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.     

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.     

Pi 2 pulsations and the auroral electrojet

Measurements of the properties of Pi 2 pulsations along a magnetic meridian at high latitudes during a number of substorms have been analyzed for their relationship to the auroral electrojet. It is found that the maximum Pi 2 pulsation amplitudes are closely associated with the instantaneous position of the electrojet. That is, the average pulsation amplitude in the Pi 2 band as well as the amplitudes of pulsations at specific frequencies in the band have maximum amplitudes at latitudes close to the instantaneous electrojet location. Stations equatorward of the electrojet tend to observe a classical Pi 2 waveform concurrent with the onset of the substorm electrojet. Stations near the electrojet observe a broad spectrum of pulsations indicating a multiplicity of sources. Stations poleward of the initial electrojet position see little pulsation activity until the electrojet moves overhead. The appearance of large amplitude Pi 2 pulsations at a station which was poleward of the electrojet at the onset of a substorm appears to be coincident with the arrival of the poleward border of the electrojet.     

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.     

ULF pulsation mode coupling in the ionosphere and magnetosphere

The effect of realistic ionospheric Hall conductances on axisymmetric toroidal mode hydromagnetic wave resonances is investigated. The toroidal modes couple to evanescent poloidal modes near the ionospheres such that the composite modes resonate at the constant frequencies of the corresponding single-field-shell resonances for zero Hall conductance. A model for these composite modes is developed which has narrow but finite latitudinal resonance widths such as to make the modes valid solutions of the hydromagnetic equations. The modes also suggest that “shell” solutions can realistically describe such properties of real pulsations as frequency, damping, phase variation along the field-line and node-antinode behaviour at the ionospheres. Estimates of ionospheric coupling strength are obtained and compared with magnetospheric coupling strength. It is found that magnetospheric coupling dominates ionospheric coupling for any single non-axisymmetric mode. However, ionospherically coupled axisymmetric modes should be necessary components of the Fourier sum of modes required to model any real pulsation of low to moderate apparent azimuthal wave number.Estimates of the range of magnetospheric coupling strength are obtained for pulsations under a variety of conditions.     

Development of an auroral absorption substorm: Studies of substorm related absorption events in the afternoon-early evening sector

We discuss the effects in ionospheric absorption of particle precipitation observed in the afternoon-early evening sector during substorms with onset in the midnight sector. All events considered here occurred during magnetically disturbed periods, K_p > 3. For many of the substorm events a smooth southward moving absorption bay is seen in the midnight and evening sectors about 1 h preceeding the onset. The magnetic pulsation activity is low during this preceding bay.

After substorm onset near magnetic midnight the precipitation region may expand with a sharp onset at the front towards the West in spatially confined regions at high and low L-values separately with about equal velocities. The observations are consistent with a model of westward expansion of the energetic electron precipitation in two regions, aligned parallel to the auroral oval, at high and low L-values of about L 6 and L 4.8.

The westward expanding absorption activity correlates well with local magnetic variations. In magnetic pulsations PiB events are seen at high latitudes simultaneously with the westward moving onsets while at low latitudes IPDP pulsations are observed during the active part of the absorption events. Later in the substorm event a slowly varying absorption event (SVA) is sometimes observed at the lower L-values, L 3–4.     

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.     

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 range of Pi2 propagation at low latitudes

The longitudinal range over which Pi2 pulsations are propagated at low latitudes is investigated and found to vary from one substorm to another. The results show that the longitudinal extent over which Pi2's can be detected varies from a narrow range of longitudes either side of the 23 L.T. meridian in some cases to almost all longitudes around the Earth in other cases. The longitudinal range of propagation does not appear to be a function of substorm intensity.     

