Evidence for cyclotron maser emission from the Sun and stars

In recent years radiation has been observed from planets, Sun and stars that is best explained by the cyclotron maser instability; in fact, all celestial bodies that might feasibly emit and be detected by their cyclotron maser radiation have been detected. Here we review those observations, the developments in the theory, the recent work on the effiency of energy transfer by cyclotron maser radiation, and some recent and future observations that might demonstrate whether the mechanism is energetically important in solar and stellar flares.

     

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.     

太阳射电爆发现象中的电子回旋脉泽辐射

太阳射电爆发(Solar Radio Burst, SRB)是太阳高能电子与背景等离子体相互作用产生的感应辐射现象,其多样的动力学谱类型及其复杂的精细结构反映了辐射源区磁等离子体结构状态丰富的物理信息,而相关辐射机制则是解读相关物理信息的关键工具.长期以来,在SRB辐射机制的研究中一直存在着争议不决的两种主要机制,即等离子体辐射机制和电子回旋脉泽(Electron Cyclotron Maser, ECM)辐射机制.近年来,针对传统的ECM辐射机制应用到SRB现象时遇到的一些主要困难,发展了由幂律谱电子低能截止驱动和包含快电子束自生阿尔文波效应的新型ECM驱动模型,并成功应用于解释各类不同SRB动力学谱的形成机制.基于这些新型的ECM辐射模型,系统地总结了ECM辐射机制在各种不同类型SRB现象中的应用,并对它们不同动力学谱结构的形成给出了一致统一的物理解释.     

A rare centimetric spike event on 10 March 2002

A group of centimetric spikes was detected during the burst of 10 March 2002 with the spectrometer of NAOC (National Astronomical Observatories of China). These centimetric spikes are resolved clearly in the time and frequency panels. A statistical analysis shows that these spikes have very short durations (5–10 ms), narrow bandwidths (20–40 MHz), very high polarization degrees (≥90%), and relatively weak flux densities (10–40 sfu). The simultaneous 2D image observations of NoRH (Nobeyama Radioheliograph) and SSRT (Siberian Solar Radio Telescope) imply that the spikes should come from a small source region, which was located in an area with a complex magnetic field. It is also found that the spikes are polarized in the X-mode. These observations suggest that the electron cyclotron maser (ECM) is the most likely mechanism of these centimetric spikes.     

A rare centimetric spike event on 10 March 2002

A group of centimetric spikes was detected during the burst of 10 March 2002 with the spectrometer of NAOC (National Astronomical Observatories of China). These centimetric spikes are resolved clearly in the time and frequency panels. A statistical analysis shows that these spikes have very short durations (5–10 ms), narrow bandwidths (20–40 MHz), very high polarization degrees (≥90%), and relatively weak flux densities (10–40 sfu). The simultaneous 2D image observations of NoRH (Nobeyama Radioheliograph) and SSRT (Siberian Solar Radio Telescope) imply that the spikes should come from a small source region, which was located in an area with a complex magnetic field. It is also found that the spikes are polarized in the X-mode. These observations suggest that the electron cyclotron maser (ECM) is the most likely mechanism of these centimetric spikes.     

A model of ultra-fast fine structures of microwave bursts

A model is presented using the electron cyclotron maser instability driven by a loss-cone distribution (taking account of the relativistic effects for electron cyclotron frequency and electron velocity as well as Landau damping and cyclotron damping) to excite the UFFS (Ultra-Fast Fine Structures) of microwave bursts and adopting a nonlinear density wave as a trigger mechanism to explain the millisecond time structure of the UFFS.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

Emission of solar radio spikes by beam-driven cyclotron resonance

The generation of bright solar radio spikes by the beam-driven cyclotron resonance maser mechanism (the resonant interaction of an electron beam with a circularly polarized wave in a background plasma under the action of a guide magnetic field) is studied. Nonlinear effects such as radiation damping and gyrophase bunching on electron energy and momentum are responsible for the enhanced direct energy conversion between the beam and the coherent wave. Factors such as beam energy spread and pitch angle distribution are analyzed. The intense maser radiation is carried at the source by the circularly polarized wave propagating along the magnetic field. Due to the magnetic field curvature, the outgoing maser radiation converts into extraordinary and ordinary modes. The extraordinary mode suffers from plasma absorption at the second harmonic layer, whereas the ordinary mode is likely to get through.     

