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Electric currents flowing in the global electric circuit are closed by ionospheric currents. A model for the distribution of the ionospheric potential which drives these currents is constructed. Only the internal electric fields and currents generated by thunderstorms are studied, and without any magnetospheric current sources or generators. The atmospheric conductivity profiles with altitude are empirically determined, and the topography of the Earth’s surface is taken into account. A two-dimensional approximation of the ionospheric conductor is based on high conductivities along the geomagnetic field; the Pedersen and Hall conductivity distributions are calculated using empirical models. The values of the potential in the E- and F-layers of the ionosphere are not varied along a magnetic field line in such a model and the electric field strength is only slightly varied because the segments of neighboring magnetic field lines are not strictly parallel. It is shown that the longitudinal and latitudinal components of the ionospheric electric field of the global electric circuit under typical conditions for July, under high solar activity, at the considered point in time, 19:00 UT, do not exceed $$9\,\upmu {\text{V/m}}$$ , and in the sunlit ionosphere they are less than $$2\,\upmu {\text{V/m}}$$ . The calculated maximum potential difference in the E- and F-layers is $$42\,{\text{V}}$$ ; the maximum of the potential occurs above African thunderstorms that are near the terminator at that time. A weak local maximum also exists above the thunderstorm area in Central America. The minimum potential occurs near midnight above the Himalayas. The potential has identical values at ionospheric conjugate points. The voltage increases to $$55\,{\text{V}}$$ at 23:00 UT and up to $$72\,{\text{V}}$$ at 06:00 UT, when local midnight comes, respectively, for the African and Central American thunderstorm areas. These voltages are about twice as large at solar minimum. With our more realistic ionospheric model, the electric fields are an order of magnitude smaller than those found in the well-known model of Roble and Hays (J Geophys Res 84(A12):7247–7256, 1979). Our simulations quantitatively support the traditional presentation of the ionosphere as an ideal conductor in models of the global electric circuit, so that our model can be used to investigate UT variations of the global electric circuit.

A mathematical model is constructed of the equatorial electrojets flowing in the ionosphere which are generated by thunderstorms occurring at low latitudes. We use the narrow definition of the global electric circuit (GEC) that includes only atmospheric and ionospheric electric fields and currents generated by thunderstorms, and ignores the contributions of all ionospheric and magnetospheric generators. The ionospheric currents which distribute charges from the thunderstorm areas to the fair weather parts of the Earth are largest in the vicinity of the geomagnetic equator. They form the specific electrojets discussed here which are in addition to the electrojets formed by the wind dynamo. A model of the ionospheric potential which drives these currents was developed earlier and this is suitably modified here. We use an empirical model of the diurnal variation of the number of lightning strikes to define the currents up to the ionosphere from the main thunderstorm areas. A two-dimensional approximation of the ionospheric conductor is based on its high conductivity along the magnetic field. The Pedersen and Hall conductivity distributions are calculated using empirical ionospheric models; the Pedersen and Hall conductances are calculated by integration along magnetic field lines and these are used in the 2-D model of the ionospheric conductor. The spatial distributions of the ionospheric electric fields and currents are obtained by numerical solution of the 2-D ionospheric current continuity equation. The positions and the directions of the electrojets are defined by the global distribution of the main thunderstorm areas, as well as of the ionospheric conductivity, and so they strongly vary with Universal Time. As far as the generation of the equatorial electrojets is concerned, the African and Asian thunderstorm areas are more effective than the American ones since they are closest to the geomagnetic equator. There are day-time electrojets, the strength of which may be up to 175 A, and night-time ones (of up to 60 A), while the total current flowing in the GEC is not larger than 1400 A at any moment of time in our model. Usually the electrojets are a few times weaker in the night-time ionosphere because of its smaller conductivity. The equatorial electrojets of the GEC thus produce magnetic perturbations on the ground, which are in the 0.1 nT range, while there are one hundred times stronger, wind dynamo-driven electrojets and other larger, space weather-associated magnetic perturbations. Nevertheless, using their specific features these magnetic perturbations could be measured, especially at the night-time geomagnetic equator when and where they are not so disguised by other ionospheric currents.

This paper examines whether the Earth-ionosphere capacitor (EIC) model is correct, by comparing observed atmospheric electrical properties with those expected for a spherical capacitor, as defined in electrostatics. The comparisons suggest that the EIC concept cannot be reconciled with, and hence cannot account for, the observations, particularly the rapid reduction of the atmospheric electric field with height that is measured. This means that the spherical EIC concept is incorrect by being too simplistic; it is thus misleading. The reason for this flawed concept is simple: the model disregards the non-uniform conductivity of the atmosphere which requires the presence of a net positive charge in the lower atmosphere that equals in magnitude the Earth's negative charge. This positive charge shields the action of the Earth's negative charge from polarizing the ionosphere positively. Thus, the lower D region ionosphere remains electrically neutral, which makes the EIC concept inappropriate.

