- Email: [email protected]
Slow and fast solar wind acceleration near solar maximum
Pergamon www.elsevier.com/locate/asr
Adv. Space Res. Vol. 30, No. 3, pp. 433-436.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02$22.00+ 0.00 PII: SO273-1177(02)00339-3
SLOW AND FAST SOLAR WIND ACCELERATION SOLAR MAXIMUM A.R. Breen’, P. Thomasson’,
NEAR
C.A. Jordan2, S.J. Tappin3, R.A. Fallows’, A. Canals’, and P.J. Moran’
’ Physics Department, University of Wales, Aberystwyth, SY23 3BZ, Wales, UK ’ Jodrell Bank Observatory, University of Manchester, MacclesJield, Cheshire SK1 I 9DL, UK 3 Birmingham University, Edgbaston Park Road, Birmingham BI5 2TT UK
ABSTRACT
2-site measurements of interplanetary scintillation (IPS) provide measurements of solar wind speed in regions of the heliosphere which are otherwise inaccessible. We present results from coordinated observations made with the EISCAT and MERLIN facilities during 1999 and 2000, covering heliocentric distances from 7 to 80 solar radii (R) predominantly in the slow solar wind. The 1999 results are compared with optical measurements from LASCO covering 4-30 R. Most slow acceleration appears to take place between 5 and 10 R, but the slow wind continues to accelerate out to 25-35 R. Some of the observations included identifiable fast flow, and in these regions acceleration began lower down and was much more rapid, with 50% of cruising speed reached by 4-5 R and acceleration complete inside 10 R - results which are similar to those from solar minimum. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION
Observations of interplanetary scintillation (IPS), in which the diffraction pattern cast across the Earth when a distant compact radio source is observed through the solar wind have been used as a flow tracer in the solar wind for over 35 years [e.g. Hewish et al, 1964; Dennison and Hewish, 1967; Armstrong and Coles, 19721. If the phase changes introduced into the incident radio waves are small (“weak scattering”) then contributions to the observed scintillation pattern from different parts of the line of sight to the source can be treated separately [e.g. Grall et al., 1996; Breen et al., 19961. As the ray-path moves closer in to the Sun, the amplitude of the scintillation pattern increases until the inner limit of weak scattering is reached. Inside this limit, in the “strong scattering” regime, it is very difficult to determine the contributions of different regions to the observed scintillation pattern [e.g. Hewish, 1989; Grall, 19951. The distance from the Sun for the transition from weak to strong scattering depends on the frequency of the radio waves received: for EISCAT observing at 0.93 GHz the transition takes place at 20-30 solar radii (R) in the slow wind; for MERLIN observing at 5 GHz, the transition lies below 7 R. The first IPS observations using the EISCAT facility at a frequency of 0.93 GHz were made in 1982 [Bourgois et al., 19851 and a regular series of measurements of solar wind velocity have been made every summer since 1991. IPS observations were made at 1.66 GHz with MERLIN in 1985 and 1992 by Rickett [1992]. The later observations included some made at 5 GHz. Measurements from EISCAT and the VLBA (at 5 and 8 GHz) were used by Grail et al. [1996] to study the acceleration of the fast solar wind over the poles of the Sun in 1994 and 1995, revealing that the fast wind accelerated very rapidly and reached its “cruising” speed by lo-15 R. Sheeley et al.[1997], Tappin and Simnett [1998] and Tappin et al. [1999] have used data from the LASCO instrument on SoHO to investigate the speed of identifiable features in white-light data. The small discrete “blobs” in the slow wind studied by Sheeley et al. [1997] showed evidence of continuous acceleration from around 2 solar radii (R) out to beyond 20 R. Tappin et al. [1999] derived outflow velocities by cross-correlating white-light intensities at two radial distances. The results indicated that the slow solar wind had reached its cruising speed by 25-30 R, with most of the acceleration taking place inside lo-15 R. Determining velocities in the fast wind is more difficult, owing to the lower densities. Breen et al. [2OOOa]compared fast wind velocities measured by EISCAT and SoHO instruments including LASCO during September 1997, and although the optical and IPS datasets did not overlap in distance the results supported the Grall et al. [ 19961 conclusions. Breen et al. [2OOOb]compared slow wind velocities from May 1999 measured by applying the Tappin et al. [1999] cross-correlation method to LASCO C2 and C3 data with IPS observations from MERLIN and EISCAT. There was good agreement between the optical and IF’S velocities which suggested that irregularities of very different scale sizes (-10000 km for the optical features and -100 km for the features producing IPS) were drifting at the same speed. Solar wind densities around this period were anomalously low [Lazarus, 20001 and there was a possibility that the acceleration 433
