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The explosive phase of the 2009 Redoubt Volcano eruption produced predominantly short duration, high-amplitude infrasound signals recorded up to 4500 km away. All 19 numbered explosive events were recorded at a local microphone (DFR, 12 km), as well as at an infrasound array in Fairbanks, Alaska (I53US, 547 km), most with high signal to noise ratios. The local microphone provides an estimate of the source parameters, and comparison between the two datasets allows the unique opportunity to evaluate acoustic source term estimation at a remote array. High waveform similarity between DFR and I53US occurs during much of the explosive phase due to strong stratospheric ducting, permitting accurate source constraints inferred from I53US data. Cross-correlation analysis after applying a Hilbert transform to the I53US data shows how the acoustic energy has passed through a single caustic, as predicted by ray theory. Similar to previous studies, significant low-frequency infrasound from Redoubt recorded at I53US is coincident with high-altitude ash emissions. The largest events also produced considerable energy at greater than 50 s periods, likely related to the initial oscillations of the volcanic plume or jet. Many of the explosive events have emergent onsets, somewhat unusual for explosive, short-duration eruptions. Comparison of the satellite-derived SO2 emissions with the relative amount of acoustic energy at I53US shows a very high, statistically significant correlation. This study reiterates the utility of using remote infrasound arrays for detection of hazardous emissions and characterization of large volcanic eruptions, and demonstrates how, under typical meteorological conditions, remote infrasound arrays can provide an accurate representation of the acoustic source.

The dependence of maximum variations of H-component negative bays at magnetic conjugate stations on a sign of the IMF B Y -component during the substorms of small and moderate intensity with |ΔH|max < 750 nT developed at B Z < 0 and |B Z / B Y | < 1 has been studied. By data from Kotzebue (Φ = 64.5°; Λ = 249.7°) and Macquarie Isl. (Φ = −64.5°; Λ = 247.8°) observatories in the eastern hemisphere and Leirvogur (Φ = 65.3°; Λ = 68.2°) and Syowa (Φ = −66.1°; Λ = 71.0°) stations in the western hemisphere it is shown that at B Y < 0 the absolute values of negative bays |ΔH|max are greater in the southern hemisphere and at B Y > 0, on the contrary, in the northern hemisphere.

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 λ630.0 nm. Auroral images at λ557.7 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° 07′N, 212° 34′E) and at Kaktovik (70° 06′N, 217° 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.

To investigate the generation and propagation mechanisms of Pi 2 magnetic pulsations, we have analyzed magnetic field data from the 210° magnetic meridian (MM) stations. We used 50 Pi 2 events that were simultaneously observed at seven stations along the 210° MM during January 1995, and focused our analysis on associated magnetic energy, ((ΔH)²+(ΔD)²)/µ0. The times when the amplitude of the magnetic energy attained the maximum (Tmax) were compared among these stations. We found that Tmax has a latitudinal dependence, especially at higher latitudes, which has not been previously reported. At Kotel'nyy (L=8.50) on the poleward side of the auroral region, Tmax occurred an average of 21 seconds earlier than Tmax at Guam (L=1.01), and often as much as one minute earlier. The existence of latitudinal variation has implications for interpretation of issues related to timing of substorm onset; it is necessary to consider the global features of Pi 2 events in the study of auroral and magnetospheric substorms.

Significant similarities have been identified in the dynamic power spectra of induction magnetometer data recorded at the conjugate cusp stations of Davis, Antarctica (mlat −74.3, mlong 101.5) and Longyearbyen, Svalbard (mlat 75.0, mlong 114.5). A 60 day period in 1993 (January–February) has been studied to characterise two common features, 1) Broadband Pc5 bursts which predominantly occur 2–4 hours before and after local magnetic noon, and which exhibit polarisations suggestive of a Kelvin-Helmholtz-like source mechanism, and 2) A resonance structure which has an arch-shaped local time dependence. Interhemisphere phase measurements indicate odd-mode toroidal standing-waves after field line resonance and azimuthal propagation effects are taken into account. Phase studies such as this may be used to study conjugacy at high latitudes and identify the location of the open and closed field line boundary, an important diagnostic parameter in space weather studies.

