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Acta Geophysica
vol. 64, no. 3, June 2016, pp. 825-840
DOI: 10.1515/acgeo-2016-0028
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Ownership: Institute of Geophysics, Polish Academy of Sciences;
© 2016 Dąbrowski et al. This is an open access article distributed under the Creative Commons
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http://creativecommons.org/licenses/by-nc-nd/3.0/.
Prospects for Solar and Space Weather Research
with Polish Part of the LOFAR Telescope
Bartosz P. DĄBROWSKI1, Andrzej KRANKOWSKI1,
Leszek BŁASZKIEWICZ1,2, and Hanna ROTHKAEHL3
1Space Radio-Diagnostics Research Centre, University of Warmia and Mazury,
Olsztyn, Poland; e-mail: bartosz.dabrowski@uwm.edu.pl (corresponding author)
2Faculty of Mathematics and Computer Sciences, University of Warmia
and Mazury, Olsztyn, Poland; e-mail: leszekb@matman.uwm.edu.pl
3Space Research Centre, Polish Academy of Sciences, Warsaw, Poland;
e-mail: hrot@cbk.waw.pl
Abstract
The LOw-Frequency ARray (LOFAR) is a new radio interferome-
ter that consists of an array of stations. Each of them is a phase array of
dipole antennas. LOFAR stations are distributed mostly in the Nether-
lands, but also throughout Europe. In the article we discuss the possibil-
ity of using this instrument for solar and space weather studies, as well as
ionosphere investigations. We are expecting that in the near future the
LOFAR telescope will bring some interesting observations and discover-
ies in these fields. It will also help to observe solar active events that
have a direct influence on the near-Earth space weather.
Key words: telescopes, LOFAR, interferometers, radio, Sun, space
weather, ionosphere.
1. INTRODUCTION
The science program of LOFAR is very broad and is organized in “Key Sci-
ence Projects” (hereafter called KSP). Currently we have the following
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KSPs: (i) epoch of reionization, (ii) surveying the low-frequency sky, (iii) the
transient radio sky, (iv) pulsar studies and surveys, (v) astroparticle physics,
(vi) magnetic fields in the universe, and (vii) solar physics and space weather
(van Haarlem et al. 2013). The calculations show that LOFAR will be a
relevant instrument for Jupiter-like planets with an active moons research. It
will be also useful in agriculture field investigations.
In this paper we will focus on solar physics and space weather, as well as
ionosphere studies by LOFAR.
2. LOFAR
The LOFAR telescope (www.lofar.org), designed and constructed by
ASTRON (the Netherlands Institute for Radio Astronomy), was officially
inaugurated by Her Majesty Queen Beatrix in June 2010. It is a large radio
interferometer operating in the frequency range 10-240 MHz (corresponding
to wavelengths of 30.0-1.2 m). LOFAR consists of an array of dipole an-
tenna stations distributed throughout Europe. Each station contains two an-
tenna fields: LBA (Low Band Antennas) and HBA (High Band Antennas).
The LBA occupies an area with a diameter of 80 m (for full configuration –
96 antennas) and operates in the frequency range 10-90 MHz. The HBA an-
tennas occupy an area with a diameter of 62 m (for full configuration – 96
tiles) and operate in the frequency range 110-240 MHz. The lack of receiver
in the frequency range 90-110 MHz is a result of using two different front-
ends systems (LBA and HBA) and incomparably more electromagnetic in-
terferences (EMI) observed in this radio band. Both LBA and HBA consist
of 48 or 96 elements per station. Typical international LOFAR station was
presented in Fig. 1.
The total number of LOFAR stations is 50, 38 of which are in the Neth-
erlands and 12 international stations are located in Germany (6 stations), Po-
land (3 stations), and one station each in France, Sweden, and UK (Fig. 2).
Most of the LOFAR stations are located in the Netherlands, 24 of them are
parts of the “core”; 6 stations out of that 24 were located on island of about
350 m diameter – “Superterp”. It is situated in the central part of the core.
