ALBERT

Description: Magnetic holes (MHs) are depressions of the magnetic field magnitude. Turner et al. (1977) identified the first MHs in the solar wind and determined an occurrence rate of 1.5 MHs/d. Winterhalter et al. (1994) developed an automatic identification criterion to search for MHs in Ulysses data in the solar wind between 1 AU and 5.4 AU. We adopt their criterion to expand the search to the heliocentric distances down to 0.3 AU using data from Helios 1 and 2 and up to 17 AU using data from Voyager 2. We relate our observations to two theoretical approaches which describe the so-called linear MHs in which the magnetic vector varies in magnitude rather than direction. Therefore we focus on such linear MHs with a directional change less than 10º. With our observations of about 850 MHs we present the following results: Approximately 30% of all the identified MHs are linear. The maximum angle between the initial magnetic field vector and any vector inside the MH is 20º in average and shows a weak relation to the depth of the MHs. The angle between the initial magnetic field and the minimum variance direction of those structures is large and very probably close to 90º. The MHs are placed in a high β environment even though the average solar wind shows a smaller β. The widths decrease from about 50 proton inertial length in a region between 0.3 AU and 0.4 AU heliocentric distance to about 15 proton inertial length at distances larger than 10 AU. This quantity is correlated with the β of the MH environments with respect to the heliocentric distance. There is a clear preference for the occurrence of depressions instead of compressions. We discuss these results with regard to the main theories of MHs, the mirror instability and the alternative soliton approach. Although our observational results are more consistent with the soliton theory we favour a combination of both. MHs might be the remnants of initial mirror mode structures which can be described as solitons during the main part of their lifetime.

Description: 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 νsw 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 dkm in kilometers increases with distance. The thickness drg normalized to the proton gyro radius decreases by a factor of 50 between 0.3 and 19 AU, from 201.3 rg down to 4.3 rg. 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. This indicates that RDs propagating parallel to B play a special role. In addition, in only a few cases is [υ] parallel to [B / ρ], which is required by the MHD theory for RDs. The DDs have strongly enhanced values of proton gyro radius rg for ω ~ 90°. In contrast, in the inner heliosphere, only a small increase in rg with ω is observed.Key words. Interplanetary physics (discontinuities; interplanetary magnetic fields) – Space plasma physics (discontinuities)