Magnetic holes in the solar wind between 0.3 AU and 17 AU

Magnetic holes (MHs) are depressions of the magnetic field magnitude. Turne r et al. (1977) identified the first MHs in the solar wind and determined a n occurrence rate of 1.5 MHs/d. Winterhalter et al. (1994) developed an aut omatic identification criterion to search for MHs in Ulysses data in the so lar wind between 1 AU and 5.4 AU. We adopt their criterion to expand the se arch 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 w hich the magnetic vector varies in magnitude rather than direction. Therefo re we focus on such linear MHs with a directional change less than 10 degre es. 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 20 degrees in average and shows a weak relation to the depth of the MHs. Th e angle between the initial magnetic field and the minimum variance directi on of those structures is large and very probably close to 90 degrees. The MHs are placed in a high beta environment even though the average solar win d shows a smaller beta. The widths decrease from about 50 proton inertial l ength 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 beta of the MH environments with respect to the helioce ntric distance. There is a clear preference for the occurrence of depressio ns instead of compressions. We discuss these results with regard to the mai n theories of MHs, the mirror instability and the alternative soliton appro ach. Although our observational results are more consistent with the solito n theory we favor a combination of both. MHs might be the remnants of initi al mirror mode structures which can be described as solitons during the mai n part of their lifetime.