Proton Pitch Angle Distributions Research Articles

Ion cyclotron wave growth calculated from satellite observations of the proton ring current during storm recovery

Using Explorer 45 observations of pitch angle distributions of ring current protons as a function of radial distance and energy during the recovery phase of a large geomagnetic storm, we have calculated wave, growth rates and the total wave energy gain of ion cyclotron waves propagating parallel to the geomagnetic field from high latitude toward the equator. We conclude that the observed proton fluxes and pitch angle anisotropies are sufficient to cause significant growth. In addition, we show that the observed patterns of proton pitch angle distributions as a function of energy, radial distance, and time (Williams and Lyons, 1974a, b) are a natural consequence of the spectrum of waves which can grow and propagate under the existing conditions of storm recovery.

Characteristics of auroral proton precipitation observed from sounding rockets

A summary of auroral proton observations from 12 sounding rockets launched from Churchill Research Range is presented. Although these rockets were launched into different auroral conditions and at various local times, certain characteristics were observed in all flights. Energy spectra were peaked in the 5- to 10-keV energy intervals with typical intensities near 10$sup 5$ cm$sup -2$ s$sup -1$ sr$sup -1$ keV$sup -1$. Proton pitch angle distributions in the loss cone were isotropic at all energies less than 20 keV. At higher energies, distributions were either isotropic or anisotropic peaked near 90degree pitch angle. Temporal and spatial structure was observed in proton precipitation with typical time scales of the order of 100 equatorial gyroperiods and scale lengths of the order of 100 gyrodiameters. No obvious electron proton correlations were apparent in any of the data. Observations of interactions of the primary proton beam with the neutral atmosphere are presented. Beam spreading effects due to charge exchange interactions are demonstrated, and equilibrium fractions for protons in the atmosphere are measured and compared with relevant laboratory observations. The implication of these results concerning auroral proton source regions and precipitation mechanisms is discussed. (AIP)

Explorer 45 observations of 1- to 30-Hz magnetic fields near the plasmapause during magnetic storms

Observations have been made of <30-Hz magnetic fluctuations, some of which appear to be ion cyclotron waves, during initial, main, and recovery phases of magnetic storms. Such storm-related observable low-frequency events occur very infrequently, in only 10 instances in more than 2000 hours of data, including approximately 100 passes through storm time ring currents. All events were observed while the satellite was near the plasmapause and in the proton ring current. The wave amplitudes were typically 1 γ and as large as 5 γ in the frequency range from 1 to 3 Hz, with lower amplitudes in the 3- to 30-Hz range. On the basis of the spectral shape and intensity of the fluctuations the waves were presumed to be primarily ion cyclotron waves. Reasonable agreement was found between calculated ring current proton lifetimes while the protons were under the influence of ion cyclotron waves and observed proton lifetimes during parts of the December 1971 and August 1972 magnetic storms. These observations during times when the ring current and plasmasphere were close and often overlapping do not prove that the waves were generated by the ion cyclotron instability or that the ion cyclotron waves were necessarily responsible for proton ring current loss. The observations do suggest that ion cyclotron waves are unstable in such cases and that proton lifetimes are consistent with loss from ion cyclotron turbulence. Further theoretical (ion cyclotron wave growth limits) and observational (proton energy and pitch angle distribution changes during these or similar events) studies are required before the role of the ion cyclotron instability in ring current dynamics can be completely determined.

Further aspects of the proton ring current interaction with the plasmapause: Main and recovery phases

Recent storm recovery phase results from Explorer 45 (Williams and Lyons, 1974) show the existence of nearly isotropic proton pitch angle distributions with apparently empty loss cones at high equatorial L values and a transition to a rounded pitch angle distribution at lower L values, the transition altitude decreasing with decreasing energy. These recovery phase results were used to describe the hot-cold plasma interaction in the vicinity of the plasmapause region. It was concluded on the basis of low-altitude loss cone observations that a region of strong plasma turbulence exists during storm recovery phase at altitudes above Explorer 45 apogee (5.24 RE). We present in this report direct Explorer 45 observations during main phase that are consistent with the observation of the turbulent (full loss cone) region. We also present recovery phase data consistent with outward expansion of the plasmapause region due to refilling from the ionosphere.

