The first detection of the solar U+III association with an antenna prototype for the future lunar observatory
Lev Stanislavsky, Igor Bubnov, Alexander Konovalenko, Peter Tokarsky, Serge Yerin
aa r X i v : . [ a s t r o - ph . I M ] F e b Research in Astronomy and Astrophysics manuscript no.(L A TEX: ms0442˙v1.tex; printed on February 15, 2021; 2:19)
The first detection of the solar U+III association with an antennaprototype for the future lunar observatory
Lev Stanislavsky , Igor Bubnov , , Alexander Konovalenko , Peter Tokarsky and Serge Yerin , Institute of Radio Astronomy, Kharkiv, Ukraine; [email protected] V.N. Karazin Kharkiv National University, Kharkiv, Ukraine
Received 2020 December 11; accepted 2021 January 29
Abstract
We report about observations of the solar U+III bursts on 5 June of 2020 by meansof a new active antenna designed to receive radiation in 4-70 MHz. This instrument can serveas a prototype of the ultra-long-wavelength radiotelescope for observations on the farside ofthe Moon. Our analysis of experimental data is based on simultaneous records obtained withthe antenna arrays GURT and NDA in high frequency and time resolution, e-Callisto networkas well as by using the space-based observatories STEREO and WIND. The results from thisobservational study confirm the model of Reid and Kontar (2017).
Key words:
Sun: corona — Sun: radio radiation — methods: data analysis — telescopes
Lev Stanislavsky et al.
Its important advantage is a feasibility of preliminary approbation on Earth, receiving radio emission infrequencies close to ionospheric cutoff and higher. The present paper is just devoted to real observationsas a test of this antenna demonstrating a scope of this instrument for exploring the Universe at ultra longwavelengths in radio emission. Consequently, we observed and study the solar type U radio burst associatedwith type III bursts that are useful to examine possible models of the event.
The great interest in the development of small-sized active dipoles for low-frequency radio astronomy hasnoticeably intensified theoretical and experimental studies of antenna technology, amplifiers and relatedcomponents (see Konovalenko et al. 2016 and references therein). Over fifteen years of experience inoperating active dipoles in the GURT ground-based radio array showed their reliability and validity ofscientific results. These dipoles, being small in size, provide optimal “radio astronomical sensitivity”, whichis determined primarily by the contribution of the amplifier temperature to the noise temperature of theactive dipole. In the case of a noise-free amplifier, the noise temperature of the active dipole is equal tothe antenna temperature, obtained from observations of the Galactic background radio emission. A goodresult is considered to achieve the contribution of the amplifier temperature to the noise temperature of theactive dipole no more than 10% - 25%. In this way, the corresponding studies were carried out to develop aprototype of the ultra-long-wavelength broadband antenna for radio astronomy purposes in which the Moonis an instrument location.By numerical simulations of the antenna prototype and performing measurements of its parameters,we have studied the active-dipole antenna of complex geometry, located above partially conductive ground(Tokarsky 2017). The sketch of this antenna is shown in Fig. 1. Consequently, the computer model of thisantenna was developed, which allowed us to obtain its characteristics (impedance, energy parameters, radi-ation pattern) in the operating frequency domain of 1 to 70 MHz. As a part of these studies, the dipole andthe amplifier were designed and manufactured as an active antenna, capable of receiving cosmic radiationin the frequency band 4-70 MHz. The test measurements of the Galactic background radiation were suc-cessfully conducted at the Radio Astronomical Observatory of S. Ya. Braude in terrestrial conditions. Theantenna design was the following. The dipole arm lengths along the midline (ABC in the frontal projectionof Fig. 1) are 2.8 m, the angle γ of inclination to the horizontal plane is 45 ◦ , and the dipole terminals arelocated at h = 1.7 m above ground. It is made of steel tubes having the diameter of 23 mm. As an an-tenna amplifier, the low-noise amplifier (LNA) with PHEMT transistors is used (Korolev 2014). The LNAnominal gain is G amp =
22 dB, the effective noise temperature is about 50 K, and the 3rd order nonlineardistortion coefficient is more than 30 dB/ µ V. The LNA is supplied with the voltage of 5 V at the currentconsumption of 40 mA.The spectrum records of solar radio emission were obtained by using the receiver DSP-Z (short abbre-viation for digital spectropolarimeters of type Z) which is a standard device of the radio telescope UTR-2(Zakharenko et al. 2016). This allows performing the real-time FFT analysis in two independent channelswhose operating frequency band is 0-33 MHz. Since the prototype antenna can receive radio emission upto 70 MHz, we applied a special technique for records of radio emission in the frequency band twice as he first detection of the solar U+III association 3
Fig. 1
Sketch of the antenna prototype (a) with its frontal projection (b).
