Narrowband frequency-drift structures in solar type IV bursts
We have established the Zao Solar Radiospectrograph (ZSR), a new solar radio observation system, at the Zao observatory of Tohoku University, Japan. We observed narrowband fine structures with type IV bursts with ZSR on 2 and 3 November 2008. The observed fine structures are similar to fiber bursts in terms of the drift rates and the existence of emission and absorption stripes. Statistical analysis of the drift rates, however, shows that the observed fine structures are different from the ordinary fiber bursts as regards the sense and the magnitude of their drift rates. First, the observed drift rates include both positive and negative rates, whereas ordinary fiber bursts are usually characterized by negative drift rates. Second, the absolute values of the observed drift rates are tens of MHz s−1, whereas the typical drift rate of fiber bursts at 325 MHz is approximately −9 MHz s−1. In addition, all fine structures analyzed have narrow emission bands of less than 17 MHz. We also show that the observed narrowband emission features with drift rates of approximately 40 MHz s−1 can be interpreted as the propagation of whistler-mode waves, which is the same process as that underlying fiber bursts.
Key wordsSun radio type IV burst fiber burst fine structure
Solar type IV bursts are intense radio phenomena that follow solar flares. They have wideband continuous spectra that extend from meter into decimeter wavelengths (e.g., Wild et al., 1963). Following their discovery by Elgarøy (1959), many observations of type IV bursts have revealed that they are accompanied by several kinds of fine structure. One of the most prominent fine-structure in type IV bursts are fiber bursts. They were first detected by Young et al. (1961). They are usually referred to as ‘intermediate drift bursts’ (IMDs). Detection of the same kind of burst has also been reported by Thompson and Maxwell (1962), Slottje (1972), Aurass et al. (1987), and many others. Recently, observations in the decimeter and microwave range have been made by many authors. For example, Benz and Mann (1998) investigated fiber bursts in the frequency range of 1–3 GHz. Chernov et al. (2001) and Fernandes et al. (2003) reported observational results with high resolution at frequencies above 1 GHz. The main characteristics of fiber bursts are as follows: (i) their drift rate is usually negative, (ii) the drift/frequency ratio, ǀ(d f/dt)/ f ǀ, is on the order of 0.04–−0.1 s−1, (iii) their instantaneous bandwidth is approximately 2% of the emission frequency (Benz and Mann, 1998), and (iv) they are accompanied a paralleldrift absorption band in the background continuum radiation (e.g., Chernov, 2006). Kuijpers (1975) proposed an emission mechanism for fiber bursts consisting of the interaction of a Langmuir and a whistler-mode wave. This produces a left-handed polarized ordinary (L-O) mode wave that can escape from the emission region. According to this theory, the drift rate of the emission frequency is interpreted by the group velocity of the whistler-mode waves. The proposed mechanism is generally accepted, although it has also attracted some criticism (e.g., Bernold and Treumann, 1983). Recently, Aurass et al. (2005) showed the validity of this process for estimating magnetic-field strength with fiber bursts. Previous studies have also proposed another emission mechanism which involves Alfvén velocity as the speed of the motion of emission source (e.g., Treumann et al., 1990; Kuznetsov, 2006; Karlický et al., 2013). Observations and the theoretical background of fiber bursts are reviewed by Chernov (2006).
