On the strength of electric currents and zonal magnetic fields at the top of the Earth''s core: Methodology and preliminary estimates

A new method is described for estimating: (a) the meridional electric current density, j_θ, (b) the vertical growth rate of the zonal magnetic field, ∂B_φ/∂r, or its scale-height, B_φ/∂B_φ/∂r) and (c) the vertical growth rate of the vertical current density, ∂j_r/∂r, at a few isolated points on the top surface of the Earth's core from observations of the internal geomagnetic field at the Earth's surface. The theoretical technique rests on combining unaccelerated, gravitationally-driven Boussinesq fluid dynamics of the core with frozen-flux electromagnetism, the mantle being treated as a spherically symmetric insulator.

Insertion into this theory of main field models for epochs 1965, 1975 leads to preliminary values for these quantities of magnitude: (a) j_θ 1 A/m², (b) ∂B_φ/∂r 10⁻⁶ T/m or B_φ/(∂B_φ/∂r) 10 m, (c) ∂j_r/∂r 10⁻⁶ A/m³. Some geophysical implications of these estimates are discussed.     

The measurement of electric fields in clouds

Summary The theory and practice of construction of field mills for static and slowly varying electric fields are discussed. A developed design for a differential field mill suitable for use inside thunderclouds is presented with methods for optimization of the device under usual conditions. This design has been tested both in clouds and in fair weather fields, comparing in the latter case with normal ground based electric field meters.     

Two observed consequences of penetration electric fields

The influence of penetration electric fields (PEF) on storm-time energetic particles in the inner magnetosphere and on the stability of plasma in the low-latitude ionosphere is widely recognized. We describe two consequences of PEFs, regularly observed during magnetic storms that indicate their persistence throughout the main phases. These are (1) the presence of equatorial plasma bubbles (EPB) across the evening local time sector during main phases and their absence throughout recovery, and (2) detections of low-energy ion precipitation in the dawn sector equatorward of the auroral electron boundary.     

A three-dimensional model of ionospheric F-region

The three-dimensional stationary in solar-terrestrial system numerical model of the global ionosphere at F-region altitudes is developed. The input parameters are: the structural parameters of empirical thermospheric models (temperature, composition), electric fields, solar UV-radiation spectrum, corpuscular flows at high latitudes. The model includes the calculations of thermospheric circulation, electron density, electron and ion temperatures. The model reproduces the main morphological peculiarities of the distribution of thermospheric circulation and ionospheric plasma parameters. A comparison analysis of the results of the model calculation corresponding to different thermospheric models (DTM, MSIS, Jacchia-77, MSIS-83) is carried out. It is shown, that thermospheric circulation systems are the effective indicator of faithfulness of thermospheric models.     

Generation of superDreicer electric fields in the solar chromosphere

The electric field generation at the front of the current pulse, which originates in a coronal magnetic loop owing to the development of the Rayleigh–Taylor magnetic instability at loop footpoints, has been considered. During the τ_A ≈ l/V _A ≈ 5?25 s time (where l is the plasma plume height entering a magnetic loop as a result of the Rayleigh–Taylor instability), a disturbance related to the magnetic field tension B _?(r,t), “escapes” the instability region with the Alfvén velocity in this case. As a result, an electric current pulse I_z(z ? V _A t), at the front of which an induction magnetic field E _z, which is directed along the magnetic tube axis and can therefore accelerate particles, starts propagating along a magnetic loop with a characteristic scale of Δξ ≈ l. In the case of sufficiently large currents, when B _? ²/8π > p, an electric current pulse propagates nonlinearly, and a relatively large longitudinal electric field originates E _z ≈ 2I _z ³ V _A/c ⁴a²B_z ²l, which can be larger than the Dreicer field, depending on the electric current value.     

Effects of geomagnetic activity on the mesospheric electric fields

The results of three series of rocket measurements of mesospheric electric fields carried out under different geomagnetic conditions at polar and high middle latitudes are analysed. The measurements show a clear dependence of the vertical electric fields on geomagnetic activity at polar and high middle latitudes. The vertical electric fields in the lower mesosphere increase with the increase of geomagnetic indexes K_p and K_p. The simultaneous increase of the vertical electric field strength and ion conductivity was observed in the mesosphere during geomagnetic disturbances. This striking phenomenon was displayed most clearly during the solar proton events of October, 1989 accompanied by very strong geomagnetic storm (K_p = 8+). A possible mechanism of generation of the vertical electric fields in the mesosphere caused by gravitational sedimentation of charged aerosol particles is discussed. Simultaneous existence in the mesosphere of both the negative and positive multiply charged aerosol particles of different sizes is assumed for explanation of the observed V/m vertical electric fields and their behaviour under geomagnetically disturbed conditions.Paper Presented at the Second IAGA/ICMA (IAMAS) Workshop on Solar Activity Forcing of the Middle Atmosphere, Prague, August 1997     

