Location and parameters of a microwave millisecond spike event

A typical microwave millisecond spike event on November 2, 1997 was observed by the radio spectrograph of National Astronomical Observatories (NAOs) at 2.6–3.8 GHz with high time and frequency resolution. This event was also recorded by Nobeyama Radio Polarimeters (NoRP) at 1–35 GHz and Radio Heliograph (NoRH) at 17 GHz. The source at 17 GHz is located in one foot-point of a small bright coronal loop of YOHKOH SXT and SOHO EIT images with strong photospheric magnetic field in SOHO MDI magnetograph. It is assumed that the electron cyclotron maser instability and gyro-resonance absorption dominate, respectively, the rising and decay phase of the spike event. For different harmonic number of gyro-frequency or magnetic field strength, a fitting program with free plasma parameters is used to minimize the difference between the observational and theoretical values of the exponential growth and decay rates for a given spike. The plasma parameters at third harmonic number are more comparable to their typical values in solar corona. Hence, it is able to provide a diagnosis for the source parameters (magnetic field, density, and temperature), the properties of radiations (wave vector and propagation angle), and the properties of non-thermal electrons (density, pitch angle, and energy). The results are also comparable with the diagnosis of the gyro-synchrotron radiation model, the frequency drift rates and a dipole magnetic field model, as well as the YOHKOH SXT and SOHO MDI data. This study is supported by the NFSC project nos. 10333030 and 10273025, and “973” program with no. G2000078403.     

Calculations of Coronal Magnetic Field Parallel and Perpendicular to Line-of-Sight in Microwave Bursts

Based on the approximations of the non-thermal gyro-synchrotron radiation used by G.A. Dulk, and K.A. Marsh, Astrophs. J. 259, 350, 1982, the author analyses calculations of the propagation angle and coronal magnetic field self-consistently with brightness temperature, spectral index, frequency, turnover frequency, and polarization degree in solar microwave bursts. Hence, the coronal magnetic fields parallel and perpendicular to the line-of-sight in the sources of microwave bursts are calculated theoretically, and discussed in an event observed by the Nobeyama Radio Polarimeters (NoRP).     

一个复杂太阳射电爆发及其微波源和远紫外冕环特征的初步研究

利用太阳射电宽带频谱仪(0．7-7．6GHz)于2001年10月19日观测到的复杂太阳射电大爆发，呈现出许多有趣的特征，结合NoRH(Nobeyama Radio Heliograph)的高空间分辨率射电成像观测及TRACE(Transition Region and Coronal Explorer)在远紫外(EUV)波段的高空间分辨率成像观测资料，分析了该爆发的射电频谱特征和微波射电源的演化以及它们与复杂的EUV日冕环系统的关系，该爆发是一个双带大耀斑的射电表征．前一部分以宽带(从厘米到米波)爆发为主，机制是回旋同步辐射，所对应的是环足源的辐射；后一部分以窄带(分米到米波)分米波爆发为主，机制是等离子体辐射和回旋共振辐射的联合，对应的是环顶源的辐射。     

Observations of bright coronal points at wavelengths of 5.2 and 1.76 cm

Based on two-dimensional solar images obtained with the Siberian Solar Radio Telescope and the Nobeyama Radio Heliograph and using YOHKOH soft X-ray images, we investigate bright coronal points. The principal microwave emission mechanism of these points is shown to be the thermal bremsstrahlung of an optically thin plasma. The fact that, in several cases, bright coronal points do not coincide at two wavelengths can be explained by imaging peculiarities of the Nobeyama Radio Heliograph rather than by physical factors.     

