Electron Cyclotron Maser Instability Research Articles

Electron cyclotron maser instability by evolving fast electron beams in the flare loops

The electron cyclotron maser instability (ECMI) stands as a pivotal coherent radio emission mechanism widely implicated in various astrophysical phenomena. In the context of solar activity, ECMI is primarily instigated by energetic electrons generated during solar eruptions, notably flares. These electrons, upon leaving the acceleration region, traverse the solar atmosphere, forming fast electron beams (FEBs) along magnetic field lines. It is widely accepted that as these FEBs interact with the ambient plasma and magnetic fields, they give rise to radio and hard X-ray emission. Throughout their journey in the plasma, FEBs undergo modifications in their energy spectrum and velocity spatial distribution due to diverse energy loss mechanisms and changes in ambient plasma parameters. In this study, we delve into the impact of the evolving energy spectrum and velocity anisotropic distribution of FEBs on ECMI during their propagation in flare loops. Our findings indicate that if we solely consider the progressively flattened lower energy cutoff behavior as FEBs descend along flare loops, the growth rates of ECMI decrease accordingly. However, when accounting for the evolution of ambient magnetic plasma parameters, the growth rates of ECMI increase as FEBs delve into denser atmosphere. This underscores the significant influence of the energy spectrum and velocity anisotropy distribution evolution of FEBs on ECMI. Our study sheds light on a more comprehensive understanding of the dynamic spectra of solar radio emissions.

Impact of Electron Precipitation on Brown Dwarf Atmospheres and the Missing Auroral H3+ Emission

Recent observations have demonstrated that very low-mass stars and brown dwarfs are capable of sustaining strong magnetic fields despite their cool and neutral atmospheres. These kilogauss field strengths are inferred based on strong, highly circularly polarized gigahertz radio emission, a consequence of the electron cyclotron maser instability. Crucially, these observations imply the existence of energetic nonthermal electron populations, associated with strong current systems, as are found in the auroral regions of the magnetized planets of the solar system. Intense auroral electron precipitation will lead to electron collisions with the H2 gas that should generate the ion H3+ . With this motivation, we targeted a sample of ultracool dwarfs, known to exhibit signatures associated with aurorae, in search of the K-band emission features of H3+ using the Keck telescopes on Maunakea. From our sample of nine objects, we found no clear indication of H3+ emission features in our low-to-medium-resolution spectra (R ∼ 3600). We also modeled the impact of an auroral electron beam on a brown dwarf atmosphere, determining the depth at which energetic beams deposit their energy and drive particle impact ionization. We find that the H3+ nondetections can be explained by electron beams of typical energies ≳2–10 keV, which penetrate deeply enough that any H3+ produced is chemically destroyed before radiating energy through its infrared transitions. Strong electron beams could further explain the lack of UV auroral detections and suggest that most or nearly all of the precipitating auroral energy must ultimately emerge as thermal emissions deep in brown dwarf atmospheres.

Radio emission as a stellar activity indicator

Radio observations of stars trace the plasma conditions and magnetic field properties of stellar magnetospheres and coronae. Depending on the plasma conditions at the emitter site, radio emission in the metre- and decimetre-wave bands is generated via different mechanisms, such as gyrosynchrotron, electron cyclotron maser instability, and plasma radiation processes. The ongoing LOFAR Two-metre Sky Survey (LoTSS) and VLA Sky Survey (VLASS) are currently the most sensitive wide-field radio sky surveys ever conducted. Because these surveys are untargeted, they provide an opportunity to study the statistical properties of the radio-emitting stellar population in an unbiased manner. Here we perform an untargeted search for stellar radio sources down to sub-mJy level using these radio surveys. We find that the population of radio-emitting stellar systems is mainly composed of two distinct categories: chromospherically active stellar (CAS) systems and M dwarfs. We also seek to identify signatures of a gradual transition within the M-dwarf population, from chromospheric or coronal acceleration close to the stellar surface similar to that observed on the Sun to magnetospheric acceleration occurring far from the stellar surface similar to that observed on Jupiter. We determine that radio detectability evolves with spectral type, and we identify a transition in radio detectability around spectral type M4, where stars become fully convective. Furthermore, we compare the radio detectability versus spectra type with X-ray and optical flare (observed by TESS) incidence statistics. We find that the radio efficiency of X-ray and optical flares, which is the fraction of flare energy channelled into radio-emitting charges, increases with spectral type. These results motivate us to conjecture that the emergence of large-scale magnetic fields in CAS systems and later M dwarfs leads to an increase in radio efficiency.