Table 1
Data Number of Size, Class Location,sunspots MH Magn. degree2020/06/03 2 70 α S24E712020/06/04 2 100 α S24E582020/06/05 3 130 β S24E442020/06/06 5 110 β S22E332020/06/07 6 100 β S23E202020/06/08 7 100 β S24E072020/06/09 4 70 β S24W062020/06/10 1 50 α S24W212020/06/11 1 50 α S22W352020/06/12 1 50 α S26W462020/06/13 1 60 α S25W602020/06/14 1 20 α S25W732020/06/15 1 10 α S26W86 wide as the receiver can perform in each channel separately. While one channel received radio signals in thefrequency range 4-33 MHz, another operated within 33-66 MHz. Their combination gave the spectrum ofradio emission within 4-66 MHz. Based on the technique, the frequency and time resolutions were 4 kHzand 100 ms, respectively, in each channel.
Radio emission of solar bursts is often divided into types from the analysis of their observed frequencydrift rate. The most numerous among the solar bursts are bursts of type III (see, for example, Reid andRatcliffe 2014 with references therein). They are caused by high velocity electron beams accelerated byunstable magnetic fields of the solar atmosphere, and because of this, their frequency drift rate is extremelyhigh. Typically, the type III bursts manifest a monotonic shape in dynamic spectra, going from high to lowfrequencies, showing the motion of beams to Earth. However, sometimes the closed magnetic fields on theSun can change this exhibition in dynamic spectra. Their overall spectral signature resembles the letters Jand U, and the bursts are therefore called the type J and the type U, respectively (Fokker 1970, Labrum
Lev Stanislavsky et al. β class, but decaying, after 10 June itmoved to the α class until it disappeared completely, going to the far side of the Sun. Note that the B6 (faint)flare occurred in this region at ∼ ∼
20 MHz to ∼
15 MHz. Dueto low solar activity, in general, the cutoff frequency of the Earth’s ionosphere at our latitude is noticeablylower than 10 MHz (in some days even reaching frequencies of 2 MHz). This makes it possible to conductradio observations at frequencies pretty close to those that are planned to record with lunar observatories.Therefore, the received bursts were confidently detected at extremely low frequencies close to 6 MHz,despite numerous natural and artificial radio interference. The much more detailed spectrum above 8 MHzwas provided by recording this event with the GURT antenna array (Fig. 3). This active antenna consists of25 cross-dipoles, five in each row and column. Dipole arms have the east-west and north-south orientations.The instrument serves for receiving radio emission within 8-80 MHz (Konovalenko et a. 2016). As thesensitivity of the GURT array is higher than one dipole antenna has, its dynamic spectrum represents many he first detection of the solar U+III association 5
Fig. 2 (Color online) Dynamic spectrum of the solar U+III bursts (observed on 5 June 2020)with help of the antenna prototype intended for ultra-long-wavelength records. The prominentnarrow vertical and horizontal lines indicate numerous intensive radio disturbances generated bynatural lightning discharges and broadcast stations, respectively.
