The structure and origin of magnetic clouds in the

Authors: Bothmer V Schwenn R

Publish Date: 1998/01/31

Volume: 16, Issue: 1, Pages: 1-24

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Abstract

Abstract 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 03 and 1 AU 85 of the MCs were associated with fastforward interplanetary shock waves supporting the close association between MCs and SMEs solar mass ejections Seven MCs were identified as direct consequences of Heliosdirected SMEs and the passage of MCs agreed with that of interplanetary plasma clouds IPCs identified as whitelight brightness enhancements in the Helios photometer data The total plasma and magnetic field pressure in MCs was higher and the plasmaβ lower than in the surrounding solar wind Minimum variance analysis MVA showed that MCs can best be described as largescale quasicylindrical 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 eastwest direction The largescale flux tube model for MCs was validated by the analysis of multispacecraft 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 overexpansion 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 03 and 42 AU proportional to the distance R from the Sun as R08 R in AU The density decrease inside MCs was found to be proportional to R–24 thus being stronger compared to the average solar wind Four different magnetic configurations as expected from the fluxtube concept for MCs have been observed in situ by the Helios probes MCs with left and righthanded 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 Earths orbit during 1984–1991 showed a preference for NSclouds 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 largescale expanding quasicylindrical 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 whitelight features of SMEs and the measured volumes of their interplanetary counterparts suggest that MCs may not simply be just Hαprominences but that SMEs likely convect largescale coronal loops overlying the prominence axis out of the solar atmosphere

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