06-07-2012, 01:38 PM
HELIOGRAPH OF THE UTR-2 RADIO TELESCOPE
[attachment=26971]
Abstract
The broadband analog-digital heliograph based on the UTR-2 radio telescope is
described in detail. This device operates by employing the parallel-series principle
when five equi-spaced array pattern beams which scan the given radio source (e.g.
solar corona) are simultaneously shaped. As a result, the obtained image presents
a frame of 5 × 8 pixels with the space resolution 25 ′× 25 ′ at 25 MHz. Each pixel
corresponds to the signal from the appropriate pattern beam. The most essential
heliograph component is its phase shift module for fast sky scanning by pencil-
shape antenna beams. Its design, as well as its switched cable lengths calculation
procedure, are presented, too. Each heliogram is formed in the actual heliograph just
by using this phase shifter. Every pixel of a signal received from the corresponding
antenna pattern beam is the cross-correlation dynamic spectrum (time-frequency-
intensity) measured in real time with the digital spectrum processor. This new
generation heliograph gives the solar corona images in the frequency range 8-32
MHz with the frequency resolution 4 kHz, time resolution to 1 ms, and dynamic
range about 90 dB. The heliographic observations of radio sources and solar corona
made in summer of 2010 are demonstrated as examples.
e-mail: alexstan[at]ri.kharkov.ua
1 Introduction
Comprehensive information on the physical processes that accompany solar
activity can be obtained only with engaging of an extensive scope of observation
means. At the meter wavelengths, radio spectrographs, heliographs
and polarimeters are widely used to meet this purpose [1]. A worse situation
is the case for decameter wavelengths. In particular, the heliograph
realization is technically a rather labor intensive problem. Obtaining the
high-resolution images (being one of the major demands in application of
heliographs) at that low frequencies requires the huge size antennas (> 1
km). At the same time, the heliograph can help obtain new useful information
on the extremely multifarious types of bursts being rather specific
to this range of frequencies.
Any heliograph has essential advantages in the case it ensures the possibility
of simultaneous observations at several frequencies. Multi-frequency
measurements of the positions of sources of Type II and III bursts allow
to immediately determine the velocity of shock waves and this of electron
beams which are propagated in the solar corona. Of substantial interest
are the measurements of the velocity of electron beams generating Type III
and IIIb bursts at different trajectory phase. The positional measurements
at several frequencies will probably permit evaluation of the magnitude of
visible displacement of burst sources with respect to their true position
caused by the refraction and scattering of radio waves in corona.
Thus, the heliograph permits observation of an angular structure of
burst sources and its evolution during their lifetime, as well as to identify
the bursts with the appropriate activity regions and measure their heights
above the photosphere. The position measurements of double Type IIIb-
III, III-III bursts and drifting pairs allow to find out whether the both
components are excited in the same place of the corona and at which
plasma frequency. Of special interest are the investigations of properties
and determination of the positions of coronal mass ejections, frequently
associated with the Type II bursts. This phenomenon is considered a key
one in the problems of solar-terrestrial relationships and space weather.
No less interesting direction of the research efforts are observations of
the two-dimensional brightness distributions across the quiet Sun at the
decameter waves. At the same time, as is known, just at the heights
where the decameter radiation is generated there exists the most interesting
corona region where the solar wind originates. The here-presented far from
exhaustive list of problems capable of solving with the heliograph makes
its construction a rather attractive idea. Especially, in case if there is an
opportunity of using the available antenna system suitable for this purpose.