A model for the distribution of the ionospheric electric potential which drives the currents which close the global electric circuit is constructed. Only the internal electric fields and currents generated by thunderstorms are studied. The atmospheric conductivity profiles with altitude are empirically determined, and the topography of the Earth’s surface is taken into account. A two-dimensional approximation of the ionospheric conductor is based on large conductivities along the geomagnetic field; the Pedersen and Hall conductivity distributions are calculated using the empirical models IRI, MSIS and IGRF. The maximum calculated voltage difference in the ionosphere under typical conditions for July, under low solar activity, at 19:00 UT, is about 85 V. In our model the potential has identical values at ionospheric conjugate points. With our more realistic ionospheric model, the electric fields are found to be an order of magnitude smaller than those of the well-known model of Hays and Roble [9].

[1] We use VLF electromagnetic wave data measured by the DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) satellite at an altitude of about 700 km to check for the presence of statistically significant changes of natural wave intensity (due to signals from lightning) related to preseismic activity. All the relevant data acquired by DEMETER during almost 6.5 years of the mission have been analyzed using a robust two-step data-processing schema. This enables us to compare data from the vicinity of about 8400 earthquakes with an unperturbed background distribution based on data collected during the whole DEMETER mission and to evaluate the statistical significance of the observed effects. We confirm previously reported results of a small but statistically significant decrease of the wave intensity (by ∼2 dB) at frequencies of about 1.7 kHz. The effect is observed for a few hours before the times of the main shocks; it occurs during the night. The effect is stronger between March and August, at higher latitudes and for the positions of hypocenters below the sea. We suggest an explanation based on changed properties of the lower boundary of the ionosphere, which leads to a decrease of the intensity of lightning-generated whistlers observed at the spacecraft altitude. This effect might result from a lowering of the ionosphere associated with an increase in the electrical conductivity of the lower troposphere due to an additional ionization of air molecules at the Earth's surface prior to earthquakes.

This paper gives a resume of the papers written in English which (a) describe some of the recording instruments in use at the Nagycenk Observatory (NCK) since the International Geophysical Year (IGY 1957–1958) and up to the present time, (b) summarise the most important and different types of observations associated with thunderstorms which have been made there, and (c) discuss their various geophysical interpretations. The paper describes the main results which have been obtained in four areas of thunderstorm associated atmospheric and geospace science within the context of Earth system science. These relate to the following parameters of atmospheric electricity: the vertical electric potential gradient just above the Earth’s surface and the air–Earth current as well as the point discharge current, Schumann resonance (SR) signals of the Earth-ionosphere cavity at 8, 14 and 20 Hz, transient luminous events (TLEs), and some aspects of the behaviour of the ionosphere. Deductions from these data sets are concerned with the global lightning activity and the conductivity of the air, with diurnal, seasonal, annual and long-term variations of the SR amplitudes and resonant frequencies in terms of migrating thunderstorm centres, with transient SR excitations and with sprites and other TLEs, and with ionospheric disturbances. The paper closes with some thoughts on future research directions based on the observations at NCK and Sopron and the results achieved since the IGY.

An apparent discrepancy is pointed out - at all heights, and by up to an order of magnitude - between the height profiles of atmospheric conductivity derived at AC using ELF propagation studies, especially from information on Schumann resonance of the Earth-ionosphere cavity, and using a model of the DC global atmospheric electric circuit. This serious issue is resolved by creating a hybrid profile of these two mid-latitude profiles, the first of which refers to conditions by day and the second by night. This hybrid profile is thus a first order attempt to represent globally averaged conditions. Close to the Earth’s surface, where the resistance of the atmosphere is largest, the properties of the DC global model exert the greatest influence, whereas in the middle atmosphere, at heights between 40 and 100 km, full wave computations show that the AC results are the more crucial. The globally averaged hybrid profile presented here has some limitations, and the physical reasons for these are addressed. They are due to the presence of aerosol particles of ice and/or of meteoric material which reduce the ionospheric D-region conductivity by an order of magnitude over only ~2 km of height, thereby causing ledges of ionisation. In the context of the globally averaged profile, published observations of the ionospheric effects of the giant gamma-ray flare from SGR 1806-20 (a neutron star having an enormously large magnetic field) occurring at 21:30 U.T. on December 27, 2004, are briefly discussed.

