Detection of a cyclotron line in the radio spectrum of a solar active region and its interpretation

Observations of the active region AR 7962 obtained at 2–32 cm on the RATAN-600 radio telescope on May 10–12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background of the smooth spectrum of the local source associated with sunspots. This narrow-band emission is identified with a bright, pointlike, high-frequency source at 1.7 cm recorded on maps made using the Nobeyama radio telescope. The characteristics of the observed line (lifetime 3 days, brightness temperature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the third harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be due to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data can be used to probe the magnetic field, kinetic temperature, and electron density in the radiation source in the corona.     

Solar 3.04 cm hydrogen line emission revealed in observations of the active region NOAA 10105

RATAN-600 observations of a microwave source located above the active region NOAA 10105 obtained on September 7–20, 2002 with a frequency resolution of ～10% have revealed a spectral feature near 3.04 cm that can be interpreted as a neutral hydrogen line. This feature was observed September 11, 2002, in both absorption and emission, and was detected in the spectra of various portions of the source (sunspot, flocculus, and background). The maximum line depth of (35 ± 5)% of the source brightness was observed at the start of the observations (9.2^h UT) in the flocculus in absorption. The line intensity decreased rapidly with time, becoming less than the measurement errors by 9.7^h UT. It is most likely that the 3.04 cm emission is related to a 2B chromospheric flare (M2.2 X-ray burst) observed at ～7.5^h UT in the floccular field, near the main sunspot of NOAA 10105. In this case, the total duration of the event was about two hours. These observations are consistent with earlier statistical studies, and refines these based on data with higher spatial resolution. Recommendations for further observational studies of the solar 3.04-cm hydrogen line are presented; requirements for theories of the 3.04 cm line taking into account nonequilibrium states of the active-region plasma are indicated.     

The properties of long-term sunspot oscillations

It is shown that neglecting the motion of sunspots in the plane of the sky in pixels of SOHO MDI magnetograms obtained for the vertical direction results in false periods of 700–1300 min in the long-term oscillations of the magnetic fields of sunspots observed near the central meridian (the Y artefact). The oscillation mode proposed by Efremov, Parfinenko, and Solov’ev in 2012 to be the lowest-frequency sunspot mode is an artefact. A proposed technique for monitoring this artefact using wavelet transforms can be used to study oscillation periods in the range 15 min < T < 500 min. The observational dependence of the oscillation frequency of the sunspot magnetic field on the field strength is constructed using observations of 45 sunspots. This dependence shows a multimode behavior that is consistent with earlier ground observations. One interpretation of this dependence based on the existence of four geometrical oscillation modes detected earlier is proposed.     

The three-dimensional structure of the corona above active region NOAA 9591 from microwave observations

We study the active region NOAA 9591 using observations at 1.92–10.17 cm obtained on two large Russian radio instruments: the RATAN-600 radio telescope and the Siberian Solar Radio Telescope. The active region was associated with an isolated spot at the photospheric level, whose magnetic field had a well-defined Delta configuration. The radio observations show that the structure of the coronal source located above the spot cannot be described in a simple (unipolar) cyclotron model. A comparison with X-ray observations indicates that the three-dimensional structure of the corona above the spot can be represented as a strongly elongated loop whose apex resembles a cusp brightening (a qualitative model for the structure is presented). Unexpectedly, radiation with a high degree of polarization (～25%) was detected far (～100 000 km) from the photosphere. The need for a quantitative model for coronal sources above the strong Delta-configuration magnetic fields, which are known to play an important role in active solar processes (flares, phenomena such as coronal-mass ejections) is outlined. Thanks to its simple morphology, which enabled the identification of a pure Delta configuration, the active region NOAA 9591 provides high-quality observational material for the creation of such a model.     

