John V. Olson

Pi2 pulsations and substorm onsets : A review

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 onset 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.

Uncertainties associated with parameter estimation in atmospheric infrasound arrays

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.

Listening to the secret sounds of Earth's atmosphere

A new global network is breathing life into a dormant branch of geophysics. The study of infrasound, or long-period acoustic signals in the atmosphere was bustling in the 1950s and 1960s. Prior to 1963, almost all nuclear tests occurred in the atmosphere. After 1963, the USSR and U.S. signed the Limited Test Ban Treaty (LTBT), which eliminated all atmospheric nuclear tests. During the era of atmospheric nuclear testing, infrasound research was in demand, since the massive explosions produced strong, long-period acoustic waves that were globally observed and could be used to locate and describe the nuclear tests. Interest in this branch of geophysics waned with the end of atmospheric testing.

An automated procedure for near‐real‐time Kp estimates

The Kp index indicates geomagnetic disturbances in a simple manner. The index is derived from magnetic field data acquired at 11 ground stations distributed worldwide at subauroral latitudes (49°–62°), has values ranging from 00 to 90 in 28 steps, and is given at 3-hour intervals. Kp is widely used to study the dynamic relationship between the solar wind and the magnetosphere, to empirically specify the location of the plasmapause and other plasma regions/boundaries, and also as input to various models of the magnetosphere and ionosphere. Currently, the official Kp index is delivered with a delay of many days, so it is not useful for near-real-time monitoring of the state of the magnetosphere. We have developed an algorithm to derive an estimated Kp, denoted Kpest, using magnetometer data from nine ground stations that can transmit data in near real time. The algorithm is fully automated and includes a data-cleaning routine, a quiet-day-curve routine, and a routine to convert magnetic field deviations to Kpest. We have evaluated the performance of Kpest using archived magnetometer data and the official Kp. When data from all of the nine stations are available, the linear correlation coefficient between Kpest and Kp is 0.93. In addition, we find that a similarly high correlation between Kpest and Kp can occur when data from only one to three stations are used. We conclude that an automated procedure using data from a small number of ground stations can generate Kp estimates that are reliable in the context of near-real-time monitoring of space weather.

Distribution of wave packet sizes in microbarom wave trains observed in Alaska

This work reports on a study of the distribution of wave packet sizes contained in intervals of continuous microbarom activity. Microbaroms are a class of atmospheric infrasound that is characterized by narrow-band, nearly sinusoidal, waveforms with periods near 5 s. They are known to be generated by marine storms, presumably through a nonlinear interaction of surface waves, however the detailed analysis of the process is still incomplete. The data analyzed were obtained using the University of Alaska infrasound array of four microphones located in central Alaska. Because of the narrow-band feature of the microbarom signals, the Hilbert transform is applicable as a method for finding phase breaks in the signal. The phase breaks are interpreted as the demarcation of the boundaries of wave packets. When applied to long sequences of microbaroms a broad distribution of packet lengths is found that diminishes monotonically with length and has a mean near 10 cycles and a variance nearly as large. The distribution function decreases exponentially with packet length. The distribution of packet sizes is influenced by the presence of multiple sources and multiple propagation paths between the sources and the sensor array. Identification of individual packets should open the way to a more detailed analysis of microbarom wave trains. After separating the wave train into individual wavelets the intermicrophone correlation is estimated as a function of microphone separation. As has been observed in earlier microbarom studies, a decrease in correlation was observed for microphone pairs orthogonal to the direction of propagation when compared to correlations between microphones spaced along the direction of wave propagation.

Explosion localization via infrasound

Two acoustic source localization techniques were applied to infrasonic data and their relative performance was assessed. The standard approach for low-frequency localization uses an ensemble of small arrays to separately estimate far-field source bearings, resulting in a solution from the various back azimuths. This method was compared to one developed by the authors that treats the smaller subarrays as a single, meta-array. In numerical simulation and a field experiment, the latter technique was found to provide improved localization precision everywhere in the vicinity of a 3-km-aperture meta-array, often by an order of magnitude.

High-latitude Observations of Infrasound from Alaska and Antarctica: Mountain Associated Waves and Geomagnetic/Auroral Infrasonic Signals

The Geophysical Institute at the University of Alaska Fairbanks has established and operated seven different infrasonic microphone arrays in Alaska, Canada, Sweden, and Antarctica from 1965 to the present in a continuing effort to study natural sources of infrasound, at high latitudes, in the pass band from 0.015 to 10 Hz. Recently, in association with the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization [ADD REF TO CHAPTER 2], modern 8-microphone infrasound arrays, with digital data-acquisition at 20 Hz, were installed as I55US in Windless Bight, Antarctica (2001) and as I53US in Fairbanks, Alaska (2002). Coherent infrasonic signals, observed over the period 2000–2008 at both stations, are studied here for both mountain associated waves (MAW) and unique high trace-velocity signals associated with geomagnetic and auroral activity at high latitude regions.

Discrimination of near-field infrasound sources based on time-difference of arrival information

A computationally efficient method for discriminating between near- and far-field infrasound sources using array time-difference of arrival (TDOA) information is described. Rather than assess wave-front curvature, the discriminant quantifies the statistical departure of TDOA information from that of a plane wave passing the array. Since the method constrains neither the functional form nor the amplitude characteristics of a signal it is suited for discrimination of signals across large-aperture infrasound arrays. Experimental results confirm theoretical predictions to a range of order ten array apertures. The discriminant is applied to data from an Antarctic infrasound array.

Spatiotemporal characteristics of cusp latitude spectra

In this study we analyze the spatiotemporal characteristics of spectra generated from 754 days of cusp latitude magnetometer data (Longyearbyen and selected Magnetometer Array for Cusp and Cleft Studies (MACCS) stations). In order to distinguish between the presence of spatial (fixed in magnetic local time) and temporal (fixed in universal time) signatures in cusp latitude spectra we develop a simple test using trace power, polarization, and ellipticity spectra. On the basis of this test we find evidence for both spatial and temporal signatures in cusp latitude spectra. We find that the trace power spectrum is dominated by temporal information; however, the polarization and ellipticity spectra contain unambiguous spatial structure. Temporal information in cusp latitude spectra is carried primarily by broadband Pc3 (10–50 mHz) noise, while spatial information is carried by polarized Pc3 (1–10 mHz) pulsations. Additionally, we establish a state-space measure as a quantitative means of discriminating the spatial passage of the cusp and boundary regions by ground-based magnetic means. The measure is based on the difference between daily polarization spectra (centered on local magnetic noon) and the mean polarization spectra for a given station. This procedure replaces previous determinations which were made from spectra “by-eye”.

Performance of an infrasound source localization algorithm

We present a performance analysis of a method of acoustic source localization based on time difference of arrival (TDOA) information for an arbitrary array of sensors. The method begins with the construction of a vector containing estimates of time delays for each unique sensor pair in the array via cross correlation. An optimization in the space of geographic location and sound speed is then conducted to minimize the difference between the observed vector and one calculated from a search in that space. The technique uses an analytic method of seeding based on the acoustic analog of a light cone. The source localization procedure is tested as a function of a number of parameters, including array aperture, number of sensors, sample rate, signal‐to‐noise ratio, and signal type. The technique is shown in application to real and synthetic infrasound signals and its statistical behavior is given.