Two Yohkoh SXT full-sun images. The soft X-ray intensity excluding flares plotted against time

Acton (1996) has examined almost a four year span of coronal data - excluding flare emission, obtained since launch. He finds that the expected indices (e.g. full disc X-ray intensity, X-ray active region area) correlate well with the declining cycle. However average plasma temperature (Te) is poorly correlated. The relations between several of the indices involved are shown in Figure 2. Although Te and Emission Measure (EM) simply exhibit scatter and individually correlate poorly with other cycle-dependent parameters, there is an excellent correlation between Te and pressure as represented by Te x (EM)Oe5. This may indicate that a common heating mechanism is operative throughout the cycle which delivers energy throughout the corona in a manner

that does not depend on the rate of magnetic flux emergence but rather on the quantity of

material being heated. Much further work needs to be done with this important data base. Meantime the concentration of bright X-ray emission in active regions is forcefully indicated.

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3. X-RAY EMISSION FROM A NORTH POLAR CORONAL HOLE Observations of coronal holes near the sun are important for determining the plasma parameters in the source region of the high speed solar wind. Observations at X-ray wavelengths are difficult because of low electron densities within the hole and the possibility that other coronal structures in the line of sight can be confused with the faint coronal hole emission. A north polar coronal hole was observed by Yohkoh on 1992 October 3. The X-ray image is shown in Figure 3 from which it is clear that the hole region is essentially free from

Fig. 3. The corona observed by Yohkoh on 3 Oct., 1992. A well developed polar coronal hole is evident. JASR 19:12-D

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contamination by high latitude diffuse structures. This is confirmed by a synoptic plot of the images which demonstrates that the hole was similarly free of contamination half a solar rotation before the observation. Details of the work are given by Foley et al. (1997) and so will not be described here. Plots of radial intensity and temperature extending to 1.8 R, are compared with predictions of models by Withbroe (1988) which were based mainly on Skylab EUV data and on white light density measurements. Reasonable agreement is obtained with the model that is appropriate for solar minimum conditions. Temperature data are also consistent with a value at 1.5 R, deduced from in-situ Solar Wind Xon Composition Spectrometer (SWIGS) measurements by the Ulysses spacecraft while at high heliographic latitudes (Ko et al., 1997). For faint signal levels scattering of X-rays from the rest of the disc by SXT must be allowed for. While the SXT exhibits lower scattering at off-axis angles than did the Skylab instrument, it is necessary to correct the coronal hole image for the the effect of large angle scattering. Pre launch calibrations (Martens et al., 1995) combined with in-flight observations of scattering halos of several flares which occurred at up to 2000 arc set off axis (Hara et al., 1994), have been used to construct the point spread function of the telescope. Mariska (1978) finds evidence in Skylab data for the presence of lower temperature material (T, I 1 MK) close to the limb and this is supported by Foley et al., 1997 who suggest that better agreement with the Yohkoh intensity estimates would be obtained by using model values of temperature lower than those given by Withbroe (1988) for R I 1.05 R,. 4. DIFFUSE CORONAL STRUCTURES In addition to Active Regions and Coronal Holes, Yohkoh images also include diffuse patches of emission often visible at high latitudes. We have observed several of these regions with the SXT and they exhibit generally similar properties. Typical examples, observed on the limb during a spacecraft offpoint on 1992 August 26, are shown in Figure 4. The region outlined on the south-east limb was investigated in more detail. Careful observation and alternative displays of the data show traces of loop-like structures near the base of the region but no complete loops can be resolved. For determination of the plasma properties, the region outlined in Figure 4 was exposed through two SXT filters - Al and Al/Mg/Mn, and the resulting data were aligned to < 2 arc set and corrected for the effects of CCD saturation, dark noise and particle hits. The small excluded region contained a coronal bright point. Two exposures taken a few minutes apart were summed to yield a total exposure time of 121 s. So as to enhance the statistical quality, the data were further summed into radial arcs of constant distance from sun centre. The contribution of X-ray scattering from the rest of the disc was calculated using the point spread function and subtracted. The radially averaged signal was plotted against radial distance for both filters and the filter ratio technique (Tsuneta et al., 1991) with the plasma emlssivity calculations of Mewe et al. (1985, 1986) were used to compute radially averaged values of T, and Emission Measure (EM). These are shown plotted against RIR, in Figure 5. While EM falls with height above the limb in the manner that would be expected given the fall-off in coronal density, Te rises from a value of 1.8 MK at the limb and levels off at - 2.2 MK in the range 1.5 to 1.9 R,. Assuming that the diffuse region is a static structure which emerges radially, ne - 3.107 cms3 at 1.8 R,. Although no such loops can be resolved, a single hypothetical semi-circular loop of height 0.8 R, would have a full-length Lf - 9.1010 cm and a pressure P - 9.10-3 dyn cm-2. The measured value of T, is almost

