SOHO Joint Observations Programme 93 ==================================== Acceleration Profile of the Slow Solar Wind ___________________________________________ (Coordinated study for Second Whole Sun Month Campaign) Authors: - Andy Breen (EISCAT), Rainer Schwenn (LASCO) _______ Max-Planck Institut für Aeronomie, Katlenburg-Lindau, D-37189, Germany - Alice Lecinski (HAO Mk.3) High-Altitude Observatory, Boulder, Colorado Submitted: 18 May 1998 Instruments: LASCO/EISCAT/HAO Mk.3 coronagraph 1. Background _____________ One of the primary missions of SOHO is to study the origin and acceleration of the solar wind. Considerable progress has been made towards measuring the acceleration profile of the fast wind (e.g. Grall et al, 1996; SOHO JOP 068), but the slow solar wind poses its own problems. Observations with the LASCO C2 and C3 coronagraphs show the existance of moving structures in the slow wind. Sheeley et al (1997) suggested that these could be used as flow tracers for the slow wind, the moving intensity features being carried 'like leaves in the wind'. The results of the measurements made by Sheeley et al suggested that the slow wind accelerated at a constant velocity of around 4 m s-2 though most of the 30 R field of view of the LASCO instrument. Velocities of ~300 km s-1 were present by 30 R, although there was considerable scatter in the data. Measurements of slow wind velocity using two-site radio-scattering methods (Interplanetary Scintillation or IPS) suggest that two populations of slow wind may exist - a "fast-accelerating" component which reaches its final velocity inside 25 R and a "late-accelerating" component with an acceleration profile similar to that of the intensity enhancements seen in LASCO results (e.g. Breen et al 1996a, 1996b). Direct comparison of IPS and optical measurements of slow wind speed will, by allowing direct comparisons of measurements made simultaneously, remove a number of the uncertainties remaining from these earlier observations. Both IPS and optical-difference measurements use irregularities in the solar wind as flow tracers but the scale sizes of the irregularities are very different (~100 km for IPS, ~1 R for the LASCO optical difference measurements). The small-scale irrelgularities to which IPS is sensitive are likely to be accelerated up to the bulk solar wind speed more rapidly than the large irregularities seen by LASCO, but are also more likely to be biased upwards by the effects of Alfvén wave propagation (e.g. Klinglesmith, 1997). As the flux of Alfvén waves in the slow wind is reletively low and as the region sampled by EISCAT (generally outside 20 R) lies outside the Alfvén point of the slow wind the IPS mesurements should provide an accurate estimate of the bulk velocity of the solar wind. The overlap of the EISCAT and LASCO C3 fields of view will permit direct comparisons and intercalibration of velocities seen by the two instruments, while the overlap of the LASCO C3 and C2 fields of view will allow the calibrated vel The combination of overlapping EISCAT and LASCO measurements make this JOP a powerful tool for measuring the acceleration profile of CMEs. 2. Overview of Observing Programme __________________________________ Scheduling This JOP should be run during the Second Whole Sun Month period (1-28 August 1998), when other SOHO and ground-based instruments will provide supporting datasets. EISCAT measurements are restricted to cetain alignments of the receiving sites reletive to the Sun, so the best time to run this JOP would be when the greatest number of low-latitude radio sources are observable close to the Sun. On this basis, the best time for this JOP would be on 8-10 August, when 5-6 good low-latitude radio sources are observable by EISCAT each day, with at least two sources lying inside 33 R. Whenever possible, multiple observations of these sources will be made. A list of the proposed EISCAT observations is attached (table 1). LASCO C2 and C3 coverage is requested on 8, 9, 10 August. LASCO observing periods should begin ~7 hours before the start of the EISCAT observations and cover as many as possible of the EISCAT observations. Date Source flux Dist.