Ion Pitch Angle Distributions Research Articles

Plasmasphere and its interaction with the ring current

In the last few years (after 1976) new instruments have been used for plasmaspheric studies (‘mutual-impedance’ probe, ion energy-mass spectrometers) which have led to new results. The existence in the plasmasphere of previously unknown ions (He2+, O2+) was revealed and the pitch-angle distributions of ions with energies 10–20 eY were measured. New measurements confirmed the existence of a ‘hot’ (‘warm’) zone in the outer plasmasphere and of a midday-midnight asymmetry of the plasmasphere. The source of outer plasmasphere heating is apparently the ring-current decay; electrons are heated less than ions. The plasmasphere is not only the medium of ring current decay, but it appears that it is one of thering-current ion sources.

Pulsations in magnetic field and ion flux observed at L = 4.5 on August 5, 1972

After the unusually strong compression of the earth's magnetosphere associated with the August 1972 geomagnetic storm, large amplitude ULF pulsations were observed for several hours in space by Explorer 45 magnetic field and particle instruments near L = 4.5 and at magnetic observatories on the ground over a large range of latitudes. Spectral analysis of Explorer 45 magnetometer data suggests that a compressional mode oscillation coupled to a transverse mode oscillation. Enhancements in amplitude of a 300‐s period wave near 0040 UT August 5 coincide with an intensification of 100‐ to 1000‐Hz magnetic and electric field oscillations and with the appearance of enhancements of fluxes of energetic ions. During this period the ion pitch angle distribution in each available energy channel (24–300 keV) followed a periodic sequence, apparently synchronized with the magnetic pulsations, from normal trapping (highest fluxes near 90° and lowest near 0° and 180°) to a nearly isotropic particle distribution. During the transitions the particle flux near 90° pitch angle was alternately larger earthward of the satellite (before isotropy) and larger radially outward from the satellite (after isotropy). Intense field‐aligned fluxes of lower energy ions (E ≤ 5 keV) were observed periodically throughout the interval. Possible configurations of the magnetosphere consistent with the wave and particle observations are discussed, the most likely candidate being the presence of a wavelike boundary near the satellite. It is possible that the satellite sensed the low‐latitude boundary layer at L = 4.5 during this period of extreme magnetospheric compression.

Observations of field‐aligned energetic electron and ion distributions near the magnetopause at geosynchronous orbit

On August 28, 1978, the dayside magnetopause crossed the geosynchronous satellite GEOS 2 several times during a geomagnetically disturbed period, and clear signatures of the interconnection of field lines through the magnetopause were observed. The MPAE particle spectrometer provided high time resolution observations of the distribution of energetic electrons (E > 22 keV) and ions (E > 27 keV). Magnetometer data were used to determine the location of GEOS 2 relative to the magnetopause. The pitch angle distributions of ions and electrons were found to be strongly asymmetric with respect to 90°, and the asymmetries have been interpreted in terms of field‐aligned particle streaming. Evidence is provided for the first time for electron streaming inside the magnetopause which continues for many bounce periods. It is concluded that magnetospheric field lines opened, at least for brief time intervals, as a consequence of interconnection with magnetosheath field lines. Comparisons of electron spectra provide evidence that the streaming electrons observed in the magnetosheath originate in the magnetosphere.

Effects of Titan on trapped particles in Saturn's magnetosphere

The close fly‐by of the Voyager 1 spacecraft to the Saturnian satellite Titan provided an opportunity for the low energy charged particle (LECP) experiment to investigate the influences of Titan on the magnetosphere energetic particle distributions. Magnetic field data from the magnetometer (MAG) experiment are used with angular distribution data from LECP to study the changes in the pitch angle distributions of ions (40 < Ei < 220 keV), and electrons (26 < Ee < 61 ReV). Before and after the Titan encounter the ions are observed to exhibit a bulk motion in the direction expected of corotation of the particles with the planetary magnetic field. The magnetosphere corotation velocity outside the influence of Titan is deduced to be ∼100 to 150 km/s, less than the ∼200 km/s expected for rigid corotation. Titan appears to disrupt the corotation motion of the ions: the fluxes of ions in the corotation direction are appreciably diminished in intensity, while those observed in the equatorial plane perpendicular to this direction are relatively unaffected. The resulting distribution and trajectory modeling conclusions suggest that the convection electric field is effectively absent in the wake of Titan. Before and after the Titan encounter the electrons are observed to exhibit pitch angle distributions peaked at ∼90°, representative of trapped particles. The presence of Titan produces a reduction in the fluxes of the electrons and flatter distributions with pitch angle.

