2nd LOFAR KSP meeting Potsdam 2009 July Experience of NRH observations: which benefit for LOFAR KSP ? A. Kerdraon Observatoire de Paris - LESIA - USN

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP: outline Interferometry –Baselines, field of view –Fringe stopping, Sun motion –Calibration Time & frequency sampling, polarization Perturbations: Ionosphere and RFI Data formats and software

Nançay Radioheliograph General characteristics –Frequency range: MHz –648 baselines from 50 to 3200m (25 to 4,800  –Spatial resolution: ~4 to 0.3 arcmin (depending on frequency, declination, snapshot/synthesis) –Field of view: from 3 to 0.5 degrees –Stokes I and V –Time resolution: 5 ms* number of frequencies

Ext2 Nançay Radioheliograph array configuration Ext1Ext0 H1H2H7H8 H16 NS1 NS2 NS12 NS23 North South 1600 m 1248 m NS8 A0A1A2A3 NS24 « Est-West » antennae Mhz 1 polarization « North-South » antennae 5 m diameter Mhz 2 polarizations « Est-West Extension » antenna (Ext0) « North-South Extension » antenna (NS24) 7 m diameter Mhz 2 polarizations « Est-West Extensions » antennae (Ext1, 2) 10 m diameter Mhz 2 polarizations « Anti Aliasing » antennae Log Periodic Mhz 2 polarizations 1200 m

2nd LOFAR KSP meeting Potsdam 2009 July Nançay Radioheliograph: East - west array flat antennas Low gain antennas: (~wide band dipoles) Severe sensitivity limitation at high frequency One linear polarization

2nd LOFAR KSP meeting Potsdam 2009 July Nançay Radioheliograph: 5m antennas (north-south array)

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP: Interferometry U-V coverage –The solar corona is a broad source: u-v min < 30  Main problem: negative bowl due to poor uv sampling around the origin. Strong impact on quiet Sun T B. –Diffusion of radio waves in the corona broadens sources: baselines > km are not useful (probably) Field of view: > 6 degrees (> inverse of UV min) –CMEs may be observed at very high altitudes –To a lesser extent, type III also –This is a primary beam problem

2nd LOFAR KSP meeting Potsdam 2009 July Bastian et al. (2001)

2nd LOFAR KSP meeting Potsdam 2009 July CleanDirty Negative bowl removed (hardly) by CLEAN. Accuracy of low T B in coronal holes ? UV min ~70

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP: Interferometry Fringe stopping and coordinates –Absolute accuracy ~1 arsec (better if we use long baselines) –Sun motion UT/ST: it is better to make the fringe stopping in UT, but that can be corrected oofline (NRH uses UT) Sun hour angle/declination slow variations: up to 1 arcmin / hour. –Can also be done offline ( NRH uses one solar center coordinate per day, and makes the corrections offline) Imaging: –The preferred mode is snapshot –Earth rotation synthesis increases the quality of quiet corona thermal emission.

11 juillet 2008 : MHz. refait 27 mai

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP: Interferometry Calibration –Ideally: gains to a few %, phases to a few degrees NRH problems: no strong point source in the sky. –We use most often a model of Cygnus A. Problems come from the small antenna sizes, the strange arrays configuration and the simplified correlator (which dont make all the possible correlations) LOFAR should be much better. –Polarization calibration is done by a rotation of the antennas (there is no polarized calibrator). Related problem: crosstalk between the 2 polarization of the antennas should be as low as possible (or corrected ?): instrumental polarization should be <1% (Type III polarization…) –It is difficult (impossible) to calibrate in the presence of an active sun: the best answer is stability, at least for 24 hours. Have a common frequency between LOFAR and NRH (151 MHz ?)

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : Interferences MHz band Nançay (interference survey antenna) Wide band example

2nd LOFAR KSP meeting Potsdam 2009 July MHz band Nançay (interference survey antenna) Narrow band examples Special Issues at Low frequencies: Interferences

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : Interferences NRH has no RFI mitigation capability –Study for FASR (experimental interferometer) The classic system with banks of narrow filters can remove medium level low bandwidth telecom signals, with simple detection of low bandwidth signals. It is more difficult for powerful interferences Solar obervations are special: –Not sensitive to low level interferences – Detection of RFI based on the power level is not possible, due to solar bursts. We try to have the best status in the (very few) band ~allocated to radioastronomy: –74, 151, 327, 408, 610 MHz.

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : Interferences For LOFAR KSP: –Make a simple ~real time RFI mitigation, avoid storing lots of small bandwidth correlations. –The situation is getting worst in the metric band, with digital audio and video broadcast: Everybody should work in his country to get the best legal protection of the astronomy bands ( is it too late ?).

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : 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

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : 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 Their effect is VERY sensitive to the height of the sun (10° is a bad value). They are frequent. –There are TID: « Bubbles » isolated disturbances NRH see phase shifts (100°) crossing the arrays in ~30 sec.

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : Ionosphere Possible corrections –For NRH: almost none Try to follow a stable source on the sun, if any (noise storm). It is difficult to measure motions on the quiet sun emission. –For LOFAR: ? Ionosphere model based on motions of radio sources (equivalent to multi object adaptative optics). Needs one source per square degree, not convenient for solar observations. At low frequencies, you have to correct not only motions, but also scintillations.

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : time/frequency sampling 0.1 sec, 200 kHz bandwidth, 5 to 20 frequencies –With 20 frequencies, it is possible to have a raw spectrum of different sources. With the 200 kHz bandwidth, only I and V are required –In a spectrograph mode (one or a few stations), 4 Stokes make sense if the bandwidth is <10 kHz. Burst / monitoring (= integrated) modes –Both need the same number of stations, observing time, correlator resources. Is monitoring mode a convenient quicklook to the observations?

2nd LOFAR KSP meeting Potsdam 2009 July NRH -> LOFAR KSP : Data formats Store visibilities, not images –Processing algorithms may be improved –Possibility to make images in wider fields Use standard FITS (Soho headers ?, Aips compatible?) –Think to quick look products to facilitate data access Integrated data (standard images?) movies For solar studies, essential capabilities are: –Movies –Merging with other solar observations –Sources detection and tracking –Integration in Solarsoft –Specific multiscale deconvolution Storage: compression with loss –Integrate when time variations with time are small + manual decision for exceptionnal events.

NRH -> LOFAR KSP : the end 2nd LOFAR KSP meeting Potsdam 2009 July Thank you