We propose a mechanism to explain suggested links between seismic activity and ionospheric changes detected overhead. Specifically, we explain changes in the natural extremely low-frequency (ELF) radio noise recently observed in the topside ionosphere aboard the DEMETER satellite at night, before major earthquakes. Our mechanism utilises increased electrical conductivity of surface layer air before a major earthquake, which reduces the surface-ionosphere electrical resistance. This increases the vertical fair weather current, and (to maintain continuity of electron flow) lowers the ionosphere. Magnitudes of crucial parameters are estimated and found to be consistent with observations. Natural variability in ionospheric and atmospheric electrical properties is evaluated, and may be overcome using a hybrid detection approach. Suggested experiments to investigate the mechanism involve measuring the cut-off frequency of ELF "tweeks", the amplitude and phase of very low frequency radio waves in the Earth-ionosphere waveguide, or medium frequency radar, incoherent scatter or rocket studies of the lower ionospheric electron density. © 2009 Elsevier Ltd.

A description is given of the global atmospheric electric circuit operating between the Earth’s surface and the ionosphere. Attention is drawn to the huge range of horizontal and vertical spatial scales, ranging from 10−9 m to 1012 m, concerned with the many important processes at work. A similarly enormous range of time scales is involved from 10−6 s to 109 s, in the physical effects and different phenomena that need to be considered. The current flowing in the global circuit is generated by disturbed weather such as thunderstorms and electrified rain/shower clouds, mostly occurring over the Earth’s land surface. The profile of electrical conductivity up through the atmosphere, determined mainly by galactic cosmic ray ionization, is a crucial parameter of the circuit. Model simulation results on the variation of the ionospheric potential, ∼250 kV positive with respect to the Earth’s potential, following lightning discharges and sprites are summarized. Experimental results comparing global circuit variations with the neutron rate recorded at Climax, Colorado, are then discussed. Within the return (load) part of the circuit in the fair weather regions remote from the generators, charge layers exist on the upper and lower edges of extensive layer clouds; new experimental evidence for these charge layers is also reviewed. Finally, some directions for future research in the subject are suggested.

Our colleague A. P. Nickolaenko has pointed out an unfortunate, and obvious, misprint in our paper. The error is in line 5 of page 12 showing values for s0, which corresponds to the value of the surface conductivity in units of S/m. These values should not be negative.

Electrical processes coupling the atmosphere and ionosphere differ from all other such processes, because both DC and AC effects operate at (almost) the speed of light. Electrostatic and/or electromagnetic field changes in different parts of the global atmospheric electrical circuit, which cause these “teleconnections”, arise from thunderstorms, lightning and sprites and, at higher latitudes, from the precipitation of Van Allen belt energetic electrons and interactions of the solar wind with the magnetosphere. Some observational, theoretical and modelling studies of interest are outlined.

The purpose of this paper is to model numerically the ionisation created in the mesosphere above a strong horizontal (cloud-to-cloud) lightning discharge, and thus to understand better the formation of elves and related columniform (spatially structured) sprites. Such ionisation depends upon height, via the ratio of the VLF/LF wave electric field (radiated by the current in the discharge) to the atmospheric neutral density. The simulation shows local maxima on the order of 10 km apart due to the interference between direct waves radiated by the discharge current, waves reflected by the ground and waves reflected by the ionosphere. Parametric studies are carried out varying the frequency, amplitude, height, length and duration of the current. For waves ⩾30 kHz generated by a current ⩾50 kA , localised peaks of electron density occur from 75 to 85km altitude, particularly for discharges longer than 15 km. From such peaks, time-varying “stalactites” on the bottom of the ionosphere, streamers could develop and propagate downwards. These could become columniform sprites in the presence of the quasi-electrostatic field due to the negative charge remaining in the thundercloud after a positive cloud-to-ground discharge.

It has recently been suggested that successive intense lightning-electromagnetic pulse (EMP) events could cause significant large-scale changes to the properties of the nighttime lower ionosphere. In order to examine this quantitatively, data on lightning detected over the United States are combined with the output from a simulation code. During the course of a night strong lightning-EMP events can lead to significant (∼100% or even greater) increases in the electron density of the lower ionosphere, with the largest increases at ∼90 km altitude. Regions with significant decreases in electron density are also possible. It is shown that changes in the electron temperature of the lower ionosphere are unlikely to be significant. The time required to produce large-scale changes of ionospheric electron density above an active thunderstorm may explain the observation of a thunderstorm “modification time” before red sprite activity is initiated.

We review the theory, as well as satellite and ground-based data testifying to the high efficiency of parametric interactions of whistler mode waves and lower-hybrid resonance (LHR) and ion-acoustic or ion-cyclotron waves. Some applications of the parametric effects to diagnostics of the ionosphere and the magnetosphere are discussed.

