Electron Cyclotron Maser Mechanism Research Articles

Electron Cyclotron Maser Emission in Solar Radio Bursts

Radio bursts are ubiquitous in the cosmic plasma. Solar radio emission mainly comes from the outer atmosphere of the sun. It is an induced radiation phenomenon generated by the interaction between energetic electrons and solar atmospheric plasma. Different dynamic spectra of solar radio bursts (SRBs) contain physical information of the plasma structure and state in the radiation source region. Therefore, the radiative mechanism of radio bursts has always been the object of research. There are two kinds of coherent radiation mechanisms related to solar radio bursts: one is the plasma radiation mechanism based on electron Langmuir frequency; the other is the electron cyclotron maser (ECM) radiation mechanism based on the electron cyclotron frequency. Although these two radiation mechanisms were proposed almost at the same time, based on the understanding of the coronal environment and the ECM mechanism at that time, the ECM radiation mechanism did encounter some difficulties in explaining SRBs. Until 1979, Wu & Lee introduced the relativistic effect and used the ECM radiation to explain the earth’s Auroral Kilometric Radiation (AKR). Since then, the ECM emission has attracted wide attention. Considering some difficulties in applying the ECM emission mechanism to SRBs, we proposed a series of modified models in recent years. Firstly, the cutoff in the energy spectrum of the power-law electrons can effectively drive the ECM instability without relying on the anisotropic distribution of electron velocity. Secondly, considering the influence of Alfvén wave perturbations which are prevalent in space and celestial plasmas, a self-consistent ECM emission mechanism excited by energetic electron beams is developed. On this basis, this paper summarizes the application of the ECM emission mechanism in traditional SRB phenomena from type I to V and microwave SRBs in recent years.

Modification of “Pressed” Atmospheres in Active Regions of Ultracool Stars

Ultracool stars usually have active regions, which is confirmed by their high-power radiofrequency emission modulated by the star axial rotation. The interpretation of this emission is commonly based on the electron cyclotron maser mechanism realized in the active regions. A plasma mechanism of radiofrequency emission is not considered, because ultracool star atmospheres are tightly “pressed” against the star surface, and the plasma frequency is much lower than the electron gyrofrequency (fL ≪ fB) at the coronal levels. This paper explores active regions of ultracool stars for the possible existence of a system of coronal magnetic loops carrying electric current generated by photospheric convection. It is shown that current dissipation induces a temperature increase inside the loops to about 107 K, which causes an increase in the scale of height of the inhomogeneous atmosphere and, at the coronal levels, effectuates condition fL ≫ fB, at which the plasma mechanism of radiofrequency emission prevails over the electron cyclotron maser mechanism. The magnetic loop parameters, intensity of electric currents generated by the photospheric convection, and efficiency of plasma heating inside the magnetic loops are evaluated on the example of the brown dwarf TVLM513-46546. The scale of the height of the modified atmosphere, which appears to be comparable to the star radius, is calculated; it is shown that the soft X-ray flow created by the hot modified atmosphere inside a coronal magnetic loop is about equal to that observed for brown dwarf TVLM513-46546.

154 MHz Detection of Faint, Polarized Flares from UV Ceti

Abstract We have detected four flares from UV Ceti at 154 MHz using the Murchison Widefield Array. The flares have flux densities between 10 and 65 mJy—a factor of 100 fainter than most flares in the literature at these frequencies—and are only detected in polarization. The circular polarized fractions are limited to % at 3σ confidence and two of the flares exhibit polarity reversal. We suggest that these flares occur periodically on a timescale consistent with the rotational period of UV Ceti. During the brightest observed flare, we also detect significant linear polarization with a polarization fraction %. Averaging the data in 6 minute, 10 MHz frequency bins we find that the flux density of these flares does not vary over the 30 MHz bandwidth of the Murchison Widefield Array; however, we cannot rule out finer time-frequency structure. Using the measured flux densities for the flares, we estimate brightness temperatures between K, indicative of a coherent emission mechanism. The brightness temperature and polarization characteristics point to the electron cyclotron maser mechanism. We also calculate the flare rates given our four observed flares and compare them to flare rates for the set of M dwarf stars with known 100–200 MHz flares. Our measurement is the first for flares with intensities mJy at 100–200 MHz.

