Realistic Magnetic Field Model Research Articles

Hourglass Magnetic Field of a Protostellar System

An hourglass-shaped magnetic field pattern arises naturally from the gravitational collapse of a star-forming gas cloud. Most studies have focused on the prestellar collapse phase, when the structure has a smooth and monotonic radial profile. However, most observations target dense clouds that already contain a central protostar, and possibly a circumstellar disk. We utilize an analytic treatment of the magnetic field along with insights gained from simulations to develop a more realistic magnetic field model for the protostellar phase. Key elements of the model are a strong radial magnetic field in the region of rapid collapse, an off-center peak in the magnetic field strength (a consequence of magnetic field dissipation in the circumstellar disk), and a strong toroidal field that is generated in the region of rapid collapse and outflow generation. A model with a highly pinched and twisted magnetic field pattern in the inner collapse zone facilitates the interpretation of magnetic field patterns observed in protostellar clouds.

Detecting ALP wiggles at TeV energies

Axions and axion-like-particles (ALPs) are characterised by their two-photon coupling, which entails so-called photon-ALP oscillations as photons propagate through a magnetic field. These oscillations lead to distinctive signatures in the energy spectrum of high-energy photons from astrophysical sources, allowing one to probe the existence of ALPs. In particular, photon-ALP oscillations will induce energy dependent oscillatory features, or “ALP wiggles”, in the photon spectra. We propose to use the discrete power spectrum to search for ALP wiggles and present a model-independent statistical test. By using PKS 2155-304 as an example, we show that the method has the potential to significantly improve the experimental sensitivities for ALP wiggles, and that the ALP wiggles may be detected using the Cherenkov Telescope Array (CTA) for optimistic values of the photon-ALP coupling constant and the magnetic field. Moreover, we discuss how these sensitivities depend on the modelling of the magnetic field. We find that the use of realistic magnetic field models, due to their larger cosmic variance, substantially enhances detection prospects compared to the use of simplified models.

Distributed magnetic attitude control for large space structures

The utility of distributed magnetic torque rods for the attitude control of a large space structure is investigated. Distributed arrays of actuators offer advantages such as distributing structural loads, increasing fault tolerance, allowing structures to be designed modularly, and additionally the actuators may be integrated with on-orbit fabrication strategies. First, distributed torques are shown to effectively rotate highly flexible structures. This is compared with torques applied to the centre-of-mass of the structure, which cause large surface deformations and can fail to enact a rotation. This is demonstrated using a spring–mass model of a planar structure with embedded actuators. A distributed torque algorithm is then developed to control an individually addressable array of actuators. Attitude control simulations are performed, using the array to control a large space structure, again modelled as a spring–mass system. The attitude control system is demonstrated to effectively detumble a representative 75 × 75 m flexible structure, and perform slew manoeuvres, in the presence of both gravity-gradient torques and a realistic magnetic field model.

Structure-preserving techniques in accelerator physics

To very good approximation, particularly for hadron machines, charged-particle trajectories in accelerators obey Hamiltonian mechanics. During routine storage times of eight hours or more, such particles execute some revolutions about the machine, oscillations about the design orbit, and passages through various bending and focusing elements. Prior to building, or modifying, such a machine, we seek to identify accurately the long-term behaviour and stability of particle orbits over such large numbers of interactions. This demanding computational effort does not yield easily to traditional methods of symplectic numerical integration, including both explicit Yoshida-type and implicit Runge–Kutta or Gaussian methods. As an alternative, one may compute an approximate one-turn map and then iterate that map. We describe some of the essential considerations and techniques for constructing such maps to high order and for realistic magnetic field models. Particular attention is given to preserving the symplectic condition characteristic of Hamiltonian mechanics.

Quasiperiodic 1‐Hour Alfvén Wave Resonances in Saturn's Magnetosphere: Theory for a Realistic Plasma/Field Model

AbstractWe model the omnipresent quasiperiodic 60 min waves in Saturn's outer magnetosphere as field line resonances adopting a realistic magnetic field model and a measurement‐based plasma density distribution using Alfvén wave resonance theory for arbitrary field geometries (Singer et al., 1981). The modeled eigenfrequencies for the second and higher modes are roughly independent of invariant latitude, mapping into large regions of the magnetosphere up to at least 20 RS, and the third and fourth harmonic modes having close to 1 h eigenperiod. The model predicts the normalized amplitudes of these higher modes of the magnetic field perturbations at high latitudes to exceed the amplitudes at the plasma sheet, in agreement with the observations of more frequent occurrences at mid‐to‐high latitudes. The periods of the higher order field line resonances in the outer magnetosphere differ considerably from those in a dipole model, illustrating the importance of a realistic field model.

