Journal of geomagnetism and geoelectricity

The O₁ Component of Geomagnetic Lunar Daily Variation in H at Alibag, 1927-1970

N. S. SASTRI, B. R. ARORA, D. R. K. RAO

1975 Volume 27 Issue 2 Pages 85-94
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.85

JOURNAL FREE ACCESS

Show abstractHide abstract

The O₁ component of geomagnetic lunar daily variation, L(O₁), in the horizontal intensity at Alibag for the period 1927-1970 is determined by the generalized method of Winch. Significant partial tides, comparable to the phase law tides, are observed. An increasing trend is indicated in the range of L(O₁) with increasing sunspot number. The principal harmonics of L(O₁) show a marked increase in amplitude from apogee to perigee.

View full abstract

Download PDF (957K)
Sudden f_min Enhancements and Sudden Cosmic Noise Absorptions Associated with Solar X-Ray Flares

Teruo SATO

1975 Volume 27 Issue 2 Pages 95-112
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.95

JOURNAL FREE ACCESS

Show abstractHide abstract

Sudden f_min enhancements (SF_mE's) and sudden cosmic noise absorptions (SCNA's) associated with increments of X-ray fluxes during solar flares are studied on the basis of X-ray flux data measured by SOLRAD 9 and 10 satellites. Some statistical analyses on SF_mE's observed at five observatories in Japan, corresponding to increased X-ray fluxes in the 1-8Å band are made for 50 solar flare events during the period January 1972 to December 1973, and value of f_min is expressed as functions of cos χ (χ; solar zenith angle) and 1-8Å band X-ray flux. Similar study is also made for SCNA's observed by 30MHz riometer at Hiraiso for 15 great solar flare events during the same period, together with 27.6MHz riometer data reported by SCHWENTEK (1973) and 18MHz data published by DESHPANDE and MITRA (1972b). It is found that f_min value (MHz) and SCNA value (L, dB) of a radio wave with frequency f (MHz) are related to X-ray flux (F₀, erg cm^-2sec^-1) in the 1-8Å band and to cos χ, by following approximate expressions,
f_min (MHz)=10F₀^1/4cos^1/2χ,
L (dB)=4.37×10³f^-2F₀^1/2cos χ,
respectively. Blackout seems to occur for F₀ values causing f_min's greater than about 5MHz. It is shown that these expressions can be derived from a brief theoretical calculation of radio wave absorption in the lower ionosphere. Also it is suggested that threshold X-ray fluxes in the 1-8Å band which may produce a minimum SF_mE (2MHz), blackout and minimum SCNA (0.27-0.36dB for 30MHz noise) are 1.6×10^-3, 6.2×10^-2 and (3-8)×10^-3erg cm^-2sec^-1, respectively, for cos χ=1.

View full abstract

Download PDF (1726K)
Relation between Lower Ionospheric Electron Density Profiles and Cosmic Noise Absorption during Auroral Zone Disturbances

Shigeru MIYAZAKI

1975 Volume 27 Issue 2 Pages 113-129
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.113

JOURNAL FREE ACCESS

Show abstractHide abstract

Eight rockets among a total of twenty-one rocket experiments by the 12th, 13th and 14th Japanese Antarctic Research Expedition parties were launched during auroral disturbances at night at Syowa Station, Antarctica between 1971 and 1973. The electron density profiles in the lower ionosphere between about 50km and 130km under various disturbed conditions were obtained by rocket-borne radio frequency probes and electrostatic probes.
The collision frequency profile is derived from the electron density profiles and cosmic noise absorption measurements. There exists a maximum absorption layer in the region between 75km and 85km, and this location of the layer is not strongly dependent on the magnitude of the ionospheric disturbance. An increase in the electron density, especially in the D and E region between about 75km and 110km has a good correlation with cosmic noise absorption; the following empirical formulae are obtained from the observational results:
Δn_e=exp(0.205x+4.67),
h_m=-2x+100,
where Δn_e(cm^-3) is the maximum increment of the electron density from the quiet level, h_m(km) is the height of the maximum increment of the electron density and x(dB) is the cosmic noise absorption.

View full abstract

Download PDF (1603K)
On the Sudden Disappearance of Equatorial Sporadic E

K. SEN GUPTA, B. V. KRISHNA MURTHY

1975 Volume 27 Issue 2 Pages 131-138
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.131

JOURNAL FREE ACCESS

Show abstractHide abstract

The sudden disappearance of equatorial sporadic E at Trivandrum (dip-0.6°S) is investigated. It is found that the phenomenon does not exhibit a clear lunar time dependence. The sudden disappearance occurs mainly in the afternoon period and during magnetically quiet as well as disturbed periods. Its association with the changes in the range of horizontal magnetic field is studied and the results are presented.

View full abstract

Download PDF (730K)
Areal Coverage of Spurious Reversals of the Earth's Magnetic Field

C. G. A. HARRISON, E. RAMIREZ

1975 Volume 27 Issue 2 Pages 139-151
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.139

JOURNAL FREE ACCESS

Show abstractHide abstract

The Laschamp event ended between 8000 and 20000 years ago. In several other parts of the world, excursions of the Earth's magnetic field have been seen at about this time interval, but at other parts, notably in the Aegean sea, there is no evidence for a reversal or excursion of the field during this time. By using a two dipole model, we have shown that it is possible to produce an apparent reversal at one point on the Earth's surface, which produces deviations in the direction of the magnetic field over a rather small area around the position of reversal. The two dipoles used were a centered axial dipole to model the main magnetic field, and a dipole located at the top of the core, to produce the reversed field above it. Our models show that the data from the Aegean sea are not contradictory with the Laschamp event provided that it is a pseudo reversal. We have also shown that if the non-dipole field is caused by horizontal electric currents at the top of the core, then pseudo reversals are more likely to occur at high latitudes than at low latitudes.

View full abstract

Download PDF (1103K)
Electromagnetic Core-Mantle Coupling Associated with Changes in the Geomagnetic Dipole Field

Hidehumi WATANABE, Takesi YUKUTAKE

1975 Volume 27 Issue 2 Pages 153-173
Published: 1975
Released on J-STAGE: March 12, 2010

DOIhttps://doi.org/10.5636/jgg.27.153

JOURNAL FREE ACCESS

Show abstractHide abstract

On a shelluar earth model electromagnetic coupling between the mantle and the core is investigated when the geomagnetic dipole field changes its intensity. Besides electromagnetic interaction between the dipole change and the relative slip of the mantle to the core, coupling of the dipole change with shear motions within the core is considered. If, in the core, the dipole change is limited within a surface layer shallower than a few hundred kilometers, the electromagnetic interaction gives the same order of magnitudes and phases of mantle oscillation as suggested from observation for three different periods, 8000, 400 and 65 years, provided that the electrical conductivity of the bottom part of the mantle is 10^-9 to 10^-8emu.
It is shown that mean motion of the surface shells of the core thus calculated is compatible with the observed variations in the drift velocity of the geomagnetic secular change. Except for surface shells, those in the deep interior is confirmed to oscillate almost with the same angular velocity, like a rigid rotation, for all the periods.

View full abstract

Download PDF (1718K)

Register with J-STAGE for free!