Magnetic fluctuations in the 22–100 millihertz (Pc3) band are a consistent indicator of the presence of the cusp in the overhead ionosphere at high latitudes. Correlation of the signals from a variety of instruments have shown that the sources of these pulsations are local (ionospheric) rather than distant (magnetospheric) [Engebretson et al., 1990]. Modulated electron precipitation is presumed to be the source of the fluctuations through the modulations in ionospheric conductivity that they produce. Olson and Szuberla [1997] used data from a pair of cusp stations to deduce the scale size of the precipitating beams using a simple model in which the beams were assumed to have circular cross section. They obtained an upper bound for the coherence length of the order of 200 km. In this paper we extend the analysis of Olson and Szuberla by incorporating data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) magnetometer array and the Australian ANARE antarctic sites to give a broader range of station separations. Using a statistical approach we computed the cumulative distribution function of the interstation coherence and from that distribution we established a measure of coherence, CL. The result of this analysis is a coherence that diminishes with inter-station distance as CL ≈ 1.4 exp(−S/250) where S is the station separation in km. When this result is interpreted in the context of the simple model mentioned above we find a coherence length of 140–180km.

The Sounding of the Cleft Ion Fountain Energization Region (SCIFER) experiment was conducted to investigate the ionospheric origin of the Cleft Ion Fountain (CIF). In the previous decade several high altitude spacecraft studies concluded that the CIF is the principal source of mass for the magnetosphere, especially O+. Yet the ionospheric cleft in the altitude range between 1000 km and 2000 km had not been explored since the ISIS spacecraft experiments in the 1970s. SCIFER was designed to fill that gap with instrumentation that provided continuous spatial/temporal resolution two orders of magnitude better than that achieved by previous orbiting spacecraft.

Magnetic field data from the 210° magnetic meridian chain of stations have been analyzed to examine which component of the DL and DP fields dominates main impulses of sc and si observed on the ground. The DL and DP fields correspond to transfer processes from the magnetopause through the magnetosphere to the Earth's surface and through the polar ionosphere into the equatorial ionosphere, respectively. Northern and Southern Hemisphere asymmetry of sc and si disturbances appears at low and middle latitudes. Amplitude ranges of the sc and si main impulses observed at low-latitude conjugate pairs (L = 2. and 1.6) in the summer hemisphere are about twice as wide as those in the winter hemisphere. The equatorial enhancement of sc and si main impulses can be seen at Yap island (o = -0.3 °) during the daytime at 0600-1800 LT. These observational facts can be interpreted by invoking an asymmetry in the Northern and Southern Hemisphere twin-vortex-type ionospheric currents driven by the DP field, i.e., by invoking enhanced ionospheric conductivities in the summer hemisphere. The instantaneous penetration of sc and si-associated DP fields through the ionosphere from the polar region to the equator must play an important role in the energy transfer of sc and si disturbances from the solar-wind region to the magnetic equator.

After a recent discussion with colleagues about the role of ‘practice’ in proposed revisions to our graduate curriculum, I sat down and listed all of the different practices that had consumed my time over the previous few days, focusing on those that I would consider particular to my work as a humanities scholar. Most of them likely would be legible to the public as the kinds of work ‘humanities people’ do: starting to read Becoming Undone, the latest book by Elizabeth Grosz (2011) on Charles Darwin, and then rereading parts of Marx’s Grundrisse in preparation for an upcoming conference presentation on Marxism and New Media; revising (again) the third chapter for my book manuscript, and sketching out an outline for this essay; preparing a lecture on social memory and archives for an upcoming class; and trudging through the IRB (institutional review board) paperwork for a new ethnographic project involving medicine, gender, and visual culture.

During the 1993–1994 Winter, simultaneous measurements were conducted in Alaska of the radio spectrum from 0.1 to 4.8 MHz and geomagnetic activity. Night-time geomagnetic disturbances, consisting of bays in the DC magnetic field and rapid variations recorded with the induction coil, were found to be correlated with prompt absorption features in the LF/MF/HF radio spectrum and with three types of natural emissions at LF/MF/HF frequencies. During February–March, 1994, 66 out of 73 magnetic signatures meeting certain criteria were associated with LF/MF/HF absorption events and 30 of the 73 were associated with natural emissions. The radio spectrum was sometimes observed to be sensitive to relatively weak magnetic activity.

