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Volume 41, Issue 12
Displaying 1-4 of 4 articles from this issue
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Girija RAJARAM1989 Volume 41 Issue 12 Pages 1001-1023
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSFrom a survey of the published final values of the geomagnetic index Dst for the period 1958-1972, we found long time intervals of over 25-30 days, during which this index remained consistently positive (Dst+) or negative (Dst-). A study is made of relevant parameters on the ground, in the magnetosphere, in the solar wind and on the Sun to seek out systematic features associated with the two conditions. In order to eliminate factors arising from seasonal and solar cycle variations, we selected pairs of Dst+ and Dst- which involve successive months of the same year, or the same month of two successive years.
Three parameters which show a systematic difference between Dst+ and Dst- intervals are found to be 1) the state of solar photospheric magnetic fields 2) the flux density of solar MeV protons measured in the magnetosphere and 3) the southward component of the interplanetary magnetic field. While the effect of the last on geomagnetic activity has been well-discussed in the literature, it is suggested that the correlations of the first two to the conditions of Dst+ and Dst- demand a careful scrutiny of the solar-terrestrial relationship.View full abstractDownload PDF (2137K) -
Takahiro OBARA, Hiroshi OYA1989 Volume 41 Issue 12 Pages 1025-1042
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA topside sounding experiment by the PPS system on board the EXOS-C (Ohzora) satellite made successful observations in the polar ionosphere. From analyses of the sonograms the following results have been obtained.
i) Associated with the particle precipitation into the polar cusp region, small scale irregularities causing the spread F phenomena with electron density enhancement have been generated.
ii) In the polar cap ionosphere, there are also small-scale irregularities of the electron density causing the spread F phenomena. These irregularities have also been associated with an enhancement of the electron density. The origin of these irregularities in the polar cap is in the polar cusp region, and the irregularities are transported into the polar cap region by convection from the cusp region.
iii) In the midnight polar cap region, an electron density depression region, called a plasma hole, has been sometimes observed. The magnetic activity index corresponding to detection of the plasma hole always indicates a very low value. This is interpreted to occur as the result of a reduced plasma convection in the polar cap region. While the polar cap plasma escapes continuously due to the effect of the polar wind and losses through the attachment process, the supply is insufficient at the ionospheric level with the lack of convection, causing the plasma hole.View full abstractDownload PDF (6880K) -
Hiromasa YAMAMOTO, Tadao MAKINO, Ichiro NAITO, Hiroyuki SEKIGUCHI1989 Volume 41 Issue 12 Pages 1043-1056
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe transmission of the O2 1.27μm airglow to the ground is determined by coordinated measurements of a filter radiometer on board the S-310-16 rocket and a ground-based grating monochrometer. The inferred transmission of 6.3±3.4% for a zenith direction is in good agreement with the calculated value of 6.5% by EVANS et al. (1970). The zenith intensity of the 1.27μm nightglow is deduced to be 58±14kR, which is consistent with the rocket experimental result obtained by EVANS et al. (1972).View full abstractDownload PDF (1079K) -
A Numerical ExperimentK. VEERASWAMY, U. RAVAL1989 Volume 41 Issue 12 Pages 1057-1075
Published: 1989
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe presence of Mesozoic-Gondwana sediments below the cover of Deccan flood basalt, as inferred on the basis of Deep Seismic Sounding (DSS) (KAILA, 1986), needs to be complemented by other geophysical methods. In view of this and the increasing emphasis on combined seismic and electrical probing (JONES, 1987), a Magneto-Telluric (MT)-Audio Magneto-Telluric (AMT) coverage of the DSS profiles on the Deccan Traps is suggested. In order to gain a quantitative insight and aid the field campaign, a theoretical analysis of the geoelectromagnetic problem is presented here. A shallow subsurface section (up to 2-3km depth) under the Ujjain-Mahan profile, deduced from the refraction data, has been simulated. The electromagnetic response of a 2-D sedimentary inhomogeneity sandwiched between the upper basaltic layer and the granitic basement is computed using a finite element algorithm and for the plausible parametric values taken from field studies (SUBRAHMANYAM, 1987) and the literature (CLARK, 1966). Quantitative estimates of induction characteristics along the selected profile are obtained for variations in a) thicknesses and conductivities of the trap, b) conductivities of sediments, c) conductivities of the basement and d) the period of exitation. The comparison of results for the E- and B-polarization modes reveal that the latter is significantly more sensitive to the dip of a contact fault controlling the subsurface sedimentary structure under the Ujjain-Malian section. The analysis also takes into account the influence of the sloping interfaces and topography at the surface and deeper levels.View full abstractDownload PDF (1485K)
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