A beam pulsed amplifier mechanism responsible for effective amplification of short very low frequency (VLF) electromagnetic pulses is proposed. Effective amplification near the magnetic equator outside the plasmasphere is considered. A conditional growth rate of short electromagnetic pulses is calculated. Obtained results can explain some important features of the oblique electromagnetic chorus emissions without hiss-like radiation background.
We examine speciﬁc features of the realisation of the beam pulse ampliﬁer (BPA) mechanism of chorus excitation in the density ducts that have a width of the order of 100– 300 km with refractive reﬂection.The dispersion characteristics of whistler emissions in a planar duct under conditions for the fulﬁlment of the Wentzel–Kramers–Brillouin (WKB) approximation and refractive reﬂection from the “walls” of the duct are analysed. It is shown that in the enhanced duct, discrete spectral elements of chorus with a narrow angular spectrum along the external magnetic ﬁeld can be excited at frequencies somewhat lower than half of the electron cyclotron frequency. In the depleted duct at frequencies somewhat higher than half of the electron cyclotron frequency, chorus with a narrow angular spectrum along the magnetic ﬁeld can be excited. The proposed model explains the possibility of excitation of chorus with small angles of the wave normal when the BPA mechanism is implemented.It is noted that the properties of chorus, such as the intensity and a typical angle of the wave normal, can be different for the lowerand upper-band chorus.
Simultaneous records of VLF (very low frequencies) emissions have been carried out at two ground-based stations located at similar geomagnetic latitudes near L ~ 5.5 and spaced in the longitude by ~ 400 km, Kannuslehto (KAN) in Finland and Lovozero (LOZ) in Russia, using quite similar VLF receivers with two calibrated orthogonal air-core loop antennas. We found that the general spectral properties of the VLF chorus emissions at these two stations were similar and typically have right-hand polarization. Contrary to VLF chorus, the short-period VLF emissions (periodic emissions, PE) in which separated spectral elements are repeated with the periodicity of 3-4 s were mostly left-hand polarized. Usually, these waves propagated in the north-south direction. We suppose that PEs are generated inside of the plasmasphere by the cyclotron instability under a quasi-linear relaxation of the energetic electron distribution function. However, sometimes PE occurred only at an individual station. We speculated that this could be due to the influence of the local inhomogeneities to the VLF waves during the propagation through the ionospheric trough to the ground. Unusual series of short-duration (10-100 s) bursts of VLF emissions, lasting several hours, were also found in the morning under very quiet geomagnetic conditions (Kp ~ 0-1). Generally, these emissions were observed simultaneously at KAN and LOZ showing both right-hand and left-hand polarization, and different arrival directions provided the rather extended ionospheric exit area.
We propose a simple explanation of the prolonged existence of pancake-like electron velocity distributions inthe radiation belts. The pancake-like distribution function is characterized by a longitudinal particle velocity (along themagnetic field) of the order of the thermal velocity of the background plasma. The parameters of the tablet-like distribution function with a characteristic longitudinal particle velocity of the order of 20 Alfvèn velocities are refined. Such distribution functions can occur in the middle magnetosphere near the magnetic equator with appropriate sources of energetic particles. The stability of these distributions is examined. The results agree with known experimental data.
The problems of reflection and transmission of a whistler wave incident in the nighttime ionosphere from above are considered. Numerical solution of the wave equations for a typical condition of the lower ionosphere is found. The solution area comprises both the region of strong wave refraction and a sharp boundary of the nighttime ionosphere ( 100 km). The energy reflection coefficient and horizontal wave magnetic field on the ground surface are calculated. The results obtained are important for analysis of the extremely low-frequency and very low-frequency (ELF–VLF) emission phenomena observed from both the satellites and the ground-based observatories.