Electron Cyclotron Maser Emission Research Articles

Cyclotron Maser Radiation in Space and Laboratory Plasmas

AbstractOne of the best known coherent radio emission mechanisms is the electron cyclotron maser instability. In this article we will demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration and propagation in space and laboratory plasmas. These include electron ring distributions, horseshoe or crescent shaped electron distribution functions. Planetary and stellarmagnetospheres are examples of where horseshoe or crescent shaped electron distributions can be found and astrophysical shocks produce ring shaped electron distribution functions. In the laboratory horseshoe or crescent shaped distributions are produced whenever an electron beam propagates into a stronger magnetic field region. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron cyclotron maser emission from solar coronal funnels?

The sun is covered by a network of supergranular cells. The convective motion of these cells leads to the formation of strong magnetic fields at the cell boundaries. At larger heights in the solar transition region and low corona, this magnetic field geometry expands rapidly within a short distance, and forms the magnetic structure of funnels. This field line geometry represents a magnetic mirror, and since the plasma density strongly increases with depth in the transition region, the electron velocity distribution function (VDF) can develop a loss cone. Within such a coronal funnel, the plasma frequency can have smaller values than the electron cyclotron frequency, ωp < Ωe. These are the necessary conditions for the generation of X-mode waves through the electron cyclotron maser mechanism. Since there is some observational evidence for radio emission from the supergranular network, it is of interest to investigate the possibility of this plasma wave generation in a quiet stellar atmosphere in detail. In this paper, a kinetic model is used to calculate the electron VDF in a coronal funnel. A method is derived to determine wave growth rates from the electron VDF. Its application on the coronal funnel VDF indeed results in X-mode wave growth. However, it is also found that wave absorption by higher-order resonances at larger heights in the atmosphere plays an important role.

Electron-cyclotron maser emission from white dwarf pairs and white dwarf planetary systems

By analogy to Jovian radio emissions powered by the electromagnetic interaction between Jupiter and its moons, we propose that close magnetic-nonmagnetic white-dwarf pairs and white-dwarf planetary systems are strong radio sources. A simple model is developed to predict the flux densities of radio emission generated by a loss-cone-driven electron-cyclotron maser. The radio emission from these systems has high brightness temperatures, is highly polarized, and varies on a periodic cycle following the orbital rotation. Masers from magnetic-nonmagnetic white-dwarf pairs, with orbital periods <10 min, are expected to be detectable over a wide range of radio frequencies. Terrestrial planets in close orbits about magnetic white dwarfs, with orbital periods $\la 30$ hr, can also produce detectable radio emission, thus providing a means to identify Earth-sized extrasolar planets.

Effect of Random Inhomogeneities on Electron Cyclotron Maser Emission

The spectral properties of solar radio spikes are believed to be tightly related to the spike source inhomogeneity: in particular, the spikes display a rather large scatter of spectral bandwidth of the emission lines both within one event and from one event to another. This paper studies the effect of random magnetic and density inhomogeneities on the spectral properties of the electron cyclotron maser emission produced by superthermal electrons, which is likely responsible for solar radio spikes. Magnetic irregularities with a broad distribution over spatial scales are shown to provide considerable spectral broadening that is not necessarily small compared to the natural bandwidth, even for small magnetic inhomogeneities. The developed renormalization scheme allows us to describe quantitatively the spectral shape of the emission line when the broadening is large. Moreover, the presence of stronger magnetic inhomogeneities gives rise to the splitting of the emission line into two peaks, in agreement with the observed noninteger harmonic ratio between spectrally separated spike bands. Typical values of the spectral bandwidth are found to be fairly consistent with spike observations. The developed theory suggests new possibilities for random magnetic field diagnostics. The estimated magnitudes of the random magnetic field at the spike source look rather reasonable.