Following some lightning flashes from energetic thunderclouds, blue jets and red sprites are observed in the atmosphere above the cloud and into the ionosphere. In order to understand the physical processes leading to these and associated phenomena, both the temporal and spatial evolution of the electric (and magnetic) fields due to the thundercloud and the lightning discharge are modelled. These numerical simulations are carried out either using a quasi-electrostatic code or an electromagnetic code, with appropriate boundary conditions and grids. The redistribution of electric charge and the electromagnetic pulse due to the lightning, can accelerate electrons, which collide with neutrals and ions, heating them, and also ionising the atmosphere. Runaway electrons and/or electrical breakdown of the atmosphere can also occur. The first and second positive bands of molecular nitrogen are excited appreciably if sufficient energy is produced. The situation is strongly nonlinear. The results (see also Cho and Rycroft, this issue) show the temporal and spatial development of (a) the electric field divided by the neutral gas density, and (b) the energy density of optical emissions (up to 10 13 photons m −3 s −1 ). They show that energy propagates up to the ionosphere in 0.3 ms, at the speed of light. A ring of optical emissions is created, the outer rim of which propagates horizontally in the ionosphere at an altitude ∼ 90 km, reaching a radial distance of 280 km in a further 0.7 ms. At the same time, the intense electric field at > 07 km altitude above the thundercloud creates a much enhanced ( ∼ 10 3 ×) electron density (with a radius up to 25 km) which lasts for several ms. This heated region modifies the amplitude and phase characteristics of VLF radio waves propagating in the Earth-ionosphere waveguide.

Computersimulations are carried out to study the ‘sprite’ onset mechanism. Both electrostatic and electromagnetic codes are developed to calculate the electric field structure and optical emission intensity between the top of the thundercloud and the ionosphere. The optical emission is composed of two structures. One peaks at 70 km height and its lateral dimension is 50–60 km; the other peaks at 90 km height and the lateral dimension extends beyond 200 km. It is found that the nitrogen first positive band, which has a red colour, dominates over the nitrogen second positive band except at the bottom of the optical emission. The upper part of the optical emission is caused by a horizontally travelling electromagnetic pulse induced by a lightning discharge current. The lower structure is caused by electrostatic effects induced by the unneutralized charge left after the lightning discharge current flows. The electromagnetic codes developed can simulate the self-consistent response of the upper atmosphere to the lightning discharge current. The electrostatic treatment can predict only the optical emission at heights less than ∼80 km. The optical emission intensity has a strong nonlinear dependence on the electric field strength through the enhanced electron density, and is increased for a long discharge path, a large current, and a short pulse. Also, the higher the lightning discharge is initiated, the brighter the optical emission is, because the electrostatic field is stronger for high altitude lightning.

A quantitative theory of the generation of ELF/VLF signals, which are caused by the ionospheric demodulation of modulated signals from broadcasting MF/HF transmitters, is developed. Two cases are considered when the ionospheric current, modulated by broadcasting signals, is a one-dimensional linear current (the polar electrojet), and when it is an extended two-dimensional current sheet. It is shown that the main demodulation effect is caused by the formation of an ionospheric travelling wave antenna. In the case of the polar electrojet, this antenna radiates ELF/VLF waves in two directions, one of which coincides with the line connecting the broadcasting transmitter with the ELF/VLF receiver, and the other one is in a direction which is the mirror image of this line relative to the current direction. In this case the polarisation of the ELF/VLF magnetic field component varies widely. In the case of the extended ionospheric current, the demodulation takes place along the line connecting the MF/HF transmitter with the ELF/VLF receiver, and the horizontal ELF/VLF magnetic field component is normal to this line. Amplitude and phase distortions in the ELF/VLF signal are absent below a frequency ⪅ 1 kHz and appear above ∼ 1 kHz. Quantitative estimates of the amplitude of the ELF/VLF signal give a value ∼0.15pT which is close to the experimentally observed value.

Geomagnetic field variations and electromagnetic waves of different frequencies are ever present in the Earth's environment in which the Earth's fauna and flora have evolved and live. These waves are a very useful tool for studying and exploring the physics of plasma processes occurring in the magnetosphere and ionosphere. Here we present ground-based observations of natural electromagnetic emissions of magnetospheric origin at very low frequency (VLF, 3–30 kHz), which are neither heard nor seen in their spectrograms because they are hidden by strong impulsive signals (sferics) originating in lightning discharges. After filtering out the sferics, peculiar emissions are revealed in these digital recordings, made in Northern Finland, at unusually high frequencies in the VLF band. These recently revealed emissions, which are observed for several hours almost every day in winter, contain short (~1–3 min) burst-like structures at frequencies above 4–6 kHz, even up to 15 kHz; fine structure on the 1 s time scale is also prevalent. It seems that these whistler mode emissions are generated deep inside the magnetosphere, but the detailed nature, generation region and propagation behaviour of these newly discovered high latitude VLF emissions remain unknown; however, further research on them may shed new light on wave-particle interactions occurring in the Earth's radiation belts.

