Ionosphere Cavity Research Articles

Electric Fields and Plasma Processes in the Auroral Downward Current Region, Below, Within, and Above the Acceleration Region

The downward field-aligned current region plays an active role in magnetosphere-ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. A wealth of related phenomena, including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from sounding rockets and satellites, such as Freja, FAST, Viking, and Cluster, to illustrate the characteristics of the electric fields and related parameters, at altitudes below, within, and above the acceleration region. Special emphasis will be on the high-altitude characteristics and dynamics of quasi-static electric field structures observed by Cluster. These structures, which extend up to altitudes of at least 4-5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former are found to occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at soft plasma boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft indicates that the formation of the electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current often seen is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures appear to be decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The mapping depends on the current-voltage relationship in the downward current region, which is highly non-linear and still unclear, as to its specific form.

ELF Attenuation Factor Derived from Distance Dependence of Radio Wave Amplitude Propagating from an Artificial Source

The distance dependence was used of the artificial extremely low frequency (ELF) radio signal measured in the Atlantic Ocean for establishing the attenuation rate at a particular frequency. We demonstrate that value obtained is in accord with the literature data, as well as with the model based on the Schumann resonance measurements. Deviations do not exceed the standard inaccuracy conditioned by the finite signal to noise ratio. The attenuation factor thus obtained might be used for testing models of the Earth - ionosphere cavity. Absolute calibrations of measurements provided the effective current moment of the field source.

The Schumann resonance: A tool for exploring the atmospheric environment and the subsurface of the planets and their satellites

The propagation of extremely low frequency (ELF) electromagnetic waves and resonance phenomena in the Earth atmosphere has been extensively studied, in relation with ionospheric dynamics, and thunderstorm and lightning activities. A similar investigation can be performed for any other planet and satellite environment, provided this body is wrapped into an ionosphere. There are, however, important differences between Earth and other bodies, regarding the surface conductivity, the atmospheric electron density, the ionospheric cavity geometry, and the sources of electromagnetic energy. In a first approximation, the size of the cavity defines the range of the resonance frequency; the electron density profile, up to the upper atmospheric boundary, controls the wave attenuation; the nature of the electromagnetic sources influences the field distribution in the cavity; and the body surface conductivity, which gives the reflection and transmission coefficients, indicates to what extent the subsurface can be explored. The knowledge of the frequencies and attenuation rates of the principal eigenmodes provides unique information about the electric properties of the cavity. Instruments capable of monitoring the electromagnetic environment in the ELF range are, therefore, valuable payload elements on balloons, descent probes and landers. We develop models for selected inner planets, gaseous giants and their satellites, and review the propagation process of ELF electromagnetic waves in their atmospheric cavities, with a particular emphasis on the application of the Schumann resonance observation to subsurface studies. The instrumentation suitable for monitoring the electromagnetic environment in geophysical cavities is briefly addressed.

A case study on the ELF characterization of the Earth–ionosphere cavity: Forecasting the Schumann resonance intensities

Schumann Resonances (SR) are the electromagnetic (EM) phenomena which occur in the cavity formed by the conducting Earth and the ionosphere, with peak mode frequencies close to 8, 14, 20, 26, 32 Hz, etc. [Price, C., Melnikov, A., 2004. Diurnal, seasonal and inter-annual variations in the Schumann resonance parameters. Journal of Atmospheric and Solar-Terrestrial Physics 66, 1179–1185]. The spectral characteristics of the SR modes are defined by their resonant mode amplitudes, center frequencies and half-widths [Price, C., Melnikov, A., 2004. Diurnal, seasonal and inter-annual variations in the Schumann resonance parameters. Journal of Atmospheric and Solar-Terrestrial Physics 66, 1179–1185]. The objective of this work is two fold: (i) to investigate the characteristics of SR parameters obtained at Şarköy in Turkey and (ii) to model the dynamical characteristics of the SR processes by forecasting the first SR mode power values using a neural network (NN) model. The NN model is revised version of the METUNN [Tulunay, Y., Tulunay, E., Senalp, E.T., 2004a. The neural network technique—1: a general exposition. Advances in Space Research 33, 983–987].

A new numerical model for the simulation of ELF wave propagation and the computation of eigenmodes in the atmosphere of Titan: Did Huygens observe any Schumann resonance?

