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| Titel |
Magnetic holes in the solar wind between 0.3 AU and 17 AU |
| VerfasserIn |
K. Sperveslage, F. M. Neubauer, K. Baumgärtel, N. F. Ness |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 7, no. 3/4 ; Nr. 7, no. 3/4, S.191-200 |
| Datensatznummer |
250004257
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| Publikation (Nr.) |
 copernicus.org/npg-7-191-2000.pdf |
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| Zusammenfassung |
| 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. |
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