434
A. R. Breen et al.
profile of the slow wind observed during this period was not representative of more typical conditions. Accordingly a second series of IPS observations using EISCAT and MERLIN were carried out in May 2000. The observations carried out in May 1999 and May 2000 were dominated by the slow wind, but significant coronal holes were present during both periods, giving rise to detectable fast flow. In this paper we discuss the observations of slow and fast wind velocities between 4 and 100 R during May 1999 and May 2000.
OBSERVATIONS The 1999 observations were made from EISCAT, MERLIN and LASCO between 2”d and 17* May and were coordinated so that the different instruments observed flow along similar streamlines of flow (within *lo” latitude and lying off the same limb). The May 2000 observations from EISCAT and MERLIN were made between 1 lth and 24 May 2000 off the south-west limb of the Sun and were coordinated in the same way as the 1999 observations, with flow along similar streamlines traced outwards from the corona. (a) USC0 observations (I999 only): The LASCO C2 and C3 instruments observed the solar corona off the south-east limb of the Sun for 12 hours per day on 91hand 1 lth May 1999, producing an image every 12 minutes. This was the finest time resolution possible when using C2 and C3 together. The results were divided into two overlapping distance ranges from the Sun and cross-correlated to obtain estimates of the flow speed. (b) MERLIN observations (1999 and 2000): The strong radio source 0318+164 (CTA-21), with its small angular size and well-studied structure, is an ideal source for high-frequency IPS observations of the innermost regions of the solar wind. In early May 1999 0318+164 lay off the south-east limb of the Sun, moving closer to the Sun and towards higher latitudes with time. The observations on 4’h May 1999 were made using the Cambridge and Knockin telescopes of MERLIN, operating at a central frequency of 1.66 GHz, a single polarisation (left circular) of equivalent bandwidth 10 MHz being sampled at 100 Hz. On 9* and 11” May 1999 the Cambridge and Jodrell Bank Mk.2 telescopes were used at a central frequency of 5 GHz. In mid-May 2000 0318+164 lay off the south-west limb of the Sun, moving away from the Sun and towards lower latitudes during the campaign interval. The source was observed at 5 GHz using the Cambridge and Knockin telescopes on 12’h May 2000 and with Knockin and Jodrell Bank Mk.2 on 15* and 16” May 2000.
Figure 1: (a)Ray-paths for EISCAT and MERLIN measurements of IPS during May 1999 projected back to 2.5 R using a constant-velocity ballistic model and overlaid on a map of white-light intensity constructed from LASCO C2 images at 2.5 R off the east limb. (b) Ray-paths for EISCAT and MERLIN measurements of IPS during May 2000, projected back to 2.5 R and overlaid on a west limb LASCO C2 white-light map in the same way as in Fig. la. Only data taken off the SE limb of the Sun in May 1999 and off the SW limb of the Sun in May 2000 are discussed in this paper. (c) EISCAT observations EISCAT made observations of IPS off the south-east limb of the Sun every day from 2”d May 1999 to 15h May 1999 and off the south-west limb between llth and 281h May 2000. The data were received on one polarisation over a 5 MHz bandwidth (8 MHz at the Tromso and Sodankyla sites in 1999), centred on 931.5 MHz. Initial sampling was at 10 kHz, with the data subsequently averaged to give a sample every 0.01 s.