Fig. 3 (Color online) Dynamic spectrum of many solar bursts at ∼ ∼
16 MHz and ∼ Lev Stanislavsky et al.
Time [UT] F r equen cy [ M H z ] Time [UT] F r equen cy [ M H z ] Fig. 4 (Color online) Dynamic spectra of radio emission recorded with thesolar space-based observatories WIND and STEREO A on 5 June of 2020,according to https://solar-radio.gsfc.nasa.gov/data/wind/rad2 and https://solar-radio.gsfc.nasa.gov/data/stereo/new summary.Interestingly, this result manifests an important feature of this event. We observe the separation of electronbeams into two parts. One part, due to the deflection of beams by solar magnetic fields back to the Sun,caused the emergence of the U-shaped burst, whereas another followed in the opposite direction to Earth,producing traces on the dynamic spectra, characteristic for the solar bursts of type III, merging together. Thetypes of solar bursts, observed simultaneously, may be called the U+III bursts, since they were connectedwith the same place (AR12765) on the Sun and generated by the same group of associated electron beams.The U+III event is superimposed to a series of weaker type III bursts.To confirm this concept, we have found the U-shaped burst in the observations made on other suit-able radio telescopes, which had a similar low-frequency range for recording solar radio emission. Oneof e-Callisto network stations (Benz et al. 2009), located in Greenland (ISR Kellyville observatory nearKangerlussuaq, 66 ◦ ′ ′′ N, 50 ◦ ′ ′′ W), recorded the solar U-shaped burst at the same timethat we did (see Table 2). Let us mention a little about hardware features of this station for solar observa-tions: the frequency domain of 10 to 95 MHz is divided into 200 frequency channels each of which has theband of 475 kHz, and the time resolution is 250 ms. The antenna consists of one broadband low-frequencyantenna intended for the Long Wavelength Array (LWA, see http://lwa.phys.unm.edu). The spectra obtainedby the e-Callisto station (see Fig. 5) are morphologically identical to the spectrum obtained by our instru-ments, which had noticeably both better sensitivity and higher frequency-time resolution. There is no doubtthat the records show the same event, manifesting the solar U-shaped burst. Moreover, fragments of thisevent was also detected with other e-Callisto stations. It is also worth mentioning that this group of solar he first detection of the solar U+III association 7
Time [UT] F r equen cy [ M H z ] Time [UT] F r equen cy [ M H z ] Fig. 5 (Color online) Dynamic spectra of radio emission in right (RP) and left (LP) handedpolarizations from the solar observations on 5 June of 2020 with the e-Callisto network stationin Greenland. In the upper part of the spectra, the track similar to a type III burst can be seen,separating from the type U burst branch with the negative frequency drift rate.bursts was recorded by the Nanc¸ay Decametric Array (NDA) in France. The dynamic spectrum is shownin Fig. 6. The radio telescope provides daily observations of solar radio emission at 10-80 MHz. Its 2 × According to Reid and Kontar (2017), the generation of radio emission in the form of the solar U burstssimultaneously with type III bursts requires strictly defined conditions. Initially, having a negative velocitygradient, an injected electron beam cannot generate radio emission because of its stable distribution functionto Langmuir wave production. Nevertheless, when it propagates through solar corona, a positive velocitygradient becomes its feature, making the beam unstable and producing a solar burst. The propagation effectsdetermine a starting height with which the burst is nascent. The value is 0.6 R ⊙ (from the solar photosphere).Although it is dependent on parameters of beams, its value may be taken as an estimate. If one assumes thatthis conjecture is true, with its help we can calibrate the electron density model of solar corona (see moredetails below). Before 0.6 R ⊙ an electron beam does not produce radio emission, but after that three regimesare possible. The electron beam with too low density still does not emit. In the second regime, for moderatedensities, a beam travels along open magnetic field lines, producing only type III bursts. The third regime, Lev Stanislavsky et al.