[1] A quantitative model of the global atmospheric electric circuit has been constructed using the PSpice electrical engineering software package. Currents (~1 kA) above thunderstorms and electrified rain/shower clouds raise the potential of the ionosphere (presumed to be an equipotential surface at 80 km altitude) to ~250 kV with respect to the Earth's surface. The circuit is completed by currents flowing down through the fair-weather atmosphere in the land/sea surface and up to the cloud systems. Using a model for the atmospheric conductivity profile, the effects of both negative and positive cloud-to-ground (CG) lightning discharges on the ionospheric potential have been estimated. A large positive CG discharge creates an electric field that exceeds the breakdown field from the ionosphere down to ~74 km, thereby forming a halo, a column sprite, and some milliseconds later, from ~67 km down to ~55 km at ~60 ms after the discharge, a "carrot" sprite. Estimates are made of the return stroke current and the thundercloud charge moment change of a +CG discharge required to exceed the threshold breakdown field, or the threshold field for creating and sustaining negative or positive streamers. The values for breakdown at 80 km altitude are 35 kA and 350 C.km, (Coulomb.kilometers), respectively, and those at 70 km altitude are 45 kA and 360 C.km, respectively. The different temporal and spatial developments of the mesospheric electric field distinguishing between column and carrot sprites agree with the latest deductions from recent observations. The current flowing in the highly conducting sprite reduces the ionospheric potential by ~1 V.

We give a brief overview of our present understanding of the global circuit formed by the atmosphere, the ionosphere and the solid and ocean surface of the Earth. The circuit is driven by currents (∼1 kA) above both thunderstorms and electrified shower clouds up to the ionosphere, which is presumed to be an equipotential surface (∼250 kV with respect to the Earth). This large scale circuit is completed by currents (∼2 pA/m2) flowing down through the fair weather atmosphere to the Earth’s surface and up into the atmosphere below the electrically active clouds. Using a realistic profile of the atmospheric electric conductivity, we have generated a model of this equivalent circuit using the PSpice electrical engineering software package. The effects of model lightning discharges on the circuit have been estimated numerically. Model sprites exhibiting mesospheric discharge processes have been constructed, first for negative cloud‐to‐ground discharges (−CGs), then for discharges transferring positive charge to...

High energy radiation from tens of keV to tens of MeV in energy, the transition region from x‐rays to gamma rays, has been observed in association with high electric fields in air in many contexts: from the ground, associated with thunderstorms and with individual flashes of natural and rocket‐triggered lightning [1, 2, 3, 4, 5, 6, 7, 8] from sparks in the laboratory [9, 10, 11, 12], from Earth orbit in the form of millisecond Terrestrial Gamma‐ray Flashes (TGFs) [13, 14], and at intermediate altitudes from aircraft and balloons [15, 16, 17, 18, 19]. The gamma radiation in all cases is almost certainly bremsstrahlung generated by the collision of accelerated electrons with nuclei in the atmosphere. The penetrating gamma‐rays can provide a mechanism for coupling between the troposphere, mesosphere, ionosphere and magnetosphere in the form of energetic particles and ionization.

Synoptic, long-term observational programmes provide a wide diversity of information on the solar-terrestrial environment. Such data sets are particularly valuable in certain locations, on the ground, or in space. Further, such data sets are necessary for identifying the baseline from which anthropogenic changes may occur, for characterising transient events, for placing the results of short-term experiments in context, and also for carrying out statistical analyses. Results for all of these four examples can be interpreted according to present understanding of the physics of solar-terrestrial phenomena. Inadequacies are identified and understanding is thereby improved. New hypotheses are put forward and tested; new modelling schemes and new observational techniques are devised.Some examples are presented, dealing with different physical parameters of solar, interplanetary, magnetospheric, ionospheric, thermospheric and geomagnetic phenomena. Considerations of mass, momentum and energy transfer across the magnetopause, and of energy dissipation in the polar, auroral and midlatitude upper atmosphere are emphasised, on time scales ranging from a few minutes to a century. Such studies are essential for the success of the Solar Terrestrial Energy Programme, STEP, the research aspects of which are underpinned by synoptic programmes. They will open the door to more successful predictions of the solar-terrestrial environment, an ever-developing requirement for mankind at the end of the twentieth century.