Variations of the trapped particle population and of cutoff and pitch angle distribution of simultaneously observed magnetospheric solar protons during substorm activity

The behavior of solar protons (1.5–2.7 MeV) and trapped electrons (>1.5 MeV) observed by a low-altitude polar-orbiting satellite (Azur) during the occurrence of an isolated substorm (March 29–30, 1970) is examined. Onset of the substorm is determined and its development is discussed by using energetic and high-energy particle data from the ATS 5 satellite. The comparison of equatorial and low-altitude electron data shows that the behavior of the trapped electron population is consistent with the adiabatic model given by Lezniak and Winckler (1970). Observation of an isotropic pitch angle distribution of the solar protons leads to the conclusion that there is evidence for strong pitch angle scattering of these particles in the pseudotrapping region. We also observed during the substorm a large change in the slope of the solar particle flux profile at cutoff that occurred simultaneously with a variation of the trapping boundary. It is suggested that this effect might explain the often observed ‘deep entry’ of solar protons into closed drift shells well below the nominal cutoff.

Geomagnetically trapped alpha particles: 3. Low-altitude outer zone alpha-proton comparisons

The outer zone population of trapped α particles and protons was measured off the equator [B/B0 = 1.3–2.3] in the L interval 2 ≤ L ≤ 4 during 1969. The α particle spectra are best represented by a power law with spectral indices between 2.4 and 4.0 over the energy range 0.85 ≤ Eα ≤ 9.0 MeV. The α particle and proton spectra have the same slope at the same total energy. The distribution functions for a particles and protons have similar shapes, and they suggest that the location of the source of these particles is at L > 4. The proton pitch angle distribution at small pitch angles is flatter than near the equator. The α/p ratio is, at best, very weakly dependent upon pitch angle over the range of small pitch angles covered [18°–37°]. The α/p ratio peaks near L = 2.5 for Eα 2.8 at all E/A. The L and energy dependence of the ratios can be explained in terms of the observed proton and α particle energy spectra. The observed α/p ratios are in agreement with previous measurements. It is suggested that pitch angle scattering may be the primary source of the low-altitude ions. The measurements are compared with recent theoretical predictions.

HF absorption near the polar cap edge during PCA events

Sometimes during PCA events the hf (3 to 30 MHz) absorption observed at a riometer station near the polar cap edge decreases, although it remains constant or even increases at a higher-latitude station. The absorption stays depressed for a few hours and then returns to or near the predecrease level. The effect often occurs in the local forenoon hours, is called the midday recovery, and has been attributed to an increase in geomagnetic cutoff energy alpha f solar protons or to development of an anisotropic pitch angle distribution of solar protons. Two recoveries are discussed in relation to satellite proton observations along meridians near those of the ground station. (WDM)

Observations of proton spectra (1.0 ≤Ep≤ 300 kev) and fluxes at the plasmapause

Detailed proton spectral and pitch angle distribution observations have been obtained from instrumentation flown aboard the NASA Small Scientific Satellite, S3-A (Explorer 45), launched from Kenya, Africa, into an elliptical orbit having an apogee of 5.24 RE, a perigee of 220 km, an inclination of 3.5°, and a period of 7.82 hours. Data used in this initial report are from two proton detectors and a three-axis fluxgate magnetometer. Data from the magnetometer are used to routinely display the particle data as a function of local pitch angle. The lower energy proton instrument consists of an electrostatic analyzer-channeltron configuration measuring proton energies from 0.8 to 30 kev in 16 energy intervals.

