Eiscat measurements of interaction regions in the solar wind

Adv. Space Res. Vol. 20, No. I, pp. 21-30, 1991 8 1997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177197 $17.00 + 0.00 PIT: SO273-1177(97)00475-4

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EISCAT MEASUREMENTS OF INTERACTION REGIONS INTHE SOLAR WIND A. R .- Breen,* W. A. CoIes,** R.,R. Grail,** M. T. Klinglesmith,*+ J.’ Markkanen,*** P. J. Moran,” C. A. Varley* and P. J. S. Williams* * Adran FjZseg, Pr#ysgol Cymru Aberystwyth, SY23 3B2, Wales, U.K. ** Department of Electrical and Computer Engineering, University of California San Diego, CA, U.S.A. *** EISCAT Sodunkyla, SF-99600. Finland

ABSTRACT EISCAT measurements of interplanetary scintillation (IPS) have shown that the solar wind is normally dominated by distinct fast and slow components of flow. A minority of observations show velocities intermediate between those of fast and slow streams. The properties of these events are consistent with ob$ervations of the compression region at the leading edge of a co-rotating interaction region (CIR) or, in a minority of cases, with the passage of a @1997 COSPAR. Published by E&w&r Science Ltd. coronal mass ejection (CME) through the solar wind. INTRODUCTION Two+tation measurements of interplanetary scintillation (IPS), in which simultaneous, observations are made from widely separated sites’, provide a powerful technique for determining the velocity of the solar wind at a wide range of latitudes and distances from the SUI&~. The velocity of the solar wind is estimated largely from the time lag at which the auto- and cross- correlation functions of the signals received at the two sites intersect, with the accuracy increasing as the baseline between the sites increase,s. The three .widely separated antennas of the EISCAT system make baselines of up lo 390 km available for IPS observationsk5 and allow two components of velocity present in the IPS line of sight to be resolved 697. IPS measurements from EISCAT and direct measurements from Ulysses8 60th indicate that the solar wind is dominated by distinct fast (700-800 km s-l) and slow (300-400 km s-l) components. Fast flow has long been known to originate in coronal hole&‘o, so it is possible to use white-light and soft X-ray images of the Sun to determine which parts of the lPS line bf sight lie in regions of fast flow. Furthermore, IPS observation&7 and indirect spacecraft measurements11 show that the acceleration of the solar wind is very rapid, - taking place inside about 10 solar radii (R) in the case of the fast stream. As the solar wind is known to flow in a direction close to radial at distances from the Sun of more than 6 R, it is possible to assume a constant velocity and simple spherical expansion in mapping each line of sight from source to antenna down into the upper corona. This analysis allows the contributions from different parts of the line of sight to the received signals to be disentangled and the true velocities and weights of the fast and slow streams td be estimated, which would net be possible if the solar wind contained many different velocities 6. INTERACTION

REGIONS

AND TRANSIENTS

EISCAT and Ulysses both see a significant minority of intermediate velocities, particularly at mid-latitudes where coronal holes extend towards the solar equator. We consider that most intermediate velocities arise from interaction regions in the solar wind, where the rotation of

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the Sun carries a fast stream under a slow stream. These co-rotating interaction regions (CIRs) are familiar from spacecraft observations and are characterised by a dense region of compressed plasma at the leading edge of the fast stream and a low-density region at the trailing edge where the two flows are diverging. The degree of compression at the leading edge of the fast stream increases as the angle between the spirals of magnetic field carried by the fast and slow streams steepens - that is, with increasing distance from the Sun. At large distances from the Sun shocks develop at the leading edge of the fast stream r2, but in the region observable by EISCAT .low for such dramatic effects to be oroduced. (i

