Ducted propagation of hydromagnetic waves in the F2 layer

Calculations of the properties of the ionospheric duct centered at the F₂ layer are carried out with a view to investigating the ducted propagation of Pc1 micropulsations in directions out of the geomagnetic meridian plane. For a horizontally uniform ionosphere, duct properties are found to be essentially the same in all horizontal directions. Propagation characteristics of ducted waves, however, vary according to ionospheric and sunspot conditions. In practice, therefore, it is expected that horizontal propagation over a large recording network is not isotropic because of the diurnal changes in the ionosphere.     

Ducted propagation of hydromagnetic waves in the upper ionosphere

This paper presents a theoretical investigation of the ducting of Pc1 hydromagnetic waves in an ionospheric layer situated above the F₂ region. Theoretical calculations show that this upper ionospheric duct may also sustain horizontal propagation of Pc1 pulsations over appreciable distances. It is found that there is a low-frequency cutoff as in the case of the F₂ layer waveguide. The group velocity of waves in the upper ionospheric duct is considerably greater, and dispersion is more pronounced compared to the ducted propagation in the F₂ region.     

Propagation characteristics of hydromagnetic waves in various layers of the ionosphere

This paper derives the basic propagation characteristics of hydromagnetic waves in various layers of the ionosphere. It is shown that propagation in the upper ionosphere and the F₂ layer is largely isotropic. In the lower region of the ionosphere there are two possible modes of propagation, both being anisotropic. Propagation characteristics of waves in this lower region, however, are relatively independent of the direction of horizontal propagation. Calculations of intrinsic wave attenuation show that ducted propagation of Pc 1 signals over appreciable horizontal distances may only take place in the upper layers of the ionosphere.     

On the vertical electric component of the geomagnetic pulsation field

The existence of a close correlation between the atmospheric electric field and geomagnetic pulsations in the range of Pc 2–4 is demonstrated experimentally and theoretically (from the horizontal propagation of pulsations). The need to take this into account in any geological-geophysical interpretation of experimental data is stressed.     

Ionospheric dispersion and Pc 1 pulsations

Dispersion measurements were performed on geomagnetic pulsation data recorded over an Australasian network in a search for evidence of ionospheric dispersion of Pc 1 signals. A method of analysis was adopted in which the slope of emission elements of a selected Pc 1 event are examined individually. It has been found that there are no significant ionospheric dispersion effects for propagation between middle and low latitudes. Magnetospheric propagation paths calculated from dispersion measurements show large variations and are not considered generally reliable.     

Some properties of geomagnetic field pulsations in the 0.1–1.0-Hz frequency range: A quantitative description and comparison with the satellite and ground-based observations

By using an image-dipole magnetic field model for a variety of plasma density profiles we have studied the latitude effect of the 0.1–1.0-Hz hydromagnetic wave propagation in the Earth's magnetosphere. On comparing the results of signal group delay time calculations for dipole and model magnetic fields with ground and satellite observations we obtain some propagation characteristics of Pc1s and localize the regions of their generation. Our results show that most high-latitude Pc1 events are generated in the outer magnetosphere in accordance with ground and satellite observations and theoretical considerations. The non-dipole geometry of the geomagnetic field in the outer magnetosphere (at geomagnetic latitudes φ₀ > 66°, L > 6) has a significant effect on the hydromagnetic wave propagation.     

Source structure and dynamics of Pc 1 pulsations at low latitudes

Structured Pc 1 signals propagate in the ionospheric F2 region duct from their secondary sources at high latitudes to lower latitudes. Propagation directions to low latitude stations can be inferred from measurements of polarization parameters. The analysis of five events recorded at two low latitude stations (L = 1.9) are presented. Direction of arrival measurements are used to investigate the spatial and temporal structure of Pc 1 sources. Results show a close relationship between the structure of events identified in the frequency-time representation and direction of arrival measurement patterns. Multiple sources are sometimes indicated.     