Many radio spectrometers for observations of solar radio bursts have been operated in the meter-wave range, such as Phoenix in Switzerland (Benz et al., 1991), Brazilian Solar Spectroscope (BSS) in Brazil (Sawant et al., 2001), Hiraiso Radio Spectrograph (HiRAS) in Japan (Kondo et al., 1995), and Culgoora in Australia (Prestage et al., 1994). Since the resolution of these wide-band spectrometers is often limited to a time resolution of 0.1 s and a frequency resolution of 1 MHz, it is not sufficient to investigate the fine-structure details of radio bursts such as fiber bursts, which have approximately 1 MHz of instantaneous bandwidth at a frequency near 300 MHz. There are radiospectrographs with higher time resolution of milliseconds (e.g., Fu et al., 1995; Jiřička and Karlický, 2008; Dąbrowski et al., 2011), which have revealed a new class of fine structures of radio bursts (e.g., Magdalenić et al., 2006). However, the time resolution is still not adequate in the meter wavelength range, especially for metric type IV bursts. In addition, the frequency resolution in those observations with high-time resolution has still remained up to 1 MHz. Higher-resolution observations and more detailed analyses are required to understand of fine-structured radio bursts, as well as particle and wave dynamics in the solar corona.
In this regard, we established new spectral observations in the range of 315–332 MHz at an extremely high resolution of 100 kHz and 10 ms. The details of the instrumentation are described in Section 2, the statistical characteristics of the observed fine structures are covered in Section 3 and our interpretation is presented in Section 4. Section 5 gives conclusions of the present study.
A new instrumentation for the observations of solar radio emission in the meter-wave range, the Zao Solar Ra-diospectrograph (ZSR), was established in June 2008 at the Zao observatory (latitude: 38°06′, longitude: 140°32′ E, altitude: 685 m) of Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Japan. The observation system is based on system for observation of Jovian synchrotron radiation by Watanabe et al. (2005). The ZSR system is fed by a 4 × 2 stacked 27-element cross-Yagi antenna manufactured by Creative Design Corp., covering a frequency range around 327 MHz. The antenna effective area is about 20 m2, with a half-power beam width of about 8°. The antenna has a typical altazimuth mount and follows the Sun continuously.
The ZSR’s observation parameters.
27-element, 8-stack Yagi antenna
Antenna effective area
Total system gain
Minimum detectable sensitivity
About 75 % of events show positive drift rates.
The distribution of the drift rate has a peak in the range of 20–40 MHz s−1, and only 2.6 % of events have the drift rates of approximately −9 MHz s−1, a typical value for fiber bursts.
In the physics of solar radio emissions, various emission mechanisms have been studied such as gyro-synchrotron, bremsstrahlung, and plasma emissions. In the observed frequency range, plasma emissions have been mainly considered (e.g., Iwai et al., 2013). In the plasma emission theory, the frequency drift of emission features can be explained by (i) non-thermal electron beams, which are common in type III bursts (e.g., Suzuki and Dulk, 1985), or (ii) whistler-mode waves, thought to be the origin of fiber bursts (e.g., Kuijpers, 1975). Therefore, positive drift rates are interpreted as downward motions of the either emission source. In the following sections, we discuss the applicability of these two ideas in the context of the observed level of fine structure from the point of the amplitude of the drift rates.
4.1 Non-thermal electron beams
It is well known that the temperature in the corona after the start of a solar flare often reaches about 10 MK (e.g., Fernandes et al., 2012). Thus, the derived electron energy is at a comparable level to that of the flare related plasma, which seems too small to be a non-thermal electron beam generating radio emission. Therefore, we conclude that fine-structure events with a drift rate of a few tens of MHz s−1 are not caused by non-thermal electron beams. It should be noted that the magnetic field lines along which the emission sources move are assumed to be completely perpendicular to the solar surface. If the magnetic lines are oblique to the radial direction, the speed of the motion of the wave source corresponding to the observed drift rate can be larger than the estimated value. We do not rule out non-thermal electron beams moving along oblique magnetic field lines as a possible wave source of the event with a drift rate of a few tens of MHz s−1
On the other hand, events characterized by the higher drift rates, especially above 100 MHz s−1 which corresponds to about 7.6 keV of the energy of electrons, seem to have their origin in such an electron beam because they can have the higher speed and be an non-thermal electron beam.
