The detailed structure of ion velocity space distributions in the plasma sheet boundary layer (PSBL) in the distant geomagnetic tail has been investigated. Three separate, tailward‐streaming ion ...populations have been observed simultaneously in PSBL crossings in which the inner edge of the PSBL was identified as a slow mode shock: cold, low‐energy, ions presumably of ionospheric origin that fill much of the tail lobes, and two more energetic populations. The more energetic of these latter populations, which was concentrated in the outer (lobe) layers of the PSBL, had a “kidney bean” shape. The less energetic population had a well‐defined low‐energy cutoff that decreased with increasing penetration into the PSBL from the lobe. The sources of these two populations may be cold lobe ions accelerated in the current sheet near the distant neutral line and plasma sheet ions that leak across the shock, respectively.
Ion acceleration Terasawa, T
Advances in space research,
1995, 1995-1-00, 19950101, Letnik:
15, Številka:
8
Journal Article
Recenzirano
The terrestrial bow shock is an important “test site” for theories of astrophysical particle acceleration. Based on the “calibration” performed here, we can safely apply the acceleration theory to ...shocks in other remote objects, such as shocks propagating in solar flare sites, the terminating shock of the solar wind, supernova shocks, and shocks around active galactic nuclei, etc. An overview of the stochastic ion acceleration process at the terrestrial bow shock is presented. A summary of the remaining problems on the injection, acceleration, and spatial distribution is also given.
The nonlinear evolution of a large‐amplitude incoherent Alfvén wave is studied via one‐dimensional magnetohydrodynamic simulation. The initial wave magnetic field is given as a superposition of ...circularly polarized Alfvén waves with the same helicity and propagation direction but with different wave numbers. What we observe is contrary to the previous belief that incoherent Alfvén waves are stable against decay: In a low β plasma (β = 0.2) we can clearly see the growth of backscattered Alfvén waves, which are opposite in helicity and propagation direction from the original Alfvén waves. In a high β plasma (β = 2.0), on the other hand, no backscattered Alfvén waves are observed. These results are consistent with the expectation from the theory of the parametric decay instability developed for a coherent Alfvén wave. We also show that incoherent Alfvén waves in the solar wind can decay parametrically in the region of 4–20 RS where β is sufficiently low. For the decay process to work, the period of Alfvén waves should be less than several minutes.
Two types of the plasma sheet boundary layer (PSBL) crossing during a geomagnetically active period are analyzed in detail to investigate relationship between the ion beams, low-frequency (0.01–2
Hz) ...electromagnetic waves, and cold ion energization in PSBL using the Geotail data. In response to changes in the solar wind conditions and auroral activity inferred from the AL index, the properties of PSBL ions changed dramatically: During the less disturbed period, we observed counter-streaming thermalized beams with no significant wave activity in the low-frequency range. The thermalized beams may be generated from the distant magnetotail and have undergone thermalization before reaching the Geotail position of
X
∼
−30
R
E. During the periods of enhanced auroral activity with southward IMF, collimated freshly injected earthward beams are observed together with the large-amplitude electromagnetic waves having a peak around 0.025
Hz (∼0.1
Ω
ci). Cold ions are energized during the periods of large-amplitude low-frequency fluctuations. It is suggested that fresh beams from the near-Earth neutral line (NENL) play an important role in the cold ion energization.
We present a general theory for the pitch angle scattering and velocity diffusion of particles in the field of a spectrum of waves in a magnetized plasma. We use the test particle theory to analyze ...the particle motion. We examine the form of diffusion surfaces and give analytical expressions for the resonance width and bounce frequency. The resonance widths are found to vary strongly as a function of harmonic number. The resulting diffusion can be quite asymmetric with respect to pitch angle of 90°. The conditions for the onset of pitch angle scattering and energy diffusion are explained in detail. We also address some of the known shortcomings of the standard quasi‐linear theory and show ways to overcome them. In particular, we find the often stated quasi‐linear gap at 90° to exist only under very special cases. For instance, oblique wave propagation can easily remove the gap. The conditions for the existence of the gap is described in great detail. A new diffusion equation which takes into account the finite resonance widths is also discussed. The differences between this new theory and the standard resonance broadening theory is explained. The emphasis is on the simplicity of the theory and its power in making quantitative predictions.
In the lobe/mantle regions of Earth's magnetotail (<210 RE), the Geotail spacecraft sometimes observed multicomposition ion flows consisting of both ionospheric (H+/He+/O+) and solar wind (H+/He++) ...ions. Statistics on the He+ beams show that they were observed under similar conditions to those of the O+ beams reported previously. Namely, the heavy ion beams of ionospheric origin tend to exist during geomagnetically active periods in the mantle‐like regions of high plasma beta, where ions of solar wind origin are the major component. The total duration of identified He+ events amounts to 3% of the total observation time in the lobe/mantle region, while that of O+ events amounts to 13%. A remarkable point is that on a short timescale, the He+ and O+ beams often appear to exist nearly exclusively. That is to say, their densities sometimes vary in an opposite sense, even when He+ and O+ coexist. If the O+ and He+ ions are given the same energy in a source region, the initial distribution function of He+ has twice the peak velocity of that of O+, and their alternating appearance and density anticorrelation in the lobe/mantle are easily explained due to the velocity filter effect. Thus the anticorrelation of their densities may suggest that ionospheric ions have undergone an energization which leads to a different velocity for different ion species. A mechanism leading to the same velocity, on the other hand, would require an alternating enhancement of the He+ and O+ fluxes in a source region to explain the anticorrelation of the densities.
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