Auroral Infrasonic Waves Research Articles

Studying possibilities for the classification of infrasonic signals from different sources

Atmospheric infrasonic signals were classified on the basis of data obtained in the United States (University of Alaska, Fairbanks) and in the Antarctic region (Windless Bight) from 1980 to 1983. The data archive included five classes of signals from different sources: explosions, mountain associated waves, microbaroms, volcanic infrasound, and auroral infrasonic waves. This classification was based on the theory of testing statistical hypotheses. The possibilities of separating these classes were studied. It was shown that the signals (from the archive used) that are characteristic of explosions and volcanic activity can be rather easily separated from those characteristic of mountain associated waves, microbaroms, and auroral infrasonic waves.

Auroral Infrasonic Waves and Poleward Expansions of Auroral Substorms at Inuvik, N.W.T., Canada

Summary Observations at Inuvik (70.4 dipole latitude) have shown that supersonic motions of auroral arcs that sweep across the zenith from south to north during poleward expansions of auroral substorms do not generate observable auroral infrasonic waves. This is in contrast to the fact that equatorward supersonic motions of similar auroral arcs do produce large amplitude infrasonic bow waves. These results imply an asymmetry in the basic generation mechanism of infrasound within the auroral electrojet arcs. The morphology of auroral infrasonic wave (AIW) substorms has been determined from auroral zone observations at College, Alaska (64.6 dipole latitude) by comparison of all sky camera (ASC) photographs, surface magnetometer observations and riometer records of auroral cosmic noise absorption events (Wilson 1969a, b, c, 1970). Supersonic motion of large-scale auroral forms in a direction transverse to the long axis of the auroral arcs was found to be a necessary condition for the production of AIW by an auroral arc (Wilson & Nichparenko 1967). This led to the development of a ' shock wave ' model to explain the directional and amplification properties of the radiation of infrasonic waves by auroral arcs in supersonic translation (Wilson 1967). It was shown that if a pressure pulse is produced within the auroral arc by an unspecified mechanism that is of constant amplitude and phase in the frame of reference of the moving arc, then a ' shock wave ' would be produced by constructive interference of the wavefronts as the source moved across the sky. It has been pointed out that ' bow wave ' is a better name for this phenomenon (Chimonas & Peltier 1970). Shock conditions may prevail in the E region of the aurora; however, by the time the infrasound reaches the ground the wave amplitude is only a few dyn/cm-2. Magnetometer records have been used to relate the translational speeds of auroral electrojet arcs to the generation of AIW (Wilson 1969b). The AIW was found to arrive at the surface from six to eight minutes after zenith passage of an auroral electrojet arc in supersonic translation. The horizontal trace velocity V,, of the AIW was shown to have the same direction and speed as the auroral sources motion (Wilson 1967). Further evidence for the association of AIW with supersonic auroral motion that develops along the auroral oval (Feldstein & Starkov 1967) during the expansion phase of the average auroral substorm (Akasofu 1968) was found by a comparison of auroral sudden-onset cosmic noise absorption events and AlW (Wilson 1970). An infrasonic observatory was set up at Inuvik, N.W.T. in Canada at dipole latitude 70.4 (geographic co-ordinates 6835'N, 133W) with an array of four microphones (see Cook & Young 1962, for equipment description), a three-component magnetometer, an all-sky auroral camera and a riometer for measuring cosmic noise

Observations of Acoustic Aurora in the 1-16 Hz Range

Summary Acoustic aurora have been heard by long-term residents of the artic. They have also been recorded on microbarographs. Acoustic events associated with aurora are now reported in the near infrasonic range (1–16 Hz) at Barrow, Alaska. These observations were made with the aid of a resonant detector achieving a high signal-to-noise ratio similar to that used in extending the rocket-grenade technique to 109km. Over 100 impulsive events of a quasi-repetitive nature were recorded on a patrol basis during January 1970. Acoustic events were correlated with disturbed magnetic conditions and optical aurora but uncorrelated with lower-frequency auroral microbarograph events at College or Inuvik. It is hoped that these initial observations will persuade interested parties to a more complete study of this phenomena and encourage an explanation of the generation mechanism for auroral infrasonic waves.