To date, there exist not too many radio telescopes permitting to obtain
2D heliograms of the Sun and other sky radiation sources. In the microwave
range, they are represented by the Nobeyama heliograph (Japan)
operating at two frequencies, 17 and 35 GHz [2], and the RATAN-600 based
heliograph (Russia) operating at 3.75 GHz [3]. Roughly within these frequency
ranges, the observations are also made at the Siberian Solar Radio
Telescope (Russia) [4] at 5.7 GHz, and at the Owens Valley Radio Observatory
interferometer (USA) within 1-18 GHz, whose whole range being split
into 86 individual subbands [5]. The anticipated Chinese heliograph being
designed for radio measurements within 0.4-15 GHz [6] can be considered
belonging under the promising instruments, too. For centimeter and meter
wavelength radio astronomy investigation, used are the Nancay radioheliograph
(France) [7], operating only at frequencies 169, 327 and 408 MHz,
and the Gauribidanur radioheliograph (India) which surveys at some individual
frequencies within 40-150 MHz range [8]. Since 1969 till 1984,
the Culgoora radioheliograph (Australia) [9] was functioning at frequencies
43, 80, 160 and 327 MHz. Note that very few instruments were used
at the decameter wavelengths. In this respect we may mention the Clark
Lake Teepee-Tee radio telescope (USA) [10] at which the measurements at
some individual frequencies within 15-125 MHz were made. Though for
now, it is already put out of service. It will be observed also that developing
the antenna arrays capable of operating low frequencies (decameter
wavelengths) is connected with heavy technological and methodological
difficulties. This frequency range is subject to very heavy noise conditions,
the ionosphere exerts an essential impact on the behavior of decameter
wave propagation, and achieving the narrow beam pattern requires antennas
of huge areas. For instance, the Nancay Decameter Array (Nancay,
France) [11] is effectively used for getting dynamic spectra of solar radio
emission, while because of its small sizes practically never used for heliographic
observations. On account of these reasons, nowadays only a few
decameter range antenna systems exist capable of performing heliographic
observations, though they also have limited capacities, and thus need upgrading,
or even new antenna systems should be built. Suffice it to mention
the promising projects LOFAR [12, 13] and LWA [14] which are to be put
into service in the near future. Therefore the researches carried out with
the UTR-2 based heliograph are of great scientific interest, making up for
a deficiency in the knowledge of physical processes on the Sun and/or in
the circumsolar space. Note that until now, the UTR-2 radio telescope
remains the world-largest and most effective instrument operating within
8-32 MHz, and it is expected to remain that kind performer in long-term
future. Accordingly, its intensive design improvement, instrumentation
and usage (this including in heliographic mode, too) promises a vast deal
of useful astrophysical information.
The high solar activity is frequently accompanied by sporadic radio
emission in the meter and decameter wavelength ranges [1, 15]. Some of
its types (I, II, III and IV) are common for these ranges. At the same
time, they have their inherent specificity. For instance, the Type I bursts,
most frequent at meter waves, are practically not observed at the decameter
wavelengths. On the other hand, the short-lived stria-bursts, drifting pairs,
Type IIIb bursts are met only at frequencies below 60 MHz, though the
latter, along with Type III bursts, are the most numerous events of the
solar decameter radiation [16].
The solar decameter sporadic radiation has begun to be systematically
studied with the work of Ellis and McCulloch [17]. Since then, a substantial
progress was reached in understanding its nature. Meanwhile, the theoretical
formulation of the mechanism and conditions of solar burst generation
cannot yet give, in most cases, an adequate representation. Therefore further
analysis of new experimental data, including those observed with the
heliograph, is of current importance.
The present paper is devoted to the UTR-2 heliograph construction, its
general functional scheme and discussion of its most important features.
In Section 2 we start with the history of heliograph studies in our institute.
The basis of any radioheliograph, the UTR-2 based heliograph making no
exception, is the antenna system. In Section 3, this latter brief description
as applied to the heliograph design, as well as the detailed analysis
of the modern configuration of a two-dimensional decameter wavelength
heliograph based on the UTR-2 T-shaped antenna system (in Sections 4
and 5), are presented. Next, Section 6 is aimed at design description of the
most important UTR-2 based heliograph element, namely the phase shifter
for fast beam scanning. The phase shifter essential parts are the switched
coaxial delay-line cables. In Section 7 the method is described for calculating
the lengths of these switched cables. The heliograph control unit helps
to generate the pulse train for switching the fast scanning phase shifter and
scan markers generation (see Section 9). Consequently, the UTR-2 array
pattern beam is scanning the desired sky area, and the heliograph scan
sector format is discussed in Section 8. Further, in Section 10 we consider
the operation features of the heliograph receiver-recorder. It is based on
a multichannel digital spectropolarimeter (DSP). In Section 11 we present
the preliminary results of test observations made with the considered instrument
in summer of 2010. In conclusions, we summarize the heliograph
development and prospects for its future application in radio astronomy
observations at decameter wavelengths.