The effect of damping on geomagnetic pulsation amplitude and phase at ground observatories

We present some results from a model of forced oscillations of the magnetosphere. The purpose of this work is to examine the effects and consequences of damping on geomagnetic pulsations as observed on the ground. The aim of the current work is to quantify the amount of damping applicable to geomagnetic pulsation waveforms. Ionospheric conductivities vary with latitude and time of day and this variation will effect the damping of geomagnetic pulsations. The variations in ionospheric conductivities are taken into account to predict the changes in amplitude and phase of geomagnetic pulsations over an extended latitudinal array of ground observatories. Three situations are modelled where the damping factor γ/ω_n, which is related to the amplitude loss per cycle, is different: (i) γ/ω_n approximately equal to 0.01, this corresponds to the ionospheric Joule damping of Newton et al. (1978); (ii) λ/ω_n equal to 0.1, this value is consistent with the empirically determined day-time damping factors from the observed latitude-dependent transient decays of the pulsation single effect events discussed by Siebert (1964). The value of 0.1 as the damping factor is taken as typical of day-time conditions and its effect on amplitude and phase for continuous pulsations is considered; and (iii) λ/ω_n is latitude-dependent; three different levels of damping are used appropriate for the night-time conditions associated with the auroral electrojet, plasmatrough and plasmasphere.The results from the model suggest that observationally determined damping factors are greater than those computed from ionospheric Joule damping alone. The model also illustrates the broadening of the latitudinal resonance width with increasing damping and the reducing of the phase change across resonance to less than 180°. The model also successfully reproduces features of pulsation single effect events and Pi2 pulsations.     

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.     

Magnetic pulsation Pi2 and substorm onset

Coincidence between the onset of sudden brightening of the auroral arc in the auroral oval and the onset of Pi2 magnetic pulsation in low latitudes is examined based on the auroral data obtained at a chain of stations in Alaska and the Pi2 data obtained at the low-latitude station Onagawa. The result shows that the low-latitude Pi2 occurs almost simultaneously with the sudden brightening of the auroral arc, i.e. the onset of an auroral substorm (T = 0). It is concluded that the onset of substorms can be identified quite well with the onset of the low-latitude Pi2.     

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.     

An ionospheric origin for Pi 1 micropulsations

Cosmic noise absorption pulsation events observed with fast response riometers at Macquarie Island in the southern auroral zone have almost always been accompanied by Pi 1 micropulsations. A cross-spectral analysis of fast response riometer data and vertical component induction magnetometer data for one such event showed that, after the low frequency components are removed, the absorption A(t) is better correlated with the absolute value of the field Z(t) than with the recorded quantity $\text{d}\text{Z}\text{dt}$. The peaks in Z(t) lag those in A(t) by one second while A(t) lags $\text{d}\text{Z}\text{d}\text{t}$ by abou second. Furthermore, many of the pulsations in Z(t) show a similar time asymmetry to that commonly observed in c.n.a. pulsations, viz. a more rapid onset time than decay time.These results are taken as evidence that the Pi 1 micropulsations observed from the ground during the recovery phase of an auroral substorm are brought about by fluctuations in the ionospheric currents which give rise to the magnetic bay, these fluctuations being due to conductivity changes resulting from particle precipitation. The lag between A(t) and Z > (t) is attributed to the self-inductance of the electrojet.     

Simple Pi burst micropulsation events and associated aurora at two sites in Alaska

Type Pi magnetic-field pulsation bursts were selected for which the associated aurorae were relatively simple and stable and occurred in the ionosphere between College and Fort Yukon in alaska. Power spectral-density traces for College and Fort Yukon HandD were computed and were studied relative to the aurora and to more complex events presented in earlier studies. The power spectral-density traces associated to simpler aurora were found to be consistent with the assumption of simpler 3-dimensional current systems as generators of the Pi waves. The spectra of associated precipitation pulsations had a peak near 10mHz in common with the magnetic field spectra in all events, and also near 3 mHz in one event. The precipitation pulsations at 3 and 10mHz may have enhanced the magnetic field spectra at those frequencies through modulation of the ionospheric resistance to the current.     