435
Slow and Fast Solar Wind Acceleration Near Solar Maximum
RESULTS During May 1999 and May 2000 the corona was complex in structure, with multiple streamers occupying most of the region around the Sun. This would suggest that the solar wind would be mainly slow - as was indeed observed - but in during both months coronal holes extended across the southern hemisphere of the Sun, and these provided the source regions of narrow streams of fast solar wind [e.g. Breen et al., 20011. The ray-paths for EISCAT and MERLIN IPS observations during May 1999 and 2000 are projected down to 2.5 R and overlaid on maps of coronal white-light intensity in Figure la and lb respectively. Regions of the ray-paths lying above coronal holes are assumed to be immersed in fast streams of solar wind. This allows the velocities of the fast and slow streams to be detemined independently [e.g. Breen et al., 19961.
I
3
SoHO!LASCOC2 andC3 EISCAT 93I .5 MHz MERLIN 1.66GHz and5 GHz
0’8 (a)
.‘.30
I
a
5 ‘00 cx
0
MERLIN: 75” S
I
60 90 Heliocentricdistance (solar radii)
30
90 Heliocentric distance (solar radii)
(b)
60
I
Figure 2: (a) LASCO, MERLIN and EISCAT measurements of slow wind velocity between 4 R and 100 R, May 1999 and (b) MERLIN and EISCAT measurements of slow wind velocity between 7 R and 110 R, May 2000.
3ll P
v 0
1
EISCAT May 2000 EISCAT May 1999 MERLIN May 2000 MERLIN Mav 1999 SoHOlLASCO Mav 1999
10
Heliocentric distance (solar radii)
Figure 3: LASCO, MERLIN and EISCAT measurements of fast-component velocities between 4 and 100 R during May 1999 and 2000 Slow stream velocities from May 1999 and May 2000 are shown in Figure 2a and 2b respectively. The large uncertainties in flow speeds seen in some of these measurements - particularly during May 2000 - arise mainly from the difficulty in determining the exact boundaries of fast and slow streams. In several cases the ray-path lies above and along the boundary of a coronal hole and in these cases the observation will include contributions from regions with very different velocities. Nonetheless a clear trend is apparent in the data from both months, with velocities increasing from -100 km s-’ or less inside 7 R to reach a cruising speed in the region of 300-400 km s“ by 25-30 R.
CONCLUSIONS The slow wind results from the May 1999 and 2000 campaigns show the same pattern, with slow wind speed increasing from around 100 km s*’ at 10 R to 300-400 km s-’ by 25-30 R, with no detectable increase in velocity outside this distance. The irregularities observed by IPS and optical methods in 1999 were drifting at comparable speeds, and the acceleration profile reported for these data by Breen et al. [2OOOb]is consistent with that seen in the radio observations
436
A. R. Breen et al.
from 2000, suggesting that slow wind in early May 1999 was not atypical in its velocity profile. A significant spread in slow wind velocities is seen in both datasets, which we interpret as intrinsic variations in slow wind speed - the slow wind
is known to be much more variable in velocity and density than the fast wind. A more serious difftculty is the large uncertainty in some of the measurements, introduced when the ray-path for the observation lies partially above the boundary of a coronal hole and so passes through regions where steep velocity gradients are present. These difficulties are greater under near-solar maximum conditions when coronal holes are transient phenomena and are therefore difficult to
locate in maps constructed from white-light data over a solar rotation. More detailed modelling will be needed to reduce these uncertainties. The fast wind only occupied a small proportion of the ray-path in these observations, but nonetheless a weak fast component was detectable in the May 1999 LASCO and IRS data and in the May 2000 IFS data, as shown in figure 3. Once again the irregularities observed by LASCO and IPS had very similar drift velocities, suggesting that they were drifting at the background flow speed. Acceleration was much more rapid than in the slow wind, with 500 km s-t reached by 6-7 R and no detectable acceleration outside IO-12 R. These results provide the first set of measurements of solar maximum fast wind velocity covering the whole distance range from 4-100 R, and are consistent with those reported by Grail et al. [ 19961 at solar minimum.
