R. E. Erlandson

A statistical study of Pc 1-2 magnetic pulsations in the equatorial magnetosphere 2. Wave properties

A statistical study of narrowband transverse 0.1- to 4.0-Hz magnetic pulsations, essentially Pc 1–2 (0.1–5.0 Hz), occurring from L=3.5 to L=9, |MLAT| 7 have not previously been discussed and are found to exhibit remarkable polarization behavior. The A.M. events have the highest X observed in the data base, with X averaging 0.4 to 0.5, but most remarkably, they are linearly polarized at all magnetic latitudes sampled. Given the high normalized frequency, the equatorial linear polarization of the A.M. events cannot be explained in terms of a crossover from left- to right-hand polarization occurring during propagation from low to high magnetic field strengths. Oblique propagation or the effects of multiple reflection through the wave growth region might lead to linear polarizations. The results suggest that a new examination of EMIC wave generation specifically addressing the properties of this A.M. population may be needed.

Observations of electromagnetic ion cyclotron waves during geomagnetic storms: Wave occurrence and pitch angle scattering

This paper addresses the occurrence of low-L (3.5 < L < 5) electromagnetic ion cyclotron (EMIC) waves during magnetic storms. Using ∼10 years of data, a statistical analysis indicates that EMIC waves in the equatorial magnetosphere occur 5 times more often during magnetic storms than during quiet times. EMIC wave occurrence during magnetic storms is important because EMIC waves pitch angle scatter energetic protons and thus represent a ring current loss mechanism. Observations in the equatorial magnetosphere, acquired by the Dynamic Explorer 1 (DE 1) satellite, are used to show the simultaneous occurrence of EMIC waves and enhanced proton fluxes in the loss cone. The study involves three cases selected only on the basis of observed EMIC wave activity and the availability of energetic ion data. The observations show that EMIC waves indeed scatter protons into the loss cone. Model calculations of the loss cone heat-flux density compare favorably to the observed proton flux in the loss cone. These calculations are then used to estimate the effect of EMIC waves on ring current energy loss, assuming an effective area of wave activity and the duration of the wave activity. It is found that large-amplitude, long-duration EMIC waves can produce the observed recovery in the Dst index.

Ion energization mechanisms at 1700 km in the auroral region

Observations obtained by the Freja satellite at altitudes around 1700 km in the high-latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations ...

Freja observations of electromagnetic ion cyclotron ELF waves and transverse oxygen ion acceleration on auroral field lines

Extremely low-frequency (ELF) magnetic and electric field plasma wave emissions were recorded on 2 October 1993 on auroral field lines by the Magnetic Field Experiment during Freja orbit 4770. The ELF wave frequencies were below the local oxygen gyrofrequency (25 Hz) and between the helium and proton gyrofrequencies (100 to 400 Hz). The ELF waves, interpreted as electromagnetic ion cyclotron (EMIC) waves, were observed in a region of inverted-V-type electron precipitation. The EMIC waves were correlated over time with auroral and lower energy ({approximately} 100 eV) electrons, which are both possible sources of free energy, and also with transversely accelerated oxygen ions. The waves above the helium gyrofrequency were more closely correlated with the transverse oxygen ion acceleration than the waves below the oxygen gyrofrequency. These observations are consistent with a scenario in which electron beams generate EMIC waves, which then produce transverse oxygen ion acceleration through a gyroresonant interaction. 16 refs., 4 figs.