Propagation of auroral infrasonic waves studied by ray-tracings

On the basis of a ray tracing method the propagation and the attenuation of an auroral infrasonic wave are studied. Relations between the direct and reflected waves recorded at the Syowa Station, Antarctica, are clarified with regard to; (1) the delay time, (2) the intensity ratio, and (3) trace velocities. The time required for a wave to travel from the source to the ground is calculated as a function of a source altitude. The retardation time of the wave arrival behind the zenith crossing of the source current is deduced. A method is proposed for estimating the altitude of a source current from the retardation and a trace velocity of the wave. It is concluded that the existence of a supersonic equatorward motion of an electrojet which continues for a certain distance is necessary for the observation of auroral infrasonic waves. This distance must exceed at least 60 km equatorwards from the zenith to enable the direct wave to be observed and with total length of 930 km to enable the reflected wave to be observed. From these conditions it is also concluded that the infrasonic wave is not seen in mid latitudes and the reflected wave is a rare phenomenon.

Temporal and spatial changes of polar substorms and infrasonic wave emissions

By use of auroral infrasonic wave (AIW) records observed at the Syowa station, Antarctica, the relations between the AIW emissions and the auroral substorms were studied separately in the cases of emissions in the dusk, dawn and midnight. For this purpose, contour maps of the horizontal magnetic vectors at the time of AIW emissions were compared with the equivalent current systems. The followings are found. The existence of substorm activity is always found at the time of AIW emissions, but its strength has only minor relations to their occurrence. The condition for the occurrence of the emission depends largely on the manner how the disturbed region moves. The AIWs are emitted when the disturbed region moves with a supersonic speed and the trace direction of the wave roughly coincides with the direction of the traveling disturbance. While the morning and the evening AIWs are emitted from the rear region of the traveling disturbance, the midnight one from the front region. The AIW emissions tend to synchronize with the overhead crossing of the electrojet current and at that time the current causes the rotational change in the horizontal magnetic vectors on the ground.

Auroral infrasonic waves and the auroral electrojet

Using models of a line current and a band current with a parabolic intensity distribution across the width, techniques to deduce the speed, the direction of motion and the zenith crossing time of the electrojet from observations of surface magnetic perturbations are studied. From the current motions deduced by these techniques and the observed traces of the Auroral Infrasonic Waves (AIW), the following four facts are established. 1. (1) The time between the zenith crossing of the current and the arrival of AIW at the ground is reasonable. 2. (2) The direction of travel of AIW is considered to be parallel to the current drift or parallel to the current flow. 3. (3) The trace velocity of AIW is equal to the calculated drift velocity of the current. 4. (4) AIW can be produced only by a line (or a narrow band) current. Several minutes before the AIW arrive at the ground, the existence of certain motions of westward current which satisfy the AIW emission conditions proposed by Wilson (1972) have been confirmed. However there are several cases in which a succession of two equatorward supersonic zenith crossings of westward current have induced only one AIW.

An auroral infrasonic substorm investigation using Chatanika radar and other geophysical sensors

Characteristics of the supersonic auroral arcs within the 0905 UT 2 April 1973 substorm were determined using data from (1) all-sky cameras; (2), surface magnetometers, (3) multispectral scanning photometers, (4) 30MHz riometers, (5) Chatanika incoherent-scatter radar, (6) Homer auroral radar, and (7) infrasonic microphone arrays at College and Stevens Village in Alaska. These data were analyzed to determine the properties of an auroral electrojet arc that generates auroral infrasonic waves (AIW). An arc that was show to be the source of an AIW was found to have the following characteristics: (1) a velocity of 500 m/sec traveling from an azimuth of 350°; (2) an intensity in 4278 A of 26 Kr, (3) a maximum electron density of 2.8 × 10 6 el/cm 6 at 100km height, (4) an equivalent westward line current of 2.8 × 10 6 A, (5) orientation of ΔH parallel to the AIW direction of travel and perpendicular to the arc's long axis, (6) a characteristic energy of the primary auroral electron spectrum of 3.0keV, and (7) an energy deposition rate for the auroral pdarticles of 100 erg/cm 2 sec.