Phenomenology and periodicity of radio emission from the stellar system AU Microscopii

Stellar radio emission can measure a star’s magnetic field strength and structure, plasma density, and dynamics, and the stellar wind pressure impinging on exoplanet atmospheres. However, properly interpreting the radio data often requires temporal baselines that cover the rotation of the stars, orbits of their planets, and any longer-term stellar activity cycles. Here we present our monitoring campaign on the young, active M dwarf AU Microscopii with the Australia Telescope Compact Array between 1.1 and 3.1 GHz. With over 250 h of observations, these data represent the longest radio monitoring campaign on a single main-sequence star to date. We find that AU Mic produces a wide variety of radio emission, for which we introduce a phenomenological classification scheme predicated on the polarisation fraction and time-frequency structure of the emission. Such a classification scheme is applicable to radio emission from other radio-bright stars. The six types of radio emission detected on AU Mic can be broadly categorised into five distinct types of bursts, and broadband quiescent emission. We find that the radio bursts are highly circularly polarised and periodic with the rotation period of the star, implying that the emission is beamed. It is therefore most likely produced by the electron cyclotron maser instability. We present a model to show that the observed pattern of emission can be explained by emission from auroral rings on the magnetic poles. The total intensity of the broadband emission is stochastic, but we show that its circular polarisation fraction is also periodic with the rotation of the star. Such a periodicity in the polarised fraction of emission has not been observed on an M dwarf before. We present a qualitative model to describe the periodicity in the polarisation fraction of the broadband emission, using low-harmonic gyromagnetic emission. Using a simple qualitative model, we infer a magnetic obliquity of at least 20° from the observed variation in polarisation fraction. Finally, we show that the radio emission might be evolving on long timescales, hinting at a potential stellar magnetic activity cycle.

Waves and plasma emissions excited by ring-beam energetic electrons interacting with weakly magnetized plasmas

Ring-beam energetic electrons can drive two different kinetic instabilities, including the bump-on-tail instability by the beam component and the electron cyclotron maser instability by the ring component. It is critical to understand how the two instabilities develop and interact, and how their competition for free energy affects the wave modes and further plasma emission process. Here, we present fully kinetic particle-in-cell simulations of the ring-beam interaction with weakly magnetized plasmas of coronal conditions. We found that both the beam-Langmuir (BL) mode and the upper-hybrid (UH) mode can be excited efficiently, along with significant fundamental and harmonic plasma emissions. In comparison with the corresponding pure-beam case, in the ring-beam case, the BL mode has a suppressed range of k⊥, slower damping and thus stronger intensity, and the fundamental emission is considerably stronger, while the harmonic emission has comparable intensity yet being more directional. We further investigated the effect of the pitch angle (α) of the ring-beam distribution, found that the (1) BL mode is more limited in the range of k⊥ with larger α and (2) the UH mode is excited in localized blobs that can be clustered with a straight line whose slope is ∼cot α. Most of these intriguing results can be interpreted with the linear kinetic theory.

Excitation of extraordinary modes inside the source of Saturn’s kilometric radiation

The electron cyclotron maser instability (ECMI) of extraordinary mode waves was investigated with the parameters observed in Saturn’s kilometric radiation (SKR) sources. Previous studies employed simplified dispersion relations, and did not consider the excitation of the relativistic (R) mode. This mode is introduced by considering the relativistic effect in plasmas consisting of both cold and hot electrons. Using particle-in-cell simulations, we investigated the excitation of R and X modes based on the measured data. Using the reported value of the density ratio of energetic to total electrons ne/n0 = 24%, the most unstable mode is the R mode. The escaping X-mode emissions are amplified only if the energetic electrons are dominant with ne/n0 ≥ 90%. For these cases, only the X mode is excited and the R mode disappears due to its strong coupling. The results are well in line with the linear kinetic theory of ECMI. The properties of both the R and X modes are consistent with the observed SKR emissions. This raises questions about the nature of the measured electric field fluctuations within “presumed” SKR sources. The study provides new insights into the ECMI process relevant to SKR emission mechanisms.

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.