4.2 Whistler-mode waves
It is demonstrated that whistler-mode waves with a group velocity corresponding to the observed drift rate of order 40 MHz s−1, which is not typical for the fiber bursts, can exist and that the observed drift emission can be generated by whistler-mode waves. In this study, the frequency of whistler-mode waves of 0.25 ωce at which the group velocity is maximal is taken account, whereas the frequency of whistler-mode waves was assumed approximately 0.1 ωce in previous studies of fiber bursts (e.g., Chernov et al., 2008). This is the main reason why the fine structures with a drift rate of a few tens of MHz s−1 is explained by the same mechanism of typical fiber bursts which have approximate −9 MHz s−1 in the observed frequency range.
In the present study, we assumed that whistler-mode waves are excited by energetic electrons generated in the associated flare. Here we consider the plasma environment in the coronal region. In the observed coronal region where the plasma frequency is around 325 MHz, the ratio of the plasma frequency to the cyclotron frequency, ωp/ωce is estimated to be 10. This condition is similar to the plasma environment in the terrestrial magnetosphere, where whistler-mode waves are generated under the presence of energetic electrons of a few to tens of keV having anisotropic velocity distribution (e.g., Kennel and Petschek, 1966). Since keV electrons should be ubiquitous in the coronal region after flare, we can expect the generation of whistler-mode waves at the frequency range around 0.25 ωce assumed in the present study.
Events that exhibit faster drift rates than about 100 MHz s−1, on the other hand, cannot be explained by the whistler-mode waves. They are assumed to be generated by nonthermal electron beams as well as type III bursts. The characteristic drift rate that divides the two generation mechanisms discussed above is not clear from the histogram in Fig. 4.
To investigate fine-structure details of solar type IV bursts, we established a new radio observation system at the Zao observatory of Tohoku University. Narrowband fine structures that are difficult to detect based on observations with typical spectral resolutions were observed in type IV bursts on 2 and 3 November 2008. Statistical analysis of the drift rate shows that they are different from ordinary fiber bursts in both sense and magnitude. First, the observed drift rates exhibit both positive and negative rates, whereas ordinary fiber bursts usually have negative drift rates. Second, the absolute values of the observed drift rates are tens of MHz s−1, while the typical drift rate of fiber bursts at 325 MHz is approximately −9 MHz s−1 (e.g., Benz and Mann, 1998). In addition, the all analyzed fine structures have narrow emission bands of less than 17 MHz. The observed narrowband emissions features with drift rates of a few tens of MHz s−1 at 325 MHz can be interpreted by the generation process of fiber bursts (Kuijpers, 1975); i.e., the emission features are thought to be caused by whistlermode waves propagating in the corona. These observed narrowband events are difficult to detect with the traditional resolution of 1 MHz and 0.1 s. These results imply that the higher resolution used in the present study is necessary to investigate fine structures accompanying type IV bursts.
Based on the observations using ZSR, only one event was detected, which consists of the radio burst with narrowband fine structure. The number of events is too small to discuss their occurrence rate. In addition, the ZSR’s frequency range is too narrow to survey their association with other radio bursts in different frequency range and the variance of the coronal loop structures and the distribution of energetic electrons obtained through imaging observations such as with Hinode, STEREO, SDO, and the Nobeyama Radio Heliograph. This problem can be overcome with new wide-band and high-resolution observations of solar radio bursts, such as obtained with IPRT/AMATERAS (Iwai et al., 2012).
The ZSR instrumentation is supported by the Planetary Plasma and Atmosphere Research Center, Graduate School of Science, Tohoku University. The author wishes to thank Drs. Maki Akioka and Tetsuro Kondou who provided radio spectra observed at HiRAS. This work was carried out as part of a joint research program with the Solar-Terrestrial Environment Laboratory at Nagoya University. This work was also supported by the Global COE (Center of Excellence) program, ‘Global Education and Research Center for Earth and Planetary Dynamics’, at Tohoku University (Principal Investigator: Prof. Eiji Ohtani)
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