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Volume 49, Issue 8
Displaying 1-7 of 7 articles from this issue
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Birbal Singh1997 Volume 49 Issue 8 Pages 995-1000
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe characteristics of very high dispersion (D ∼ 54-70 sec1/2) whistlers recorded simultaneously in a low latitude ground station at Agra (geomag. lat. 17°1'N, L =1.15) during the night-time observation on 2 April, 1979 are studied and their propagation paths are determined using the curve fitting technique. The results show that some of these whistlers are high latitude whistlers which propagated along the magnetic field line of L = 3.6 while the others are one hop and three hop mid-latitude whistlers which propagated along the field lines between L = 1.9 to 2.5. Some of these whistlers are recorded simultaneously in our ground station. The dispersion and occurrence characteristics of these high dispersion whistlers are compared with those of the majority of relatively low dispersion ducted whistlers (D ∼ 30-35 sec1/2) which were recorded in our station during the same period of observation and then we discuss the propagation characteristics of high dispersion whistlers in terms of nonducted Pro-Longitudinal mode (PL mode) of propagation.View full abstractDownload PDF (21748K) -
Yuji Yamada, Masahiko Takeda, Tohru Araki1997 Volume 49 Issue 8 Pages 1001-1012
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe occurrence frequency of preliminary impulse (PI) of geomagnetic sudden commencements was statistically studied using the geomagnetic records of H-component at Fredericksburg (FRD; 49.0° geomagnetic latitude) and San Juan (SJG; 29.2°). In the daytime, positive PI predominantly occurs before noon and negative PI in the afternoon. These features, which are the same as those of high-latitude PI, indicate that high-latitude ionospheric currents responsible for PI extends toward middle and low latitudes. Almost all PIs detected in the nighttime are negative at FRD though the occurrence rate is much smaller than in the daytime. The diurnal variation of the occurrence rates of PI shows a drastic seasonal change at SIG, which is probably caused by a seasonal variation of the ionosphere. No correlation between the occurrence rates and the solar activity was found at either observatories. Ionospheric currents which are driven by a pair of field-aligned currents were numerically calculated on the assumption of the stationary state, and the geomagnetic field variation caused by the three-dimensional currents is simulated. Although the symmetric distribution of the field-aligned currents with respect to the noon-midnight meridian has been assumed, the calculated diurnal variation of magnetic field exhibits a severe deflection from the symmetry in middle and low latitudes.View full abstractDownload PDF (30398K) -
Eiji Mochizuki, Yukiko Yokoyama, Itsujiro Shimizu, Yozo Hamano1997 Volume 49 Issue 8 Pages 1013-1033
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSSpherical harmonic expansion is affected by uneven distribution of observation points. A weighting scheme is proposed in order to reduce the effects. The weights of observation points are determined in order to homogenize the distribution. Spherical harmonic expansion of the synthetic geomagnetic data is demonstrated; the weighting works in terms of decrease of truncation error of the expansion. The method is applicable to find a new observation site and to prioritize present geomagnetic observatories.View full abstractDownload PDF (42239K) -
Stanislav I. Bragivsky1997 Volume 49 Issue 8 Pages 1035-1048
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe conditions for obtaining a realistic geodynamo model which accurately describes the dynamo really existing in the core of the Earth are discussed. The problem of model selection originates from the fact that the equations governing the geodynamo contain unknown parameters which should be found by fitting the geodynamo solution to available observational parameters. The system of geodynamo equations, taken from the paper by Braginsky and Roberts (1995) is provided here with short explanations. The set of unknown parameters of a geodynamo model is established. A new non-dimensional parameter is introduced which measures the relative strength of the sources driving the core convection from the “top” and from the “bottom”. Available observational parameters necessary for fitting the simulated geodynamo model to observations are discussed, particularly those which originate from the analysis of MAC-waves in the Earth core.View full abstractDownload PDF (47189K) -
Shin-ichiro Kondo1997 Volume 49 Issue 8 Pages 1049-1063
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSBased on the viewpoint of fluctuation growth, we analyze and discuss the geomagnetic field reversal, focusing on a three-disk dynamo model, mainly on the effect of mutual inductance on dynamic behaviors. If mutual inductance is large, which means strong interaction between neighboring disks and their associated circuits, then a very slight fluctuation of electric current is found to be greatly enhanced after a short incubation time. On the contrary, after along incubation time, fluctuation growth is very slow when mutual inductance is small. Therefore the degree of interaction is related to the length of incubation time: the stronger the interaction, the shorter incubation time. The analogy between the disk dynamos and the geomagnetic field reversal patterns implies slight fluctuations are greatly enhanced during the period for the past 25 Ma, whereas fluctuation growth is very slow during the period of the Cretaceous Superchron, implying that mutual inductance is dependent on time. Hence, disk dynamo calculations including time dependence of mutual inductance are found to explain geomagnetic field reversal pattern qualitatively. From the mathematical viewpoint, numerical solutions for time dependent mutual inductance is found to be obtained by frequency modulation (FM) over those for time independent mutual inductance.View full abstractDownload PDF (29631K) -
Y. Ogawa, Y. Honkura1997 Volume 49 Issue 8 Pages 1065-1071
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSWe made audiomagnetotelluric sounding to image the subsurface structure across the Atera fault, which is a 66 km long active strike-slip fault in central Japan. A major feature of the resistivity model is existence of two dipping conductors at 0.5-2.0 km depth: one dipping southwest and the other northeast. These imply fractured zones, created either by strike slip movements or by vertical ones. An audiomagnetotelluric imaging is suited for mapping subsurface fractures zones in active fault regions.View full abstractDownload PDF (20379K) -
Y. Ogawa, Y. Honkura, F. Ohtani, H. Kuroki, Y. Mitsuhata1997 Volume 49 Issue 8 Pages 1073-1078
Published: August 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSWe collected magnetotelluric data in the Nikko volcanic area in the period range from 0.01 to 10 sec. Impedance data were decomposed with the regional strike direction N0°E and were analized by two-dimensional inversions including static shifts as model parameters. The major model features are: (1) southern conductor at 8 km depth which is consistent with the seismic S-wave reflectors, (2) northern conductor located at 5 km depth, consistent with the seismic attenuation zones, and (3) a resistitive gap in between, consistent with the magnetic body inferred from aeromagnetic data. The two conductors probably imply trapped free water at around 400°C. The resistor in between implies the volcanic body, which intruded the crust and may have broken the regime of trapped free water.View full abstractDownload PDF (17223K)
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