Balloon measurement of vertical and horizontal atmospheric electric fields

Summary Twenty-eight balloons have been flown in Canada to measure vertical and horizontal electric fields at balloon altitudes. A horizontal electric field of magnitude typically between 10 and 50 millivolts/meter has been measured at night at high latitudes in association with auroras and magnetic disturbances. Its origin is in the magnetosphere and it maps to balloon alitudes with small attenuation according to Maxwell's equations. The vertical electric field at the balloon displays variations as the horizontal ionospheric field changes in order to maintain . Thus, magnetospheric processes affect both vertical and horizontal atmospheric electric fields and the potential differences induced by these processes may be comparable to weather induced potential differences. Weather processes have also been observed to produce large horizontal electric fields at balloon altitudes. Methods of distinguishing horizontal fields of ionospheric origin from those of weather origin are investigated with the conclusion that a determination of the source of a given event can often if not generally be made.This paper was presented byU. Fahleson     

Calculation of atmospheric electric fields penetrating from the ionosphere

The spatial distributions of electric fields and currents in the Earth’s atmosphere are calculated. Electric potential distributions typical of substorms and quiet geomagnetic conditions are specified in the ionosphere. The Earth is treated as a perfect conductor. The atmosphere is considered as a spherical layer with a given height dependence of electrical conductivity. With the chosen conductivity model and an ionospheric potential of 300 kV with respect to the Earth, the electric field near the ground is vertical and reaches 110 Vm⁻¹. With the 60-kV potential difference in the polar cap of the ionosphere, the electric field disturbances with a vertical component of up to 13 V m⁻¹ can occur in the atmosphere. These disturbances are maximal near the ground. If the horizontal scales of field nonuniformity are over 100 km, the vertical component of the electric field near the ground can be calculated with the one-dimensional model. The field and current distributions in the upper atmosphere can be obtained only from the three-dimensional model. The numerical method for solving electrical conductivity problems makes it possible to take into account conductivity inhomogeneities and the ground relief.     

Differential mill for measuring electric fields and conductivities in thunderclouds

Summary A differential rotating mill for the measurement of electric fields has been designed and built. The instrument was specifically designed to be insensitive to the effects of space charge and frictional charging so that it could operate in the environment of an active thundercloud. The electric field range of the device is 100 to 250,000 volts per meter, (however, corona should occur before the maximum measurable field is reached) and the conductivity range is approximately (depending on the electric field) 10^–11 to 10^–13 mhos/meter.Forty of these devices have been parachuted into active, and potentially active, thunderclouds around Tucson, Arizona. Although the presently available measurements are too few for an unequivocal conclusion, the data strongly support the presence of very high conductivities in thunderstorms.This research was supported by the Atmospheric Sciences Section, National Science Foundation under Grant GA-701.     

Ionospheric response to variable electric fields in small-scale auroral structures

High time and space resolution optical and radar measurements have revealed the influence of electric fields on E-region electron density profiles in small-scale auroral structures. Large electric fields are present adjacent to auroral filaments produced by monoenergetic electron fluxes. The ionisation profiles measured within and beside the auroral filaments show the effects of plasma convection due to electric fields as well as the consequences of the response time to large and dynamic fluxes of energetic electrons. Without high-resolution optical measurements, the interpretation of the radar data is limited.     

Electromagnetic response function for the semi-annual variations estimated by the single-station method using the night-time values of the geomagnetic fields

The geomagnetic night-time values were used to estimate the electromagnetic response function Q1 for half-year period. If the spatial structure of the source field can be described by the approximation, one can estimate the Q1 value using the single-station Z/H method. This technique enables us to carry out regional deep geomagnetic sounding by the method. The data used for analyses are geomagnetic night-time values for about, typically, 26 years from 5 good-quality stations and for several years from 34 stations distributed over the globe. The results indicate that the night-time values yield more reliable response estimates for half-year period compared to the usual estimates obtained from daily means. It implies that the approximation for the night-time fields holds good for the half-year period, but the daily means are not suitable for estimating the response function of the semi-annual variations by using the single-station method. Source field analyses for daily means data and night-time means data have also been carried out in this paper.     

