Notes

Outline

Special Issues in low frequency solar radio observation. A. KERDRAON Observatoire de Paris Dynamic range Interferences Calibration Ionospheric disturbances

Special Issues at Low frequencies: Dynamic range Bursts expected in the range 10-2 to 104 SFU Dynamic in RF channels (before correlation) Wide bandwidth channels: interference is the main problem, the power level may be dominated by interferences ~constant over a wide frequency band. Narrow bandwidth channels: Assuming Tsys = noise + sky background + For a large antenna (50 m2), dynamic range is 300 - 106 K (40 db) For a small antenna (1m2 - dipole), it is ~23 db

Special Issues at Low frequencies: Dynamic (ctd.) Power variations may be > 20 db/sec. Implications: High dynamic electronics needed Automatic attenuators or very fast AGC may be be needed in narrow band channels. Correlation is better made by 1 bit correlator +power measurement (or many bits correlator - challenging ?) If a spectrum is computed before correlation, it will have the dynamic of narrow channels (neglecting interferences) Note: Dynamic on images (Tb) may be much higher

Special Issues at Low frequencies: Interferences (ctd.) Low frequencies  ( < 2 GHz) are crowded: Very powerful transmitters (TV and FM) (emitters ~100 kW) Low power transmissions and GSM (emitters of 10 - 100 W) Satellites Protected (for radioastronomy) bands are very few and difficult to keep free of interferences: 151, 327, 408, 610, 1400 MHz… with bandwidth from 3 to a few ten MHz, in principle . Satellites emissions may occur very close (149.9 MHz) to astronomy band Antennas provide usually a poor rejection ( 10 db)

Special Issues at Low frequencies: Interferences 150 - 250 MHz band Nançay (interference survey antenna) Wide band example

Special Issues at Low frequencies: Interferences 150 - 152 MHz band Nançay (interference survey antenna) Narrow band examples

Special Issues at Low frequencies: Interferences(ctd.) Strong interferences: may be the main challenge: Power of TV is 80 db above quiet sun emission at 200 MHz (Nançay) A stop band filter may be required for each strong emitter The power is very high in the wide band electronic Very high dynamic components are needed in order to avoid intermodulation (which can generate thousands of parasitic lines) That level may be to high for digital filtering and interference excision. Site quality is very important.

Special Issues at Low frequencies: Interferences(ctd.) Mean level interferences ( <40db above quiet sun) At the lower end of the band (< 300 MHz): It is very difficult to find a 1 Mhz clean band Interferences have a small bandwidth ( 5-20 kHz) This imply: Small observing bandwidth, very good filters. Digital interference excision will require high dynamic, high resolution spectra and filters. There are (at present) less problems above 300 MHz… Don ’t forget internal interferences, mainly from high speed digital electronics.

Special Issues at Low frequencies: Calibration General Based on stability (over days and weeks) Stability is achieved by buried electronics and transmission lines Stability allows observation of calibrators outside sun observing time. Calibration provides complex gains for each antenna and frequency valid for days and weeks.

Special Issues at Low Frequencies: Calibration (cdt.) Calibrators Very few (5 ?) strong calibrators All of them have structure above 1000 l. Calibrators are not polarized No signal on X-Y correlators: corrected by rotating some antennas by 45 degrees during calibrations. Calibrators flux decreases strongly with increasing frequency

Special Issues at Low frequencies: Calibration Nançay present solution: Uses only Cygnus A radio-galaxy, which gives a strong signal at 5000 l and 500 MHz Uses a 2 components model, and fits fringes Accuracy could be better for long baselines Starting a new procedure based on redundancies and short baselines: antennas are calibrated step by step, and almost no source model is needed.

Special Issues at Low frequencies: Calibration Conclusions Phase/gain stability is mandatory Self-cal on calibrators should be the best solution The array design is important: Redundancies are useful Antennas involved only in long baselines are difficult to calibrate.

Special Issues at Low frequencies: Ionosphere Ionosphere at 164 MHz Very severe case (includes some distorsion) In most cases: smaller motion and no distorsion. Likely to occur at low site angle

Special Issues at Low Frequencies: Ionosphere Generalities Density inhomogeneities due to Travelling Ionospheric Disturbances (TIDs) may affect radio observations at dam to dm wavelength. TIDs most often due to gravity waves, sometimes to other phenomena (including magnetosphere). Effects are proportionnal to f-2 Gravity waves are neutral atmosphere phenomenon , which couples through collisions to electrons and ions

Special Issues at Low Frequencies: Ionosphere Observations Many techniques, including radio interferometry Large database in Nançay (C. Mercier), sun and other sources, day and night Very large database in Los Alamos, made with a 9 antennas VLBI array and satellites beacons (A. Jacobson) Some observations at VLA (cosmic radio sources)

Special Issues at Low frequencies: Ionosphere Ionospheric gravity wave schema ~horizontal wave plan ~horizontal energy propagation ~horizontal atmosphere motions Amplitude increases with decreasing density

Special Issues at Low Frequencies: Ionosphere Properties of gravity waves: Auroral origin:  T ~1h, lhoriz ~1000 km, Vf > 400 m/sec southward, usually at night. Meteorological origin: T = 15-60 mn, lhoriz = 150-250 km, Vf ~100 m/sec, usually southward Mostly during the day, quickly damped over ~1000 km. Origin (local) is strongly site dependant TEC gradient 2. 1011 m-3 typically Electronic density changes (1-10%) occur in F2 layer(  ~250 km)

Special Issues at Low Frequencies: Ionosphere Gravity effects on observations Due to first and second derivative of TEC Very dependent on geometry: maximum when line of sight is along wave fronts, small when perpendicular to wavefronts, or //B. Typical image motion +/- 1 arcmin at 169 MHz, typical timescale: 30 mn.  At least 10x in worst conditions (+ some image distorsion) 10x less at nightime. Focusing may occur at low frequencies, exceptionnal cases up to 50 MHz at low elevation.

Special Issues at Low Frequencies: Ionosphere Conclusions: In order to minimize most effects, it is important to avoid low elevation angles (high latitudes, sunrise and sunset) Remaining effects: Distortion should be corrected by self-cal (but who can self-cal quiet sun emission ?). Image shifts should be detected by images correlation, or observation of stable small source (Noise Storms), and corrected within less than 1 beamwidth. For transient bursts (ex. Type II/III/IV/V bursts) in the absence of Noise Storms, there is no trivial position correction.