[1] The recent explosive eruptions of Okmok and Kasatochi volcanoes provide an opportunity to use seismic, local infrasound, distant infrasound array, and remote sensing data in concert to better monitor volcanoes in the Aleutian Arc and to better understand the source processes. The eruption of Okmok Volcano began on 12 July 2008 and included a seismically active phase that lasted continuously for about 10 h. In contrast, the eruption of Kasatochi which began on 7 August 2008 consisted of five explosive events that lasted from 26 to 68 min each and had a cumulative duration of 3.4 h. Given the event times by local seismic stations, the corresponding infrasound signals were found in the data recorded by local infrasound sensors and by distant infrasound arrays. Signals from the Okmok eruption were detected by three International Monitoring System (IMS) arrays as far away as 4400 km; signals from the Kasatochi eruption were detected at greater distances up to 5200 km away by seven infrasound arrays including the ones that detected the event at Okmok Volcano. Back azimuth propagation and a simple acoustic wave propagation model in unison with known event times were used to confirm that the planar, acoustic signals recorded at the arrays had originated from the eruptions. The infrasound array data reflected the differences in eruption styles between Okmok and Kasatochi as the signals from Kasatochi were of shorter duration, of greater amplitude, and detected over greater distances. The infrasound array data were also able to distinguish between two types of tremor episodes that occurred at Kasatochi Volcano based on atmospheric disturbance.

[1] A FDTD model of infrasound propagation in a realistic atmosphere is used to provide quantitative interpretation of the recently reported infrasound signatures from pulsating aurora. The pressure perturbations observed on the ground are analyzed as a function of energy flux of precipitating auroral electrons and geometry and altitude localization of the source. The results indicate that fluxes on the order of 50 erg/cm2/s are needed to explain pressure waves magnitudes of 0.05 Pa observed on the ground. This energy is unlikely to be provided exclusively by precipitating electrons, and Joule heating associated with the electrojet modulated by the pulsating aurora may be responsible for part of the deposited energy.

We have found a consistent correlation between ground based ultra low frequency (ULF) magnetic and optical measurements of the dayside cusp/boundary layer system which, we believe, may allow ground based identification of the boundary layers and central cusp. We have based our analysis on data taken at Longyearbyen Svalbard, Norway. Under normal magnetic conditions, as the station rotates beneath the auroral zone each day it passes under the ionospheric projection of the cusp-boundary layer system, near magnetic noon at approximately 0830 UT. During the passage of the cusp-boundary layer system a distinctive sequence of magnetic spectral features is observed. The boundary layer passage is characterized by a sharp increase in the power spectral density of broad band, unpolarized noise, combined with a polarized low frequency (1–3 mHz) signal. This is followed by a period of greatly reduced power spectral density of the unpolarized noise. A narrow band polarized signal in the 3–5 mHz band persists throughout this quiet period which is observed to be predominantly left hand polarized before local magnetic noon. This narrow band feature also appears to rise in frequency as the cusp passes overhead, and then falls again after noon. Optical observations are strongly correlated with the magnetic data. We believe the observed features have their source in the ionospheric footprint of the boundary layers and central cusp. The polarized signals may arise from Alfven waves spawned at the magnetopause which propagate down the magnetic field lines that map to the boundary layers. As the cusp may move with changing geomagnetic activity, observations of the features noted here may not be seen on every day. The statistical question of how often, and under what conditions, these features appear is an ongoing research topic. We plan to address this statistical question in a future manuscript.

This paper describes the geomagnetic pulsations observed at the high-latitude ionosphere during the experiment dealing with the ionospheric generation of ELF/VLF electromagnetic waves. The experiments were conducted in June and October of 1987, and there was clear evidence of geomagnetic pulsations intermixed with the ELF/VLF signals generated. This was manifested in both the magnitude and phase data. This paper discusses the detectability of pulsations based on the ELF/VLF generation experimental technique and a method of characterizing the pulsation being observed. A simple simulation model is introduced to facilitate the interpretation of the data, and the procedure for characterizing the pulsation is described.