Plausible association of distant late M dwarfs with low-frequency radio emission

We present the serendipitous discovery of eight distant (> 50 pc) late M dwarfs with plausible associated radio emission at 144 MHz. The M-dwarf nature of our sources has been confirmed with optical spectroscopy performed using HET/LRS2 and Subaru/FOCAS, and their radio flux densities are within the range of 0.5−1.0 mJy at 144 MHz. Considering the radio-optical source separation and source densities of the parent catalogues, we suggest that it is statistically probable that the M dwarfs are associated with the radio emission. However, it remains plausible that for some of the sources the radio emission originates from an optically faint and red galaxy hiding behind the M dwarf. The isotropic radio luminosities (∼1017 − 18 erg s−1 Hz−1) of the M dwarfs suggest that if the association is real, the radio emission is likely driven by a coherent emission process produced via plasma or electron-cyclotron maser instability processes, which is potentially caused by binary interaction. Long-term monitoring in the radio and high-resolution radio follow-up observations are necessary to search for any variability and pinpoint the radio emission to determine whether our tentative conclusion that these ultracool dwarfs are radio emitting is correct. If the low-frequency radio emission is conclusively associated with the M dwarfs, this would reveal a new population of optically faint and distant (> 50 pc) radio-emitting M dwarfs.

Fine Structures of Radio Bursts from Flare Star AD Leo with FAST Observations

Radio bursts from nearby active M-dwarfs have been frequently reported and extensively studied in solar or planetary paradigms. Whereas, their substructures or fine structures remain rarely explored despite their potential significance in diagnosing the plasma and magnetic field properties of the star. Such studies in the past have been limited by the sensitivity of radio telescopes. Here we report the inspiring results from the high time-resolution observations of a known flare star AD Leo with the Five-hundred-meter Aperture Spherical radio Telescope. We detected many radio bursts in the 2 days of observations with fine structures in the form of numerous millisecond-scale sub-bursts. Sub-bursts on the first day display stripe-like shapes with nearly uniform frequency drift rates, which are possibly stellar analogs to Jovian S-bursts. Sub-bursts on the second day, however, reveal a different blob-like shape with random occurrence patterns and are akin to solar radio spikes. The new observational results suggest that the intense emission from AD Leo is driven by electron cyclotron maser instability, which may be related to stellar flares or interactions with a planetary companion.

Hunting for exoplanets via magnetic star–planet interactions: geometrical considerations for radio emission

ABSTRACT Recent low-frequency radio observations suggest that some nearby M dwarfs could be interacting magnetically with undetected close-in planets, powering the emission via the electron cyclotron maser (ECM) instability. Confirmation of such a scenario could reveal the presence of close-in planets around M dwarfs, which are typically difficult to detect via other methods. ECM emission is beamed, and is generally only visible for brief windows depending on the underlying system geometry. Due to this, detection may be favoured at certain orbital phases, or from systems with specific geometric configurations. In this work, we develop a geometric model to explore these two ideas. Our model produces the visibility of the induced emission as a function of time, based on a set of key parameters that characterize magnetic star–planet interactions. Utilizing our model, we find that the orbital phases where emission appears are highly dependent on the underlying parameters, and does not generally appear at the quadrature points in the orbit as is seen for the Jupiter–Io interaction. Then using non-informative priors on the system geometry, we show that untargeted radio surveys are biased towards detecting emission from systems with planets in near face-on orbits. While transiting exoplanets are still likely to be detectable, they are less likely to be seen than those in near face-on orbits. Our forward model serves to be a powerful tool for both interpreting and appropriately scheduling radio observations of exoplanetary systems, as well as inverting the system geometry from observations.

ECMI resonance in AKR revisited: Hyperbolic resonance, harmonics, and wave–wave interaction

Recapitulation of the resonance condition for the fundamental and higher electron cyclotron harmonics in the electron cyclotron maser instability (ECMI) enables radiation below and confirms the possibility of radiation in a narrow band above harmonics n > 1. Near n = 1 resonance on the confined lower X-mode branch, amplification is supported by the decrease in phase and group speeds. Confined slow large-amplitude quasi-electrostatic X-modes non-linearly modulate the plasma to form cavitons until self-trapped inside them at a further increasing wavenumber. They undergo wave–wave interaction, enabling escape into free space in the second harmonic band below n = 2. At a sufficiently large parallel wavenumber (oblique propagation), the fundamental resonance n = 1 is hyperbolic, a possibility so far missed but vital for an effective ECMI in the upward current region. Here, the resonance hyperbola favorably fits the loss-cone boundary, the presumably important ECMI upward-current source-electron distribution, to stimulate ECMI growth at available auroral electron energies.

