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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.

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We have analyzed the coronal dimmings for seven fast (> 600 km/s) coronal mass ejections (CMEs) occurring between 23 April and 9 May which were associated with flares from NOAA active region (AR) 8210. Each of these CMEs had at least one group of interplanetary radio bursts associated with them. These dimming regions were identified by their strong depletion in coronal EUV emission within a half hour of the estimated time of CME lift‐off. They included areas which were as dark as quiescent coronal holes as well as other regions with weaker brightness depletions. While the location of the active region and the associated flare did not correspond well with the coronagraph observations, we found that the extended dimming areas in these events generally mapped out the apparent “footprint” of the CME as observed by white‐light coronagraph. We briefly discuss the implications of these results on models of CME topology.

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The LASCO and EIT instruments on the SOHO spacecraft have provided an unprecedented set of observations for studying the physics of coronal mass ejections (CMEs). They provide the ability to view the pre-event corona, the initiation of the CME and its evolution from the surface of the Sun through 30 R ⊙ . An example of the capability of these instruments is provided in a description of a single event (Dere et al., 1997). During the first 2 years of operation of LASCO and EIT on SOHO, a substantial fraction, on the order of 25 to 50%, of the CMEs observed exhibited structure consistent with the ejection of a helical magnetic flux rope. An examples of these has been reported by Chen et al. (1997) and Dere et al. (1999). These events may be the coronal counterpart of magnetic clouds discussed by Burlaga et al.(1981) and Klein and Burlaga (1982). They analyzed observations of magnetic fields behind interplanetary shocks and deduced that the field topology was that of a helical flux rope.Recently, we have explored a number of the consequences of the helical flux rope description of these types of CMEs. Vourlidas et al. (1999) examined the energetics of CMEs with data from the LASCO coronagraphs on SOHO. The LASCO observations provide fairly direct measurements of the mass, velocity and dimensions of CMEs. Using these basic measurements, we determined the potential and kinetic energies and their evolution for several CMEs that exhibited a flux-rope morphology. Assuming magnetic flux conservation ('frozen-in' fields), we used observations of the magnetic flux in a variety of magnetic clouds near the Earth to determine the magnetic flux and magnetic energy in CMEs near the Sun. Figure 1 shows these quantities for a few representative flux rope CMEs. In general, we find that the potential and kinetic energies increase at the expense of the magnetic energy as the CME moves out, keeping the total energy roughly constant. This demonstrates that flux rope CMEs are magnetically driven. Furthermore, since their total energy is constant, the flux rope parts of the CMEs can be considered to be a closed system above ∼ 2 R ⊙ . Subramanian et al. (1999) examined images from LASCO to study the relationship of coronal mass ejections (CMEs) to coronal streamers. We wished to test the

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