Solar corona observations at decameter wavelengths Artem Koval Institute of Radio Astronomy Kharkov, Ukraine

Introducton Solar radio emission consists of two components: thermal (continuum “background” radiation) non-thermal (bursts, noise storms, flares and other disturbing features) The observations of the quiet Sun at decameter wavelengths are imposed restriction by the fact that the radiation is much weaker in comparison with the burst activity. Thus, the principal requests for radio astronomical instrument are very large dynamic range and high sensitivity as well as carrying-out observations during periods of reduced activity (sunspot minimum). UTR-2 radio telescope is an appropriate instrument satisfying basic demands 2

Quiet-Sun Radio Emission: Overview of Observations Frequency Radio astronomical (MHz) Reference instrument _ 80.0 Sheridan (1970) Culgoora Radio Heliograph 60.0 Aubier et al. (1971) Arecibo Radio Telescope 36.9 Aubier et al. (1971) – 29.3 Aubier et al. (1971) – 25.0 Abranin et al. (1976) UTR-2 Radio Telescope 73.8 Kundu et al. (1977) Clark Lake Radio Telescope 26.3 Kundu et al. (1977) – 73.8 Erickson et al. (1977) – 57.7 Erickson et al. (1977) – 38.1 Erickson et al. (1977) – 25.8 Erickson et al. (1977) – 19.0 Erickson et al. (1977) – 73.8 Thejappa & Kundu (1992) Clark Lake Radio Heliograph 50.0 Thejappa & Kundu (1992) – 38.5 Thejappa & Kundu (1992) – 75.0 Ramesh et al. (2000) Gauribidanur Radio Telescope 34.5 Subramanian (2004) – 77.0 Ramesh et al. (2006) – 51.0 Ramesh et al. (2006) – 3

UTR-2 Radio Telescope, Ukraine Effective area m 2 N-S and E-W arms 12 sections Frequency band MHz The beam sizes ~25 ´× 25 ´ 4

Experimental equipment W N S DSP UTR-2 antenna array declination - hour angle - latitude of UTR-2 Digital Signal Processor 5

DSP block scheme ADC block Channel1 Channel 2 FPGA matrix SOFTWARE GPS receiver Hard disk + Visual data presentation 6

DSP characteristics ADC sampling frequency 66 MHz Operating bandwidth 8-33 MHz Max. number of output frequency channels 8192 Frequency resolution 4 kHz Time resolution from 0.2 msec up to 1 sec Dynamic range 90 dB 7

Multi-beam scanning North South Pencil-shaped beams I II III IV V Date Number of scans September, 2010 per day ´ Total number: 26 The scanning was realized using these beams 8

One-dimensional scans of the quiet Sun Example of time-frequency profiles from scans along solar equator on September 6, Time zero corresponds to 08:59:15 UT 9

Solar spectrum of quiet Sun (corona) at decameter wavelengths The equation of the line: 10

Chiniese RHNancy RH, NDA Culgoora RH Gauribidanur RH Nobeyama RH RATAN-600 RH, SSRT Ukraine RH LOFAR OVRO Clark Lake 11

Heliograph’s general functional scheme 12

Principle of heliogram construction Extra phase shift module – fast change of antenna pattern position along U. Extra phase shift module – fast change of antenna pattern position along U. W E Multi-beam UTR-2 regime: Ω(θ, φ) ≈ 25´× 25´ at 25 MHz Multi-beam UTR-2 regime: Ω(θ, φ) ≈ 25´× 25´ at 25 MHz I II III IV V IIIIIIIVVVIVIIVIII N S 13

Heliograph field of view Number of image elements 5  8 Separating markers Separating markers Time of cadre composition 0.24 sec, 2 min 4 min The field sizes ~ 2.5  3.3  14

Examples of heliographic observations 3C348 (Freq. 21 MHz; August 29, 2010) 3C123 (Freq. 21 MHz; September 3, 2010) 15

Examples of heliographic observations Solar corona (Freq. 21 MHz; August 29, 2010; 07:20 UT) 16

Conclusion  The two-dimensional heliograph of decameter wavelengths represents unique radio astronomical tool (the most low-frequency and multichannel).  The radio heliograph is capable to produce the images of the “quiet” Sun as well as to track for displacement of radiation sources in the solar corona.  The preliminary results of heliographic observations of point sources (3C123, 3C348) as well as solar corona are presented.  The new working mode of the UTR-2 radio telescope – one dimensional heliograph – has been introduced as a supplementary, multitasking tool for radio astronomical measurements, in particular to investigate a radio emission from the “quiet” Sun.  The values of integrated flux density were obtained and extended to long- wavelength region of “quiet” Sun radiation spectrum indicating good agreement with results in others (meter and so on) wavelengths. 17

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