Evolution of the solar wind structure over a solar cycle: Interplanetary scintillation velocity measurements compared with coronal observations

Abstract

Sixteen years of solar wind observations via the technique of interplanetary scintillation 0PS) are presented. By an ecliptic comparison with in situ spacecraft observations, these data are shown to be valuable estimates of the large-scale slowly evolving structures in the solar wind speed, but to underestimate the speed in small-scale or rapidly evolving structures. These 1PS observations allow the large structures to be studied over solar latitudes from 60 ø north to 60 ø south over more than a solar cycle. The o•ervations are presented as half-yearly averages in Cartington longitude and latitude. These are compared with plots of coronal density estimated from coronameter observations and radial magnetic field estimated from solar magnetograph observations and the potential field method. •n low and declining solar activity, there is the expected relationship between fast wind and 1ow'clensity open-field regions, and between slow wind, dense corona and proximity to the neutral sheet. The dipolar field component wanders up to 30 ø from the rotation axis and is followed by dipolar distributions of density and velocity. Near solar maximum, average wind speeds are uniformly slow over the entire range of latitudes covered, the coronal density becomes more spherically distributed (though it retains a somewhat lower density over the solar poles), and the neutral sheet ranges over all latitudes and sometimes forms disconnected surfaces. The relationship between wind speed, density and angular separation from the neutral sheet are then broken. The results confirm the controtiing influence of the coroo•l magnetic field in dete•g the threedimensional structure of the solar wind. INTRODUCFIONIn the more than twenty years that spacecraft have been exploring the solar wind an enormous wealth of information has been acquired on its properties. However, the great bulk of the observations come from spacecraft in orbits near the ecliptic. Thus direct information on the solar wind out of the ecliptic comes only from a few spacecraft. In particular, Pioneer 11 and Voyager I have spent extended periods of time at latitudes in the range 10 ø to 20 ø solar latitude. Remote sensing techniques such as comet tail and radio propagation observations, in contrast, are not constrained in latitude but are more difficult to interpret as they provide less direct measurements of the basic plasma parameters. This paper presents the results of a long-i•'m study of the solar wind velocity using the method of interplanetary scintillations 0PS) and a comparison of the large-scale velocity structure with that of the coronal density and magnetic field. Comet observations have shown some latitudinal variation of solar wind speed [Brandt et al., 1975], but the sporadic nature of comet passages makes them difficult to use for a long-term study. Similarly, 2 years of early IPS observations at Cambridge, England showed conflicting results on wind speed versus latitude [Hewish and Symotuts, •969]. The results of the long-term IPS study at the University of C...

Heliospheric tomography using interplanetary scintillation observations: 1. Combined Nagoya and Cambridge data

“…Thus point P Carrington maps often only gave good information concentrated close to the heliographic equator and relied on the occasional solar pole crossing of a high ecliptic latitude source to provide information at other than the equator. Often the point P analysis was averaged over periods of 6 months [Rickett and Coles, 1991] to show general trends in the data especially at high solar latitudes. This is not as necessary using the tomographic modeling analyses.…”

“…Until now, these maps have been produced using the so-called "point P" approximation, where the material responsible for the observed phenomenon is assumed to be located at the point where each line of sight passes closest to the Sun [Coles et al, 1980;Rickett and Coles, 1991;. In this approximation each observation is related to a single solar latitude and longitude by tracing the point P to a given heliocenttic distance along its Archimedean spiral.…”

Long term variability in solar wind velocity and IMF intensity and the relationship between solar wind parameters & geomagnetic activity

“…He also found significant variation in B and V from one cycle to the next. Rickett and Coles (1991) observed that near solar maximum, average wind speeds were uniformly low. Bruno et al (1994) studied the changes in selected solar wind parameters at 1 AU through two solar activity cycles, separating the data into parts corresponding to fast and slow solar wind.…”

Latitudinal gradients of solar wind speed around the ecliptic: Systematic displacement of the streamer belt

“…[3] Polar coronal holes start growing after solar maxima and attain their largest extension towards the heliographic equator during the declining phase of the solar cycle [Newkirk and Fisk, 1985;Kojima and Kakinuma, 1990;Rickett and Coles, 1991]. Simultaneously, large latitudinal gradients in SW speed arise close to the equator between the fast SW of coronal holes and the slow SW of the streamer belt extending around the minimum speed locus.…”