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Decameter Stationary Type IV Burst in Imaging Observations on the 6th of September 2014
录入者:  |  时间:2016-06-02 10:06:03  |  作者:Koval A.  |  浏览次数:

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

(Koval A., Stanislavsky A., Chen Y.,Feng S.,Konovalenko A., andVolvachYa.)

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.

SFM

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.

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.

UTR(NORTH ARM)

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.

3D cube

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., andVolvachYa. 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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