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30% more than the 1.6 MK value expected from the scaling law Tmax = 1.4.103 (PxLf)“*33 (Rosner et al., 1978, RTV) where P is the loop pressure in dyn cmT2. Using pressure and length values appropriate to a

Fig. 4. A region of diffuse coronal emission observed by Yohkoh above the SE limb on 26 Aug., 1992. Fig. 5. Electron temperature and plasma emission measure radially averaged and plotted against height for the region of diffuse emission in Figure 4. height of 0.5 R,, the expected scaling law value of Te is 1.9MK which is closer to the observed value. However T, is observed to increase with height up to 0.5 R. and to level off thereafter. Hence closed loops of increasing height which follow the RTV relation cannot reproduce the observed behaviour of temperature and density in the diffuse structure, Further an assumption of hydrostatic equilibrium does not yield a good fit to the observations. Several other diffuse coronal regions observed by Yohkoh show a similar behaviour of Te with height although the value of Tmax varies by around 10% - 2.0 to 2.4 MK (Foley et al., 1995). it is almost always the case that diffuse structures seen by Yohkoh underlie extended streamer-like structures seen in coronagraph data.. This is true for the 1992 August 26 example considered above as is clear from a superposition of the image from the Mauna Loa K coronameter on the Yohkoh image (Figure 6). Thus the

Fig. 6. A superposition of Yohkoh SXT data and Mauna Loa K-coronameter data for 26 Aug. which shows that the diffuse X-ray emission region underlies a coronal streamer seen in white light.

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plasma observed by Yohkoh although apparently retained in the corona, seems to merge into streamer structures which may be partially open. This may explain the Ulysses observations of systematic variation in the “frozen-in” electron temperature of the solar wind plasma with both wind velocity and heliographic latitude (Geiss et al., 1995). More recently Von Steiger (Private Communication, 1996) has carried out a preliminary analysis of data from both the north and south polar passes. His results are shown in Figure 7 as plots of solar wind He ion velocity (v,) against heliographic latitude from the Ulysses plasma instrument (Phillips et al., 1995) and of the electron temperature (To) deduced from measuring the relative concentrations of Oxygen ions with the SWICS instrument. For heliographic latitudes 1/300/ the velocity data indicate values of - 800 km s- 1 appropriate to the high speed wind streams that originate in polar coronal holes. At these times the “frozen-in” electron temperature value is about 1.2 MK. However at lower latitudes, the ion velocity reaches values below 400 km s-l while T, values up to - 2.5 MK are deduced from the Oxygen ionization stage concentrations. ‘Ihe rapid fluctuations in v, and Te are believed to be caused by the spacecraft moving between high and low speed wind streams and thus between plasmas with low and high values of T,. While the slow wind ion concentration data require the kind of analysis that has been carried out for the high speed data by Ko et al., 1996 before more generally valid estimates of “frozen-in” T, can be made, the similarity of the Oxygen-derived temperature values with those measured by Yohkoh in diffuse structures close to the sun reinforce suggestions that plasma from streamers is the source of the slow solar wind. 5. CORONAL MASS EIECTIONS (CMEs) Because they are spatially large phenomena with comparatively low intensity per unit surface area, CMEs are hard to observe and a detailed understanding of their origin and evolution is hampered by the difficulty of combining the results of relevant space and ground based observations. Thus actual mass ejections have been well observed in the plane of the sky by ground based coronagraphs but the source structures are best observed at X-ray and EUV wavelengths on the disc. It has until recently been difficult for observers using these different wavelengths and techniques to agree on what they have seen. However solar disc signatures are being recognized in Yohkoh images (Hudson, 1996) and a recent review by Hundhausen (1997) has demonstrated that an increasingly integrated view of these important events is being developed. Until the advent of light coronagraphs temperature coronal - usually of greater

Yohkoh, CMEs had principally been observed with either ground or space-based white which detect photospheric continuum emission scattered by free electrons in the high plasma. The ejection involves the disruption of a large region of coronal magnetic field extent than that of an active region. Often the field topology is that required to contain

prominence material though the field lines are not always loaded with cool prominence plasma. The CME.

