Search

We analysed a GOES C6.2 class solar flare that occurred on 27 October 2003 at the heliographic position S20E29. Utilising a database developed from a simplified 3D magnetic loop model, we inferred a set of geometric and physical parameters to characterise the flare. This model, based on general properties of known solar flares and adjusted for instrument resolution, was used to replicate observations from the Nobeyama Radio Polarimeter (NoRP) spectra and the Nobeyama Radioheliograph (NoRH) brightness maps. We identified a possible range of parameters by comparing observed spectra and brightness distribution maps with modelled counterparts. The analysis involved computing a weighted mean of the one hundred best-fit model parameters, ranked by increasing χ 2 values.Across six discrete time intervals representing the gradual, impulsive, and decay phases of the flare, we observed slight variations in values for ten analysed parameters, with notable exceptions at the burst maxima, including an energy spectral index of approximately ∼ 2.34 , a photospheric magnetic field strength of around ∼ 2 308 G , and a non-thermal electron density of about ∼ 10 7.11 cm - 3 . In general, our results replicated the soft-hard-soft behaviour in the gradual-peak-decay phases of a solar flare. Therefore, we infer that the physical parameters (magnetic field strength, energy spectral index, and non-thermal electron density) were accurately recovered. However, fitting geometric parameters such as inclination, asymmetry, azimuth, loop radius, loop height, and footpoint separation proved challenging due to the limited resolution of the instruments, which affects the precise determination of loop-like flare geometries. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter Array (ALMA) allows for solar observations in the wavelength range of 0.3–10 mm, giving us a new view of the chromosphere. The measured brightness temperature at various frequencies can be fitted with theoretical models of density and temperature versus height. We use the available ALMA and Metsähovi measurements of selected solar structures (quiet sun (QS), active regions (AR) devoid of sunspots, and coronal holes (CH)). The measured QS brightness temperature in the ALMA wavelength range agrees well with the predictions of the semiempirical Avrett–Tian–Landi–Curdt–Wülser (ATLCW) model, better than previous models such as the Avrett–Loeser (AL) or Fontenla–Avrett–Loeser model (FAL). We scaled the ATLCW model in density and temperature to fit the observations of the other structures. For ARs, the fitted models require 9%–13% higher electron densities and 9%–10% higher electron temperatures, consistent with expectations. The CH fitted models require electron densities 2%–40% lower than the QS level, while the predicted electron temperatures, although somewhat lower, do not deviate significantly from the QS model. Despite the limitations of the one‐dimensional ATLCW model, we confirm that this model and its appropriate adaptations are sufficient for describing the basic physical properties of the solar structures. [ABSTRACT FROM AUTHOR]

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.

Newly available high-resolution imaging of solar radio emission at many closely spaced frequencies and times provides new physical insight into the processes, structure, and dynamics of the solar atmosphere. The observational advances have spurred renewed interest in topics dating from the early days of solar radio astronomy and have led to considerable advances in our knowledge. Highlights of recent advances include the following: Quantitatively measuring the dynamic magnetic field strength, particle acceleration, and hot thermal plasma at the heart of solar flares and hinting at the processes that relate them. Resolving in space and time the energization and transport of electrons in a wide range of contexts. Mapping the magnetized thermal plasma structure of the solar chromosphere and corona over a substantial range of heights in active and quiet regions of the Sun. This review explains why solar radio imaging spectroscopy is so powerful, describes the body of recent results, and outlines the future work needed to fully realize its potential. The application of radio imaging spectroscopy to stars and planets is also briefly reviewed. [ABSTRACT FROM AUTHOR]

