Baolin Tan

Distinct propagating fast wave trains associated with flaring energy releases

Context. Large-scale fast waves with perturbation of the EUV emission intensity are well resolved in both temporal and spatial scale by SDO/AIA. These waves are prone to propagate along the magnetic field line. Aims. We aim to probe the link between propagating fast wave trains and flaring energy releases. By measuring the wave parameters, we reveal their nature and investigate the potential to diagnose the energy source and waveguide. Methods. The spatial and temporal evolution of the wave amplitude and propagating speed are studied. The correlation of individual wave trains with flare-generated radio bursts is tested. Results. The propagating wave pattern comprises distinct wave trains with varying periods and wavelengths. This characteristic signature is consistent with the patterns formed by waveguide dispersion, when different spectral components propagate at different phase and group speeds. The wave train releases are found to be highly correlated in start time with the radio bursts emitted by the nonthermal electrons that were accelerated in bursty energy releases. The wave amplitude is seen to reach the maximum midway during its course. This can be caused by a combined effect of the waveguide spread in the transverse direction and density stratification. The transverse amplitude distribution perpendicular to the wave vector is found to follow approximately a Gaussian profile. The spatial structure is consistent with the kink mode that is polarised along the line-of-sight. The propagating speed is subject to deceleration from ∼735−845 km s −1 to ∼600 km s −1 . This could be caused by the decrease in the local Alfven speed and/or the projection effect.

The Microwave Pulsations and the Tearing Modes in the Current-Carrying Flare Loops

Solar microwave observations of the X3.4 Flare/CME event observed in Chinese solar broadband radiospectrometer (SBRS/Huairou) on 2006 December 13 show a series of very short period pulsations (VSP) with the period of <1.0 s in the frequency range of 2.60-3.80 GHz. Many pulsating events have the period of only several tens of milliseconds. These pulsations are quasi-periodic, broad bandwidth, and ubiquitous during all the phases of the flare/CME event. Based on theoretical analysis of the temporal behavior of the resistive tearing mode in the electric current-carrying flare loops, we propose that microwave pulsations are a result of the modulation of the tearing-mode oscillations in the current-carrying flare loops. Our calculation of the period of the tearing-mode oscillations are in good agreement with the observations.

OSCILLATIONS IN A SUNSPOT WITH LIGHT BRIDGES

The Solar Optical Telescope on board Hinode observed a sunspot (AR 11836) with two light bridges (LBs) on 2013 August 31. We analyzed a two-hour Ca II H emission intensity data set and detected strong five-minute oscillation power on both LBs and in the inner penumbra. The time-distance plot reveals that the five-minute oscillation phase does not vary significantly along the thin bridge, indicating that the oscillations are likely to originate from underneath it. The slit taken along the central axis of the wide LB exhibits a standing wave feature. However, at the center of the wide bridge, the five-minute oscillation power is found to be stronger than at its sides. Moreover, the time-distance plot across the wide bridge exhibits a herringbone pattern that indicates a counter-stream of two running waves, which originated at the bridges sides. Thus, the five-minute oscillations on the wide bridge also resemble the properties of running penumbral waves. The five-minute oscillations are suppressed in the umbra, while the three-minute oscillations occupy all three cores of the sunspots umbra, separated by the LBs. The three-minute oscillations were found to be in phase at both sides of the LBs. This may indicate that either LBs do not affect umbral oscillations, or that umbral oscillations at different umbral cores share the same source. It also indicates that LBs are rather shallow objects situated in the upper part of the umbra. We found that umbral flashes (UFs) follow the life cycles of umbral oscillations with much larger amplitudes. They cannot propagate across LBs. UFs dominate the three-minute oscillation power within each core; however, they do not disrupt the phase of umbral oscillation.

