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Imaging Solar Wind Scintillation

Submitter: Richard Fallows
Description: Interplanetary scintillation (IPS - the scintillation of a compact radio source due to density variations in the solar wind) is a phenomenon commonly used to observe the solar wind throughout the inner heliosphere. Typically the measurements are simple, consisting of intensity time series' recorded simultaneously from individual radio receivers, which can then be cross-correlated and modelled to find a number of solar wind parameters, such as the solar wind velocity(s) crossing the lines of sight.

However, the full weak-scattering model usually applied contains quite a number of parameters, several of which have to be assumed. Amongst these parameters are those describing the density variations themselves: Usually, these are assumed to be cigar-shaped and elongated in the direction of the interplanetary magnetic field, itself assumed to be radial in direction from the Sun. The solar wind velocity is also assumed to be radial in direction. However, this can change in the presence of a Coronal Mass Ejection (CME).

The many stations of LOFAR mean that it is possible to visualise and distentangle this information in a way that would be impossible otherwise. Individual snapshots from the movie are effectively u-v plots, put into radial and tangential directions from the Sun. Individual points are the values of the cross-correlation functions from different baselines for a certain time-lag, where the value of the positive time-lag is placed at the location corresponding to a positive value of the radial baseline, and the value of the equivalent negative time-lag is placed simultaneously at the location corresponding to a negative value of the radial baseline. A contour grid is then applied to all the data points and imaged.

The images show the spatial structure of the scintillation pattern itself. The main blob is a convolution of the solar wind density structure with that of the radio source; it is elongated in an off-radial direction corresponding to an interplanetary magnetic field structure which has been rotated due to the presence of a CME.

The whole blob moves in the radial direction with a speed corresponding to that of the CME. At a certain point it separates into two, the faster one is the CME, whereas the slower one is moving with the background solar wind and appears to be moving in an off-radial direction as a result of the CME passage.

This is probably the first time that IPS has ever been viewed in this way!
Copyright: ASTRON
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