The propagation of extremely low frequency (ELF) electromagnetic waves in the Earth's ionospheric cavity and the associated resonance phenomena have been extensively studied, in relation with lightning activity. We perform a similar investigation for Titan, the largest moon of Saturn. There are important differences between Earth and Titan, as far as the cavity geometry, the atmospheric electron density profile, and the surface conductivity are concerned. We present an improved 3D finite element model that provides an estimate of the lowest eigenfrequencies, associated quality factors ( Q-factors), and ELF electric field spectra. The data collected by the electric antenna of the Permittivity, Waves, and Altimetry (PWA) instrument reveals the existence of a narrow-band signal at about 36 Hz during the entire descent of Huygens upon Titan. We assess the significance of these measurements against the model predictions, with due consideration to the experimental uncertainties.

Magnetic polarization of the Schumann resonances: An asymptotic theory

Horizontal components of extremely low-frequency (ELF) fields in the gyrotropic Earth–ionosphere cavity are expressed through the fields of an isotropic cavity. A linear approximation of the perturbation method with respect to two small parameters is used. These are the impedance z 0 of the isotropic ionosphere and the ratio γ≡ z 2/ z 1 of the off-diagonal to diagonal elements of the surface impedance matrix of the gyrotropic ionosphere. It is shown that the effect of splitting of the Schumann resonance frequencies caused by the existence of the d.c. geomagnetic field results in an “apparent” angular shift of all ground-based sources of ELF radiation toward the geographic West (“refraction of ELF fields in the gyrotropic Earth–ionosphere cavity”) and excitation of an elliptical magnetic polarization. An analytical relation for the mentioned phenomena is derived. Explicit expressions for polarization characteristics of the electromagnetic ELF noise excited by an arbitrary number of equatorial thunderstorm centers are obtained. Expected values of the “apparent” angular shift for these sources are estimated. Analysis of the estimated magnitudes shows that locations of thunderstorm centers and isolated Q-bursts determined with the use of multi-component ELF measurements should be corrected to exclude the “gyrotropic refraction” effect. Equations to calculate the necessary corrections are presented. The results are generalized to the case of an arbitrary latitudinal location of the sources.

Recent studies of Schumann resonance and ELF transients

Schumann resonance (SR) is the global electromagnetic phenomenon occurring in the Earth – ionosphere cavity at frequencies of 8, 14, 20 Hz, etc (the ELF band). Phenomenon was named after the author of the pioneer publication on the planetary resonance. Since natural electromagnetic signal originates from lightning strokes distributed worldwide, parameters of oscillations reflect the average condition of ionospheric plasma, spatial distribution and current intensity of thunderstorms. ‘Regular’ lightning strokes occur at a rate of 50 events per second, and their electromagnetic radiation forms the SR background. An irregular or transient component exists caused by relatively rare powerful strokes (the rate is about one event per minute or even less) that provide pulses ten times higher than an ordinary discharge. Three orthogonal field components (vertical electric and two crossed horizontal magnetic fields) are recorded at a Schumann resonance field site, and the power spectra are obtained in the frequency band up to 40 Hz averaged over a few minutes with the typical frequency resolution of 0.1 Hz. The main objective of report is to review the recent results in the Schumann resonance studies. The report consists of two parts; one of them summarizes the experimental data obtained during last 3 – 5 years, while the second describes model studies of ELF radio propagation. We try to mention the most important results in the Schumann resonance. The report surveys the data obtained by using ‘classical’ power spectra acquired at different observatories. Power spectra give information on the properties of the lower ionosphere boundary and on the dynamics of the global thunderstorms. For instance, the sudden changes can be detected in the space weather such as Solar Proton Events (SPE); the regular seasonal motion of thunderstorms is monitored, which is associated with the ground surface temperature; unusual changes might be found in the SR spectra prior to disastrous earthquakes, etc. The cross-spectra of both magnetic fields and magnetic and electric fields (Poynting vector) might be obtained when three field components are recorded. Cross-spectra enhance the coherent fields and reduce the noise thus presenting a more exact information on both the source location and the SR parameters. Elliptical polarization of horizontal magnetic field is measured via the cross-spectra technique, and recent comparison with the model computations proved the detection of Schumann resonance line splitting. Accumulated experimental data present a wide area for resolving the inverse electromagnetic problems directed to establishing an unknown instant distribution of the global lightning activity. Resonance parameters measured worldwide allowed for constructing the effective globally averaged models of the lower ionosphere and the dispersion relation for ELF radio waves. Recent model developments are connected with applications of finite element methods to the resonance problem. Simultaneously, the ‘classical’ spectral field representation was transformed into the time domain providing an effective computational algorithm. Analytical time domain solution allowed generating the quasi–random pulse successions modeling the ELF radio noise. The model time domain ELF signals might be synthesized with predetermined properties. In the conclusion of review, we note a renewal of interest towards the extra-terrestrial Schumann resonance studies. The Huygens probe mission on Titan stimulated interest to distant detection of feasible lightning strokes at the planets and their global location by using the planetary resonance concurrently with studies of ionospheres.