On the Use of Different Magnetic Field Models for Simulating the Dynamics of the Outer Radiation Belt Electrons During the October 1990 Storm

AbstractDuring geomagnetic storms, the magnetic field in the outer belt is known to be significantly distorted by the solar wind, such that the outer belt dynamics can be greatly impacted by the magnetic field topology. In this context, we develop a reduced Fokker‐Planck code that includes the effects related to realistic magnetic field models (by using the existing dedicated LANLGeoMag library) through the steps of preprocessing, postprocessing, and, for the first time, during the computation of the reduced Fokker‐Planck diffusion equation itself. We perform the solutions of the reduced Fokker‐Planck equation in the framework of the geomagnetic storm that occurred from 9 October to 15 October 1990. With the use of Combined Release and Radiation Effects Satellite observations, the magnetic field model is shown to strongly affect the way of conciliating theory with observations (processing steps). More specifically, we explain analytically and numerically why the use of a dipole field can lead to misleading interpretations on the local enhancements (attributed to local acceleration) displayed by the electron distribution function, resulting in inaccurate simulations results at large L‐shells. The consideration of a realistic field does not produce any artificial peaks. With such corrected data sets, a great part of the dynamics can be described by radial transport and is thus better reproduced by the simulations. This crucial importance of the field geometry is further emphasized with the calculation of unidirectional, omnidirectional, and integral electron fluxes, and their accuracy is quantified thanks to dedicated metrics.

Frequency variability of standing Alfvén waves excited by fast mode resonances in the outer magnetosphere

AbstractCoupled fast mode resonances (cFMRs) in the outer magnetosphere, between the magnetopause and a turning point, are often invoked to explain observed discrete frequency field line resonances. We quantify their frequency variability, applying cFMR theory to a realistic magnetic field model and magnetospheric density profiles observed over almost half a solar cycle. Our calculations show that cFMRs are most likely around dawn, since the plasmaspheric plumes and extended plasmaspheres often found at noon and dusk can preclude their occurrence. The relative spread (median absolute deviation divided by the median) in eigenfrequencies is estimated to be 28%, 72%, and 55% at dawn, noon, and dusk, respectively, with the latter two chiefly due to density. Finally, at dawn we show that the observed bimodal density distribution results in bimodal cFMR frequencies, whereby the secondary peaks are consistent with the so‐called “CMS” frequencies that have previously been attributed to cFMRs.

Numerical modelling of the lobes of radio galaxies in cluster environments – II. Magnetic field configuration and observability

We describe three-dimensional magnetohydrodynamical modelling of powerful radio galaxies in realistic poor cluster environments. This modelling extends our earlier work on the hydrodynamics of radio galaxies as a function of their cluster environment to consider the magnetic field configuration in the lobes and its observational consequences, using a realistic model for the magnetic field in the intracluster medium, very high density contrast in the lobes and high numerical resolution. We confirm, now with a realistic magnetic field model, that lobes have characteristic trajectories in the radio power/linear size diagram which depend strongly on their environment. We investigate the detailed evolution of polarized emission, showing that the lobes evolve from the initially ordered field configuration imposed by our boundary conditions to one in which the longitudinal field comes to dominate. We obtain simulated observations of polarization whose properties are quantitatively consistent with observations. The highly spatially intermittent magnetic field also reproduces the observation that inverse-Compton emission from lobes is much smoother than synchrotron. Our simulations allow us to study the depolarizing effect of the external medium on the lobes, and so to demonstrate that Faraday depolarization from environments of the type we consider can reproduce the integrated fractional polarization properties of large samples and the observed preferential depolarization of the receding lobe.