Internal gravity waves over an oscillating sunspot

Internal gravity waves excited in the overlying atmosphere by the proper vertical oscillations of an entire sunspot are studied. For simplicity, the oscillations of the sunspot are introduced into the model through the lower boundary conditions, by specifying oscillations in the vertical velocity. The characteristic radius of the oscillating region is assumed to equal the spot size and the frequency to correspond to the long-period natural oscillations of the spot. Results of numerical computations are presented. It is shown that internal gravity waves propagate nearly in the horizontal direction. Therefore, immediately above the spot, the wave energy drops exponentially with the height, in good agreement with the available observational data. The level where the amplitude and energy density of the wave are maximum rises slowly with distance from the spot.     

The altitude structure of the coronal magnetic field of AR 10933

The magnetic fields of solar active regions are analyzed using a method based on comparing the spatial structures of the reconstructed magnetic field and of the radio emission of the active region. Two approaches are used: comparing the radio size of the active region and the corresponding size calculated using the reconstructed magnetic field, and comparing the radio spectra that are observed and calculated using the reconstructed magnetic field. Overall, the calculated sizes and spectra correspond fairly well to the observational data, making it possible to estimate physical parameters of the emitting region, such as the electron density and temperature.     

Sign reversals of the polarization of the microwave emission of sunspot radio sources

Thermal cyclotron emission features from a sunspot-associated source in a solar active region are considered in the framework of a three-level temperature model with a hot layer. The images of the source have a complex fine structure, with rings with different brightness temperatures and different signs of circular polarization. The proposed model suggests the possibility of a double or multiple reversal of the sign of polarization, as well as a significant increase of the fluxes in the 3–4 cm range, as is typical of active regions associated with proton flares. Energetic particles of the halo cannot provide the required temperatures of the layers. Alternative heating mechanisms are discussed.     

Dissipation of diamagnetic currents and plasma heating in coronal magnetic loops

SOHO and TRACE data have shown that the coronal plasma is heated most actively near sunspots, in magnetic loops that issue from the penumbral region. The source of heating is nonuniform in height, and its power is maximum near the footpoints of the magnetic loops. The heating process is typically accompanied by the injection of dense chromospheric plasma into the coronal parts of the magnetic loops. It is important that the radiative losses cannot be compensated for via electron thermal conduction in the loops, which have temperatures of 1.0–1.5 MK; therefore, some heating source must operate throughout the entire length of the loop, balancing radiative losses and maintaining a quasi-steady state of the loop over at least several hours. As observations show, the plasma density inside the loops exceeds the density of the ambient plasma by more than an order of magnitude. It is supposed that the enhanced plasma density inside the loops results from the development of the ballooning mode of a flute-type instability in the sunspot penumbra, where the plasma of the inner sunspot region, with β _i ? 1, comes into contact with the dense chromospheric plasma, which has β _e ? β _i (β is the gas-to-magnetic pressure ratio). As the chromospheric plasma penetrates into the potential field of the sunspot, the generated diamagnetic currents balance the excess gas pressure. These currents efficiently decay due to the Cowling conductivity. Even if neutrals are few in number in the plasma (accounting for less than 10^?5 of the total mass density), this conductivity ensures a heating rate that exceeds the rate of the normal Joule dissipation of diamagnetic currents by 7–8 orders of magnitude. Helium is an important factor in the context of plasma heating in magnetic loops. Its relatively high ionization potential, while not forbidding dielectronic recombination, ensures a sufficiently high number of neutrals in the coronal plasma and maintains a high heating rate due to the Cowling conductivity, even at coronal temperatures. The heating results from the “burning-out” of the nonpotential component of the magnetic field of the coronal magnetic loops. This mechanism provides the necessary heating rate for the plasma inside the loops if the loops are thin enough (with thickness of the order of 10⁵–10⁶ cm). This may imply that the observed (1–5) × 10⁸-cm-thick loops consist of numerous hot, thin threads. For magnetic loops in hydrostatic equilibrium, the calculated heating function exponentially decreases with height on characteristic scales a factor of 1.8 smaller than the total-pressure scale height, since the scale heights for the total pressure and for the ⁴He partial pressure are different. The heating rate is proportional to the square of the plasma pressure in the loop, in agreement with observational data.