Double Layers and Ion Phase-Space Holes in the Auroral Upward-Current Region

The dynamic evolution of the boundary between the ionosphere and auroral cavity is studied using 1D and 2D kinetic Vlasov simulations. The initial distributions of three singly ionized species (H+, O+, e-) are determined from space-based observations on both sides of an inferred strong double layer. The kinetic simulations reproduce features of parallel electric fields, electron distributions, ion distributions, and wave turbulence seen in satellite observations in the auroral upward-current region and, for the first time, demonstrate that auroral acceleration can be driven by a parallel electric field supported, in part, by a quasistable, strong double layer. In addition, the simulations verify that the streaming interaction between accelerated O+ and H+ populations continuously replenished by the double layer provides the free energy for the persistent formation of ion phase-space holes.

Electric properties and related physical characteristics of the atmosphere and surface of Titan

The permittivity, waves and altimetry (PWA) instrument was designed for the investigation of the electric properties and other related physical characteristics of the atmosphere of Titan, from an altitude around 140 km down to the surface. PWA carried sensors to measure the atmospheric conductivity, and record electromagnetic and acoustic waves up to frequencies of 11.5 and 6.7 kHz, respectively. PWA also measured the relief roughness during the descent and the permittivity of the surface after touchdown. The measurements and the results of the preliminary analysis are presented. An ionized layer is detected at altitudes above 50 km, using two independent techniques, and the presence of free electrons in the upper atmosphere is confirmed. An electric signal at around 36 Hz is observed throughout the descent, but it is not yet confirmed that this emission is unambiguously related to a resonance of the ionospheric cavity. The relative dielectric constant of Titan's surface material is nearly 2 and the electric conductivity 4×10 −10 S m −1. The electric properties of the surface seem to evolve after touch-down, possibly due to a local warming of the landing site by the Huygens Probe body.

Ionospheric Alfvén resonator excitation due to nearby thunderstorms

A theory of midlatitude Ionospheric Alfvén Resonator (IAR) excitation due to random cloud‐to‐ground lightning discharges is developed. Electromagnetic wave radiated from the lightning discharges penetrates into the ionosphere, thereby exciting the shear Alfvén and magnetosonic waves in the F region of ionosphere. The IAR arises due to wave reflection from the Alfvén velocity gradients in the topside ionosphere. Typically, the ionospheric resonance cavity accumulates the shear Alfvén wave energy with periods from 1 s to a few tenths of seconds. To proceed analytically, a suitably idealized plane‐stratified model of the medium was used that ignores the magnetic field line curvature and dip angle but includes plasma conductivity variations with altitude. The thunderstorm centers distributed around a ground‐recording station is assumed to be statistically independent sources of the lightning activity, which is a stochastic Poisson process. The lightning onset time and the current moment is supposed to be a random value, while the shape and duration of return strokes are deterministic. Model calculations of the IAR spectrum due to nearby thunderstorm activity were applied to interpret ULF observation made at Karimshino station (52.94°N, 158.25°E) in Kamchatka peninsula. It is shown that the sharp impulses which are in one‐to‐one correspondence with the appearance of the spectral resonance structure (SRS) in dynamic spectrograms can be the result from nearby lightning discharges followed by impulse IAR excitation. The correlation functions and power spectra of the IAR due to random lightning discharge process is studied both analytically and numerically. We found that the nearby thunderstorms in the range of 1000–2000 km make a main contribution to the SRS signature of the midlatitude IAR, whereas the remote/tropic thunderstorm activity is of minor importance. It is not inconceivable that there may exist other permanent mechanisms of the midlatitude IAR excitation, for example, the high‐altitude neutral wind in the E region might also be operative.

Lightning and climate: A review

Research on regional and global lightning activity and the global electrical circuit is summarized. This area of activity has greatly expanded through observations of lightning by satellite and through increased use of the natural (Schumann) resonances of the Earth–ionosphere cavity. The global electrical circuit provides a natural framework for monitoring global change on many time scales. Lightning is responsive to temperature on many time scales, but the sensitivity to temperature appears to diminish at the longer time scales.

