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Decameter Stationary Type IV Burst in Imaging Observations on the 6th of September 2014

Koval A., Stanislavsky A., Chen Y.,  Feng S.,  Konovalenko A., and  Volvach  Ya.


Solar bursts are ones of significant signatures of solar activity. Particularly, the type IV bursts frequently associated with the most powerful and large-scale solar events such as solar flares and coronal mass ejections (CMEs). These bursts represent broadband continuum emissions with a variable time structure and have two distinct classes: moving (type IVm) and stationary (type IVs) bursts. Usually each stationary radio burst is the broadband flare-related continuum emission. It is believed that this long-lasting radiation is thought to be produced by energetic electrons trapped within coronal structures such as magnetic arches. So far, following accomplished review of scientific papers the spatial features of type IVs bursts' sources at low frequencies (meter-decameter wavelength range) of observations still remain unexplored.


In the study we present results of the first-of-its-kind positional measurements of a solar stationary type IV burst in decameter wavelengths (frequencies below 30 MHz) expanded by heliographic observations in meter wavelength domain.  In our research we consider the event occurred on 6 September 2014. The multi-frequency in-depth analysis demonstrates the complex nature of the considered event. Particularly, on synthesized dynamic spectrum the stationary type IV radiation appears in form of two visibly separated low- (LF )and high-frequency  (HF) type IV bursts (see Figure 1). The slow and wide CME playing a special role in the event took place during the type IV emission. 

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Figure 1. Dynamic spectrum of the type IV burst on 6 September 2014. The combined spectrum is obtained from records of ORFEES (450-144 MHz), e-CALLISTO (144-100 MHz), NDA (80-33 MHz), and UTR-2 (33-17 MHz) instruments. The UTR-2 contribution was shorter in time than ones of other telescopes, as its observation session was finished at 13:00 UT. Different components of the radio burst as well as other registered radio events have been labeled on the figure.


We mainly focus on low-frequency imaging measurements of type IV radio emission taking with the help of UTR-2 Radio Heliograph (Kharkiv, Ukraine) for the first time within 18.5-31 MHz supplemented by Nancay Radio Heliograph (NRH) observations at several working frequencies from 150 MHz up to 327 MHz. Using radio imaging measurements provided by the NRH and the UTR-2 Radio Heliograph we investigated spatial features of corresponding HF and LF radio sources of stationary type IV bursts. Figure 2 shows time series of synthesized images.


We revealed that the radio source locations changed with the CME propagation from the Sun as well as the HF sources and the LF sources are in good alignment with each other above the CME-associated AR. We provided several convincing arguments confirming that the HF-LF type IV bursts are common broadband stationary type IV radio emission and their sources belong to single radio source.


According to the heliographic observations we suggest the burst source was confined within a high coronal loop, which was a part of the CME. In such a way, the type IV radiation emitted from a single high-lying loop with one foot located around the AR from which the CME emerged. The loop can be delineated by connecting the HF and LF sources as shown in Figure 2. The NRH measurements demonstrate a movement of the HF radio emission source inside the magnetic loop close to its base. The UTR-2 heliographic images show very similar features, but only higher, in the outer corona. According to UTR-2 radio images the uttermost altitudes for 20 MHz, 25 MHz, 30 MHz emission layers were around 4.5RS, 3.9 RS, and 3.0 RS, correspondingly. The angular evolution may indicate the response of the presumed loop structure to the CME disturbance.

 

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Figure 2. Composite time series from SOHO/LASCO C2 running-difference and SDO/AIA (171 Å channel) direct images overplotted by the locations of radio emission maxima obtained from UTR-2 intensity radio heliograms and from NRH brightness temperature images. The spatial evolution of high-frequency and low-frequency parts of the type IVs sources is shown with respect to the CME propagation in time.


The main research includes solar spectral and imaging measurements carried out by the UTR-2 radio telescope in the range 18.5-31.0 MHz. The spectral analysis of the type IVs radio emission was performed together with radio data from higher frequencies observed by NDA (33.0-80.0 MHz), e-CALLISTO (100-144 MHz) and ORFEES (144-450 MHz). All this allows us to consider many aspects of the complex event on 6 September 2014, especially the spatial characteristics and dynamical evolution of radio sources of the type IVs radio bursts.


The two-dimensional (2D) radio heliograph based on the UTR-2 radio telescope (see Figure 3) is a radio astronomical instrument for regular low-frequency solar observations within 8-33 MHz. The radio heliograph is an updated device for obtaining two-dimensional images of brightness distribution of radio emission from the Sun. The field of view of the heliograph covers the spatial sector 2.1°×3.3° in sky. The example of UTR-2 radio heliograms is shown in Figure 4. In this case the data were collected in a so-called three-dimensional (3D) cube, along two spatial coordinates and at frequency. Usually the processing of imaging measurements obtained by low-frequency radioastronomical instruments  (for example LOFAR, UTR-2) is complicated procedure because of peculiarity of radio image creation at low frequencies (antenna side lobes effect, radio frequency interferences, and others). In our study, after meticulous treatment of huge amount of UTR-2 heliograms (about 1.5×106) we presented radio images showing spatial structure of stationary type IV burst for the first time in decameter wavelength range.  

 

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Figure 3. View of “North” arm of UTR-2 antenna array situated near Kharkiv city in Ukraine. The white island in the background is one section of developing Giant Ukrainian Radio Telescope (GURT) which covers 8-80 MHz frequency range of observations.   

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Figure 4. Three-dimensional angular structure of the type IV burst at 12:00:24 UT in the frequency range 18.5-31.0 MHz according to the UTR-2 Radio Heliograph data. In these frames the radio emission increases within 25-29 MHz. The intensity value is expressed in relative units. The white circle indicates the solar disk.

 

The paper “Decameter Stationary Type IV Burst in Imaging Observations on the 6th of September 2014” by Koval A., Stanislavsky A., Chen Y.,  Feng S.,  KonovalenkoA., and  Volvach  Ya. has been accepted by the Astrophysical Journal and will be published in the near future.

(download link: http://arxiv.org/abs/1606.00990)


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