2 Brief History of Heliograph Studies in the UTR-2 Observatory
The first positional observations of solar radiation with the use of a twodimensional
heliograph based on the UTR-2 radio telescope were made
during July 31 to August 11, 1976 [18]. At that time, the enhanced radiation
was associated with emerging of the active McMath No. 14352 region
on the eastern limb which, while moving westwards, was intersecting the
entire solar disk. For the aforesaid period of observations, numerous Type
III and IIIb bursts, Type IIIb-III bursts, as well as short-lived narrowband
stria-bursts being components of the Type IIIb burst chains were
recorded. The daily two-hour measurements centered for the local midday
were made at the operating frequency 25 MHz, and also in parallel
by the “North-South” (N-S) antenna array to which output the dynamic
spectrograph for the 23.5-25.5 MHz band was mounted. The recorded heliograms
permitted constructing the histograms of coordinate distribution
of effective centers of Type III and stria-bursts for hour angle (h) and declination
(δ) in the selected days for different positions of solar disk active
regions. The positional data on the average coordinates of burst sources
centers were used to obtain the day-to-day dependence of the radiating region
position. The motion velocity of the active region, crossing the solar
disk westwards, permitted estimates of the average heights Rs (being radial
distances from the Sun’s center) of the Type III burst and stria-burst
sources during the radio burst. The heights Rs were determined by several
methods which yielded close results. In the meter and centimeter wavelengths,
the radial distances of the coronal emitting regions were calculated
in papers [19, 20]. The Rs values estimated with the UTR-2 heliographic
observations have appeared comparable to each other and corresponded to
those first obtained at decameters during the interference measurements at
the Clark Lake Radio Observatory (CA, USA) at 30 MHz while investigating
a number of solar noise storms [21]. The heliograms have permitted the
conclusion that the stria-bursts of IIIb chains and Type III bursts appear
about the same place on the Sun’s image plane.
Further research efforts, assisted by the UTR-2 antenna system operating
the mode of a two-dimensional heliograph, were largely concentrated
on studying the Type IIId radiobursts with echo components
[22, 23, 24, 25, 26, 27, 28]. The decameter Type IIId bursts were first
recorded at the UTR-2 observations of July 6, 1973. Narrow-band elements
of this emission fine structure – the diffuse stria-bursts – are a
variety of ordinary short-lived stria-bursts, of which the dynamic spectra
of Type IIIb bursts are patterned. The principal feature of diffuse striabursts
consists in that their form largely depends on heliolongitude of the
coronal sources region. When the Type IIId source is in the near-limb
region, the stria-bursts with steep fronts and sharp peaks are observed
monotonically damping in a few seconds. With the active region approach
to the central solar meridian, the time splitting occurs, i.e. the echo-like
component of bursts with growing delay appears. The echo-burst delay
time becomes maximum when the emitting region traverses the central
meridian. The appearance of an extra burst echo-component and the delay
variation with heliolongitude are qualitatively explained by using a
simple model of a point pulsed source placed into a spherically symmetric
corona and emitting at the plasma frequency second harmonic [29].
Using this kind model in interpreting the origin of Type IIId bursts was
suggested in papers [22, 23]. However, the theoretically calculated values
were poorly coordinated with those UTR-2 measured. The assumption [28]
was therefore made that the echo-component of stria-bursts is formed not
merely due to the reflection from a deeper layer of a spherically symmet2
BRIEF HISTORY OF HELIOGRAPH STUDIES IN THE UTR-2 8
ric corona, but also as a result of the refraction of radio waves on some
large-scale coronal structures. This hypothesis has been supported in [30]
where the author has mathematically simulated the shaping of Type IIId
bursts. During the systematic researches of the Sun with the UTR-2 array
system and positional observations with the two-dimensional heliograph
for almost two decades, more than 10 solar Type IIId burst storms were
recorded. It will be observed that Type IIId radio bursts are rather rare
events, unlike Type III bursts being most mass events of the decameter
sporadic solar radiation. With the complex observations using the twodimensional
heliograph, in June 1984 during a week-long Type III storm,
about 1000 bursts were recorded at 25 MHz. The data obtained allowed
the statistical sample manipulation of decameter events by employing the
cluster analysis [31]. For the statistical procedure of sample ordering and
clustering, used were such parameters of bursts as time delay between the
two sequent bursts, burst duration at half-power level, maximum intensity
and degree of circular polarization.