Pc 1–2 observations of heavy ion effects by synchronous satellite ATS-6

Ion cyclotron waves generated in the magnetosphere by the ion cyclotron instability of protons are thought to be the origin of Pc 1–2 geomagnetic pulsations. Propagation characteristics of these waves have been measured using ATS-6 synchronous satellite magnetometer wave data. Of particular interest are the wave spectra, polarization properties, and wave diagnostics; all are characteristic of propagation in a cool ambient magnetospheric plasma containing He⁺ and O⁺ heavy ions.     

Multipoint observations of low latitude ULF Pc3 waves in south-east Australia

Geomagnetic pulsations recorded on the ground are the signatures of the integrated signals from the magnetosphere. Pc3 geomagnetic pulsations are quasi-sinusoidal variations in the earth’s magnetic field in the period range 10–45 seconds. The magnitude of these pulsations ranges from fraction of a nT (nano Tesla) to several nT. These pulsations can be observed in a number of ways. However, the application of ground-based magnetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the earth’s magnetosphere. The solar wind provides the energy for the earth’s magnetospheric processes. Pc3–5 geomagnetic pulsations can be generated either externally or internally with respect to the magnetosphere. The Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers were established in south-east Australia over a longitudinal range of 17 degrees at L = 1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400–700 km/s. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing oscillations of magnetospheric field lines.     

Local time asymmetry in the characteristics of Pc5 magnetic pulsations

The wave characteristics of Pc5 magnetic pulsations are analyzed with data of OGO-5, ISEE-1 and -2 satellites. The toroidal modes (δB_D >δB_H) of Pc5 pulsations are observed at a higher magnetic latitude in the dawnside outer magnetosphere. The compressional and poloidal modes (δB_z.dfnc; ～ δB_H >δB_D) of Pc5 pulsations are mostly observed near the magnetic equator in the duskside outer magnetosphere. This L.T. asymmetry in the occurrence of dominant modes of Pc5's in space can be explained by the velocity shear instability (Yumoto and Saito, 1980) in the magnetospheric boundary layer, where Alfvénic signals in the IMF medium are assumed to penetrate into the magnetospheric boundary layer along the Archimedean spiral. The asymmetrical behaviour of Pc5 pulsation activity on the ground across the noon meridian can be also explained by the ionospheric screening effect on the compressional Pc5 magnetic pulsations. The compressional modes with a large horizontal wave number in the duskside magnetosphere are expected to be suppressed on the ground throughout the ionosphere and atmosphere.     

Wave packet propagation in an amplifying medium and its application to the dispersion characteristics and to the generation mechanisms of Pc 1 events

The formulae which give the propagation characteristics of a wave packet in a dispersive and amplifying medium, are established. Application is made to the propagation of Pc 1 elements through a magnetosphere constituted of a cold plasma and a high energy proton population. It is shown that the spectral shape, in a frequency-time coordinate system, of the Pc 1 elements is related to two terms : v = d²ω/dk², which represents the variation of the group velocity with frequency and which depends only on the cold plasma characteristics, and μ = -d²γ/dk², in which γ is the amplification coefficient depending on the frequency and which is related to the high energy particle distribution function. When v ? μ, only the usual dispersion effects occur, but a new method is found for determining the line of force on which the micropulsations are generated, without making any assumption about the cold plasma density distribution inside the magnetosphere. It is also possible to deduce some characteristics about the high energy proton distribution. Theoretical computations are presented, which give the frequency variation of the amplification coefficient as a function of the e-folding energy and the anisotropy factor of these high energy protons. Applications are made to ~30 pearl events which are analysed in detail according to this theory. When μ ? v, other effects do appear. After a preliminary phase, the pearl elements can become parallel for a while, or even re-erect before lying again; the duration of each element gives an indication about the number of interacting particles. The conditions for the validity of the quasi-linear theory, and some other non-linear effects related with the interpretation of Pc 1 micropulsations are also discussed.     