Plasma pitch angle distributions near the substorm injection front

Earthward propagation of substorm‐associated, abrupt plasma changes has been observed near synchronous orbit (Moore et al., 1981). We report data obtained from the University of New Hampshire hot plasma instrument on the ATS‐6 spacecraft during January 1980 that provide electron and ion pitch angle distributions in the vicinity of such an injection front. Evidence is found of symmetric atmospheric source cones for few 100‐eV electrons after front passage. This supports the concept of atmospheric electron degradation of the hot high altitude plasma, and the proposal that the injection front is a moving precipitation‐flow boundary between the hot plasma and the cooler plasma, which has become spectrally degraded via interaction with the atmosphere. In contrast, ion source cone distributions are more sporadic and not clearly associated with the injection front passage. The enhanced hot plasma electron intensities that appear in association with front passage exhibit a modest field‐aligned anisotropy with a minimum at 90° pitch angle and secondary minima at small and large pitch angles characteristic of symmetric loss cones. This is consistent with mirror compression of these electrons on inward‐collapsing field lines. Ions of comparable energies do not exhibit the field alignment, probably because their bounce periods are comparable to or longer than the time scale of collapse so that their second invariants are not conserved.

Low‐energy ion pitch angle distributions in the outer magnetosphere: Ion zipper distributions

Ion pitch angle distributions, measured in a near‐synchronous orbit, are predominantly field aligned at low energies and predominantly peaked perpendicular to the magnetic field at higher energies. The transition from field‐aligned fluxes to fluxes peaked predominantly perpendicular to the magnetic field occurs over a very narrow energy range. These ion distributions have been observed at all local times between 5.3 and 7.8 RE. This transition energy correlates with the deep minimum observed in the ion spectra. There is no apparent correlation between the ion transition energy and magnetic local time, L, Kp, or Dst. However, the transition energy does respond to observed particle injections. The transition energy decreases prior to injection, increases abruptly at injection by as much as 10–20 keV, and then decreases slowly after injection. Fresh low‐energy ions are supplied at injection and decrease in intensity over several hours to instrument threshold level. Ion drift trajectory calculations indicate that the low‐energy component below the transition energy drifts in from the nightside plasma sheet via local morning to the dayside. The high‐energy component, above the transition energy, arrives on the dayside via local evening.

Low-Energy Charged Particles in Saturn's Magnetosphere: Results from Voyager 1

The low-energy charged particle instrument on Voyager 1 measured low-energy electrons and ions (energies >/= 26 and >/= 40 kiloelectron volts, respectively) in Saturn's magnetosphere. The first-order ion anisotropies on the dayside are generally in the corotation direction with the amplitude decreasing with decreasing distance to the planet. The ion pitch-angle distributions generally peak at 90 degrees , whereas the electron distributions tend to have field-aligned bidirectional maxima outside the L shell of Rhea. A large decrease in particle fluxes is seen near the L shell of Titan, while selective particle absorption (least affecting the lowest energy ions) is observed at the L shells of Rhea, Dione, and Tethys. The phase space density of ions with values of the first invariant in the range approximately 300 to 1000 million electron volts per gauss is consistent with a source in the outer magnetosphere. The ion population at higher energies (>/= 200 kiloelectron volts per nucleon) consists primarily of protons, molecular hydrogen, and helium. Spectra of all ion species exhibit an energy cutoff at energies >/= 2 million electron volts. The proton-to-helium ratio at equal energy per nucleon is larger (up to approximately 5 x 10(3)) than seen in other magnetospheres and is consistent with a local (nonsolar wind) proton source. In contrast to the magnetospheres of Jupiter and Earth, there are no lobe regions essentially devoid of particles in Saturn's nighttime magnetosphere. Electron pitch-angle distributions are generally bidirectional andfield-aligned, indicating closed field lines at high latitudes. Ions in this region are generally moving toward Saturn, while in the magnetosheath they exhibit strong antisunward streaming which is inconsistent with purely convective flows. Fluxes of magnetospheric ions downstream from the bow shock are present over distances >/= 200 Saturn radii from the planet. Novel features identified in the Saturnian magnetosphere include a mantle of low-energy particles extending inward from the dayside magnetopause to approximately 17 Saturn radii, at least two intensity dropouts occurring approximately 11 hours apart in the nighttime magnetosphere, and a pervasive population of energetic molecular hydrogen.