ACKNOWLEDGEMENTS We would like to thank the director and staff of EISCAT for the 0.93 GHz IPS data used in this study. EISCAT is supported by the scientific research councils of Finland, France, Germany, Japan, Norway, Sweden and the U.K. We would like to give particular thanks to B.J. Rickett and W.A. Coles for making their experimental programmes, analysis routines and expertise available to us. MERLJN is a National Facility operated by the University of Manchester on behalf of PPARC. We would like to thank the Director and staff of Jodrell Bank for making available necessary additional equipment. The LASCO white-light data was made available by the LASCO consortium. SOHO is an ESA/NASA joint mission. Three of us (ARB, RAF, PJM) were supported by PPARC during the period when this work was carried out. AC was supported by the University of Wales.
REFERENCES Armstrong J.A., and W.A. Coles, Analysis of three-station interplanetary scintillation data, J. Geophys. Res., 77, pp. 4602-4610. 1972 Bourgois, G., W.A. Coles, G. Daigne, J. Silen, T. Turenen and P.J.S. Williams, Measurements of solar wind velocity using EISCAT, Astron. Astrophys., 144, pp. 452462, 1985 Breen, A.R., W.A. Coles, R.R. Grall, M.T. Klinglesmith, J. Markkanen, P.J. Moran, B. Tegid and P.J.S. Williams, EISCAT measurements of the solar wind, Ann. Geophys., 14, pp, 1235-1245, 1996 Breen, A.R., C.F. de Forest, B.J. Thompson, J.F. McKenzie, A. Modigliani, P.J. Moran, C.A. Varley and P.J.S. Williams, Comparisons of interplanetary scintillation and optical measurements of solar wind acceleration with model results, Advances in Space Research, 26(S), pp.781-784,200Oa Breen, A.R., S.J. Tappin, CA. Jordan, P. Thomasson, P.J. Moran, R.A. Fallows, A. Canals and P.J.S. Williams, Simultaneous Interplanetary Scintillation and Optical measurements of the acceleration of the Slow Solar Wind, Annales Geophysicue, 18, pp.995-1002,200Ob Breen, A.R., A. Canals, R.A. Fallows, P.J. Moran and M. Kojima, Large-scale structure of the Solar Wind from Interplanetary Scintillation measurements during the rising phase of cycle 23, Adv. Space. Res., this issue, 2001 Dennison, P.A., and A. Hewish, The solar wind outside the plane of the ecliptic, Nature, 213, pp.343-346, 1967 &all, R.R., Remote sensing observations of the solar wind near the Sun, Ph.D. Thesis, University of California, San Diego, 1995 Grall, R.R., M.T. Klinglesmith, W.A. Coles, A.R. Breen, P.J.S. Williams and R. Esser, Rapid acceleration of the polar solar wind, Nature, 379,429-432, 1996 Hewish, A., P.F. Scott and D. Willis, Interplanetary scintillation of small-diameter radio sources,Nature, 203, pp.1214-1217, 1964 Hewish, A., A user’s guide to scintillation, J. Atmos. Terr. Phys., 51, pp.743-750, 1989 Lazarus, A. J., The Day The Solar Wind Almost Disappeared, Science, 287, pp.2172-2173,200O Rickett, B., IPS observations of the solar wind velocity and microscale density irregularities in the inner solar wind, in Solar Wind 7, Proceedings of the 3rd COSPAR Colloqium held in Goslur, Germany, 16-20 September 1991, COSPAR Colloquia Series, Volume 3, E. Marsch and R. Schwenn, eds., Pergamon, pp.255-258, 1992 Sheeley, N.R. et al., Measurements of flow speeds in the corona between 2 and 30 R, Astrophys. J., 484,472-478, 1997 Tappin, S.J. and G.M. Simnett, in Proc. 31st ESIAB Symposium, ESA-SP-415, ed. A. Wilson, pp.1 17-121, 1998 Tappin, S.J., GM. Simnett and M.A. Lyons, A determination of the outflow speeds in the lower solar wind, Astron. Astrophys., 350, pp.302-309, 1999
Adv. Space Res. Vol. 30, No. 3, pp. 433-436.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/02$22.00+ 0.00 PII: SO273-1177(02)00339-3
SLOW AND FAST SOLAR WIND ACCELERATION SOLAR MAXIMUM A.R. Breen’, P. Thomasson’,
NEAR
C.A. Jordan2, S.J. Tappin3, R.A. Fallows’, A. Canals’, and P.J. Moran’
’ Physics Department, University of Wales, Aberystwyth, SY23 3BZ, Wales, UK ’ Jodrell Bank Observatory, University of Manchester, MacclesJield, Cheshire SK1 I 9DL, UK 3 Birmingham University, Edgbaston Park Road, Birmingham BI5 2TT UK
ABSTRACT