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which typically involves the release of more, than 10 *3 kg of material, “opens” the magnetic field and blows much of the involved plasma out of the corona. The loss of material should leave a void in the corona and this has indeed provided an X-ray signature for these events. The association with flares remains uncertain but long duration events (LDEs) are often observed and may be related to the formation of post event loops filled with high temperature plasma. Because of the large volumes involved, the related X-ray emission which is often not detected by GOES, will be faint. However Yohkoh observations have clearly detected the formation of large post event arcades following CMEs (Hudson et al., 1996) and, as we will see below, the reduction in X-ray emission or “dimming” associated with the loss of plasma from the corona. Thus it is clear that the GOES broad band whole sun X-ray detectors lack the sensitivity to detect CMEs and the socalled “flare myth” might be better termed the “GOES myth”. One of the first events observed by the Yohkoh SXT to exhibit coronal dimming occurred on 1994, February 27. The image and related light curves are shown in Figure 8. The associated flare was a long duration event

Fig. 8. Early image of an over the limb event on 27 Feb., 1994 and light curves for the two regions, A and B, indicated on the image (LDE) which occurred about 6 deg over the limb. It is therefore not possible to see the large post event structures which may well be associated The light curves refer to the summed intensities in the two boxes labelled A and B. Region B has an intrinsically fainter signal since it lies higher in the corona and so from 09:06 onwards, the signal begins to rise again due to the contribution of scattered X-rays. The scattering contribution to the signal in region A is not detected until 0!3:16 which is close to the time of maximum of the LDE as registered by GOES. However both plots show an initial decline which is due to the sweeping of material out of the corona by the CME. Most of the ejected material probably originated low in the corona and was mainly directed through region A. An event observed by Hiei et al. (1993) provides post-facto evidence for the coronal dimming signature of mass ejection. Yohkoh SXT, Mauna Loa Ho prominence monitor and K-coronameter observations together gave a strong indication that a mass ejection occurred at the south-west limb between January 23 and 24, 1992. A large prominence which was visible at 21.13 UT on January 23 was not seen at 18:45 UT on January 24. Both white light and X-ray observations indicated that a major rearrangement took place in the corona in

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this time interval. Thus an arch and cavity overlying the prominence were replaced by a brighter cusped helmet streamer while the Yohkoh X-ray images shows that the change took place between 08:07 UT and 09:20 UT on January 24. The disappearance of the prominence and the large scale rearrangement of the magnetic field imply the occurrence of a mass ejection. This conclusion is supported by the dimming data (Hudson et al., 1996) which are shown in Figure 9. The intensities plotted refer to the two regions shown on the image. Both intensities drop after 06:OOUT indicating the onset of the ejection event as material is swept out of the corona. Flux from the region above the newly formed helmet later increases since the new feature moves outward at an speed of -10 km/s. Based on the reduction in flux, Hudson (1996) estimates the mass as

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- 1012 kg which is typical of the range observed for CMEs. Thus observations of coronal dimming at X-ray wavelengths provides a new means of detecting CMEs which will prove useful given the difficulty of maintaining continuous coronagraph coverage by ground-based instruments. Both of the events occurred at the limb and so it was not easy to observe the launching process. Lemen et al. (1976) have used the SXT to locate the solar origins of events that were identified as CMEs in interplanetary space by instruments on the Ulysses spacecraft on the basis of bidirectional streaming by suprathermal electrons (Gosling et al., 1994). Seven events registered by Ulysses corresponded to solar flares (2), large arcades on the disc (3) and arcades behind the limb (2). Two of these events - the long duration flare (LDE) of February 20, 1994 and the large arcade event of April 14, 1994, have been studied in detail by Plunkett and Simnett (1997). The X-ray LDE lasted for up to six hours and involved a pair of active regions. These were later linked to each other and to the surrounding corona by a system of post-flare loops which were visible for more than 24 hours. The LDE began seven minutes after the eruption of a filament which had been visible along the inversion line connecting the two active regions for several days. Thus the involvement of a filament-containing field configuration and in this case the eruption of a filament, are important

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characteristics of the CMF!Zphenomenon. An even larger scale two ribbon event occurred on April 14, 1994 in which a large arcade formed over the southern polar crown region during a period of several hours. This global restructuring of the southern polar corona was observed by Yohkoh and is illustrated in Figure 10. Although there was no associated filament eruption and the intensity enhancement was barely detectable in GOES, a CME signature was observed at Ulysses and a weak LDE signature was obtained from the SXT images by Hudson et al. (1995). Although very different in spatial scale at the sun, both these events led to major geomagnetic storms that lasted for several days.