MICROWAVE ZEBRA PATTERN STRUCTURES IN THE X2.2 SOLAR FLARE ON 2011 FEBRUARY 15

A zebra pattern (ZP) structure is the most intriguing fine structure on the dynamic spectrograph of a solar microwave burst. On 2011 February 15, an X2.2 flare event erupted on the solar disk, which is the first X-class flare since the solar Schwabe cycle 24. It is interesting that there are several microwave ZPs observed by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) at a frequency of 6.40-7.00 GHz (ZP1) and at a frequency of 2.60-2.75 GHz (ZP2) and by the Yunnan Solar Broadband Radio Spectrometer (SBRS/Yunnan) at a frequency of 1.04-1.13 GHz (ZP3). The most important phenomenon is the unusual high-frequency ZP structure (ZP1, up to 7.00 GHz) that occurred in the early rising phase of the flare and the two ZP structures (ZP2, ZP3) with relatively low frequencies that occurred in the decay phase of the flare. By scrutinizing the current prevalent theoretical models of ZP structure generations and comparing their estimated magnetic field strengths in the corresponding source regions, we suggest that the double plasma resonance model is the most probable one for explaining the formation of microwave ZPs, which may derive the magnetic field strengths at about 230-345 G, 126-147 G, and 23-26 G in the source regions of ZP1, ZP2, and ZP3, respectively.

QUASI-PERIODIC WIGGLES OF MICROWAVE ZEBRA STRUCTURES IN A SOLAR FLARE

Quasi-periodic wiggles of microwave zebra pattern (ZP) structures with periods ranging from about 0.5 s to 1.5 s are found in an X-class solar flare on 2006 December 13 at the 2.6-3.8 GHz with the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou). Periodogram and correlation analysis show that the wiggles have two to three significant periodicities and are almost in phase between stripes at different frequencies. The Alfven speed estimated from the ZP structures is about 700 km s–1. We find the spatial size of the wave-guiding plasma structure to be about 1 Mm with a detected period of about 1 s. This suggests that the ZP wiggles can be associated with the fast magnetoacoustic oscillations in the flaring active region. The lack of a significant phase shift between wiggles of different stripes suggests that the ZP wiggles are caused by a standing sausage oscillation.

MICROWAVE QUASI-PERIODIC PULSATION WITH MILLISECOND BURSTS IN A SOLAR FLARE ON 2011 AUGUST 9

A peculiar microwave quasi-periodic pulsation (QPP) accompanying a hard X-ray (HXR) QPP of about 20 s duration occurred just before the maximum of an X6.9 solar flare on 2011 August 9. The most interesting aspect is that the microwave QPP consists of millisecond timescale superfine structures. Each microwave QPP pulse is made up of clusters of millisecond spike bursts or narrowband type III bursts. There are three different frequency drift rates: the global frequency drift rate of the microwave QPP pulse group, the frequency drift rate of the microwave QPP pulse, and the frequency drift rate of individual millisecond spikes or type III bursts. The physical analysis indicates that the energetic electrons accelerating from a large-scale highly dynamic magnetic reconnecting current sheet above the flaring loop propagate downward, impact the flaring plasma loop, and produce HXR bursts. The tearing-mode (TM) oscillations in the current sheet modulate HXR emission and generate HXR QPP; the energetic electrons propagating downward produce Langmuir turbulence and plasma waves, resulting in plasma emission. The modulation of TM oscillation on the plasma emission in the current-carrying plasma loop may generate microwave QPP. The TM instability produces magnetic islands in the loop. Each X-point will be a small reconnection site and will accelerate the ambient electrons. These accelerated electrons impact the ambient plasma and trigger the millisecond spike clusters or the group of type III bursts. Possibly, each millisecond spike burst or type III burst is one of the elementary bursts (EBs). A large number of such EB clusters form an intense flaring microwave burst.

A physical explanation of solar microwave Zebra pattern with the current-carrying plasma loop model

The microwave Zebra pattern structure is an intriguing fine structure on the dynamic spectra of solar type IV radio bursts. Up to now, there is no perfect physical model for the origin of the solar microwave Zebra pattern. Recently, Ledenev et al. (Sol. Phys. 233:129, 2006) put forward an interference mechanism to explain the features of microwave Zebra patterns in solar continuum events. This model needs a structure with a multitude of discrete narrow-band sources of small size. Based on the model of a current-carrying plasma loop and the theory of tearing-mode instability, we propose that the above structure does exist and may provide the main conditions for the interference mechanism. With this model, we may explain the frequency upper limit, the formation of the parallel and equidistant stripes, the superfine structure and intermediate frequency drift rate of the Zebra stripes. If this explanation is valid, the Zebra pattern structures can reveal some information of the motion and the inner structures of the coronal plasma loops.

