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1.
Energetic protons haying ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to the geomagnetic field lines, in the ring current region by exciting Mode 1 arid Mode 2 nonresonant instabilities and a resonant instability. Mode 1 nonresonant instability has frequencies around ~4 Hz with transverse wavelengths of ~(8–80) km, and it is likely to occur in the region L = (7–8). Mode 2 nonresonant instability can generate frequencies ~(850–1450) Hz with transverse wavelengths ~(2–20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950–1250) Hz and (30–65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (4–6). The low-frequency modes driven by energetic protons could attain maximum saturation electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is suggested that the turbulence produced by the low-frequency modes may cause pitch angle scattering of ring current protons in the region outside the plasmapause resulting in the ring current decay. 相似文献
2.
The waves, propagating nearly transverse to the ambient magnetic field, with frequencies near the harmonics of the proton-cyclotron frequency are studied in an inhomogeneous plasma with protons having loss-cone distributions. Three types of drift cyclotron instabilities have been studied: (i) non-flute instability; (ii) B-resonant instability; and (iii) non-resonant instability. Increases of loss-cone and density gradient increase the growth rates of all three instabilities. Increases in the positive temperature gradient and t (ratio of thermal pressure of trapped protons to magnetic field pressure) have a stabilizing effect on the non-flute and non-resonant instabilities and a destabilizing effect on the B-resonant instability. The non-resonant instability has an interesting feature: a particular harmonic can be excited in two separate bands of unstable wave numbers. These instabilities can play an important role in the dynamics of the ring current and the inner edge of the plasma sheet region of the magnetosphere. The discrete turbulence generated by them would give rise to precipitation of protons on the auroral field lines, which may contribute to the excitation of diffuse aurora. These instabilities may be relevant to the observation of harmonic waves at 6R
E by Perrautet al. (1978). 相似文献
3.
A generation mechanism for 1–30 Hz waves of the second category, observed near the plasmapause by Taylor and Lyons (1976), is suggested in terms of a resonant electron instability. The instability arises because of the resonant interaction between the ring current electrons outside the plasmapause and the ordinary mode drift waves. The instability can generate waves in the frequency range from 0.45 to 35.0 Hz in the region between L = 4.5 and 5.5. The instability can also explain satisfactorily the other properties such as no changes in the proton distributions, the direction of the wave magnetic field and the localization of the region of wave activity, associated with these waves. 相似文献
4.
Shear flow instability is studied in the Earth’s magnetotail by treating plasma as compressible. A dispersion relation is derived from the linearized MHD equations using the oscillating boundary conditions at the inner central plasma sheet/outer central plasma sheet (OCPS) interface and OCPS/plasma-sheet boundary layer (PSBL) interface, whereas the surface-mode boundary condition is used at the PSBL/lobe interface. The growth rates and the real frequencies are obtained numerically for near-Earth (\midX\mid\sim10-15 RE) and far-Earth (\midX\mid\sim100 RE) magnetotail parameters. The periods and wavelengths of excited modes depend sensitively on the value of plasma-sheet half thickness, L, which is taken as L=5 RE for quiet time and L=1 RE for disturbed time. The plasma-sheet region is found to be stable for constant plasma flows unless MA3>1.25, where MA3 is the Alfvén Mach number in PSBL. For near-Earth magnetotail, the excited oscillations have periods of 2–20 min (quiet time) and 0.5-4 min (disturbed time) with typical transverse wavelengths of 2–30 RE and 0.5-6.5 RE, respectively; whereas for distant magnetotail, the analysis predicts the oscillation periods of \sim8-80 min for quiet periods and 2–16 min for disturbed periods. 相似文献
5.
The region of bouncing electron beams in the earth’s magnetosphere can be unstable against a non-resonant electromagnetic lower hybrid instability. The instability is purely growing in the rest frame of the plasma, and can be excited either by the temperature anisotropy or the drift velocity of the bouncing electron beams. The growth rates of the instability decrease with the increase of cold electron density. Consequently the growth rate is maximum at the equator where the cold electron density is minimum. The intense turbulence generated by this instability could broaden the bouncing electron beams thereby explaining the observed wider cone width of the beams at the equator. The instability could generate magnetic pulsations in the frequency range of orderPc 1?Pc3 with typical wavelength ≈ (3–10) km in the, magnetosphere during magnetic storms or substorms. 相似文献
6.