Solar accelerated electron beams, a component of space weather, are emitted by eruptive events at the Sun. They interact with the ambient plasma to grow Langmuir waves, which subsequently produce radio emission, changing the electrons' motion through space. Solar electron beam–plasma interactions are simulated using a quasilinear approach to kinetic theory to probe the variations in the maximum electron velocity [ Ξ ] responsible for Langmuir wave growth between the Sun's surface and 50 R⊙ above the surface. We find that it peaks at 5 R⊙ at 0.38 c and decreases as r − 0.5 to 0.16 c at 50 R⊙. The role of the initial beam density [ n beam ] and velocity spectral index [ α ] on the energy density of the beam and Ξ is extensively studied. We show that a high spectral index yields a lower Ξ , while a high n beam yields a higher Ξ , and vice versa. We observe at different energy channels that below 60 keV, electrons arrive up to 0.75 minutes earlier than expected at 13 R⊙ while higher energy electrons propagate scatter free in the plasma. A special focus on the associated Type III radio burst shows that the energy range [ Δ E ] of electrons producing Langmuir waves evolves from 7 keV to 1 keV between 0 and 28 R⊙. Understanding the transport effect on the electron beam kinetics and arrival time at Earth has space weather implications. The results of this simulation can be tested against readily available in-situ data from Solar Orbiter and Parker Solar Probe. [ABSTRACT FROM AUTHOR]

Using the radio spectra of the 2 April 2022 eruptive flare, we analyze a group of highfrequency type III bursts by our new wavelet method. In this analysis, we found a multi-periodicity of these bursts that is interpreted by the electron beams accelerated in the fragmented magnetic reconnection in the rising magnetic rope. We propose that each period in these type III bursts is a result of the periodic interaction of sub-ropes formed in the rising magnetic rope. In each interaction, the period depends on the diameter of interacting sub-ropes and local Alfvén velocity. This interpretation is supported by detection of the specific EUV structure which was, according to our knowledge, observed for the first time. All proposed processes occur in the rising magnetic rope. Thus, this flare deviates from the standard flare model, where the main magnetic reconnection is located below the rising magnetic rope. [ABSTRACT FROM AUTHOR]

Solar radiation is emitted in the wide frequency range from gamma to radio emission. Hence, joint observations from different instruments and frequency ranges let us to complete full representation of the evolution of solar events and confirm authenticity of data from specific instruments. In this article, we present comparative analysis data from Solar Spectropolarimeter of Meter Range wavelengths (SSMD) and other instruments. For the investigation we have chosen three solar events registered with SSMD in 2019: type III radio bursts on May 6 from 05:05 UT to 05:12 UT, type II radio bursts on May 6 from 05:12 UT to 05:19 UT, and type III radio bursts on April 14 from 05:03 UT to 05:21 UT. For comparison we apply data from the following instruments: SSMD, SRH (Siberian Radioheliograph), SDO/AIA (Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO)), Wind/WAVES (WAVES investigation on the WIND spacecraft), ground-based spectropolarimeter AMATERAS, spacecraft GOES-14. As a result, we can conclude that they are in good correlation with each other and correspond to outstanding characteristics for each event. The investigation also confirms the SSMD data authenticity.

The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of the Sun. In particular, ALMA’s ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimeter wavelengths and future polarization measurements has great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long-standing questions about the structure, dynamics, and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun’s highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modeling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimizing ALMA’s solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enable the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modeling of the Sun and their potential and challenges, together with selected examples.

An investigation of the generation mechanism for stripes-pattern radio spectra is important for an understanding of the dynamics of non-thermal electrons in several astronomical objects, including the Sun, Jupiter, and the Crab Pulsar. A new analytical study is carried out to identify the plasma characteristics of fiber- and zebra-pattern emission sources without an underlying density or magnetic model. The analysis demonstrates that the source region of the stripes emission is located underneath the reconnection point, where the ratio s of the instability growth rate to the electron gyrofrequency ωc does not equal unity; that is, s  = k ⊥ v ⊥/ωc ≠ 1. When | s | < 1, the plasma condition of the source region becomes k ⊥ v ⊥ < ωp < ωc, where ωp is the plasma frequency, and the emission source is likely to produce a fiber radio burst. For | s | > 1, the plasma condition of the source region is ωc < ωp < k ⊥ v ⊥, and the emission source is likely to produce zebra-pattern emission. This indicates that the magnetic field in the source region of zebra-pattern radio emission is weak and it is relatively high in the source region of fiber-pattern emission. An approach is applied to estimate the plasma parameters of a zebra-pattern emission source observed on 2011 June 21. The behaviour of the blasted medium, which is produced by magnetic reconnection, is investigated. The results show that the blasted medium propagates isothermally as a sausage-like wave for a short time during the emission. The study discusses the conditions for producing different types of striped radio emission and provides a simple computational approach that could be useful in a number of astronomical contexts. [ABSTRACT FROM AUTHOR]