Longitudinal variation in E- and F-region ionospheric trends

A novel technique is used to examine northern hemisphere midlatitude longitudinal variations in ionospheric long-term trends. Differences in hour-by-hour monthly median ionospheric parameters between equilatitudinal observatory pairs are analysed for long-term trends, thus eliminating at source the large solar cycle and geomagnetic variability that normally hinders ionospheric trend calculations. The results confirm the finding of Bremer [1998. Trends in the ionsopheric E- and F-regions over Europe. Annales Geophysicae 16, 698–996] that there are longitudinal variations in the F-region altitude trend across Europe, but suggest the influence of a stationary wave-like feature between 3°W and 104°E. Possible causes such as scaling errors, insufficient account of changes in ionisation underlying the F-region, varying gravity wave fluxes, and secular change in the geomagnetic field are ruled out. The data suggest that the longitudinal variation may reflect long-term changes in a large-scale stationary feature induced via non-migrating tides induced by latent heat release in the troposphere.Significant differences in the long-term trend of E-region peak plasma frequency between observatories were also found. These E-region differential trends varied with solar zenith angle reaching over 0.3 MHz per decade between Juliusruh and Moscow at midday in summer.     

ELF-VLF atmospheric waveforms under night-time ionospheric conditions

Tweek atmospherics generated by lightning discharges and propagated in the night-time Earth-ionosphere waveguide, have often very pronounced dispersive features near the first few waveguide cut-off frequencies (f_cm\simm\dot{s}f_c1, f_c1\sim1.6-1.9 kHz, m=1, 2, ...,), being very extended in time, and have rather large amplitudes of oscillations with periods corresponding to the narrow vicinity of the cut-off frequencies. In this paper an analytical approach is developed to describe the waveform of distant tweeks. It is based on the solving of the Maxwell equations in two qualitatively different regions, whose changes are related in the first instance to the changes in the relative magnitudes of the displacement current and components of the conduction currents, and the following asymptotic matching of the solutions in the transitional region. The analytical night-time waveguide model accounts for both anisotropy and vertical inhomogeneity of the low ionosphere. The model is valid for upper ELF - lower VLF range and is well suitable for the analysis of the QTE_m modes in the cut-off frequency regions, which determine the most important part of the tweek spectra and tweek amplitudes. The influence of the different ionospheric heights on the tweek characteristics is determined. The efficiency of the tweek generation by cloud-to-cloud discharge is also evaluated.     

Polar lacuna on ionograms. E- or F-region processes?

Lacuna on ionograms indicate conditions where the usual ionospheric reflections of vertically propagating radio waves transmitted from and subsequently received by an ionosonde are missing over a range of frequencies. Reflections from the lowermost E-region should still be observable contrary to the more usual cases where absorption processes in the underlying D-region removes the low-frequency part of the ionospheric reflections appearing in ionograms. It is emphasized below that lacuna cases related to E-region processes should be distinguished from lacunae related to processes in the F1 and F2 regions. This distinction appears to have been neglected in a recent paper.     

Behavior of electric potential fields over random conductivity,layered earth models

Solutions are derived for the potential distributions over one-layer and two-layer random conductivity earth models for the direct current resistivity method. Monopole and Wenner arrays are considered. It is shown that the random potential due to a spatially stationary Gaussian random conductivity is also Gaussian but not spatially stationary. The ensemble and sample statistics of the random kernel and apparent resistivity are examined. Representative curves of these functions are presented to show the effects of a random conductivity profile on them. It is shown that the effect of the random nature of the conductivity profile on an apparent resistivity sounding curve is greatest for small electrode spacings.     

Response of ionospheric electric fields to variations in the interplanetary magnetic field

The STARE system (Scandinavian Twin Auroral Radar Experiment) provides estimates of electron drift velocities, and hence also of the electric field in the high-latitude E-region ionosphere between 65 and 70 degrees latitude. The occurrence of drift velocities larger than about 400 m/s (equivalent to an electric field of 20 mV/m) have been correlated with the magnitude of the Interplanetary Magnetic Field (IMF) components B_z and B_y at all local times. Observation days have been considered during which both southward (B_z<0) and northward (B_z>0) IMF occurred. The occurrence of electric fields larger than 20 mV/m increases with increases in B_z magnitudes when B_z<0. It is found that the effects of southward IMF continue for some time following the northward turnings of the IMF. In order to eliminate such residual effects for B_z<0, we have, in the second part of the study, considered those days which were characterized by a pure northward IMF. The occurrence is considerably lower during times when B_z>0, than during those when B_z is negative. These results are related to the expansion and contraction of the auroral oval. The different percentage occurrences of large electric field for B_y>0 and B_y<0 components of the IMF during times when B_z>0, clearly display a dawn-dusk asymmetry of plasma flow in the ionosphere. The effects of the time-varying solar-wind speed, density, IMF fluctuations, and magnetospheric substorms on the occurrence of auroral-backscatter observations are also discussed.