Solar Astronomy with LOFAR - First Steps OSRA Working Group Day Frank Breitling AIP Potsdam, 27.11.2008

Contents Introduction to LOFAR The Solar Key Science Project First Steps – Report from “The 1st GLOW Interferometry School” in Garching

The LOw Frequency Array (LOFAR) 1. Introduction to LOFAR 3/28 The LOw Frequency Array (LOFAR) Is a new radio interferometer in the range of 30-240 MHz with baselines from 1-1000 km with thousands of antennas in Europe centere in Exloo, Netherlands total costs 150 Mio. € commissioning phase has started first light in 2006

1. Introduction to LOFAR 4/28 The LOFAR Consortium LOFAR – Netherlands (~20 institutions) Institutes: ASTRON (head), Geophysical, Agrotechnological, ... Universities: Groningen, Leiden, Amsterdam, Radloub, ... 18 core + 18 remote stations, partially operational GLOW – German LOng Wavelength consortium MPIfR Bonn, AIP, MPA Garching, TLS Tautenburg, FZ Jülich Universities: Bonn, Bochum, Bremen, Köln, ... 5 stations funded, some already operational E-LOFAR – European Extension negotiations in progress ca. 10 stations planed

Comparison with other Experiments 1. Introduction to LOFAR 5/28 Comparison with other Experiments + LOFAR has 8 simultaneous beams, => observations and calibration at the same time

Station of the AIP in Potsdam-Bornim 1. Introduction to LOFAR 6/28 Station of the AIP in Potsdam-Bornim Site at the Leibniz-Institut für Agrartechnik Potsdam-Bornim (ATB)

1. Introduction to LOFAR 7/28 LOFAR Remote Stations 96 Low-Band antennas (30-80 MHz) 96 High-Band antennas (120-240 MHz) 200m x 100m array Remote location (no trains, highways, industry, etc.) Costs 750.000 EUR Low-Band High-Band

1. Introduction to LOFAR 8/28 Data Processing Station output: 8 beams x 4 MHz => 3 Gbit/s Station network: 10 Gbit/s Ethernet Central Correlator Blue Gene (IBM Supercomputer) at University of Groningen Max. input 400 Gbit/s Groningen

KSP: Solar Physics & Space Weather with LOFAR 2. The Solar Key Science Project 9/28 KSP: Solar Physics & Space Weather with LOFAR Collaboration & Management plan: Frank Breitling Dr. Alexander Warmuth AIP

Radio Emission from the Corona 2. The Solar Key Science Project 10/28 Radio Emission from the Corona Thermal: 106K Non-thermal: Flares, CMEs Radio frequency given by Plasma Frequency =>f only depends on density N, N determined by the height f= 1 2π N e 2 m e ε 0

Information from Radio Spectra 2. The Solar Key Science Project 11/28 Information from Radio Spectra Observations of solar radio bursts: Coronal density model (Mann et al., 1999) Radio Spectrogram ® height-time diagram Frequency drift rate ® velocity of the source

Information from LOFAR 2. The Solar Key Science Project 12/28 Information from LOFAR Images with high resolution: Current image resolution: ~5' LOFAR image resolution: ~1', only limited by turbulence in the corona LOFAR will achieve unprecedented resolution at 30-240 MHz

Radio Burst and Space Weather 2. The Solar Key Science Project 13/28 Radio Burst and Space Weather Emission from Flares & CMEs can be a threat for Devices on earth, e.g.: navigation systems, power lines, pipe lines Astronauts, Satellites LOFAR can determine Direction of emission to earth Observe the interaction with interplanetary space be ideal instrument for MWL-campaigns Flare at sun Interaction with magnetosphere LOFAR can make important contributions to space weather studies

Desired Observation Modes for Sun Monitoring 2. The Solar Key Science Project 14/28 Desired Observation Modes for Sun Monitoring

