Propagation of Z-mode and whistler-mode emissions observed by Interball 2 in the nightside auroral region

A case study of VLF (very low frequency) natural emissions propagating at h igh altitude above the nightside auroral zone is presented. The analysis is based on data of the MEMO experiment on board the Interball 2 spacecraft. In its VLF band the device records waveforms of three magnetic and two elec tric field components covering a frequency range 1-20 kHz. Several analysis methods are applied in order to obtain the details on the wave propagation and mode structure and to identify possible source regions. We demonstrate that observed band-limited emission consists of Z-mode and whistler-mode w aves. Downgoing Z-mode waves are found just above the lower cutoff at the L =0 frequency. A possible source may be connected with the electron cyclotro n resonance mechanism taking place in the nightside sector at altitudes abo ve 21,000 km. Upgoing whistler-mode waves with upper cutoff at the local pl asma frequency are probably generated by upgoing electrons in the auroral r egion at altitudes lower than the actual satellite position, i.e., below 18 ,000 km.