ISSN:0094-8276    

DOI:10.1029/GL010i008p00601

年代:1983

数据来源: WILEY

摘要:

Recent observations of electrostatic waves associated with whistler mode chorus emissions provide evidence that electrons are being trapped by Landau resonance interactions with the chorus. In this paper we discuss the trapping, acceleration and escape of electrons in Landau resonance with a whistler mode wave packet. We show that acceleration can occur by both inhomogeneous and dispersive effects. The maximum energy gained is controlled by the points where trapping and escape occur. Large energy changes are possible if the frequency of the wave packet or the magnetic field strength increase between the trapping and escape points. Various trapping and escape mechanisms are discussed.

ISSN:0094-8276    

DOI:10.1029/GL010i008p00603

年代:1983

数据来源: WILEY

摘要:

A simulation study of VLF triggered emissions has been carried out in order to investigate quantitative changes of the velocity distribution function of resonant electrons caused by a combined action of nonlinear phase‐trapping and geomagnetic inhomogeneity. It is a counter‐part of the an analytic theory proposed by Roux and Pellat. The result basically supports their idea which is based on a simplified assumption. It also gives detailed information of more complicated behavior of both trapped and untrapped particles. It is pointed out that the contribution by electrons which are trapped in the midst of the whistler wave train is as important as that by electrons trapped at the wave‐front. It is further revealed that deformation of the velocity distribution function is caused not only by trapped electrons but also by untrapped electrons perturbed by a whistler triggering wave. The well‐known characteristics of pulse length dependence of rising or falling tone characteristics of triggered emissions is interpreted by the present simulation

ISSN:0094-8276    

DOI:10.1029/GL010i008p00607

年代:1983

数据来源: WILEY

摘要:

Electron precipitation events induced by discrete VLF whistler mode waves have previously been detected by photometers at Siple Station, Antarctica. This paper presents the first observations of ionospheric optical emissions correlated with VLF waves at the conjugate location, near Roberval, Quebec. Since most whistlers recorded at Siple or Roberval originate in the north, Roberval affords a clear perspective on the direct precipitation induced during the first pass of the wave as it propagates southward. For such a wave the direct precipitation and that induced in the "mirrored mode" by the returning two‐hop wave should differ in arrival time by roughly twice the wave propagation time between hemispheres, while at Siple the effects of the direct and mirrored modes may overlap in time. A well defined series of observations of structured λ4278 optical emissions was observed on August 30, 1979 in the aftermath of an intense magnetic storm. The optical emissions were found to lead the arrival time of the two‐hop waves by about 0.7 s instead of lagging the local waves by about 1‐2 s as had been previously observed for whistler driven events at Siple. The observed arrival time relationships are consistent with the predictions of a cyclotron resonance interaction model, and thus support previous observations of x‐rays at Roberval. The importance of the first pass of the wave is further emphasized by an approximate proportionality between the amplitude of the VLF waves recorded at Siple and the intensity of the optical emission bursts at Roberval. Although structured optical emissions correlated with wave bursts can clearly be detected at Roberval, relatively large magnetospheric particle fluxes may be required to produce suc

ISSN:0094-8276    

DOI:10.1029/GL010i008p00611

年代:1983

数据来源: WILEY

摘要:

The first direct observations of the precipitation of radiation belt electrons by the controlled injection of VLF signals from a ground based transmitter were recently reported from data acquired in the SEEP (Stimulated Emission of Energetic Particles) experiment. That outstanding example of time‐correlated wave and electron data has now been enhanced by the finding of four additional modulated events out of 65 satellite passes when one of the U. S. Navy VLF transmitters at Cutler, Maine (NAA) or at Annapolis, Maryland (NSS) was being modulated in a 3s ON/2s OFF format. During each of these events the fluxes of precipitating electrons were observed repeatedly to display a characteristic time behavior with respect to the transmitter modulation: a relatively slow rate of increase after start of the ON period leading to a maximum about 2 seconds later. Details of this consistent pattern and the statistics of occurrence of modulation events are presented along with comparisons of the absolute fluxes of precipitating electrons observed during normal transmitter operation with those recorded when one of the transmitters was modulate

ISSN:0094-8276    

DOI:10.1029/GL010i008p00615

年代:1983

数据来源: WILEY

摘要:

