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.     

Long-term prediction of Pc 1 geomagnetic pulsation occurrences

The monthly occurrences of Pc 1 geomagnetic pulsations at California stations over the 18.5 yr interval January 1955–June 1973, were spectral analyzed to obtain the characteristics of the major periodicities. There were eight lines in the spectrum with amplitudes significantly above the background noise and the periods, amplitudes and phases of the corresponding periodicities were determined from the spectral data. The periods of the eight lines, listed in order of their relative amplitudes, were ～14yr, 40.1 m, 13.8 m, 5.72 m, 3.08 m, 2.96 m, 10.5 m and 5.94 m. The eight periodicities were used to predict the trend of Pc 1 occurrences at middle latitudes over the remainder of the decade. It appears that the present moderately high rate of occurrence will remain approximately constant during 1974. After 1974 the rate should increase and reach a maximum level in 1977. From 1978 to 1980 a steady decrease in the rate of occurrence is indicated. There are no times when the level of Pc 1 occurrences drops nearly to zero.     

A generation mechanism for Pc 5 micropulsations in the morning sector

In the companion paper (Lam and Rostoker, 1978) we have shown that Pc 5 micropulsations are intimately related to the behaviour and character of the westward auroral electrojet in the morning sector. In this paper we show that Pc 5 micropulsations can be regarded as LC-oscillations of a three-dimensional current loop involving downward field-aligned current flow near noon, which diverges in part to form the ionospheric westward electrojet and returns back along magnetic field lines into the magnetosphere in the vicinity of the ionosphere conductivity discontinuity at the dawn meridian. The current system is driven through the extraction of energy from the magnetospheric plasma drifting sunwards past the flanks of the magnetosphere in a manner discussed by Rostoker and Boström (1976). The polarization characteristics of the pulsations on the ground can be understood in terms of the effects of displacement currents of significant intensity which flow near the F-region peak in the ionosphere and induced currents which flow in the earth. These currents significantly influence the magnetic perturbation pattern at the Earth's surface. Model current system calculations show that the relative phase of the pulsations along a constant meridian can be explained by the composite effect of oscillations of the borders of the electrojet and variations in the intensity of current flow in the electrojet.     

Modelling of Pc5 pulsation structure in the magnetosphere

Magnetohydrodynamic resonance theory is used to model the structure of the magnetospheric and ionospheric electric and magnetic fields associated with Pc5 geomagnetic pulsations. In this paper the variation of the fields across the invariant latitude of the resonance are computed. The results are combined with calculations of the variation along a field line to map the fields down to the ionosphere. In one case the results are compared with measurements obtained by the STARE auroral radar and show good agreement. The relationship between the width of the resonance region and ionospheric height-integrated Pedersen conductivity is computed and it is shown how auroral radar measurements of Pc5 oscillations could be used to determine ionospheric height-integrated Pedersen conductivity. It is pointed out that from these calculations it would be possible to identify the field line on which a satellite was located by comparing a Pc5 pulsation observed by the satellite, and the same pulsation observed by STARE.     

The longitudinal phase variation of mid-latitude Pc3-4 micropulsations

The longitudinal phase variation of Pc3-4 micropulsations has been investigated using data from three stations at geomagnetic latitude ～54°, in the British Isles. With one exception, the events analysed showed a phase change of ? 10° per degree of longitude. Apparent longitudinal phase velocities were in the approximate range 250–350 km/sec with a general tendency to decrease with increasing period. In most cases the Western station was leading in phase and there was no obvious diurnal pattern. The significance of these results to theoretical work is discussed.     

Observations of a quiet-time Pc5 wave in the outer magnetosphere

We report an observation of the radial profile of a Pc5 magnetic pulsation and the associated energetic electron flux oscillations from 10 to 18 Re, recorded by the IMP-5 satellite at 19.00 M.L.T. on 21 March 1970. The Pc5 pulsation was mainly compressional and occurred during extremely quiet geomagnetic conditions. Fluxes of energetic electrons detected above three energy thresholds (18, 45, and 80 keV) were found to oscillate out of phase with magnetic field intensity. One new result is that both the wave amplitude and the wave period increased with radial distance. Second, the electron flux oscillation amplitude was roughly proportional to magnetic field fluctuation amplitude and wave period. The wave event is found to be interpreted better as an ion drift wave because of lack of polarization reversal. The characteristics of energetic electron flux oscillations are shown to agree qualitatively with theoretical calculations of the kinetic perturbation of distribution functions by compressional waves.     

