Strong Pitch Angle Diffusion Research Articles

Evidence for Strong Pitch-Angle Diffusion within the Polar Cusps

AS is well known, the classical zero-Larmor radius Chapman-Ferraro theory of the magnetosphere1–3 neither accounts for the observed penetration of low-energy solar wind plasma through the dayside magnetopause into the polar magneto-sphere4, 5 nor explains the specific properties5–9 of this plasma entry. On the other hand, because gradients of the magnet field3 tangential to the magnetopause are an inherent property of the zero-Larmor radius theory of the magnetosphere, the former can probably be understood on the basis of finite Larmor radius theory that takes into account the drift motion of the particles. The dominant role of the tangential gradients in the formation of the theta-model of the geomagnetic tail has recently been established10. The tangential gradients on the dayside magnetopause point in the direction of the polar neutral points, which are the sources and sinks of almost all of the magnetic field lines that make up the magnetopause. Thus the geometry of the magnetospheric field allows for lines of maximum gradient drift perpendicular to the magnetopause that connect the neutral points along the line of demarcation with the neutral sheet of the tail. We propose that magneto-sheath plasma enters the magnetosphere along these zones of maximum tangential gradient drift velocity, thereby identifying them with the polar cusps. Then some of the characteristics of the observed particle population within the polar cusps can be explained if one further assumes that the inward drift motion of the particles couples to strong pitch-angle diffusion11. This assumption is partially justified by the measured pitch-angle distributions in the distant polar cusps which suggest that the distribution near the loss cone is more isotropic than that of the magnetosheath. It is also natural to assume that the polar cusp region will be highly unstable against various kinds of plasma waves and hence wave-particle interactions leading to pitch-angle diffusion could occur there.

Dayside auroral-oval plasma density and conductivity enhancements due to magnetosheath electron precipitation

Demonstration that magnetosheath electrons precipitating into the dayside auroral oval are a significant source of ionization and consequently will lead to electrical conductivity enhancements within the oval. By assuming that the electrons are maintained isotropic by strong pitch-angle diffusion as they precipitate into the ionosphere, the precipitation heat flux can be simply related to solar-wind energy density and consequently to the level of magnetic activity. For quiet solar-wind conditions, the heat fluxes of 1 to 10 ergs/sq cm/sec expected and observed lead to height-integrated Pedersen conductivity enhancements of 4 to 15 mhos. During magnetic storms the conductivity enhancements could increase by a factor of 3 to 5. Since the precipitating electrons are soft, the Hall conductivity enhancements are smaller than the Pedersen conductivity enhancements. For typical electric fields the computed conductivity enhancements lead to field-aligned currents bounding the enhancements in order-of-magnitude agreement with observation. The topside ionosphere should also have a density enhancement over the auroral oval on the dayside.

Strong pitch-angle scattering of energetic electrons in the presence of electrostatic waves above the ionospheric trough region

Simultaneous measurements of energetic electrons and electrostatic waves in the midlatitude trough region indicate that events characterized by strong pitch-angle diffusion in the altitude range 2000-4500 km occur in the presence of ELF electrostatic waves attributed to a current-driven ion cyclotron instability. Scattering by the electrostatic waves is sufficient to account for the strong pitch-angle scattering commonly observed by low-altitude satellites. The resulting precipitation creates a sharply defined, high-latitude boundary for energetic electrons as observed by low-altitude satellites. This boundary is similar to the feature generally referred to as the high-latitude trapping boundary. However, the precipitation boundary occasionally occurs on closed field lines at a significantly lower latitude than the trapping boundary. When present, the precipitation boundary is most sharply defined in the nighttime hemisphere.

Observations of precipitating protons in the energy range 2.5 kev≤E≤200 kev

Abstract : Measurements of precipitating protons in the energy range 2.5 - 200 keV have been made on Ov1-15 (1968-59A), a low-altitude polar satellite, with a combination of electrostatic analyzers and integral foil detectors. During disturbed times, precipitating proton fluxes in the range 10 million - 100 million/sq cm-sec are seen. On several occasions, it has been possible to compare the spatial distribution of precipitation with the plasmapause profile taken by Ogo 5 during all phases of magnetic storm activity. It is found that the near-earth boundary of maximum proton precipitation lies just inside the plasmapause. If observed pitch-angle distributions are classified according to whether the loss cone is nearly empty or otherwise, it is found that the corresponding 40 keV flux intensities and L values separate into two classes in a way described by weak vs strong pitch-angle diffusion. (Author)

The acceleration and precipitation of Van Allen outer zone energetic electrons

The first part of this paper presents the experimental results of a particle correlation experiment conducted between the magnetosphere and the auroral zone. The precipitation time profiles and increases of outer zone energetic electron fluxes are extremely well correlated. The enhancement of electron fluxes observed at synchronous altitudes is predominantly in the 50- to 150-kev energies. The increases are non-adiabatic, because they are not correlated with δB/δt. The variations observed in higher energy electron fluxes are generally correlated with variations in the local magnetic field and hence can be accounted for by adiabatic theories. Energy spectrums of the equatorial and auroral zone electrons are similar, with typical e folding energies of about 20 kev for both regions. Lifetimes of the non-adiabatic electrons are typically 10³ sec but may be as short as ∼200 sec. The equatorial electron pitch angle distribution is anisotropic and peaked toward 90°. The anisotropy in the pitch angle distribution changes with time and shows dependence on the electron flux. Finally, the energy spectrum in the equatorial plane is pitch angle dependent, with a steeper spectrum for smaller pitch angle particles. The second part of this paper presents theoretical interpretations of the experimental results, which can be consistently interpreted in terms of local acceleration by using the strong pitch angle diffusion model.