The STEREO heliospheric imager: how to detect CMEs in the heliosphere

Advances in Space Research (Impact Factor: 1.18). 01/2005; 36(8):1512-1523. DOI: 10.1016/j.asr.2005.01.024

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: On 15-17 February 2008, a CME with an approximately circular cross section was tracked through successive images obtained by the Heliospheric Imager (HI) instrument onboard the STEREO-A spacecraft. Reasoning that an idealised flux rope is cylindrical in shape with a circular cross-section, best fit circles are used to determine the radial width of the CME. As part of the process the radial velocity and longitude of propagation are determined by fits to elongation-time maps as 252±5 km/s and 70±5° respectively. With the longitude known, the radial size is calculated from the images, taking projection effects into account. The radial width of the CME, S (AU), obeys a power law with heliocentric distance, R, as the CME travels between 0.1 and 0.4 AU, such that S=0.26 R0.6±0.1. The exponent value obtained is compared to published studies based on statistical surveys of in situ spacecraft observations of ICMEs between 0.3 and 1.0 AU, and general agreement is found. This paper demonstrates the new opportunities provided by HI to track the radial width of CMEs through the previously unobservable zone between the LASCO field of view and Helios in situ measurements.
    Annales Geophysicae 01/2009; · 1.52 Impact Factor
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    ABSTRACT: We discuss features of coronal mass ejections (CMEs) that are specific to heliospheric observations at large elongation angles. Our analysis is focused on a series of two eruptions that occurred on 2007 January 24-25, which were tracked by the Heliospheric Imagers (HIs) onboard STEREO. Using a three-dimensional (3-D) magneto-hydrodynamic simulation of these ejections with the Space Weather Modeling Framework (SWMF), we illustrate how the combination of the 3-D nature of CMEs, solar rotation, and geometry associated with the Thomson sphere results in complex effects in the brightness observed by the HIs. Our results demonstrate that these effects make any in-depth analysis of CME observations without 3-D simulations challenging. In particular, the association of bright features seen by the HIs with fronts of CME-driven shocks is far from trivial. In this Letter, we argue that, on 2007 January 26, the HIs observed not only two CMEs, but also a dense corotating stream compressed by the CME-driven shocks. Comment: 5 pages, 2 figures, accepted for ApJ Letter
    The Astrophysical Journal 08/2008; · 6.73 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


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