Pi 2 pulsations and the eastward electrojet: A case study

We report the results of a case study of two Pi 2 pulsations observed near the eastward electrojet by the Scandinavian Magnetometer Array. The power of the two Pi 2 pulsations, calculated using a standard Fast Fourier Transform method, peaks near the centre of the eastward electrojet. For both events there is a strong latitudinal gradient in the power poleward of the equatorward border of the electrojet. The sense of polarisation is predominantly clockwise at the northern stations and anticlockwise at the southern stations although the reversal from clockwise to anticlockwise does not occur at a constant latitude. For the first event the polarisation reversal occurs at higher latitudes in the western half of the array; for the second the polarisation reversal occurs at higher latitudes at the edges of the array. The polarisation reversal does not appear to be related to the location of the eastward electrojet. Equivalent current vectors of the Pi 2 pulsations, obtained by rotating the band pass filtered data through 90°, exhibit clear vortex structures in both events. The vortices change sense of direction at half the period of the Pi 2 pulsation. A simple model for the ionospheric electric field in accord with the field line resonance theory reconstructs the basic features of the observed Pi 2 equivalent current system. We thus conclude that Pi 2 signatures in the region of the eastward electrojet and far away from the auroral break-up region are governed by the field line resonance mechanism.     

Pi2 pulsations in the magnetosphere

Several substorms were observed at Explorer 45 in November and December 1971, and January and February 1972, while the satellite was in the evening quadrant near L = 5. These same substorms were identified in ground level magnetograms from auroral zone and low latitude stations. The satellite vector magnetic field records and rapid run ground magnetograms were examined for evidence of simultaneous occurrence of Pi2 magnetic pulsations. Pulsations which began abruptly were observed at the satellite during 7 of the 13 substorms studied and the pulsations occurred near the estimated time of substorm onset. These 7 pulsation events were also observed on the ground and 6 were identified in station comments as Pi2. All of the events observed were principally compressional waves, that is, pulsations in field magnitude. There were also transverse components elliptically polarized counter-clockwise looking along the field line. Periods observed ranged from 40 to 200 sec with 80 sec often the dominant period.     

Nonpotential waves in the auroral electrojet

An ionospheric plasma instability in the auroral electrojet region believed to be responsible for electromagnetic waves with angular frequency smaller thanv _i is discussed. The threshold current velocityU _t and oscillation growth rate are found for the realistic physical situation when a nonuniform plasma density and an ionization function are considered. It is found that the gradient of plasma density n may be neglected in the expressions forU _t and and that the spectral density of fluctuations in the density of the plasma agrees well with the experimental data.     

The distribution of stably trapped magnetospheric plasma as determined from Pi2 micropulsations and the auroral electrojet index, AE

The distribution of stably trapped plasma is determined as a function of equatorial geocentric distance, R. Observed Pi2 micropulsation periods and the magnitudes of their associated magnetic bays, measured by the auroral electrojet index, AE, are used in determining this distribution. Plasma density is found to vary inversely as approximately the sixth power of R. When Pi2 period-AE relations are studied for varying time lags, the pulsation periods are found to define prolonged intervals of abnormal AE levels. Thus the periods may be potentially useful in forecasting geomagnetic disturbances or abnormally quiet intervals. A qualitative physical model is proposed to explain the observational relations.     

Pi 2 pulsations: High latitude results

A review of recent experimental results from studies of high latitude Pi 2 pulsations indicates that these pulsations are fundamentally related to the initiation of the auroral breakup and substorm. At high latitudes, the Pi 2's show their peak intensities in the region where the breakup begins and appear to remain in this region after the breakup has spread poleward. In addition, the Pi 2's occur simultaneously with, or before all other ionospheric phenomena associated with the breakup. The field aligned and ionospheric currents associated with the Pi 2 resemble those of a typical substorm, but the ionospheric currents are phase shifted compared to the field aligned current. The periodic oscillations of the Pi 2's are probably caused by a reflection of the initial field aligned current pulse from the auroral ionosphere. This pulse is trapped on dipolar field lines leading to multiple reflections from North and South auroral ionospheres.     

