Procedures are developed for specifying the polarization characteristics of n-dimensional waves, and in particular three-dimensional waves of geophysical interest. We show that when a wave is in a pure state or is totally polarized, all the polarization information can be represented by a single vector u in an n-dimensional unitary space. Simple measures of the degree of polarization of the wave are constructed from the characteristic equation of the spectral matrix S. These measures are functions only of the scalar invariants of S and consequently S need not be diagonalized. If S represents a purely polarized wave, the unitary vector which contains the polarization information about the wave can be obtained directly from S using any 2n -1 equations of n2 possible equations. By multiplying by a phase-factor this unitary vector can be written in the form u = rl t ir2 where r, and r2 are orthogonal vectors in a real space. For an elliptically polarized wave, rl and r2 locate the major and minor axes of the ellipse, and the ellipticity is given by the ratio of their magnitudes. The polarization parameters of ULF magnetic waves at the Earth's surface are computed from one set of five equations (n = 3) and compared with parameters calculated using established techniques.

No abstract

Abstract. Pi2 pulsations have been the subject of continuous study since they were recognized to be an integral part of the magnetospheric substorm. With the advent of arrays of ground instruments the nature of the Pi2 has begun to be understood. As adopted by the 13th General Assembly of the International Union of Geodesy and Geophysics in 1963, Pi2 is a designation that includes impulsive pulsations in the period range from 40 to 150 s. The Pi2 signal encompasses a class of pulsations that represents two fundamental processes. The first process is the sudden generation of field-aligned currents in association with the disruption of cross-tail currents in the plasma sheet and their subsequent effects on the ionosphere. The ionosphere appears to be something more than a passive load for this electrodynamic impulse. It responds, sending currents back into a magnetosphere whose topology is changing and, perhaps producing the feedback necessary to cause the explosive growth of the substorm current system. Oscillations of these currents are detected across the nightside of the Earth at onse• as the midlatitude and high-latitude portions of Pi2. The second process is the impulse response of the inner magnetosphere to the compressional waves that are generated at substorm onset. Traveling inward, they stimulate field line resonances and surface waves at the plasmapause and excite global oscillations in the inner magnetosphere. The two processes produce wave modes that couple and cross-couple threading energy into the inner magnetosphere and ultimately to the ground. The purpose of this review is to construct a phenomenological overview of the Pi2.

A set of 61 relatively uncontaminated Pc 4‐5 micropulsations from the interval July 1 through October 10, 1974, were examined for longitudinal phase changes. The data were obtained from the east‐west line of magnetometers operated by the University of Alberta over the summer and early fall of 1974. From our analysis a direct correlation between longitudinal phase shift and frequency is found which is of the form m = (1.4 ± 0.4) f + 0.26, where f is the frequency in millihertz and m is the azimuthal wave number derived from the measured phase shift. This relationship indicates a phase velocity which is independent of frequency and whose magnitude is Vϕ ≃ 14 km/s. The sign of the phase shift, and thus the direction of apparent phase velocity, changes abruptly 1–2 hours before local noon. The signals appear to propagate away from this prenoon region toward the terminators; that is, they move westward in the morning hours and eastward during the afternoon hours. Both the magnitude and the sign of this phase velocity are consistent with proposed generation mechanisms which invoke the Kelvin‐Helmholtz instability at the magnetopause boundary.

This study describes a method for determining the statistical confidence in estimates of direction-of-arrival and trace velocity stemming from signals present in atmospheric infrasound data. It is assumed that the signal source is far enough removed from the infrasound sensor array that a plane-wave approximation holds, and that multipath and multiple source effects are not present. Propagation path and medium inhomogeneities are assumed not to be known at the time of signal detection, but the ensemble of time delays of signal arrivals between array sensor pairs is estimable and corrupted by uncorrelated Gaussian noise. The method results in a set of practical uncertainties that lend themselves to a geometric interpretation. Although quite general, this method is intended for use by analysts interpreting data from atmospheric acoustic arrays, or those interested in designing and deploying them. The method is applied to infrasound arrays typical of those deployed as a part of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization.