In this report, we consider the amplification of electromagnetic ion cyclotron waves along a geomagnetic field line in the multicomponent magnetosphere, assuming that the waves propagate parallel to the background magnetic field. We find it is possible for the ring-current protons (energy ∼10–100 keV), which supply the free energy to stimulate the waves, to resonate with the waves not only in the equatorial region but also off the equator. An instability, caused by a thermal anisotropy, may occur in separated regions on and/or off the equator. The positions of the source regions along the wave path depend on the concentration of cold heavy ion species. The significant off-equator source regions may be located at geomagnetic latitudes where the waves, with frequencies greater than the He+ gyrofrequency on the equator, are in a local He+ pass band.

[1] We report here on infrasound signals observed using the infrasound microphone array at Fairbanks, Alaska that are apparently associated with the presence of pulsating auroras near the zenith. We have investigated the frequency domain coherence between the luminous intensity of the pulsating auroras and the high trace-velocity infrasound received at the earth's surface. Video data of the aurora display from an All-Sky video camera on the night of December 5, 2003 was compared with the pressure waveform data from the eight-microphone infrasonic array at Fairbanks. Enhanced coherence between the two signals was observed when a propagation delay time for the infrasound sample with respect to the video data sample was used. The frequency domain coherence was found to be high between the pulsating aurora intensity above the array and the high trace-velocity infrasound signals at the surface in the pass band from 0.03 to 0.08 Hz. Ray-tracing studies indicate that such high trace-velocity infrasound signals originate from sources within 35 km of the zenith above the array for a source height of 110 km. The presence of pulsating aurora patches within this area during the time interval of received infrasound and the high coherence between the video and infrasound data lead us to believe that periodic heating of the atmosphere by pulsating aurora is the actual source of the observed high trace-velocity infrasound.

[1] The north-south asymmetry of the amplitude of ULF pulsations in the Pc 3–5 band is studied using magnetic field data from the magnetically conjugate stations at L ∼ 5.4: Kotzebue (KOT) in the northern hemisphere and Macquarie Island (MCQ) in the southern hemisphere. We obtained the following results for the northward (H) component of magnetic pulsations: (1) The north to south power ratio shows a maximum in the northern winter and a minimum in the northern summer. This “seasonal variation” is stronger at higher frequencies (Pc 3 and Pc 4 frequencies). (2) The north to south power ratio for the Pc 4 and Pc 5 frequency band is basically greater than 1.0 for all seasons. This “positive offset” is stronger at lower frequencies. The “seasonal variation” implies that the magnetohydrodynamic (MHD) waves incident from the magnetosphere are more strongly shielded when the ionospheric conductivity is higher. The “positive offset” may result from the difference of the background magnetic field intensity between KOT and MCQ.