(R)Lat Long Limb Start Stop Sites (Jy) -of closest approach-- ---- UT ---- --------to Sun---------- ------------------------------------------------------------------------------------- 980808 0854+201 1.6 21 31 1 W 0622 0637 KS 0830+241 0.6 47 28 1 W 0637 0652 KS 0842+185 1.4 29 7 358 W 0822 0837 KS 0832+185 0.9 37 3 258 W 0852 0907 KS 1008+075 6.3 59 -15 212 E 0955 1010 TS 0854+201 1.6 22 30 358 W 1207 1222 TK 0801+142 2.5 64 -15 353 W 1307 1322 KS 0954+177 1.1 38 25 193 E 1420 1435 KS 0854+201 1.6 23 29 355 W 1800 1815 KS 980809 0854+201 1.6 25 27 348 W 0622 0637 KS 0842+185 1.4 32 7 345 W 0815 0830 KS 0832+185 0.9 40 3 348 W 0850 0905 KS 0854+201 1.6 25 27 346 W 0905 0920 TS 1008+075 6.3 56 -16 197 E 0950 1005 TS 0854+201 1.6 25 27 345 W 1222 1237 TK 0954+177 1.1 35 28 178 E 1440 1455 KS 0854+201 1.6 26 26 341 W 1800 1815 KS 980810 0854+201 1.6 28 24 334 W 0645 0700 KS 0842+185 1.4 36 6 332 W 0815 0830 KS 0832+185 0.9 44 3 332 W 0845 0900 KS 0854+201 1.6 28 24 333 W 0915 0930 TS 1008+075 6.3 53 -17 182 E 0940 0955 TS 1042+120 3.2 74 9 191 E 1022 1037 KS 0854+201 1.6 29 24 331 W 1237 1252 TK 0954+177 1.1 32 31 162 E 1505 1520 KS 0854+201 1.6 29 23 328 W 1800 1815 KS Table 1: EISCAT observations during proposed JOP Instrument Participation ------------------------ Participation is required from EISCAT and LASCO (C2 and C3). MDI data will aid in interpretation of the results as it can be used to constrain coronal expansion models (e.g. Mikic and Linker, 1996; Linker et al, 1998; Breen et al, 1998b) but it is anticipated that these data will be available as part of the WSM campaign. LASCO (contact Rainer Schwenn) EISCAT (contact Andy Breen) Mk.3 (contact Alice Lecinski) Detailed Observing Plan ----------------------- LASCO C1, C2 and C3 coronagraphs should run in standard synoptic mode, producing images every 30 minutes (C1) or 60 minutes (C2, C3). The HAO Mk.3 coronagraph will also run in normal synoptic mode. EISCAT will observe 8-9 sources/day, in accordance with the timetable given above. The observations during the period of this JOP form a sub-section of an on-going comparison of EISCAT and LASCO measurements of the slow wind. EISCAT will be making measurements of solar wind velocity almost every day from 1998/05/16 to 1998/09/30 while synoptic LASCO measurements will be made throughout the summer. References __________ Breen, A.R., W.A. Coles, R.R. Grall, U.-P. Løvhaug, J. Markkanen, H. Misawa and P.J.S. Williams, EISCAT measurements of Interplanetary Scintillation, J. Atmos. Terr. Phys., 58, pp.507-519, 1996a 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, 1996b Breen, A.R., Z. Mikic, J.A. Linker, A.J. Lazarus, B.J. Thompson, P.J. Moran, C.A. Varley, P.J.S. Williams, D.A. Biesecker and A. Lecinski, Interplanetary scintillation measurements of the Solar Wind: linking coronal and in-situ observations, submitted to J. Geophys. Res., March 1998 Grall, R.R., W.A. Coles, M.T. Klinglesmith, A.R. Breen and P.J.S. Williams, Rapid acceleration of the Polar Solar Wind, Nature, 379, pp.429-432, 1996 Linker, J.A., Z. Mikic, D.A. Biesecker, R.J. Forsyth, A.J. Lazarus, A. Lecinski, P. Riley, A. Szarbo and B.J. Thompson, Magnetohydrodynamic modelling of the solar corona during Whole Sun Month, submitted to J. Geophys. Res., Febuary 1998 Mikic, Z. and J.A. Linker, The large-scale structure of the solar corona and inner heliosphere, Solar Wind Eight - A.I.P. Conference Proceedings, 382, p.104, 1996 Sheeley, N.R., Y.-M. Wang, S.H. Hawley, G.E. Brueckner, K.P. Dere, R.A. Howard, M.J. Koomen, C.M. Korendyke, D.J. Michels, S.E. Paswaters, D.G. Socker, O.C. St. Cyr and D. Wang, Measurements of Flow Speeds in the Corona between 2 and 30 R, Astrophys. J., 484, p.472, 1997