ESA Science & Technology - NTC amplification associated with small local density depletions

Electric field spectrogram of Cluster C2 for the period from 04:30 to 04:44 UT, 9 July 2001, in the frequency range 2 kHz - 80 kHz.

The plasma frequency (Fp) is directly related to the electron density N_e reminding Fp ~ 9 N_e^1/2.  Fp is precisely identified by the sounder and by a signature in electrostatic emissions (low frequency cut off). During the first 10 minutes, it increases smoothly from 12 kHz to 20 kHz. Meanwhile, non-thermal continuum is observed between 28 to 36 kHz, decreasing smoothly in frequency. Large irregular density structures are then observed. Patchy bright emissions are observed at frequencies apparently in continuity with the NTC. These emissions are enhanced in density depletions.