Nanoflares and the shock formation of magnetohydrodynamic waves in the solar chromosphere have been considered as key physical mechanisms of the heating of the chromosphere and corona. To investigate candidates of their signature in the mm-wavelength, a tiny active region located on the solar disk was observed with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm, coordinated with observatories on orbit including Hinode SOT spectro-polarimeter in the Cycle 4 solar campaign (19 March 2017). ALMA’s spatial resolution was moderate, far from the best performance, but it provided stable conditions that are suitable to investigate temporal variations in the mm-wavelength. We determined that the noise level is less than 20 K (σ) over 1 hour in the 20-s cadence time series of synthesized ALMA images. The time variations with amplitudes above the noise level were observed throughout the field of view, but variations exceeding 200 K, corresponding to energy input to the chromosphere on the order of 1020-22 erg, were localized in two locations. One location was on the polarity inversion line, where tiny concentrated magnetic patches exist in weak field and a tiny magnetic flux may be emergent. The other location was at the outer edge of a bipolar magnetic region, which was under development with a successive series of magnetic flux emergence. This observation suggests that nanoflare-class energy inputs in the chromosphere can occur associated with emerging flux activities.

Using the radio spectra of the 2 April 2022 eruptive flare, we analyze a group of highfrequency type III bursts by our new wavelet method. In this analysis, we found a multi-periodicity of these bursts that is interpreted by the electron beams accelerated in the fragmented magnetic reconnection in the rising magnetic rope. We propose that each period in these type III bursts is a result of the periodic interaction of sub-ropes formed in the rising magnetic rope. In each interaction, the period depends on the diameter of interacting sub-ropes and local Alfvén velocity. This interpretation is supported by detection of the specific EUV structure which was, according to our knowledge, observed for the first time. All proposed processes occur in the rising magnetic rope. Thus, this flare deviates from the standard flare model, where the main magnetic reconnection is located below the rising magnetic rope.

The possibility of generating diffuse radiation in extended astronomical media by plasma turbulence is investigated under the assumption that the turbulence can be understood as an ensemble of small-scale magnetic filaments (narrow current sheets) forming a texture around a large number of magnetic depletions or voids. On astronomically microscopic scales, the dilute high temperature medium (plasma) is to be considered ideally conducting, forming a collection of Josephson junctions between two such adjacent quasi-superconductors. The oscillation frequency of those junctions depends on the part of the spectrum that contributes to the oscillation, causing weak radio backgrounds. Lowest Josephson frequencies/energies near zero may become sources of quasi-stationary magnetic fields.

We report on a multi-wavelength analysis of the 26 January 2014 solar eruption involving a coronal mass ejection (CME) and a Type-II radio burst, performed by combining data from various space and ground-based instruments. An increasing standoff distance with height shows the presence of a strong shock, which further manifests itself in the continuation of the metric Type-II burst into the decameter–hectometric (DH) domain. A plot of speed versus position angle (PA) shows different points on the CME leading edge traveled with different speeds. From the starting frequency of the Type-II burst and white-light data, we find that the shock signature producing the Type-II burst might be coming from the flanks of the CME. Measuring the speeds of the CME flanks, we find the southern flank to be at a higher speed than the northern flank; further the radio contours from Type-II imaging data showed that the burst source was coming from the southern flank of the CME. From the standoff distance at the CME nose, we find that the local Alfv́en speed is close to the white-light shock speed, thus causing the Mach number to be small there. Also, the presence of a streamer near the southern flank appears to have provided additional favorable conditions for the generation of shock-associated radio emission. These results provide conclusive evidence that the Type-II emission could originate from the flanks of the CME, which in our study is from the southern flank of the CME. [ABSTRACT FROM AUTHOR]