The remaining 14 stations in the Netherlands, called “remote”, are located
outside the core at a distance up to 90 km. Each one of the International
LOFAR Telescope (ILT) stations is connected by broadband ~10 Gb/s net-
work with data centre in the Netherlands where the correlator is located.
When stations are not working in ILT mode, the data from each station can
be transferred to local data center (van Haarlem et al. 2013).
The angular resolution of the whole LOFAR network depends on the
baseline and frequency of observations. For the baseline around 1550 km the
angular resolution of the instrument is 0.1 arcsec at 240 MHz (the highest
frequency of LOFAR observations) and about 3.2 arcsec at a frequency of
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
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Fig. 1. The configuration of a typical international LOFAR station. The left circle
represents the LBA antennas and the right one depicts HBA tiles. Picture taken from
ASTRON technical documentation.
Fig. 2. International LOFAR stations network in Europe. It is formed by50 stations,
of which 38 are in the Netherlands and the rest are scattered across Europe.
POLFAR includes three stations: Bałdy, Borówiec, and Łazy each of them is
marked by a special code (given in parentheses).
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10 MHz (the lowest frequency of LOFAR observations). In the case of inde-
pendent observations, with the use of only three Polish stations (as a separate
interferometer), the angular resolution will be around 0.5 arcsec for
240 MHz (Błaszkiewicz et al. 2016, van Haarlem et al. 2013).
The maximum spectral channel width is 195.3125 kHz, which corre-
sponds to one subband. To provide higher spectral resolution, subband can
be split into several channels – usually 16, 64, 256 or 2048. The time resolu-
tion is calculated by taking the inverse of the frequency resolution. For typi-
cal observations by LOFAR telescope we use 256 channels per subband and
we obtain a maximum time resolution of 1.3 ms. For 2048 channels (maxi-
mum channels per sub band) we obtain a maximum time resolution of 10 ms
(van Haarlem et al. 2013).
The expected sensitivity of the LOFAR depends of the array configura-
tions. For the LOFAR core with an 8 hour integration time and an effective
bandwidth of 3.66 MHz is 9.0 mJy for 30 MHz and 0.38 mJy for 180 MHz
(van Haarlem et al. 2013). Some additional information about the technical
aspects of LOFAR can be found in Błaszkiewicz et al. (2016).
3. POLFAR
POLFAR – Polish LOFAR Consortium – was established in 2007. POLFAR
consists of the following institutions: University of Warmia and Mazury
(Olsztyn); Jagiellonian University (Kraków); Space Research Center, Polish
Academy of Sciences (Warszawa); Szczecin University; Nicolaus Coperni-
cus University (Toruń); Nicolaus Copernicus Astronomical Center, Polish
Academy of Sciences (Warszawa); University of Zielona Góra; Wrocław
University of Environmental and Life Sciences; and Poznań Supercomputing
and Networking Center.
In 2014 a contract for construction of three new LOFAR stations in Po-
land was signed between ASTRON and POLFAR. These stations were built
at Bałdy (53°35′45″N, 20°35′26″E, station code: PL612) operated by the
University of Warmia and Mazury (Fig. 3), Borówiec (52°16′37″N,
17°04′28″E, station code: PL610), operated by the Space Research Center of
the Polish Academy of Sciences, and Łazy (49°57′53″N, 20°29′23″E, station
code: PL611) operated by the Jagiellonian University. Such a number of
LOFAR stations allows creating an independent interferometer, that can be
used when these stations do not take part in the LOFAR network observa-
tions. Construction of the three new LOFAR stations in Poland will signifi-
cantly improve the resolution and sensitivity of the whole LOFAR
interferometer. The longest baseline in the LOFAR network is situated be-
tween Chilbolton (UK) and Łazy; it is around 1550 km (Krankowski et al.
2014).
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
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Fig. 3. The LOFAR station in Bałdy (code: PL612).