Cyclotron Maser Radiation from Astrophysical Shocks

One of the most popular coherent radio emission mechanisms is electron cyclotron maser instability. In this article we demonstrate that electron cyclotron maser emission is directly associated with particular types of charged particle acceleration such as turbulence and shocks commonly inferred in astrophysical plasmas.

Optically Thick Gyrosynchrotron Emission from Anisotropic Electron Distributions

We present a numerical study of optically thick gyrosynchrotron emission produced by fast electrons with anisotropic pitch-angle distributions. The anisotropy is found to affect substantially both radiation spectra and polarization. The harmonic nature of gyrosynchrotron emission becomes more pronounced for the anisotropic case. The effect is entirely provided by a reduction of the self-absorption coefficient due to pitch-angle anisotropy; thus, it is essentially different from electron cyclotron maser emission, which is due to wave amplification under negative absorption. The paper studies the dependences of the emission on the involved parameters and discusses possible applications of the results to observations of microwave solar radio bursts.

Long-duration Coherent Radio Emission from the dMe Star Proxima Centauri

AbstractThe Australia Telescope and Anglo-Australian Telescope were used in May 2000 to record the radio and optical emissions from the dMe flare star Proxima Centauri. Eight bright optical flares over a two-day interval resulted in no detectable excess short-term radio emission at 1.38 and 2.50 GHz. However, a slowly declining 1.38 GHz emission over the two-day interval was nearly 100% right circular polarised and was restricted to a relatively narrow bandwidth with total intensity (I) and circular polarisation (V) varying significantly over the 104 MHz receiver bandwidth. These are the first observations to show that highly-polarised narrowband flare star emission can persist for several days. This signature is attributed to sources of coherent radio emission in the star's corona. Similarities with various solar radio emissions are discussed; however, it is not possible with the existing observations to distinguish between fundamental plasma emission and electron–cyclotron maser emission as the responsible mechanism.

High resolution studies of intermediate drift bursts

SUMMARY We have analyzed IMD bursts recorded in the decimetric frequency band (- 1000 - 2500 MHz) for the first time with high time resolution of 20 - 50 ms and frequency less than 10 MHz. The main characteristics determined for the individual IMD bursts are: (i) the total duration is N 200 - 2000 ms; (ii) the frequency bandwidth, Av 1~ 50 - 150 MHz; (iii) the instantaneous bandwidth, Au/v N 2 - 12 %; (iw) all individual IMD bursts show negative frequency drift rate as ti 21 -30 - -270 MHz s-l; and (u) the ratio b/u ” -0.20 - -0.02 s-l. We discussed the application of Alfven wave models based on plasma emission and electron cyclotron maser emission mechanisms to IMD bursts observations. We find that the model with plasma emission can satisfy the requirements on the density and the magnetic field of the emission region for generation of the decimetric IMD bursts. The magnetic field strength in the source was estimated as N 30 ~ 150 G. Finally, we concluded that the maser model is not operative in the frequency range of our observations and also the whistler wave model predicts a dependence between ti and Y that does not agree with the present observations. ACKNOWLEDGEMENTS The BSS project is supported by INPE, CNPq and FAPESP. F.C.R. Fernandes and V. Krishan ac- knowledge receiving scholarship from MCT-CNPq (PCI-382499/2002-06) and FAPESP (01/06031-8), respec- tively. The participation of H.S. Sawant in 34th COSPAR Scientific Assembly was supported by FAPESP (01/00056-9). Thanks are due to the referees for suggestions and comments. REFERENCES Aurass, H., G. P. Chernov, M. Karlicky, et al., On a sequence of remarkable fine structures in the type IV burst of 24 April, 1985, Solar Phys., 112, 547-357, 1987. Benz, A. O., and G. Mann, Intermediate drift bursts and the coronal magnetic field, As&on.