fb Cawington

Longitude

Figure 1 (a) White-light map derived from HA0 coronagraph measurements centred on 5” September 1991, showing the line of sight for EISCAT observations of 1229+020 (at 87 R) projected down on to the corona. The letters F, I and S represent fast, intermediate and slow velocity flow respectively, measured by the IMP-8 spacecraft at 1 AU (215 R). (b) EISCAT correlation functions for the observation corresponding to (a). The observed correlation functions are shown as broken lines and the results of the model fit as solid lines. Figure 1a shows a white-light map of the corona derived from High-Altitude Observatory measurements centred on 5th September 1991. The superimposed lines are the line of sight for the EISCAT observation of the strong radio source 1229+020 on 5th September 1991, projected down to the corona with assumed velocities of 800 (line 1) and 400 km s-l (line 2). The tick-marks show the angle along the line of sight at 10” intervals from the point of closest approach to the Sun (marked by a triangle). The crossed circle marks the position of the Earth. Both projections take the line of sight across the boundary of a low-latitude coronal hole, with the sector above the hole extending over an angle from +lO” to -25”, where negative angles are towards the Earth. The main region of fast flow is nearer to the Earth than the slow flow, which on the west limb of the Sun will carry the fast flow under the slow region as the Sun rotates. The geometry indicates that the fast stream is overtaking a region of slow flow. Figure 1b shows the auto- and cross-correlation functions of the scintillations observed by the EISCAT sites at Kiruna and Sodankyla during the observation corresponding to figure la. The dotted lines denote the average autocorrelation function of the scintillations observed at each site and the cross-correlation function of the scintillations observed at the two sites, with the solid lines showing the results of the two-dimensional scattering model. The fit of the model to the data is extremely good. Moreover, the results show an undisturbed slow stream at 380 km s-t but a ‘fast’ stream which is considerably slower and produces much more scintillation than expected. We are therefore confident that the observations of 1229+020 on 5th September 1991 show a compressed interaction region at the leading edge of the fast stream. The observation shown in figure 1 was at a radial distance of 87 R, but intermediate velocities associated with fast stream/slow stream interaction regions have been observed as close to the Sun as 27 R, as shown in figure 2. Figure 2a shows a white-light map centred on 24th August 1994: the superimposed white lines are the lines of sight for EISCAT observations of 1042+120 on 24th August 1994, projected down onto the corona with assumed velocities of 800 km s-t (line 1) and 400 km s-t (line 2). Figure 2b shows the corresponding correlation functions. The results of the scattering model suggest that the line of sight includes a normal slow stream with a velocity of about 360 km s-t, but the ‘fast’ stream is much slower than normal and gives rise to much more scintillation. There

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results are very similar to those shown in figure 1 and strongly suggest that CIRs begin to develop as close to the Sun as 27 R.

Carrington Longitude

Figure 2 (a) White-light map derived from HA0 coronagraph measurements centred on 24th August 1994, showing the line of sight for EISCAT observations of 1042+120 (at 27 R) projected down on to the corona. (I?) The broken lines show the correlation functions from the EISCAT observation corresponding to (a), with the solid lines showing the results of the scattering model fit. Another possible cause of intermediate velocities are coronal mass ejections (CMEs), which are ‘bubbles’ of closed magnetic flux ejected into the solar wind. CMES have been observed at a wide range of velocities (between 100 and 1200 km s-t) and vary considerably in density’“. CMEs are characterised by a closed field structure in which density irregularities at the nose of the event are elongated perpendicular to the direction of flow, instead of parallel to the magnetic field. As a result, for a suitable spacing of the two antennas the scattered diffraction pattern gives rise to a negative correlation between the recorded scintillations at zero la,=t4, This ‘undershoot’ is conspicuous in figure 3, which shows the passage of a fast transient through the IPS line of sight, seen in observations of 0318+164 made on three successive days in 1995. On 28th May (figure 3a) a single peak dominates the crosscorrelation function, corresponding to a solar wind velocity of about 390 km s-l. There is a small bulge in the correlation function at a shorter time lag (-0.75 s), suggesting the presence of a faster secondary velocity. The coefficients of cross-correlation at short lags are very small or negative. By the next day (figure 3b) the correlation function is shows a second, very pronounced peak at a short time lag and there is a clear negative lobe close to zero lag. The second peak corresponds to a component of velocity perpendicular to the IPS line of sight of about 650 km s-t. By 30th May (figure 3c) the fast peak had almost vanished and there was only a very small negative lobe in the correIation function at short lags. The negative lobe in the correlation function at short lags strongly suggests that the transient high-soeed flow seen on 29th Mav corresnonded to the oassaae of a CME across the line of siaht.