Auroral infrasonic wave emissions on a night of low geomagnetic disturbance

Auroral infrasonic wave emissions on a night of low geomagnetic disturbance

The multiplicity of auroral infrasonic wave forms

During a period of fairly low planetary magnetic index (Kp varied from 1 + to 2) across the study period on June 25, 1969, numerous low-amplitude unidirectional wave trains of infrasound were recorded at the Geophysical Institute in College, Alaska. By using a mean horizontal transit speed of auroral infrasonic waves of 243 m/s the origin of these pressure disturbances is shown to be various locations within the auroral electrojet. Because of the intermittent nature of the coherent low-amplitude signal reception and the presence of many nearly coherent pressure oscillations on this occasion it is suggested that upper atmospheric winds could exert the dominant influence to enhance or suppress propagation of auroral infrasonic waves from ionosphere to ground.

Infrasonic wave generation by aurora

A comparison of the characteristics of auroral infrasonic waves (AIW) in the passband from 10 to 100 sec. period is given for trans-auroral zone stations. The morphology of 2 Hz auroral infrasound observed at Kiruna, Sweden is described. An important asymmetry in the generation of AIW bow waves, with respect to poleward or equatorward motion of auroral electrojet arcs, is shown to be related to the morphology of the ionospheric E-region electric field during an auroral substorm. Auroral radar studies of AIW source arcs are presented. Incoherent scatter radar observations of electron density profiles of auroral arcs are used to describe the differences in arcs that do or do not produce AIW in an attempt to explain the asymmetry in AIW generation by supersonic auroral arcs. An infinite line-current induction model of an auroral electrojet moving transverse to its axis above a conducting earth is used to determine the relationship of an AIW to the surface magnetic perturbation produced by the electrojet. The role of the auroral electrojet in AIW production is discussed in a study of polar magnetic substorms wherein westward traveling auroral surges and the eastern ends of westward auroral electrojets were found to be strong sources of AIW. Theoretical work on the generation of AIW's is reviewed and evaluated with respect to the observed characteristics of AIW substorms. Lorentz force and Joule heat are discussed as probable source terms for the production of the acoustic energy within an auroral electrojet. The important unsolved problems in understanding auroral infrasound are outlined.

Chatanika radar investigation of high latitude E‐region ionization structure and dynamics

Simultaneous Chatanika incoherent scatter radar, auroral all‐sky camera, and photometric data are utilized to delineate the structure and dynamics of the auroral E layer for various types of disturbances. The vertical distribution of ionization (Ne(h) profiles) is presented and discussed for various disturbance phenomena, such as the great geomagnetic storm of August 2–9, 1972, intense auroral particle precipitation events (March 16, 1972, March 2, 1973), and auroral infrasonic wave disturbances, April 2, 1973. Ne(h) profiles for quiet conditions and for a slowly varying disturbance (the July 10, 1972, solar eclipse) are shown for comparison purposes.

The motions of peaks in ionospheric auroral absorption and auroral infrasonic waves

The average direction of motion of ‘peaks’ in ionospheric auroral absorption events changes with local time in a manner similar to that of the direction of propagation of auroral infrasonic bow waves (AIW) as observed in the auroral zone. A study of the morphology of motions of auroral absorption peaks can thus give information on the variation with local time of the motion of auroral arcs that generate AIW.

Trans-auroral zone auroral infrasonic wave observations

The auroral infrasonic wave (AIW) substorm morphologies are compared for two trans-auroral zone stations, Inuvik, N.W.T. Canada (70°·4 dip lat) and College, Alaska (64°·6 dip lat), that lie along the same magnetic meridian with a north-south separation of 738 km. Statistical studies of the number of AIW received at College over a 5 yr period and at Inuvik over a 2 yr period as well as studies of individual auroral substorms observed at both stations have shown that in the morning sector many more AIW are observed at College than at Inuvik. This difference is related to the changing location of the westward auroral electroject with local time (Weins and Rostoker, 1973). The distribution of frequency of occurrence of AIW horizontal trace velocity V η is presented for College data together with a discussion of the effects on the distribution of (1) source speed, (2) wind shear, (3) geometry of the AIW mach cone with respect to the observing station, (4) the filtering of AIW with high ray path apogees and (5) the decrease in AIW amplitude with increasing mach number.