     

Form of the latitude distribution of sunspot activity

The spatial (latitude) distribution of sunspots is studied, including its dependence on solar activity. It is shown that the latitude distributions of sunspots for a given year can be approximately described by the normal law, with its variance being a linear function of the current level of solar activity. Thus, an increase in activity is accompanied by an expansion of the zone of solar activity, in good agreement with earlier results. As the solar activity increases, the width of the zone of sunspot generation and the latitude maximum of the sunspot density grow somewhat more slowly than the number of sunspots, in agreement with observations. The results obtained can be used to reconstruct the spatial distributions of sunspots in the past, interpret the magnetic activity of stars, and address the requirements of the dynamo theory in the form of constraints imposed on models of cyclicity.     

Short-period variations in the proper motions of sunspots from SOHO (MDI) observations

Short-period (1–60 min) variations in the coordinates of the centers of gravity of isolated sunspots are analyzed. The sunspot coordinated were determined using two sets of observational data—magnetograms and intensities—obtained by SOHO (MDI) on December 6, 1998, from 01:00 to 21:57 UT with temporal resolution 60 s and spatial resolution 0.6″/pixel. A slow drift in the sunspot coordinates was removed using a low-frequency filter with a 61-min integration window. The guiding errors (RMS～0.014″) were determined by analyzing correlated motions in pairs of sunspots, and were removed from the time series before determining the sunspot proper motions. Based on the calculated power spectra for the sunspot proper motions, two period intervals containing appreciable power were identified. One coincides with the well-known 5-min acoustic solar oscillations. The concentration of power in this interval is greater for the coordinate variations derived the magnetograms than those derived from the intensities; the harmonic amplitude for some peaks reaches ～±30 km. The other spectral interval corresponds to periods exceeding 30 min. Overall, the rms short-period variations in the sunspot proper motions are 9.9±2.2 and 16.7±7.6 km (0.014″±0.003″ and 0.024″±0.010″) for the magnetogram and intensity data, respectively.     

Study of the Magnetic Properties of Sunspot Umbrae

Astronomy Reports - The paper presents the results of the study of sunspots, obtained by the authors and other researchers in recent years. The results regarding the atmosphere above sunspot...     

Modeling the frequency dependence of the durations of solar radio spikes

The dependence of the durations of millisecond narrow-band solar radio spikes τ on the frequency f at which they are emitted is studied. A regression analysis of available experimental data at 0.3–3 GHz yields the relationship τ ∝ f ^?1.29±0.08, refining the known Güdel-Benz law. This relationship differs at a statistically significant level from the intuitive expectation τ ∝ f ^?1; this requires a physical interpretation. The frequency dependences of the pulse durations for various modes of cyclotron maser emission near the first, second, and third harmonics of the electron cyclotron frequency have been modeled. The model relationships for different cyclotron maser emission modes are different; only the results of the model for the second harmonic of the extraordinary wave are consistent with the observed behavior. Thus, this wave is a probable candidate for the dominant mode in spike bursts.     

Phase dependence of the spectral properties of the X-ray binary 4U0115+63: Observations with the Beppo-Sax satellite

The phase dependence of the spectral properties of the X-ray source 4U0115+63 is analzed based on X-ray observations obtained with the Beppo-Sax observatory. The spectrum contains a feature that is interpreted as a cyclotron absorption line. Deep lines of the first three orders were always observed, while the fourth-order line was detected only at certain phases. The parameters of the cyclotron feature depend strongly on the phase, while the remaining spectrum varies only relatively weakly with phase. The cyclotron lines of various orders are appreciably non-equidistant, and the second-order line is nearly always deeper than the first-order line. Possible physical conditions in the active region of the source that could give rise to the observed spectrum are discussed.     