4. LOFAR AS A TOOL FOR IONOSPHERE STUDY
One of the challenges in the design and operation of the LOFAR telescope is
the calibration of the influence of the ionosphere which, at low frequencies,
is not uniform and can change within minutes – timescales shorter than the
length of observations (Krankowski et al. 2007).
At low frequencies (< 300 MHz), at which LOFAR works, the dominant
effects of the ionosphere are refraction, propagation delay (Fig. 4) and Fara-
day rotation. For the LOFAR telescope, the ionosphere is the main source of
phase errors in the visibilities (Thompson et al. 2001, Intema et al. 2009).
LOFAR specifications of importance for ionosphere research are presented
in Table 1.
Table 1
Specifications of the LOFAR telescope for ionospheric research
(Gaussiran II et al. 2004)
Characteristic LOFAR telescope
Temporal resolution 1 s
Horizontal resolution 2 m
Vertical resolution 2 m
Relative accuracy < 0.001 TECU (Total Electron Content Units)
The delay per array element depends on the total electron content (TEC)
along the line-of-sight through the ionosphere, and therefore on antenna
position and viewing direction. The calibration of observations at low fre-
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830
Fig. 4. The ionosphere causes propagation delay differences between array elements,
resulting in phase errors (Δφ) in the visibilities. The delay per array element depends
on the line-of-sight through the ionosphere, and therefore on antenna position and
viewing direction. The calibration of observations at low frequencies requires phase
corrections that vary over the field-of-view of each antenna (Intema et al. 2009).
quencies requires phase corrections that vary over the field-of-view of each
antenna (Intema et al. 2009). Calibration methods that determine just one
phase correction for the full viewing cone of each antenna (like self-
calibration) are therefore insufficient. So, special methods of calibration are
required and these methods can give us the ionospheric details at the time of
observation as an additional result (Sotomayor-Beltran et al. 2013).
The calibration methods used in data reduction of the LOFAR network
are based on adapting of system to ionospheric conditions. This method is
using a known radio source to correct for ionospheric phase distortion on
each receiving element of interferometer with the use of ionospheric state
predictions. The correction of phase during calibration is just the amendment
to the model used and together with the model it is giving the real
ionospheric state during observations in source direction for each telescope
component. It should also be converted to TEC number and serve in surveys
on the structure of the ionosphere (Gaussiran II et al. 2004).
The ionosphere changes during the observation within the station beam,
and direction-dependent calibration is required (van der Tol and van der
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
831
Veen 2007). Also the ionosphere is different for different stations and causes
different distortions at different stations. Identifying and removing these ra-
dio wave distortions is an excellent opportunity for measurements of the
TEC along the line of sight of each LOFAR antenna (Gaussiran II et al.
2004).
Currently, most data about the state of the ionosphere comes from the in-
ternational network of ground-based GPS receivers, operating in real-time.
However, the spatial and temporal sampling of available GPS data is not suf-
ficient for complete calibration of radio astronomical measurements and is
not reliable enough for ionospheric modeling. However, linking real-time
GPS data to the data reduction scheme of telescopes such as LOFAR, as well
as linking ionospheric monitoring using known radio sources to tomographic
inversion of the ionosphere from GPS measurements, will provide ad-
vantages to both disciplines.
Gaussiran II et al. (2004) perorate that tomographic techniques can be
used to invert the thousands of changing and independent TEC measure-
ments produced by LOFAR into three-dimensional electron density specifi-
cations above the array. These specifications will have higher spatial and
temporal resolutions than are routinely available by other techniques. These
specifications will be used to investigate small-scale changes of ionospheric
irregularities, equatorial plasma structures, and ionospheric waves. In addi-
tion, LOFAR will improve the understanding of the solar drivers of the iono-
sphere by simultaneously measuring the solar radio burst and the TEC
(Gaussiran II et al. 2004).
5. SUN OBSREVATIONS AT METRIC WAVELENGTHS
Solar radio observations are made at a large range of wavelengths. They are
divided into microwaves (f > 3 GHz), decimeter/meter (f < 3 GHz), dekame-
ter (f < 30 MHz), and hectometer/kilometer (f < 3 MHz) (Warmuth and
Mann 2005). All this is also a result of solar radiation properties in different
wavelength ranges.
