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Volume 43, Issue 7
Displaying 1-6 of 6 articles from this issue
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R. G. RASTOGI, P. V. KOPARKAR, A. PATIL, B. M. PATHAN1991 Volume 43 Issue 7 Pages 549-561
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe paper describes the results of daytime VHF scintillations at a number of stations within the electrojet region in India. Slow and weak scintillations (about 1dB peak to peak fluctuations) are observed almost on 90% of occasions during the midday hours at the station on the magnetic equator. The average occurrence of Es-q type of scintillation decreases very rapidly with increasing dip latitude. The occurrence of Es-q type of scintillations is precisely coincident with the period when the equatorial electrojet defined by ΔH (Trivandrum minus Alibag) is positive. These observations clarify the apparent discrepancy of earlier observations of daytime VHF radio scintillations at Huancayo and Ootacamund.View full abstractDownload PDF (1213K) -
Yukiko YOKOYAMA, Takesi YUKUTAKE1991 Volume 43 Issue 7 Pages 563-584
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSAmong the geomagnetic secular variations of several decades, the variation with a period of about 60 years, is considered to be one of the most predominant features. Spherical harmonic analyses were made of the annual mean values of the three components at 21 magnetic observatories, and the annual sets of the Gauss coefficients were obtained during the interval from 1910 to 1983. Then the time series of the Gauss coefficients were analyzed by the Sompi spectral method. It turned out that 1) The amplitudes of the Gauss coefficients vary with periods of approximately 60 years. 2) The “sixty year variation” is of internal origin. 3) The variation occurs on a global scale. 4) The variation is commonly observed in almost all the Gauss coefficients of spherical harmonic degree up to 4.5) The variation observed in each mode varies roughly in phase (or antiphase) except a few modes.View full abstractDownload PDF (1968K) -
C. G. A. HARRISON, J. R. SOUTHAM1991 Volume 43 Issue 7 Pages 585-599
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSMagnetic field gradient measurements for geophysical purposes have been made with much less frequency than total field observations. Yet there are considerable advantages in gradient measurements, especially if the objective is to study magnetic anomalies produced by magnetization contrasts in the crust and possibly upper mantle of the Earth. This is because gradient measurements enhance short wavelength signals relative to long wavelength signals. The power spectrum of a magnetic gradient measurement along any line is the same as the power spectrum of the field measured along the same line multiplied by the wavenumber. This means that signals from crustal sources are greatly enhanced over those from core sources, and also that diurnal variations of the field, which appear as very long wavelength features in most observations, are heavily attenuated compared with the crustal signal. To make full use of gradient measurements it is desirable to orient the gradient instrument. If orientation can be achieved, then it is possible to determine the lineation direction of lineated features using just one profile running at some angle to the feature lineation. This is similar to the technique for determining lineation direction from the measurement of field components, but the requirement for orientation is much less severe. Even if orientation cannot be achieved, there are rotationally invariant quantities which can be formulated from the gradient tensor which are of potential use in the interpretation of magnetic anomaly sources.View full abstractDownload PDF (1405K) -
The Case of a Long Scattering Mean Free PathTakasuke SAKAI1991 Volume 43 Issue 7 Pages 601-606
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSFor the case of a long scattering mean free path, the solution of pitch angle transport equation using the method of KOTA et al. (1982) (KMJKGO) is compared with the simulation which follows the trajectories of particles in a simulated random magnetic field. The comparison shows that the KMJKGO method gives similar time profiles of the intensity to those obtained by the simulation, even with the anisotropic injection of particle at the source. It is found that the time profile is very sensitive to the direction of the velocity cone with respect to the ambient magnetic field at the point of injection, even though the same scattering mean free path is used. Thus, the direction of the velocity cone at the injection of the particles may be an important parameter when interpreting the propagation of solar cosmic rays from the time profile of intensity. This is especially true for a long scattering mean free path.View full abstractDownload PDF (523K) -
R. G. RASTOGI, A. PATIL, S. ALEX1991 Volume 43 Issue 7 Pages 607-611
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe paper describes the comparison of individual measurements of the vertical plasma drift velocities in the F region of the ionosphere, VzF, computed from the doppler shift of 50MHz backscatter echoes at Jicamarca, Peru during the post-sunset hours with the corresponding values of the vertical velocity of the F layer computed from the rate of change of the minimum virtual height of the F layer, ∂h′minF/∂t, at Huancayo, Peru.
It is found that there is a remarkable coincidence in the time of the reversal of vertical drift velocities of plasma measured by VHF incoherent radar and the F region vertical drift computed from the HF ionosonde. Statistically, the F layer vertical drift from ionogram data is about two-third of the incoherent scatter radar plasma drift velocities.
Such a calibration would be greatly useful to study electric fields during sunset hours in the equatorial ionosphere at different longitudes where long series of ionospheric data are available.View full abstractDownload PDF (475K) -
G. K. RANGARAJAN1991 Volume 43 Issue 7 Pages 613-618
Published: 1991
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA detailed analysis of the indices An and As of geomagnetic activity is made to bring out the nature of the North-South asymmetry in geomagnetic activity. A 10% excess activity in the northern hemisphere is shown to be a real feature for all the years in the epoch 1964-1984. Seasonal variation in the asymmetry exhibits the expected northern summer (winter) maximum for the positive (negative) asymmetry. Equinoxial maxima for marginally positive asymmetry is also shown. A well-defined Universal-Time daily variation is present in the asymmetry when the northern hemisphere is more active.View full abstractDownload PDF (559K)
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