The paper reviews recent advances in studies of electric discharges in the stratosphere and mesosphere above thunderstorms, and their effects on the atmosphere. The primary focus is on the sprite discharge occurring in the mesosphere, which is the most commonly observed high altitude discharge by imaging cameras from the ground, but effects on the upper atmosphere by electromagnetic radiation from lightning are also considered. During the past few years, co-ordinated observations over Southern Europe have been made of a wide range of parameters related to sprites and their causative thunderstorms. Observations have been complemented by the modelling of processes ranging from the electric discharge to perturbations of trace gas concentrations in the upper atmosphere. Observations point to significant energy deposition by sprites in the neutral atmosphere as observed by infrasound waves detected at up to 1000 km distance, whereas elves and lightning have been shown significantly to affect ionization and heating of the lower ionosphere/mesosphere. Studies of the thunderstorm systems powering high altitude discharges show the important role of intracloud (IC) lightning in sprite generation as seen by the first simultaneous observations of IC activity, sprite activity and broadband, electromagnetic radiation in the VLF range. Simulations of sprite ignition suggest that, under certain conditions, energetic electrons in the runaway regime are generated in streamer discharges. Such electrons may be the source of X- and Gamma-rays observed in lightning, thunderstorms and the so-called Terrestrial Gamma-ray Flashes (TGFs) observed from space over thunderstorm regions. Model estimates of sprite perturbations to the global atmospheric electric circuit, trace gas concentrations and atmospheric dynamics suggest significant local perturbations, and possibly significant meso-scale effects, but negligible global effects.

[1] We present ground-based observations of banded structures (BS) in auroral hiss. This BS hiss was detected at Porojarvi station in Northern Finland, mainly in the local evening from 19 to 23 MLT, during magnetically quiet periods; half of all the events observed corresponded to AE < 50 nT. The structures, which resemble those observed by low-altitude satellites, can be related to the cyclotron resonant interaction of VLF waves at frequencies near the lower-hybrid resonance with suprathermal protons in the upper ionosphere and magnetosphere. The maximum probability distribution of band spacing for the events observed at Porojarvi is reached in the frequency range 200–300 Hz, which corresponds to proton gyrofrequencies at L ∼ 6 at altitudes of 4000–3000 km. We found that the BS hiss was detected in the region of evening-side downward field-aligned currents and rather intense precipitation of ions with energies 1–30 keV, detected by DMSP spacecraft. We propose that the observed BS hiss can be formed not only due to absorption at the proton gyroharmonics, as is usually assumed for similar structures observed onboard satellites, but also due to generation by energetic protons with an anisotropic velocity distribution.

Several processes acting below, in and above thunderstorms and in electrified shower clouds drive upward currents which close through the global atmospheric electric circuit. These are all simulated in a novel way using the software package PSpice. A moderate negative cloud-to-ground lightning discharge from the base of a thunderstorm increases the ionospheric potential above the thundercloud by 0.0013%. Assuming the ionosphere to be an equipotential surface, this discharge increases the current flowing in the global circuit and the fair-weather electric field also by 0.0013%. A moderate positive cloud-to-ground lightning discharge from the bottom of a thunderstorm decreases the ionospheric potential by 0.014%. Such a discharge may trigger a sprite, causing the ionospheric potential to decrease by ∼ 1 V . The time scales for the recovery of the ionospheric potential are shown to be ∼ 250 s , which is of the same order as the CR time constant for the global circuit. Knowing the global average rate of lightning discharges, it is found that negative cloud-to-ground discharges increase the ionospheric potential by only ∼ 4 % , and that positive cloud-to-ground discharges reduce it by ∼ 3 % . Thus, overall, lightning contributes only ∼ 1 % —an almost insignificant proportion—to maintaining the high potential of the ionosphere. It is concluded that the net upward current to the ionosphere due to lightning is only ∼ 20 A . Further, it is concluded that conduction and convection currents associated with “batteries” within thunderclouds and electrified shower clouds contribute essentially equally ( ∼ 500 A each) to maintaining the ionospheric potential.

Optical methods of studying the atmosphere are one valuable means of investigating atmospheric behaviour at heights ranging from less than 1 km to several hundred km. Some examples are given of results from various optical experiments carried out at Halley, Antarctica (76oS, 27oW;L=4.2), as is a consideration of the results of some complementary experiments. By combining observations made using different techniques, an improved understanding of atmospheric, ionospheric and magnetospheric processes is obtained.