It should be recalled that way back in the mid-nineties, the radioastronomy
observations with the UTR-2 array were made only near to several
fixed frequencies: 10.0, 12.5, 14.7, 16.7, 20.0 and 24.8 MHz, while
the broadbandness of this antenna system is much greater. On the other
hand, in the capacity of a recorder, e.g. for the heliograph, the FTAP-
2 facsimile set was used recording the signal on a electrochemical paper
with the dynamic range making only 10-16 dB. Such a situation urgently
demanded extensive design improvement of the whole UTR-2 hardware
facility. When the UTR-2 antenna amplifier system had been upgraded
in the late nineties, the measurements became possible in the continuous
frequency band of 10-30 MHz and more [32]. Thereupon, an exigency of
installation of appropriate recording equipment arose. With that end in
view, for multi-frequency observations a 60-channel spectrum analyzer was
put into service which included 60 separate frequency-tunable within 10-
30 MHz radio receivers with the 4-10 kHz bandwidth. The next step was
applications of digital multichannel (1000 and more channels) spectrum
analyzers (digital receivers). These latter already allowed covering the entire
UTR-2 array frequency range with the frequency separation of about
4 kHz, as well as recording on digital storage media and perform varied
signal computer processing [33, 34, 35].
The very first configuration of the UTR-2 array based heliograph was
as follows. By means of a special beam control system of the UTR-2 array
it became possible to quickly scan the sky area under investigation. Its
image was recorded through signal injection from the outputs of N-S and
E-W (“East-West”) antennas into the sum-difference device. The sum and
difference signals were amplified by individual receivers with square-law
detectors. The outputs of square-law detectors were in opposition, thus
providing the difference of their output voltages. This difference voltage
then fed the final recorder input. Actually, the radio astronomy research
efforts are basically carried out using the digital receiver-recorder, being a
digital spectropolarimeter (DSP) capable of signal receiving and processing,
and data recording on the PC hard disk. Surprisingly, we still face
the fact that though the UTR-2 based heliograph was designed long ago
and used in diverse radioastronomy researches, its complete and thorough
description had been postponed because of ever-lasting upgrading its hardware
environment. For this long period, the heliograph has been essentially
reengineered, new possibilities for its application appeared, and things have
reached an exigent necessity point to describe the heliograph development
at long last review.
[attachment=26971]
Abstract
The broadband analog-digital heliograph based on the UTR-2 radio telescope is
described in detail. This device operates by employing the parallel-series principle
when five equi-spaced array pattern beams which scan the given radio source (e.g.
solar corona) are simultaneously shaped. As a result, the obtained image presents
a frame of 5 × 8 pixels with the space resolution 25 ′× 25 ′ at 25 MHz. Each pixel
corresponds to the signal from the appropriate pattern beam. The most essential
heliograph component is its phase shift module for fast sky scanning by pencil-
shape antenna beams. Its design, as well as its switched cable lengths calculation
procedure, are presented, too. Each heliogram is formed in the actual heliograph just
by using this phase shifter. Every pixel of a signal received from the corresponding
antenna pattern beam is the cross-correlation dynamic spectrum (time-frequency-
intensity) measured in real time with the digital spectrum processor. This new
generation heliograph gives the solar corona images in the frequency range 8-32
MHz with the frequency resolution 4 kHz, time resolution to 1 ms, and dynamic
range about 90 dB. The heliographic observations of radio sources and solar corona
made in summer of 2010 are demonstrated as examples.
e-mail: alexstan[at]ri.kharkov.ua
1 Introduction
Comprehensive information on the physical processes that accompany solar
activity can be obtained only with engaging of an extensive scope of observation
means. At the meter wavelengths, radio spectrographs, heliographs
and polarimeters are widely used to meet this purpose [1]. A worse situation
is the case for decameter wavelengths. In particular, the heliograph
realization is technically a rather labor intensive problem. Obtaining the
high-resolution images (being one of the major demands in application of
heliographs) at that low frequencies requires the huge size antennas (> 1
km). At the same time, the heliograph can help obtain new useful information
on the extremely multifarious types of bursts being rather specific
to this range of frequencies.
