The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a five telescope package, which has been developed for the Solar Terrestrial Relation Observatory (STEREO) mission by the Naval Research Laboratory (USA), the Lockheed Solar and Astrophysics Laboratory (USA), the Goddard Space Flight Center (USA), the University of Birmingham (UK), the Rutherford Appleton Laboratory (UK), the Max Planck Institute for Solar System Research (Germany), the Centre Spatiale de Leige (Belgium), the Institut d'Optique (France) and the Institut d'Astrophysique Spatiale (France). SECCHI comprises five telescopes, which together image the solar corona from the solar disk to beyond 1 AU. These telescopes are: an extreme ultraviolet imager (EUVI: 1-1.7 R ), two traditional Lyot coronagraphs (COR1: 1.5-4 R and COR2: 2.5-15 R ) and two new designs of heliospheric imagers (HI-1: 15-84 R and HI-2: 66-318 R ). All the instruments use 2048 × 2048 pixel CCD arrays in a backside-in mode. The EUVI backside surface has been specially processed for EUV sensitivity, while the others have an anti-reflection coating applied. A multi-tasking operating system, running on a PowerPC CPU, receives commands from the spacecraft, controls the instrument operations, acquires the images and compresses them for downlink through the main science channel (at compression factors typically up to 20×) and also through a low bandwidth channel to be used for space weather forecasting (at compression factors up to 200×). An image compression factor of about 10× enable the collection of images at the rate of about one every 2-3 minutes. Identical instruments, except for different sizes of occulters, are included on the STEREO-A and STEREO-B spacecraft.

Abstract. 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 0X8 (R in AU). The density decrease inside MCs was found to be proportional to R À2X4 , 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 coun...

Abstract. Coronal mass ejections (CMEs) are a direct consequence of the dynamic nature of the solar atmosphere. They represent fundamental processes in which energy is transferred from the Sun into interplanetary space, including geospace. Their origin, 3D structure and internal magnetic field configuration are to date not well understood. The SOHO spacecraft, launched by the end of 1995, has provided unprecedented data on CMEs since instruments switched on in 1996. From a detailed investigation of the full set of LASCO (Large Angle Spectroscopic Coronagraph) observations from 1996 to the end of 2002, a set of structured CME events has been identified, which exhibits white-light fine structures likely indicative of their internal magnetic field configuration and possible 3D structure. Their source regions in the low corona and photosphere have been inferred by means of complementary analyses of data from the Extreme-Ultraviolet Imaging Telescope (EIT) and Michelson Doppler Imager (MDI) on board SOHO, and ground-based Hα measurements. According to the results of this study, structured CMEs arise in a self-similar manner from pre-existing small scale loop systems, overlying regions of opposite magnetic polarities. From the characteristic pattern of the CMEs' source regions in both solar hemispheres, a generic scheme is presented in which the projected white-light topology of a CME depends primarily on the orientation and position of the source region's neutral line on the solar disk. The paper also provides information about the white-light characteristics of the analysed CMEs, such as angular width and position angle, with respect to their source region properties, such as heliographic location, inclination and length, including the frequency and variation of these parameters over the investigated time period.