Effects of Fully Relativistic Condition on Electron Cyclotron Maser Emission

The electron cyclotron maser (ECM) instability is a very important nonthermal radiation mechanism. It has been developed by proposing various electron distribution functions as well as the relativistic resonance condition, called the semirelativistic correction. Taking account of the relativistic effects of both the velocity distribution of energetic electrons and the resonance condition, called the fully relativistic correction, the present paper investigates the ECM instability driven by a power-law electron distribution with a low-energy cutoff. The results show that (1) both in the semirelativistic and fully relativistic cases, the growth rate and relative frequency bandwidth of ordinary (O) and extraordinary (X) modes show a positive correlation with cutoff energy E c , i.e., the peak frequency decreases with increasing E c ; (2) the peak frequency ratio (H peak/F peak) of the harmonic and fundamental waves is always ∼2; (3) compared with the semirelativistic case, the fully relativistic case has a larger growth rate (for both the O and X mode) and a smaller peak frequency (only for the O mode) for energy > 50 keV, and there is almost no difference at lower energy for the two cases; (4) the peak frequency of the X1 mode can be higher than its cutoff frequency in a strongly magnetized plasma, implying that the X1 mode emission may escape more easily for a higher E c and stronger magnetic field. These results can be helpful for us to understand better the physics of radio bursts from the Sun and other objects.

Plasma emission induced by ring-distributed energetic electrons in overdense plasmas

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

Interferometric Imaging, and Beam-Formed Study of a Moving Type-IV Radio Burst with LOFAR

Type-IV radio bursts have been studied for over 50 years. However, the specifics of the radio emission mechanisms is still an open question. In order to provide more information about the emission mechanisms, we studied a moving Type-IV radio burst with fine structures (spike group) by using the high-resolution capability of the Low-Frequency Array (LOFAR) on August 25, 2014. We present a comparison of Nançay Radioheliograph (NRH) and the first LOFAR imaging data of the Type-IV radio burst. The degree of circular polarization (DCP) is calculated at frequencies in the range 20 – 180 MHz using LOFAR data, and it was found that the value of DCP gradually increased during the event, with values of 20 – 30%. LOFAR interferometric data were combined with white-light observations in order to track the propagation of this Type-IV burst. The kinematics shows a westward motion of the radio sources, slower than the CME leading edge. The dynamic spectrum of LOFAR shows a large number of fine structures with durations of less than 1 s and high brightness temperatures (T_{ mathrm{B}}), i.e., 10^{12} – 10^{13} K. The gradual increase of DCP supports gyrosynchrotron emission as the most plausible mechanism for the Type IV. However, coherent emissions such as Electron Cyclotron Maser (ECM) instability may be responsible for small-scale fine structures. Countless fine structures altogether were responsible for such high T_{mathrm{B}}.

Wave Emission of Nonthermal Electron Beams Generated by Magnetic Reconnection

Magnetic reconnection in solar flares can efficiently generate nonthermal electron beams. The energetic electrons can, in turn, cause radio waves through microscopic plasma instabilities as they propagate through the ambient plasma along the magnetic field lines. We aim at investigating the wave emission caused by fast-moving electron beams with characteristic nonthermal electron velocity distribution functions (EVDFs) generated by kinetic magnetic reconnection: two-stream EVDFs along the separatrices and in the diffusion region, and perpendicular crescent-shaped EVDFs closer to the diffusion region. For this purpose, we utilized 2.5D fully kinetic Particle-In-Cell code simulations in this study. We found the following: (1) the two-stream EVDFs plus the background ions are unstable to electron/ion (streaming) instabilities, which cause ion-acoustic waves and Langmuir waves due to the net current. This can lead to multiple-harmonic plasma emission in the diffusion region and the separatrices of reconnection. (2) The perpendicular crescent-shaped EVDFs can cause multiple-harmonic electromagnetic electron cyclotron waves through the electron cyclotron maser instabilities in the diffusion region of reconnection. Our results are applicable to diagnose the plasma parameters, which are associated to magnetic reconnection in solar flares by means of radio wave observations.

Radio masers on WX UMa: hints of a Neptune-sized planet, or magnetospheric reconnection?