2-site measurements of interplanetary scintillation (IPS) provide measurements of solar wind speed in regions of the heliosphere which are otherwise inaccessible. We present results from coordinated observations made with the EISCAT and MERLIN facilities during 1999 and 2000, covering heliocentric distances from 7 to 80 solar radii (R) predominantly in the slow solar wind. The 1999 results are compared with optical measurements from LASCO covering 4-30 R. Most slow acceleration appears to take place between 5 and 10 R, but the slow wind continues to accelerate out to 25-35 R. Some of the observations included identifiable fast flow, and in these regions acceleration began lower down and was much more rapid, with 50% of cruising speed reached by 4-5 R and acceleration complete inside 10 R - results which are similar to those from solar minimum. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION
Observations of interplanetary scintillation (IPS), in which the diffraction pattern cast across the Earth when a distant compact radio source is observed through the solar wind have been used as a flow tracer in the solar wind for over 35 years [e.g. Hewish et al, 1964; Dennison and Hewish, 1967; Armstrong and Coles, 19721. If the phase changes introduced into the incident radio waves are small (“weak scattering”) then contributions to the observed scintillation pattern from different parts of the line of sight to the source can be treated separately [e.g. Grall et al., 1996; Breen et al., 19961. As the ray-path moves closer in to the Sun, the amplitude of the scintillation pattern increases until the inner limit of weak scattering is reached. Inside this limit, in the “strong scattering” regime, it is very difficult to determine the contributions of different regions to the observed scintillation pattern [e.g. Hewish, 1989; Grall, 19951. The distance from the Sun for the transition from weak to strong scattering depends on the frequency of the radio waves received: for EISCAT observing at 0.93 GHz the transition takes place at 20-30 solar radii (R) in the slow wind; for MERLIN observing at 5 GHz, the transition lies below 7 R. The first IPS observations using the EISCAT facility at a frequency of 0.93 GHz were made in 1982 [Bourgois et al., 19851 and a regular series of measurements of solar wind velocity have been made every summer since 1991. IPS observations were made at 1.66 GHz with MERLIN in 1985 and 1992 by Rickett [1992]. The later observations included some made at 5 GHz. Measurements from EISCAT and the VLBA (at 5 and 8 GHz) were used by Grail et al. [1996] to study the acceleration of the fast solar wind over the poles of the Sun in 1994 and 1995, revealing that the fast wind accelerated very rapidly and reached its “cruising” speed by lo-15 R. Sheeley et al.[1997], Tappin and Simnett [1998] and Tappin et al. [1999] have used data from the LASCO instrument on SoHO to investigate the speed of identifiable features in white-light data. The small discrete “blobs” in the slow wind studied by Sheeley et al. [1997] showed evidence of continuous acceleration from around 2 solar radii (R) out to beyond 20 R. Tappin et al. [1999] derived outflow velocities by cross-correlating white-light intensities at two radial distances. The results indicated that the slow solar wind had reached its cruising speed by 25-30 R, with most of the acceleration taking place inside lo-15 R. Determining velocities in the fast wind is more difficult, owing to the lower densities. Breen et al. [2OOOa]compared fast wind velocities measured by EISCAT and SoHO instruments including LASCO during September 1997, and although the optical and IPS datasets did not overlap in distance the results supported the Grall et al. [ 19961 conclusions. Breen et al. [2OOOb]compared slow wind velocities from May 1999 measured by applying the Tappin et al. [1999] cross-correlation method to LASCO C2 and C3 data with IPS observations from MERLIN and EISCAT. There was good agreement between the optical and IF’S velocities which suggested that irregularities of very different scale sizes (-10000 km for the optical features and -100 km for the features producing IPS) were drifting at the same speed. Solar wind densities around this period were anomalously low [Lazarus, 20001 and there was a possibility that the acceleration 433