EFFECTS OF THE TEMPERATURE ANISOTROPY ON THE MASER INSTABILITY EXCITED BY LOWER ENERGY CUTOFF BEHAVIOR

The electron–cyclotron maser (ECM) conventionally driven by velocity anisotropies of energetic electrons trapped in magnetic fields is one of the most important radio-emission mechanisms in astrophysics. Recently, Wu and Tang proposed that a proper lower energy cutoff behavior of power-law electrons can effectively excite the ECM emission. This paper considers effects of temperature anisotropy on this new ECM mechanism. The results show that the growth rates of the ECM emissions increase with β⊥0 and β∥0, where β⊥0 and β∥0 are the perpendicular and parallel velocity spreads (in units of the light velocity c) of the energetic electron beam, respectively. Moreover, the growth rates of O1 and X2 modes both sensitively depend on the ratio of the electron–cyclotron frequency to the plasma frequency Ω and reach their extremum values at Ω ≃ 1.5 for the O1 mode and at Ω ≃ 1.0 for the X2 mode. Meanwhile, as the mean velocity of the electron beam βs (in units of c) increases, the growth rate of the O1 mode remains approximately constant and that of the X2 mode decreases considerably.

Observations of radio pulses from CU Virginis

Abstract The magnetic chemically peculiar star CU Virginis is a unique astrophysical laboratory for stellar magnetospheres and coherent emission processes. It is the only known main-sequence star to emit a radio pulse every rotation period. Here we report on new observations of the CU Virginis pulse profile in the 13- and 20-cm radio bands. The profile is known to be characterized by two peaks of 100 per cent circularly polarized emission that are thought to arise in an electron–cyclotron maser mechanism. We find that the trailing peak is stable at both 13 and 20 cm, whereas the leading peak is intermittent at 13 cm. Our measured pulse arrival times confirm the discrepancy previously reported between the putative stellar rotation rates measured with optical data and with radio observations. We suggest that this period discrepancy might be caused by an unknown companion or by instabilities in the emission region. Regular long-term pulse timing and simultaneous multiwavelength observations are essential to clarify the behaviour of this emerging class of transient radio source.

Millisecond Microwave Spikes: Statistical Study and Application for Plasma Diagnostics

We analyze a dense cluster of solar radio spikes registered at 4.5-6 GHz by the Purple Mountain Observatory spectrometer (Nanjing, China), operating in the 4.5-7.5 GHz range with 5 ms temporal resolution. To handle the data from the spectrometer, we developed a new technique that uses a nonlinear multi-Gaussian spectral fit based on X-2 criteria to extract individual spikes from the originally recorded spectra. Applying this method to the experimental raw data, we eventually identified about 3000 spikes for this event, which allows us to make a detailed statistical analysis. Various statistical characteristics of the spikes have been evaluated, including the intensity distributions, the spectral bandwidth distributions, and the distribution of the spike mean frequencies. The most striking finding of this analysis is the distributions of the spike bandwidth, which are remarkably asymmetric. To reveal the underlaying microphysics, we explore the local-trap model with the renormalized theory of spectral profiles of the electron cyclotron maser (ECM) emission peak in a source with random magnetic irregularities. The distribution of the solar spike relative bandwidths calculated within the local-trap model represents an excellent fit to the experimental data. Accordingly, the developed technique may offer a new tool with which to study very low levels of magnetic turbulence in the spike sources, when the ECM mechanism of the spike cluster is confirmed.