The dynamic cusp

A unique alignment of the Viking satellite with respect to a network of magnetometers in Greenland has provided the opportunity to study the relationship of pulsations and plasma characteristics in the dayside cusp. Observations in the interplanetary medium were not available during the event studied here, but particle data from the DMSP satellite and hot plasma observations from Viking provide strong evidence that the IMF had a strong northward component. The presence of Pc 1 bursts, Pc 4–5 pulsations, and a tailward traveling twin vortex pattern of ionospheric convection suggests that the magnetosphere may have been temporarily compressed. Magnetic field data acquired at synchronous altitude from GOES 5 and on the ground from Huancayo support this suggestion. Plasma with ion dispersion characteristics associated with a cusp during southward IMF was detected by Viking over a 3.5° range of latitude. The presence of standing Alfven waves and ring current ions suggests that this “cusplike” plasma was observed on closed geomagnetic field lines. As Viking moved further poleward, it detected a different region of plasma with characteristics associated with a cusp during northward IMF. The presence of plasma on closed field lines with “southward IMF” ion dispersion characteristics can be explained with a poleward moving plasma source. We suggest that the magnetosphere, during a northward IMF, is temporarily compressed by a solar wind pressure enhancement that produces the Pc 1 bursts, Pc 4–5 pulsations, and ionospheric vortices. As the magnetosphere recovers to its “precompressed” shape, the source of cusp plasma will move poleward until it reaches an equilibrium position for northward IMF. The Viking satellite, following in the wake of this source, will detect plasma with “southward IMF” characteristics until it reaches the latitude of the actual “northward IMF” cusp. These observations support the view that the shape of the magnetosphere may rarely be static but is often changing as a result of the delicate and variable balance between the solar wind and geomagnetic field.

Viking magnetic and electric field observations of periodic Pc 1 waves: Pearl pulsations

Pearl pulsations, with an average repetition period of 60 s, were recorded using the magnetic and electric field experiments on the polar-orbiting Viking satellite. The wave event occurred on September 30, 1986, during Viking orbit 1212 at 1030 MLT, from L = 3.6 to L = 4.1, and at an altitude of 13,500 km. Electron density observations obtained from Viking show that the waves were generated at the plasmapause and at lower amplitudes in the plasmasphere. The wave Poynting flux, calculated using the magnetic and electric field, indicated that the waves generally were propagating downward toward the ionosphere, although upward Poynting fluxes were observed. Clear evidence of upward propagating waves, associated with downward propagating waves reflected at the ionosphere, was not observed. Linear convective growth rates suggest that the anisotropic ions which provide the free energy have a perpendicular temperature around 15 keV. The repetition period, calculated using the measured electron density and magnetic field strength at Viking, is consistent with the double-hop transit time for ion cyclotron waves which propagate along field lines from one hemisphere to the other. However, the absence of upward propagating wave packets implies that the upper limit of the wave ionospheric reflection coefficient is on the order of 10 to 20%. Alternative mechanisms for producing the observed repetition are also investigated and include a periodic generation model of pearl pulsations at the ion bounce period.

Pc 1 waves in the ionosphere: A statistical study

A statistical study of Pc 1 waves has been performed using electric field data recorded in the ionosphere by the Dynamics Explorer 2 satellite. The study was performed by applying an automated wave detection algorithm to over 900 hours of data recorded at invariant latitudes (INV) greater than 40° from December 6, 1981, to February 16, 1983. A total of 390 Pc 1 waves in the electric field were identified using a selection technique based on spectral peak detection in the frequency range from 0.2 to 6.0 Hz. Most events were observed at frequencies between 0.4 and 2.0 Hz and in the dawn (0400–0600 MLT) and noon (1000–1500 MLT) sectors from 50° to 62° INV. Detection of events at high latitudes (INV > 65°) was limited by noise associated with auroral zone electric fields. Significant differences in the properties of ionospheric Pc 1 waves were observed in the dawn and noon sectors. First, noon sector waves were clearly ordered by the equatorial gyrofrequencies (ƒ 10 (mV/m)2/Hz, in the noon sector compared with just 23% in the dawn sector. Third, for events with a magnetic field component, the median value of ΔE/ΔB was 410 km/s in the dawn and 330 km/s in the noon sector, although the median value of the ratio between (ΔE/ΔB) and the Alfven velocity was similar in both sectors. Fourth, more short-duration events (2–8 s) were observed in the noon than in the dawn sector. We conclude that the source of noon sector ionospheric Pc 1 waves is electromagnetic ion cyclotron waves generated in the equatorial magnetosphere. The source of the dawn sector waves is less certain, as many waves were observed only in the electric field.

