The Whole Sun Month In-Situ working group has focussed on gathering the available data sets and identification of "manageable" research tasks. A great deal of the data were presented in the Sumannen et al. AGU poster. The goal of this poster was to attempt to identify the solar wind measurements with their solar source regions by projecting the solar wind measurements back to their source longitude using a ballistic (constant velocity) approximation. Of course, a strong high-speed region associated with the equatorial coronal hole was apparent at 1 AU in both WIND and SOHO data.
Two aspects of the heliospheric structure during WSM attracted the group's attention: the return of Ulysses to variable solar wind latitudes, and the prevalence of high-speed streams on the "quiet side" of the sun (see Carrington Longitudes 0 - 180 in the synoptic maps). Ulysses re-entered the slow-speed solar wind near the WSM period, having completed a pass over the north pole of the sun. During WSM, the spacecraft was located approximately 40 degrees behind the west limb of the sun and 27-29 degrees to the north of the solar equator. Pete Riley has done an analysis of this data set, and had two key results. First, a simple constant-velocity ballistic mapping of the solar wind at 4.2 AU does not represent a "realistic" corona at distances of a few solar radii. The interactions of adjacent regions of solar wind become more important the further the wind travels. In order to extrapolate the solar wind data back to the source longitude, it was necessary to apply a fluid dynamic code. Second, Ulysses saw no evidence of the equatorial coronal hole structure, implying that the craft was within the northern polar coronal hole boundary during its pass through the equatorial coronal hole longitude.
Alan Lazarus and John Steinberg brought up the question of the high-speed stream structure observed by WIND/SWE and SOHO/CELIAS during this period. As expected, the equatorial coronal hole was associated with high-velocity solar wind, while the "quiet side" of the sun, around Carrington Longitudes 0 - 180, had as close to solar minimum magnetic structure as can be expected. Still, the quiet side at a few solar radii is not necessarily the quietest at 1 AU or at 4.2 AU. Arik Posner demonstrated that most of the energetic electrons and protons observed by SOHO/COSTEP were associated with the steepening of these high-speed streams into corotating interaction region (CIR) shocks beyond 2 AU. Interestingly enough, the equatorial coronal hole had almost no energetic particle flux associated with it - it was the quietest part of the rotation!
Andy Breen's EISCAT measurements during this period also indicate possible regions of mixed solar wind speeds at these longitudes. The IPS (interplanetary scintillation) measurements allow the in-situ group a little freedom to leave the orbits to which we are confined, doing several scans at latitudes which represent the transition from the high-speed to the slow-speed solar wind.
The discussion then turned to identifying the source regions on the sun - could these CIR's be identified with coronal holes? Current sheet boundaries? Bends in the current sheets? Regions of changing polarity? People usually identify the source regions of slow-speed solar wind with boundaries between closed and opened field lines. Todd Hoeksema and Xuepu Zhao have tried to do a field extrapolation with MDI data which shows some evidence of one or two open field regions near the suspected source longitudes of the CIR's.
Mikic and Linker examined the results of their model to attempt to demonstrate which regions on the sun (polar or equatorial) corresponded to the in-situ measurements. There was again some success, possibly identifying other source regions, but there was still no clear reason for the large number of CIR's that were observed.
The group will focus on several pressing questions. First, can we identify a clear signature of these streams - high-speed running into slow-speed? There should be predictable density, buildup, velocity gradients, and deflection of radial flows due to this interaction. If some of the high-speed streams are transient, the gradient in velocity will be more in the radial direction than vxB direction, and the pressure/density structure should be different. Second, there are clear differences in composition and temperature between coronal hole solar wind and slow-speed wind. Do the high-speed streams have similar compositions/temperature to coronal hole regions? Third, we need to more closely compare our data sets, ensuring that each observation makes sense within the context of the others. Finally, as the modellers attempt to identify the source regions, are there diagnostics from the coronal instruments which support their findings?
Arik Posner's ESLAB symposium paper on CIR structure: Fluxes of MeV particles at Earth's Orbit and their Relationship with the Global Structure of the Solar Corona: Observations from SOHO
"Ulysses observations of the northward extension of the heliospheric current sheet" by Bob Forsyth, A. Balogh, E.J. Smith and J.T. Gosling
Jack Gosling's CIR review from 1997
Vic Pizzo's 3-D models, papers from 1997 and Solar Wind 8 conference and Pizzo, V. J., and J. T. Gosling, 3-D simulation of high-latitude interaction regions: Comparison with Ulysses results, Geophys. Res. Lett., 21, 2063-2066, 1994.
For energetic particle acceleration: Fisk and Lee, ApJ 237, 620, 1980.
Concerning the composition at CIRs : Wimmer et al paper, JGR, this year.
CIR's and sun-earth effects: A. H. McAllister and N. U. Crooker's "CME's, CIR's and Geomagnetic Storms," from Coronal Mass Ejections, AGU Geophysical Monograph Series 99, N. Crooker, J. A. Joselyn, and J. Feynman, eds., 1997.
CIR's and IPS: Andy Breen et al., Interaction Regions in The Solar Wind," Ann. Geo., in press 1998
IPS and mixed-velocity regions on the sun: Woo and Habbal [GRL, 24, 1159, 1997] and Habbal et al., Ap. J., Nov 1 1997.
To get a good idea of basic CIR structure,Schwenn, R., Large-scale structure of the interplanetary medium, in Physics of the Inner Heliosphere I, edited by R. Schwenn and E. Marsch, pp. 99-181, Springer-Verlag, New York, 1990.
Hundhausen, A. J., Coronal Expansion and Solar Wind, SpringerVerlag, New York, 1972.
Crooker, N. U., and E. W. Cliver, Postmodern view of M-regions, J. Geophys. Res., 99, 23,383-23,390, 1994.
Burlaga, L. F., Interplanetary stream interfaces, J. Geophys. Res., 79, 3717-3725, 1974.
Gosling, J. T., J. R. Asbridge, S. J. Bame, and W. C. Feldman, Solar wind stream interfaces, J. Geophys. Res., 83, 1401-1412, 1978.