The electron–cyclotron maser for astrophysical application

The electron–cyclotron maser is a process that generates coherent radiation from plasma. In the last two decades, it has gained increasing attention as a dominant mechanism of producing high-power radiation in natural high-temperature magnetized plasmas. Originally proposed as a somewhat exotic idea and subsequently applied to include non-relativistic plasmas, the electron–cyclotron maser was considered as an alternative to turbulent though coherent wave–wave interaction which results in radio emission. However, when it was recognized that weak relativistic corrections had to be taken into account in the radiation process, the importance of the electron–cyclotron maser rose to the recognition it deserves. Here we review the theory and application of the electron–cyclotron maser to the directly accessible plasmas in our immediate terrestrial and planetary environments. In situ access to the radiating plasmas has turned out to be crucial in identifying the conditions under which the electron–cyclotron maser mechanism is working. Under extreme astrophysical conditions, radiation from plasmas may provide a major energy loss; however, for generating the powerful radiation in which the electron–cyclotron maser mechanism is capable, the plasma must be in a state where release of susceptible amounts of energy in the form of radiation is favorable. Such conditions are realized when the plasma is unable to digest the available free energy that is imposed from outside and stored in its particle distribution. The lack of dissipative processes is a common property of collisionless plasmas. When, in addition, the plasma density becomes so low that the amount of free energy per particle is large, direct emission becomes favorable. This can be expressed as negative absorption of the plasma which, like in conventional masers, leads to coherent emission even though no quantum correlations are involved. The physical basis of this formal analogy between a quantum maser and the electron–cyclotron maser is that in the electron–cyclotron maser the free-space radiation modes can be amplified directly. Several models have been proposed for such a process. The most famous one is the so-called loss-cone maser. However, as argued in this review, the loss-cone maser is rather inefficient. Available in situ measurements indicate that the loss-cone maser plays only a minor role. Instead, the main source for any strong electron–cyclotron maser is found in the presence of a magnetic-field-aligned electric potential drop which has several effects: (1) it dilutes the local plasma to such an extent that the plasma enters the regime in which the electron–cyclotron maser becomes effective; (2) it generates energetic relativistic electron beams and field-aligned currents; (3) it deforms, together with the magnetic mirror force, the electron distribution function, thereby mimicking a high energy level sufficiently far above the Maxwellian ground state of an equilibrium plasma; (4) it favors emission in the free-space RX mode in a direction roughly perpendicular to the ambient magnetic field; (5) this emission is the most intense, since it implies the coherent resonant contribution of a maximum number of electrons in the distribution function to the radiation (i.e., to the generation of negative absorption); (6) it generates a large number of electron holes via the two-stream instability, and ion holes via the current-driven ion-acoustic instability which manifest themselves as subtle fine structures moving across the radiation spectrum and being typical for the electron–cyclotron maser emission process. These fine structures can thus be taken as the ultimate identifier of the electron–cyclotron maser. The auroral kilometric radiation of Earth is taken here as the paradigm for other manifestations of intense radio emissions such as the radiation from other planets in the solar system, from exoplanets, the Sun and other astrophysical objects.

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.

Radio Emission from Ultrashort-Period Double Degenerate Binaries

AbstractTiming measurements of periodic X-ray pulses from two ultrashort-period double degenerate binaries, RX J1914+24 and RX J0806+15, show that the rates of change of their orbital periods are consistent with gravitational radiation losses. This contradicts the predictions of models which invoke mass transfer between the two white dwarfs. The X-ray emission is, therefore, unlikely to be powered by accretion processes. The unipolar inductor model explains the source of X-ray emission as electrical dissipation at the base of a flux tube, which connects the magnetic white dwarf to its companion. This model is most consistent with the observed X-ray pulse properties. A similar current system exists in the Jupiter–Io system, where a mildly relativistic electron current produces an auroral footprint at the base of the Io flux tube and highly polarized beamed radio emission by means of the electron cyclotron maser mechanism. Detection of radio emission from RX J1914+24 and RX J0806+15 would thus provide further support for the unipolar inductor model. We present theoretical predictions, based on a loss-cone-driven electron cyclotron maser model, of radio fluxes from systems with parameters similar to RX J1914+24 and RX J0806+15.