Modeling ionospheric super‐fountain effect based on the coupled TIMEGCM‐SAMI3

AbstractRecently, efforts have been undertaken to develop a coupled thermosphere‐ionosphere‐plasmasphere model based on two well‐established models, namely, the Thermosphere‐Ionosphere‐Mesosphere General Circulation Model (TIMEGCM) developed at the National Center for Atmospheric Research and the SAMI3 ionosphere model developed at the Naval Research Laboratory. This paper presents the first results from the coupled model on the investigation of a prompt penetration electric field (PPEF) event that took place on 9 November 2004. The coupled model eliminates two major upper boundary limitations of the stand‐alone TIMEGCM, e.g., the upper boundary height and the prescribed O+ fluxes at the upper boundary. It is found that the F‐layer peak height is raised above 800 km in response to the large PPEF. The O+ fluxes in the top ionosphere vary drastically during the course of the PPEF, with strong upward and downward fluxes with a magnitude greater than 109 cm−2 s−1 in localized regions. For the first time, the coupled model allows us to simulate and visualize the super‐fountain effect on a global scale. Future model development is also envisaged, including the implementation of a more realistic magnetic field model and a fully two‐way coupling between neutrals and ions.

Intensification of dayside diffuse auroral precipitation: contribution of dayside Whistler-mode chorus waves in realistic magnetic fields

Abstract. Compared to the recently improved understanding of nightside diffuse aurora, the mechanism(s) responsible for dayside diffuse aurora remains poorly understood. While dayside chorus has been thought as a potential major contributor to dayside diffuse auroral precipitation, quantitative analyses of the role of chorus wave scattering have not been carefully performed. In this study we investigate a dayside diffuse auroral intensification event observed by the Chinese Arctic Yellow River Station (YRS) all-sky imagers (ASI) on 7 January 2005 and capture a substantial increase in diffuse auroral intensity at the 557.7 nm wavelength that occurred over almost the entire ASI field-of-view near 09:24 UT, i.e., ~12:24 MLT. Computation of bounce-averaged resonant scattering rates by dayside chorus emissions using realistic magnetic field models demonstrates that dayside chorus scattering can produce intense precipitation losses of plasma sheet electrons on timescales of hours (even approaching the strong diffusion limit) over a broad range of both energy and pitch angle, specifically, from ~1 keV to 50 keV with equatorial pitch angles from the loss cone to up to ~85° depending on electron energy. Subsequent estimate of loss cone filling index indicates that the loss cone can be substantially filled, due to dayside chorus driven pitch angle scattering, at a rate of ≥0.8 for electrons from ~500 eV to 50 keV that exactly covers the precipitating electrons for the excitation of green-line diffuse aurora. Estimate of electron precipitation flux at different energy levels, based on loss cone filling index profile and typical dayside electron distribution observed by THEMIS spacecraft under similar conditions, gives a total precipitation electron energy flux of the order of 0.1 erg cm−2 s−1 with ~1 keV characteristic energy (especially when using T01s), which can be very likely to cause intense green-line diffuse aurora activity on the dayside. Therefore, dayside chorus scattering in the realistic magnetic field can greatly contribute to the YRS ASI observed intensification of dayside green-line aurora. Besides wave induced scattering and changes in the ambient magnetic field, variations in associated electron flux can also contribute to enhanced diffuse aurora emissions, the possibility of which we cannot exactly rule out due to lack of simultaneous observations of magnetospheric particles. Since the geomagnetic activity level was rather low during the period of interest, it is reasonable to infer that changes in the associated electron flux in the magnetosphere should be small, and consequently its contribution to the observed enhanced diffuse auroral activity should be small as well. Our results support the scenario that dayside chorus could play a major role in the production of dayside diffuse aurora, and also demonstrate that changes in magnetospheric magnetic field should be considered to reasonably interpret observations of dayside diffuse aurora.

Bounce‐averaged diffusion coefficients due to resonant interaction of the outer radiation belt electrons with oblique chorus waves computed in a realistic magnetic field model

We present the results of calculations of the bounce‐averaged pitch angle, mixed, and momentum diffusion coefficients in a dipole and two realistic field models (the T01s model for quiet and storm conditions). We consider resonant interactions of the outer radiation belt electrons with oblique chorus waves. We demonstrate that on the dayside, the use of a realistic magnetic field versus a dipole field only makes a significant difference for small equatorial pitch angles at energies larger than E = 1 MeV. On the nightside, the differences between the scattering rates calculated in the Tsyganenko and dipole models can reach several orders of magnitude at various equatorial pitch angles for electrons with E ≥ 0.5 MeV. The most significant changes in the scattering rates computed in the realistic and dipole magnetic fields occur during the geomagnetically active conditions. On the nightside, for E ≥ 0.5 MeV, the diffusion coefficients calculated in the Tsyganenko field show significant scattering near the edge of the loss cone that can produce loss of electrons to the atmosphere, while in the dipole model there is no scattering at small equatorial pitch angles. Our computations in the realistic field show that resonant interactions between electrons with E ≤ 1 MeV and chorus waves can be an effective net loss mechanism on both the dayside and the nightside. To explain the differences in the scattering rates associated with a change in the magnetic field model, we present the contribution of various resonant harmonics to the diffusion and examine the changes in the resonance condition.