[1] In order to investigate the characteristics of Pi 2 propagation observed in the high-latitude region, ground magnetometer data obtained at high-latitude CPMN stations were analyzed. The power of magnetic perturbation, (ΔH)2 + (ΔD)2, were calculated for Pi 2 events observed at four stations from 11 February through 20 April 1996 and for Pi 2s observed at two stations from 1 January through 27 March 1997. The times when the power of Pi 2s reached the maximum and the maximum amplitudes were compared among stations. The results are as follows. Pi 2s observed at Kotel'nyy (KTN: MLAT = 69.94°, MLON = 201.02°) reached the maximum amplitude earlier than those at lower-latitude station Tixie (TIK: MLAT = 65.67°, MLON = 196.88°), though the amplitudes were smaller at KTN than at TIK on average. The time lag from KTN to TIK has two peaks in its distribution; the primary and the secondary peaks are located around 10 s and 35 s, respectively. The mean value of the whole distribution of the time lag from KTN to TIK is about 20 s. Ultra Violet Image (UVI) data obtained by the Polar satellite was available during the second period; the UVI data indicate that it was when the auroral oval was located equatorward to KTN that the Pi 2 amplitude tended to reach the maximum earlier at KTN than at TIK. This observational result is important because Pi 2 was observed earlier in the polar cap region rather than in the auroral region. That is to say, Pi 2 was observed earlier in the polar cap region, which is not directly connected with the source region of Pi 2 via the magnetic field line. Longitudinal characteristics of Pi 2 propagation were derived by using data from longitudinally separated stations TIK, Chokurdakh (CHD: MLAT = 64.67°, MLON = 212.12°) and Kotzebue (KOT: MLAT = 64.52°, MLON = 249.72°). The result indicates the existence of the longitudinal center of Pi 2 propagation. The average magnetic local time of the center is estimated to have been ∼22.5 MLT; eastward (westward) of the center, Pi 2 exhibited an eastward (westward) propagation. The temporal and spatial developments of the Pi 2 propagation along the auroral zone were derived in an empirical manner. That is, the MLT (set to zero at the propagation center) dependence of the maximum amplitude time and the maximum amplitude itself of Pi 2 were derived in an empirical manner. As a result it is concluded that in the premidnight sector (i.e., around 22.5 MLT), KTN is the most probable location that observes the maximum amplitude of Pi 2 earliest among the CPMN stations located along 210° magnetic meridian. Our results show that the low-latitude Pi 2, which has often been used as a time indicator of substorm onset, is often delayed from the Pi 2 observed in the premidnight polar cap region. The present results imply that the consideration of high-latitude Pi 2s in addition to low-latitude Pi 2s can provide a new insight into the substorm onset timing. Thus it is necessary to consider the global features, especially Pi 2s observed in higher-latitude region, for studying substorm onset timing issues.

[1] Infrasonic waves associated with the November 3, 2002, magnitude 7.9 earthquake in Alaska were detected by the microphone array (I53US) in Fairbanks, Alaska. The epicenter of the earthquake lay some 150 km south of the Fairbanks array. Both seismic and acoustic signals were recorded successively by the microphone array. The fault rupture of the earthquake moved east-southeast along the Denali-Totschunda system fault for approximately 294 km. The northern side of the fault moved to the east and vertically upward relative to the southern side. Displacements up to 8.8 meters were measured at various points along the fault [Eberhart-Phillps et al., 2003]. The seismic P-wave signal arrived at the array approximately 27 seconds after initiation of the rupture with a trace velocity of 5 km/sec. This was followed, some 12 minutes later, by an infrasonic wavetrain that had an average trace velocity of 0.34 km/sec and lasted about ten minutes. The azimuth of arrival of the infrasonic waves steadily moved eastward throughout the wave train apparently following the rupture's leading edge along the fault. The largest infrasonic signals, reaching over 12 Pa, came from the azimuth of the regions of largest ground motion along the fault. The source of the infrasound is taken to be the sudden local motion of the mountains in the Alaska Range along the Denali fault as they responded to the eastward-propagating fault rupture.

[1] All-sky auroral data at Kotzebue (KOT, 64.4° MLAT), Alaska, and magnetic field data from both high- and low-latitude stations are used to demonstrate the relation between multiple auroral brightenings and Pi 2 magnetic pulsations during an isolated substorm. At least five Pi 2 wave packets are identified in the magnetic field data around the end of the growth phase. Auroral luminosity variations do not show oscillations in the Pi 2 frequency range, though each Pi 2 wave packet has good correspondence with each small-scale auroral brightening and poleward motion. This fact may contradict the idea of high-latitude Pi 2 generation by oscillations of substorm field-aligned currents. We discuss possible reasons of this discrepancy from localization of the Pi 2 current system and low-energy electrons as the current carrier.

A ULF pulsation event is discussed on the basis of experimental observations of electric and magnetic field measurements as well as particle measurements from the DE 1 spacecraft. The observations were made near the magnetic equator in a space covering a large range of L shells and magnetic latitudes, and comparisons are made to ground observations. Azimuthal oscillations are observed following gradually decaying long-period compressional waves. Weak interaction between magnetic shells indicates that the source is probably weak, and ground data on magnetic pulsations showed strong signals that did not necessarily correspond to the quasisinusoidal pulsations observed in space. Azimuthal pulsations observed by the spacecraft indicate that there was a plasma density gradient beyond the plasmapause. The ULF pulsations were probably affected by changes in the magnetic field and solar-wind dynamic pressure, and their periods are found to be linked to geomagnetic latitude.