Chaotic temporal variations in cosmic masers

Electron cyclotron maser emission is an accepted physical mechanism for generating coherent planetary and stellar radio emissions. Observational data has indicated evidence of nonlinear and chaotic temporal variability in some cosmic masers such as solar microwave spikes. The nonlinear and chaotic characteristics of cosmic masers can be attributed to plasma turbulence, such as Alfvén chaos, embedded in the emission region. We report a chaos theory of Alfvén waves which can account for chaotic acceleration of electrons in the source region of cosmic masers. Two types of Alfvén intermittency are identified: Pomeau-Manneville intermittency and crisis-induced intermittency. Since Alfvén waves may be responsible for accelerating electrons that emit maser radiations, the chaotic dynamics of Alfvén waves may be the origin of chaotic time variations of cosmic masers. Hence, we suggest that Alfvén intermittency may cause intermittent temporal fluctuations which can be observed in cosmic masers.

Microwave plasma emission of a flare on AD Leo

An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as T_b ~ 5x10^10 K (with values T_b > ~3x10^13 K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of n ~ 2x10^11 cm^-3 and B ~ 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r ~ 7x10^8 cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T >= 2x10^7 K and density n_ext <= 10^-2 n. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.

Broadening of electron cyclotron maser emission lines in a nonuniform magnetic field

The formation of cyclotron maser emission lines in a non-uniform (regular or random) magnetic field is studied. In the presence of sufficiently small inhomogeneity, the line shape can be described by a broadened Gaussian profile. In the case of stronger inhomogeneity, the initial Gaussian profile splits into two Gaussian components, which could be observationally perceived as “harmonics.” A relation between the distribution of local magnetic trap sizes and the distribution of the spectral widths of solar radio spikes is derived. Possible applications of the results to the interpretation of solar radio spikes and related problems are discussed.

A z mode electron‐cyclotron maser model for bottomside ionospheric harmonic radio emissions

A model is proposed for recent ground‐based observations of auroral roar emissions, detected at 2Ωe and 3Ωe, where Ωe is the local electron cyclotron frequency in the source region, between 200 and 500 km above the Earth's surface. Electron cyclotron maser emission is a likely mechanism to account for these emissions because it naturally produces coherent radiation at harmonics of Ωe. A theory for auroral roar emissions has already been proposed, whereby maser‐generated second (X2) and third (X3) harmonic x mode radiation is amplified in the source region by multiple reflections off the walls of the density cavity in which they are produced. After many reflections the X2 and X3 waves propagate along the density cavity to a ground‐based observer. However, it is demonstrated here with ray‐tracing calculations that it is highly probable that maser‐generated X2 and X3 radiation is reabsorbed at lower altitudes and thus cannot be detected at the ground. An indirect maser mechanism is proposed instead, where maser‐generated z mode waves at Ωe grow to high levels in the source region and then undergo repeated nonlinear wave‐wave coalescence to produce second‐ and third‐harmonic waves that propagate directly to the ground. The z mode waves must satisfy the necessary kinematic constraints to produce observable second‐ and third‐harmonic radiation. The dependence of the z mode maser on the temperature and functional form of the unstable electron distribution is discussed, along with the conditions required for the coalescence processes to proceed and produce the observed levels of radiation.

On the generation of auroral radio emissions at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations

Electron cyclotron maser emission has been suggested as a possible explanation of ground‐based observations of auroral radio waves at twice and three times the lower ionospheric electron cyclotron frequency (2ƒce and 3ƒce). Preliminary theoretical work concluded that direct maser excitation at the first and second harmonics of the free space extraordinary mode (X2 and X3) could exceed collisional ionospheric damping rates, although growth rates were low. Recently, however, ground‐based observations indicate that these emissions are predominantly O mode polarized. Therefore a reassessment of the lower ionospheric cyclotron maser as an emission mechanism is necessary. In this paper, kinetic cyclotron maser instability calculations are extended to include the trapped Z mode for varying thermal energies and auroral potential drops within the context of a realistic ionospheric electron distribution model. It is found that when the local upper hybrid frequency is in the close vicinity of 2ƒce or 3ƒce, harmonic Z mode (Z2 and Z3) growth rate is shown to be exceedingly high. This finding suggests that an indirect mechanism that involves linear or nonlinear mode conversion of the excited harmonic Z mode into the ordinary (O) mode is a possible emission mechanism.

