On “geometric” effects in the microwave radiation of active regions passing through the solar disk

Two basic “geometric” effects manifest themselves as slow variations in the properties of the microwave radiation of the active region passing through the solar disk, namely 1) sign inversions of the circular polarization and 2) characteristic peaks of the brightness temperature at certain longitudes on both sides of the central solar meridian and depression near it. These effects are related to the magnetic directivity of the cyclotron radiation governing in the centimeter wavelength range above large spots. The concept of “normal” passage of the active region through the solar disk is introduced in this paper on the basis of studying the mentioned effects observed at two wavelengths, 5.2 cm (Siberian Solar Radiotelescope, Buryatiya) and 1.76 cm (Radioheliograph in Nobeyama, Japan), to separate the “normal,” or “geometric” effects, in the behavior of the microwave radiation from the evolutional effects that may cause high-power solar flares. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 8, pp. 641–659, August 2008.     

Features of Short-Period Oscillations of Microwave Emission from the Solar Active Region before the Flare

The radiation of microwave sources above sunspots at a frequency of 17 GHz gives information about the parameters of solar plasma in the regions where the magneti-field strength is B ~ 2000 G in the transition region between the chromosphere and corona. Short-period oscillations (with a period of several minutes) of microwave emission from solar active regions (ARs) reflect wave processes in magnetic flux tubes of sunspots. Short-period oscillations of microwave emission from AR NOAA 12242 before two flares on December 17, 2014 are analyzed. This analysis is based on solar radio images obtained by means of the Nobeyama Radio Heliograph with a 10″?15″ two-dimensional spatial resolution. The radio maps of the whole solar disk were synthesized in a nonstandard mode with a cadence of 10 s and an averaging time of 10 s. An increase in the power of about ten-minute oscillations of microwave radiation approximately 40 to 50 min before the M1.5 flare (01: 00 UT) is found. On the same day, an increase in the power of ten-minute oscillations is observed about 60 min before the M8.7 flare (04: 42 UT). This effect is similar to the effect found earlier by two groups of authors independently for three-minute oscillations—namely, they observed a sharp increase in three-minute oscillations 15 to 20 min before the radio burst accompanying the flare. The effect in question may be interpreted as an relationship ofMHD waves propagating along the magnetic flux tube of a sunspot and the onset of the solar flare.     

Manifestation of microflares in modulation of the microwave radiation of the coronal magnetic loops

We reveal a modulation the frequency of which varies quasi-periodically with a variation period of about 150 s when analyzing the low-frequency modulation of the intensity of the solar microwave radiation. It is shown that this modulation can be a manifestation of the microflares occurring in the coronal magnetic loop. The interaction of 5-min photospheric oscillations with currentcarrying loops results in modulation of the electric current in the loop and, as a result, generation of the inductive electric field. This leads to the emergence of a group of runaway electrons and their acceleration by the electric field. The most favorable conditions for the acceleration appear near the loop top, where the Dreicer field is minimum. When the electrons accelerated in the region near the top of the loop reach its footpoints, a microflare is observed. The radiation loss of the loop and the energy which is released in the loop as a result of the microflares are compared It is shown that for some loop parameters, the radiation loss can be compensated completely and the heating can exceed the radiation loss.     

Analysis of the uniform magnetic field free-electron laser with scalarized photons in the microwave-spectral region

The recently developed concept of scalarized photons (formally photons of any polarization) is used to analyze the spontaneous and stimulated emission in the uniform magnetic field free-electron laser in the microwave spectral region. In fact, this free-electron laser is the simplest of many other, wiggler and wiggler-free free-electron lasers whose analyses could be done with scalarized photons in the small signal regime and whose physical parameters can be conveniently chosen for radiation to be generated in the microwave spectral region. As to the uniform magnetic field free-electron laser, which is treated here in some detail, with the electron beam energy of up to 10 MeV and the uniform magnetic field of up to 4 Tesla, the radiation (occurring with the fundamental and higher harmonic frequencies) can cover easily a 10 to 10,000 GHz spectral region.     

Power of microwave generation in an ultrarelativistic electron beam in the regime of virtual cathode formation in an externally applied magnetic field

A numerical study of the output power characteristics of microwave radiation from a relativistic electron beam (REB) with a virtual cathode in the presence of externally applied longitudinal magnetic field is performed. Typical dependences of the output microwave power of the relativistic vircator system on the external magnetic field strength are obtained, showing a number of local maxima. It is found that the characteristic behavior of the radiation power is determined by the conditions and mechanisms of virtual cathode formation in the presence of external longitudinal magnetic field and a self-magnetic REB field.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Giant magnetoresistance oscillations induced by microwave radiation and a zero-resistance state in a 2D electron system with a moderate mobility

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10⁶ cm²/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10⁶ cm²/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation.     

