Rapid changes observed in a comet’s plasma tail

Team of researchers from National Astronomical Observatory of Japan, Stony Brook University (The State University of New York) and Tsuru University, reported highly resolved find details of this comet captured in B-band in 2013 (Subaru Telescope's Image Captures the Intricacy of Comet Lovejoy's Tail). They used I-band filter which includes H₂O+ line emissions and V-band filter which includes CO+ and H₂O+ line emissions. During the observations, the comet exhibited very rapid changes in its tail in the course of only 20 minutes. Such extreme short-term changes are the result of the comet's interactions with the solar wind where charged particles constantly sweeping out from the Sun. The reason for the rapidity of these changes is not well understood.

Dr. Jin Koda, the principal investigator of these nights, says "My research is on galaxies and cosmology, so I rarely observe comets. But Lovejoy was up in the sky after my targets are gone on our observing nights, and we started taking images for educational and outreach purposes. The single image from the previous night revealed such delicate details along the tail, it inspired us further to take a series of images on the following night. As we analyzed the images, we realized that the tail was displaying rapid motion in a matter of only a few minutes! It was just incredible!"

The plasma tail of a comet forms when gas molecules and atoms coming out from the comet encounter the solar wind. Changes and disturbances in the solar wind can affect the behavior and appearance of this plasma tail, causing it to form clumps of ionized material. The material in the plasma tail departed from the comet's coma and floats away on the solar wind. At these times, the plasma tail can take on a "kinked" or twisted look.

A good candidate for the detailed study must be a bright comet with an orbit that takes it close enough to the Sun to form a plasma tail. In addition, the best viewing angles for astronomers to capture views of plasma tail changes occur when the comet also approaches close to Earth. As a result, comets that allow good viewing of the plasma tail are relatively rare -- about one or two per year. During its passage, the Comet Lovejoy's plasma tail was almost perpendicular (83.5 degrees) to the line of sight from Earth. That made it a prime candidate for close-up observations of its plasma tail structure using Suprime-Cam.

Another discovery is that clumps located in the plasma tail at about 300 thousand kilometers from the nucleus moved fairly slow speed at about 20-25 kilometers per second. That is much slower than reported in other comets, such as P/Halley, which gave off clumps that moved as fast as 58 kilometers per second or the value 44 +/- 11 kilometers per second as derived from several bright comets in the past.

The speed of the solar wind ranges from 300 to 700 kilometers per second and the intensity and velocity that the comet encounters depends on where it is located with respect to the Sun. The solar wind helps to accelerate the clumps in the tail out away from the Sun. Eventually the clumps in the comet's tail reach this high speed. The observation team thinks they witnessed the beginning of the acceleration of the clumps by the solar wind.

It is still under study how these ion clumps form and what parameters determine the initial speed of them. Dr. Masafumi Yagi, the first author of the paper noted "Comets are often observable only during the twilight as they come near the Sun. On the other hand, it becomes difficult to observe faint objects like galaxies during the twilight hours because of the brighter sky background. Well designed telescope scheduling like this case makes an effective use of the Subaru Telescope's time and will enable us to collect more data of comets when the opportunity arises in the future."

Their research paper titled "Initial Speed of Knots in the plasma Tail of C/2013 R1 (Lovejoy)" will be published in Astronomical Journal in its March 2015 issue.