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

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1. Introduction

Magnetoionic theory predicts two modes of propagating electromagnetic radiation, the ordinary and extraordinary mode. The ordinary mode has the larger group velocity. Hence a sharply peaked unpolarized signal propagating through a plasma will arrive in ordinary mode first, and the extraordinary mode will be delayed. The detection of a delay would open exciting possibilities for coronal investigations: (i) The sense of polarization of the delayed mode uniquely defines the predominant direction of the longitudinal magnetic field in the medium of propagation. (ii) The delay is proportional to the longitudinal component of magnetic field. (iii) If the polarization originates in the source and is imbedded in the dispersing medium, the delay and the observed polarization define the predominant mode of emission. The dominant mode is an important characteristic of the emission mechanism. Thus, its determination can discriminate between different processes.

The two modes may not originate in the source of emission. One mode only may be emitted and be partially transformed into the other during propagation. In the following the source of emission is distinguished from the spatial origin of the polarization, e.g. the site of depolarization, from where the two modes propagate independently.

Delays between the two circular modes can originate in various ways. Some metric type III bursts start with high circular polarization and show a declining degree of polarization toward the end of the burst (Gopala Rao 1965; Slottje 1974; Santin 1976). This amounts to a delay between the peaks of the left and right circular modes of up to one second. It is much bigger than propagation in the corona could produce and is interpreted as an effect of the different cutoff frequencies of the two modes. For a given frequency the ordinary mode can escape from closer to the plasma frequency. Fast particles arrive first at a given height and excite fast Langmuir waves having frequencies close to the plasma frequency. Therefore the ordinary mode is excited first and is more intense (Benz et al. 1979).

Another previously reported case of delays between modes are fine structures of decimetric type IV bursts. Chernov & Zlobec (1995) find no consistent trend in the sign of the delays and interpret them by multipath effects and quasi-transverse regions. Furthermore they consider the delays as too large for dispersion effects by the corona. Wentzel et al. (1986) failed to detect a delay between modes in metric type I bursts with an upper limit of 4 ms.

Here we report on delays in the arrival times of the two circular modes in narrowband spikes observed in the decimeter range. Narrowband spikes of solar radio emission have been known for more than three decades (cf. Benz 1986 for a review). They have been discovered in the various frequency bands as soon as the necessary time resolution became available and form a separate type of emission distinguishable in the spectrum from other short emissions (e.g. Isliker & Benz 1994). Their typical duration decreases from 100 ms at 300 MHz to less than 10 ms at 3 GHz (Güdel & Benz 1990). Spikes are the shortest solar radio emissions and thus the ideal signal to search for group velocity delays.

Spike bursts from the center of the solar disk tend to be highly polarized. The average polarization significantly decreases toward the limb (Güdel & Zlobec 1991). At low frequencies, Benz & Güdel (1987) have noticed a definite relation between the sense of observed spike polarization and the polarity of the leading spot of the associated active region. Assuming that the polarity of the emission region is the same as the leading spot, they find that the spikes are emitted in the ordinary mode. Güdel & Zlobec (1991) distinguish between metric spikes, occurring mostly below 300 MHz in association with type III bursts, and decimetric(or microwave) spikes above 300 MHz in association with hard X-rays. For the decimetric type and the same assumptions they find extraordinary emission mode. From their findings one may conclude that metric and decimetric spikes are different modes of emission or suffer different propagation conditions.

The total bandwidth of spikes is extremely small. Typical values range from 0.2-3% (Csillaghy & Benz 1993). Estimates of the source size based on bandwidth and VLBI observations (Benz et al. 1996) suggest values of the order of 100 km. This leads to brightness temperatures of the spike radiation up to [FORMULA] K, being strong evidence for a coherent emission process. Csillaghy & Benz (1993) find no difference in the frequency dependence of duration and bandwidth in metric and decimetric spikes. This may be taken to indicate that both kinds of spikes are produced by the same mechanism.

Several emission mechanisms for spikes have been proposed, but none is generally accepted. The electron maser has been suggested by many authors (e.g. Holman et al. 1980; Melrose & Dulk 1982; Aschwanden 1990; Robinson 1991; Kuncic & Robinson 1992). It only operates at a very low ratio of thermal to magnetic energy density, [FORMULA], and predicts extraordinary mode except for a restricted range of [FORMULA]. Upper hybrid, z-mode and Bernstein-mode instabilities have been proposed by Zhelznyakov & Zaitsev (1975), Vlahos et al. (1983), Tajima et al. (1990), Güdel & Wentzel (1993), and Willes & Robinson (1996). These waves require wave-wave coupling to be converted into propagating radio emission, which then is predominantly in ordinary mode.

High time resolution observations of decimetric narrowband spikes are presented in the following section. In Sect. 3 the method is described how the delay has been measured for the first time and how its accuracy was estimated. The results are compared with theoretical expectations and discussed in view of the physics of spike emission and propagation in Sect.  4.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998