The solar radio emission is a source of information about the structure
and dynamics of the solar atmosphere and it can be divided into: (i) quiet
Sun component, (ii) slowly varying component, and (iii) sporadic (burst)
component (Kundu 1965).
The solar radio burst emission is mainly a plasma emission that is emit-
ted with the local plasma frequency of the source region in corona, or one of
its first harmonics. The plasma frequency depends on the electron density
and is given by:
[
]
3
8.98 10 Hz ,
pe
N=×
ν
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832
where Ne is the electron density in cm–3 (Melrose 1985). It shows that the
emissions on different frequencies come from different layers of the solar
atmosphere (the electron density decreases with altitude in the solar atmos-
phere); the longer the wavelength, the higher the layer where it is formed.
A commonly used coronal density model is the one proposed by
Newkirk (1961):
()
4.32
010 s
R
R
e
NR N=×
α
with N0 = 4.2 × 104 cm–3, where R is the radial distance from the solar sur-
face (normalized to the solar radius Rs), and the enhancement factor
α
=
1 ÷ 4 (depending on whether the burst takes place in the quiet corona or near
an active region). So the density model of the solar corona provides a link
between frequency and height.
If we use the density model of the heliosphere proposed by Mann et al.
(1999), which is necessary to obtain information about changes in the elec-
tron density with the distance from the Sun, then it turns out that the source
of radio waves that can be observed by LOFAR, operating in the frequency
range 10-240 MHz, are located in the middle and higher corona. At
240 MHz, the highest frequency at which LOFAR operates corresponds to
the height R = 1.17 Rs. It is also the lowest altitude in the solar atmosphere
where we can study radio bursts. It corresponds to approximately
120 000 km above the photosphere. In this region of the solar atmosphere we
observe radio burst responding to Coronal Mass Ejections (CMEs). The
study of such phenomena is particularly important for space weather because
they can trigger ionospheric storms (Mann et al. 2007).
6. SOLAR RADIO BURST AT METRIC WAVELENGTHS
In general, we distinguish five main types of solar radio bursts, from type I
up to type V (Wild and McCready 1950, Wild 1950a, b). The LOFAR oper-
ating in the frequency range 10-240 MHz where can be observe all these
events. In this chapter, we will briefly describe each of them. The basic clas-
sification of solar radio bursts in the frequency range 25-400 MHz is pre-
sented in Fig. 5.
6.1 Type I solar radio bursts
Type I solar radio bursts are narrowband (a few MHz), in general non-
drifting and of short duration (< 1 s). This type of bursts appears only at met-
ric wavelengths. They usually occur in large numbers, forming irregular
structures superposed on a continuous background. This so-called noise
storms can last from a few tens of minutes to several days. Long duration is
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
833
Fig. 5. Schematic diagram shows the basic classification of solar radio bursts in the
frequency range 25-400 MHz (based on Ganse et al. 2012).
a characteristic feature that distinguishes it from other types of solar radio
emission. Their emission mechanism is the fundamental-frequency plasma
emission (Melrose 1975).
6.2 Type II solar radio bursts
Type II radio bursts occur in a wide frequency range, from metric to kilome-
ter wavelengths (Nelson and Melrose 1985). This type of bursts is character-
ized by slowly drifting (rate) from 0.1 up to 1 MHz/s, from high to low
frequencies, which means that the exciting agent moves up out of the corona
(Warmuth and Mann 2005).
The typical type II bursts show the two-band emission on dynamic spec-
trum consisting of the fundamental emission band and the harmonic emis-
sion band at about twice the frequency of the fundamental one. The
fundamental emission refers to the plasma frequency of the emission region,
slowly decreasing over time as the coronal/interplanetary shock travels out-
wards into the heliosphere (Cane et al. 1987, Nelson and Melrose 1985). The
radio bursts of this type are generated by magneto-hydrodynamic shock
waves, which propagate through the solar corona and interplanetary space.
