5.5.2 Ion Velocities
The ion tails exhibit large scale structure that is observed to move down the tail.
Tracking of these features has allowed study of the velocity and acceleration of the
ions in the tail. The results have been broadly confirmed by more recent high
resolution spectroscopic observations that have determined the Doppler shift of
specific lines. Feature tracking of the tail of 1P/Halley during the 1986 apparition
gave ion accelerations of around 3–4 m s
À2 such that ion velocities were
approaching 200 km s
À1 around 45 million kilometres from the nucleus (Celnik
and Schmidt-Kaler 1987). However, this is highly variable and probably strongly
dependent upon the solar wind conditions and the ion production rates.
5.5.3 Disconnection Events
The basic structure of the tail in steady-state can be understood from numerical
models (Fig. 5.15). However, solar wind conditions are rarely stable and it is now
clear that changes in the solar wind interaction with the comet can lead to spectacular
phenomena in the plasma tail. Disconnection events (e.g. Figure 5.15) give the
impression that the whole plasma tail is “disconnecting” from the head of the
comet and flowing away.
The Niedner-Brandt sequence describes four phases of a disconnection event
based upon observation (e.g. Brandt and Chapman 2004; see also Jia et al. 2007). In
phase I, the tail becomes narrower than usual, and tail rays can be observed. In phase
II, the tail breaks from the nucleus. In phase III, the old tail recedes from the nucleus
and a new tail forms. In phase IV, the old tail disappears and the comet resumes its
usual appearance.
In the mid-1980s the idea of solar wind magnetic sector boundary crossings
provoking disconnection events gained some traction. However, the number of
observed tail disconnection events is too high to be explainable solely by
interplanetary sector boundary crossings (Jockers 1985). Alternatively, increases in
the ambient solar wind pressure might result in plasma instabilities. We now know
that changes in solar wind conditions do have consequences. Vourlidas et al. (2007)
made the first direct imaging of the interaction of a comet (2P/Encke) with a coronal
mass ejection (CME) in the inner heliosphere with high temporal and spatial
resolution and observed a clear, correlated, disconnection of the tail. They concluded
that the disconnection is driven by magnetic reconnection between the magnetic field
entrained in the CME and the interplanetary field draped around the comet.
As noted above, disconnection is often accompanied by the appearance of quasilinear tail rays. Bundles of these structures may appear either roughly symmetrically
to the tail axis or on one side of the main tail only. C/1973 XII Kohoutek was studied
by Jockers (1985) and, in this case, one-sided rays were most frequently observed.
5.5 Spectroscopy and Imaging of Plasma Tails
423
The ion tails exhibit large scale structure that is observed to move down the tail.
Tracking of these features has allowed study of the velocity and acceleration of the
ions in the tail. The results have been broadly confirmed by more recent high
resolution spectroscopic observations that have determined the Doppler shift of
specific lines. Feature tracking of the tail of 1P/Halley during the 1986 apparition
gave ion accelerations of around 3–4 m s
À2 such that ion velocities were
approaching 200 km s
À1 around 45 million kilometres from the nucleus (Celnik
and Schmidt-Kaler 1987). However, this is highly variable and probably strongly
dependent upon the solar wind conditions and the ion production rates.
5.5.3 Disconnection Events
The basic structure of the tail in steady-state can be understood from numerical
models (Fig. 5.15). However, solar wind conditions are rarely stable and it is now
clear that changes in the solar wind interaction with the comet can lead to spectacular
phenomena in the plasma tail. Disconnection events (e.g. Figure 5.15) give the
impression that the whole plasma tail is “disconnecting” from the head of the
comet and flowing away.
The Niedner-Brandt sequence describes four phases of a disconnection event
based upon observation (e.g. Brandt and Chapman 2004; see also Jia et al. 2007). In
phase I, the tail becomes narrower than usual, and tail rays can be observed. In phase
II, the tail breaks from the nucleus. In phase III, the old tail recedes from the nucleus
and a new tail forms. In phase IV, the old tail disappears and the comet resumes its
usual appearance.
In the mid-1980s the idea of solar wind magnetic sector boundary crossings
provoking disconnection events gained some traction. However, the number of
observed tail disconnection events is too high to be explainable solely by
interplanetary sector boundary crossings (Jockers 1985). Alternatively, increases in
the ambient solar wind pressure might result in plasma instabilities. We now know
that changes in solar wind conditions do have consequences. Vourlidas et al. (2007)
made the first direct imaging of the interaction of a comet (2P/Encke) with a coronal
mass ejection (CME) in the inner heliosphere with high temporal and spatial
resolution and observed a clear, correlated, disconnection of the tail. They concluded
that the disconnection is driven by magnetic reconnection between the magnetic field
entrained in the CME and the interplanetary field draped around the comet.
As noted above, disconnection is often accompanied by the appearance of quasilinear tail rays. Bundles of these structures may appear either roughly symmetrically
to the tail axis or on one side of the main tail only. C/1973 XII Kohoutek was studied
by Jockers (1985) and, in this case, one-sided rays were most frequently observed.
5.5 Spectroscopy and Imaging of Plasma Tails
423