The possible escape of electron cyclotron maser radiation from hot stellar coronas through a window of transparency

The main argument against the idea that the intense radio emission observed from active regions on the Sun and flare stars is electron-cyclotron maser (ECM) radiation is that such radiation should be strongly absorbed in higher-lying layers where the condition for the cyclotron resonance at harmonics of the electron gyrofrequency is fulfilled. Cyclotron absorption lowers the efficiency of ECM radiation virtually to zero for a broad range of angles between the direction of propagation of the radiation and the magnetic field. Less severe absorption is possible only in narrow angular “windows” along (for ordinary and extraordinary waves) and perpendicular to (for ordinary waves) the magnetic field. However, the ECM radiation that is generated does not fall into these windows of transparency due to the kinematic conditions corresponding to coronal magnetic traps. We investigate the efficiency of induced scattering of ECM radiation on ions in the equilibrium plasma in the source. Under certain conditions, induced scattering leads to the formation of a condensate of ECM radiation with the direction of its wave vectors approximately along the magnetic field, enabling the escape of the radiation through windows of transparency. The most favorable conditions for this phenomenon are realized for ordinary waves. We estimate the optical depths of the sources of the ECM radiation to the scattering and the angular width of the condensate for ordinary and extraordinary waves for the cases of the flare radio emission of the star AD Leo and the sources of type I noise storms in the solar corona. In both cases, the polarization of the emergent radiation should correspond to the ordinary wave.     

基于小波变换的郑州降雨量与太阳黑子活动关系分析

为探究太阳黑子活动与地区降雨量的关联性,采用连续小波变换方法,分析了1980年以来郑州地区(新郑站点)降雨量和太阳黑子数的数据变化,并对不同时段两者的相关性进行研究;进一步对两者进行交叉小波变换和小波相干谱分析,并根据两者关系通过太阳黑子数观测值对降雨量进行预测。结果表明：(1)不同时段降雨量和太阳黑子数的相关性存在正负差异的现象。降雨量的第一主周期尺度是21 a,在此主周期尺度下得到14 a主周期;太阳黑子数的第一主周期尺度是16 a,在此主周期尺度下得到11 a主周期(与经验值相符)。降雨量与太阳黑子数的主周期相差了3 a,因此导致两者的相关性在不同时段存在正负差异。(2)降雨量与太阳黑子数在1992—2008年的8～12 a时间尺度上关联性显著,且降雨量比太阳黑子数存在规律性的时间滞后,两者在2～4 a和7～10 a的时间尺度上关联性较好;降雨量比太阳黑子数分别在1991—2004年和2006—2013年这两个时间段上呈现规律性的滞后,两者在其他时间段的各时间尺度上关联性不明显。(3)根据延迟年数经验公式,由太阳黑子数观测值对降雨量进行预测,最近的降雨量峰年在2022年附近,与2...     

Peculiarities of polarized radio emission of solar active regions

Spectra of solar-flare active regions displaying peculiarities in their polarized radio emission observed on the RATAN-600 radio telescope at 2–16 GHz are considered. An appreciable dip of the circularly polarized emission (Stokes parameter V) in the middle of the microwave range (6–12 GHz), sometimes with a reversal of the sign of the polarization, is unusual. In some cases, the ordinary emission also dominates at long microwave wavelengths. Expected peculiarities of the frequency structure of microwave sources are calculated in simple models with loops in the form of hot and cool tori. Numerical calculations of these spectra show that the above features of the polarized emission can be explained by the presence of a hot region in the solar corona. It is shown that the parameters of the spectrum of the polarized emission can be used to determine the magnetic field in this hot region and the product of the relative magnetic-field gradient and the loop thickness.     