The 13 June 2010 event was chosen as an example to find spots on a CME-related shock where type II radio bursts were generated. We used the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) data to find the shock and calculate the emission measure distribution over the solar limb in order to obtain various plasma characteristics ahead of and behind the shock front. A region was found in the shock where the electron density jump X on the shock front, the Alfvén Mach number M a and the shock velocity V s h reach a maximum simultaneously. Moreover, the calculated value of X in this region was found to be closest to the value of X r b based on type II radio burst data, (X r b) = N 2 / N 1 = (f u / f l) 2 , where N 2 and N 1 are the electron densities at the upstream and downstream shock regions, f u and f l are the radio emission frequencies at the upper and lower band of the dynamic spectrum, in the second harmonic region. Based on these findings, we hypothesized that it is this shock region that is the source of type II radio bursts (type II RB). This region moves at an angle of + 20 ∘ from the center of the circle approximating the shock, relative to the direction through the shock middle. The type II radio burst source velocity is shown to be close to the CME-related shock velocity. This can be regarded as indirect evidence of the shocks being the source of type II radio bursts. A dependence, N 1 (R s h) , has been obtained in the + 20 ∘ direction, where R s h is the shock front location. This dependence is shown to differ noticeably from the coronal background electron densities obtained by Newkirk (Astrophys. J. 133, 983, 1961) and Saito et al. (Ann. Tokyo Astron. Obs. 12, 53, 1970) density models. [ABSTRACT FROM AUTHOR]

From 2018 Oct 12 to 13, three successive solar eruptions (E1–E3) with B-class flares and poor white light coronal mass ejections (CMEs) occurred from the same active region NOAA AR 12724. Interestingly, the first two eruptions are associated with Type II radio bursts but the third is not. Using the soft X-ray flux data, radio dynamic spectra and dual perspective EUV intensity images, we comparatively investigate the three events. Our results show that their relevant flares are weak (B2.1, B7.9, and B2.3) and short-lived (13, 9, and 14 min). The main eruption directions of E1 and E2 are along ~45° north of their radial directions, while E3 primarily propagated along the radial direction. In the EUV channels, the early speeds of the first two CMEs have apparent speeds of ~320 and ~380 km s−1, which could exceed their respective local Alfvén speeds of ~300 and ~350 km s−1. However, the CME in the third eruption possesses a much lower speed of ~160 km s−1. These results suggest that the observed Type II radio bursts in the eruptions E1 and E2 are likely triggered by their associated CMEs and the direction of eruption and the ambient plasma and magnetic environments may take an important place in producing Type II radio burst or shock as well.

We analyze the relations between various combinations of peak fluxes and fluences of solar microwave bursts at 35 GHz recorded with the Nobeyama Radio Polarimeters during 1990–2015, and corresponding parameters of proton enhancements with E>100 MeV exceeding 0.1 pfu registered by GOES monitors in near-Earth environment. The highest correlation has been found between the microwave and proton fluences. This fact reflects a dependence of the total number of protons on the total duration of the acceleration process. In the events with strong flares, the correlation coefficients of proton fluences with microwave and soft X-ray fluences are higher than those with speeds of coronal mass ejections. The results indicate a statistically larger contribution of flare processes to acceleration of high-energy protons. Acceleration by shock waves seems to be less important at high energies in events associated with strong flares, although its contribution probably prevails in weaker events. The probability of a detectable proton enhancement was found to directly depend on the peak flux and duration of a microwave burst. This can be used for diagnostics of proton enhancements based on microwave observations.

