Mike,
The faint tail of this comet consists of gas which has come from the nucleus and which has been ionised by sunlight. This ionised gas is called a plasma and it is affected by the magnetic field that is carried along by the solar wind. Gusts in the solar wind can compress the magnetic field lines in the ion tail so that they effectively short circuit releasing a lot of energy that can sever the tail from the head.
The image below is from tonight and it has a field of view of 2.3×1.7deg. There is a lot of structure in the ion tail and this provides a direct measurement of the interplanetary magnetic field in the vicinity of the comet.
Nick.
