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

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

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

4. 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

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

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

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

8. 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

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

10. Radio Emission from the Corona
2. The Solar Key Science Project /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

11. Information from Radio Spectra
2. The Solar Key Science Project /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

12. Information from LOFAR
2. The Solar Key Science Project /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 MHz

13. Radio Burst and Space Weather
2. The Solar Key Science Project /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

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

15. Solar Science Data Center
2. The Solar Key Science Project /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)

16. First Steps - Report from the GLOW Interferometry School
Date&Place: Nov 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

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

18. 3. First Steps /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 

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

20. 3. First Steps /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

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

22. UV-Plot of the Calibrator Source
3. First Steps /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.

23. Calibration of the Data
3. First Steps /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.

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

25. Final result after CLEAN-ing
3. First Steps /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.

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

27. The Paradox of Radio Interferometry
Comment /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

28. Comment /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)