Interferometric phase velocity measurements in the auroral electrojet

A double-probe electric field detector and two spatially separated fixed-bias Langmuir probes were flown on a Taurus-Tomahawk sounding rocket launched from Poker Flat Research Range in March 1982. Interesting wave data have been obtained from about 10s of the downleg portion of the flight during which the rocket passed through the auroral electrojet. Here the electric field receiver and both density fluctuation (δn/n) receivers responded to a broad band of turbulence centered at 105 km altitude and at frequencies generally below 4 kHz. Closer examination of the two (δn/n) turbulent waveforms reveals that they are correlated, and from the phase difference between the two signals, the phase velocity of the waves in the rocket reference frame is inferred. The magnitude and direction of the observed phase velocity are consistent either with waves which travel at the ion sound speed (C_s) or with waves which travel at the electron drift velocity. The observed phase velocity varies by about 50% over a 5 km altitude range—an effect which probably results from shear in the zonal neutral wind, although unfortunately no simultaneous neutral wind measurements exist to confirm this.     

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Location of source regions with respect to the auroral electrojets

Substorm onsets and intensifications are accompanied on a one-to-one basis by a Pi 2 magnetic pulsation burst. The source region for these pulsations is generally thought to lie in the region of substorm disturbance in the auroral oval. In this paper we outline the characteristics of Pi 2 pulsations in regions near the substorm enhanced electrojet but removed from the locale of the westward travelling surge. We show that a resonance region for the pulsations lies at the equatorwad edge of the westward electrojet, which in the evening sector marks the locus of the Harang discontinuity. Finally we show examples where the maximum amplitude of the Pi 2 is located at or equatorward of the southern border of the eastward electrojet or at the southern border of the westward electrojet. This is clear evidence for the coupling of wave energy into the L-shells far distant from the source of the energy. Mechanisms for Pi 2 generation are discussed in the context of the results presented in this paper.     

Latitude variation of the spectral components of auroral zone Pi2

Evidence is presented from spectral analysis of Pi2 pulsations detected during a substorm by the University of Alberta meridian chain of magnetometers to support the conclusion that at auroral latitudes there is no apparent correlation between the principal spectral components of Pi2 pulsations and the latitude of the observations. From these data we infer that the Pi2 magnetic variations observed at the Earth's surface are not generated by simple MHD eigenoscillations of magnetospheric field. As well, the data show clear contributions to the Pi2 pulsation spectrum by ionospheric currents. These observations lead to the suggestion that Pi2 pulsation spectra are produced by the sudden changes in magnetospheric and ionospheric current systems which take place at the beginning of a substorm.     

A phase and amplitude study of auroral pulsations

To supplement a rocket investigation of the auroral green line, data from some auroral pulsations have been analyzed by direct integration of the time-dependent continuity equations for each of the sources now thought to contribute. It is shown that indirect processes are not incompatible with studies of auroral pulsations, and that a non-negligible contribution to the green-line intensity comes from dissociative recombination. Calculations using the theoretical O(¹S) lifetime show that the remaining green line must come from a direct or very fast process; however, if the O(¹S) lifetime can be reduced, significant portions of the green line can come from the transfer of energy from N₂(A³Σ_u⁺) to atomic oxygen.     

Empirical formula to relate the auroral electrojet intensity with interplanetary parameters

An empirical formula has been constructed using the results of correlative analyses to determine in what form the AL index, as a measure of the intensity of the westward auroral electrojet, depends on interplanetary parameters. The formula thus obtained shows that AL is mainly determined by B_sV² where B_s is the southward component of the IMF and V is the solar wind velocity, and is modulated in a characteristic way by the combined effect of the east-west component of the IMF and the tilt angle of the Earth's dipole axis toward the Sun-Earth line. In contrast, effects of the solar wind density and the IMF variability were found to be insignificant.Implications of the empirical formula are discussed mainly in relation to the problem of the location in the dayside magnetosphere of the region where the reconnection process to initiate the substorm takes place.     