The role of drift orbit bifurcations in energization and loss of electrons in the outer radiation belt

[1] Radiation levels in Earth's outer electron belt (L ≳ 2.5) vary by orders of magnitude on the time scales ranging from minutes to days. Multiple acceleration and loss processes operate across the belt and compete in defining its global variability. One such process is the drift orbit bifurcation effect. Caused by coupling of the drift and bounce motions, it breaks the second adiabatic invariant of radiation belt electrons producing their transport in radius and pitch angle. In this paper we investigate implications of drift orbit bifurcations to the global state and variability of the outer electron belt. For this purpose we use three-dimensional test particle simulations of electron guiding center motion in a realistic magnetic field model. We show that even at most quiet solar wind conditions bifurcations affect a broad range of the belt penetrating inside the geosynchronous orbit. This has an important practical implication for the analysis of experimental particle data: since the third adiabatic invariant is undefined for bifurcating orbit, the electron phase space density cannot be expressed in terms of three adiabatic invariants. We show that long-term transport of electrons due to drift orbit bifurcations is a complex combination of large ballistic jumps and small-amplitude diffusion in the second invariant and radial location. To model long-term transport, we derive an empirical map of the second invariant and radial jumps at bifurcations. The map can also be implemented by other radiation belt models, which cannot directly account for the physics of drift orbit bifurcations. Drift orbit bifurcations can produce electron losses through the magnetopause escape and through pitch angle scattering into the atmospheric loss cone. Most electrons, however, can stay quasi-trapped in the bifurcation regions for very long time periods. The pitch angle and radial transport due to drift orbit bifurcations lead to their meandering back and forth across the region producing mixing and recirculation of particle populations with different initial conditions. We show that this recirculation can greatly amplify electron energization by radial diffusion. Compared to the diffusion alone, the combined action of radial diffusion and drift orbit bifurcations can double electron energization at each recirculation cycle. Our results suggest that drift orbit bifurcations can play an important role in the buildup of increased electron fluxes in the storm recovery phase.

Chorus-driven resonant scattering of diffuse auroral electrons in nondipolar magnetic fields

[1] We perform a comprehensive analysis of resonant scattering of diffuse auroral electrons by oblique nightside chorus emissions present along a field line with an equatorial crossing of 6 RE at 00:00 MLT, using various nondipolar Tsyganenko magnetic field models. Bounce-averaged quasi-linear diffusion coefficients are evaluated for both moderately and actively disturbed geomagnetic conditions using the T89, T96, and T01s models. The results indicate that inclusion of nondipolar magnetic field leads to significant changes in bounce-averaged rates of both pitch angle and momentum diffusion for 200 eV to 10 keV plasma sheet electrons. Compared to the results using a dipole field, the rates of pitch angle diffusion obtained using the Tsyganenko models are enhanced at all resonant pitch angles for 200 eV electrons. In contrast, for 500 eV to 10 keV electrons the rates of pitch angle scattering are enhanced at intermediate and/or high pitch angles but tend to be considerably lower near the loss cone, thus reducing the precipitation loss compared to that in a dipole field. Upper band chorus acts as the dominant cause for scattering loss of 200 eV to 2 keV electrons, while lower band chorus scattering prevails for 5–10 keV electrons, consistent with the results using the dipole model. The first-order cyclotron resonance and the Landau resonance are mainly responsible for the net scattering rates of plasma sheet electrons by oblique chorus waves and also primarily account for the differences in bounce-averaged diffusion coefficients introduced by the use of Tsyganenko models. As the geomagnetic activity increases, the differences in scattering rates compared to the dipole results increase accordingly. Nonnegligible differences also occur particularly at high pitch angles for the diffusion rates between the Tsyganenko models, showing an increase with geomagnetic activity level and a dependence on the discrepancy between the Tsyganenko model fields. The strong dependence of bounce-averaged quasi-linear scattering rates on the adopted global magnetic field model and geomagnetic activity level demonstrates that realistic magnetic field models should be incorporated into future modeling efforts to accurately quantify the role of magnetospheric chorus in driving the diffuse auroral precipitation and the formation of electron pancake distributions.

