The structure and origin of magnetic clouds in the solar wind

Plasma and magnetic ®eld 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 identi®ed 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 identi®ed as direct consequences of Helios-directed SMEs, and the passage of MCs agreed with that of interplanetary plasma clouds (IPCs) identi®ed as whitelight brightness enhancements in the Helios photometer data. The total (plasma and magnetic ®eld) pressure in MCs was higher and the plasma-b lower than in the surrounding solar wind. Minimum variance analysis (MVA) showed that MCs can best be described as largescale quasi-cylindrical magnetic ̄ux tubes. The axes of the ̄ux tubes usually had a small inclination to the ecliptic plane, with their azimuthal direction close to the east-west direction. The large-scale ̄ux 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 con®guration. 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 R (R in AU). The density decrease inside MCs was found to be proportional to Rÿ2:4, thus being stronger compared to the average solar wind. Four di€erent magnetic con®gurations, as expected from the ̄ux-tube concept, for MCs have been observed in situ by the Helios probes. MCs with leftand 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 ®eld 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 ®lament disappearances (disparition brusques, DBs). The magnetic con®gurations of the ®laments, as inferred from the orientation of the prominence axis, the polarity of the overlying ®eld lines and the hemispheric helicity pattern observed for ®laments, 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 ̄ux tubes in the solar wind, most likely caused by SMEs associated with eruptions of large quiescent ®laments. We suggest that the hemispheric dependence of the magnetic helicity structure observed for solar ®laments can explain the preferred orientation of MCs in interplanetary space as well as their solar cycle behavior. However, the whitelight features of SMEs and the measured volumes of their interplanetary counterparts suggest that MCs may not simply be just Ha-prominences, but that SMEs likely convect large-scale coronal loops overlying the prominence axis out of the solar atmosphere.