Long-term predictions of Pc 1 geomagnetic pulsations: Comparison with observations

Comparison is made between the measured monthly occurrences of pc 1-active days in California during the interval 1970–1976 and predictions of these occurrences by Soviet and U.S. researchers. It is shown that successful long-term predictions of the monthly occurrences can be made for up to 3 yr ahead and probably also for longer intervals depending on the need. These predictions could have application in cooperative programs of observation, in magnetotelluric studies, and in communications.     

High-latitude Pc1–2 geomagnetic pulsations and their connection with location of the dayside polar cusp

The result of investigating high-latitude Pc1–2 pulsations are presented in this paper. They show that these unstructured oscillations are typical in intervals of low magnetic activity for regions of projections of the dayside cusp on the Earth's surface. The morphological properties of these pulsations, namely the character of their diurnal variations and dependence of their amplitude and frequency of occurrence on magnetic activity on different latitudes, suggest methods of utilization for tracing the location of the equatorial boundary of the dayside cusp. It is suggested that Pc1–2 pulsations are generated mainly in the dayside magnetosheath on field lines, crossing the magnetopause and entering in the dayside cusp. The possible mechanism of generation is the ion-cyclotron instability of plasma of finite pressure (β_⊥ ? 1) and with anisotropic temperature (T_⊥ > T_∥).     

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.     

Comment on “a generation mechanism for Pc5 micropulsations in the morning sector” by Gordon Rostoker and Hing-Lan Lam

It is shown that the combined spatial and temporal oscillation of the auroral electrojet proposed by Rostoker and Lam (1978) gives rise to a second harmonic in the ground recorded pulsation record. An alternate spatial and temporal variation which does not inherently produce higher harmonics is proposed. It is suggested that both proposals require further theoretical and/or experimental investigation.     

The effects of density gradients on the convective amplification of upper hybrid waves in the magnetosphere

Intense (? 10 mVm^?1) electrostatic plasma waves near the upper hybrid frequency have been observed between ± 50° magnetic latitude during spacecraft plasmapause crossings. We present wave growth rate and three-dimensional convective amplification calculations which suggest how intense upper hybrid (IUH) events can occur over such a wide range of latitudes. The effects of wave refraction are shown to be crucial to the proper calculation of convective amplification.We first calculate upper hybrid wave growth for an IUH event at 10° MLAT during which a complete electron distribution function with a loss cone feature was measured simultaneously with the waves. We show that a parallel density gradient may be necessary to account for the observed amplification. Without such a density gradient, the dipole magnetic field gradient would quickly refract the wave vector component parallel to the local field lines out of the unstable region in wave vector space. Upon mapping the distribution function observed at 10° MLAT to other latitudes by conserving the electrons' magnetic moments, we then find that the mapped distribution could produce large amplification at higher latitudes only if there is an appropriate parallel density gradient. At the equator, the long magnetic field gradient scale length enables large amplitudes to be attained without a density gradient.The results of our UH ray tracing analysis are related to theories and observations of magnetospheric continuum radiation.     

Propagation of Pc 1 pulsations in off-meridian directions

Experimental investigations on the horizontal propagation of Pc 1 pulsations in directions out of the geomagnetic meridian have been carried out using data recorded over a three-year period at four widely-separated middle and low latitude stations. The results obtained show that Pc 1 signals propagate in off-meridian directions, and that horizontal propagation characteristics in the early morning hours have small directional changes.     