Man’s first, and most dramatic, activities in this field were to conduct high altitude (400...500 kin) nuclear explosions in the late 1950s and early 1960s. The Argus experiment created charged particles which affected the Van Allen trapped radiation belts. The Starfish experiment of 9 July 1962 had. a higher yield (l.L~Megaton (Mt)) than the Argus explosions (~1 kt); an intense artificial radiation belt was created, and this lasted longer (many years) since the source was at a lower latitude (Johnson Island, Pacific Ocean) than that used for the earlier Argus shots (South Atlantic). Three Soviet explosions in October and November 1962 made artificial radiation belts which were less intense than that due to Starfi~h. Because their source was at a higher latitude (Siberia) these belts decayed rather rapidly. Interesting effects have been observed in the VLF and ELF radio bands after high—altitude nuclear explosions. At the instant of detonation of Starfish, electromagnetic signals were radiated; a whistler was received in the opposite hemisphere. Only seconds after the explosion, artificial aurorae were observed in New Zealand and the absorption of cosmic radio noise by the Alaskan ionosphere was increased. The peak absorption occurred within one minute, with recovery to normal conditions taking a few hours. After a few minutes the ionosonde in Jamaica indicated greater absorption in the lower ionosphere. Also some minutes after the explosion the Q value of the resonant (8 Hz) earth-ionosphere cavity decreased; after about three hours it returned to its normal value. The change may be explained, not only by the enhancement of ionisation below the explosion, but also by the world—wide (low-latitude) ionisation produced by the trapped energetic electrons, arising from neutron decay, drifting eastwards in longitude. Synchrotron radiation from the ~ 2 million electron volt (MeV) electrons produced by the explosion was detected by radioastronomy observatories at low latitudes.

The signals at an altitude of 500–600 km from two ground-based transmitters (GBR, NAA) at frequencies of 16.0 kHz and 17.8 kHz, respectively, have been studied in experiments aboard the satellites Ariel 3 and Ariel 4. Refraction of whistler mode energy in a direction perpendicular to the local magnetic meridian has been observed and measured. The direction of the refraction reverses between day-time and night-time and correlates with the reversal of that component of the refractive index vector due to cross meridian gradients of electron density. Ionospheric and magnetospheric ray tracings have provided estimates of the cross meridian electron density gradients required to give the observed refraction.

Solar-terrestrial physics is concerned with the near-Earth space environment, that is with the solar wind, magnetosphere, ionosphere and thermosphere. It deals with this region, now often termed geospace, in both its ideal, steady-state situation and its transient response to dramatic events occurring on the Sun or in the interplanetary medium. I start by asking four questions about this topic: why? where? when? and how? Why do we study solar-terrestrial physics? Nine reasons which illustrate the value of solar-terrestrial physics to Man nowadays are as follows: (i) geospace is the only natural, cosmic plasma accessible to Man for carrying out in situ physical investigations; (ii) the results of such detailed studies may be applied to other plasma physical situations occurring elsewhere in the Universe; (iii) both astronauts and sensitive electronic components or instruments, aboard Earthorbiting satellites or the Space Station, have to operate satisfactorily in the charged particle environment of space; (iv) the lifetimes of satellites are determined by the density of the thermosphere, which can vary by a factor of a hundred or more as a function of solar and geomagnetic activity; (v) observations made by a radar altimeter or a synthetic aperture radar aboard a satellite are affected by the ionospheric plasma; (vi) radio communications are upset by solar flares and by energetic charged particle events; (vii) large electromotive forces are induced by magnetospheric transients, in long conductors; these cause large electric currents to flow which can disrupt electrical power grid systems and telephone cables, or accelerate corrosion in pipelines; (viii) aeromagnetic prospecting for natural resources, particularly at high latitudes, is upset by geomagnetic disturbances; and (ix) laboratory plasma devices, designed for fusion, are susceptible to plasma instabilities which can be well studied in geospace. Such research is thus not only of relevance to more universal problems or of purely academic interest; it takes on increasing practical importance as we use geospace nowadays for an increasing number and diversity of satellites. I now consider briefly the where, the when and, particularly, the how of solar-terrestrial physics. Details will be filled in by other contributors. The outermost atmosphere of the Sun is the corona at a temperature of over one million kelvins. It is the source of the omnipresent solar wind plasma, a mixture of electrons and ions that is electrically neutral on average. The expanding supersonic solar wind, with a speed in excess of one million kilometres per hour, encounters the Earth's magnetic field (figure 1). This

The paper discusses how profiles of electron and/or ion distributions that are produced by two different computer models can be smoothly coupled together. The first of these models is the empirical International Reference Ionosphere which produces a vertical profile of ionospheric parameters up to an altitude of 1000 km. The second is a physically-based, diffusive equilibrium model of the plasmasphere based upon the theoretical work of Angerami and Thomas /1/, in which plasma is constrained to move parallel to the Earth's magnetic field lines. Some problems associated with this work are considered, as are some initial results.