Coherent Emission in Astrophysics: A Critique

The applications of ‘coherent emission’ to radioastronomical sources is discussed critically. The applications considered are electron cyclotron maser emission applied to certain solar and stellar radio emissions, pulsar radio emission, and suggested coherent emission from active galactic nuclei.

Electron-cyclotron maser theory for extraordinary Bernstein waves

Electron-cyclotron maser emission is investigated in the regime where wave growth in the electrostatic Bernstein modes dominates (ωp/Ωe&gt;1.5). A semirelativistic growth rate is derived assuming that the wave dispersion is dominated by a cool background electron distribution and the instability is driven by a low-density hot loss-cone-like electron distribution. The properties of Bernstein wave growth are most strongly dependent on the relative temperatures of the hot and cool electron distributions. For Thot/Tcool[gsim ]10, the fastest growing Bernstein waves are produced at frequencies just below each cyclotron harmonic in Bernstein modes lying below the upper-hybrid frequency. For Thot/Tcool[lsim ]10, additional Bernstein modes above the upper-hybrid frequency are excited, with wave frequencies in each excited mode lying significantly above the corresponding cyclotron harmonic. The dependence of Bernstein wave growth on the relative hot and cool electron number densities and emission angle is also discussed.

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 1015 K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.

Elliptically polarized Jovian decametric radiation: An investigation of the electron cyclotron maser mechanism

A model for the elliptical polarization of Jovian decametric radiation is presented, based on the electron cyclotron mechanism. The aim is to determine whether the observed elliptical polarization is consistent with the radiation being generated by mildly relativistic electrons streaming along converging magnetic field lines. The growth rate for electron cyclotron maser emission is considered, assuming a drifting DGH distribution function for the streaming electrons. Constraints on the allowable parameters of this distribution function are made by the observed polarization, timescale, bandwidth, and angular range of the radiation.

On the harmonic structure of solar radio spikes

This paper analyzes the frequency structure of the bands of electron-cyclotron maser instability. The calculations show that each term of the series describing the growth rate provides a double-peak structure, if we accept a nonthermal electron distribution with a ‘two-side’ loss-cone. The ratio of central frequencies is found to be non-integer in general case. We conclude that the harmonic structure of solar radio spikes observed in a number of events can be imitated by electron-cyclotron maser emission of mildly relativistic electrons with a power-law momentum distribution.

Electron-Cyclotron maser emission from streaming distributions

Motivated by the need to explain observed elliptically polarized emission from Jupiter, the mechanism of electron-cyclotron maser emission is considered for drifting electron distributions, where the electrons stream with a non-zero mean velocity parallel to the magnetic field lines. An analytical expression for the semirelativistic growth rate is derived and its properties analysed in detail for waves generated in the magneto-ionic modes. The main features of the growth rate are discussed, on the basis of a geometric analysis using resonant ellipses.

Electron cyclotron maser emission at oblique angles

Observations of elliptically polarized bursts of decametric radio emission from Jupiter with axial ratios of the polarization ellipse 0.2 ≲ T ≲ 0.7 imply emission at angles 45° ≲ θ ≲ 80° to the magnetic field. Emission at oblique angles ≲ 60° is not expected in the conventional theory of electron cyclotron maser emission. It is argued that electron cyclotron emission at a given θ requires electrons with parallel velocity concentrated around υ ‖ = υ cos α = c cos θ. A mechanism is proposed that might produce an appropriate “spiraling beam” distribution, with a peak in velocity space at speed υ = υ 0 and pitch angle α = α 0 ≠ 0.