A cometís tale
posted: January 28, 2015

Over the last few weeks, sky gazers have been enjoying a relatively bright and colorful comet in the northern skies. Comet Lovejoy, named after its discoverer Terry Lovejoy (Australia), has been the source of many beautiful pictures that can be admired on the web (see e.g. Spaceweather.com comet gallery).

The comet was closest to Earth on 7 January, passing above the ecliptic (the Sun-Earth plane) 2 days later which resulted in better viewing conditions for observers on the northern hemisphere. At the time, the comet was at a distance of about 200 million kilometers from the Sun (see diagram below). Many observers reported it even visible to the naked eye.

Interestingly, on 8 January, an abrupt change in the comet's ion tail could be observed. An ion tail forms when ultraviolet sunlight rips one or more electrons from gas atoms in the particle cloud around the comet's nucleus, turning them into ions (charged particles). The solar wind then carries these ions straight outward and away from the Sun along the magnetic field lines that are draped around the comet. This is why an ion tail is always pointing away from the Sun. Hence, it is no surprise that when there is a sufficiently strong disturbance in the solar wind, this tail can get detached from the comet. This is called a disconnection event (DE). Such a disconnection event occurred on 8 January (see annotated images below).

The intriguing question is if we can trace back the source of the perturbation in the solar wind. As it turns out, the culprit might have been the same curious, strong disturbance that created a major geomagnetic storm on 7 January at Earth (see this news item). Indeed, the comet was more or less in the same pathway of the disturbance as the Earth (see the diagram above). At the observed speed of 480-500 km/s at Earth, it would take about 28-30 hours for the solar wind to transit the remaining 50 million kilometers to the comet. This would put the timing of the encounter of the disturbance with the comet somewhere between 08:00 and 09:00UT on 8 January, nicely fitting Nick Howes 11:15 UT image, taken just after the DE had started. The magnetic field of the disturbance had also enough enough punch to detach the comet's tail. Its magnetic field was twice as strong than that of passing high speed streams from coronal holes (e.g. a component of resp. 20 nT vs. 10 nT). See the annotated diagram underneath.

Credits - The original images of the comet can be found at the Spaceweather.com comet gallery: Michael Jäger, Nick Howes, and Rolando Ligustri.