On the latitudinal extent of chorus emissions as observed by the Polar Plasma Wave Instrument

[1] The statistical distribution of chorus wave power in the off-equatorial region is evaluated using data from the Plasma Wave Instrument (PWI) Sweep Frequency Receiver (SFR) on board the Polar spacecraft. Maps of average wave power in the meridional plane divided into four local time sectors are presented. The geomagnetic dependence of wave power is examined, and substorm activity and enhanced solar wind speed result in distinctly different wave distributions. The maximum latitudinal extent of chorus as a function of latitude and L* is estimated within the orbit constraints of the spacecraft, and on the basis of this the corresponding minimum resonant energy for first-order relativistic cyclotron resonance is calculated using a realistic magnetic field model.

Ion composition in the plasma trough and plasma plume derived from a Combined Release and Radiation Effects Satellite magnetoseismic study

The mass and energy carried in the magnetosphere by heavy ions, O+ in particular, are known to increase as geomagnetic activity increases. However, the ion composition in the magnetosphere has not been fully specified since measurements of the flux of different ion species from the ionospheric thermal energy (below 1 eV) to the ring current energy (above 100 keV) are difficult with single‐particle instruments. We used mass density determined by a magnetoseismology technique and the electron density derived from measured plasma wave spectra to investigate the ion composition and total mass density for Combined Release and Radiation Effects Satellite (CRRES) orbit 962 (27−28 August 1991). This orbit occurred during a geomagnetic storm and included afternoon passes through the plasmasphere, the plasma trough, and a plasma plume, where these plasma regions were identified using the electron density ne. In the magnetoseismology analysis, we determined the fundamental frequency of the toroidal standing Alfvén waves fT1 from the electric and magnetic field data and then inferred the corresponding total mass density ρtotal at the satellite by solving an MHD wave equation with a realistic magnetic field model and a realistic assumption for the mass distribution along the field line. The value of fT1 changed little when the spacecraft moved between the plasma trough and the plasma plume, implying the dominance of heavy ions in the plasma trough. From the values of ne and ρtotal, we derived quantities associated with O+ by assuming that the plasma consisted of three ions, H+, He+, and O+. In the plasma trough, O+ ions are found to carry a number density of ∼10 cm−3, ∼50% of the number density, and ∼90% of the mass density. On the other hand, O+ is found to be much less dominant in the plasma plume. Our results are consistent with DE‐1 studies of the formation of an oxygen torus at the outer edge of the H+ plasmapause during geomagnetic active periods and with GEOS‐1 and GEOS‐2 studies that reported strong dependence of O+ density on geomagnetic activity and on solar extreme ultraviolet flux. In addition, our events indicate that the plasma plume boundary, defined in terms of the number density of electrons or light ions (H+ and He+), may not exhibit similar structure in the total mass density that can be readily detected using magnetoseismology techniques.

Evolution of the proton ring current energy distribution during 21–25 April 2001 storm

Three ring current models are used to follow the evolution of the proton ring current during the 2001‐04‐21–25 storm: The ring current model combined with tracing particles numerically in the drift approximation by Ganushkina et al. (2005), the empirical model of proton fluxes in the inner magnetosphere developed by Milillo et al. (2003), and the kinetic ring current‐atmosphere interaction model (RAM) by Liemohn et al. (2001). The paper focuses on the effects of the electric and magnetic field models and initial particle distributions on the final energy distribution. We examine a variety of large‐scale magnetic field and convection electric field models as well as substorm‐associated, smaller‐scale, and time‐varying electric fields. We find that (1) using more realistic magnetic field models leads to reduction of the ring current energy content by ∼30%; (2) details of the global convection field have little influence on the overall ring current evolution; (3) smaller‐scale impulsive electric field have profound effects on the ring current evolution, particularly with regard to the acceleration of the higher‐energy particles; and (4) in the ring current models, the choice of the initial and boundary conditions have significant effects on the modeled ring current intensity and energy spectrum.

Electron pitch angle distributions following the dipolarization phase of a substorm: Interball‐Tail observations and modeling

We investigate Interball‐Tail observations of electron pitch angle distributions after the dipolarization phase of a substorm. For 10 keV electrons we observe beamlike, coniclike, and perpendicularly peaked distributions at L ∼ 11, L ∼ 9, and L ∼ 7, respectively. We examine the efficiency of betatron heating and Fermi acceleration associated with adiabatic transport of the electrons during the substorm dipolarization phase. This dipolarization phase was modeled using transition between different Kp levels within a realistic magnetic field model. The calculations reproduce well the evolution of the high‐energy electron flux in the parallel and perpendicular directions. They also reproduce well the pitch angle distribution observed by Interball‐Tail at 10 keV, after the dipolarization phase. It is shown that Fermi acceleration is the leading process, compared to betatron heating. The production of the coniclike distributions is narrowly linked to the existence of a transition region between dipolelike and taillike magnetic fields, at about L ∼ 9.

