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.

[1] The Solar and Heliospheric Observatory (SOHO) mission's white light coronagraphs have observed nearly 7000 coronal mass ejections (CMEs) between 1996 and 2002. We have documented the measured properties of all these CMEs in an online catalog. We describe this catalog and present a summary of the statistical properties of the CMEs. The primary measurements made on each CME are the apparent central position angle, the angular width in the sky plane, and the height (heliocentric distance) as a function of time. The height-time measurements are then fitted to first-and second-order polynomials to derive the average apparent speed and acceleration of the CMEs. The statistical properties of CMEs are (1) the average width of normal CMEs (20°< width 120°) increased from 47°(1996; solar minimum) to 61°(1999; early phase of solar maximum) and then decreased to 53°(2002; late phase of solar maximum), (2) CMEs were detected around the equatorial region during solar minimum, while during solar maximum CMEs appear at all latitudes, (3) the average apparent speed of CMEs increases from 300 km s À1 (solar minimum) to 500 km s À1 (solar maximum), (4) the average apparent speed of halo CMEs (957 km s À1) is twice of that of normal CMEs (428 km s À1 ), and (5) most of the slow CMEs (V 250 km s À1 ) show acceleration while most of the fast CMEs (V > 900 km s À1 ) show deceleration. Solar cycle variation and statistical properties of CMEs are revealed with greater clarity in this study as compared with previous studies. Implications of our findings for CME models are discussed.

Abstract. An earth-directed coronal mass ejection (CME) was observed on May 12, 1997 by the SOHO Extreme ultraviolet Imaging Telescope (EIT). The CME, originating north of the central solar meridian, was later observed by the Large Angle Spectrometric Coronagraph (LASCO) as a "halo" CME: a bright expanding ring centered about the occulting disk. Beginning at about 04:35 UT, EIT recorded several CME signatures, including dimming regions close to the eruption, post-eruption arcade formation, and a bright wavefront propagating quasi-radially from the source region. Each of these phenomena appear to be associated with the same eruption, and the onset time of these features corresponds with the estimated onset time observed in LASCO. We discuss the correspondence of these features as observed by EIT with the structure of the CME in the LASCO data.

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[1] Many broadside coronal mass ejections (CMEs) propagate almost radially beyond the first couple of solar radii, and their angular widths remain nearly constant while propagating through the corona. Assuming that these characteristics hold true for halo CMEs that originate far from solar limbs, some useful geometric and kinematic properties of halo CMEs may be reproduced using a simple geometrical model of a CME as a cone. The cone model uses three free parameters, characterizing the angular width and the central position of the halo CME. These geometric properties can be determined by matching the observed halos at a series of times with the modeled halos for a series of radial distances. The kinematic properties, the radial velocity and acceleration, of the halo CME can also be determined on the basis of the series of times and radial distances. These properties are important for predicting the geoeffectiveness of a halo CME and cannot be observed directly with currently available instrumentation. As a test, the geometric and kinematic properties of the 12 May 1997 halo CME have been inferred using the cone model. This shows that the cone model does provide a new way of testing our understanding of halo CMEs, though there are limitations for some halo CMEs.

