Astronomy

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Astron

Initiator: ASTRON Netherlands Institute for Radio Astronomy

eu  SNN

This project was co-financed by the EU, the European Fund for Regional Development and the Northern Netherlands Provinces (SNN), and EZ/KOMPAS.

Technical Description

Observations needed

To perform the planned observations the following observation modes have been specified according to the capabilities of LOFAR. The number of frequencies was limited by the I/O bandwidth to the correlator. One LOFAR station cannot cover the full LOFAR frequency rage, therefore several stations are combined to cover the full frequency range. The loss of sensitivity and deterioration of station beams are not critical for solar observations, since the active sun is a strongest radio source in the sky.

Spectrometer Mode

LOFAR can provide spectroscopic data at a millisecond rate but only a time resolution of 0.01s is needed for solar observations. In the whole frequency range 30-80 MHz and 120-240 MHz, LOFAR can measure the intensity with a spectral resolution of 100 kHz, i.e. 170MHz/100kHz=1700 channels. An intensity value is represented by 2 bytes. With these parameters the expected data rate is b_s = 340kB/s = 1.224 GB/h. To cover the full frequency range, multiple stations have to be combined.

Monitoring mode

The monitoring mode is used for the routine imaging. These observations are only done at four frequencies, two in low- and two in high-band.
Frequency Nr.
1
2
 3 4
Frequency [MHz]  40  80 120
240

The cadence is 1 per minute. The expected data rate in monitoring mode is b_m = 26kB/s = 100MB/h.

A simulated web page of the monitoring mode is shown in figure 4. It contains the radio maps offered by LOFAR and in addition an H-alpha image from the solar observatory Kanzelhöhe and a simultaneous dynamic radio spectrum.


Figure 4. Simulated web page of the monitoring mode.

Burst Mode

The burst mode is used for recording of solar radio bursts with a high spectral and temporal resolution. Observations at 22 different frequencies are defined to resolve the solar activity at different coronal height levels. The frequencies are given in the table below.

Low Band
Frequency Nr.
 1 2
3
 4 5
6
7
8
9
Frequency MHz
40 45
50
55
60 65
70
75

80

High Band
Frequency Nr. 10
11
12
13
14
15
16
17
18
19
20
21
22
Frequency MHz
 120 130
140
150
160
170
180
190
200
210
220
230
240
The number of observed frequencies can be traded for image cadence if the I/O capabilities of LOFAR's correlator are exceeded. The switching between the monitoring and the burst mode can possibly be triggered by an external instrument acting as a "burst bell". This can be a remote LOFAR station operated in a stand-alone mode, or a solar spectrometer that preferably covers higher frequencies, e.g. up to 800 MHz, that correspond to lower heights in the solar corona. Thus, a burst can be detected before it reaches LOFAR's frequency range.
The expected data rate in burst mode is B_r =b_b *t_r =200MB/s = 360GB/h. A ring buffer of 2 hours is planned.

Estimated Data Volume 

 Data Type
Data Rate [MB/s]
Data Rate [GB/h]
 Burst Mode
100
360
 Monitoring Mode
0.03
0.1
 Spectrometer Mode
0.34
1.2
ASTRON initiated LOFAR as a new and innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. 
Development: Dripl | Design: Kuenst   © copyright 2014 Lofar