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.     

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.     

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.     

Toward some new directions in Pc5 pulsation theory—I. Consequences of ionosphere hall current

The Pc5 pulsation model of Rostoker and Lam (1978), which was constructed on the assumption of a non-axisymmetric magnetosphere-ionosphere, is extended to account for the consequence of the ionosphere Hall current. The purpose of the extension is to determine if the more difficult equivalent extension of the hydromagnetic wave theory would be profitable by obtaining some insight into the probable results of such an extension. The new model predicts that the natural resonant frequency is altered by the ionosphere conductivities. This alteration is examined in detail. Another prediction is that the east-west current flow in the ionosphere differs in phase from the north-south current. This phase difference contributes to the polarization properties of the ground-observed pulsations. These results are consequences of the behaviour of the resonant circuit and not that of the signal source. The relationship of this work to that using the traditional approach is discussed.     

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.     

Long period Pc5 pulsations

The characteristics of long period Pc5 pulsations (frequency 3·33-1·67 mHz; period 300–600 sec) for stations in the subauroral, auroral and polar zones are studied for 1967. These pulsations occur mainly in the auroral and polar zones with one morning and one evening peak; in the cusp region they occur most frequently near local noon. The evening peak gets stronger and appears farther away from noon with increasing geomagnetic activity. Periods are shorter and amplitudes larger in the morning compared to the evening hours. Only in a small latitudinal belt (60–70°) do the periods tend to increase with latitude. Amplitudes are almost always maximum near the central line of the auroral zone and drop much more sharply towards lower latitudes than towards higher latitudes. Considerable diurnal variations and also variations with magnetic activity are found to exist in the occurrence-latitude and amplitudelatitude profiles. In all the three regions the occurrence and the amplitude of these pulsations increase with magnetic activity to a certain level after which results become uncertain. Periods either do not change very much or at some stations decrease as activity increases.     

On the pulsations of Ez (AIR) in the frequency range of Pc 1

The records of the pulsations of E_z(air) and magnetic components during a pc 1 event are discussed.     

A compressional Pc4 pulsation observed by three satellites in geostationary orbit near local midnight

We describe the observation of a magnetic pulsation with a period of 55 s, recorded at geostationary orbit by three satellites (ATS 6, SMS 1 and SMS 2) in the local time sector 2100–2400. We use magnetic data from all three spacecraft and also plasma data from ATS 6. The pulsation had a large compressional magnetic component which appeared to be balanced by pressure fluctuations in the hot ring current plasma which were in antiphase with the magnetic variations. This allows the wave to be guided along a field line. From the plasma data we are also able to obtain estimates of the field line displacement and hence the electric field, which enables us to conclude that this is a second harmonic field line resonance. We find that the wave has a very short East-West (E-W) wavelength (m?100) and a westward azimuthal group velocity of about 30 km s^?1. The most probable source for this wave is a bounce resonant interaction with ring current protons. The characteristics of this wave are in many ways similar to those of giant pulsations observed on the ground. ATS 6 was near the inner edge of the ring current electrons and as the wave converted the 10 keV electron Alfvén layer back and forth across ATS 6, we were able to estimate the Alfvén layer energy gradient and obtain a value of 1 keV in 1000 km. This gradient is considerably steeper than that predicted by a steady uniform convection electric field.     

Longitudinal characteristics of Pc5 magnetic pulsations

Data from an East-West line of magnetometer stations stretching approximately along 67° geomagnetic latitude from western Alberta (290° geomagnetic longitude) to western Quebec (350° geomagnetic longitude) in Canada have been used to study the longitudinal characteristics of Pc5 geomagnetic pulsations. This paper concerns the analysis of 3 days' data of relatively intense pulsational activity which occurred around the middle of October in 1976. The intensity variations of Pc5 activity on longitude and time clearly show that the activity is localized in longitude in the morning sector and confused in the afternoon sector. Pulsational activity in the morning sector for two of the events studied appears to be markedly enhanced across the dawn terminator and midway through the pre-noon quadrant. A study of the longitudinal phase variation indicates that the eastern stations lead in phase before noon and lag in phase after noon. This implies that the signals propagate away from noon toward the dawn-dusk meridian. A systematic reversal in the sense of polarization in the horizontal plane was observed when the line of stations rotated across noon. The polarization characteristics in the vertical planes of the events recorded by stations in eastern Canada between 318° and 350° geomagnetic longitude appear to be stationary with respect to time suggesting that the polarization characteristics of pulsations are influenced by geoelectric structures. The implications of these morphological features will be discussed.     

On the pulsation amplitude of cepheid variables

On the basis of more abundant data a relation logPV/logR for cepheid variables (Fernie, 1965) is constructed. A linear relation between logP V and logR for classical cepheids is found, which perhaps has a break at R=10R . On the logR/logP diagram thes-cepheids (Efremov, 1968) show a distinct sequence. Alls-cepheids present a relative variation of the radii R/R0.075. The existence of non-s-cepheids with R/R0.075 raises a point about the evolutionary place of these stars (see Efremov, 1968). One could suppose that cepheids with logP>1.1 pulsate in the first overtone.     

