L. J. Cahill

Magnetometer array for cusp and cleft studies observations of the spatial extent of broadband ULF magnetic pulsations at cusp/cleft latitudes

We have used magnetometer data from 10 locations in Arctic Canada and Greenland, covering over 5 hours in magnetic local time at magnetic latitudes from 75° to 79°, to characterize the dayside patterns of enhanced long-period ULF (10- to 600-s period) wave power at cusp/cleft latitudes. We conclude the following: (1) In agreement with earlier single-station studies, we find that the most common wave type is broadband noise (Pi 1-2). Distinct Pc 3-4 activity and more sustained monochromatic Pc 5 activity are most apparent when this broadband noise is weak. (2) Multistation observations also make clear that strong, broadband Pi 1-2 signals are both temporally and spatially structured: Although their amplitude is somewhat larger near local noon and near nominal cusp latitudes, they often occur simultaneously (to within a few minutes) at all stations. They are thus not local signals, and cannot be interpreted as evidence of passage of an auroral region or boundary over an individual magnetic observatory. In particular, we have found no evidence for a distinctive “cusp” signature in broadband ULF waves in this frequency range. (3) The occurrence of strong broadband Pi 1-2 signals at these latitudes appears to be controlled largely by solar wind velocity. We found good correlations between the occurrence of strong Pi 1-2 signals and high solar wind velocity, and we also noted some dependence on the cone angle of the interplanetary magnetic field for moderate to low solar wind velocities. We speculate that there may be an additional dependence on enhanced levels of trapped plasma in regions topologically connected to the very high latitude dayside ionosphere, such as the entry layer, high-latitude dayside field minimum regions, or plasma mantle. Available satellite data on the level of trapped energetic electron fluxes at geosynchronous orbit showed that broadband power levels appeared to correlate with enhanced flux levels on the time scale of days, but not on shorter time scales, suggesting that any such dependence is not directly related to substorm injections.

Explorer 45 observations of 1‐ to 30‐Hz magnetic fields near the plasmapause during magnetic storms

Observations have been made of <30-Hz magnetic fluctuations, some of which appear to be ion cyclotron waves, during initial, main, and recovery phases of magnetic storms. Such storm-related observable low-frequency events occur very infrequently, in only 10 instances in more than 2000 hours of data, including approximately 100 passes through storm time ring currents. All events were observed while the satellite was near the plasmapause and in the proton ring current. The wave amplitudes were typically 1 γ and as large as 5 γ in the frequency range from 1 to 3 Hz, with lower amplitudes in the 3- to 30-Hz range. On the basis of the spectral shape and intensity of the fluctuations the waves were presumed to be primarily ion cyclotron waves. Reasonable agreement was found between calculated ring current proton lifetimes while the protons were under the influence of ion cyclotron waves and observed proton lifetimes during parts of the December 1971 and August 1972 magnetic storms. These observations during times when the ring current and plasmasphere were close and often overlapping do not prove that the waves were generated by the ion cyclotron instability or that the ion cyclotron waves were necessarily responsible for proton ring current loss. The observations do suggest that ion cyclotron waves are unstable in such cases and that proton lifetimes are consistent with loss from ion cyclotron turbulence. Further theoretical (ion cyclotron wave growth limits) and observational (proton energy and pitch angle distribution changes during these or similar events) studies are required before the role of the ion cyclotron instability in ring current dynamics can be completely determined.

High‐latitude ground observations of Pc 1/2 micropulsations

A ground-based survey of Pc 1/2 (0.1-0.4 Hz) and Pc 1 micropulsations throughout 1986 has provided evidence for the location of the Pc 1/2 source region. Data were taken from three high-latitude stations, located at South Pole (−75° geomagnetic latitude; 1530 UT local noon), Sondre Stromfjord (+74°, 1330 UT LN) and Siple (−61°, 1700 UT LN). The study revealed a diurnal occurrence pattern for waves in the 0.1-0.4 Hz band (Pc 1/2) and showed that the pattern was not due to the effects of sunlight on the ionosphere but instead from a postnoon magnetospheric source region. On the basis of the latitudinal occurrence patterns of the waves above and below 0.4 Hz, it is concluded that the waves observed on the ground above 0.4 Hz come primarily from plasmapause latitudes, while the source of the Pc 1/2 lies between the plasmapause and the magnetopause. The estimate of source locations for waves above and below this frequency, combined with the typically sharp upper frequency limit of waves in the 0.1-0.4 Hz band (Pc 1/2) are interpreted as evidence that He+ ions in the outer magnetosphere influence propagation and possibly wave growth. These results are compared with those of Anderson et al. [1990, 1992a,b], who showed with a spacecraft study that Pc 1 are more commonly observed beyond L = 7 than in regions closer to the Earth. It is concluded that many of the waves above the He+ gyrofrequency from the outer magnetosphere do not always reach the ground. An extensive search for correlations between Pc 1/2 occurrence and solar wind pressure and magnetic field orientation showed no direct connection between solar wind parameters and Pc 1/2 generation. They may instead be amplified by plasma sheet ions that drift sunward on the dusk side of the magnetosphere [Kaye and Kivelson, 1979; Anderson and Hamilton, 1993] and undergo ion-cyclotron resonance in the afternoon sector. This mechanism is consistent with the diurnal pattern and apparent source location of the Pc 1/2.