Abstract. We report the properties of all the 841 coronal mass ejections (CMEs) observed by the Solar and Heliospheric Observatory (SOHO) Large Angle Spectroscopic Coronagraph (LASCO) C2 and C3 white-light coronagraphs from January 1996 through June 1998, and we compare those properties to previous observations by other similar instruments. Both the CME rate and the distribution of apparent locations of CMEs varied during this period as expected based on previous solar cycles. The distribution of apparent speeds and the fraction of CMEs showing acceleration were also in agreement with earlier reports. The pointing stability provided by an L-1 orbit and the use of CCD detectors have resulted in superior brightness sensitivity for LASCO over earlier coronagraphs; however, we have not detected a significant population of fainter (i.e., low mass) CMEs. The general shape of the distribution of apparent sizes for LASCO CMEs is similar to those of earlier reports, but the average (median) apparent size of 72 ø (50 ø ) is significantly larger. The larger average apparent size is predominantly the result of the detection of a population of partial and complete halo CMEs, at least some of which appear to be events with a significant longitudinal component directed along the SunEarth line, either toward or away from the Earth. Using full disk solar images obtained by the Extreme ultraviolet Imaging Telescope (EIT) on SOHO, we found that 40 out of 92 of these events might have been directed toward the Earth, and we compared the timing of those with the Kp geomagnetic storm index in the days following the CME. Although the "false alarm" rate was high, we found that 15 out of 21 (71%) of the Kp _> 6 storms could be accounted for as SOHO LASCO/EIT frontside halo CMEs. If we eliminate three Kp storms that occurred following LASCO/EIT data gaps, then the possible association rate was 15 out of 18 (83%). IntroductionThe dynamic processes taking place in the rarified atmosphere of our nearest star are sufficient motivation for many researchers to examine the Sun, the hellosphere, and planetary magnetospheres as plasma physics laboratories. But recent research connecting severe geomagnetic disturbances directly with coronal mass ejections from the Sun [e.g., Gosling, 1993] has renewed interest in a more systemic approach to the arcane specialties of solar and space physics (e.g., collection of This manuscript describes recent observations of coronal mass ejections near the Sun. These sporadic ejections of material through the Sun's atmosphere into interplanetary space can be detected remotely (both by imaging and by inference) at many wavelengths across the electromagnetic spectrum (e.g., X ray, EUV, Ha, and radio). Also the plasma, particle, and magnetic properties of ejected material can be measured in situ in the heliosphere. However, the phrase "coronal mass The understanding of the origin, observation, and effects of CMEs has benefited from significant effort during the past 25 years, and the reader is directed to any of the r...

Abstract. We investigate the global large amplitude waves propagating across the solar disk as observed by the SOHO/Extreme Ultraviolet Imaging Telescope (EIT). These waves appear to be similar to those observed in Ha in the chromosphere and which are known as "Moreton waves," associated with large solar flares [Moreton, 1960[Moreton, , 1964. Uchida [1968] interpreted these Moreton waves as the propagation of a hydromagnetic disturbance in the corona with its wavefront intersecting the chromosphere to produce the Moreton wave as observed in movie sequences of Ha images. To search for an understanding of the physical characteristics of these newly observed EIT waves, we constructed a three-dimensional, time-dependent, numerical magnetohydrodynamic (MHD) model. Measured global magnetic fields, obtained from the Wilcox Solar Observatory (WSO) at Stanford University, are used as the initial magnetic field to investigate hydromagnetic wave propagation in a three-dimensional spherical geometry. Using magnetohydrodynamic wave theory together with simulation, we are able to identify these observed EIT waves as fast mode MHD waves dominated by the acoustic mode, called magnetosonic waves. The results to be presented include the following: (1) comparison of observed and simulated morphology projected on the disk and the distancetime curves on the solar disk; (2) three-dimensional evolution of the disturbed magnetic field lines at various viewing angles; (3) evolution of the plasma density profile at a specific location as a function of latitude; and (4) computed Friedrich's diagrams to identify the MHD wave characteristics.

Abstract. A complete halo coronal mass ejection (CME) was observed by the SOHO Large-Angle and Spectrometric Coronagraph (LASCO) coronagraphs on May 12, 1997. It was associated with activity near Sun center, implying that it was aimed earthward. Three days later on May 15 an interplanetary shock and magnetic cloud/flux rope transient was detected at the Wind spacecraft 190 RE upstream of Earth. The long enduring southward magnetic fields associated with these structures triggered a geomagnetic storm. The CME was associated with a small coronal arcade that formed over a filament eruption with expanding double ribbons in Hot emission. The flare was accompanied by a circular EUV wave, and the arcade was flanked by adjacent dimming regions. We surmise that these latter regions marked the feet of a flux rope that expanded earthward into the solar wind and was observed as the magnetic cloud at Wind. To test this hypothesis we determined key parameters of the solar structures on May 12 and compared them with the modeled flux rope parameters at Wind on May 15. The measurements are consistent with the flux rope originating in a large coronal structure linked to the erupting filament, with the oppositepolarity feet of the rope terminating in the depleted regions. However, bidirectional electron streaming was not observed within the cloud itself, suggesting that there is not always a good correspondence between such flows and ejecta.