“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.
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.
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.
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)