Abstract.We present simultaneous measurements of thermospheric winds, auroral emissions, and ionospheric currents over Alaska, obtained from four separate instruments. Thermospheric (F region) wind maps were recorded by an all-sky imaging Fabry-Perot spectrometer located at Poker Flat and observing at A630.0 nm. Auroral images at •557.7 nm were obtained from the low-resolution visible imager on board the Polar satellite. White-light all-sky auroral images were recorded by ground-based all-sky cameras located in Alaska at Poker Flat (65 ø 07'N, 212 ø 34'E) and at Kaktovik (70 ø 06'N, 217 ø 24'E). Finally, the east-west component of the ionospheric F region plasma convection was inferred using the Alaskan meridian chain of magnetometers. Montage images of these four data sets are presented, projected onto a geographic map of the Alaskan region. We examine a 10-hour period during the Alaskan local nighttime of February 10, 1997. These montages illustrate a close relationship between spatial structures occurring in the aurora, in the ionospheric plasma convection, and in the F region wind field. Latitudinal shear of the geomagnetic zonal wind, often observed in the premidnight time sector, was seen to be associated with both the equatorward and poleward boundaries of the discrete aurora. We focus particularly on a period commencing just after 0900 UT, when a strong shear in the zonal wind was observed to sweep southward across Alaska. After magnetic midnight the wind field was dominated by the emergence of the "cross-polar jet" from the polar cap. This overwhelmed any wind features associated with local auroral processes.

Geomagnetic pulsations associated with the polar cusp have been detected at Cape Parry, Northwest Territories, Canada, using flux gate and induction magnetometers. Data from this station show enhanced power almost every day near local noon. It is this enhancement which is taken as the signature of the polar cusp. Maximum signal levels are observed prior to local noon, although the signals are present for several hours on each side of local noon. The spectrum of pulsations near the cusp is broadband, reaching from a few millihertz to above 1 Hz. On average the power spectrum decreases with frequency as f-2.6. Within this general decrease there are often found regions of enhanced power, principally near 5 mHz and near 40 mHz. We have identified the 5-mHz band with fluctuations in ionospheric currents associated with the cusp boundary and its motions. The 40-mHz band is presumed to be due to direct penetration of the hydromagnetic radiation in the cusp to the ground. From the measured spectrum we infer an rms fluctuation amplitude of the cusp currents of approximately 3 x 103 A, and from this we estimate the total power dissipated by these fluctuations of approximately 4 x 10 7 W. 10,055

Abstract. To investigate the generation and propagation mechanisms of Pi 2 magnetic pulsations, we have analyzed magnetic field data t?om the 210 ø magnetic meridian (MM) stations. We used 50 Pi 2 events that were simultaneously observed at seven stations along the 210 ø MM during January 1995, and tbcused our analysis on associated magnetic energy, ((/X/--D2+(AD)2)/kt 0. The times when the amplitude of the magnetic energy attained the maximum (T,,,,x) were compared among these stations. We tbund that T,,,,,. has a latitudinal dependence. especially at highel' latitudes. which has not been previously reported. At Kotel'nyy (L=8.50) on the poleward side of the auroral region. T,,,,.,. occurred an average of 21 seconds earlier than T,,,,,. at Guam (L=I.01). and often as much as one minute earlier. The existence of latitudinal variation has implications tbr interpretation of issues related to timing of substorm onset; it is necessaD to consider the global t•atures of Pi 2 events in the study of auroral and magnetospheric substorms. We present ncx• 11ndings on the latitudinal dependence o1' Pi 2 energy translbr. especially at highel' latitudes and near the plasmapause. These findings can affect the interpretation of some results concerning substorm onset-timing issues. Statistical AnalysisThe magnetic pressure, or magnetic energy density, is defined as B-/2t, to. The magnetic energy carned by MHD waves is expressed by ((A/Dr+ (AD)2+ (ALD2)//a0 . That is, the variation of the magnetic energy is proportional to the squared amplitude of magnetic field variation. For Pi 2 events used in this paper, the contribution to the magnetic energy fi'om the Z component is small and negligible, and when the Z component has constant and significant contribution to the magnetic energy, the induction effect caused by ground conditions is considered. rather than the external source. Thus.•xe use ((_XH) 2 + (,_XD)2)//a0 as the estimate of variation in We note that the ZYK data had a timing error during Jan. -Mar. 1995. According to Kikuchi and Araki [1979]. the variation in the electric field accompanying a DP-2-type sudden commencement (sc) is transmitted instantaneously fi'om high latitude to the equator by Earth-ionosphere waveguide mode. Thus. by using three sc events that occurred in the interval, we have adjusted the timing of the ZYK data 1619