Field and thermal plasma observations of ULF pulsations during a magnetically disturbed interval

ULF pulsations were observed by DE 1 between 1600 and 1830 UT, October 31, 1982, during a magnetically disturbed interval. Ground observations suggested that the pulsations were excited by a sudden increase in the solar wind velocity and pressure. During the pulsation interval DE 1 traveled near apogee from −55 to −20° geomagnetic latitude and from L ∼ 13 to L ∼ 4 at about 0900 LT. The waves observed were azimuthal oscillations preceded by gradually decaying long period compressional waves which lasted for more than 1 hour. Phase relations between magnetic and electric field oscillations and calculated Poynting flux indicate that in the outer magnetosphere (L > 8) DE 1 observed propagating waves which contained strong poloidal components, while the quasi-sinusoidal toroidal waves seen later for L < 10.3 were standing along field lines. The toroidal waves appeared as four wave packets, each of which corresponded to a region with a distinct plasma distribution. The observed wave periods decreased with L over an extended magnetospheric region. The seemingly weak interaction between magnetic shells suggests that the source was a broadband one. Magnetometer data from several high latitude observatories located near the footpoints of the magnetic shells crossed by DE 1 were also examined. The magnetic pulsations on the ground contained many frequency components, and the waves seen most strongly in space were often not the strongest signals seen on the ground near the same field lines. The broadband nature of the ground pulsations indicates that the stations also detected oscillations of the adjacent field lines. The major frequencies seen at ground stations seemed to be roughly constant for about 2 hours but L dependent. This suggests that the changing periods seen in space by DE 1 were clearly L related and not temporally varying.

Observations of the plasma environment during an active ionospheric ion beam injection experiment

Abstract The electrodynamics of the release of neutral beams of a few tens of eV to 200 eV Argon ions in the upper ionosphere is becoming clearer as a result of several rocket flights each having a different location of the Argon release with respect to the diagnostic payload. A volume of about 10 meters radius centered on the Argon release payload is highly turbulent which scatters the Argon ions to angles well beyond the 30 degree half angle emission cone. Broadband wave activity is measured in this volume. The superthermal neutralizing beam electrons become magnetized in this volume for across-field plasma releases. If this volume is not neutralized by field-aligned electron transport it becomes charged and decelerates the Argon ions. Ambient electrons are accelerated near or within this volume to energies of several hundred eV. The mechanism for doing this appears to be due to the wave turbulence rather than any payload neutralization effect. In a much larger volume of cross-field dimension equal to the Argon ion gyroradius (a few hundred meters) the wave activity is predominately near the lower hybrid frequency and also consists of narrow-band waves at ion gyrofrequecies. It is in this volume that ambient ions are accelerated perpendicular to the local magnetic field.

Statistical study of hydromagnetic chorus events at very high latitudes

Hydromagnetic (HM) chorus events are ULF waves with typical frequencies from 0.2 to 0.6 Hz; their frequency-time spectrum consists of a combination of band-limited unstructured emissions and discrete elements. These waves are one of a class of short-period ULF emissions (in the Pc 1 and 2 frequency range) that may be of value in ground-based identification of the footpoint of magnetospheric boundary regions. As part of an extensive survey of Pc 1 and 2 waves at very high latitudes, we have identified all occurrences of HM chorus in nearly a full years data from South Pole Station (−74.2° geomagnetic latitude, local noon ∼ 1530 UT) and McMurdo, Antarctica (−80.2° geomagnetic latitude, local noon ∼ 2030 UT) during 1990, and at McMurdo and Sondrestromfjord, Greenland (74.2° geomagnetic latitude, local noon ∼ 1330 UT) during 1988. In agreement with previous studies, these events tend to occur within a few hours of local noon. Earlier studies reported that HM chorus events are typically seen in ground records at auroral zone latitudes and suggested that they originated in the outer dayside magnetosphere. Study of DMSP particle boundary data indicated that the events reported here occurred during conditions of extremely contracted auroral ovals and are thus also consistent with an outer dayside magnetospheric source region. However, the occurrence of HM chorus events in this high-latitude data set was limited to the months of October through May, with an occurrence peak in February, and no events were found in the months of July through September in either of the years studied. Since both hemispheres are covered, this suggests not a seasonal variation (with late winter minimum) but rather an apparent annual variation. We speculate that such an annual variation in very high latitude occurrence of HM chorus may be related to the MLT-UT offset of the Earths geomagnetic and geographic poles, in conjunction with wave propagation cutoffs at the high-latitude magnetic field minima occurring on dayside magnetic field lines very near the magnetopause.

Waves generated in the vicinity of an argon plasma gun in the ionosphere

This paper describes results of a lofted plasma gun experiment, where the instrumentation to observe waves generated by the gun was carried on the same platform as the gun. Previous experiments, where the gun has been separated from the measuring instrumentation has shown a very intense and broad band range of waves generated from the interation of the plasma from the plasma gun with the ionosphere. Wave activity was observed, with large harmonic content, in the frequency range 10 to 1000 Hz. Much of this wave activity could not be related to cyclotron frequencies of argon or other gases known to be present in the ionosphere. This report presents initial results from analysis of particle and wave measurements made by instrumentation carried with the plasma gun.