Helical Cerenkov effect, a novel radiation source

The observability of the helical Cerenkov effect as a novel radiation source is discussed. Depending on the value of the index of refraction of the medium, the strength of the uniform magnetic field, and the electron beam energy, helical Cerenkov radiation can occur in the same spectral regions as the ordinary Cerenkov effect, that is, from microwave to visible wavelengths. From the kinematics point of view, I argue that for a microwave wavelength of 10^-1cm this effect should be observable in a medium with an index of refraction of 1.4, with a beam energy of 3 MeV, and a uniform magnetic field of 4 T. On the specific level, however, for the sake of simplicity, I discuss the observability of this effect for visible light with the central wavelength of 5×10^-5 cm which can be achieved with 2 MeV in beam energy, silica aerogel as a medium (with an index of refraction of 1.075), and uniform magnetic fields from 5 to 10 T. For a 10-T magnetic field, I calculate that in the visible region of 250 to 750 nm an electron will produce a photon per 10 cm of traveled length. As to the stimulated helical Cerenkov emission, I estimate that respectable gains are possible even if the beam passes close to the dielectric rather than through it. In addition to being potentially a new radiation source, the helical Cerenkov effect could possibly be used as a detector of radiation by energetic electrons that are trapped in a medium by strong magnetic fields     

Magnetoresistance oscillations due to Zener tunneling and microwave radiation in a 2D electron gas in GaAs quantum well with AlAs/GaAs superlattices barriers

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces magnetoresistance oscillations periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the covered frequency range depends on the microwave radiation power.     

Prevalence of the Ordinary Mode in Microwave Radio Emission from Solar Active Regions

We propose an interpretation of some observations of the ordinary (o) mode in the microwave emission of slowly-varying (s) radio sources associated with active regions in the solar atmosphere. As a rule, the extraordinary (e) mode dominates in the s-component, which is fully explained by the generally accepted theory of its origin. This theory is based on the cyclotron and bremsstrahlung mechanisms of radiation of thermal electrons in an inhomogeneous plasma of the solar atmosphere above sunspots, where the magnetic field decreases and the kinetic temperature increases with height. It is shown in this work that the prevalence of the ordinary mode is due most probably to the presence of a region with a negative gradient of the kinetic temperature in the atmosphere above the sunspot. We analyze the qualitative frequency dependences of the brightness temperature and polarization of the radiation of a hot and cold (compared to the ambient plasma) filament in the s-component source, as well as the possible reasons for the polarization reversal at a certain frequency. Special attention is paid to the active region NOAA 4741 with the unipolar sunspot described in [1, 2]. Criteria permitting one to judge which of two effects (linear wave coupling or presence of a region with a negative temperature gradient) is responsible for the o-mode in the s-component are discussed.     

Optimization of the parameters of a virtual-cathode oscillator with an inhomogeneous magnetic field

A two-dimensional numerical model is used to study the generation of powerful microwave radiation in a vircator with an inhomogeneous magnetic field applied to focus a beam. The characteristics of the external inhomogeneous magnetic field are found to strongly affect the vircator generation characteristics. Mathematical optimization is used to search for the optimum parameters of the magnetic periodic focusing system of the oscillator in order to achieve the maximum power of the output microwave radiation. The dependences of the output vircator power on the characteristics of the external inhomogeneous magnetic field are studied near the optimum control parameters. The physical processes that occur in optimized virtual cathode oscillators are investigated.     

Influence of external nonuniform magnetic field on the spectral characteristics of a virtual cathode oscillator’s output radiation

The influence of an external non-uniform magnetic field created using a periodic magnetic focusing system on the spectral characteristics of a virtual cathode oscillator’s output radiation is investigated by means of numerical simulation. It is shown that raising the magnetization intensity of a single magnetic ring or the number of magnetic rings leads to an increase in the output microwave radiation power spectrum’s irregularity. As consequence, the output microwave signal exhibits wide-band oscillations with a bandwidth close to one octave.     

Localized pits and peaks in forbush decrease, associated with stratified structure of disturbed and undisturbed magnetic fields

Summary Forbush decrease (FD) is generally interpreted as a result of diffusion-convection of cosmic rays in a disturbed interplanetary magnetic field associated with the magnetohydrodynamic shock wave caused by solar flare. In this paper, we point out that a large number of FDs contain an isolated region or regions with pit-type time profile, in which cosmic rays are not in a diffusion-convection state but in a trapped state in undisturbed, uniform and strong magnetic field perpendicular to the solar wind. The trapped state is also characterized with a large ratio of the magnetic to ion thermal energy. The median duration time of the state is about 8 hours. About half of these states are associated with the northward (or southward) magnetic field, while the other half with the eastward (or westward) magnetic field. Flares responsible for the former state seem to be concentrated in an eastward region from about 30°W on the solar disk, while those for the latter state seem rather symmetric with respect to the centre of the solar disk. It is suggested that the trapped state is produced inside a magnetic tube of force which is not of a small scale such as that of the magnetic bubble pointed out by Klein and Burlaga, but of a large scale, having a horseshoe structure with its ends supposed to be connected to somewhere in an inner region near the Sun and with its cross-section supposed to be of a thin filament with radial and transverse dimensions of ≈0.1 a.u. and ≈1.1 a.u. at the Earth’s orbit. This belt-like tube of force is supposed to be produced on the solar surface or near the Sun and to be carried out by solar wind in a frozen state, trapping in itself low-density cosmic rays near the Sun. In addition to the pits, we point out also the existence of some peaks which are observed not only in the trapped region but also in a region of extremely disturbed magnetic field neighbouring in between two trapped regions. It is suggested that cosmic rays in the region of the latter type are supposed to be guided freely (or easily) from outer space through a path with similarly disturbed magnetic state, and therefore, they could maintain their density in the region always higher than in the neighbouring regions. Two kinds of cosmic-ray-guiding mechanism in the above can be regarded as being at opposite poles.     