The locations of the depressed emission in microwaves, EUV and soft X-rays are compared with each other and with the location of the plasma outflow in the active region (AR) 8535 on the Sun. We found that two open-field regions overlap the regions of depressed emission near the AR's sunspot. These two open-field regions are simulated with the potential-field source-surface (PFSS) model under radial distances of RSS = 1.8 R⊙ and RSS = 2.5 R⊙. Each open-field region is located between the arcades of the loops of the same magnetic polarity. The former open-field region covers the region of the plasma outflow, which is thus useful for the tests on connection to the heliosphere. The utmost microwave depression of the intensity in the ordinary mode (the Very Large Array 15 GHz observations) also overlaps the region of the plasma outflow and thus indicates this outflow. The lasting for eight days depression in soft X-rays and the SOHO EIT 2.84×10−8 m images are attributed to the evacuation of as hot coronal plasma as T ≥ 2 × 106 K from the extended in height (“open”) magnetic structures. We conclude that the AR 8535 presents the sunspot atmosphere affected by the large-scale magnetic fields.

The analysis of the spectral properties and dynamic evolution of a CME/shock event observed on November 1st 2003 in white-light by the LASCO coronagraph and in the ultraviolet by the UVCS instrument operating aboard SOHO, has been performed to compute the properties of some important plasma parameters in the middle corona below about 2R⊙. Simultaneous observations obtained with the MLSO/Mk4 white-light coronagraph, providing both the early evolution of the CME expansion in the corona and the pre-shock electron density profile along the CME front, were also used to study this event. By combining the above information with the analysis of the metric type II radio emission detected by ground-based radio spectrographs, we finally derive estimates of the values of the local Alfvén speed and magnetic field strength in the solar corona.

In this paper we investigate the solar activity at the radio frequency (37 GHz) using an extensive data series (solar radio maps) from the Metsähovi Radio Observatory. This paper aims to present this unique solar radio map collection to the public knowledge. The data set covers the years from 1978 to 2011 (solar cycles 21–24). We investigate the long-term solar activity on the ground of the distribution of solar radio brightenings and the differential rotation of the Sun.

Interplanetary electron beams are unstable in the solar wind and they generate Langmuir waves at the local plasma frequency or its harmonic. Radio observations of the waves in the range 4-256 kHz, observed in 1994-2010 with the WAVES experiment onboard the WIND spacecraft, are statistically analyzed. A subset of 36 events with Langmuir waves and type III bursts occurring at the same time was selected. After removal of the background, the remaining power spectral density is modeled by the Pearson system of probability distributions (types I, IV and VI). The Stochastic Growth Theory (SGT) predicts log-normal distribution for the power spectrum density of the Langmuir waves. Our results indicate that SGT possibly requires further verification.

A brief historical review of the development of solar physics and solar radio astronomy in Latvia and its current status is presented.

The hard X-ray time profiles of most solar eruptive events begin with an impulsive phase that may be followed by a late gradual phase. In a recent article (Aurass et al. in Astron. Astrophys. 555, A40, 2013), we analyzed the impulsive phase of the solar eruptive event on November 3, 2003 in radio and X-ray emission. We found evidence of magnetic breakout reconnection using the radio diagnostic of the common effect of the flare current sheet and, at heights of ±0.4 R, of a coronal breakout current sheet (a source site that we called X). In this article we investigate the radio emission during the late gradual phase of this event. The work is based on 40 - 400 MHz dynamic spectra ( Radio Spectrograph, Observatorium Tremsdorf, Leibniz Institut für Astrophysik Potsdam, AIP) combined with radio images obtained by the French Nançay Multifrequency Radio Heliograph (NRH) of the Observatoire de Paris-Meudon. Additionally, we use Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) flux records, and Solar and Heliospheric Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO) and Extreme ultraviolet Imaging Telescope (EIT) images. The analysis shows that the late gradual phase is subdivided into two distinct stages. Stage 1 (lasting five minutes in this case) is restricted to reoccurring radio emission at source site X. We observe plasma emission and an azimuthally moving source (from X toward the NE; speed≈1200 km s) at levels radially ordered against the undisturbed coronal density gradient. These radio sources mark the lower boundary of an overdense region with a huge azimuthal extent. By the end of its motion, the source decays and reappears at point X. This is the onset of stage 2 traced here during its first 13 minutes. By this time, NRH sources observed at frequencies≤236.6 MHz radially lift off with a speed of ≈ 400 km s (one third of the front speed of the coronal mass ejection (CME)) as one slowly decaying broadband source. This speed is still observable in SOHO/LASCO C3 difference frames in the wake of the CME four hours later. In stage 2, the radio sources at higher frequencies appear directly above the active region with growing intensity. We interpret the observations as the transit of the lower boundary of the CME body through the height range of the coronal breakout current sheet. The relaxing global coronal field reconnects with the magnetic surroundings of the current sheets that still connect the CME in its wake with the Sun. The accelerated particles locally excite plasma emission, but can also escape toward the active region, the CME, and the large-scale solar magnetic field. The breakout relaxation process may be a source of reconnection- and acceleration rate modulations. In this view, the late gradual phase is a certain stage of the coronal breakout relaxation after the release of the CME. This article is, to our best knowledge, the first observational report of the coronal breakout recovery. Our interpretation of the radio observations agrees with some predictions of magnetic breakout simulations ( e.g. Lynch et al. in Astrophys. J. 683, 1192, 2008). Again, combined spectral and imaging radio observations give a unique access to dynamic coronal processes that are invisible in other spectral ranges. [ABSTRACT FROM AUTHOR]