Pc3-4 geomagnetic pulsations at very low latitude in Brazil

In order to investigate Pc3-4 geomagnetic pulsations at very low and equatorial latitudes, L=1.0 to 1.2, we analyzed simultaneous geomagnetic data from Brazilian stations for 26 days during October-November 1994. The multitaper spectral method based on Fourier transform and singular value decomposition was used to obtain pulsation power spectra, polarization parameters and phase. Eighty-one (81) simultaneous highly polarized Pc3-4 events occurring mainly during daytime were selected for the study. The diurnal events showed enhancement in the polarized power density of about 3.2 times for pulsations observed at stations close to the magnetic equator in comparison to the more distant ones. The phase of pulsation observed at stations near the magnetic equator showed a delay of 48-62° in relation to the most distant one. The peculiarities shown by these Pc3-4 pulsations close to the dip equator are attributed to the increase of the ionospheric conductivity and the intensification of the equatorial electrojet during daytime that regulates the propagation of compressional waves generated in the foreshock region and transmitted to the magnetosphere and ionosphere at low latitudes. The source mechanism of these compressional Pc3-4 modes may be the compressional global mode or the trapped fast mode in the plasmasphere driving forced field line oscillations at very low and equatorial latitudes.     

The characteristics of Pc 3 and Pc 4 geomagnetic micropulsations recorded at Sanae,Antarctica

Digital dynamic spectra of micropulsations recorded at SANAE (L ～ 4) show that Pc 3 pulsations have frequencies which decrease throughout the day. Both the onset frequency and the rate of decrease of frequency depend on the level of magnetic activity during the previous night. The variation of Pc 3 amplitudes and frequencies is explained in terms of the position of the plasmapause and the associated Pc 3 resonance region in the plasmatrough.For Pc 4 pulsations a constant frequency is observed on most days and it is not possible to infer the presence of a Pc 4 resonance region.     

Magnetic resistivity and Hall currents effects on the stability of a self-gravitating plasma of varying density in variable magnetic field

The effect of Hall currents have been studied on the instability of a stratified layer of a self-gravitating finitely conducting plasma of varying density. It is assumed that the plasma is permeated by a variable horizontal magnetic field stratified vertically. The stability analysis has been carried out for longitudinal mode of wave propagation. The solution has been obtained through integral equation approach. The dispersion relation has been derived and solved numerically. It is found that both the Hall currents and finite conductivity have a destabilizing influence on the growth rate of the unstable mode of disturbance.     

Method of locating the Pc 1 source and hot plasma parameters in the generation region

A method of estimating hot and cold plasma parameters using ground Pc 1 observations is proposed. The values of both hot and cold plasma concentrations, proton energy and temperature anisotropy in the Pc 1 generation region are calculated. It is shown that the most appropriate (effective) indicator of the L-shell, where Pc 1 are generated, is the group delay of wave packets.     

Short-term prediction and a new method of classification of Pc 1 pulsation occurrences

Because of their known tendency to occur in the interval 2–7 days after the start of a geomagnetic storm, Pc 1 pulsations (0.2–5 Hz) are particularly well suited for a method of occurrence prediction based on the comparison of running means of a geomagnetic activity index. By comparing the running mean of a short interval (～ 2 days) of activity data with the mean of a longer interval (～ 5 days), it is possible to isolate the intervals of declining activity that contain a large proportion (if 66%) of Pc 1 pulsation occurrences. Assuming the real time availability of a daily activity index, predictions can be made for 3–10 days ahead of the probability of Pc 1 occurrences. The method of prediction generalizes the previous observations on the relation between Pc 1 pulsations and geomagnetic storms, and one of its important features is its ability to divide Pc 1 pulsation occurrences into a unified system of categories. It is probable that this system can be exploited to provide new information about the pulsations.     