The horizontal propagation of Pc1 pulsations in the ionosphere

The propagation properties of the various elements of the plane-wave angular spectrum of a Pc1 pulsation signal in the ionosphere are determined by a full-wave numerical analysis. A spectral component is characterized by the wave-vector azimuthal direction, and the Snell constant S. The isotropic R-mode transmission coefficient to ground is fairly flat for S ? 400, but thereafter (S > 500) drops rapidly with increasing S. Coupling of energy from the field-guided L-mode to the R-mode occurs along the entire length of the L-mode trajectory within the ionospheric duct in which the R-mode can propagate. Within the duct, the R-mode attenuation is determined largely by R to L-mode coupling, which is larger for E-W than for N-S azimuths, especially for steep angles of incidence (S < 100). This should lead to enhanced injection of energy into E-W high altitude, high velocity paths, but to higher E-W attenuation at oblique angles. For oblique propagation (S ? 200) horizontal group velocities are slightly higher than the Alfvén phase velocity at the F-layer peak, but about twice as high for steep angles (S ≈ 100).     

Studies of geomagnetic micropulsations observed at Ahmedabad during the magnetic storm of August 4 to 6, 1972 by power spectrum method

The magnetic field variations observed at Ahmedabad during the magnetic storm of August 4 to 6, 1972 are subjected to power Spectrum Analysis. The storm is divided in five parts depending upon its morphology and phase. Prominent peaks are observed in the range of 60–300 sec periods. The prominent periods and their respective power are studied in relation to the progress and phase of the storm. These are then related to the condition of the magnetosphere and the interplanetary medium observed at the time of the storm.     

A sub-class of pi 1 micropulsations associated with the diurnal transit of the neutral sheet

A composite pi 1-hydromagnetic noise class of micropulsations is found to have pronounced local time occurrence maxima at 0930 and 2130. It is shown that the night-time events tend to occur within a geomagnetic local time ‘slot’, the extent of which correlates quasi-linearly with the width of the neutral sheet recombination slot. No such correlation exists for night-time pi 1 bursts. The suggested energy source for these night-time pi 1-hydromagnetic noise events is geotail field-line merging and the concomitant injection and acceleration of the geotail plasma.     

Vorticity in the magnetosphere

It is shown that Birkeland current and vorticity in the magnetosphere are intimately related, suggesting the importance of taking explicit account of vorticity, particularly velocity shear, when considering magnetospheric motions. An equation of motion for the magnetosphere coupled to the ionosphere is derived. It is suggested that experience with MHD fluids generally might fruitfully be brought to bear on certain problems in the magnetosphere to answer the question, not ‘why a sheet of Birkeland current,’ but rather ‘why a localised velocity shear.’     

Sodium in the Jovian magnetosphere

Observations of sodium D-line emission from Io and the magnetosphere of Jupiter are reported. A disk-shaped cloud of sodium is found to exist in the Jovian magnetosphere with an inner edge at about 4R and an outer edge at about 10R . The gravitational scale height above the equatorial plane is a few Jovian radii. The data are interpreted in terms of a sputtering model, in which the sodium required to maintain the cloud is sputtered off the surface of Io by trapped energetic radiation-belt protons. Conditions on the atmospheric density are obtained. The Keplerian orbits attainable by such escaping sputtered atoms can provide the observed spatial distribution. The required 500-keV proton flux required to provide the 1–10 keV protons which will sputter the sodium at the surface of Io is consistent with the limiting trapped flux determined by ion-cyclotron turbulence.Publication No. 1410, Institute of Geophysics and Planetary Physics, University of California, Los Angeles 90024, Cal., U.S.A.     

On the occurrence of Pi 2 micropulsations

Related investigations concerning the source and occurrence of Pi 2 micropulsations indicate that (1) Pi 2 peak occurrence is near 2230 LMT, before (after) which the initial disturbance vector orientation is primarily northeast (northwest) in the northern hemisphere; (2) peak occurrence is near 2330 (2030) LMT during intervals of low (high) magnetic activity; (3) also, solar wind pressure is directed at a streaming angle of 188° (165°) during low (high) magnetic activity; (4) Pi 2 rate of occurrence peaks when 1 + ? Kp ? 2?. The diurnal variation of occurrence and of the initial disturbance vector orientation may be caused by a magnetospheric Pi 2 source located near the 2230 LMT meridian. Comparison of Pi 2 peak occurrence time and solar wind variations with magnetic activity indicates possible relationships of the two. The fact that processes generating Pi 2 appear optimum, when 1 + ? Kp ? 2?, is consistent with the inverse relation of the yearly Pi 2 occurrence frequency to solar activity.