Pi 3 magnetic pulsations associated with substorms

Using magnetic data from the North American IMS network at high latitudes, Pi 3 pulsations are analysed for a period of $\text{4}\text{1}\text{2}$ continuously-disturbed days. The data were obtained from 13 stations in the Alaska and Fort Churchill meridional chains and in the east-west chain along the auroral zone. In the past, Pi 3 pulsations associated with substorms have been classified into two sub-categories, Pi p and Ps 6. However, we find that Pi 3's which have longer periods than Pi p and which are different from Ps 6 are more commonly observed than these two special types. Power spectra, coherence and phase differences are compared among the stations. Results show that noticeable differences for latitudinal dependence of period and amplitude exist among midnight, morning and late-evening Pi 3 pulsations. Results for Pi 3 occurring near midnight indicate that the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the center of the westward auroral electrojet. On the other hand, for Pi 3 pulsations occurring in the morning, the periods at which the power spectral density is a maximum are longest, and the amplitude largest, near the poleward edge of the westward electrojet. Furthermore, for Pi 3 pulsations occurring in the late evening, their periods are longer and their amplitudes larger near both the Harang discontinuity and the poleward edge of the westward electrojet than near its center. Correlations between pairs of adjoining stations are better in the polar cap than at auroral latitudes. It is also found from hodograms that the sense of polarization often varies from one station to another for the same event, and that the time duration in which the same rotational sense is maintained is shorter near midnight than in the morning and late evening. It is suggested that the source regions of the morning and late-evening Pi 3's lie on the electrojet boundaries; that is at the Harang discontinuity (in the evening) and at the poleward edge of the westward electrojet (in the morning and evening). The generation of midnight Pi 3 pulsations, centered at a location within the westward auroral electrojet appears to be associated directly with the generation of that electrojet.     

Simultaneous ground-satellite observations of Pi 2 magnetic pulsations and their high frequency enhancement

Pi 2 magnetic pulsations are a frequent occurrence at the earth's surface and have been shown to be clearly correlated with substorm expansion onset. These pulsations are also observed in space at synchronous orbit at the same time as they are seen on the ground at the satellite conjugate point. In this brief report we describe three days in 1969 on which Pi 2 magnetic pulsations were simultaneously observed at the synchronous satellite ATS 1 and at Tungsten, N.W.T., Canada, near the foot of the ATS 1 magnetic field line. These Pi 2 bursts all exhibit the characteristic waveform and frequency, as well as an ～0.3 Hz enhancement, at both locations. This high frequency enhancement appears to be an integral part of Pi 2 bursts both on the surface and at synchronous orbit and should be considered in the development of models of generation mechanisms.     

Correlations between ground observations of Pi 2 geomagnetic pulsations and satellite plasma density observations

Ground observations of Pi 2 geomagnetic pulsations are correlated with satellite measurements of plasma density for three time intervals. The pulsations were recorded using the IGS network of magnetometer stations and the plasma density measurements were made on board GEOS-1 and ISEE-1. Using the technique of complex demodulation, the amplitude, phase and polarisation characteristics of the Pi 2 pulsations are observed along two meridional profiles; one from Eidar, Iceland (L = 6.7) to Cambridge, U.K. (L = 2.5) and the other from Tromso, Norway (tL = 6.2) to Nurmijarvi, Finland (L = 3.3). The observed characteristics of the Pi 2 pulsations are then compared with the plasma density measurements. Close relationships between the plasmapause position and the position of an ellipticity reversal and a variation in H component phase are observed. A small, secondary amplitude maximum is observed on the U.K./Iceland meridian well inside the position of the projection of the equatorial plasmapause. The primary maxima on the two meridians, in general occur close to the estimated position of the equatorward edge of a westward electrojet. Using the plasma density measurements, the periods of surface waves at the plasmapause for two intervals are estimated and found to be in good agreement with the dominant spectral peaks observed at the ground stations near the plasmapause latitude and within the plasmasphere. The polarisation reversal, together with phase characteristics, spectral evidence and the agreement between the theoretical and observed periods leads to the suggestion that on occasions a surface wave is excited on the plasmapause as an intermediate stage in the propagation of Pi 2 pulsations from the auroral zone to lower latitudes.     

The transient response mechanism and Pi2 pulsations at substorm onset—Review and outlook

Pi2 pulsations are nowadays thought to be transient hydromagnetic signals associated with the build-up of a new stationary magnetosphere-ionosphere coupling system after the sudden formation of the substorm current wedge. To illustrate this transient response mechanism, we will first briefly describe the substorm current circuit. Subsequently, we will demonstrate that the gross characteristics of high-latitude Pi2 can be explained by the sudden switch-on of this current wedge during substorm onset if its westward expansion is taken into account. We will conclude by discussing some additional phenomena and processes (like conductivity and electron density gradients, kinetic Alfvén waves, ionospheric polarization electric fields, and mode coupling) which have to be included into a realistic model for Pi2 pulsations and thus timedependent magnetosphere-ionosphere coupling.     