A satellite study of dayside auroral conjugacy

Abstract. A study of dayside auroral conjugacy has been done using the cleft/boundary layer auroral particle boundaries observed by the DMSP-F7 satellite in the southern hemisphere and the global UV auroral images taken by the Viking spacecraft in the northern hemisphere. The 22 events have been studied on the basis of an internal IGRF 1985 magnetic field; it is shown that there is a displacement of up to 4° in latitude from the conjugate points with the northern aurora appearing to be located poleward of the conjugate point. No local time dependence of the north-south auroral location difference was seen. The use of a more realistic magnetic field model for tracing field lines which incorporates the dipole tilt angle and Kp index, the Tsyganenko 1987 long model plus the IGRF 1985 internal magnetic field model, appears to organize the data better. Although with this external plus internal model some tracings did not close in the opposite hemisphere, 70% of those that did indicated satisfactory conjugacy. The study shows that the degree of auroral conjugacy is dependent upon the accuracy of the magnetic field model used to trace to the conjugate point, especially in the dayside region where the field lines can either go to the dayside magnetopause near the subsolar point or sweep all the way back to the flanks of the magnetotail. Also the discrepancy in the latitude of northern and southern aurora can be partially explained by the displacement of the neutral sheet (source region of the aurora) by the dipole tilt effect.

A satellite study of dayside auroral conjugacy

<strong class="journal-contentHeaderColor">Abstract.</strong> A study of dayside auroral conjugacy has been done using the cleft/boundary layer auroral particle boundaries observed by the DMSP-F7 satellite in the southern hemisphere and the global UV auroral images taken by the Viking spacecraft in the northern hemisphere. The 22 events have been studied on the basis of an internal IGRF 1985 magnetic field; it is shown that there is a displacement of up to 4^° in latitude from the conjugate points with the northern aurora appearing to be located poleward of the conjugate point. No local time dependence of the north-south auroral location difference was seen. The use of a more realistic magnetic field model for tracing field lines which incorporates the dipole tilt angle and <i>Kp</i> index, the Tsyganenko 1987 long model plus the IGRF 1985 internal magnetic field model, appears to organize the data better. Although with this external plus internal model some tracings did not close in the opposite hemisphere, 70% of those that did indicated satisfactory conjugacy. The study shows that the degree of auroral conjugacy is dependent upon the accuracy of the magnetic field model used to trace to the conjugate point, especially in the dayside region where the field lines can either go to the dayside magnetopause near the subsolar point or sweep all the way back to the flanks of the magnetotail. Also the discrepancy in the latitude of northern and southern aurora can be partially explained by the displacement of the neutral sheet (source region of the aurora) by the dipole tilt effect.

Hydromagnetic wave activity detected by the GEOS 2 Double‐Probe Experiment during 1979

I make a comparative survey of wave period and amplitude measurements for hydromagnetic disturbances detected by the GEOS 2 double‐probe experiment in 3 months during 1979. Interpretation of these data is aided through knowledge of the average mass to charge ratio of the ion component of the background magnetospheric plasma. These ratios are obtained from comparison of the observed period to a WKB period estimate based on a realistic magnetic field model, and measured plasma parameters. For data recorded in January 1979, event amplitudes and occurrence were greatest in the early afternoon, corresponding to background electron densities in the range 5–10 cm−3, and mass to charge ratios between about 1.6 and 6. Values of mass to charge ratio decrease systematically with local time, with high early morning values indicating a preponderance of heavy ions (possibly O+) at the times of the wave activity. By dusk, mass to charge ratios are close to unity, pointing to the dominance of H+ ions. For the magnetically quiet month of July 1979, amplitudes and occurrence were greatest in the morning hours, with the effects of plasma trough filling and depletion on wave periods clearly visible. Qualitative estimates show that, for this month, no clear trend exists in the local time variations of mass to charge ratio. Wave activity registered in the disturbed equinoctial period of March 6 to April 5, 1979, indicates strong excitation, particularly in the morning hours, with a broad spread in period, and no systematic change in mass to charge ratio through local time.