 |
| Titel |
The structure and origin of magnetic clouds in the solar wind |
| VerfasserIn |
V. Bothmer, R. Schwenn |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 16, no. 1 ; Nr. 16, no. 1, S.1-24 |
| Datensatznummer |
250013098
|
| Publikation (Nr.) |
 copernicus.org/angeo-16-1-1998.pdf |
|
|
|
|
|
| Zusammenfassung |
| Plasma and magnetic field data from the
Helios 1/2 spacecraft have been used to investigate the structure of magnetic
clouds (MCs) in the inner heliosphere. 46 MCs were identified in the Helios data
for the period 1974–1981 between 0.3 and 1 AU. 85% of the MCs were associated
with fast-forward interplanetary shock waves, supporting the close association
between MCs and SMEs (solar mass ejections). Seven MCs were identified as direct
consequences of Helios-directed SMEs, and the passage of MCs agreed with that of
interplanetary plasma clouds (IPCs) identified as white-light brightness
enhancements in the Helios photometer data. The total (plasma and magnetic
field) pressure in MCs was higher and the plasma-β lower than in the
surrounding solar wind. Minimum variance analysis (MVA) showed that MCs can best
be described as large-scale quasi-cylindrical magnetic flux tubes. The axes of
the flux tubes usually had a small inclination to the ecliptic plane, with their
azimuthal direction close to the east-west direction. The large-scale flux tube
model for MCs was validated by the analysis of multi-spacecraft observations.
MCs were observed over a range of up to ~60° in solar longitude in the ecliptic
having the same magnetic configuration. The Helios observations further showed
that over-expansion is a common feature of MCs. From a combined study of Helios,
Voyager and IMP data we found that the radial diameter of MCs increases between
0.3 and 4.2 AU proportional to the distance, R, from the Sun as R0.8
(R in AU). The density decrease inside MCs was found to be proportional to R–2.4,
thus being stronger compared to the average solar wind. Four different magnetic
configurations, as expected from the flux-tube concept, for MCs have been
observed in situ by the Helios probes. MCs with left- and right-handed magnetic
helicity occurred with about equal frequencies during 1974–1981, but
surprisingly, the majority (74%) of the MCs had a south to north (SN) rotation
of the magnetic field vector relative to the ecliptic. In contrast, an
investigation of solar wind data obtained near Earth's orbit during 1984–1991
showed a preference for NS-clouds. A direct correlation was found between MCs
and large quiescent filament disappearances (disparition brusques, DBs). The
magnetic configurations of the filaments, as inferred from the orientation of
the prominence axis, the polarity of the overlying field lines and the
hemispheric helicity pattern observed for filaments, agreed well with the in
situ observed magnetic structure of the associated MCs. The results support the
model of MCs as large-scale expanding quasi-cylindrical magnetic flux tubes in
the solar wind, most likely caused by SMEs associated with eruptions of large
quiescent filaments. We suggest that the hemispheric dependence of the magnetic
helicity structure observed for solar filaments can explain the preferred
orientation of MCs in interplanetary space as well as their solar cycle
behavior. However, the white-light features of SMEs and the measured volumes of
their interplanetary counterparts suggest that MCs may not simply be just
Hα-prominences, but that SMEs likely convect large-scale coronal
loops overlying the prominence axis out of the solar atmosphere. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|