Decimetric Spike Bursts versus Microwave Continuum

We analyze properties of decimetric spike bursts occurring simultaneously with microwave gyrosynchrotron continuum bursts. We found that all of the accompanying microwave bursts were highly polarized in the optically thin range. The sense of polarization of the spike clusters is typically the same as that of the optically thin gyrosynchrotron emission, implying preferential extraordinary wave-mode spike polarization. Optically thick spectral indices of the continuum in spike-producing events were not observed to be larger than 2.5, suggesting low or absent Razin suppression. This implies that the plasma frequency-to-gyrofrequency ratio is systematically lower in the spike-producing bursts than in other bursts. The spike cluster flux density is found to be tightly correlated with the high-frequency spectral index of the microwave continuum for each event, while the flux-to-flux correlation may not be present. We discovered strong evidence that the trapped fast electrons producing the microwave gyrosynchrotron continuum have an anisotropic pitch-angle distribution of the loss cone type in the spike-producing bursts. The spike clusters are mainly generated when the trapped electrons have the hardest and the most anisotropic distributions. The new properties are discussed against the currently available ideas about emission processes and models for spike generation. We conclude that the findings strongly support the electron cyclotron maser mechanism of spike emission, with characteristics agreeing with expectations from the local-trap model.

Electron‐Cyclotron Maser Driven by Charged‐Particle Acceleration from Magnetic Field–aligned Electric Fields

We present a detailed description of the auroral kilometric radiation (AKR) source region based on observations from the Fast Auroral SnapshoT (FAST) satellite and discuss how these new results may pertain to solar and stellar radio sources. FAST satellite observations are directly within the AKR source region and have unprecedented spatial and temporal resolution. They confirm many of the fundamental elements of the electron-cyclotron maser mechanism but with substantial modification. The most important modification is that the emissions do not draw their energy from a loss-cone instability; rather, the radiation results from an unstable horseshoe or distribution. The most far-reaching implication is that the electron-cyclotron maser is directly associated with a particular type of charged particle acceleration, a magnetic field-aligned (parallel) electric field in a dipole magnetic field. These findings change several of the characteristics of the electron-cyclotron maser mechanism and may necessitate reanalysis of some astrophysical radio sources. Under the shell instability, radio emissions with brightness temperatures ~1014 K, the steady state limit of the loss-cone instability, may be continuous. Through observations, we demonstrate that source brightness may be as high as 1020 K in steady state. A moderately or strongly relativistic beam may result in broadband emissions. A loss cone is not required, so the radiation source may be high above the stellar or planetary surface. Although the generation is in the X mode with k|| = 0, we suggest that the radiation, guided by a density cavity that is created by the parallel electric field, efficiently converts to the R mode, which experiences substantially lower absorption at higher harmonics. These findings also suggest that parallel electric fields may be a fundamental particle acceleration mechanism in astrophysical plasmas.

Kinetic theory of the slow-wave ECM

Abstract The general kinetic theory of slow-wave electron cyclotron maser (ECM) interaction is presented. Using the Vlasov-Maxwell equations the perturbation indicted on the electron distribution function by the action of slow hybrid waves is obtained. A general expression for the power transfer between an electron beam of any equilibrium distribution function and the hybrid fields is derived, which can reveal all the interactions that potentially can occur in the system. The effect of the electron guiding-centre drift and the development of a dipole beam current is also considered. To illustrate the physics of the interaction, the theory is applied to the case of a filamentary cold electron beam with exclusively axial initial electron momentum placed on the axis of the system, and expressions for the starting beam current and the frequency pulling due to the beam presence are derived for the case of the slow-wave ECM mechanism. Numerical examples are presented showing that an electron beam with an accele...

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.

