Hardware and software complex and methods of observing VLF radiation in Kamchatka

The paper is devoted to the description of the hardware-software complex developed at the Institute of Cosmophysical Research and Radio Wave Propagation FEB RAS and installed in the region of Karymshina river (Elizovo raion, Kamchatksiy kray, Russia). The hardware-software complex is aimed at receiving the natural electromagnetic radiation in VLF range to investigate electromagnetic radiation sources and detect the features of signal propagation along the Earth surface and during its passage though the Earth ionosphere and magnetosphere. Special attention was focused on the detection accuracy of azimuthal arrival angles of the radiation from height lightning discharges responsible for whistler occurrences observed in the VLF range. Detailed description is given for the receiving antenna complex, the method for detection of radiation arrival azimuth, VLF direction finder calibration, checking the complex operation when receiving signals from correct time radio stations and from lightning discharges recorded by the WWLLN global network. The receiving antenna complex consists of two magnetic antennas, arranged vertically with the plane in the north-southern and east-western directions, and an electric antenna. Each magnetic antenna contains 98 coils of copper wire and has the geometric square of 48 m$^2$. The electric rode antenna has the height of 9 m from its upper part to its basis. When receiving signals, two method for determination of VLF radiation source azimuths were applied, the amplitude and spectral ones. For the effective operation, the simplest one, the amplitude method, was applied. For the most accurate determination of the azimuth, the spectral method was developed. It takes more time to make calculations by it but it allows one to improve the accuracy of azimuth determination by more than one order compared to the amplitude method. Application of the spectral method made it possible to investigate the relation of height lightning strokes with the activity of thunderstorms of meteorological and volcanic origin and showed that height lightning discharges are the sources of whistlers. A software complex was developed to record and store the data. It includes client and server ends. The server processes the data flow from the ADC and makes correct time referencing. The client end is responsible for full archive organization and its storage. Magnetic antennas are calibrated by sending noise and sinusoidal signals on magnetic and electric antennas.