Excitation of the ionospheric resonance cavity by thunderstorms

Excitation of the middle latitude ionospheric Alfvén resonator (IAR) due to global thunderstorm activity is considered. The lightning discharges in thunderstorm centers are modeled in terms of a stochastic Poisson process. The lightning onset time and the current moment amplitude are supposed to be random values, while the shape of current moment is assumed to be a given function. Electromagnetic TM mode radiated from the lightning discharges penetrates into the ionosphere via mode coupling to the shear Alfvén and magnetosonic wave modes in the E layer of the ionosphere. The ionospheric resonance cavity can trap and accumulate the shear Alfvén wave energy with periods from one second to a few tenths of seconds, thereby exciting the IAR resonances. Using a simplified model of the topside ionosphere, we have analyzed a possibility for the IAR excitation far from the tropical thunderstorm region. The low‐frequency power spectra of the IAR on the ground surface is studied both analytically and numerically. It is shown that the IAR power spectra exhibit spectral resonance structure (SRS) only during the nighttime conditions. Furthermore, it is found that the calculated spectra are one or two orders of magnitude lower than that observed at Karimshino station in Kamchatka region. This fact suggests that the SRS is not simply due to stochastic lightning discharge process but that some generation mechanisms for the IAR excitation at middle latitudes might also be operative.

Response of the Earth–ionosphere cavity resonator to the 11-year solar cycle in X-radiation

Global changes in the fundamental Earth's Schumann resonances (SR) have been identified on the time scale of the 11-year solar cycle (SC). The first multi-station, multi-field component, multi-mode experimental evidence for solar X-ray modulation of the Earth–ionosphere waveguide is presented. Long-term SR observations covering one solar minimum and two solar maxima, accumulated under different experimental conditions in Antarctica, Hungary and the United States, have been analyzed. The variations of both SR frequencies and quality ( Q ) factors display pronounced maxima during solar maxima #22 and #23, and corresponding minima during the solar minimum of 1996–1997. The observed variations are interpreted on the basis of the uniform extremely low frequency (ELF) mode theory and the presence of two characteristic ionospheric layers responsible for ELF propagation (Greifinger and Greifinger, 1978: Radio Science 13, 831–837; Mushtak and Williams, 2002: Journal of Atmospheric and Solar Terrestrial Physics 64, 1989–2001). The hundredfold changes in the solar X-ray flux dominate the variations in the conductivity profile within the upper characteristic ELF layer (the 90–100 km portion of the E-region), with a negligible effect on the lower characteristic layer (the 50–60 km portion of the D-region). The general increase/decrease of this conductivity by up to one order of magnitude over the solar cycle is responsible for changes in the wave phase velocity and, consequently, in the observed SR frequencies by several tenths of Hz. The change in the scale height of the conductivity profile by several tenths of percent modulates the reflection/dissipation effects within the upper characteristic layer and, therefore, the Q-factors of the resonator. The physical role of the X-radiation in influencing SR is also demonstrated on short ( ∼ 1 h ) time scales, when occasionally variations in X-radiation comparable to the 11-year solar cycle are experienced.

Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster

Abstract. Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ≈1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ≈10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ≈10µA/m2, and upward electron beams with characteristic energies of a few hundred eV to a fewkeV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20s, too short for monitoring the evolution of the structure, but yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.

Suppression of convective cell generation by Alfvén waves in the ionospheric auroral cavity

A theory for the parametric interaction of inertial Alfvén wave (IAW) packets with large-scale convective cells accounting for the nonzero ratio of the Alfvén and light velocities is presented. It is shown that in the auroral plasma cavity where the Alfvén velocity has its maximum value, the parametric interaction of IAWs with convective cells is substantially reduced. In this region the Alfvén velocity is comparable to the speed of light and thus the displacement current cannot be neglected. This current plays a stabilizing role and results in a decline of the parametric instability growth rate. On the contrary, the incorporation of the finite width of the IAW packet leads to increase in the convective cell growth rate. A compact expression for the instability growth rate and the optimal conditions for the fastest growing convective mode are derived. The relevance of the theory to recent spacecraft measurements in the Earth’s ionosphere is pointed out.