A simple method is proposed to investigate the stability of a charge neutral magnetopause current sheet with respect to the tearing-mode instability. This method may serve as a useful tool in understanding the processes of local opening of the closed magnetosphere. 相似文献
7.
E. Costa E. Echer M.V. Alves B.T. Tsurutani F.J.R. Simões F.R. Cardoso G.S. Lakhina 《Journal of Atmospheric and Solar》2011,73(11-12):1405-1409
We present a new method of calculating cross-field diffusion of charged particles due to their interactions with interplanetary magnetic decreases (MDs) in high heliospheric latitudes. We use a geometric model that evaluates perpendicular diffusion to the ambient magnetic field as a function of particle's gyroradius, MD radius, ratio between fields outside and inside the MD, and a random impact parameter. We use Ulysses magnetic field data of 1994 to identify the MDs and get the empirical size and magnetic field decrease distribution functions. We let protons with energies ranging from 100 keV to 2 MeV interact with MDs. The MD characteristics are taken from the observational distribution functions using the Monte Carlo method. Calculations show that the increase in diffusion tends to saturate when particles' gyroradius becomes as large as MD radii, and that particles' gyroradius increases faster than diffusion distance as the energy of the particles is increased. 相似文献
8.
In the framework of non-linear fluid theory we use a lower hybrid (LH) wave of the form
as a pump which interacts with the small fluctuations with the low-frequency vibrations
i
or =0, where
i
, is the hydrogen ion-cyclotron (HIC) gyrofrequency. The ponderomotive force generated by the beating of the high-frequency pump wave 0 and the sideband LH waves (±0) produces a non-linear coupling between the high- and low-frequency motions of electrons and ions. Under certain conditions the HIC waves and the zero-frequency waves both become parametrically unstable and start to grow. These excited waves then heat the ions by stochastic acceleration in the transverse direction, thus explaining the formation of ion comics along the auroral field lines. Electrons would be heated in the parallel direction directly by the pump field as well as by low-frequency waves. Thus a single mechanism can explain the existence of ion-cyclotron waves, zero-frequency waves, ion conics, and energetic electrons along the auroral field lines. 相似文献
9.
G. S. Lakhina 《Astrophysics and Space Science》1990,165(1):153-161
The velocity shear of ion beams observed in the polar cusp region can drive the kinetic Alfvén modes unstable. A hot ion beam can excite both a resonant kinetic Alfvén wave instability and a nonresonant coupled Alfvén ion-acoustic wave instability. For the case of a cold ion beam only the latter instability is excited. For the altitude range of 5–7R
e
, velocity shearS0.04–1.0 is needed to excite the kinetic Alfvén wave instabilities. HereS=(dV
B
/
cB
dx), whereV
b
is the streaming velocity,and
cB
is the gyrofrequency of the bean ions. The excited modes have frequencies, in the satellite frame of reference, in the ULF frequency range. The noise generated by the velocity shear-driven Alfvén modes is electromagnetic in nature. These modes have a substantial component of parallel electric fields and, therefore, they can play an important role in the ionosphere-magnetosphere coupling process occurring in the polar cusp region. 相似文献
10.
Nonlinear propagation of strong low-frequency waves, as emitted by pulsars or compact galactic nuclei at their rotation frequencies, in a magnetized plasma is investigated. It is shown that even rather small amplitude waves can drive electrons to ultrarelativistic energies. In the limit when the electrons are ultrarelativistic but the ions are immobile, two types of circularly polarized waves (i.e., ± modes) are excited. In the wave zone of the Crab pulsar, both the electric field ( 3 V m–1) and the wavelength (108 m) of the - mode are larger, by an order of magnitude, than those of the + wave mode. Both ± modes can become modulationally unstable due to their nonlinear interaction with density fluctuations induced by the electrostatic waves. 相似文献