Use of position sensitive detectors at energies below 1 MeV: Application to a fast determination of the pitch angle distribution of protons inside the magnetosphere

Experimental data which were obtained in the laboratory are presented, concerning the possible use of a position sensitive detector (P.S.D.) to determine the pitch angle and energy distribution of protons inside the magnetosphere. The pitch angle α is determined by putting a slit in front of the P.S.D. Each value of α is therefore associated with a value x of the abscissa of the particle impact on the P.S.D. A division of the two impulses (one being proportional to the particle energy E, and the other to the product of x by E) gives the value of x, hence of α. It is shown that the measurement of x is improved if one takes into account the correlated noise between the two impulses, E and xE. Experimental evidence is given that with such a system, one can achieve an angular resolution of 15° for E ≈ 0.1 MeV and 2.5° for E ≈ 1 MeV. Practical application to spin stabilized satellites is also considered.

Access of solar protons to the Earth's polar caps

Energetic solar proton observations in the interplanetary medium by Explorer 33 and Explorer 35 and over the polar caps by Injun 5 during the period from September 1968 through March 1970 have been examined in detail. The solar proton intensities observed over the polar regions were compared with the interplanetary intensities on an absolute basis. The high polar latitude (HPL) proton intensities tracked the magnetic field aligned interplanetary intensities well (solar-antisolar intensities as determined from the sectored Explorer data). The tracking was in agreement with the open magnetosphere model about 90% of the time - i.e., the intensities observed on the HPL magnetic lines of force, which would be connected to magnetic lines of force from the sun in an interconnected geomagnetic and interplanetary magnetic field, were in agreement with the maximum intensity of the interplanetary proton pitch angle distribution. The intensities observed at the HPL region of the opposite polar cap were in agreement with the minimum intensity of the interplanetary pitch angle distribution. The low polar latitude intensities generally tracked the maximum interplanetary intensity very well.

Observations of magnetic-field-aligned auroral-electron precipitation

A sounding rocket was launched in the early evening into an auroral display during a 50-γ positive bay. The flight path carried the rocket over the two most northerly arcs in a band of parallel east-west oriented arcs, into a region of low activity to the north of the band, and then into a large fold northeast of the launch site. Several times during the flight, intense field-aligned electron pitch-angle distributions (i.e., peaked near 0°) were observed; these regions were found to be coincident with the northern boundary of the band of auroral precipitation. No equivalent distributions were observed between arcs to the south. Electron intensities of about 1011 (cm² sec ster kev)−1 peaked near β = 0 were observed at 0.55 kev, and electrons near this energy were conservatively estimated to carry a current of 2 × 10−4 amp/m². This electron current sheet was estimated to have a large east-west extent and a north-south thickness of 10 km, and under certain conditions would produce a transverse magnetic field of 10³ γ. Field-aligned 7.8-kev electrons were also observed in the boundary region with intensities (∼3 × 109 (cm² sec ster kev)−1), a factor of 10 larger than that observed at any other time in the flight. Energetic electron (E > 25 kev) pitch-angle distributions were found to be generally antisotropic peaked near 90°; however, at times of maximum intensities, the distributions approached isotropy. Energetic proton (E > 30 kev) intensity profiles obtained during the flight were featureless, and pitch-angle distributions were anisotropic, peaked near 90°. No correlation with electron intensities was apparent; however, the proton pitch-angle distributions gradually shifted toward isotropy on two occasions of intense electron precipitation (near the center of the arc). This minor correlation wag not considered statistically significant. Low-energy ion precipitation was below the limit of detectability, and an upper limit on the primary ion beam was set at 5 × 105 (cm² sec ster kev)−1 and the 1- to 20-kev energy range. Most characteristics of particle precipitation observed in the boundary region are shown to be consistent with a local acceleration from a parallel electric field with a potential drop greater than 7 kv.

Pitch-angle distributions of protons and helium ions in the magnetosphere

Pitch-angle distributions of protons and helium ions in the magnetosphere

Variations in the doppler profile of Hα in aurorae

Variations in the observed Doppler profile of Hα emitted from high latitude aurorae are reported. On one particular night the half-width of the horizon profile ranged from about 15 to 23 A. The variations are interpreted as changes in the pitch angle and velocity distributions of the incident protons. With an assumed pitch angle distribution of the form cos n θ the index n seems to vary from below 0 to about 6. Theoretical considerations show that this may indicate that the protons are accelerated in the vicinity of the Earth.