Total current of the auroral electrojet estimated from the IMS Alaska meridian chain of magnetic observatories

Based on magnetic data from the IMS Alaska meridian chain of observatories, the total current of the westward auroral electrojet flowing across the meridian is estimated by using two independent methods. The first one is a simple integration of the north-south component of magnetic perturbations along the meridian, providing the quantity F in units of nT·km. The other is to use the forward method, providing the total current I in units of A. It is shown that F and I have nearly identical time variations. Thus, by normalizing the two quantities and taking the numerical value of F in units of nT·km, it is possible to estimate the total electrojet current flowing across a magnetic meridian, with an accuracy of 90%, by using the latitudinal profile of the H component, namely I (A) = 2.0 F (nT·km).     

Influence of the east-west component of the interplanetary magnetic field on the intensity of the auroral electrojet

Analysis of interplanetary data has been made to evaluate the influence of the Biny component of the IMF on the magnitude of the AL index, especially with reference to the contribution of B_y in the reconnection process between the IMF and geomagnetic fields in the dayside part of the magnetosphere. The results of the analysis showed that the effect of Biny on AL is predominantly of a different type from that expected by the current theory of reconnection, and the B_y effect of the latter type was found to be much less significant in magnitude than the theory predicts. The discrepancy may be resolved if the region where the reconnection takes place has an elongated shape.     

The relationship of pc 5 micropulsation activity in the morning sector to the auroral westward electrojet

Data from a line of magnetometers stretching along a corrected geomagnetic meridian ～ 302°E through western Canada are used to study the relationship between the convection westward electrojet and Pc 5 micropulsations in the morning sector. It was found that the dominant spectral bands in the Pc 5 range occur within the same latitudinal range occupied by the electrojet. The intensity contours and the character of the polarization parameters clearly show that the Pc 5 activity tracks the westward convection electrojet. The Pc 5 activity is found to be enhanced in conjunction with rapid reconfigurations of the electrojet. Evidence of spatial oscillations of the borders of the electrojet and variations in the intensity of the electrojet is presented. It is concluded from our study that the Pc 5 activity in the morning sector is closely related to the convection westward electrojet and its associated three-dimensional current system.     

Polarization characteristics of Pi 2 pulsations and implications for their source mechanisms: Influence of the westward travelling surge

This paper expands the earlier results of Rostoker and Samson (1981), who noted that there are two latitudinal areas of Pi 2 localization near the high latitude, substorm enhanced electrojets. The detailed study presented here outlines the morphology of the polarizations of the Pi 2's in and near the westward travelling surge. There are two latitudinal areas of Pi 2 localization. A poleward Pi 2 predominates within the surge and to the East, whereas an equatorward Pi 2 predominates equatorward and West of the surge. These Pi 2 localizations appear to correlate with the substorm enhanced westward and eastward electrojets respectively. However, the maximum in the Pi 2 power does not always coincide with the center of the electrojet. The poleward Pi 2 has largest amplitudes to the East of the head of the westward travelling surge. This Pi 2 shows a latitudinal polarization reversal from clockwise on the equatorside (viewed down on H-D plane) to counterclockwise on the poleside of a latitudinal demarcation line, which occurs just poleward of the initial breakup. This demarcation line is usually equatorward of the most poleward expansion of the surge. To the West of the surge front, where the equatorward Pi 2 predominates, there is again a latitudinal polarization reversal but in this case the polarization is counterclockwise equatorward and clockwise poleward of the demarcation line. This demarcation is equatorward of that for the poleward Pi 2, and appears to lie at the latitude of the initial breakup. Consequently, the westward travelling surge appears to mark the longitudinal transition from equatorward to poleward Pi 2. The elliptical polarization of the Pi 2's is most likely caused by azimuthai (longitudinal) expansion of the field-aligned currents in the surge, in association with reflection of the field-aligned current pulses from northern and southern high latitude ionospheres.     

An estimate of the Earth's induction effects on the vertical polarization of the Pc5 pulsations recorded in auroral regions

It is assumed that the generation mechanism of Pc5's is somehow related to oscillations of a current system whose ionospheric path is the westward (or the eastward) electrojet. The Earth induction effects on the vertical polarization of Pc5 pulsations recorded in auroral regions are then modelled as the response of a flat layered Earth to an overhead uniform band of current whose intensity oscillates in time with periods ranging from 2 to 10 min. It is shown that polarization ellipses are opened in the H-Z planes by induction alone with a maximum ellipticity of the order of 0.15 at distances from the centre of the electrojet comparable to its width.