Analysis of Pc 3, 4 micropulsation wave forms recorded at Calgary in September 1969 shows a tendency toward signal enhancement in cases where the interplanetary magnetic field is at a small angle to the sun-earth line. Scatter plots of hourly Pc 3, 4 amplitudes exhibit a definite trend toward large signals when this angle is less than 50 or 60 degrees, and a corresponding disappearance of significant amplitudes when the angle is greater than 60 degrees. There is, however, an appreciable variability in individual cases. Power density spectrograms improved the correlation of pulsation strength with low angle in some cases.

In this paper, we present a generalization of the Stokes parameters and Stokes vectors for n-dimensional second-order stationary vector processes of the form ${\bf x}^T ( t ) = [ x_1 ( t ),x_2 ( t ), \cdots ,x_n ( t ) ]$. The Stokes parameters for $n = 2$ have been used for some time in describing the polarization characteristics of photons, but there has apparently been little attempt to generalize these parameters to arbitrary dimensions, with arbitrary bases. The Stokes parameters for $n = 2$ are found by expanding the density matrix in terms of the identity matrix plus the three Pauli spin matrices. The coefficients of this expansion are the Stokes parameters, and the ordered set of these coefficients is the Stokes vector. The basis set for $n = 2$ is trace-orthogonal and Hermitian, allowing a direct generalization to trace-orthogonal sets for $n > 2$. We show how these basis sets can be constructed from the outer products of complete sets of orthonormal vectors in unitary spaces. The Stokes vector sp...

On 16 March 1980 a rocket launched from Poker Flat, Alaska, carried a new type of high-explosive barium shaped charge to 571 km, where detonation injected a thin disk of barium vapor with high velocity nearly perpendicular to the magnetic field. The purpose of the experiment, named “King Crab,” was to advance knowledge of the instabilities, waves, and optical and magnetic effects produced. The TV images of the injection are spectacular, revealing three major regimes of expanding material which showed early instabilities in the neutral gas. The most unusual effect of the injection is a peculiar rayed barium-ion structure lying in the injection plane and centered on a 5 km “black hole” surrounding the injection point. Preliminary computer simulations show a similar rayed structure development due to an electrostatic instability.

A rocket launched from Poker Flat, Alaska, carried a new type of high-explosive barium shaped charge to 571 km, where detonation injected a thin disk of barium vapor with high velocity nearly perpendicular to the magnetic field. The TV images of the injection are spectacular, revealing three major regimes of expanding plasma which showed early instabilities in the neutral gas. The most unusual effect of the injection is a peculiar rayed barium-ion structure lying in the injection plane and centered on a 5 km 'black hole' surrounding the injection point. Preliminary electrostatic computer simulations show a similar rayed development.

We show two examples of magnetic pulsation recordings which illustrate that IPDP type wave events can occur simultaneously in the evening and morning sectors of the magnetosphere. This is a further confirmation that the generation of morning IPDP's is also associated with the enhancement of magnetospheric electric fields and energetic plasma during substorms.

The paper describes an experiment to determine whether the radial orientation of the interplanetary magnetic field (IMF) is associated with ULF activity in the Pc 3-4 range. Data are obtained from base levels, undisturbed intervals, IMF and disturbance selection, and trigonometric correlation. The results obtained are discussed, noting particularly that for low Kp, the probability of enhanced amplitude noise rises as IMF orientation with respect to the nominal solar wind flow decreases in both Pc 3 and Pc 4 channels.

