[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 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.

[1] Full halo coronal mass ejections (CMEs) erupting from the side of the Sun facing Earth, i.e., frontside full halo CMEs, are considered to be a likely cause of major, transient geomagnetic storms. However, this hypothesis has not been tested over a full solar cycle. We compare all frontside full halo CMEs observed during the first half of solar cycle 23, from 1996 to the end of 2000, with moderate or larger storms at Earth. We show that the association of frontside full halo CMEs with such storms tends to decrease from 1997 to 2000, though this decreasing trend is not monotonic. We examine the locations of the frontside full halo CMEs from 1996 to 2000 with respect to two kinds of coronal closed field regions: bipolar closed field regions between opposite-polarity open field regions and unipolar closed field regions between like-polarity open field regions. We find that even during solar maximum when the occurrence frequency of the two kinds of regions is nearly the same, the central positions of the frontside full halo CMEs are mostly located under the bipolar coronal streamer belt, suggesting that most full halo CMEs originate in the bipolar coronal helmet streamers that are sandwiched between coronal holes having opposite magnetic polarity. Because the inclination of the heliospheric current sheet increases toward solar maximum, the fraction of CMEs emitted into the ecliptic decreases, and the inclination of associated flux ropes increases. These effects help to explain the solar cycle effect on the storm effectiveness of frontside full halo CMEs.

[1] The streamer belt region surrounding the heliospheric current sheet (HCS) is generally treated as the primary or sole source of the slow solar wind. Synoptic maps of solar wind speed predicted by the Wang-Sheeley-Arge model during selected periods of solar cycle 23, however, show many areas of slow wind displaced from the streamer belt. These areas commonly have the form of an arc that is connected to the streamer belt at both ends. The arcs mark the boundaries between fields emanating from different coronal holes of the same polarity and thus trace the paths of belts of pseudostreamers, i.e., unipolar streamers that form over double arcades and lack current sheets. The arc pattern is consistent with the predicted topological mapping of the narrow open corridor or singular separator line that must connect the holes and, thus, consistent with the separatrix-web model of the slow solar wind. Near solar maximum, pseudostreamer belts stray far from the HCS-associated streamer belt and, together with it, form a global-wide web of slow wind. Recognition of pseudostreamer belts as prominent sources of slow wind provides a new template for understanding solar wind stream structure, especially near solar maximum.

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