The directions of propagation, in the earth-ionosphere waveguide, of multi-component two-hop whistlers recorded on 10 July 1972 by four VLF goniometer receivers in eastern Canada have been determined. Using the bearings of these great circle paths, triangulation of several whistler exit-points has been accomplished. The L-values of the whistler exit-points determined by this method are systematically lower than those expected from their nose frequencies, by ~ 0.6. Various explanations are discussed for this effect. The most satisfactory is that the whistler waves leave through the side of the ducts (in which they had propagated for most of their path through the magnetosphere) at an altitude of a few thousand kilometres, and then are refracted to lower L-values before exiting from the lower ionosphere. The results are consistent with both the duct termination altitude predicted by Bernhardt and Park (1977) for the appropriate conditions and also with the observed upper cut-off frequency of the whistlers.

Fluctuations of the fundamental and harmonic frequencies of the Schumann (earth-ionosphere cavity) resonances measured near Tromso, Norway, on 28 March 1979 are examined on a timescale of ≲ 20 minutes. Their relationships with two sudden ionospheric disturbance events and one other geophysical event is investigated. The experimentally observed frequency variations are, in some cases, at variance with the theoretical predictions of Madden and Thompson (1965).

One-hop whistlers were recorded simultaneously at three high latitude stations in northern Norway during February 1978 using VLF goniometer receivers. Triangulation of the azimuthal bearings of whistlers received during an auroral substorm from 22.00 to 22.37 UT on 12 February located their whistler exit-points about 100 km to the north-east of Tromso, corresponding to an L -value for all the determinations of 6.4 ± 0.2. The frequency-time profiles of the same whistlers were analysed using the curve-fitting procedure of Tarcsai ( J. atmos. terr. Phys. 37 , 1447, 1975) to determine their L -value of ducted propagation. These were found to lie in the L -value range 2.8–4.0, assuming a diffusive equilibrium distribution along the field lines. The L -values determined for the whistlers' exit-points were thus found to be considerably greater than that corresponding to the field line along which they were ducted. This discrepancy explained by the whistlers first following a field-aligned ducted path through the plasmasphere and then, after being reflected by sporadic-E ionisation in the lower ionosphere, following a sub-protonospheric path ( Carpenter et al., J. geophys. Res. 69 , 5009, 1964) to higher latitude. It is shown by curve-fitting to whistler frequency—time profiles obtained by ray-tracing that such a path combination yields whistler spectra consistent with those observed.

ELF and VLF radio signals were recorded in the afternoon to early morning (local time) between 24 March and 4 April 1979, in Northern Scandinavia. Apart from signals of natural origin, timing signals, i.e. six pips of equal duration of 105 ± 8 ms, at 1 kHz ± 0.5 Hz, were observed on the hour UT. Such signals only occur on days of relatively high geomagnetic activity during enhanced auroral electrojet activity. They are believed to be generated by non-linear demodulation (self-detection) of signals from two or more amplitude modulated transmitters in the USSR, operating at 173, 200, 236, 263 and 657 kHz. The simplest explanation for the observations is provided by the three transmitters operating at 173 kHz.

Observations of naturally occurring very low frequency radio signals were made in Newfoundland during the solar eclipse of 7 March 1970. Some of the atmospherics received were tweeks, normally characteristic of night-time propagation conditions, each of which has a sharp low frequency cut-off which gives information on the height of the ionospheric reflector. This varies from 69 km at the start of the eclipse to 76 km at maximum totality, and to 69 km at the end; these heights are somewhat less than those measured at totality by rocket methods, since the effects of the conical perturbation to the ionosphere are integrated over the propagation path from source to receiver.

We have conducted experiments measuring ELF and VLF radio signals in the afternoon to early morning (local time) between 24 March and 4 April 1979 in Northern Scandinavia. Apart from those of a natural origin (atmospherics, chorus, and so on), we received timing signals, six pips which occurred on the hour UT; the duration of each pip, of frequency 1 kHz±0.5 Hz, was 105±8 ms. A spectrogram of these is shown in Fig. 1. We have established that these are not due to non-linear demodulation of strong higher frequency radio signals in any part of the equipment. We consider that they are produced by non-linear demodulation of signals from two or more amplitude modulated transmitters in the USSR operating at 173, 200, 236, 263 and 657kHz. The demodulation occurs only during enhanced auroral electrojet activity.

Observations of the polar regions from space have led to significant contributions in a variety of scientific disciplines: geophysics, geodesy, geology, glaciology, meteorology, climate, oceanography, biology, and the physics of the upper atmosphere, ionosphere, and magnetosphere. Some results in these diverse fields were described in papers presented at the recent joint COSPAR (Committee on Space Research) and SCAR (Scientific Committee on Antarctic Research) Workshop on Satellite Observations of the Antarctic: Past, Present, and Future. Other results were presented at the COSPAR Symposia on Space Observations for Climate Studies and on Achievements of the International Magnetospheric Study. Each of these meetings was held during the 25th Plenary meeting of COSPAR, which took place in Graz, Austria, from June 25 to July 7, 1984.