14-Apr-94 01: 1656 UT

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Fig. 10. The formation of a large arcade over the southern polar crown won April 14th which followed a CME whose interplanetary effects were registered by Ulysses from between April 20 and 23.

6. CONCLUSIONS The SXT has accumulated a large data base in more than five years of the declining solar cycle from September, 1991. The active region emission in particular correlates well with other activity indicators but there is no strong correlation with plasma temperature. This suggests that the nature of the coronal heating mechanism does not change through the cycle. However X-ray emission does correlate well with coronal pressure. Emission has been detected from a north polar coronal hole. The radial variation of intensity and temperature agree well with the model of Withbroe (1988) and with recent Ulysses observations of “frozenin” temperature by Ko et al (1997). Details are given by Foley et al (1997). The radial variation of intensity and temperature have also been measured for diffuse patches of emission at high latitudes. Temperature rises to - 2.2 MK and levels off in the height range 1.5 R, to 1.9 R,. The hydrostatic equilibrium assumption

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does not fit the observations. Although traces of loop-like structures can be discerned, closed loops which follow the usual scaling laws (e.g. RTV, 1978) cannot reproduce the observed behaviour of temperature and density in the diffuse structures. These regions must be heated at heights 2 1.5 R, and observed peak temperature values compare well with those estimated by Ulysses in slow solar wind streams. Yohkoh SXT observations have established coronal dimming as a new signature for the outward propagation of material in CMEs. The launching of CMEs involves magnetic field configurations that are capable of containing tllament material in the corona although cool material is not always present. SXT observations show the formation of post-CME arcades which can on occasion involve an entire solar hemisphere REFERENCES Acton, L.W., Proc 9th Cool Star Workshop, To appear in Proc Astron. Sot. Pacific, (1996). Foley, C. A., Culhane, J.L., Acton, L.W.. Submitted to Astrophys. J. (1997). Foley, C.A.. Culhane, J.L., Acton, L.W.. In Press, Proc. IAU Colloq. 153, Makuhari, Japan, (1995). Geiss, J., et al., Science, 26% 1033-1036, (1995). Gosling, J. et al., Geophys. Res. Lett.. 21, 2271, (1994) Hara., H. et al., Pub. Astron. Sot. Japan, 46, L493, (1994). Hiei, E., Hundhausen, A.J., Sime, D.G., Geophys. Res. Lett., 20,24, (1993). ‘l&on.

H.S., Haisch, B. and Strong, K.T., J. Geophys. Res., 100,3473,(1995).

Hudson, H. S., Proc 153rd Colloquium on Magnetodynamic Phenomena in the Solar Atmosphere, eds Uchida, Y.. Kosugi. T. and Hudson, H.S., Kluwer Academic Publishers, p 89, (1996) Hudson, H.S., Acton, L.W ., Freeland, S.L., Astrophys. J., 470,629, (1996) Hundhausen, A., J. in “Cosmic Winds and the Eeliosphere”, eds Jokipii, J.R., Sonett, C.P. and Giampapa, M.S., Univ. of Arizona Press, (1997). Ko, Y-K., Fisk, L.A., Geiss, J., Gloeckler, G., Guhathakurta, M., Submitted to Solar Physics, (1997). Lemen, J.R. et al., In Press, Proc. Solar Wind 8, Meeting at Dana Point, California (1996). Mariska, J.T., Astrophys. J. 225, 252-258, (1978). Mewe, R., Gronenschild. E.H.B.M., Van den Oord, G.H.J., A&on. Astrophys.Suppl. 62, 197, (1995). Mewe, R., Lemen, J. R., Van den Oord, G.H.J., Astron. Astrophys.Suppl. 63,5 11, (1996). Martens, P.C., Acton, L.W., Lemen, J.R., Solar Physics, 157, 141, (1995). Phillips, J.L., et al., Science, 268, 1030-1033, (1995). Plunkett, S.P. and Simnett, G.M., In Press, Astron. Astrophys., 1997 Rosner, R., Tucker, W.H., Vaiana, G.S., Astrophys. J., 220,643, (1978). Tsuneta, S., et al., Solar Physics, 136, 37-67, (1991). Withbroe, G. L. Astrophys. J., 325,442-467, (1988).