Dual-filament initiation of a Coronal Mass Ejection: Observations and Model

We propose a new model for the initiation of solar coronal mass ejections (CMEs) and CME-associated flares. The model is inferred from observations of a quiescent filament eruption in the north-western quadrant of the solar disk on 4 September 2000. The event was observed with the Siberian Solar Radio Telescope (5.7 GHz), the Nobeyama Radioheliograph (17 GHz) and SOHO/EIT and LASCO. Based on the observations, we suggest that the eruption could be caused by the interaction of two dextral filaments. According to our model, these two filaments merge together to form a dual-filament system tending to form a single long filament. This results in a slow upward motion of the dual-filament system. Its upward expansion is prevented by the attachment of the filaments to the photosphere by filament barbs as well as by overlying coronal arcades. The initial upward motion is caused by the backbone magnetic field (first driving factor) which connects the two merging filaments. Its magnetic flux increases slowly due to magnetic reconnection of the cross-interacting legs of these filaments. If a total length of the dual-filament system is large enough, then the filament barbs detach themselves from the solar surface due to magnetic reconnection between the barbs with oppositely directed magnetic fields. The detachment of the filament barbs completes the formation of the eruptive filaments themselves and determines the helicity sign of their magnetic fields. The appearance of a helical magnetic structure creates an additional upward-directed force (second driving factor). A combined action of these two factors causes acceleration of the dual-filament system. If the lifting force of the two factors is sufficient to substantially extend the overlying coronal magnetic arcade, then magnetic reconnection starts below the eruptive filament in accordance with the classical scheme, and the third driving factor comes into play.     

Magnetic Properties of Metric Noise Storms Associated with Coronal Mass Ejections

Using Nancay Radioheliograph (NRH) imaging observations, combined with SOHO/Michelson Doppler Imager (MDI) magnetogram observations and coronal magnetic field extrapolation, we studied the magnetic nature of metric noise storms that are associated with coronal mass ejections (CMEs). Four events are selected: the events of 2000 July 14, 2001 April 26, 2002 August 16 and 2001 March 28. The identified noise storm sources cover or partially cover the active regions (ARs), but the centers of storm sources are offset from the ARs. Using extrapolated magnetic field lines, we find that the noise storm sources trace the boundary between the open and closed field lines. We demonstrate that the disappearance of noise storm source is followed by the appearance of the burst source. The burst sources spread on the solar disk and their distributions correspond to the extent of the CME in LASCO C2 field of view. All the SOHO/Extreme Ultraviolet Imaging Telescope (EIT) dimmings associ- ated with noise storm sources are located at the periphery of noise storms where the magnetic lines of force were previously closed and low-lying. When the closed field becomes partially or fully open, the basic configurations of noise storm sources are changed, then the noise storm sources are no longer observed. These observations provide the information that the variations of noise storms manifest the restructuring or reconfiguring of the coronal magnetic field.     

A complex solar radio burst and the characteristics of its related microwave sources and EUV coronal loops

On 2001 Oct. 19, a very complex solar radio burst with a host of interesting features was observed with a broadband (0.7–7.6 GHz) solar radio spectrometer. Combining with the data of NoRH (Nobeyama Radio Heliograph) and TRACE (Transition Region and Corona Explorer), the spectral features of the radio burst, the evolution of the microwave radio sources, and relations with the complex EUV coronal loops are analyzed. The burst is the radio manifestation of a large double-ribbon flare; it consisted of two stages. The earlier stage was dominated bya broadband burst in the centimeter-meter waveband from gyro-synchrotron emission of sources at the footpoints of the loop. The later stage was dominated by a narrow-band decimeter wave burst in the decimetermeter waveband, from a combination of plasma emission and gyro-resonance emission from sources in the top of the loop.     

