Lunar daily geomagnetic variations at Tucson

Summary. Mean hourly values of the geomagnetic elements for Tucson, Arizona, have been analysed for the M ₂ lunar tide for the period 1909–1975. Results for the average diurnal variation are presented, and changes with season, sunspot cycle and magnetic activity are discussed.     

On the location of the ionospheric current systems responsible for the lunar and solar magnetic variations

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From a study of the sunspot cycle influence on the lunar ( L ) and solar ( S ) daily geomagnetic variations, Malin, Cecere & Palumbo have suggested that the electric currents responsible for these variations do not flow in the E -region as has hitherto been accepted. Indeed they suggested that the L current may even flow in the F 2 region. The consequences of such a suggestion are considered and it is shown that an L current in the F -region is highly unlikely from the dynamical point of view. The evidence for E -region currents is presented and it is suggested that the variation of E -region electron density used by Malin et al . to indicate conductivity changes in that region may not be a reliable indicator of such changes.     

Contributions from oceanic and ionospheric dynamos to the lunar daily variation at Alibag

Summary. The contributions from the oceanic and ionospheric dynamos, L_o and L_I respectively, to the geomagnetic lunar daily variation, L , at Alibag, a coastal station in the Indian equatorial region, are calculated from the L harmonics derived from a 41–yr long series of observations. The analysis in the calendar months shows a steady and significant ocean dynamo contribution in the vertical component, Z, in all the months except April. Examination, by an analysis of the data year by year, of the association of L_o and L_I with varying solar and magnetic activities reveals, surprisingly, a stable correlation between the magnetic activity index A _P and the oceanic part in the horizontal and vertical components but not in declination, which probably indicates the influence of induced currents, along the latitudes, on L _o.     

The annual and daily variations of the Dst index

Summary. This investigation concerns both the annual variation and the daily variation of the Dst index for 18 years. The data display a 6-month wave, a 12-month wave and a 24-hr wave the phase of which apparently does not vary in the course of the year. This contrasts with the variations observed with planetary indices derived from subauroral K indices, which undergo a modulation in sin²Ψ_M (Ψ_M is the angle between the solar wind and the dipole axis) with the well-known change in phase of the daily variation from one solstice to another. The origin of the 6-month wave is attributed to the mechanism suggested by Malin & Isikara: the 6-month variation of the mean latitude of the ring current. The 12-month wave may be due to the northern latitude of most of the Dst observatories. The daily variation is probably due to the asymmetric part of the ring current which is not averaged out because of the non-uniform longitudinal distribution of the Dst observatories.     

On the anomalous features of the geomagnetic quiet-day field variations at Nagpur, India

The quiet-day geomagnetic field variation data from the recently commissioned Nagpur geomagnetic observatory, which has augmented the currently active latitudinal chain of Indian magnetic observatories, are analysed for the year 1993- The variations for diurnal frequencies (Sq) recorded at Nagpur do not follow the expected trend with latitude. This is most conspicuous in the northward horizontal ( X ) component. The anomalous behaviour at Nagpur is also seen in the diurnal harmonic amplitudes when compared with those of the neighbouring stations Alibag (south of Nagpur) and Ujjain (north of Nagpur). This behaviour is attributed to the presence of electrically conducting anomalous sources in the vicinity of Nagpur. The anomalous internal source is inferred to be located at relatively shallower depths and is highly localized.     

Long-period geomagnetic variations and mantle conductivity: an inversion using Bailey's method

Summary. We have implemented an algorithm which is based on Bailey's solution of the inverse problem of electromagnetic induction in the Earth. The study was motivated by recent determinations of very long period data and also benefited from recent redeterminations of high frequency data. The algorithm has been successfully tested to provide reliable estimates of conductivity down to a depth of 2000 km, using synthetic data in the period range from 4 days to 11 years. Smooth data sets, which are required for the inversion, were constructed from various sources. At a given depth, the range of inverted models is less than one order of magnitude. Due to the lack of high frequency data, the conductivity of the upper 600 km of the mantle, which is found to be of the order of 10⁻¹Ω⁻¹ m⁻¹, may be overestimated. The algorithm performs well in the middle mantle, where conductivity rises steadily from 1 to 50 Ω⁻¹ m⁻¹. The lack of very low frequency data and limitations of the algorithm prevent one from obtaining meaningful estimates in the lower mantle. However, the study of the propagation of the late 1960s secular variation acceleration provides an estimate of the mean conductivity of the whole mantle. Thus, a complete mantle profile can be constructed. It is found that deep mantle conductivity probability does not exceed a few hundred Ω⁻¹ m⁻¹.     

The inverse problem of geomagnetic induction at a fixed frequency

Summary. A method for the determination of the electrical conductivity of the Earth is developed when the components of the response function on the basis of spherical harmonics for a fixed frequency are known. By writing the differential equation for the field inside a spherically symmetric conductor as a finite difference equation, it is shown that the formal solution of the latter for the response function has a Thiele representation in the degree of the harmonics. This property enables one to calculate the conductivity at a finite number of points using a continued-fraction expansion of the response function.     

The pole-tide signal in the geomagnetic field at a low-latitude station

summary . The equilibrium pole-tide's signal is identified at a period around 14.22 month (427 day) in geomagnetic elements H and Z at a low-latitude coastal station, Alibag, in the Indian region. The signal however, could not be detected clearly in the element D . Consistent amplitudes and phases, over the hours, of this tide in H spectra of both'all days'and'IQ days'indicate that the origin of the tide in the geomagnetic field cannot be due to modulation of an external current system. It is suggested that the probable source of excitation may be the'ocean dynamo'.