We have made a survey of ULF emissions associated with sudden compressions of the magnetosphere due to sudden impulses (SI or ssc) on the basis of induction coil magnetometer recordings collected in Finland and Alaska during 1976–1979. We show several examples to illustrate the utility in classifying the emissions according to their spectral characteristics. The most common emissions are Pc1 pulsations. These show a maximum occurrence in the noon-afternoon sector and may undergo drastic variations in form as a function of latitude along one meridian. Pulsations which occur both in Finland and Alaska, at the same latitude separated by 175°, show very little similarity. A SI amplitude threshold of approximately 10 nT is necessary for the stimulation of Pel pulsations, however, if there is previous Pc1 activity this threshold is lowered. As SI amplitudes rise above 25 nT the probability of observing ULF emissions rises dramatically. We note that SI often trigger band-limited bursts of ULF emissions near 1 Hz. These bursts are short-lived with durations of a few minutes and occur a few minutes after the SI. They are most common during years of high solar activity. We conclude that the stimulation of different types of ULF emissions by SIs reflects the state of the magnetosphere at the time of the SI.

We present a description of a formalism for the detection and analysis of polarization states of arbitrary three-dimensional waveforms based upon the extensions of state vector concepts used in the descriptions of photons in optics and quantum mechanics. The formulation is based upon the interpretation of the spectral matrix which represents the waveform as a density matrix. Once this connection is established the power representing a general information state may be estimated. Expansion of the spectral matrix in a basis set of trace-orthogonal matrices leads to the Stokes vector representation of the signal waveform. This compact representation provides very direct routes to the estimation of various important signal parameters such as polarization, ellipticity, etc. The techniques are applied to a set of ULF waveforms which are scanned for the presence of magnetospheric resonances. The location of the resonant L shell with respect to the magnetometer station is established verifying the power and utility of the analysis techniques.

We have studied induction magnetometer data recorded at College, Alaska for the occurrence of ULF emissions associated with sudden impulses (SI). In this study we surveyed three years of data (1977–1979 inclusively) and found that for the 76 SI's reported in IAGA bulletins, 32 were found to be followed by ULF emissions in the College data. While the 76 SI's occurred at all local times those which were associated with ULF emissions at College peaked in occurrence near local noon. We have attempted to interpret these observations in terms of a simple model based upon a Chapman-Ferraro double-dipole model of the magnetosphere. Using this model an estimate of field compressions associated with SI's can be made and from this the increases in the temperature anisotropy and plasma beta may be estimated. This simple model predicts maximum growth rate near noon on high latitude field lines, just inside the magnetopause. Further, inspection of growth rate curves for varying plasma anisotropy and beta leads us to the conclusion that an increase in anisotropy is the primary cause of the ULF emissions observed.

In this paper we report on a study of the association between the polarization states of Pcl and ULF pulsations (primarily IPDP) and electron precipitation recorded at College, Alaska in 1983. This study indicates that there is no direct association of precipitating electrons with Pcl pulsations exhibiting left-hand polarization at the ground. We examined 33 pulsation events which occurred in the afternoon magnetosphere during the first four months of 1983. Twenty-five of the pulsations were right-hand polarized and eight were left-hand polarized. Particle precipitation was observed during 16 of the right-hand polarized events and four of the left-hand polarized events. These results indicate that the handedness of a pulsation event is not a good predictor of associated electron precipitation. Recent investigations have shown that the propagation of Pcl waves in a multicomponent plasma can account for mixed polarization signatures on source field lines. Our results then indicate that the identification of a source field line based upon polarization information alone may be incorrect.

It is shown that the observed local time variation of dayside geomagnetic micropulsations is consistent with the Kelvin-Helmholtz generation mechanism operating at the magnetopause. The variation of the angle between the interplanetary magnetic field and the magnetopause around the magnetosphere causes variations in the magnetosheath magnetic field, which in turn lead to local time variations in micropulsation amplitudes. Morning sector pulsations are expected to be larger than afternoon sector pulsations. Furthermore, large-amplitude pulsations are expected to be more frequently observed when the angle between the interplanetary magnetic field and the solar wind velocity in front of the bow shock is small.

