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Astron. Astrophys. 355, 804-808 (2000)

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1. Introduction

Jovian decametric (DAM) radio emission essentially appears to be elliptically polarized. The recent observations made at Nançay Radio Astronomy Observatory, France (Lecacheux et al. 1991; Dulk et al. 1992, 1994) show that this emission is [FORMULA] elliptically-polarized at all frequencies in the measured range of 10 to 38 MHz. The ellipticity of the emission polarization (ratio of polarization ellipse axes) varies weakly from storm to storm and average values of the ellipticity can be considered as specific signatures of the "source" to be distinguished in the central meridian longitude (CML) and Io phase diagram. For example, at frequencies near 20 MHz the average degrees of linear [FORMULA] and circular [FORMULA] polarization are [FORMULA] and [FORMULA] for the emission from the Io-B "source", while for Io-A it is [FORMULA] and [FORMULA]; for Io-C [FORMULA] and [FORMULA] (Dulk et al. 1994). The minus sign corresponds to the right-hand polarization. Moreover, during an emission storm the ellipticity is approximately constant as a function of frequency and time. There are only a few exceptions to this rule. Lecacheux et al. (1991) have studied the spectral feature, which looked like a great arc in the Io-A event which occurred on 1988 Nov.2. They found that the spectral feature was superimposed on the main emission and had a considerably higher degree of circular polarization than the other part of the burst. For this spectral feature the average degrees of polarization were [FORMULA] and [FORMULA] versus [FORMULA] and [FORMULA] in another part of the storm.

The origin of the elliptical polarization of DAM emission is studied in a number of articles (e.g. Warwick 1970; Goertz 1974; Lecacheux 1988; Lecacheux et al. 1991; Melrose & Dulk 1991; Shaposhnikov et al. 1997). All of them consider the origin of the elliptical polarization as a result of the violation of the geometrical optics approximation in the Jovian magnetosphere. These studies can be divided in two groups. The first group (Lecacheux 1988; Lecacheux et al. 1991; Melrose & Dulk 1991) assumes that violation of the geometrical optics approximation took place already in the emission source itself, and the original polarization has been retained from the emission source to the observer. In this case, the observed polarization ellipticity is only determined by the angle [FORMULA] between the magnetic field lines and the direction of the ray path at the emission point. This theory may explain the origin of the elliptical polarization and gives a means to find quite precisely the location of the source. However, it has a number of serious drawbacks. For example, following this model Leblanc et al. (1994) found that a number of the Io-C events (the average circular polarization [FORMULA]) were generated in the magnetic tubes which were in front of the instantaneous Io flux tube (IFT) at a distance of about [FORMULA]. This requires Io to extend its influence upstream of the satellite. Another group of the papers (Warwick 1970; Goertz 1974; Shaposhnikov et al. 1997) assumes that the violation of the geometrical optics approximation can take place outside the emission source and some change of the polarization ellipticity occurs along the way. In the detailed polarization model of Shaposhnikov et al. (1997) there is no hard relation between the ellipticity of the observed emission and the angle [FORMULA]. The observed ellipticity of the emission polarization is determined by two parameters: angle [FORMULA] and, mainly, the magnetospheric plasma density [FORMULA] in the transitional region (TR) where the polarization of electromagnetic modes is essentially elliptical. The second parameter [FORMULA] involved in this consideration permits us to avoid the drawbacks mentioned above. However, including the second parameter in the model makes it impossible to find the location of the emission source by the polarization observational data. None of the papers discusses the origin of the time variations of the ellipticity during the burst similar to those observed in the Io-A event of Nov.2. 1988

In the present study, in order to explain the time features of emission polarization, we develop the model of the origin of the elliptical polarization suggested by Shaposhnikov et al. (1997). In addition, we show that the observed polarization feature of the great arc gives us the possibility both to locate quite precisely the position of the emission source and to estimate the level of the magnetospheric plasma density in IFT.

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© European Southern Observatory (ESO) 2000

Online publication: March 9, 2000
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