FIGURE 2. Plasma wave spectrogram.

The initial Cassini bow shock crossing (appearing as the earliest cross on the Cassini trajectory in Fig. 1), upstream ion acoustic waves and Langmuir waves preceding the shock crossing are shown. Here the intensity of waves is shown as a function of frequency (ordinate) and time (abscissa), using the colour bar to indicate electric field spectral density. We note that the Langmuir waves are at a frequency of about 2 kHz, corresponding to an electron density of 0.05 cm^-3. Assuming a nominal solar-wind speed of 450 km s^-1, the derived solar wind dynamic pressure is 0.018 nPa. The Joy et al.¹⁶ 10% bow shock model, which assumes a dynamic pressure of 0.02 nPa, crosses the Cassini trajectory very close to this observed shock.