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Solar wind through a comet’s tail
This data plot uses solar wind speed and direction data from the Solar Wind Analyser’s proton and alpha sensor (SWA-PAS) to estimate how close the ESA/NASA Solar Orbiter spacecraft approached to the centre of Comet C/2021 A1 Leonard’s ion tail during December 2021. The plot records how close each packet of solar wind detected by SWA-PAS is thought to have got to the comet’s nucleus during its journey from the Sun to the spacecraft. The left axis gives the scale in astronomical units (au), where 1 au is the distance from the Sun to the Earth, and the same distance is shown in kilometres on the right axis.
Changes in the solar wind flow speed and direction are responsible for the variations in the plotted distance. There are short data acquisition gaps on 15 and 17 December.
Data like these can help determine the timing of the tail crossing. Comet ion detections from another SWA sensor (the Heavy Ion Sensor) lasted several days, demonstrating that the tail was millions of kilometres across. Homing in on the mid-point of the tail-crossing based on the first analysis puts it from the end of 16 December (according to the data plot shown here) to 1200-1300 UT on 17 December (based on the preliminary analysis of signatures from other instruments).
