Observation of ionospheric Alfven resonance at a middle latitude station

This paper intends to report on the statistical results on the spectral resonance structures of the ionospheric Alfven resonances (IAR) in the ULF frequency range 0.1-5.0 Hz on the basis of the analysis of long-term data obtained from July 2000 to December 2002 (2.5 years) at the Karimshino station (Kamchatka, Russia) (L=2.1) by the conventional 3-component search-coil magnetometer. We analyze both the dynamic spectra of three components and polarization spectra in order to distinguish IAR from other possible noises. The average frequency difference _F between the adjacent maxima, intensity and occurrence rate of the IAR spectra have been estimated from the averaged spectra. Early papers at middle latitudes have been based on a small data base. Based on our first long-term observation at middle latitude, new findings, especially related to the seasonal variation, have emerged from this analysis. (1) There is an evident seasonal variation in the occurrence rate with a maximum in the September-January period and an almost complete absence of IAR structures in the spring-early summer time. (2) Occurrence maximum in the diurnal variation is found at 21-23 LT in the autumn and winter. Almost all the IAR structures are observed at local nighttime. (3) The averaged _F is found to be about 0.2-0.5 Hz in the autumn period but it seems to increase up to 0.5-0.7 Hz in winter. (4) The IAR occurrence rate is inversely correlated with the Kp index of global magnetic activity. (5) The intensity of D component exceeds essentially that of H component of the IAR structures in a majority of cases. Diurnal variations of resonance frequencies in the H and D components are not always identical. Finally the mechanisms to explain the observed characteristics of the IAR have been discussed

SYNTHESIS OF THE LOW-FREQUENCY VIBRATION EXCITER

The article is devoted to synthesis of the low-frequency vibration exciter for checking of sensors of acceleration. In many areas of the modern equipment sensors of acceleration working in very low range of frequencies are widely used. For checking and graduation of such sensors the vibration exciter capable to provide rectilinear horizontal harmonic oscillations of the calibrated accelerometer in so low range of frequencies are required. Low frequencies of fluctuations cause the necessity of creation of big amplitudes of movements for ensuring the acceptable values of amplitudes of accelerations. The low-frequency electrodynamic vibration exciter with a magnetic suspension of mobile part which is a component of the National Standard of the vibration movement of the Russian Federation is so far created. However, development of the modern equipment demands expansion of frequency ranges to the area of ultralow frequencies. One of requirements shown to the vibration exciter working in the ultralow range of frequencies is increase in amplitude of horizontal movements of a mobile part as with small amplitudes the speed and acceleration of the harmonious law of the movements proportional according to the frequency of fluctuations and a square of this frequency, will have small amplitude values. One of problems of realization of a control system of the electric drive of the vibration exciter consists in that a mobile part possesses indifferent position of balance. The centre of fluctuations of a mobile part is not defined and can be in any point on magnetic conductor length. That fluctuations had the steady centre in an average point of a magnetic conductor without use of a mechanical spring, the drive is supplied with an additional control system of fluctuations, or a so-called electromagnetic spring