Effects of turbulence on the electron cyclotron-maser mechanism for solar microwave spike bursts

It is shown that small magnetic perturbations can significantly alter the rates of cyclotron growth, absorption, mode conversion, and refraction because of the sensitive dependence of these processes on the field strength in narrow layers. In particular, growth lengths are increased, absorption depths decreased, mode conversion becomes more effective, and turbulent refraction leads to isotropization of the emission. The criteria for significant effects to occur are derived and it is shown that they can be met by the few-percent field perturbations observed in coronal loops. Relative to the theory of cyclotron-maser emission in smoothly varying plasmas, perturbations enable fundamental o-mode (o1) and second-harmonic x-mode (x2) radiation to saturate more effectively, increase the chance of x1, o1, and x2 radiation escaping to infinity through absorption and mode-coupling windows, and partially isotropize radiation emitted near the x-mode cutoff. It is concluded that o1 and x1 emission are both likely to be present, and that x2 emission is possible under some circumstances. However, x1 radiation can escape only at near-parallel propagation (θ ≈ 0) or via mode conversion to the o-mode at θ ≈ 90°, whereas o1 and x2 emission can escape for a wide range of angles around θ = 0 and, under many circumstances, near θ = 90°.

First interferometric observations of solar microwave millisecond spike bursts

Observations, with one-dimensional spatial resolution, of solar microwave millisecond spikes at 2.8 GHz are reported. The observations were made with the Owens Valley frequency-agile interferometer with 20 ms time resolution. It is found that the spikes occur at a position different from that of the underlying gyrosynchrotron radio burst source. All of the spikes during the rising part of the burst come from the same one-dimensional location to within + or - 1 arcsec, despite the fact that rapid evolution in the sense and degree of circular polarization was observed. The one-dimensional position of the spikes is consistent with a location over a large sunspot, suggesting that the emission occurs in a region of strong, converging magnetic field. The observations support the suggestion that the spikes are due to the electron-cyclotron maser mechanism.

Ordinary mode radio emission from Uranus

On January 24, 1986, during the inbound trajectory to Uranus, the planetary radio astronomy instrument on board Voyager 2 detected left‐hand polarized emission for several hours at frequencies near 200 kHz. Unlike the dominant source observed on the nightside of the planet, only a single episode of this inbound emission was seen, and the emission was extremely weak. Generating less than 500 kW, it is probably the weakest freely propagating planetary radio emission thus far observed by Voyager. We associate this weak emission with a source region in the vicinity of Uranus' north (dayside) magnetic pole, where magnetic field lines point approximately in the direction of the spacecraft when the emission is observed. Therefore the wave normal angle relative to B is probably less than 90° in the generation region. We conclude that direct wave generation in the magnetoionic ordinary mode is most likely. The source location for this emission is consistent with a region magnetically conjugate to the dominant nightside source, which is located in the vicinity of the south magnetic pole. The ordinary mode emission is discussed in the context of the electron cyclotron maser mechanism.

Path‐integrated growth of auroral kilometric radiation

Using Poeverlein's graphical method, three‐dimensional ray path calculations are performed to evaluate the path‐integrated growth of auroral kilometric radiation (AKR). The ray tracing results indicate that waves whose initial wave vector lie in the local meridian plane continue to propagate in that plane and that among these waves, those with frequencies near the cutoff frequency (fR = 0) refract substantially, whereas those with frequencies well above the cutoff frequency suffer little refraction. It is also shown that waves whose initial wave vector lies outside of the local meridian plane propagate in the longitudinal as well as the radial and the latitudinal directions. The refraction of these waves is also highly dependent upon the wave frequency, i.e., waves with frequencies near fR = 0 refract substantially, whereas waves with frequencies much above fR = 0 undergo little refraction. In order to test the electron cyclotron maser mechanism as a method for generation of AKR, a typical electron distribution function measured in the auroral zone by the S3‐3 satellite is used to calculate path‐integrated growth of representative rays. The results of this study indicate that electron distribution functions like those measured by the S3‐3 satellite are not capable of amplifying cosmic noise background to the observed intensities of auroral kilometric radiation and that much steeper slopes (∂F/∂υ⊥) at the edges of the loss cone are required. The presence of such distribution functions in the auroral zone is plausible if one assumes that backscattered electrons in this region have energies less than a few hundred eV.