Correlation between the Sharp Variation of the Transport Rate of Magnetic Helicity and Solar Eruptive Events

In this Letter we report a close relationship between the variations of the transport rate of magnetic helicity (dH/dt) and a microwave burst. The latter may be regarded as a prompt signal of nonthermal energetic particles originating from the magnetic reconnection during solar flaring events. We analyze the observations of magnetograms of MDI/SOHO and SOT/Hinode and the high-cadence microwave observation at 2.84 GHz obtained by the Chinese Solar Broadband Radiospectrometer (SBRS/Huairou) of a flare/CME event that occurred in NOAA Active Region 10930 on 2006 December 13. We find that there is a sharp jump of dH/dt around the onset and quench of a microwave burst at a frequency of 2.84 GHz: the rate of dH/dt changes from negative to positive around the start of the eruption and recovers to negative when the eruption stopped. Furthermore, the temporal profile of dH/dt is consistent with that of a microwave burst. These results indicate that sharp variations of dH/dt are closely related to the solar eruption.

STATISTICS AND CLASSIFICATION OF THE MICROWAVE ZEBRA PATTERNS ASSOCIATED WITH SOLAR FLARES

The microwave zebra pattern (ZP) is the most interesting, intriguing, and complex spectral structure frequently observed in solar flares. A comprehensive statistical study will certainly help us to understand the formation mechanism, which is not exactly clear now. This work presents a comprehensive statistical analysis of a big sample with 202 ZP events collected from observations at the Chinese Solar Broadband Radio Spectrometer at Huairou and the Ondŕejov Radiospectrograph in the Czech Republic at frequencies of 1.00-7.60 GHz from 2000 to 2013. After investigating the parameter properties of ZPs, such as the occurrence in flare phase, frequency range, polarization degree, duration, etc., we find that the variation of zebra stripe frequency separation with respect to frequency is the best indicator for a physical classification of ZPs. Microwave ZPs can be classified into three types: equidistant ZPs, variable-distant ZPs, and growing-distant ZPs, possibly corresponding to mechanisms of the Bernstein wave model, whistler wave model, and double plasma resonance model, respectively. This statistical classification may help us to clarify the controversies between the existing various theoretical models and understand the physical processes in the source regions.

SMALL-SCALE MICROWAVE BURSTS IN LONG-DURATION SOLAR FLARES

Solar small-scale microwave bursts (SMBs), including microwave dot, spike, and narrow-band type III bursts, are characterized by very short timescales, narrow frequency bandwidth, and very high brightness temperatures. Based on observations of the Chinese Solar Broadband Radio Spectrometer at Huairou with superhigh cadence and frequency resolution, this work presents an intensive investigation of SMBs in several flares that occurred in active region NOAA 10720 during 2005 January 14-21. Especially for long-duration flares, the SMBs occurred not only in the early rising and impulsive phase, but also in the flare decay phase and even after the end of the flare. These SMBs are strong bursts with inferred brightness temperatures of at least 8.18 × 1011-1.92 × 1013 K, very short lifetimes of 5-18 ms, relative frequency bandwidths of 0.7%-3.5%, and superhigh frequency drifting rates. Together with their obviously different polarizations from background emission (the quiet Sun, and the underlying flaring broadband continuum), such SMBs should be individual, independent strong coherent bursts related to some non-thermal energy release and the production of energetic particles in a small-scale source region. These facts show the existence of small-scale strong non-thermal energy releasing activities after the flare maxima, which is meaningful for predicting space weather. Physical analysis indicates that a plasma mechanism may be the most favorable candidate for the formation of SMBs. From the plasma mechanism, the velocities and kinetic energy of fast electrons can be deduced and the region of electron acceleration can also be tracked.