A comparative analysis of zebra-pattern structures at frequencies from 20 to 7000 MHz

A joint analysis of several recent solar type IV radio outbursts with zebra structures and fiber bursts in their dynamical radio spectra is carried out using all available ground-based and satellite data (Yohkoh, SOHO, TRACE). Zebra structures and fiber bursts were observed at frequencies from 20 to 6500 MHz. The main relative spectral parameters and degree of circular polarization of the zebra structures and fiber bursts are nearly the same. The relative width of the zebra structures varies only slightly with frequency (≈0.003–0.005); the radio emission is radiated in the ordinary mode. New data on centimeter-wavelength zebra structures and fiber bursts testifies that they are analogous to similar structures observed at meter wavelengths. A double-plasma-resonance model for the zebra structures based on the observational dependences for the electron density and magnetic field yields a frequency dependence for the frequency separation between stripes that does not agree with the observations. Fine structure was observed together with the rise into the corona of new, hot magnetic loops, in which instabilities associated with high-frequency and low-frequency plasma waves develop. The frequency range of the fine structure in the dynamical spectra is probably determined by the extent of these new loops in the corona. The continuous transition of the fiber bursts into zebra structures and vice versa testifies to a single origin for these two structures. All the main properties of the stripes in emission and absorption can be explained if they are associated with interactions between electrostatic plasma waves and whistlers. It is possible to obtain realistic values for the magnetic-field strength of B≈160 G at a plasma level of about 3 GHz in this model.     

Active regions near the recent solar-cycle minimum: Relation between plasma heating and electrical currents

Data on small active regions on the Sun collected over three years (2007–2009) are analyzed. Under very quiescent conditions (a low X-ray background level), the shapes of the coronal loops of some active regions correspond fairly well to the shapes of magnetic-field lines calculated in a potential approximation. This is true of several active regions (e.g., the group AR 10999 in June 2008) in which no flares more powerful than B3 were observed. The radio emission of this active region detected by the RATAN-600 telescope was very weak and virtually no polarization was detected. Subflares were observed in most groups. It is demonstrated using AR 10933 (January 2007) as an example that a growth in the soft X-ray emission by up to factors of ten simultaneous with an increase in the radio flux is characteristic for such active regions. A source with the opposite polarization developed to the Northwest of the main spot in AR 10933. A series of SOHO/MDI (and also Hinode) magnetograms shows the emergence of new magnetic flux before the development of this polarized source, which continued for several hours on January 8, 2007. The current density at surfaces located at various heights is estimated based on observations of the total vector magnetic field (Hinode data) and a non-linear, force-free magnetic-field extrapolation. The height-integrated current becomes appreciably stronger at two nodes above a field neutral line, near the location of the main emerging flux. This supports the idea that the emergence of new magnetic flux is a key factor in the evolution of active regions at all stages of their existence. The development of this picture could help in elucidating the inter-relationship between current enhancements, plasma heating, and particle acceleration, in both weak active regions and strong activity complexes.     

Long-period oscillations of the line-of-sight velocities in and near sunspots at various levels in the photosphere

New observational data on long-period oscillations of the line-of-sight velocities detected via the Doppler shifts of spectral lines observed at various heights in and near sunspots are presented. The sunspots and nearby magnetic elements oscillate with periods ranging from 40 to 80 min. The oscillations in the line-of-sight velocities persist over the entire observation session (up to four hours). These results support theoretical models in which this phenomenon represents natural long-period oscillations (vertical-radial displacements) of entire magnetic elements (sunspots, pores, and magnetic knots) about some stable equilibrium positions.     

太阳活动及其对地球环境的影响

太阳活动及其对地球环境影响的研究至今已发展成一门涉及太阳物理学、空间物理学和地球物理学的边缘学科,它研究三者的关系及相互作用的过程。本文将太阳活动分成缓变型和爆发型两类,分别介绍了它们的主要成员冕洞、总辐射、太阳黑子、太阳耀斑和日冕物质抛射的性质及特征;分别讨论了这两类太阳活动对地球环境的影响,还指出了太阳活动对固体地球的作用。