A search for a general gradient in the solar wind speed at low solar latitudes

The large-scale solar wind velocity structure in the outer heliosphere has been systematically analyzed for Cartington rotations 1587-1541 (March 1972 to April 1976). Spacecraft data were taken from Imp 7/ 8 at earth, Pioneer 6, 8, and 9 near 1 AU, and Pioneer 10 and 11 between 1.6 and 5 AU. Using the constant radial velocity solar wind approximation to map all of the velocity data to its high coronal emission heliolongitude, we examined the velocity structure observed at different spacecraft for latitudinal dependence and compared it with coronal structure in soft X rays and Ha absorption features. The constant radial velocity approximation usually remains self-consistent in decreasing or constant velocity solar wind out to 5 AU, enabling us to separate radial from latitudinal propagation effects. We found several examples of sharp nonmeridional stream boundaries in interplanetary space (-.-5 ø latitude in width), often directly associated with features in coronal X rays and Ha. In one structure there is evidence for significant (up to 40 ø) nonradial flow of the plasma in the corona below the altitude of transition to super-Alfv6nic flow.

Section: Introductionmentioning

confidence: 57%

Latitude dependence of solar wind velocity observed ≳1 AU

Mitchell¹,

Roelof²,

Wolfe³

1981

“…However, this type of variation is probably of more than academic interest. Solar wind observations from 1974 to 1975 show a simple recurrent pattern of two high-speed streams imbedded in magnetic sectors of opposite polarity [Bame et al, 1977;Gosling et al, 1976;Svalgaard, 1977]. Hundhausen [1977] has suggested that this pattern reflects a spatial variation in solar wind speed similar to that assumed above, with a = 30 ø and # • 25ø; the belt of slow solar wind is centered on a magnetic neutral line encircling the sun but tilted at •30 ø to the solar equator.…”

Section: These Resultsmentioning

confidence: 99%

Solar wind spatial structure: The meaning of latitude gradients in observations averaged over solar longitude

Hundhausen¹

1978

The three-dimensional structure of the solar wind is often studied by searching for solar latitude variations in observed physical properties that have been averaged over solar longitude. The effects of longitude averaging are examined here by using general qualitative arguments and quantitative calculations for a simple class of assumed spatial variations in the solar wind speed. We show that unless the solar wind spatial structure is simply organized about the solar equator, its presence is extremely difficult to infer from longitude averages unless the observations extend to very high solar latitudes. Thus the absence of semiannual variations in ecliptic observations or the absence of large latitude gradients in radio scintillation or comet tail results do not constitute evidence against the presence of strong spatial variations (or large spatial gradients) in the solar wind.

“…For example, during the interval including January 1968 through May 1971 a gradient could not be found in data from earth-orbiting spacecraft [Diodato et al, 1974;Diodato and Moreno, 1976]. A study of the time interval 1964-1975 [Bame et al, 1977] showed the presence of an average gradient of solar wind speed with hellographic latitude during two parts of the overall period, but when the entire 12-year period was averaged, a general gradient was not apparent above background speed variations.…”

Section: Introductionmentioning

confidence: 97%

Direct observation of the latitudinal extent of a high‐speed stream in the solar wind

Schwenn¹,

Montgomery²,

Rosenbauer³

et al. 1978

Self Cite

108

Solar wind speeds measured from the Helios 1 solar probe between 0.31 and 1 AU and the earth‐orbiting Imp 7/8 satellites have been correlated with coronal holes as determined from K coronal brightness measurements. In March 1975 during perihelion passage, Helios 1 traversed the range of heliographic latitudes from −6° to +6° in a period of only 20 days. During that time the space probe crossed the northern boundary of the high‐speed stream associated with an equatorward extension of the south polar coronal hole. While this same stream continued to be observed by Imp satellites at −5° latitude, it was no longer observable from Helios 1 at +5° latitude. The conclusion is that sharp boundaries separate high‐speed flows from the surrounding solar wind. The thickness of the boundary in latitude appears to be narrower than about 10°. The local latitudinal gradient in flow speed is at least 30 km s−1 deg−1. With less confidence it can be deduced from the Helios 1 data that the actual gradients at 0.31 AU might even exceed 100 km s−1 deg−1.