Low energy ion pitch angle distributions observed on the dayside at geosynchronous altitudes

Pitch angle distributions of low energy (20–400 eV) ions observed at geosynchronous altitudes are found to have field‐aligned components almost 90% of the time on the average from dawn to midafternoon, thereafter decreasing to just less than 50%. Pancake pitch angle component occurrence frequencies vary oppositely, increasing to a peak of 40% in late afternoon. Although the occurrence frequencies for field‐aligned components display no systematic variation with energy, pancake components are observed more often in the lower energy particles. Similarly, magnetic activity is found to have little effect on the occurrence frequencies of field‐aligned components, whereas pancake components occur more frequently under magnetically quiet conditions.

Scale of electric field along magnetic field in an inverted V event

The characteristics of magnetic‐field‐aligned electrostatic‐potential differences in the structure of an inverted V event were examined by using data on the pitch angle and energy distributions of electrons and ions obtained with the polar‐orbiting S3‐3 satellite. Potential differences were found to occur simultaneously above and below the altitude of the satellite (≃7260 km). Both of these potential differences were determined from an analysis of the electron distributions. The analysis also yielded the magnetic field intensity B1 above the satellite where the potential was zero. This value of B1 indicates that the potential extends above the satellite a distance of ≃1200 km, implying an apparent electric field of ⪍1 mv/m. The total potential difference was found to have an inverted V distribution in latitude, with a peak value of ≃4.3 kV and a latitudinal width of ≃1°. Upward‐flowing H+ and 0+ ions, with pitch angles ⪍10° were observed in the structure. Their energy spectra were peaked at energies near the potential differences below the satellite but were very broad indicating perpendicular heating and an ion source within the potential region. Implications to theory are discussed.

Geomagnetically trapped radiation

This review covers the major developments in radiation-belt phenomenology of the past four years (1970–1973). This has been a period characterized by consolidation and refinement of ideas and measurements related to geomagnetically trapped particles. Significant progress has been made in understanding ion and electron pitch-angle distributions within the context of radial diffusion and pitch-angle diffusion, respectively. Comparison of alpha-particle and proton distributions has helped to clarify the relative strengths of known radial-diffusion mechanisms. Careful measurements have indicated the directional flux of cosmic-ray-albedo neutrons, which constitute (through beta decay) a major source of high-energy (≳ 20 MeV) inner-belt protons. Inclusion of radial-diffusion and geomagnetic-secular effects has brought the theory of the inner proton belt into reasonable agreement with observation. At very lowL values (L ≲ 1.2) atmospheric collisions have been found to facilitate the radial transport of 40 keV protons and 2 MeV electrons. The plasmapause has been identified as an important boundary for plasma instabilities (wave-particle interactions) that lead to particle precipitation and red-arc excitation. Suggestions have followed for artificially simulating such plasmaspheric effects by magnetospheric injection of cold barium or lithium plasma.

Electric fields parallel to the magnetic field in a laboratory plasma in a magnetic mirror field

With electrostatic probes, the electric field component E∥ along the magnetic field B was comprehensively investigated in a collisionless plasma, the density of which was of the order of 1010 cm-3. The plasma in the experiment has several properties in common with the plasma of the ionosphere/magnetosphere scaled to laboratory dimensions. It is produced by means of electron cyclotron resonance in a microwave cavity located in the magnetic field gradient in one half of a magnetic mirror field. The magnetic field strength is 3600G in the resonance zone and 1800G in the middle of the mirror field. The measurements show that a stationary E∥ exists everywhere in the plasma, where the magnetic field gradient grad11 B in the direction of the field is different from zero. The direction of E‖ is opposite to that of grad‖B. The total potential drop along B between the resonance zone and the midplane of the mirror field is of the order of kilovolts. E‖ accelerates ions along B to energies of the order of kilo electron volts. Experimental parameters of importance for the production of E‖ are the neutral gas pressure p (normally a few times 10− Torr), the microwave power (usually about 2kW), and the mirror ratio γ in the mirror region opposite to the cavity side, γ was normally <2. For γ>2·3, an instability develops and no stationary E‖ remains. As p is increased, E‖ decreases successively. In terms of the mean free path λ, it is found that λ>5−10L is a necessary condition for the existence of E‖. L is twice the distance between the cavity and the midplane of the mirror field. In the experiment, the ion and electron pitch angle distributions are forced to be different; the ion velocity is mainly parallel to B, and the electron velocity essentially perpendicular to JB, and as consequence E‖ is created. In this way an experimental demonstration is presented of the theoretically predicted relation between E‖ and the pitch angle distributions. When imposing sufficiently strong radial electric fields Er (fields perpendicular to B), the distribution of the potential along B is deformed, probably due to changes in the particle distributions caused by E‖. We think that our results strongly support the idea that Et is produced in the magnetosphere, and is at least sometimes an important mechanism for the acceleration and precipitation of auroral particles.