During 1985 magnetometer stations were operated at Cape Parry and Sachs Harbor, N.W.T., Canada in order to investigate the spectrum of geomagnetic pulsations at high latitudes and near the polar cusp. Each day, as the stations move under the dayside cusp, the level of pulsation power increases by several orders of magnitude. The spectrum is broadband, reaching from below 5 mHz to above 1 Hz with enhancements near 5 and 40 mHz. The 5 mHz power is produced, in part, by discrete pulses which may be interpreted as due to evolving current patterns in the overhead ionosphere. The pulses have amplitudes of 50–100 nT, temporal widths of a few minutes and show time delays between the two stations which indicates poleward motion at 1–5 km s −1 . Consistently, for every pulse, the time delay depends upon the component of the magnetometer used. The largest time delays occur for the Z -component and these are about twice the delays measured using the H - and D -components. This dependence may imply the current filament is evolving in shape as it moves poleward. While the number of pulses present in the data varies from day to day, presumably controlled by solar wind conditions, when they are present the pulses occur at 10–20 min intervals throughout the times when the stations are near the dayside cusp. The amplitudes, occurrence frequency and poleward motion are consistent with the identification of these pulses as the ground signatures of currents associated with flux transfer events.

The design of data‐adaptive filters requires that the noise be defined, statistically or otherwise, by parameters which allow some means of separating the noise from the signal. We consider here multichannel data in which one knows only that the noise is less polarized than the signal in a unitary space. This description of the noise is not sufficient for designing filters which are optimum in any sense; consequently, the filters may require a number of changes in the parameters before a satisfactory design can be found. Once this design has been achieved, the filters can be used to enhance waveforms of arbitrary shape, requiring little prior knowledge of the spectral content or temporal features of the signal. In contrast to many other data‐adaptive filters which give a scalar time‐series output, the filters we describe here with vector time series input have an equal number of input and output channels. A number of examples of filtered magnetic and seismic data are given in order to emphasize the wide range of uses for the filters. Some suggestions for application of the filters to multichannel seismic data are given.

Summary. 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=r1+ir2 where r1 and r2 are orthogonal vectors in a real space. For an elliptically polarized wave, r1 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.

Gingival thickness was measured in 10 subjects with healthy gingiva on the facial aspect of selected maxillary and mandibular teeth at the depth of the gingival sulcus and midway between the sulcus depth and mucogingival line. Measurement instrumentation, consisting of a differential transformer coupled to an oscillator and digital voltmeter was sensitive to the travel of a probe from the gingival surface to tooth surface or alveolar bone. The probe assembly was accurate to 0.01 mm and the average range of differences in replicate measurements under a variety of conditions never exceeded 0.15 mm. 1. Free gingival thickness averaged 1.56 mm + 0.39, attached gingival thickness averaged 1.25 mm + 0.42 and the total mean thickness for all areas measured was 1.41 mm. 2. Thickness in mandibular free and attached gingiva and maxillary free gingiva increased from anterior to posterior. Thickness in maxillary attached gingiva remained fairly constant. 3. Thickness measured at the depth of the sulcus was directly proportional to the free gingival width. 4. Thickness measured midway between sulcus depth and mucogingival junction was inversely proportional to attached gingival width.

Hourly distributions of the angle theta/sub x/B=arccos (XxB), where X is the solar ecliptic earth-sun axis and B is the interplanetary magnetic field (IMF), have been compared with hourly Pc 3 and Pc 4 signal activity at Calgary for 198 hours in September, October, and November 1969. Hours whose distributions were concentrated above or below theta/sub X/B=50/sup 0/ were correlated with a clear separation of inactive from active micropulsation intervals. The correlation was stronger for the Pc 3 than for the Pc 4 band, and hours with significant fractions, say 10--20% of their theta/sub X/B distributions below 30/sup 0/, corresponded particularly well with high amplitudes of micropulsation signals. Signals in the Pc 3 period range on the ground can therefore be used as a crude monitor of certain extremes of IMF orientation.

We report that intervals of pulsations of diminishing periods (IPDP) type plasma events which are associated with magnetospheric substorms are less frequent in the years of high solar acitivity than in years of low activity. This new result is based on two independent continuous data sets from 1974 to 1985. We show also that IPDP events occur throughout the recurrent magnetic storms in 1976, whereas they appear more sporadically in 1980, i.e., during high solar activity. It is concluded that several factors such as changes in plasmapause position, attenuation of horizontal ionospheric wave propagation, and heavy ion densities in the magnetosphere can contribute to the observed long-term variability. copyright American Geophysical Union 1988