434
A. R. Breen et al.
profile of the slow wind observed during this period was not representative of more typical conditions. Accordingly a second series of IPS observations using EISCAT and MERLIN were carried out in May 2000. The observations carried out in May 1999 and May 2000 were dominated by the slow wind, but significant coronal holes were present during both periods, giving rise to detectable fast flow. In this paper we discuss the observations of slow and fast wind velocities between 4 and 100 R during May 1999 and May 2000.
OBSERVATIONS The 1999 observations were made from EISCAT, MERLIN and LASCO between 2”d and 17* May and were coordinated so that the different instruments observed flow along similar streamlines of flow (within *lo” latitude and lying off the same limb). The May 2000 observations from EISCAT and MERLIN were made between 1 lth and 24 May 2000 off the south-west limb of the Sun and were coordinated in the same way as the 1999 observations, with flow along similar streamlines traced outwards from the corona. (a) USC0 observations (I999 only): The LASCO C2 and C3 instruments observed the solar corona off the south-east limb of the Sun for 12 hours per day on 91hand 1 lth May 1999, producing an image every 12 minutes. This was the finest time resolution possible when using C2 and C3 together. The results were divided into two overlapping distance ranges from the Sun and cross-correlated to obtain estimates of the flow speed. (b) MERLIN observations (1999 and 2000): The strong radio source 0318+164 (CTA-21), with its small angular size and well-studied structure, is an ideal source for high-frequency IPS observations of the innermost regions of the solar wind. In early May 1999 0318+164 lay off the south-east limb of the Sun, moving closer to the Sun and towards higher latitudes with time. The observations on 4’h May 1999 were made using the Cambridge and Knockin telescopes of MERLIN, operating at a central frequency of 1.66 GHz, a single polarisation (left circular) of equivalent bandwidth 10 MHz being sampled at 100 Hz. On 9* and 11” May 1999 the Cambridge and Jodrell Bank Mk.2 telescopes were used at a central frequency of 5 GHz. In mid-May 2000 0318+164 lay off the south-west limb of the Sun, moving away from the Sun and towards lower latitudes during the campaign interval. The source was observed at 5 GHz using the Cambridge and Knockin telescopes on 12’h May 2000 and with Knockin and Jodrell Bank Mk.2 on 15* and 16” May 2000.
Figure 1: (a)Ray-paths for EISCAT and MERLIN measurements of IPS during May 1999 projected back to 2.5 R using a constant-velocity ballistic model and overlaid on a map of white-light intensity constructed from LASCO C2 images at 2.5 R off the east limb. (b) Ray-paths for EISCAT and MERLIN measurements of IPS during May 2000, projected back to 2.5 R and overlaid on a west limb LASCO C2 white-light map in the same way as in Fig. la. Only data taken off the SE limb of the Sun in May 1999 and off the SW limb of the Sun in May 2000 are discussed in this paper. (c) EISCAT observations EISCAT made observations of IPS off the south-east limb of the Sun every day from 2”d May 1999 to 15h May 1999 and off the south-west limb between llth and 281h May 2000. The data were received on one polarisation over a 5 MHz bandwidth (8 MHz at the Tromso and Sodankyla sites in 1999), centred on 931.5 MHz. Initial sampling was at 10 kHz, with the data subsequently averaged to give a sample every 0.01 s.