Figure 3. EISCAT observations of 0318+164 on (a) 2gth May 1995, (b) 29th May 1995 and (c) 30th May 1995. The passage of a fast transient (seen as a secondary peak) can be clearly seen. CONCLUSIONS Away from interaction regions and transients the solar wind consists of clear fast and slow components. The fast streams are known to originate in coronal holes, which makes it possible to use white-light and soft X-ray solar images to determine which regions of the IPS line of sight lie in fast flow and which in the denser slow wind. This in turn makes it possible to determine the true velocities and densities of the fast and slow streams. .lVR ZD-1-8

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A minority of EISCAT IPS observations show velocities intermediate between those of fast and slow streams. Of 23 such events observed between 1990 and 1996, 17 came from regions in which a fast stream was overtaking a slow stream, and showed enhanced levels of scintillation from the intermediate velocity feature. We interpret these observations as compression regions at the leading edge of CIRs. 5 other observations showed no evidence of association with any long-lived structures visible in coronal white-light images: the cross-correlation functions for these observations had,conspicuous negative lobes close to zero lag, indicating that the density irregularities in the solar wind were elongated parallel ,to the IPS line of sight. Furthermore, on those occasions when the same source was observed for several successive days, the events were found to be of short duration (- 1 day). We interpret these observations as the passage of CMEs across the line of sight. We have observed intermediate velokity events with characteristics consistent with observations of the compression region of CIRs as close to the Sun as,27 R - the closest to the Sun that these events have been seen. The densities inferred from the modelling program for the intermediate velocity regions at the leading edge of CIRs are an average of the densities of the compression region and the low-density fast stream. We are currently working on a theoretically-based model of the thickness of the compression region, allowing the true densities to be estimated. ACKNOWLEDGEMENTS We would like to thank the director and staff of EISCAT for the data used in this study. EISCAT is supported by the scientific research councils of Finland, France, Germany, Japan, Norway, Sweden and the UK. Three of US (ARB, PJM and CAV) are supported by PPARC. REFERENCES I. Armstrong, J. and W.A. Coles, Analysis of three-station interplanetary scintillation data, J. geophys. Res, 77, pp.4602-46 10, 1972 2. Rickett, B.J. and W.A. Coles, Evolution of the solar wind structure over a solar cycle: Interplanetary scintillation velocity measurements compared with coronal observations, J. geophys. Res., 96, pp. 17 17- 1736, 199 1 3. Coles, W.A., Interplanetary scintillation observations of the high latitude solar wind, Space Sci. Rev., 72 l/2, pp.21 l-222, 1995 4. Bourgois, G., W.A. Coles, G. Daign, J. Silen, T. Turenen and P.J.S. Williams, Measurements of the solar wind velocity using EISCAT, As&on. Astrophys., 144, pp.452-462, 1985 5. Breen, A.R., W.A. Coles, R. Grall, U-P. Lovhaug, J. Markkanen, H. Misawa and P.J.S. Williams, EISCAT measurements of interplanetary scintillation, J. atmos. terr. Phys., 58, pp.507-5 19, 1996 6. Grab, R.R., W.A. Coles, M.T. Klinglesmith, A.R. Breen, P.J.S. Williams, Rapid acceleration of the polar solar wind, Nature, 379, pp.429-432, 1996 7. Breen, A.R., W.A. Coles, R.R. Grall, M.T. Klinglesmith, P.J. Moran, B. Tegid and P.J.S. Williams, EISCAT measurements of the solar wind, Ann. geophys., 14, pp. 1235- 1245, 1996 8. Phillips, J.L., A. Balogh, S.J. Bame, B.E. Goldsteen, J.T. Gosling, J.T. Hoeksema, D.J. McComus, M. Neugebauer, N.R. Sheeley and Y.M. Wang, Ulysses at 50” south: constant immersion in the high-speed solar wind, Geophys. Res. Lett., 12, pp.1 105-l 108, 1994 9. Neupert, W.M. and V. Pizzo, Coronal holes as sources of recurrant geomagnetic disturbances, J. geophys. Res., 79, pp.3701-3709, 1974 IO. Hundhausen, A., An interplanetary view of coronal holes, in Coronal holes and high speed wind streams, ed. J. Zirker, Boulder Colorado Associated University Press, p.225, 1977 I I. Munro, R.H. and B.V. Jackson, Physical properties of a coronal hole from 2 to 5 solar radii, Astrophys. J., 213, p.874, 1977 12. Whang, Y.C., Shock interactions in the outer heliosphere, Space Sci. Rev., 57, pp.339-388, 1991 13. Phillips, K.J.H., Guide to the Sun, pp.235239, Cambridge University Press, 1992 14. Grab, R.R., Remote sensing observations of the solar wind near the Sun, Ph.D. Thesis, University of California San Diego, 1995