Radiation mechanisms of the plasma near point L1 affected by CMEs and associated microwave bursts

When coronal mass ejections (CMEs) interact with the solar corona and the interplanetary medium, emissions at different wavelengths occur. On the basis of study of the various radiation mechanisms of space plasma in the case of absence of CMEs, the radio radiation mechanisms of the plasma close to the Lagrange point L1 and affected by large CMEs from February to August 1999 are statistically analyzed. As shown by the results, the main radiation mechanisms are the Bremsstrahlung, a small amount of cyclotron radiation and a still weaker recombination radiation. Also, solar microwave bursts which are associated with CMEs in the same period are investigated. The results show that the microwave bursts are of the gradual type as well as spike bursts, and that the chief radiation mechanisms are the Bremsstrahlung, cyclotron resonance radiation, plasma radiation and electronic cyclotron maser radiation.     

A theoretical analysis of the X-ray cyclotron absorption lines of the isolated neutron star 1E1207.4-5209

As revealed by recent observations, in the X-ray continuum of the radio quiet isolated neutron star 1E1207.4-5209 there exist several equidistant absorption lines, and their energies are, respectively, 0.7, 1.4 and 2.1 keV. According to the theory of quantum cyclotron radiation under the quadrupolar approximation developed in recent years, we have clarified some existing doubts and affirmed that these lines are electron cyclotron absorption lines instead of proton cyclotron lines. Besides, the spatial orientation of the spin axis of this neutron star has been theoretically determined.     

Harmonics of Solar Radio Spikes at Metric Wavelengths

This paper presents the latest observations from the newly built solar radio spectrograph at the Chashan Solar Observatory. On July 18, 2016, the spectrograph records a solar spike burst event, which has several episodes showing harmonic structures, with the second, third, and fourth harmonics. The lower harmonic radio spike emissions are observed later than the higher harmonic bands, and the temporal delay of the second (third) harmonic relative to the fourth harmonic is about 30?–?40 (10) ms. Based on the electron cyclotron maser emission mechanism, we analyze possible causes of the temporal delay and further infer relevant coronal parameters, such as the magnetic field strength and the electron density at the radio source.     

Propagation and absorption of cyclotron maser radiation in solar microwave spike bursts

It has been argued that the loss-cone-driven electron cyclotron maser instability can account for the properties of millisecond microwave spike bursts observed during some solar flares. However, as it propagates outward from the corona, maser radiation undergoes gyroresonance absorption when its frequency is a harmonic of the local electron-cyclotron frequency. Existing analytical models using slab geometries predict that this absorption should be sufficiently strong to prevent the radiation from being seen at the observed levels, except under highly restrictive conditions or for unrealistic plasma parameters. A more comprehensive analysis is presented here to determine if and when maser radiation can escape to produce microwave spike bursts. This analysis employs numerical raytracing and incorporates propagation and absorption of fundamental maser emission in a realistic model of a coronal flux loop. It is found that ranges of physical conditions do exist under which maser radiation can escape to an observer and that these conditions are much more limiting for fundamental emission in the extraordinary ()-mode than in the ordinary (o)-mode. Detailed investigation implies that escaping radiation in the -mode is highly directional and chiefly observable toward the center of the solar disk, while escapingo-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to radiation observable near the solar limb.     

太阳射电毫秒Spike辐射窄带宽的理论探讨

在磁拱底部非线性等离子体密度波传播期间,损失锥分布的反射电子驱动着电子迴旋maser不稳定性的增长,激励出二次谐频波模,支配着太阳射电毫秒Spike辐射。根据这个理论模型,本文着重研究了太阳射电毫秒Spike辐射的频带宽度问题。对于典型参数,计算结果发现:辐射带宽一般为几MHz到几十MHz,最高为100MHz。而且通常折射出的二次谐频z模辐射带宽较窄,而二次谐频o模辐射带宽较宽。     

Evidence of harmonic microwave radiation during solar flares

First observational evidence of harmonic radiation at microwave frequencies during solar bursts is presented for the event of April 28, 1983. The recordings between 3.1 and 19.6 GHz show a typical continuum with a spectral maximum near 5.2 GHz. Superimposed fine structures with durations in the order of some seconds exhibit a very unusual spectral behaviour. Narrow-banded intensity peaks appeared at 5.2 and 11.8 GHz which were barely visible at 3.1, 8.4 and 19.6 GHz. These structures can be interpreted as harmonic emission. Harmonic radio emission can be generated either by plasma radiation, gyroradiation, electron-cyclotron maser or by nonlinear conversion processes. However, all of those mechanisms require extreme assumptions on the source and the ambient plasma in order to account for the observations.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

猎户座中SiO脉泽分布的新模型

我们研究了Orion-IRc2SiO脉泽饱和辐射谱线轮廓，找到了很好的拟和谱线轮廓的函数．在观测结果中发现，Orion－IRc2SiO脉泽饱和辐射谱线轮廓中存在着若干峰，为了解释这一现象，我们提出了一个新的分布模型，即多重分离的旋转膨胀盘壳模型.计算结果表明，这个新模型，不仅可以解释脉泽的非饱和辐射谱，而且可以解释脉泽的饱和辐射谱.     

