Guang-Li Huang

Whistler waves in Freja observations

Several examples of whistler wave packets accompanied by density cavities are detected in F4 experiments of Freja satellite. The density depletion seems to be a fixed structure in space from the cross-correlation of two Langmuir probes. The wavelet analysis shows the frequency drift from 1 kHz ( below the low-hybrid frequency) to 200 Hz ( below the proton cyclotron frequency). The wave packets are always associated with an extremely low frequency component at 20-30 Hz ( around the oxygen ion cyclotron frequency). The method of the minimum variance is used to calculate the oblique propagation of the wave packets with an angle decreased from 31 to 17 degrees between the wave vector and the ambient magnetic field. The right polarization of the wave packets is shown in the direction of the ambient magnetic field. Moreover, the phase velocity of the wave packets is inversely proportional to the frequency. These features may support the formation of envelope solitary whistler waves.

The growth of Alfvén waves in the resistive current-driven instability

On the basis of a two-fluid, cold-plasma, linear stability calculation with linear friction between electrons and ions, the growth rate of Alfven waves is derived from the dispersion relation for a uniformly magnetized plasma, in which the Plasma resistivity and a uniform electric current carried by an electron beam are both considered. The growth rate is directly proportional to the plasma resistivity, the electric current density and the value of the parameter omega pe/Omega e (where omega pe and Omega e are the electron plasma and cyclotron frequency respectively). Moreover, the growth of Alfven waves is mainly excited in a direction nearly parallel to the ambient magnetic field. The critical value of the velocity of the electron fluid is just equal to the Alfven velocity. The results of this paper are compared with those for the linear tearing mode

Beam-plasma instability excited by non-thermal electrons with arbitrary distribution function and comparison with electron-cyclotron master instability

An equation is derived for the growth rates of the beam-plasma instability excited by non-thermal electrons with arbitrary distribution function, and it is shown that the reactive instability does not depend on the assumption of a monoenergetic distribution. Hence the properties of electromagnetic waves are calculated for the hollow beam and loss-cone distribution functions. The general characteristics and structures of the growth rates are similar to the results for the monoenergetic distribution, but there are still some differences in the relation between the growth rates and the relevant parameters, such as the ambient parameter omega(pe)/Omega e, the angle of propagation theta and the pitch angle alpha. The main purpose of this paper is to compare the properties of the beam-plasma instability (reactive) and the electron-cyclotron maser instability (kinetic) under similar ambient conditions. The calculations show that both kinds of instabilities are easily excited at larger angles of propagation with respect to the ambient magnetic field, which means that both depend mainly on the free energy of the non-thermal electrons perpendicular to the magnetic field. The magnitudes of the growth rates of the two kinds of instabilities are comparable under the same ambient conditions. However, because the non-resonant wave-particle interaction is taken into consideration for the beam-plasma instability, which makes the resonant peaks broaden and connect with each other, the spectra of the beam-plasma instability are also more complicated than that of the maser instability, and the range of the angle of propagation of the growing waves in the non-resonant case is also larger than that in the resonant case.

Attenuation of coronal magnetic fields in solar microwave bursts

Based on the observed data by the Nobeyama Radio Observatory and the nonthermal gyrosynchrotron theory, the calculated magnetic field in a loop-like radio source of the 2001 October 23 flare attenuates from hundreds to tens of Gauss,. except in the region with very weak magnetic fields. Meanwhile, the viewing angle between the magnetic field and line of sight has a similar attenuation from tens to around ten degrees, implying that the transverse magnetic component attenuates much faster than the longitudinal one. All of these results can be understood by the magnetic energy release process in solar flares. Moreover, the column density of nonthermal electrons decreases from 109-10 to 107-8 cm(-2) during the flare, except in the region with very weak magnetic fields, where its value is larger than that with strong magnetic fields due to the mirroring effect. The calculated error and harmonic number of gyrofrequency better suit the region with strong magnetic fields.