Any heliograph has essential advantages in the case it ensures the possibility
of simultaneous observations at several frequencies. Multi-frequency
measurements of the positions of sources of Type II and III bursts allow
to immediately determine the velocity of shock waves and this of electron
beams which are propagated in the solar corona. Of substantial interest
are the measurements of the velocity of electron beams generating Type III
and IIIb bursts at different trajectory phase. The positional measurements
at several frequencies will probably permit evaluation of the magnitude of
visible displacement of burst sources with respect to their true position
caused by the refraction and scattering of radio waves in corona.
Thus, the heliograph permits observation of an angular structure of
burst sources and its evolution during their lifetime, as well as to identify
the bursts with the appropriate activity regions and measure their heights
above the photosphere. The position measurements of double Type IIIb-
III, III-III bursts and drifting pairs allow to find out whether the both
components are excited in the same place of the corona and at which
plasma frequency. Of special interest are the investigations of properties
and determination of the positions of coronal mass ejections, frequently
associated with the Type II bursts. This phenomenon is considered a key
one in the problems of solar-terrestrial relationships and space weather.
No less interesting direction of the research efforts are observations of
the two-dimensional brightness distributions across the quiet Sun at the
decameter waves. At the same time, as is known, just at the heights
where the decameter radiation is generated there exists the most interesting
corona region where the solar wind originates. The here-presented far from
exhaustive list of problems capable of solving with the heliograph makes
its construction a rather attractive idea. Especially, in case if there is an
opportunity of using the available antenna system suitable for this purpose.
To date, there exist not too many radio telescopes permitting to obtain
2D heliograms of the Sun and other sky radiation sources. In the microwave
range, they are represented by the Nobeyama heliograph (Japan)
operating at two frequencies, 17 and 35 GHz [2], and the RATAN-600 based
heliograph (Russia) operating at 3.75 GHz [3]. Roughly within these frequency
ranges, the observations are also made at the Siberian Solar Radio
Telescope (Russia) [4] at 5.7 GHz, and at the Owens Valley Radio Observatory
interferometer (USA) within 1-18 GHz, whose whole range being split
into 86 individual subbands [5]. The anticipated Chinese heliograph being
designed for radio measurements within 0.4-15 GHz [6] can be considered
belonging under the promising instruments, too. For centimeter and meter
wavelength radio astronomy investigation, used are the Nancay radioheliograph
(France) [7], operating only at frequencies 169, 327 and 408 MHz,
and the Gauribidanur radioheliograph (India) which surveys at some individual
frequencies within 40-150 MHz range [8]. Since 1969 till 1984,
the Culgoora radioheliograph (Australia) [9] was functioning at frequencies
43, 80, 160 and 327 MHz. Note that very few instruments were used
at the decameter wavelengths. In this respect we may mention the Clark
Lake Teepee-Tee radio telescope (USA) [10] at which the measurements at
some individual frequencies within 15-125 MHz were made. Though for
now, it is already put out of service. It will be observed also that developing
the antenna arrays capable of operating low frequencies (decameter
wavelengths) is connected with heavy technological and methodological
difficulties. This frequency range is subject to very heavy noise conditions,
the ionosphere exerts an essential impact on the behavior of decameter
wave propagation, and achieving the narrow beam pattern requires antennas
of huge areas. For instance, the Nancay Decameter Array (Nancay,
France) [11] is effectively used for getting dynamic spectra of solar radio
emission, while because of its small sizes practically never used for heliographic
observations. On account of these reasons, nowadays only a few
decameter range antenna systems exist capable of performing heliographic
observations, though they also have limited capacities, and thus need upgrading,
or even new antenna systems should be built. Suffice it to mention
the promising projects LOFAR [12, 13] and LWA [14] which are to be put
into service in the near future. Therefore the researches carried out with
the UTR-2 based heliograph are of great scientific interest, making up for
a deficiency in the knowledge of physical processes on the Sun and/or in
the circumsolar space. Note that until now, the UTR-2 radio telescope
remains the world-largest and most effective instrument operating within
8-32 MHz, and it is expected to remain that kind performer in long-term
future. Accordingly, its intensive design improvement, instrumentation
and usage (this including in heliographic mode, too) promises a vast deal
of useful astrophysical information.
The high solar activity is frequently accompanied by sporadic radio
emission in the meter and decameter wavelength ranges [1, 15]. Some of
its types (I, II, III and IV) are common for these ranges. At the same
time, they have their inherent specificity. For instance, the Type I bursts,
most frequent at meter waves, are practically not observed at the decameter
wavelengths. On the other hand, the short-lived stria-bursts, drifting pairs,
Type IIIb bursts are met only at frequencies below 60 MHz, though the
latter, along with Type III bursts, are the most numerous events of the
solar decameter radiation [16].
