Radial and latitudinal dependencies of discontinuities in the solar wind between 0.3 and 19 AU and −80° and +10°

Abstract

Abstract. Directional discontinuities (DD) from 5 missions at 7 different locations between 0.3 and 19 AU and −80 • and +10 • in the 3D heliosphere are investigated during minimum solar activity. The data are surveyed using the identification criteria of Burlaga (1969) (B) and Tsurutani and Smith (1979) (TS). The rate of occurrence depends linearly on the solar wind velocity caused by the geometric effect of investigating a larger plasma volume if the solar wind velocity v sw increases. The radial dependence is proportional to r −0.78 (TS criterion) and r −1.28 (B criterion), respectively. This dependence is not only due to an increasing miss rate with increasing distance. The DDs must be unstable or some other physical effect must exist. After normalization of the daily rates to 400 km/s and 1 AU, no dependence on heliographic latitude or on solar wind structures is observable. This means that the DDs are uniformly distributed on a spherical shell. Normalized 64 DD per day are identified with both criteria. But large variations of the daily rate still occur, indicating that other influences must exist. The ratio of the rates of rotational (RDs) and tangential discontinuities (TDs) depends on the solar wind structures. In high speed streams, relatively more RDs exist than in low speed streams. In the inner heliosphere (r < 10 AU), no radial or latitudinal dependence of the portions of the DD types occur. 55% clear RDs, 10% clear TDs and 33% EDs (either discontinuities) are observed, but the portions differ with regard to the criteria used. In the middle heliosphere (10 AU < r < 40 AU), the DD types are more uniformly distributed. The distribution of the directional change ω over the transition evolves to an increase of smaller ω with increasing distance from the sun. The evolution is yielded by the anisotropic RDs with small ω. The spatial thickness d km in kilometers increases with distance. The thickness d rg normalized to the proton gyro radius decreases by a factor of 50 between 0.3 and 19 AU, from 201.3 r g down to 4.3 r g . In the middle heliosphere, the orientation of the normals relative to the local magnetic field is essentially uniform except for the parallel direction where no DDs occur.Correspondence to: A. Söding (soeding@geo.uni-koeln.de) This indicates that RDs propagating parallel to B play a special role. In addition, in only a few cases is [v] parallel to [B/ρ], which is required by the MHD theory for RDs. The DDs have strongly enhanced values of proton gyro radius r g for ω ∼ 90 • . In contrast, in the inner heliosphere, only a small increase in r g with ω is observed.

Discontinuities and Alfvénic fluctuations in the solar wind

“…Söding et al (2001) reported R histograms from Helios 2 data obtained in 1976, peaking at R = 0.7. These values are all significantly lower than our averages, even if we had not applied an alpha-particle correction to the mass density, which increased our values by a factor of ∼ 1.08.…”

Alfvén waves in coronal holes: formation of discontinuities in inhomogeneous magnetic fields

“…Many single-spacecraft studies of DDs in the solar wind have been published (e.g., Burlaga 1969;Smith 1973;Martin et al 1978;Tsurutani & Smith 1979;Mariani et al 1983;Neugebauer 1989;Soding et al 2001): in most cases RDs or TDs have been found to be prevalent in fast-speed or in slow-speed streams, respectively. Using multiple (three or more) spacecraft data allows for determining the normal direction by considering the times of crossing the DD for each spacecraft (Burlaga & Ness 1969;Horbury et al 2001;Knetter et al 2003Knetter et al , 2004.…”

Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU

“…Since the first observation of the DD structure in the interplanetary magnetic field (IMF) [ Ness et al , 1966], many studies have been carried out [e.g., Burlaga , 1968; Burlaga and Ness , 1968; Burlaga et al , 1969; Tsurutani and Smith , 1979]. More recently, Söding et al [2001] studied the evolution of the DD in the heliosphere, and Knetter et al [2004] performed a multi‐satellite survey of DDs. However, it is notable that the relationship between the size of the direction change across these DDs and the depth of the magnetic depression has rarely been addressed, except in the early works of Burlaga, and by collaborators toward the end of 1960's and 1970's.…”