Approximate expressions for Gyrosynchrotron radiation in Transverse Propagation

Two sets of accurate approximate expressions for the gyrosynchrotron radiation in the transverse propagation case are presented for the first time. They contain emissivity eta(nu)-/BN and absorptivity kappa(nu)-B/N for e-mode, effective temperature T-eff and frequency of peak brightness nu(p). The expressions are designed for the range 2 to 7 of electron energy spectral index delta and for the ranges from 2 to 10 and 10 to 100 of harmonic numbers s(=nu/nu(B)). Their statistical error is, respectively, +/- 18% and +/- 29% for eta(nu)-/BN and kappa(nu)-B/N for 10 less than or equal to nu/nu(B)less than or equal to 100, +/- 128% and and +/- 170% for 2 less than or equal to nu/nu(B)less than or equal to 10.Two sets of accurate approximate expressions for the gyrosynchrotron radiation in the transverse propagation case are presented for the first time. They contain emissivity ην −/BNand absorptivity κν −B/Nfor e-mode, effective temperature Teffand frequency of peak brightness νp. The expressions are designed for the range 2 to 7 of electron energy spectral index δ and for the ranges from 2 to 10 and 10 to 100 of harmonic numbers s(=ν/νB). Their statistical error is, respectively, ±18% and ±29% for ην −/BNand κν −B/Nfor 10≤ν/νB≤100, ±128% and and ±170% for 2≤ν/νB≤10.

The effects of propagation angle of waves and pitch angle of electrons on maser and beam-plasma instabilities

The effects of the propagation angle of electromagnetic waves and the pitch angle of non-thermal electrons on the electron cyclotron maser instability and beam-plasma instability are compared. It is pointed out if the direction of propagation of electromagnetic waves is opposite to the direction of the injected electrons, the maser instability will be suppressed, and hence it cannot explain the radiation with narrow bandwidth excited by energetic electrons propagating downwards in solar flares, such as millisecond spikes, blips and the type III bursts with positive-frequency drift. This discrepancy may be solved by the non-resonant wave-particle interaction in the beam-plasma instability, in which the electromagnetic waves propagating in the same direction as the injected electron beam are mainly composed of the ordinary modes (left-circular polarization), while the electromagnetic waves propagating in the opposite direction to the beam are mainly composed of the extraordinary modes (right-circular polarization). This result is compared with a typical model of a magnetic tube in solar flares, in which the non-thermal electrons propagating upwards excite spikes, blips and type III bursts in the metre and decimetre bands, while the electron beams propagating downwards excite these temporal and spectral structures in the microwave bands.

The intrinsic relation between envelope solitons and plasma instabilities

A general form of the one-dimensional nonlinear Schrodinger equation is derived; the coefficients depend mainly on the linear dispersion relation, as does the nonlinear term. Envelope solitons may only exist together with some plasma instabilities. An example of solar radio bursts is selected as evidence for envelope solitons in the electromagnetic waveband.

A self-modulation model of scalar radio emission

We considered the possibility of self-modulation instabilities in waves propagating in the solar corona. We found that such instabilities (both longitudinal and transverse) can occur in decimeter and meter radio bursts but not in optical or X-ray emissions. The model is consistent with the observed fine structures in salar meter wave bursts.

Solar radio spike bursts and electron cyclotron resonance absorption

Abstract We study the electron cyclotron resonance absorption of solar radio emission by a magnetized non-relativistic hot plasma under the coronal conditions and obtain approximate expressions for the second and higher-order harmonics, in dependence of the plasma temperature, and the angle of emission. We discuss the effect of the resonance absorption on solar spike emission. We infer that most of the energetic electrons responsible for the spike emission come from the inner corona.

Instabilities driven by relativistic electron beams in radio jets

Abstract On the basis of the dispersion equation for the motion of a relativistic electron beam in a plasma we discuss the stability of longitudinal modes. We found that the electrostatic Langmuir wave and Alfven wave are unstable and calculated their rate of growth, while high-frequency electromagnetic mode and whistle mode are stable. The two unstable modes can be used to elucidate the problems of the hot spots in radio jets, particle re-acceleration, emission mechanism and energy transport.