The solar decameter sporadic radiation has begun to be systematically
studied with the work of Ellis and McCulloch [17]. Since then, a substantial
progress was reached in understanding its nature. Meanwhile, the theoretical
formulation of the mechanism and conditions of solar burst generation
cannot yet give, in most cases, an adequate representation. Therefore further
analysis of new experimental data, including those observed with the
heliograph, is of current importance.
The present paper is devoted to the UTR-2 heliograph construction, its
general functional scheme and discussion of its most important features.
In Section 2 we start with the history of heliograph studies in our institute.
The basis of any radioheliograph, the UTR-2 based heliograph making no
exception, is the antenna system. In Section 3, this latter brief description
as applied to the heliograph design, as well as the detailed analysis
of the modern configuration of a two-dimensional decameter wavelength
heliograph based on the UTR-2 T-shaped antenna system (in Sections 4
and 5), are presented. Next, Section 6 is aimed at design description of the
most important UTR-2 based heliograph element, namely the phase shifter
for fast beam scanning. The phase shifter essential parts are the switched
coaxial delay-line cables. In Section 7 the method is described for calculating
the lengths of these switched cables. The heliograph control unit helps
to generate the pulse train for switching the fast scanning phase shifter and
scan markers generation (see Section 9). Consequently, the UTR-2 array
pattern beam is scanning the desired sky area, and the heliograph scan
sector format is discussed in Section 8. Further, in Section 10 we consider
the operation features of the heliograph receiver-recorder. It is based on
a multichannel digital spectropolarimeter (DSP). In Section 11 we present
the preliminary results of test observations made with the considered instrument
in summer of 2010. In conclusions, we summarize the heliograph
development and prospects for its future application in radio astronomy
observations at decameter wavelengths.
2 Brief History of Heliograph Studies in the UTR-2 Observatory
The first positional observations of solar radiation with the use of a twodimensional
heliograph based on the UTR-2 radio telescope were made
during July 31 to August 11, 1976 [18]. At that time, the enhanced radiation
was associated with emerging of the active McMath No. 14352 region
on the eastern limb which, while moving westwards, was intersecting the
entire solar disk. For the aforesaid period of observations, numerous Type
III and IIIb bursts, Type IIIb-III bursts, as well as short-lived narrowband
stria-bursts being components of the Type IIIb burst chains were
recorded. The daily two-hour measurements centered for the local midday
were made at the operating frequency 25 MHz, and also in parallel
by the “North-South” (N-S) antenna array to which output the dynamic
spectrograph for the 23.5-25.5 MHz band was mounted. The recorded heliograms
permitted constructing the histograms of coordinate distribution
of effective centers of Type III and stria-bursts for hour angle (h) and declination
(δ) in the selected days for different positions of solar disk active
regions. The positional data on the average coordinates of burst sources
centers were used to obtain the day-to-day dependence of the radiating region
position. The motion velocity of the active region, crossing the solar
disk westwards, permitted estimates of the average heights Rs (being radial
distances from the Sun’s center) of the Type III burst and stria-burst
sources during the radio burst. The heights Rs were determined by several
methods which yielded close results. In the meter and centimeter wavelengths,
the radial distances of the coronal emitting regions were calculated
in papers [19, 20]. The Rs values estimated with the UTR-2 heliographic
observations have appeared comparable to each other and corresponded to
those first obtained at decameters during the interference measurements at
the Clark Lake Radio Observatory (CA, USA) at 30 MHz while investigating
a number of solar noise storms [21]. The heliograms have permitted the
conclusion that the stria-bursts of IIIb chains and Type III bursts appear
about the same place on the Sun’s image plane.
