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Astron. Astrophys. 336, 769-775 (1998)

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4. Results and discussion

While low-amplitude oscillations seen through the broad Gunn g filter can be explained with artefacts, a genuine oscillation can be observed in the stimulated emission of the C3 molecular line (using a Strömgren v filter). The relative amplitude of this fluctuation is about 3 % (peak to peak) and its range of periods lies on the time scale of minutes. It can be observed in the ion tail as well as in the comet's coma where the increase of power in the vicinity of the comet's nucleus is steeper than the local intensity gradient. Both in period and amplitude this signal is remarkably similar to the one obtained with different methods from the head of comet 1973f (Kohoutek) by Isserstedt & Schlosser (1975). This similarity seen in signals from two different comets' comae suggests that the oscillations have a common origin and are thus probably induced by the solar wind. Nevertheless, the frequencies of the power peaks apparently differ between coma and tail of Hale-Bopp (Fig. 6). In contrast, this might indicate that we see the comet's response to the trigger rather than the trigger frequency itself. However, we should remember that we deal with data sets from different days which are certainly subject to severe statistical effects. Although we note that the peak frequencies are stable over the whole field of view (Fig. 8), it is known from solar observations that even the rather sharp p-mode peaks broaden and display power spectra like those in Fig. 6 when observed only over a short duration.

Thomson et al. (1995) have claimed to observe frequencies equal to those of the solar p-modes on the basis of in situ measurements of the flux density in the solar wind. The data sets presented here display oscillations up to 8 mHz (2 minutes period). This confirms the existence of solar wind fluctuations in that frequency range. On the other hand, several results - the differences between the signal from coma and tail, the frequency values of the very peaks of the power signal, and their relative sharpness with respect to the broad power band of the solar p-modes - seem to indicate that the solar p-mode signal is not the source of this fluctuation. Taking into account the limited duration of the data sets - and thus the limited resolution in the frequency domain -, one might attribute these apparent deficiencies of the observational evidence to statistical effects, though. For a discussion on the appearence of peaks at different frequencies also refer to the arguments given by Roberts et al. (1996) and Thomson et al. (1996). However, it would be useful to observe fluctuations in the coma and the ion tail of a comet simultaneously and over a longer period.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998