Excitation of the ionospheric resonance cavity by neutral winds at middle latitudes

Abstract. A new mechanism for the ionospheric Alfvén resonator (IAR) excitation at middle latitudes is considered. It is shown that the ionosphere wind system in this region is capable of sustaining the generation of geomagnetic perturbations that can be detected by ground magnetometers. The general IAR dispersion relation describing the linear coupling of the shear Alfvén and fast magnetosonic/compressional modes is obtained. The dependence of the IAR eigenfrequencies and damping rates on the perpendicular wave number and on the ground conductivity during the day- and nighttime conditions is analyzed both analytically and numerically. In order to demonstrate the IAR excitation by neutral winds the power spectra of the geomagnetic perturbation on the ground surface are calculated. Furthermore, it is found that Kolmogorov spectra of the ionospheric turbulent neutral winds and the IAR eigenfrequencies lie in the same frequency range that make it possible to enhance the IAR excitation. The relevance of the developed theoretical model to the ground-based observations is stressed.

Polarization Characteristics of Low-Frequency Resonances in the Earth–Ionosphere Cavity

We present the results of measurements and model calculations of the polarization characteristics of ULF–ELF fields in the Earth – ionosphere cavity. It is shown that the horizontal magnetic field in the cavity is elliptically polarized. The ellipticity sign remains constant in the vicinity of the first Schumann resonance. This is explained by the fact that the first resonance frequency splits into a triplet, and one side-band wave of this triplet plays the dominant role in the radio-wave propagation. It is also shown that occasional degeneracy may occur in the cavity within the framework of the hedgehog model of the geomagnetic field.

Finite element modeling of nonlinear dispersive field line resonances: Trapped shear Alfvén waves inside field‐aligned density structures

Using a new two‐dimensional nonlinear finite element model, we investigate the interaction of dispersive shear Alfvén wave (SAW) field line resonances (FLRs) and ion acoustic waves in Earth's magnetosphere. We solve the full set of nonlinear reduced MHD equations self‐consistently in arbitrary geometries. Initially, a Cartesian box model is used to demonstrate the reliability of our numerical solution in determining the linear and nonlinear evolution of FLRs. Then the full reduced MHD equations with the effects of electron inertia, ion Larmor radius correction, and electron thermal pressure are solved in dipolar and stretched magnetic topologies. We show that time‐dependent dispersion and density steepening lead to localization of a highly structured FLR within an ionospheric (equatorial) density cavity (bump). When nonlinear effects are accounted for, we find that FLRs preferentially form in regions of low wave dispersion. Field line stretching and ponderomotive density redistribution lead to a significant reduction in FLR eigenfrequencies, bringing them into the range of observations. Nonlinear effects also cause a rapid acceleration of the timescale over which small perpendicular spatial scales appear. In our model, it is shown that density perturbations can be comparable to the equilibrium background density.

Nonlinear acceleration of dispersive effects in field line resonances

Nonlinear dynamics of a dispersive field line resonance (FLR) is investigated in a dipolar magnetic geometry using a new finite element code TOPO. Time dependent dispersion and steepening of the perpendicular Alfvén velocity profile lead to the acceleration of dispersive effects and to localization of the FLR within an ionospheric density cavity. Consequently, shear Alfvén waves are trapped between turning points inside nonlinear density perturbations. This results in a complicated nonlinear FLR structure and an associated overlap of timescales for dispersion and nonlinearity. We show that nonlinear effects lead to fine scale structuring produced by the interplay of nonlinear and dispersive effects. We discuss the relevance of our results to observations of FLR's in the auroral zone.

Electric fields and currents of stable drifting auroral arcs in the evening sector

The spatial distribution of electric fields, conductances, and currents of steadily drifting medium‐scale (15–50 km) arcs in the evening sector (20–23 magnetic local time (MLT)) is obtained from European Incoherent Scatter Radar (EISCAT) and optical ground‐based measurements. The current systems of stable arcs residing in the northward convection electric field region show a consistent pattern: currents flow downward on the equatorward side of the arcs, then poleward, and upward from the arcs. In one event where the arcs are located in a region of convection reversals, the current pattern is more complicated. Most of the arcs are associated with an enhanced northward‐directed electric field region on the equatorward side of the arc, colocated with downward field‐aligned currents (FACs) and suppressed E and F region electron densities. The width of the region of the enhanced electric field is one to four times the width of the arc. In some cases, the electron density reduction is so pronounced that the region can be described as an auroral ionospheric density cavity. The electrostatic magnetosphere–ionosphere coupling model of arcs predicts that the width L of an arc is related to the ionospheric Pedersen conductance ΣP and the “field‐aligned conductance” K by . This study shows that stable medium‐scale arcs in the evening sector obey this equation. A value of K = 2 × 10−8 S m−2 is obtained for 15–35 km wide arcs. It is argued that the large value of the field‐aligned conductance cannot be interpreted in terms of the adiabatic theory. Possibly the high value of K results from nonadiabatic processes acting on the current‐carrying electrons.