WHISPER measurements of bow shock crossings
The frequency/time spectrogram data were obtained between 08:15 and 08:45 GMT (09:15 - 09:45 CET) on 22 December 2000. Each spacecraft crossed the bow shock (red/black peaks) at slightly different times, but the crossings generally occurred at around 08:25 and 08:35 GMT (09:25 and 09:35 CET). At that time, spacecraft 2 (Salsa) was more westward than the other three spacecraft, and so it spent more time in the solar wind.
The middle section of each graph - between about 08:25 and 08:35 GMT - shows a plasma frequency of 40 kHz in the solar wind (where plasma density was about 20 particles per cm3). Before and after the bow shock crossings, the Cluster spacecraft were in the magnetosheath, where plasma density was about 70 particles per cm3 and plasma frequency was about 75 kHz.
Data courtesy of Dr. Pierrette Dicriau of LPCE / CNRS, the University of Orleans, France