435
Slow and Fast Solar Wind Acceleration Near Solar Maximum
RESULTS During May 1999 and May 2000 the corona was complex in structure, with multiple streamers occupying most of the region around the Sun. This would suggest that the solar wind would be mainly slow - as was indeed observed - but in during both months coronal holes extended across the southern hemisphere of the Sun, and these provided the source regions of narrow streams of fast solar wind [e.g. Breen et al., 20011. The ray-paths for EISCAT and MERLIN IPS observations during May 1999 and 2000 are projected down to 2.5 R and overlaid on maps of coronal white-light intensity in Figure la and lb respectively. Regions of the ray-paths lying above coronal holes are assumed to be immersed in fast streams of solar wind. This allows the velocities of the fast and slow streams to be detemined independently [e.g. Breen et al., 19961.
I
3
SoHO!LASCOC2 andC3 EISCAT 93I .5 MHz MERLIN 1.66GHz and5 GHz
0’8 (a)
.‘.30
I
a
5 ‘00 cx
0
MERLIN: 75” S
I
60 90 Heliocentricdistance (solar radii)
30
90 Heliocentric distance (solar radii)
(b)
60
I
Figure 2: (a) LASCO, MERLIN and EISCAT measurements of slow wind velocity between 4 R and 100 R, May 1999 and (b) MERLIN and EISCAT measurements of slow wind velocity between 7 R and 110 R, May 2000.
3ll P
v 0
1
EISCAT May 2000 EISCAT May 1999 MERLIN May 2000 MERLIN Mav 1999 SoHOlLASCO Mav 1999
10
Heliocentric distance (solar radii)
Figure 3: LASCO, MERLIN and EISCAT measurements of fast-component velocities between 4 and 100 R during May 1999 and 2000 Slow stream velocities from May 1999 and May 2000 are shown in Figure 2a and 2b respectively. The large uncertainties in flow speeds seen in some of these measurements - particularly during May 2000 - arise mainly from the difficulty in determining the exact boundaries of fast and slow streams. In several cases the ray-path lies above and along the boundary of a coronal hole and in these cases the observation will include contributions from regions with very different velocities. Nonetheless a clear trend is apparent in the data from both months, with velocities increasing from -100 km s-’ or less inside 7 R to reach a cruising speed in the region of 300-400 km s“ by 25-30 R.
CONCLUSIONS The slow wind results from the May 1999 and 2000 campaigns show the same pattern, with slow wind speed increasing from around 100 km s*’ at 10 R to 300-400 km s-’ by 25-30 R, with no detectable increase in velocity outside this distance. The irregularities observed by IPS and optical methods in 1999 were drifting at comparable speeds, and the acceleration profile reported for these data by Breen et al. [2OOOb]is consistent with that seen in the radio observations
436
A. R. Breen et al.
from 2000, suggesting that slow wind in early May 1999 was not atypical in its velocity profile. A significant spread in slow wind velocities is seen in both datasets, which we interpret as intrinsic variations in slow wind speed - the slow wind
is known to be much more variable in velocity and density than the fast wind. A more serious difftculty is the large uncertainty in some of the measurements, introduced when the ray-path for the observation lies partially above the boundary of a coronal hole and so passes through regions where steep velocity gradients are present. These difficulties are greater under near-solar maximum conditions when coronal holes are transient phenomena and are therefore difficult to
locate in maps constructed from white-light data over a solar rotation. More detailed modelling will be needed to reduce these uncertainties. The fast wind only occupied a small proportion of the ray-path in these observations, but nonetheless a weak fast component was detectable in the May 1999 LASCO and IRS data and in the May 2000 IFS data, as shown in figure 3. Once again the irregularities observed by LASCO and IPS had very similar drift velocities, suggesting that they were drifting at the background flow speed. Acceleration was much more rapid than in the slow wind, with 500 km s-t reached by 6-7 R and no detectable acceleration outside IO-12 R. These results provide the first set of measurements of solar maximum fast wind velocity covering the whole distance range from 4-100 R, and are consistent with those reported by Grail et al. [ 19961 at solar minimum.