The ionospheric electric field during substorms — An interpretation based on non-uniform reconnection in the geomagnetic tail

It is suggested that changes in the electric field in the night-side auroral zone and polar cap observed during the expansion phase of a substorm are related to a change in the magnetospheric flow pattern. During the substorm growth phase the flow appears to be fairly uniform across the width of the magnetosphere (uniform electric field across the tail), while at expansion the observations are consistent with the magnetospheric potential drop in the tail falling across a narrow region near the dusk magnetopause. Such non-uniform electric fields in the tail have been predicted by recent theoretical work. A rather speculative interpretation of events during a magnetospheric substorm is presented.     

A model current system for the magnetospheric substorm

We propose a model three-dimensional current system for the magnetospheric substorm, which can account for the new findings of the field-aligned and ionospheric currents obtained during the last few years by using new techniques. They include (1) the ionospheric currents at the auroral latitude deduced from the Chatanika incoherent scatter radar data, (2) the field-aligned currents inferred from the vector magnetic field observations by the TRIAD satellite and (3) the global distribution of auroras with respect to the auroral electrojets appearing in DMSP satellite photographs. The model current system is also tested by a computer model calculation of the ionospheric current pattern. It is shown that the auroral electrojets have a strong asymmetry with respect to the midnight meridian. The westward electrojet flows along the discrete aurora in the evening sector, as well as along the diffuse aurora in the morning sector. The eastward electrojet flows equatorward of the westward electrojet in the evening sector. It has a northward component and joins the westward electrojet by turning westward across the Harang discontinuity. Thus, the latitudinal width of the westward electrojet in the morning sector is much larger than that in the evening sector. The field-aligned currents, consisting of two pairs of upward and inward currents (one is located in the morning sector and the other in the evening sector), are closed neither simply by the east-west ionospheric currents nor by the north-south currents, but by a complicated combination of the north-south and east-west paths in the ionosphere. The magnetospheric extension of the current system is also briefly discussed.     

Multisatellite observations of resonant hydromagnetic waves

Most measurements of long period ULF pulsations have come from ground based and single satellite observations. The observations have given strong support to the idea that these waves are resonant standing hydromagnetic waves on geomagnetic field lines. Simultaneous ground-satellite observations provide further details of the pulsation structure and are useful for examining the effect of the ionosphere on the transmission of the waves to the ground. Recently, multisatellite observations have been used to provide further insight into the nature of pulsations and we review the results obtained using this technique. Among the results presented are those from the ISEE 1 and 2 spacecraft which are closely spaced in identical orbits, making it possible to distinguish temporal from spatial structure in waves. The ISEE spacecraft have made measurements of resonant region widths and resonance harmonics. In addition, examples are shown of recent multisatellite observations of the global nature of some pulsations and the localization of Pi2 pulsations in space.     

Pi-2 pulsations as a result of evolution of an Alfvén impulse originating in the ionosphere during a brightening of aurora

It is assumed that the original impulse producing Pi-2 pulsations is generated in the ionosphere at the moment of a brightening of aurora. The electric field is known to decrease in the auroral arc almost by an order of magnitude. The electric impulse that appears will be transferred along magnetic field lines and reflected from the ionosphere of the opposite hemisphere, forming the standing Alfvén wave. The electric field impulse of 100 $\text{mV}\text{m}$ is capable of causing magnetic field oscillations of order of 100 γ. Reflection of the Alfvén impulse from the ionosphere with horizontal inhomogeneities corresponding to different forms of auroras is studied. The following is found: (a) the resonance is possible only for harmonics with the rotating vector of polarization; (b) the resonance periods appear to depend essentially on the ionospheric conductivity; this may bring a significant error into determination of the magnetospheric plasma density from the pulsation periods; (c) the auroral zone exerts a screening influence on the pulsations excited at latitudes higher than the zone itself.