Coronal Magnetography of Solar Active Region 8365 with the SSRT and NoRH Radio Heliographs

Microwave maps of solar active region NOAA 8365 are used to derive the coronal magnetograms of this region. The technique is based on the fact that the circular polarization of a radio source is modified when microwaves pass through the coronal magnetic field transverse to the line of sight. The observations were taken with the Siberian Solar Radio Telescope (SSRT) on October 21 – 23 and with the Nobeyama Radio Heliograph (NoRH) on October 22 – 24, 1998. The known theory of wave mode coupling in quasi-transverse (QT) region is employed to evaluate the coronal magnetograms in the range of 10 – 30 G at the wavelength 5.2 cm and 50 – 110 G at 1.76 cm, taking the product of electron density and the scale of coronal field divergence to be constant of 10¹⁸ cm^–2. The height of the QT-region is estimated from the force-free field extrapolations as 6.2 × 10⁹ cm for the 20 G and 2.3 × 10⁹ cm for 85 G levels. We find that on large spatial scale, the coronal magnetograms derived from the radio observations show similarity with the magnetic fields extrapolated from the photosphere.     

Very Large Array,SOHO, and RHESSI Observations of Magnetic Interactions and Particle Propagation across Large-Scale Coronal Loops

Very Large Array (VLA) observations at wavelengths of 20 and 91 cm have been combined with data from the SOHO and RHESSI solar missions to study the evolution of transequatorial loops connecting active regions on the solar surface. The radio observations provide information about the acceleration and propagation of energetic electrons in these large-scale coronal magnetic structures where energy release and transport take place. On one day, a long-lasting Type I noise storm at 91 cm was seen to intensify and shift position above the northern hemisphere region following an impulsive hard X-ray burst in the southern hemisphere footpoint region. VLA 20-cm observations as well as SOHO EIT EUV images showed evolving coronal plasma that appeared to move across the solar equator during this time period. This suggests that the transequatorial loop acted as a conduit for energetic particles or fields that may have triggered magnetic changes in the corona where the northern noise storm region was seen. On another day, a hard X-ray burst detected at the limb was accompanied by impulsive 20- and 91-cm burst emission along a loop connecting to an active region in the same hemisphere but about 5′ away, again suggesting particle propagation and remote flare triggering across interconnecting loops.     

Probing Twisted Magnetic Field Using Microwave Observations in an M Class Solar Flare on 11 February, 2014

This work demonstrates the possibility of magnetic-field topology investigations using microwave polarimetric observations. We study a solar flare of GOES M1.7 class that occurred on 11 February, 2014. This flare revealed a clear signature of spatial inversion of the radio-emission polarization sign. We show that the observed polarization pattern can be explained by nonthermal gyrosynchrotron emission from the twisted magnetic structure. Using observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope Network, and Solar Dynamics Observatory, we have determined the parameters of nonthermal electrons and thermal plasma and identified the magnetic structure where the flare energy release occurred. To reconstruct the coronal magnetic field, we use nonlinear force-free field (NLFFF) and potential magnetic-field approaches. Radio emission of nonthermal electrons is simulated by the GX Simulator code using the extrapolated magnetic field and the parameters of nonthermal electrons and thermal plasma inferred from the observations; the model radio maps and spectra are compared with observations. We have found that the potential-magnetic-field approach fails to explain the observed circular polarization pattern; on the other hand, the Stokes-\(V\) map is successfully explained by assuming nonthermal electrons to be distributed along the twisted magnetic structure determined by the NLFFF extrapolation approach. Thus, we show that the radio-polarization maps can be used for diagnosing the topology of the flare magnetic structures where nonthermal electrons are injected.     

Radio and X-ray imaging observations of solar flares and coronal transients

We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager     

Constraints on the Regimes of Electron Acceleration in Solar Flares

Astronomy Letters - Based on the observations of microwave impulsive bursts recorded at the Nobeyama Radio Observatory, we have obtained constraints on the regimes of electron acceleration in solar...     

Polarization of loop-top and footpoint sources in microwave bursts

The polarization is analyzed in four microwave bursts with one loop-top and two footpoint sources observed at 17 GHz with the Nobeyama Radioheliograph (NoRH). The loop-like structure of the four events is confirmed by simultaneous SOHO/MDI magnetograms and TRACE/EUV images or Yohkoh/SXT images. The heliocentric distance of the four events is greater than 30°. The three microwave sources in each given burst are polarized in the same sense. This may be interpreted in terms of extraordinary mode emission, taking into account the polarity of the underlying magnetic field and propagation effects, which may lead to inversion of the sense of polarization in the limbward foot and loop-top source of the flaring loop.     