In addition, they are associated with flares and CMEs, but there is no one-to-
one correspondence.
6.3 Type III solar radio bursts
Particularly important in the study of the processes that occur in solar flares
are type III radio bursts. Type III radio bursts have been identified and ex-
tracted as a separate class by Wild and McCready (1950). They appear in the
range from 1 GHz up to 10 kHz, which corresponds to a source region from
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834
the lower part solar corona up to beyond 1 AU. Type III radio bursts are
characterized by high drift rates in frequency (around 100 MHz/s) towards
lower frequencies (corresponding to upward movement of the exciting agent
in the solar corona) and short duration (seconds). At metric waves the domi-
nant drift is from high to lower frequencies. Type III radio bursts appear in
groups, of approximately 10 bursts, and their duration is a few minutes. In
the corona, the type III radio emission regions are moving with velocities
from about 0.1 to 0.6 c (Benz 2002) with c being the speed of light. Type III
radio bursts are caused by the emission of electron beams. These beams are
moving up in the solar corona along magnetic field lines stimulating the
electron plasma to oscillations with the local plasma frequency (Warmuth
and Mann 2005). Acceleration of the electron beam, responsible for generat-
ing type III radio bursts, is probable in the areas of magnetic energy release
(Aschwanden 2004).
6.4 Type IV solar radio bursts
Type IV radio bursts have been identified for the first time in 1957 by
Boischot and Denisse (1957). Type IV solar radio bursts are broadband con-
tinua associated with flares. They are divided into two categories: stationary
and moving type IV. The stationary bursts are characterized by a broadband,
long lasting continuum with fine structures, like pulsations, zebra patterns,
and fiber bursts. They can last from hours to days and they are observed in
the range from 20 MHz up to 2 GHz. The moving type IV bursts are charac-
terized by a slow drift in frequency (this corresponds to a movement of the
exciting agent with velocities of up to several 100 km/s). The duration of this
type of phenomenon is from 30 minutes up to 2 hours and it appears in the
range from 20 up to 400 MHz (Warmuth and Mann 2005).
6.5 Type V solar radio bursts
Type V radio events are the rarest type of radio bursts observed. They are
broadband continuum emissions that occur generally lower than 200 MHz
frequencies. Type V radio bursts start during or right after a group of type III
burst, with duration of a minute or so. In the type V model source, coherent
Čerenkov plasma waves are excited by fast electrons (speed around 1/3 c)
ejected from a flare and oscillating between mirror points in a magnetic trap
in the corona (Weiss and Stewart 1965).
7. SOLAR RESEARCH WITH LOFAR
Solar observations with the LOFAR telescope will be carried out in different
basic modes: (i) routine imaging, (ii) solar bursts mode, (iii) joint observa-
tion campaigns, and (iv) single stations as spectrometers (Mann 2007). For
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
835
observations in these modes we can use a single station or small number of
stations. In the following subsections we describe in detail each of these ob-
servation modes.
Unfortunately, due to the scattering of radio waves within the solar coro-
na, the spatial resolution of the images is limited to a few 10 arcsec. For this
reason, at the beginning it is necessary to use the central core and the nearest
remote stations with baseline up to a few 10 km. Once the routine imaging of
the Sun is established, it will be possible to investigate to what extent the in-
clusion of longer baselines does improve the images (Mann 2007).
7.1 Routine imaging
In the routine image Sun monitoring mode, the Sun will be observed at about
20 frequencies through the whole LOFAR band. The images on each of
these frequencies should be taken simultaneously. When it comes to the re-
search of the long-term evolution of solar active regions, the proposition is
that images would be taken with a cadence of 1/min (Mann 2007).