Ground Pc3–Pc5 wave power distribution and response to solar wind velocity variations

We examine the magnetospheric wave power in the Pc3–Pc5 range in terms of its growth and decay characteristics and its distribution in L shell in response to the interplanetary plasma bulk velocity, V_SW. We use linear and nonlinear (rank-order) correlation and filtering methods to quantify the effective coupling of the wave power to V_SW variations. These methods are applied to measurements from 26 ground magnetometers of the IMAGE array and NOAA's GOES-10 spacecraft at geosynchronous orbit, taken over 2 years of solar-maximum activity (2002–2003). We find that the ground ULF wave power is structured in the range 3.5<L<6.4 and distributed uniformly in the range 6.4<L<15 (uncertainties in L are estimated to be ±0.5). The response of the wave power to the V_SW is characterized by an increase starting 3 days before the V_SW peak, intensifying several hours before the peak, and is followed by a fast decrease in the next 2 days. The rapid decay of ULF waves has two stages: one at τ=−6±2 h before the solar wind velocity reaches its peak, and one at the V_SW peak, τ=0. We suggest that the first one is brought about by wave–particle interaction with inner-magnetospheric populations while the second one is a dV_SW/dt effect. The correlation results are confirmed by calculating the finite-impulse response, which shows clearly the decay of the ULF waves after the V_SW peak. The response of the wave power at geosynchronous orbit is remarkably similar to that of the ground wave power at comparable L shells. The above findings characterize the inner-magnetospheric response to interplanetary high-speed streams, as opposed to the more short-lived, higher-amplitude response to CMEs.     

Toward some new directions in Pc5 pulsation theory—II. Extension of the analysis to two hemispheres and determination of the model's polarization characteristics

The model of morning sector Pc5 pulsation activity developed in the companion paper is extended to account for both north and south ionospheres. The result is consistent with recent work by Allan and Knox (1979a and b). Using the spatial oscillation model proposed by McDiarmid (1979), the polarization properties of the model are determined. It is shown that when the phase of the ground induced response is taken into account, the model predicts the results observed by Lam and Rostoker (1979). Allan and Knox (1979) have also discussed the case where the wave admittance at the ionosphere equals the height integrated Pedersen conductivity (matching) and concluded that no standing wave solution exists. Here it is shown that for pulsation systems whose essential physics is described by the proposed model, the matching condition is more complicated and is, in fact, unlikely to be satisfied. Finally, it is concluded that the results obtained with our model are sufficient to warrant the development of an equivalent hydromagnetic wave model.     

Stellar pulsation: (II) multiple distinct shells

We can find the conclusion with our analysis for VLA observation of OH maser and CO (2-1) emission line that they are distributed on some different distinct shells in the circumstellar envelope, respectively.     

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.     

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.     

Period variation of Pc5 pulsations at high latitudes

Geomagnetic data for the year 1967 from seven Canadian observatories, spanning the subauroral, auroral and polar zones, have been analysed to investigate the characteristic variation of Pc5 period with several geophysical variables. Pulsations in the whole spectrum of Pc5 (period range 150–600 s) were found to occur at all of the observatories. Those with smaller periods occurred more frequently at lower latitudes while those with longer periods occurred more frequently at higher latitudes. Daily variation of the periods of Pc5 showed little change with seasons or with magnetic activity. Periods, in general, had two daily maxima which appeared at different local times in different zones. A predominant morning peak was noted at all stations except Baker Lake, where a mid-day maximum of the period was found. The Pc5 periods tended to increase with geomagnetic activity at lower latitude stations, and to decrease with activity at stations in the polar cap for low to moderateKp levels. At high activity levels these trends appeared to reverse, though results are less certain. In different seasons and for the whole year the periods increased almost linearly with latitude. However when similar analysis was done for individual hours of the day and for different magnetic activity groups, this linear relationship between period and geomagnetic latitude was not evident. Efforts to detect a 27-day recurrence tendency of Pc5 periods did not succeed.Contributions from the Earth Physics Branch No. 495.     

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.     

A new theoretical approach to the modelling of toroidal Pc5 pulsations

A model is developed to represent a toroidal mode of Pc5 geomagnetic pulsations. It is shown that this model is consistent in its predictions, such as the latitude profiles of amplitude and phase and their dependence on the height integrated Pedersen conductivity, Σ_p, with those of Walker's (1980) theory. It is also shown that this theory is relatively easily capable of accommodating (i) a variety of field line plasma mass density distributions, (ii) a variety of external excitation schemes, (iii) unequal Σ_p's at each end of the field lines and (iv) non-dipolar geomagnetic fields. The theory yields the transient as well as the steady state response, an important feature permitting application to short-lived events or to those for which the generator is amplitude modulated. It is shown, for instance, that the amplitude-latitude profile varies during the transient. It is also shown that the steady state latitude profiles of amplitude and phase are the dual of those observed as a function of frequency when the excitation frequency is scanned through a resonance. A more realistic steady state energy flow from a generator along the field lines to the ionosphere is inherent in this theory compared with that from the mode to the ionosphere which is inherent in Walker's theory.     

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.