| Abstrakt: |
We present a method to estimate electron densities in the ecliptic plane using the STEREO Heliospheric Imagers (HIs). The nature of Thomson scattering of photospheric light by solar wind electrons is such that visible‐light observations by HI provide a means to infer information about plasma density from afar. This is achieved using discrete tomography, whereby line‐of‐sight integrals from HI are used to estimate electron density in the heliosphere over a predefined grid. The technique is applied to the Earth‐impacting coronal mass ejection (CME) launched on 12 December 2008. The two vantage points afforded by STEREO are insufficient to reproduce the density structure of the CME in detail; however, the technique is successful in locating the presence of a density enhancement associated with the CME. When applied to consecutive images, we are able to use the technique as a means to track the CME through interplanetary space. From these observations we make estimates of the CME radial velocity and density profiles, the results of which are consistent with measurements made in situ by Wind at L1. This method is presented as a new approach to determine CME propagation from HI observations, one that avoids many of the assumptions of CME morphology and dynamics that are often applied when tracking CMEs in such data. We also expect that this technique will prove useful, and a test of the long‐standing heliospheric reconstruction technique employed by using single images over time (Jackson et al., 2006, https://doi.org/10.1029/2004JA010942) when views from many wide‐angle imagers become available. Plain Language Summary: Geomagnetic storms result from powerful eruptions on the Sun that travel through interplanetary space as magnetized plasma clouds and impact the Earth's magnetic field. It is beneficial to space weather forecasting to track these events between the Sun and Earth using images taken by the Heliospheric Imagers on board the National Aeronautics and Space Administration's twin STEREO spacecraft. Commonplace techniques to achieve this are to determine the front edge of these events in images from each spacecraft and then to triangulate their position. Successive images are then used to determine the plasma motion. Here, we present an alternative technique where the images from both spacecraft are combined to infer the plasma density structure in the region of space that is covered by cameras on both spacecraft. This is possible because the amount of light observed by each image pixel is determined by the total density of plasma it is observing. The information available from just two viewpoints is not enough to accurately reconstruct the plasma density. However, we are able to infer the presence of a dense plasma cloud and track it toward Earth. We make predictions about plasma speed and density and find that they are reasonably consistent with in situ observations from spacecraft near Earth. Key Points: A technique is demonstrated for producing density estimates of 12 December CME via inversion of 108 pairs of HI‐1 images from STEREO‐A and STEREO‐BThe CME morphology cannot be accurately reconstructed due to the limited information available from just two viewpointsThe CME is tracked over 3 days, and its speed and density extrapolated to L1 to validate the results against in situ data [ABSTRACT FROM AUTHOR] |
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