7.2 Solar bursts model
Solar radio bursts are observed when the LOFAR telescope operates in the
burst mode. Solar images will be taken in this mode every 0.1 s, with at least
four channels (frequencies) that include frequency pairs separated by a factor
of two in order to detect radio radiation on the fundamental and first har-
monic frequency. The integration time for a single image will not exceed 50-
100 ms. The response time of the LOFAR telescope after observing the flare
(e.g., by using another instrument) should be as short as possible, at least less
than 1 s. All images on different frequencies should be done at the same time
(Mann 2007).
The use of some spectrometer that registers the solar radio bursts at
higher frequencies than LOFAR, e.g., up to 800 MHz, is very useful (it cor-
responds to lower heights in the solar corona). Therefore, in this case it is
possible to detect the burst before it reaches the frequency band in which
LOFAR works (Mann 2007).
7.3 Joint observation campaigns
Joint observation campaigns with other ground- and space-based instru-
ments, like GREGOR, a solar telescope located at the Observatorio del Teide
on Tenerife (visible light and near infrared), Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) satellite (working in X-rays and
gamma-rays), NASA’s Solar Dynamics Observatory satellite (extreme ultra-
violet and visible light), or Atacama Large Millimeter/submillimeter Array
(ALMA) radio interferometer (millimetre and submillimetre radio waves)
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that observe the Sun in different parts of the electromagnetic spectrum, will
let us investigate different aspects of solar activity, like for example solar
flares (http://www.aip.de/groups/osra/sksp/, access: 16 June 2015).
7.4 Single stations as spectrometers
Using a single LOFAR station, spectroscopic observations of the Sun can be
performed. In this case, we obtain a so-called dynamic spectrum which
shows the relationship of time, frequency and intensity. It is assumed that in
this mode the observations are carried out in all 165 channels, each having a
width of 195 kHz. The integration time for each spectrum will be 10 ms. It is
estimated that the total time of observations in this mode will be around 8
hours per day. The temporal resolution of such observations must be high
enough, so as to be able to observe the solar radio activity phenomenon,
whose duration can be several tens of milliseconds (Mann 2007).
8. CONCLUDING REMARKS
On 17 March 2011, during LOFAR’s commissioning phase, LOFAR ob-
served a solar radio burst, for the first time. It was a type I radio burst, seen
at 150 MHz on the west limb of the Sun (van Haarlem et al. 2013).
On 28 February 2013, LOFAR observed, over a period of 30 min, multi-
ple type III radio bursts obtaining radio images as well as high-resolution
dynamic radio spectra (Fig. 6). A number of bursts were found to be located
at high altitudes, around four solar radii from the solar center in the case of
the 30 MHz burst (Morosan et al. 2014).
Fig. 6. Images of type III radio burst observed on 28 February 2013 at: (a) 50-
55 MHz, (b) 40-45 MHz, and (c) 30-35 MHz, separated by 1 s. On the radio image
was superposed the EUV image recorded by the Atmospheric Imaging Assembly
onboard NASA’s Solar Dynamics Observatory satellite. The triangle and square
symbol indicated the location of the beams; for details see Morosan et al. (2014).
The radius of the dotted white line circle is 3 solar radii. Ellipse marked by a contin-
uous white line, on the lower right corner of each figure, is the telescope beam size.
This figure was published in Morosan et al. (2014).
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PROSPECTS FOR SOLAR RESEARCH WITH THE LOFAR
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The first solar observations with the LOFAR telescope indicate that it is
well suited for solar research at low frequencies (from 10 up to 240 MHz). In
the near future it will certainly bring some interesting observations and dis-
coveries. It will also help to observe solar active events that have a direct in-
fluence on the near-Earth space weather, a good knowledge and prediction of
which is of essential importance for our ever increasing dependence on tech-
nology from and in space and for the increasing presence of humans in
space.
Ac kn ow le dg me nt s. T he Polish LOFAR stations have been funded
by the Polish Ministry of Science and Higher Education; the funds of the
large research infrastructure “Construction of the station Polish European
LOFAR radio interferometer” (grant No. 6339/IA/158/2013.1).
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Received 19 October 2015
Received in revised form 17 February 2016
Accepted 19 April 2016
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