Some features of Pc5 pulsations during a solar cycle

The morphological features of Pc5 pulsations during a solar cycle are studied using Fort Churchill data for the years 1962–1972. Some of the characteristics noted are as follows: (1) Increasing sunspot numbers show little influence on the diurnal variation of the occurrence, amplitude and the period except perhaps some noticeable change in the absolute magnitude of these parameters during different hours of the day. (2) The morning occurrence peak dominates during all phases of the solar cycle. (3) As noted earlier (Gupta 1973a), with increasing magnetic activity the day side region(s) of generation of Pc5 is found to shift closer to the subsolar point and in the midnight sector, the occurrence region (presumably the region of open and closed field lines) seemed to shift towards earlier hours with increasing magnetic activity and towards later hours with increasing sunspot numbers. (4) Despite the smaller number of data points for high magnetic activity levels the analysis indicates that the amplitude of Pc5 pulsations is directly related to all the levels of magnetic activity. (5) The periods of Pc5 pulsations show strong correlation with increasing sunspot numbers and the amplitude and occurrences are found to vary in accordance with the magnetic activity all through the cycle. (6) The annual and semi-annual variations of Pc5 parameters have been demonstrated especially for the pulsations occurring in the morning close to 8 ± 1 h LT and for those occurring near the midnight hours. (7) A suspected 27-day recurrence tendency has been clearly noticed for the occurrence, amplitude and period of Pc5 pulsations.     

The Kelvin-Helmholtz instability in the low-latitude boundary layer

The Kelvin-Helmholtz instability on the magnetopause has frequently been invoked as a mechanism for driving geomagnetic pulsations in the Pc3–Pc5 range, as well as to explain the occurrence of surface waves on the magnetopause observed by satellites. Most theories of the instability represent the magnetopause by a sharp boundary with velocity shear. In this paper a linear theory is developed which takes into account the finite thickness of the low-latitude boundary layer on the magnetopause. The theory is in a form suitable for numerical computation and can take into account the effect of gradients in the plasma pressure, magnetic field magnitude and direction, and density. Computations show that the instability is suppressed at wavelengths short compared with the scale width of the boundary. There is thus a wavelength for which the growth rate is maximum. Extensive computations have been carried out and they show that growth can take place for a very wide range of conditions. The computations confirm earlier results snowing that maximum growth occurs for a wave vector which is perpendicular to the magnetic field. For typical solar wind conditions the theory predicts wavelengths on the magnetopause of the order of 10 times the thickness of the low-latitude boundary layer and periods in the Pc3–Pc5 range. The possible non-linear development of the instability is discussed qualitatively. The predicted results are consistent with satellite observations of pulsations.     

Atmospheric wave propagations in the mesopause region observed by the OH(8,3) band,NaD, O2A(8645Å) band and OI 5577 Å nightglow emissions

Simultaneous measurements of the upper mesospheric NaD and OH(8,3) band emissions by meridional scanning photometers, and the OI 5577 Å, O₂ Atmospheric band at 8645 Å, NaD and OH(8,3) band emissions by multi-channel tilting filter type zenith photometers have been carried out at Cachoeira Paulista (22.7°S, 45.0°W), Brazil. On two nights during the period May–August 1983, the meridional scanning observations showed horizontal intensity gradients and phase propagations. The nocturnal intensity variations on one of these occasions 13–14 June 1983, which was a magnetically disturbed night with 4 ?k_p? 8, also showed vertical phase propagation. In this paper, we present these observations and discuss the possible effects of the horizontal wind system and of gravity wave propagation.     

Dependence of Pc5 micropulsation power on conductivity variations in the morning sector

For many years it has been known the that most intense and continuous Pc5 micropulsation activity occurs in the local time quadrant between dawn and noon. Recently, Lam and Rostoker (1978) have shown that Pc5 pulsations occur in the latitudinal regime occupied by the westward auroral electrojet and have suggested that part of the oscillating current system responsible for the pulsations involves upward field-aligned current at the boundary between the sunlit and dark ionosphere at local dawn. In this paper, we show that power in the Pc5 micropulsation range is markedly enhanced as one moves across the dawn terminator at 100 km from the nightside to the dayside. It is further shown that there is a significant increase in pulsation strength at ～0730 L.T.. The increase in Pc5 pulsation strength across the dawn terminator favors the concept that Pc5 micropulsations can be viewed as oscillations of a three-dimensional current loop involving downward current in the pre-noon sector diverging to flow in the ionosphere as part of the westward auroral electrojet and returning to the magnetosphere along field lines penetrating the ionosphere across the region separating the dark and sunlit ionosphere. We further suggest that the region of enhanced high energy electron precipitation shown by Hartz and Brice (1967) to maximize in the pre-noon quadrant is associated with the marked enhancement of Pc5 activity near 0730 L.T.