Based on the frequency dependence of the power-law distribution of the peak fluxes in 486 radio bursts in 1-35 GHz observed by Nobeyama Radio Polarimeters (see Song et al. in Astrophys. J. 750:160, ), we have first suggested in this paper that the events with power-law behaviors may be emitted from the optically-thin regions, which can be considered as a good measure for the flare energy release. This result is supported by that both the power-law and optical-thin events gradually increase with radio frequencies, which are well fitted by a power-law function with similar indices of 0.48 and 0.80, respectively. Moreover, a flare occurrence rate is newly defined by the power-law event number in per unit frequency. Its values in lower frequencies are evidently larger than those in higher frequencies, which just imply that most flares are trigged in higher corona. Hence, the frequency variation of power-law event number may indicate different energy dissipation rates on different coronal heights. [ABSTRACT FROM AUTHOR]

The observation results for comets C/2009 R1 (McNaught) (June 14-28, 2010) and 17P/Holmes (November 25-December 3, 2007) performed in the line of the OH molecule and at a wavelength of 18 cm are presented. The gas productivity of the OH molecule for comet C/2009 R1 (McNaught) (June 14-28, 2010) as a function of the heliocentric distance (0.42-0.62 au) is determined. [ABSTRACT FROM AUTHOR]

An explicit equation of the propagational angle of microwave emission between the line-of-sight and the local magnetic field is newly derived based on the approximated formulae of nonthermal gyrosynchrotron emission (Dulk and Marsh in Astrophys. J. 259, 350, ). The existence of the solution of propagational angle is clearly shown under a series of typical parameters in solar microwave observations. It could be used to determine the intrinsic mode and linear mode coupling in solar microwave bursts by three steps. (1) The mode coupling may happen only when the angle approximately equals to 90 degrees, i.e., when the emission propagates through the quasi-transverse region (Cohen in Astrophys. J. 131, 664, ). (2) The inversion of polarization sense due to the weakly mode coupling takes place only when the transition frequency defined by Cohen () is larger than the frequency of microwave emission, and an observable criterion of the weakly mode coupling in flaring loops was indicated by the same polarization sense in the two footpoints of a flaring loop (Melrose and Robinson in Proc. Astron. Soc. Aust. 11, 16, ). (3) Finally, the intrinsic mode of microwave emission is determined by the observed polarization and the calculated direction of local magnetic field according to the general plasma dispersion relation, together with the mode coupling process. However, a 180-degree ambiguity still exists in the direction of longitudinal magnetic field, to produce an uncertainty of the intrinsic mode. One example is selected to check the feasibility of the method in the 2001 September 25 event with a loop-like structure nearby the central meridian passage observed by Nobeyama Radio Heliograph and Polarimeters. The calculated angle in one footpoint (FP) varied around 90 in two time intervals of the maximum phase, which gives a direct evidence of the emission propagating through a quasi-transverse region where the linear mode coupling took place, while, the angle in another FP was always smaller than 90 where the mode coupling did not happen. Moreover, the right-circular sense at 17 GHz was always observed in both two FPs during the event, which supports that the transition frequency should be larger than 17 GHz in the first FP together with strong magnetic field of over 2000 Gauses in photosphere, where the weakly coupled case should happen. Moreover, there are two possibilities of the intrinsic mode in the two FPs due to the 180-degree ambiguity. (1) The emission of extraordinary (X) mode from the first FP turns to the ordinary (O) mode in the two time intervals of the maximum phase, while, the X-mode is always emitted from the second FP. (2) The inversion from O-mode to X-mode takes place in the first FP, while the O-mode keeps in the second FP. If the magnetic polarities in photosphere and corona are coincident in this event, the intrinsic mode belongs to the second case. [ABSTRACT FROM AUTHOR]