Seasonal variation of auroral infrasonic wave activity

Although there has been a large fluctuation in the degree of auroral infrasonic wave (AIW) activity from year to year, it has been possible with 6 years (1966-1972) of data to show that there is a seasonal variation in the number of AIW that is similar to the seasonal variation found for auroral activity. (WDM)

Auroral infrasonic wave-generation mechanism

The morphology of auroral infrasonic wave (AIW) substorms, as determined from infrasonic observations at Inuvik (68.4°N), College (64.9°N), and Palmer (60.8°N) along a magnetic meridian through Alaska, shows that AIW are never observed propagating in a poleward direction, even though auroral activity frequently occurred south of the stations. AIW have been shown to be infrasonic bow waves generated by supersonic westward, equatorward, or eastward motions of auroral electrojet arcs. All AIW are observed from azimuths that vary from parallel to the auroral oval to transverse to the oval in an equatorward direction depending on the local magnetic time. Specific examples of auroral poleward expansions that cross the Inuvik zenith have been studied. It was found that even highly supersonic poleward motions of arcs with strong electrojets do not produce infrasonic bow wages. When a reversal in direction of the poleward expansion occurs and the arcs subsequently move equatorward, strong AIW are observed from the arcs. This asymmetry in the occurrence of AIW with respect to direction of motion of an arc is interpreted as an intrinsic asymmetry in the generation mechanism within the auroral arcs and not as a propagation effect. It is postulated that the basic acoustic pulse within the electrojet arcs is caused by collisions with the neutral gas of positive ions that are driven by electrodynamic drift in the E region of the auroral arc. Thus Lorentz force is the coupling mechanism between the electrojet current carriers and the neutral gas. An ionization-collision process occurs within the auroral arcs for those arcs that are moving supersonically in a direction parallel to the direction of the neutral ionization drift. The resulting increase in ion density for such arcs reduces the time constant for ion drag, and thus the Lorentz-force coupling is effective in producing an infrasonic shock wave. If the supersonic translation of the primary auroral electron sheet has a component of motion parallel to the electrodynamic drift of the positive ions, an auroral infrasonic shock wave will be produced in the E-region ionosphere and propagate to the ground as a modified shock or bow wave. If, on the other hand, the auroral-arc motion is antiparallel to the drift of the positive ions, no AIW will be produced.

Auroral infrasonic and ionospheric absorption substorms

The median global pattern for the progression of auroral absorption onsets is compared with the distribution of auroral infrasonic waves along the auroral oval. There is agreement between the directions of auroral motions as determined on one hand by the pattern of absorption onsets and on the other hand by the infrasonic wave morphology. The distribution in infrasonic wave trace velocity with respect to the auroral oval is consistent with the distribution in wave-front velocity as derived from the pattern of absorption onsets.

Two-station auroral infrasonic wave observations

The morphologies of auroral infrasonic wave substorms are compared for two stations, College and Palmer, Alaska, that are 368 km apart and approximately on the same magnetic meridian. During an 11-month period of simultaneous operation, five times more auroral infrasonic waves were received at College, the northernmost station, than at Palmer. When Palmer was closest to the equatorward edge of the average auroral oval (1100–1400 UT), very few signals were received there, probably because of an acoustical skip-zone effect. A model for the intersection of the auroral infrasonic shock surface with the ground plane is developed in order to examine marked differences in the characteristics of infrasonic wave packets observed successively at College and Palmer. These differences are related to the positions of the observing sites with respect to two regions in the model, defined as the front shock region and the side shock region.

Auroral infrasonic waves

The morphology of auroral infrasonic waves at College, Alaska, is related to the temporal and spatial distributions of supersonic auroral motions that develop within the auroral oval during polar magnetic substorms. The break-up phase of the auroral substorm results in the rapid auroral motions that generate the observed infrasonic waves. Direct observations of infrasonic wave packets from overhead supersonic auroral forms, as well as the comparison of the morphologies of infrasonic waves and auroral motions are used to verify a shock wave model for the generation of auroral infrasonic waves. Electrodynamic drift and joule heating associated with the auroral electrojets in auroras that produce infrasonic shock waves are accepted as the basic processes that generate the initial acoustic pulse within a moving auroral form.