Direct generation of solar and stellar radio bursts by energetic electron maser

A fully relativistic electron maser is proposed for the explanation of certain non-thermal solar and stellar radio bursts. This mechanism (maser synchrotron) is based on a gyroresonant interaction between waves and electrons of high energies and uses the free energy contained in an electronic distribution function that peaks for energies around 1 MeV.By a calculation of the growth rates of the three electromagnetic modes, we show that the X-mode prevails for values of _p/ _cup to 2 or 3. This result is very different from the standard cyclotron maser case where such values of _p/ _clead to quench the X-mode growth. Hence, the synchrotron maser instability appears to be a direct and efficient amplification process for considerably larger physical conditions than the cyclotron maser. In addition, the radiation, emitted over the second gyroharmonic, freely propagates without a strong reabsorbtion at the 2 _clayer. All these points can constitute major advantages of this mechanism in an astrophysical context.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Simultaneous observations of the OH maser at 1665 and 1667 MHZ in ON2

Using Nancay radio telescope in Paris-Meudon Observatory the OH masers fromON2 have been observed. The observations were made at both frequencies of1665 and 1667 MHz, and both left and right circular polarizations, and onthe central position, respectively. The observational results indicate that the physical conditions in the central part are very different. Using a new model, i.e. the distinct multiple rotating and expanding disc-ring of OH maser,the peaks of spectra have been explained. The position parameters of the OHmaser spots corresponding to the peaks in the spectral profiles have beenobtained. Our fitting and computational results indicate that the differentpeaks are superposed by the maser spot radiation at different point indifferent disc-ring with different radii. Because the mentioned differencesare caused by the physical conditions of the OH maser formingregions, using the differences, we can study the physical characters of the OH maser forming regions of ON2.     

VLBI measurements of the parallax and proper motion of U Herculis

The OH 1667 maser in the circumstellar shell around the Mira variable U Her has been observed with the VLBA at 6 epochs, spread over 4 years. By using straightforward phase referencing techniques the stellar proper motion can be measured. Preliminary analysis indicates that the parallactic motion is also detected. An important question is whether the maser spots can be assumed to be fixed with respect to the star. The observations show both evidence supporting and contradicting the idea that one maser spot is the amplified stellar image.     

The radio lighthouse CU Virginis: the spin-down of a single main-sequence star

The fast rotating star CU Virginis is a magnetic chemically peculiar star with an oblique dipolar magnetic field. The continuum radio emission has been interpreted as gyrosynchrotron emission arising from a thin magnetospheric layer. Previous radio observations at 1.4 GHz showed that a 100 per cent circular polarized and highly directive emission component overlaps to the continuum emission two times per rotation, when the magnetic axis lies in the plane of the sky. This sort of radio lighthouse has been proposed to be due to cyclotron maser emission generated above the magnetic pole and propagating perpendicularly to the magnetic axis. Observations carried out with the Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this discovery show that this radio emission is still present, meaning that the phenomenon responsible for this process is steady on a time-scale of years. The emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On the light of recent results on the physics of the magnetosphere of this star, the possibility of plasma radiation is ruled out. The characteristics of this radio lighthouse provide us a good marker of the rotation period, since the peaks are visible at particular rotational phases. After one year, they show a delay of about 15 min. This is interpreted as a new abrupt spinning down of the star. Among several possibilities, a quick emptying of the equatorial magnetic belt after reaching the maximum density can account for the magnitude of the breaking. The study of the coherent emission in stars like CU Vir, as well as in pre-main-sequence stars, can give important insight into the angular momentum evolution in young stars. This is a promising field of investigation that high-sensitivity radio interferometers such as Square Kilometre Array can exploit.     

SiO maser emission in Miras

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infrared (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in very long baseline interferometry (VLBI) observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1–0.25, which is consistent with observed phase lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13 μm appear to be closely associated in three out of four phase samples.     

Generation of auroral kilometric radiation and the structure of auroral acceleration region

Generation of auroral kilometric radiation (AKR) in the auroral acceleration region is studied. It is shown that auroral kilometric radiation can be generated by the backscattered electrons trapped in the acceleration region via a cyclotron maser process. The parallel electric field in the acceleration region is required to be distributed over 1–2 R_E. The observed AKR frequency spectrum can be used to estimate the altitude range of the auroral acceleration region. The altitudes of the lower and upper boundaries of the acceleration region determined from the AKR data are respectively ～2000 and ～9000 km.