Magnetospheric chorus - Occurrence patterns and normalized frequency

The properties of the plasmapause as a VLF wave guide are studied. The guidance that occurs is a form of gradient trapping of VLF wave energy. It is shown that guiding is possible at both the inner and outer edges of the plasmapause and that more efficient guiding occurs as the plasmapause gradients become stronger. In the case of strong gradients, waves coming from a latitude range of about 8 deg are focused tightly about the plasmapause field lines, resulting in a wave intensity increase of approximately 3 dB near the magnetic equatorial plane. It is shown that plasmapause-guided waves can be observed on the ground and can echo between hemispheres, precisely as can waves guided in normal whistler ducts. The single distinguishing feature of plasmapause-guided waves is a lowered upper cutoff frequency. The results indicate that the vicinity of the plasmapause represents a natural and readily accessible region of VLF wave guidance and focusing where both passive and active VLF experiments can be studied through ground and in situ satellite measurements.

The plasmapause as a VLF wave guide

In this paper we review recent spacecraft observations of oblique whistler-mode waves in the Earth’s inner magnetosphere as well as the various consequences of the presence of such waves for electron scattering and acceleration. In particular, we survey the statistics of occurrences and intensity of oblique chorus waves in the region of the outer radiation belt, comprised between the plasmapause and geostationary orbit, and discuss how their actual distribution may be explained by a combination of linear and non-linear generation, propagation, and damping processes. We further examine how such oblique wave populations can be included into both quasi-linear diffusion models and fully nonlinear models of wave-particle interaction. On this basis, we demonstrate that varying amounts of oblique waves can significantly change the rates of particle scattering, acceleration, and precipitation into the atmosphere during quiet times as well as in the course of a storm. Finally, we discuss possible generation mechanisms for such oblique waves in the radiation belts. We demonstrate that oblique whistler-mode chorus waves can be considered as an important ingredient of the radiation belt system and can play a key role in many aspects of wave-particle resonant interactions.

Oblique Whistler-Mode Waves in the Earth’s Inner Magnetosphere: Energy Distribution, Origins, and Role in Radiation Belt Dynamics

At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms, and times of magnetic storms. Within each of these categories, there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse - Pc 1, Pc 3, Pc 4, Pc 5; quiet time compressional - Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse - Pc 1; storm compressional Pc 4, 5.

Fluctuating magnetic fields in the magnetosphere. II - ULF waves.

The mapping of thundercloud electric fields at middle and subauroral latitudes is investigated analytically as a three-dimensional boundary value problem. The electrical conductivity is represented by several piecewise exponential functions of altitude, and the anisotropy of the medium is taken into account above 70-km altitude. The geomagnetic field lines are assumed to be straight and vertical below 150-km altitude. Electric field strength at great heights depends sensitively on conductivity and thundercloud models used in the calculations. Sample calculations using representative nighttime profiles show that ‘giant’ thunderclouds can produce transverse electric fields of tens of microvolts per meter in the equatorial plane of the midlatitude magnetosphere. In the daytime, corresponding electric fields are about an order of magnitude less. These results suggest that giant thunderclouds may be an important source of localized electric fields that can form field-aligned electron density irregularities in the ionosphere and the magnetosphere.

Penetration of thundercloud electric fields into the ionosphere and magnetosphere: 1. Middle and subauroral latitudes

While the great majority of ground whistlers are interpreted as indirect evidence of magnetospheric ducts, the first direct evidence of ducts was obtained from the Stanford University broadband VLF experiments on Ogo 1 and Ogo 3. Five discrete whistler ducts were encountered by Ogo 3 on the inbound pass of June 15, 1966, between L = 4.7 and 4.1. Each duct was characterized by reception at the satellite of ducted whistlers with a distinct spectral shape, and of the high-frequency portions of whistlers (leakages) that propagated inward from outer ducts. The data were interpreted in detail by ray tracing in a model magnetosphere that includes ducts of enhanced ionization. The following conclusions resulted: (1) the L shell thicknesses of the observed ducts ranged between 0.035 and 0.070 earth radii, and the interduct separations ranged between 0.017 and 0.18 earth radii; (2) the dimension of the ducts in longitude was estimated to be of the order of 4°, or 0.3 earth radii at the equator, which is a factor of ∼4–8 greater than the L shell dimension; (3) the whistler ducts are much more likely to be enhancements than troughs; (4) the minimum enhancement factors needed to trap frequencies up to half the equatorial electron gyrofrequency are on the order of 8%, with smaller values producing upper cutoffs at lower frequencies; (5) the limited spreading of the calculated leaked rays is in general agreement with the corresponding regions of observation and relative signal amplitudes; (6) the low cutoffs of leaked signals are probably due to accessibility; (7) cyclotron and Landau interactions are likely to play a role in upper cutoffs of leaked signals; (8) the upper cutoff of ground whistlers near fH0/2 is a trapping (rather than absorption) effect; (9) the hydrostatic type of distribution of ionization along the field lines is applicable in the plasmasphere; and (10) the travel times (and frequency of minimum delay) of ducted whistlers can be calculated with good accuracy by assuming purely longitudinal propagation.

Whistler duct properties deduced from vlf observations made with the OGO 3 satellite near the magnetic equator

Analysis of ELF and VLF data from the Stanford University experiment on Ogo 4 revealed an ELF hiss band with characteristics not previously identified. The band, referred to as band-limited ELF hiss, is seen from low to medium latitudes. On the basis of wave-propagation properties, it is proposed that the BLH is generated at large wave normal angles in the equatorial region near L = 4. This model can be used to explain the characteristics of the BLH. Two mechanisms for the generation of BLH based on radiation from energetic electrons are considered.

Jacyntho J. Angerami

Papers

Whistler duct properties deduced from vlf observations made with the OGO 3 satellite near the magnetic equator

Ogo 4 observations of extremely low frequency hiss.