Further research efforts, assisted by the UTR-2 antenna system operating
the mode of a two-dimensional heliograph, were largely concentrated
on studying the Type IIId radiobursts with echo components
[22, 23, 24, 25, 26, 27, 28]. The decameter Type IIId bursts were first
recorded at the UTR-2 observations of July 6, 1973. Narrow-band elements
of this emission fine structure – the diffuse stria-bursts – are a
variety of ordinary short-lived stria-bursts, of which the dynamic spectra
of Type IIIb bursts are patterned. The principal feature of diffuse striabursts
consists in that their form largely depends on heliolongitude of the
coronal sources region. When the Type IIId source is in the near-limb
region, the stria-bursts with steep fronts and sharp peaks are observed
monotonically damping in a few seconds. With the active region approach
to the central solar meridian, the time splitting occurs, i.e. the echo-like
component of bursts with growing delay appears. The echo-burst delay
time becomes maximum when the emitting region traverses the central
meridian. The appearance of an extra burst echo-component and the delay
variation with heliolongitude are qualitatively explained by using a
simple model of a point pulsed source placed into a spherically symmetric
corona and emitting at the plasma frequency second harmonic [29].
Using this kind model in interpreting the origin of Type IIId bursts was
suggested in papers [22, 23]. However, the theoretically calculated values
were poorly coordinated with those UTR-2 measured. The assumption [28]
was therefore made that the echo-component of stria-bursts is formed not
merely due to the reflection from a deeper layer of a spherically symmet2
BRIEF HISTORY OF HELIOGRAPH STUDIES IN THE UTR-2 8
ric corona, but also as a result of the refraction of radio waves on some
large-scale coronal structures. This hypothesis has been supported in [30]
where the author has mathematically simulated the shaping of Type IIId
bursts. During the systematic researches of the Sun with the UTR-2 array
system and positional observations with the two-dimensional heliograph
for almost two decades, more than 10 solar Type IIId burst storms were
recorded. It will be observed that Type IIId radio bursts are rather rare
events, unlike Type III bursts being most mass events of the decameter
sporadic solar radiation. With the complex observations using the twodimensional
heliograph, in June 1984 during a week-long Type III storm,
about 1000 bursts were recorded at 25 MHz. The data obtained allowed
the statistical sample manipulation of decameter events by employing the
cluster analysis [31]. For the statistical procedure of sample ordering and
clustering, used were such parameters of bursts as time delay between the
two sequent bursts, burst duration at half-power level, maximum intensity
and degree of circular polarization.
It should be recalled that way back in the mid-nineties, the radioastronomy
observations with the UTR-2 array were made only near to several
fixed frequencies: 10.0, 12.5, 14.7, 16.7, 20.0 and 24.8 MHz, while
the broadbandness of this antenna system is much greater. On the other
hand, in the capacity of a recorder, e.g. for the heliograph, the FTAP-
2 facsimile set was used recording the signal on a electrochemical paper
with the dynamic range making only 10-16 dB. Such a situation urgently
demanded extensive design improvement of the whole UTR-2 hardware
facility. When the UTR-2 antenna amplifier system had been upgraded
in the late nineties, the measurements became possible in the continuous
frequency band of 10-30 MHz and more [32]. Thereupon, an exigency of
installation of appropriate recording equipment arose. With that end in
view, for multi-frequency observations a 60-channel spectrum analyzer was
put into service which included 60 separate frequency-tunable within 10-
30 MHz radio receivers with the 4-10 kHz bandwidth. The next step was
applications of digital multichannel (1000 and more channels) spectrum
analyzers (digital receivers). These latter already allowed covering the entire
UTR-2 array frequency range with the frequency separation of about
4 kHz, as well as recording on digital storage media and perform varied
signal computer processing [33, 34, 35].
The very first configuration of the UTR-2 array based heliograph was
as follows. By means of a special beam control system of the UTR-2 array
it became possible to quickly scan the sky area under investigation. Its
image was recorded through signal injection from the outputs of N-S and
E-W (“East-West”) antennas into the sum-difference device. The sum and
difference signals were amplified by individual receivers with square-law
detectors. The outputs of square-law detectors were in opposition, thus
providing the difference of their output voltages. This difference voltage
then fed the final recorder input. Actually, the radio astronomy research
efforts are basically carried out using the digital receiver-recorder, being a
digital spectropolarimeter (DSP) capable of signal receiving and processing,
and data recording on the PC hard disk. Surprisingly, we still face
the fact that though the UTR-2 based heliograph was designed long ago
and used in diverse radioastronomy researches, its complete and thorough
description had been postponed because of ever-lasting upgrading its hardware
environment. For this long period, the heliograph has been essentially
reengineered, new possibilities for its application appeared, and things have
reached an exigent necessity point to describe the heliograph development
at long last review.