We report the electron density in a plasma tail of Comet ISON (C/2012 S1) derived from interplanetary
scintillation (IPS) observations during November 1–28, 2013. Comet ISON showed a well-developed
plasma tail (longer than 2:98 107 km) before its perihelion passage on November 28. We identified a
radio source whose line-of-sight approached the ISON’s plasma tail in the above period and obtained
its IPS data using the Solar Wind Imaging Facility at 327 MHz. We used the Heliospheric Imager onboard
the Solar-Terrestrial Relation Observatory to distinguish between the cometary tail and solar eruption
origins of their enhanced scintillation. From our examinations, we confirmed three IPS enhancements
of a radio source 1148–00 on November 13, 16, and 17, which could be attributed to the disturbance
in the cometary tail. Power spectra of 1148–00 had the steeper slope than normal ones during its occultation
by the plasma tail. We estimated the electron density in the ISON’s plasma tail and found 84 cm3
around the tail axis at a distance of 3:74 107 km from the cometary nucleus and an unexpected
variation of the electron density in the vicinity of the tail boundary