一个复杂太阳射电爆发及其微波源区特征的初步研究

我国“太阳射电宽带频谱仪(0．7～7．6GHz)”于2001年10月19日观测到的复杂太阳射电大爆发，呈现许多有趣的特征。本文结合NoRH的高空分辨率成像观测资料，分析了该爆发的微波射电源区的演化特征及与射电辐射特征的关系。还发现微波源的缓慢运动，这可能与爆发所伴随的CME的形成有关。     

On Solar Intermediate Drift Radio Bursts at Decimeter and Meter Wavelength

Fiber – or intermediate drift – bursts are a continuum fine structure in some complex solar radio events. We present the analysis of such bursts in the X17 flare on 28 Oct. 2003. Based on the whistler wave model of fiber bursts we derive the 3D magnetic field structures that carry the radio sources in different stages of the event and obtain insight into the energy release evolution in the main flare phase, the related paths of nonthermal particle propagation in the corona, and the involved magnetic field structures. Additionally, we test the whistler wave model of fiber bursts for the meter and the decimeter wave range. Radio spectral data (Astrophysikalisches Institut Potsdam, Astronomical Observatory Ond?ejov) show a continuum with fibers for ≈?6 min during the main flare phase. Radio imaging data (Nançay Radio Heliograph) yield source centroid positions of the fibers at three frequencies in the spectrometer band. We compare the radio positions with the potential coronal magnetic field extrapolated from SOHO/MDI data. Given the detected source site configuration and evolution, and the change of the fiber burst frequency range with time, we can also extract those coronal flux tubes where the high-frequency fiber bursts are situated even without decimeter imaging data. To this aim we use a kinetic simulation of whistler wave growth in sample flux tubes modeled by selected potential field lines and a barometric density model. The whistler wave model of fiber bursts accurately explains the observations on 28 Oct. 2003. A laterally extended system of low coronal loops is found to guide the whistler waves. It connects several neighboring active regions including the flaring AR 10486. For varying source sites the fiber bursts are emitted at the fundamental mode of the plasma frequency over the whole range (1200?–?300 MHz). The present event can be understood without assuming two different generation mechanisms for meter and decimeter wave fiber bursts. It gives new insight into particle acceleration and propagation in the low flare and post-CME corona.     

Radio Versus EUV/X-Ray Observations of the Solar Atmosphere

This paper reviews the contrasting properties of radio and EUV/X-ray observations for the study of the solar atmosphere. The emphasis is placed on explaining the nature of radio observations to an EUV/X-ray audience. Radio emission is produced by mechanisms which are well-understood within classical physics. Bremsstrahlung tends to be dominant at low frequencies, while gyro-resonance emission from strong magnetic fields produces bright sources at higher frequencies. At most radio frequencies the images of the Sun are dominated almost everywhere by bremsstrahlung opacity, which may be optically thick or thin depending on circumstances. Where gyro-resonance sources are present they may be used as sensitive probes of the regions above active regions where magnetic field strengths exceed several hundred gauss, and this unique capability is one of the strengths of radio observations. Typically a gyro-resonance radio source shows the temperature on an optically thick surface of constant magnetic field within the corona. Since each radio frequency corresponds to a different magnetic field strength, the coronal structure can be `peeled away' by using different frequencies. The peculiarities of radio observing techniques are discussed and contrasted with EUV/X-ray techniques. Radio observations are strong at determining temperatures and coronal magnetic field strengths while EUV/X-ray observations better sense densities and reveal coronal magnetic field lines: in this way the two wavelength domains are nicely complementary.     