A statistical study of auroral electromagnetic ion cyclotron waves

This paper discusses results from a statistical study of narrowband auroral electromagnetic ion cyclotron (EMIC) waves recorded by the Freja magnetic field experiment from December 1, 1993, to November 30, 1994. The study involves data collected at all magnetic local times (MLT), invariant latitudes between 40° and 75°, and altitudes between 1200 and 1750 km. Wave events have been identified using a two-step process. First, magnetic field spectra in the frequency range between 5 and 256 Hz were surveyed using an automated procedure which detects spectral peaks. Second, the results of this automated procedure were confirmed by visually inspecting wave spectrograms which contained these spectral peaks. A total of 316 auroral EMIC wave events were identified. The occurrence of these waves peak at auroral latitudes in the premidnight sector (1800–0100 MLT). The events have a strong seasonal dependence with peak occurrence in the winter. These properties are the same as those of auroral electron precipitation, which is suggested to be the free energy source of these waves. The spectral properties of the waves were also analyzed. The wave usually contained either one or two spectral peaks. In the cases where two spectral peaks were observed one peak was below the local O+ gyrofrequency (ƒO+) and one was above the He+ gyrofrequency (ƒHe+). The observations suggest that auroral EMIC waves can be generated below ƒO+. The wave spectra were consistent with wave generation at altitudes near or above Frejas altitude. Analysis of the spectra suggest that the auroral EMIC waves originated from a source altitude between 1500 and 5500 km.

Simultaneous ground‐satellite observations of structured Pc 1 pulsations

Structured Pc 1 pulsations are investigated using simultaneous multipoint ground-satellite observations recorded on September 10, 1986, during Viking orbit 1103. The multipoint Pc 1 observations were acquired using the Viking magnetic field experiment at 13,550 km altitude from L = 5.1 to 5.5 and three Finnish ground stations at Rovaniemi, Ivalo, and Kilpisjarvi. These stations are all located within 30 min magnetic local time and 1° in latitude of the ionospheric field line footprint of the Pc 1 source field line. Structured Pc 1 pulsations between 0.5 and 1.0 Hz were observed both on the ground and in space at similar frequencies, with similar frequency dispersion, and with a similar repetition period. The wave source region, based on Viking Langmuir probe observations, was just inside the plasmapause. The wave transit time between Viking and the ground was 12 ± 2 s, where Viking leads the ground. This implies that the waves propagate downward from Viking to the ground, consistent with previous downward Poynting flux estimates.

Nonbouncing Pc 1 wave bursts

On April 11, 1986, at about 0600 UT a long Pc 1 wave event of the hydromagnetic chorus type started on the ground, as registered by the Finnish pulsation magnetometer network. The main pulsation band at about 0.3 Hz was observed for several hours. Soon after start, this band smoothly extended to higher frequencies, forming another separate wave band which finally reached up to 0.5 Hz. During the event the Viking satellite was on its southbound pass over Scandinavia, close to the MLT sector of the ground network. From 0650 until 0657 UT, Viking observed a chain of Pc 1 bursts with increasing frequency. The strongest bursts could be grouped into two separate wave regions whose properties differed slightly. The higher-latitude region had a frequency of 0.3 Hz, well in agreement with the main Pc 1 band on the ground. The lower-latitude region contained the highest frequencies observed on the ground at about 0.5 Hz. The latitudinal extent of both wave regions was about 0.5°. They had slightly different normalized frequencies, Alfven velocities, and repetition periods. Most interestingly, the repetition periods of both wave sources were too short for the bursts to be due to a wave packet bouncing between the two hemispheres. The results give new information about the high-latitude Pc 1 waves, showing that they can consist of separate repetitive but nonbouncing bursts. We suggest that the long-held bouncing wave packet hypothesis is generally incorrect and discuss two alternative models where the burst structure is formed at the equatorial source region of the waves.