Precipitation spikes of electrons, in which the energy spread of the peak is narrow (less than 50 keV) and the peak energy is a strong function of the location in L, have been observed in the region 1.5

ISSN:0094-8276    

DOI:10.1029/GL010i008p00619

年代:1983

数据来源: WILEY

摘要:

Signals launched by the NLK Jim Creek transmitter in Alaska on 18.60 and 18.65 kHz have been observed on GEOS‐1. Data for one pass over Alaska on June 11, 1977, are presented here. The peak amplitude of the signals is ∼5 pT (0.6 mV/m), which is received when the satellite is close to exact conjugacy at 7500 km altitude. While the weaker signals received at some distance from conjugacy behave as expected from linear theory, the stronger signals received closer to conjugacy have features which indicate that some non‐linear process is active. These features are: 1) a turbulent electric frequency spectrum 2) an increased electrostatic character of the waves. The threshold field amplitude of the supposed (but unidentified) non‐linear interaction i

ISSN:0094-8276    

DOI:10.1029/GL010i008p00623

年代:1983

数据来源: WILEY

摘要:

A number of examples of auroral light bursts correlated with Pc 1 "pearl" wave packets have been recorded at Siple, Antarctica (L = 4.2). The events are unique in that an individual light burst occurs at Siple not when the wave packet is recorded at Siple but rather when the wave is measured to be at the conjugate point over Roberval, Quebec. Although one can fit the observations to a model where ions are precipitated from an interaction region near the equator via cyclotron resonance, the number of ions required to produce the light and the above mentioned light burst‐ULF wave phase relationship both make the precipitation of electrons a more plausible interpretation.In an electron model, electrons are precipitated at Siple which have interacted with the Pc 1 wave packet over Roberval. Because of the South Atlantic magnetic anomaly, particle mirror points are nearly 200 km deeper in the atmosphere at Siple than at Roberval. Electrons of auroral energies (few keV) can be in longitudinal resonance with the Pc 1 waves over Roberval in the altitude range where the Alfvén speed rapidly decreases (2000 → 500 km). Low parallel velocities are possible for these electrons in this altitude range because the electrons are close to their mirror points. Those electrons in longitudinal resonance with the Pc 1 wave packet must have their mirror points lowered such that they do not reach the atmosphere at Roberval but are definitely precipitated at Siple. An interesting possibility is suggested in that these events represent the acceleration of electrons by a kinetic Alfvén

ISSN:0094-8276    

DOI:10.1029/GL010i008p00627

年代:1983

数据来源: WILEY

摘要:

Propagation characteristics of electromagnetic ion‐cyclotron waves are extremely sensitive to the relative composition of thermal ions: this can consequently control the instability properties of resonant energetic ions in the Jovian magnetosphere. The dominance of heavy ions in the Io plasma torus will suppress ion‐cyclotron instability near the equatorial plane. Wave growth, however, can still occur away from the equator in the region where thermal hydrogen is expected to predominate. A theoretical assessment of the convective L‐mode gain indicates that Voyager 1 did not enter the preferred region of instability. Evidence for instability may nevertheless be available since oblique ion‐cyclotron waves are expected to experience a natural polarization reversal to the R‐mode which is able to propagate from high latitudes to the equator. If the amplitude of the unstable L‐mode waves exceed a gamma the concomitant scattering rate of resonant energetic ions should approach the strong diffusion limit. In the absence of significant Landau damping the power spectral density of fluctuating (≳10 Hz) R‐mode electric fields detectable near the equatorial plane by the Voyager plasma wave instrument could then exceed 10−7(V/m)²Hz−1in the inner torus. Such intense ion‐cyclotron waves could account for the observed decrease in ion phase space density and the excitation of auroral emissions on field lines map

ISSN:0094-8276    

DOI:10.1029/GL010i008p00631

年代:1983

数据来源: WILEY

摘要:

This letter addresses in detail the structure of the gap near the helium cyclotron frequency observed within the statistical distribution of the frequencies of electromagnetic cyclotron waves encountered at synchronous altitudes. It is shown that, at most, two‐thirds of the gap (the high frequency portion) results from linear dispersion effects. It is suggested that at least one‐third of the gap (the low frequency portion) results from nonlinear, off‐resonant absorption of the waves by means of near cyclotron resonance with singly charged helium

ISSN:0094-8276    

DOI:10.1029/GL010i008p00635

年代:1983

数据来源: WILEY