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Astron. Astrophys. 323, 250-258 (1997)

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2. Observations

High time resolution data of narrowband spikes have been recorded with two spectrometers of ETH Zurich. The Ikarus spectrometer (Perrenoud 1982) operated during the 21st solar cycle from 100-1000 MHz. This instrument was rebuilt into the Phoenix spectrometer with a frequency range 0.1-3 GHz operating during the 22nd solar cycle (Benz et al. 1991). Both instruments observed the full Sun with a 7m parabolic disk and a linear, log-periodic feed. The circularly polarized modes are found before pre-amplification by phase-shifting the linear modes in a hybrid. Both spectrometers were frequency-agile, integrating one channel at a time with a resolution of 0.5 ms. A total of 2000 flux densities and circular polarization values can thus be measured per second. The observing frequencies and bandwidths are selectable and controlled by the system computer. One scan in frequency can contain from 1 to 500 observing channels, yielding time resolutions from 0.5 ms to 250 ms.

The receiver is calibrated daily against a reference noise source to find the relation between the digital readings at the A/D converter and antenna temperature. Also, the zero levels of left and right circular polarization are determined. The antenna up to the first switch is calibrated with the quiet Sun several times per year, using single-frequency observations from other observatories. This provides a relation between antenna temperature and physical flux units, as well as the zero point of circular polarization.

In 1982/83 and 1992 special programs have been carried out to measure with high time resolution. The Sun was monitored in a wide band to automatically register solar bursts. If a burst is detected at a given frequency, the system computer switches to a reduced list of channels near that frequency and records for a given time. Then it goes back to the broadband list to record an overview and to test whether the burst continues. This sequence is repeated until the enhanced emission vanishes. We have used the broadband data to identify spike events and to select events with intermediate and low polarization. An example of a broadband recording from 1.03 to 2.02 GHz is shown in Fig. 1. The resolution in this figure is 50 ms in time and 10 MHz in the spectrum. The spikes are marginally resolved in frequency, but not in time, in agreement with the average values found in previous work. Fig. 1 is interrupted at 13:25:45 for 90 seconds of high time resolution single frequency recording at 1.1, 1.4, 1.7 and 2.0 GHz in 250 ms intervals each.

[FIGURE] Fig. 1. Broadband observation of a narrowband spike event on 1990 January 20 observed by the Phoenix spectrometer of ETH Zurich. A gliding background of 1 s scale was subtracted. Single spikes show as bright, vertical dashes. During the gap the instrument measured 4 frequencies with high time resolution (cf. Fig. 2, top)

Examples of spike recordings with high time resolution in a single channel are shown in Fig. 2. The digitization steps are clearly visible. In the top of Fig. 2 a weakly polarized spike is shown. The left circular polarization is slightly delayed. An example of a more polarized spike is shown in the bottom of Fig. 2. The weak right polarization mode makes it much more difficult to measure the delay.

[FIGURE] Fig. 2a and b. Single-frequency recordings of single narrowband spike bursts by the Phoenix spectrometer of ETH Zurich. The time resolution is 0.5 ms. The left circular polarization mode is shown with a full curve, right circular polarization is dashed. Top: spikes on 1990 January 20 (cf. Fig. 1) at 1.4 GHz, Bottom: a spike on 1990 February 16 at 1.1 GHz.

Table 1 lists the observing parameters of the four selected events. In the event of 1982/06/04, observed by the Ikarus spectrometer, the instrument swept through the four channels every 2 ms. The observing frequencies are in the low-frequency part of an event that extended up to 1 GHz.


[TABLE]

Table 1. Overview on the parameters of the spike observations.


Only well developed, isolated bursts, well recorded over their entire lifetime, were used for the statistical analysis. Their number is given in Table 1. They were added into one time serie for each event and then cross-correlated.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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