Solar observations with single LOFAR stations C. Vocks 1. Introduction: Solar Radio radiation 2....

Solar observations withsingle LOFAR stations

C. Vocks

1. Introduction: Solar Radio radiation

2. Observations with single LOFAR stations

3. Spectrometer mode

4. Summary

Astrophysical Institute Potsdam

Solar KSP Workshop II, 25.06.2009


Solar radio radiation

The Sun is a strong radio source:• Thermal: 106 K corona• Nonthermal: Flares, CMEs

Itensities:• Thermal: some 104 Jy• Nonthermal: up to 107 Jy

• Plasma emission

Radio wave emission:

The frequency f depends onlyon the density N


Heliospheric density model

weightmolecularmean6.0~v2/GMr

)m~/Tk(v

3rr4

rrln4

v)r(vln

v)r(v

2cSc

2/1pBc

c

c2c

2

2c

2

Parker's wind equation (1958)

A special solution agrees well withdensity measurements up to 5 AU(Mann et al., 1999)


Dynamic radio spectra

Observations of solar radio bursts:

dynamic radio spectrogram height-time diagram

Frequency drift rate (phase) velocity of the source


LOFAR frequencies in the corona

2.5210

2.0120

1.8030

1.6840

1.4870

1.37100

1.24170

1.17240

r/RSf/MHz

LOFAR Frequencies:

Middle and uppercorona


Use of a single LOFAR station

Station beam (70 m Ø):

• Low band: > 3 deg• High band: > 1 deg

The Sun is essentiallya point source!

Consequences:• No imaging• Spectral intensities asfunction of time Single station as spectrometer!


Use of spectrometer data

Dynamic radio spectra:• Frequency drift rates• Heliosph. density model• Source phase speed

Insights into thephysical processesof the radio source

Resolution needed:

• Frequency: 100 kHz• Time: 10 ms


LOFAR station as spectrometer

Itensity of solar radio bursts:

• High enough for single station observations, by far!

• 107 Jy

Frequency range:• Low band: 30 – 80 MHz• High band: 120 – 240 MHz

Some constraints:• Only 32 MHz (48 MHz?) bandwidth• Either low or high band• Either 160 MHz or 200 MHz sample frequency• 160 MHz sample frequency must be used between190 and 210 MHz


Use of several LOFAR stations

Frequency coverage:

Sample frequencies:• 160 MHz for station V• 200 MHz otherwise

6 stations needed for full coverage

48 MHz station bandwidth:• 1 station less for each low and high band?


Basic spectrometer mode

Station data processing:• Station takes samples with 200 (160) MHz rate• 1024 data points are collected, Fourier-transformed

Result:• Sub-bands of 195 (156) kHz width• Values for complex amplitudes every 6.4 (5.1) µs

Temporal resolution of 0.01 s:• Average over complex amplitudes squared• Can be handled by a PC on the station level• Station electronics capable of this?

Resulting data rate:• Total number of sub-bands in the LOFAR frequency range: 912• bb = 912 * 100/s * 4B = 365 kB/s = 1.3 GB/h


Better spectral resolution

Higher frequency resolution:• Fourier-transform series of sub-band samples

For 100 kHz frequency resolution:• DFT with 2 samples sufficient• Average again over 0.01 s• Computational effort: About doubled• Station electronics capable of this?

Resulting data rate:

• be = 730 kB/s = 2.6 GB/h


Data handling

During commissioning phase:• Station not connected to network?• Write data to local disk

Operational phase:• Send data to Solar Science Data Center• Via Jülich or Groningen?

• How will the stations be controlled? (GLOW issue)

• Data reduction on the station level?

• Controlled by ASTRON, treated as sub-array?


Summary and outlook

Solar radio radiation:

Single station spectrometer mode:

• Plasma emission• LOFAR frequencies: Middle and upper corona• The Sun is a point source for single stations• Dynamic radio spectra

• Record spectral intensities as function of time, dt = 0.01 s• 4-6 stations needed for full coverage of LOFAR frequencies• Basic spectrometer mode with sub-band resolution• Simple DFT for better frequency resolution• Data handling issues

Solar observations with single LOFAR stations C. Vocks 1. Introduction: Solar Radio radiation 2....

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