Very diffused whistlers and chorus emissions observed in the equatorial magnetosphere

Wide-band VLF electric field data of DE-1 received at Kashima, Japan for a substrom period (AL = - 520nT) on May 11, 1990 are used to study effect of plasma irregularities near the plasmapause on whistlers and L-dependency of VLF risers and falling waves. The DE-1 passed obliquely through the morning equatorial magnetosphere from geomagnetic latitude of 4.5 deg N, L = 3.3, and altitude of 13,750 km to 1.0 deg S, L = 2.5, and 10,030 km. Very diffused whistlers occurred from 1.0 deg S and L = 2.54 to 5.3 deg N and L = 2.80 for 0643 to 0646 MLT (geomagnetic local time), and a first normal whistler did at 7.9 deg N and L = 2.9. This implies that the very diffused whistlers were observed at geomagnetic latitudes below 5.8 deg N and L below 2.9. Frequency rising emissions (risers) occurred at frequency from 8 to 10 kHz and from 16 to 20 kHz at L above 3.1 and falling emissions did from frequency band from 8 to 6 kHz or that from 14 to 10 kHz in double frequency bands at L = 2.9 to 3.0. In summary, the very diffused whistlers were observed at geomagnetic latitudes below 5.8 degrees and for L below 2.9, while the frequency rising emissions and falling ones were observed at L above 2.9. Non-ducted whistlers scattered by plasma irregularities have large wave normal angles for f above f(LHR) and large electric fields parallel to wave vectors. Plasma irregularities exist near the equatorial plasmapause (Horwitz et al., 1986). The Doppler-shifted spectrum broadening of VLF waves propagating in the plasma irregularity due to the satellite motion is estimated at 100 Hz by the expression given by Bell et al.,(1983) for f(LHR) = 1 kHz and satellite velocity of 5 km/s at 2.6 Re (Earth radius). Spasojevic et al. (2003) have shown high irregularity density structures within the plasmaspheric plume observed by IMAGE EUV imager in continued substorm activity, amd also the base of plasmaspheric plume closer to the plasmasphere located at L = 2.0 to 2.3 in the morning sector. The plasmaspheric plume located between 2.6 and 3.3 Re wraps around the plasmapause between 0630 and 0400 MLT. These locations of plume irregularities are similar to those of the very diffused whistlers observed by the DE-1. Therefore, the very diffused whistlers seem to be due to the Doppler-shifted spectrum broadening of normal whistlers passing through the high irregularity density structures within the plasmspheric plume in the morning sector.