On the Microwave Spike Emission of the September 6, 1992 Flare

The main aim of this paper is to estimate, from multispectral observations, the plasma parameters in a microwave burst source which was also the site of spike emission. This information is essential for the determination of the spike emission process. By analyzing one-dimensional source distributions observed with the SSRT at 5.7 GHz and correlating them with Yohkoh X-ray and Nobeyama 17 GHz images, we have concluded that the microwave emitting region was larger than the soft X-ray loop-top source, and that the origin of the burst could be explained by gyrosynchrotron emission of non-thermal electrons in a magnetic field of approximately 100 G. It has been shown that the source of 5.7 GHz spikes observed during the burst was located close to an SXR-emitting loop with high density and temperature and a relatively low magnetic field. Thus, plasma emission is the most favourable radiation mechanism for the generation of the sub-arc-second microwave pulses.     

INFERENCES ON CORONAL MAGNETIC FIELDS FROM SOHO UVCS OBSERVATIONS

The characteristics of the magnetic field ubiquitously permeating the coronal plasma are still largely unknown. In this paper we analyze some aspects of coronal physics, related to the magnetic field behavior, which forthcoming SOHO UVCS observations can help better understand. To this end, three coronal structures will be examined: streamers, coronal mass ejections (CMEs), and coronal holes.As to streamers and CMEs, we show, via simulations of the Ly- and white-light emission from these objects, calculated on the basis of recent theoretical models (Wang et al., 1995), how new data from SOHO can help in advancing our knowledge of the streamer/CME magnetic configuration. Our discussion highlights also those observational signatures which might offer clues on reconnection processes in streamers' current sheets.Coronal holes (CHs) are discussed in the last section of the paper. Little is known about CH flux tube geometry, which is closely related to the behavior of the solar wind at small heliocentric distances.Indirect evidence for the flux tube spreading factors, within a few solar radii, is here examined.     

Magnetic Field Strengths and Structures from Radio Observations of Solar Active Regions

Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at λ=1.76cm are used for estimating the magnetic field strength in the upper chromosphere, based on thermal bremsstrahlung. The results are compared with the magnetic field strength in the photosphere from observations with the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of Beijing Astronomical Observatory. The difference in the magnetic field strength between the two layers seems reasonable. The solar radio maps of active regions obtained with the Nobeyama radioheliograph, both in total intensity (I-map) and in circular polarizations (V-map), are compared with the optical magnetograms obtained with the SMFT. The comparison between the radio map in circular polarization and the longitudinal photospheric magnetogram of a plage region suggest that the radio map in circular polarization is a kind of magnetogram of the upper chromosphere. The comparison of the radio map in total intensity with the photospheric vector magnetogram of an AR shows that the radio map in total intensity gives indications of magnetic loops in the corona, thus we have a method of defining the coronal magnetic structure from the radio I-maps at λ=1.76 cm. Analysing the I-maps, we identified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field.     

The Observational Evidence on the Loop–Loop Interaction in a Flare–CME Event on April 15, 1998

The evolution of the soft X-ray and EUV coronal loops related to the April 15, 1998 solar flare–CME event is studied with multiwavelength observations including hard X-rays (BATSE), microwaves (NoRP, CNAO) and magnetograms (SOHO/MDI), as well as images from Yohkoh/SXT and SOHO/EIT at 195 Å. It is shown that: (1) two soft X-ray and EUV loops rose, crossed and turned bright, (2) near one footpoint of these loops, the background magnetic field decreased, (3) there were similar quasi periodic oscillations in the time profiles of hard X-ray and microwave emissions, which characterized the loop–loop coalescence instability, (4) after the loop–loop reconnection, two new loops formed, the small one stayed at the original place, and the large one ejected out as part of the constructed prominence cloud. Based upon these observations, we argue that the decrease of the background magnetic field near these loops caused them to rise and approach each other, and in turn, the fast loop–loop coalescence instability took place and triggered the flare and the CME.