ABSTRACT The nearby M dwarf WX UMa has recently been detected at radio wavelengths with LOFAR. The combination of its observed brightness temperature and circular polarization fraction suggests that the emission is generated via the electron–cyclotron maser instability. Two distinct mechanisms have been proposed to power such emission from low-mass stars: either a sub-Alfvénic interaction between the stellar magnetic field and an orbiting planet, or reconnection at the edge of the stellar magnetosphere. In this paper, we investigate the feasibility of both mechanisms, utilizing the information about the star’s surrounding plasma environment obtained from modelling its stellar wind. Using this information, we show that a Neptune-sized exoplanet with a magnetic field strength of 10–100 G orbiting at ∼0.034 au can accurately reproduce the observed radio emission from the star, with a corresponding orbital period of 7.4 d. Due to the stellar inclination, a planet in an equatorial orbit is unlikely to transit the star. While such a planet could induce radial velocity semi-amplitudes from 7 to 396 m s−1, it is unlikely that this signal could be detected with current techniques due to the activity of the host star. The application of our planet-induced radio emission model here illustrates its exciting potential as a new tool for identifying planet-hosting candidates from long-term radio monitoring. We also develop a model to investigate the reconnection-powered emission scenario. While this approach produces less favourable results than the planet-induced scenario, it nevertheless serves as a potential alternative emission mechanism which is worth exploring further.

Can the Auroral Kilometric Radiation be a Self‐Organized Criticality System?

AbstractWe present statistical results for the analysis of 240 Auroral Kilometric Radiation (AKR) dynamical spectra containing 3,750 short–time AKR bursts each, and recorded on‐board Interball–2 mission (POLRAD and MEMO experiments). Data were collected between December 1996 and February 1998. We found that frequency distribution of short AKR bursts as a function of their Power Spectral Density, shows for intense bursts a powerlaw fall. This is the hallmark of the Self–Organized Criticality (SOC) systems. In our case a simple analytical Logistic Growth SOC model can reproduce increments of Electron Cyclotron Maser instability expected for AKR.

Terahertz radiation generated by electron-beam-driven plasma waves in a transverse external magnetic field

It is found that the plasma waves driven by an electron beam rotating in a magnetized overdense plasma can be converted into terahertz electromagnetic radiation by new mechanisms. According to the particle-in-cell simulation results, the radiation modes include a dominant extraordinary (X) mode at twice the plasma frequency 2fp and a subordinate X mode at fp. The 2fp radiation can be generated by the coupling of a beam mode and a scattered upper-hybrid (UH) mode, or by the coupling of a beam mode and a right-handed X mode. Here, the beam mode can be a UH mode or a left-handed X mode driven by the beam under the Cherenkov condition, and the right-handed X mode can be induced by high-order electron cyclotron maser instability. The fp radiation is the right- or left-handed fundamental X mode escaping from the plasma boundary. This study also shows that the breakdown of beam modulation is responsible for the radiation attenuation. The scheme proposed in this paper can be applied in high-power THz radiation sources and diagnosis of magnetized plasmas.

Plasma Emission versus Electron Cyclotron Maser Emission due to Power-law Energetic Electrons in Differently Magnetized Coronal Plasmas

Abstract By using self-consistent 2.5-dimensional particle-in-cell simulations, we study the excitation efficiency of electromagnetic waves by power-law energetic electrons with an anisotropic pitch-angle velocity distribution, which can simultaneously trigger the Langmuir and electron cyclotron maser instabilities, in differently magnetized coronal plasmas. It is found that the (transverse) electromagnetic waves can be excited much more efficiently in the case of strongly magnetized plasmas with ω ce > ω pe than that of weakly magnetized plasmas with ω ce < ω pe, where ω ce and ω pe are the electron cyclotron frequency and the electron plasma frequency, respectively. In particular, in a weakly magnetized plasma the electromagnetic wave is hardly excited effectively via the nonlinear coupling of Langmuir waves; although the Langmuir waves can be generated by the power-law energetic electrons, implying that the so-called plasma emission does not effectively work. These results can be helpful for us to better understand the physical mechanism of solar radio bursts.

Harmonic Maser Emissions from Electrons with Loss-cone Distribution in Solar Active Regions

Abstract Electron cyclotron maser emission (ECME) is regarded as a plausible source for coherent radio radiations from solar active regions (e.g., solar radio spikes). In this Letter, we present a 2D3V fully kinetic electromagnetic particle-in-cell simulation to investigate the wave excitations and subsequent nonlinear processes induced by the energetic electrons in the loss-cone distribution. The ratio of the plasma frequency to the electron gyrofrequency ω pe/Ωce is set to 0.25, adequate for solar active region conditions. As a main result, we obtain strong emissions at the second-harmonic X mode (X2). While the fundamental X mode (X1) and the Z mode are amplified directly via the electron cyclotron maser instability, the X2 emissions can be produced by nonlinear coalescence between two Z modes and between Z and X1 modes. This represents a novel generation mechanism for the harmonic emissions in plasmas with a low value of ω pe/Ωce, which may resolve the escaping difficulty of explaining solar radio emissions with the ECME mechanism.