The effect of bound states in microwave waveguides on electromagnetic wave propagation

The transmission of a TE microwave field with a frequency ω through Λ, T, and X waveguide junctions filled with a ferromagnetic is considered. These junctions are known to have bound states with below-cutoff frequencies. A probing microwave radiation with a frequency Ω applied to the scattering region generates magnetic oscillations with frequencies ω+nΩ (where n=0, ±1, ±2, ...), which resonantly combine with the bound waveguide states. This effect provides for a new method of studying bound waveguide states and efficiently controlling the transmission of microwave radiation.     

Absolute negative resistance in a nonequilibrium two-dimensional electron system in a strong magnetic field

The effect of microwave electromagnetic radiation on the resistance of the 2D electron gas in a GaAs/AlAs heterostructure in a strong magnetic field is investigated. It is shown that, under the nonequilibrium conditions caused by microwave radiation, the aforementioned 2D system exhibits giant oscillations of its resistance with varying magnetic field. When the measuring current density is small, an increase in the microwave power leads to the appearance of an absolute negative resistance at the main minimum of these oscillations, which lies near the cyclotron resonance. The experimental data are found to be in qualitative agreement with the theory of multiphoton photoinduced impurity scattering [J. Inarrea and G. Platero, Appl. Phys. Lett. 89, 052109 (2006)].     

A nonlinear response of the BiSrCaCuO single crystal in the microwave region

Third-harmonic microwave radiation of the BiSrCaCuO superconducting single crystal was studied. Two modes of microwave field-sample interactions were observed. In a weak field, a strong increase in the intensity of radiation after switching on a constant magnetic field, a hysteresis between opposite scan directions, and different harmonic amplitudes depending on the conditions of cooling (in the presence or absence of a magnetic field) were observed. These observations can be described by the generalized Ginzburg-Landau functional taking into account higher spatial derivatives of the order parameter. At a high intensity of incident waves, a magnetic field almost did not influence third-harmonic radiation, and, accordingly, hysteresis was absent. This is likely to be evidence that, at high powers, third-harmonic radiation arises as a result of generation of vortices under the action of a high-frequency magnetic field.     

Increase in the energy efficiency of a pulsed-periodic relativistic backward wave oscillator with a modulating resonant reflector

An efficient microwave oscillator (320 MW and 7.9 GHz) that generates microwave pulses with a duration of 90 ns is developed using optimization of an electron-wave system and decompression of the longitudinal magnetic field with a maximum induction of 0.62 T in the region of an explosive-emission cathode and a lower field (0.36 T) with respect to cyclotron resonance in the slow-wave structure. In a packet (up to 10 ns) repetitively-pulsed (100 Hz) regime, the maximum conversion efficiency of the electron-beam power to microwave radiation is 27%. The mean energy of the radiation pulse (23 J) is about 18% of the pulse energy of high-voltage oscillator.     

Microwave spectroscopy of the Zeeman effect in Rydberg atoms of sodium

Experimental results of studying the spectrum of the microwave 37P-37S transition of Rydberg sodium atoms in a weak magnetic field (≤7 G) are reported. The populations of the Rydberg states were measured using the method of selective ionization with a pulsed electric field. When the magnetic field was parallel to the ionizing electric field, a good agreement between the calculated and experimental spectral shapes was observed, making it possible to determine the unknown polarization of the microwave radiation. In the case of the orthogonal configuration of the fields, the resonance structure was suppressed in the field ionization signals due to the strong influence of the magnetic field on the electron trajectories in the detection system.     

Magnetic resonance spectrum of a two-phase state in single crystals of La0.7Pb0.3MnO3 lanthanum manganite

Two absorption lines are observed over a wide temperature range below T _c in the magnetic resonance spectrum of an La_0.7Pb_0.3MnO₃ single crystal. These lines correspond to two magnetic phases in the sample. The frequency-field dependence of spectra obtained in the range of microwave radiation frequencies 10–77 GHz allows these phases to be interpreted as ferromagnetic and paramagnetic phases. The phase volume ratio depends on the temperature and the magnitude of the external magnetic field. Features in the temperature behavior of parameters of the magnetic absorption line are observed in the region of the highest magnetic resistance of the sample. The results are interpreted within the mechanism of electronic phase separation.