ACKNOWLEDGEMENTS We would like to thank the director and staff of EISCAT for the 0.93 GHz IPS data used in this study. EISCAT is supported by the scientific research councils of Finland, France, Germany, Japan, Norway, Sweden and the U.K. We would like to give particular thanks to B.J. Rickett and W.A. Coles for making their experimental programmes, analysis routines and expertise available to us. MERLJN is a National Facility operated by the University of Manchester on behalf of PPARC. We would like to thank the Director and staff of Jodrell Bank for making available necessary additional equipment. The LASCO white-light data was made available by the LASCO consortium. SOHO is an ESA/NASA joint mission. Three of us (ARB, RAF, PJM) were supported by PPARC during the period when this work was carried out. AC was supported by the University of Wales.
REFERENCES Armstrong J.A., and W.A. Coles, Analysis of three-station interplanetary scintillation data, J. Geophys. Res., 77, pp. 4602-4610. 1972 Bourgois, G., W.A. Coles, G. Daigne, J. Silen, T. Turenen and P.J.S. Williams, Measurements of solar wind velocity using EISCAT, Astron. Astrophys., 144, pp. 452462, 1985 Breen, A.R., W.A. Coles, R.R. Grall, M.T. Klinglesmith, J. Markkanen, P.J. Moran, B. Tegid and P.J.S. Williams, EISCAT measurements of the solar wind, Ann. Geophys., 14, pp, 1235-1245, 1996 Breen, A.R., C.F. de Forest, B.J. Thompson, J.F. McKenzie, A. Modigliani, P.J. Moran, C.A. Varley and P.J.S. Williams, Comparisons of interplanetary scintillation and optical measurements of solar wind acceleration with model results, Advances in Space Research, 26(S), pp.781-784,200Oa Breen, A.R., S.J. Tappin, CA. Jordan, P. Thomasson, P.J. Moran, R.A. Fallows, A. Canals and P.J.S. Williams, Simultaneous Interplanetary Scintillation and Optical measurements of the acceleration of the Slow Solar Wind, Annales Geophysicue, 18, pp.995-1002,200Ob Breen, A.R., A. Canals, R.A. Fallows, P.J. Moran and M. Kojima, Large-scale structure of the Solar Wind from Interplanetary Scintillation measurements during the rising phase of cycle 23, Adv. Space. Res., this issue, 2001 Dennison, P.A., and A. Hewish, The solar wind outside the plane of the ecliptic, Nature, 213, pp.343-346, 1967 &all, R.R., Remote sensing observations of the solar wind near the Sun, Ph.D. Thesis, University of California, San Diego, 1995 Grall, R.R., M.T. Klinglesmith, W.A. Coles, A.R. Breen, P.J.S. Williams and R. Esser, Rapid acceleration of the polar solar wind, Nature, 379,429-432, 1996 Hewish, A., P.F. Scott and D. Willis, Interplanetary scintillation of small-diameter radio sources,Nature, 203, pp.1214-1217, 1964 Hewish, A., A user’s guide to scintillation, J. Atmos. Terr. Phys., 51, pp.743-750, 1989 Lazarus, A. J., The Day The Solar Wind Almost Disappeared, Science, 287, pp.2172-2173,200O Rickett, B., IPS observations of the solar wind velocity and microscale density irregularities in the inner solar wind, in Solar Wind 7, Proceedings of the 3rd COSPAR Colloqium held in Goslur, Germany, 16-20 September 1991, COSPAR Colloquia Series, Volume 3, E. Marsch and R. Schwenn, eds., Pergamon, pp.255-258, 1992 Sheeley, N.R. et al., Measurements of flow speeds in the corona between 2 and 30 R, Astrophys. J., 484,472-478, 1997 Tappin, S.J. and G.M. Simnett, in Proc. 31st ESIAB Symposium, ESA-SP-415, ed. A. Wilson, pp.1 17-121, 1998 Tappin, S.J., GM. Simnett and M.A. Lyons, A determination of the outflow speeds in the lower solar wind, Astron. Astrophys., 350, pp.302-309, 1999