Abstract: The analysis of the spectral properties and dynamic evolution of a CME/shock event observed on November 1st 2003 in white-light by the LASCO coronagraph and in the ultraviolet by the UVCS instrument operating aboard SOHO, has been performed to compute the properties of some important plasma parameters in the middle corona below about 2R ⊙. Simultaneous observations obtained with the MLSO/Mk4 white-light coronagraph, providing both the early evolution of the CME expansion in the corona and the pre-shock electron density profile along the CME front, were also used to study this event. By combining the above information with the analysis of the metric type II radio emission detected by ground-based radio spectrographs, we finally derive estimates of the values of the local Alfvén speed and magnetic field strength in the solar corona. [Copyright &y& Elsevier]

We present a multi-wavelength study of a solar eruption event on 20 July 2004, comprising observations in Hα, EUV, soft X-rays, and in radio waves with a wide frequency range. The analyzed data show both oscillatory patterns and shock wave signatures during the impulsive phase of the flare. At the same time, large-scale EUV loops located above the active region were observed to contract. Quasi-periodic pulsations with ∼ 10 and ∼ 15 s oscillation periods were detected both in microwave - millimeter waves and in decimeter - meter waves. Our calculations show that MHD oscillations in the large EUV loops - but not likely in the largest contracting loops - could have produced the observed periodicity in radio emission, by triggering periodic magnetic reconnection and accelerating particles. As the plasma emission in decimeter - meter waves traces the accelerated particle beams and the microwave emission shows a typical gyrosynchrotron flux spectrum (emission created by trapped electrons within the flare loop), we find that the particles responsible for the two different types of emission could have been accelerated in the same process. Radio imaging of the pulsed decimetric - metric emission and the shock-generated radio type II burst in the same wavelength range suggest a rather complex scenario for the emission processes and locations. The observed locations cannot be explained by the standard model of flare loops with an erupting plasmoid located above them, driving a shock wave at the CME front. [ABSTRACT FROM AUTHOR]

We present a study of the solar coronal shock wave on 2005 November 14 associated with the GOES M3.9 flare that occurred close to the east limb (S06º E60º). The shock signature, a type II radio burst, had an unusually high starting frequency of about 800 MHz, indicating that the shock was formed at a rather low height. The position of the radio source, the direction of the shock wave propagation, and the coronal electron density were estimated using Nanqay Radioheliograph observations and the dynamic spectrum of the Green Bank Solar Radio Burst Spectrometer. The soft X-ray, Hα, and Reuven Ramaty High Energy Solar Spectroscopic hnager observations show that the flare was compact, very impulsive, and of a rather high density and temperature, indicating a strong and impulsive increase of pressure in a small flare loop. The close association of the shock wave initiation with the impulsive energy release suggests that the impulsive increase of the pressure in the flare was the source of the shock wave. This is supported by the fact that, contrary to the majority of events studied previously, no coronal mass ejection was detected in association with the shock wave, although the corresponding flare occurred close to the limb. [ABSTRACT FROM AUTHOR]