Solar Science Data Center 2. The Solar Key Science Project 15/28 Solar Science Data Center Archive of various solar data e.g. 3D models of corona by combining LOFAR & Tremsdorf data System for data analysis Provide access & service to the scientific community (Grid interface)

First Steps - Report from the GLOW Interferometry School Date&Place: 17.-19. Nov. 2008 in Garching Lectures: Condensed version of “The 11th Synthesis Imaging Workshop” of the NRAO at New Mexico Tutorials: by James Andersons (Bonn) & Enno Middelberg (Tautenburg) Software: AIPS (Astronomical Image Processing System) (standard software developed in the 1980 with FORTRAN) Data: VLA data from the Very Large Array in New Mexico from the NRAO Data Archive

Principle of Radio Interferometry 3. First Steps 17/28 Principle of Radio Interferometry X s An antenna b multiply average 90o b.s

3. First Steps 18/28 Complex Visibility DEFINE from the sine and cosine outputs of a correlator: where This gives a relationship between the source brightness I(s), and the response of an interferometer: The image intensity is the inverse Fourier-Transform of the complex visibility. By adding a phase delay τ0 the max lies in source direction. 𝐴= 𝑅 𝐶 2 + 𝑅 𝑆 2 φ= tan −1  𝑅 𝑠 𝑅 𝑐 

Examples of Visibility Functions 3. First Steps 19/28 Examples of Visibility Functions Il V(u)

3. First Steps 20/28 Radio Synthesis Extension to 2D results in UV-plane of complex visibilities Their inverse Fourier-transformation provides the radio image The inversion process is called image synthesis However since the data is not perfect, many additional steps are required to extract an image: Editing (tagging / excluding of bad data) Calibration with reference source Self calibration, where astronomer removes features with his knowledge about the source CLEAN algorithm for image cleaning

Summary of the 1st Tutorial Session: Analyzing VLA data of the Sun 3. First Steps 21/28 Summary of the 1st Tutorial Session: Analyzing VLA data of the Sun

UV-Plot of the Calibrator Source 3. First Steps 22/28 UV-Plot of the Calibrator Source Every baseline contributes two points to the UV plane. If the baselines rotate with the earth they can provide a better coverage of the UV plane.

Calibration of the Data 3. First Steps 23/28 Calibration of the Data Shift of amplitude and phase result from changes of the ionosphere. They are corrected by comparison with reference (calibrator) sources.

First Radio Image of the Sun 3. First Steps 24/28 First Radio Image of the Sun After some further operations, we received the first image of the sun.

Final result after CLEAN-ing 3. First Steps 25/28 Final result after CLEAN-ing Finally the CLEAN algorithm was applied: A data model is assumed. This is convolved with the dirty beam and subtracted. The dirty beam is the “PSF” in Fourier-Space which is known from the calibrator source.

3. First Steps 26/28 Next Steps Get hands on the LOFAR software (MeqTrees) and data (account in Groningen is requested) 10.-12. Feb: School for LOFAR Data Processing in Dwingeloo Obtain first analysis results Setup data center / get into software development process

The Paradox of Radio Interferometry Comment 27/28 The Paradox of Radio Interferometry From Young double slit we know: observation of the photon at the slit destroys the interference. But isn't that what the antennas do? How can we see interference of a radio signal? Answer: Observe at least two photons (not at the same slit). Then a superposition is observed and the path of an individual photon is not determined. =>Interference preserved Correlator

Comment 28/28 Phase Resolution Moreover, the uncertainty principle determines phase resolution Since the count rate follows Poisson statistic !The phase resolution determines the image resolution! Δ𝐸Δ𝑡≥ ℎ 2π with 𝐸=ℎνΔ𝑁,φ=2πνΔ ⇔Δ𝑁Δφ≥1 Δφ≥ 1 𝑁 = ℎν 𝑃 𝑠𝑖𝑔 τ with Psig: Signal power, τ: integration time Reference: Thompson, Moran, Swenson 2004, “Interferometry and Synthesis in Radio Astronomy” (1.4 Quantum Effect)