Table 2
High frequency cutoff for the U+III bursts, observed on June 5, 2020 at ∼ Country Location Frequency Cutoff,range, MHz MHzDenmark Copenhagen, DTU 45-100 85United Kingdom Glasgow, UOG 45-81 > > Right Handed Polarization09:15:45 09:20:45 09:25:45 09:30:45 09:35:45 09:40:45
Time, UT F r e qu e n cy , M H z dB Left Handed Polarization09:15:45 09:20:45 09:25:45 09:30:45 09:35:45 09:40:45
Time, UT F r e qu e n cy , M H z dB Fig. 6 (Color online) Dynamic spectra of solar radio emission on 5 June of 2020 observed withNDA in Nanc¸ay.the most interesting for us, is characterized by high initial electron beam densities, that is good for uprisingboth type III and type U bursts in radio emission. In this case the magnetic loop should be large enoughfor propagation effects, making the electron beam unstable, until it reaches the loop top and turns backto the Sun. This does not exclude the generation of type III bursts, due to the movement of beams alongopen magnetic field lines. As a rule, radio emission of type U bursts has a weaker and more diffuse shapefor the positive frequency drift rate branch (Aurass and Klein 1997). The fine structure of the U+III burstsmanifests many radio sources tracing a similar path through the corona so that each of the radio patternswas produced by its electron beam. he first detection of the solar U+III association 9
Using the Newkirk (one-fold) model for the solar corona (Newkirk 1961), the radial change of theelectron density is written as n e ( r ) = α · . · · (4 . / r ) , (1)where n e ( r ) is the electron density of coronal plasma in cm − , depending on the heliocentric height r insolar radii. This α -fold model well describes the electron density profile above quiet equatorial regions ( α = 1), in dense loops ( α = 4), and in extremely dense loops for α = 10 (Koutchmy 1994, Mann et al. 2018),but the value of the parameter α is indicative. As the local plasma frequency f pe (in MHz) equals f pe = . · − √ n e , (2)then the source height and the radiation frequency are linked by a simple expression r = . f − . − . α , (3)where f is the radiation frequency in MHz. As applied to our event and following Table 2, the U+III burstsstarted with ∼
85 MHz that corresponds to the heliocentric height ∼ R ⊙ (or ∼ R ⊙ from photoshere) under α ≈
4. This is in good agreement with conjectures of Reid and Kontar (2017). Note that the high-frequencycutoff finding allows calibrating the value of α for the coronal density model. The top of the U-burst reached ∼
15 MHz, i. e. the height was about 3.5 R ⊙ , taking α ≈
4. In the pivot point the instantaneous bandwidthwas about 5 MHz, permitting us to estimate the loop width at the top. It was about 0.6 R ⊙ . Recall that theactivity of solar regions depends of their size (Solanki 2003). The larger it is, the higher the probability ofoccurrence of flares, coronal mass ejections, diversity of solar bursts and their intensity. The active region12765 was developed from the unipolar form to bipolar, and we observed the only U-burst, when the size ofthis region was maximum. Thus, one can assume that each bipolar active region with the size more 110 MHwill be able to generate electron beams with high initial densities, resulting in radio emission of the U-burstssimultaneously and together with type III bursts. Finally, it should be noticed that according to Fig. 3 andFig. 6, the U+III bursts are surrounded by weaker type III bursts which were caused by electron beams withmoderate densities. This case can correspond to the second regime mentioned above. Having such an extensive set of radio and optic observations on 5 June 2020, we can confidently assertthat the event at ∼ α class) and a bipolar configuration( β class). This was due to low solar activity. The configuration of magnetic fields and the size of the activeregion were favorable to the emergence of solar bursts of such types. Observations with help of variousradio instruments allowed us to explore this event in more detail and to get reliable results, confirming themodel of this phenomenon. Moreover, this research also gave a number of new interesting footings relatedto the development of ultra-long-wavelength antennas for future lunar radio telescopes. Acknowledgements
The authors thank the WIND, STEREO, NDA and e-Callisto network teams for theirinstrument maintenance and open data access. This research was partially supported by Research Grant0120U101334, and authors acknowledge the National Academy of Sciences of Ukraine for this support.The authors are also thankful to Dorovskyy V.V. for helpful discussions.
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