The STEREO heliospheric imager: how to detect CMEs in the heliosphere
ABSTRACT The STEREO Heliospheric Imager is a wide-angle imaging system that will enable, for the first time, a view of Earth-directed coronal mass ejections (CMEs) in a field of view which also encompasses the Earth. Twin views from widely spaced platforms, combined with the out of Sun–Earth line perspective allow a unique and powerful tool for the study of CMEs and, particularly, Earth-directed CMEs. We outline the instrumental characteristics and image simulation studies which reveal the nature of the images we anticipate.
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ABSTRACT: Stereoscopic white-light imaging of a large portion of the inner heliosphere has been used to track interplanetary coronal mass ejections. At large elongations from the Sun, the white-light brightness depends on both the local electron density and the efficiency of the Thomson-scattering process. To quantify the effects of the Thomson-scattering geometry, we study an interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-light imaging. Identifiable as an inclined streak of enhanced brightness in a time-elongation map, the travelling shock can be readily imaged by an observer located within a wide range of longitudes in the ecliptic. Different parts of the shock front contribute to the imaged brightness pattern viewed by observers at different longitudes. Moreover, even for an observer located at a fixed longitude, a different part of the shock front will contribute to the imaged brightness at any given time. The observed brightness within each imaging pixel results from a weighted integral along its corresponding ray-path. It is possible to infer the longitudinal location of the shock from the brightness pattern in an optical sky map, based on the east-west asymmetry in its brightness and degree of polarization. Therefore, measurement of the interplanetary polarized brightness could significantly reduce the ambiguity in performing three-dimensional reconstruction of local electron density from white-light imaging.Solar Physics 05/2012; 285(1-2). · 3.26 Impact Factor
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ABSTRACT: The relationship of magnetic clouds (MCs) to interplanetary coronal mass ejections (ICMEs) is still an open issue in space research. The view that all ICMEs would originate as magnetic flux ropes has received increasing attention, although near the orbit of the Earth only about one-third of ICMEs show clear MC signatures and often the MC occupies only a portion of the ICME. We have performed a systematic comparison of the cases where ICME and MC signatures coincided and where ICME signatures extended significantly beyond the MC boundaries. We found clear differences in the ICME properties (eg., speed, magnetic field magnitude), in the ambient solar wind structure, and in the solar cycle dependence for these two event types. We show that the MC and the regions of ICME-related plasma in front and behind the MC have all distinct characteristics enforcing the conception that they have intrinsically different origin or evolve differently. Erosion of magnetic flux in front of the ICME may also reconfigure the initial three-part CME seen in white-light images to a more complex ICME, but the geometrical effect (i.e. the encounter through the CME leg and/or far from the flux rope center) has little contribution to the observed mismathch in the MC and ICME boundary times. We will also discuss ramifications to CME and space weather research.Annales Geophysicae 01/2013; 31:1251-1265. · 1.52 Impact Factor
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ABSTRACT: The combination of SDO and STEREO observations enables us to view much of the solar surface and atmosphere simultaneously and continuously. These near-global observations often show near-synchronous long-distance interactions between magnetic domains that exhibit flares, eruptions, and frequent minor forms of activity. Here we analyze a series of flares, filament eruptions, coronal mass ejections, and related events which occurred on 1-2 August 2010. These events extend over a full hemisphere of the Sun, only two-thirds of which is visible from the Earth's perspective. The combination of coronal observations and global field modeling reveals the many connections between these events by magnetic field lines, particularly those at topological divides. We find that all events of substantial coronal activity, including those where flares and eruptions initiate, are connected by a system of separatrices, separators, and quasi-separatrix layers, with little activity within the deep interiors of domains of connectivity. We conclude that for this sequence of events the evolution of field on the hemisphere invisible from Earth's perspective is essential to the evolution, and possibly even to the initiation, of the flares and eruptions over an area that spans at least 180 degrees in longitude. Our findings emphasize that the search for the factors that play a role in the initiation and evolution of eruptive and explosive phenomena, sought after for improved space weather forecasting, requires knowledge of much, if not all, of the solar surface field.Journal of Geophysical Research 04/2011; 116(A4):4108-. · 3.17 Impact Factor