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Astron. Astrophys. 354, 1101-1109 (2000)

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1. Introduction

1.1. Observational context

It is well known that two coordinates are of great importance when studying jovian radio emissions: the central meridian longitude (CML) which refers to the observer's position with respect to the planetary magnetic field and the Io's orbital position with respect to superior geocentric conjunction ([FORMULA]). Bigg (1964) established for the first time that the jovian decameter emissions can be attributed to Io-dependent and Io-independent components. The first one depends on Io and appears only in certain positions of the observer with respect to the Io satellite. The second component appears wherever the observer is with respect to Io. On the CML and [FORMULA] diagram several zones of enhanced occurrence probability have been labelled sources A, B, C, D (CML and Io phase coordinates for each source is mentioned in Carr et al. 1983). The Io-controlled emissions are more intense (Desch et al. 1975; Desch 1980) and have spectral characteristics like typical shape of arcs (Warwick et al. 1979; Boischot et al. 1981; Leblanc 1981; Aubier & Genova 1985). These arcs exhibit opposite curvatures and have been called vertex early arc (VEA) and vertex late arc (VLA) by Warwick et al. (1979).

The polarization of the jovian decameter emission (DAM) was studied by several authors. DAM is mainly polarized in the right-hand (RH) sense and sometimes in the left-hand (LH) sense. Kennedy (1969) reported a polarization analysis of A, B, C and D sources at fixed frequencies (10 MHz, 16 MHz, and 22 MHz). The first measurements of the Stokes parameters had been made by the spectropolarimeter associated to the Nançay Decameter Array (NDA) in a wide frequency band (Boudjada 1991). This receiver allows to analyse the linear and circular degrees of polarization of A and B sources (Lecacheux et al. 1991) and the orientation of the ellipse of polarization (Boudjada & Lecacheux 1991; Shaposhnikov et al. 1999). Using the spectropolarimeter data several authors characterized in more details the relationship between the spectral shape and the associated polarizations (Boudjada & Genova 1991; Dulk et al. 1992; Dulk et al. 1994; Leblanc et al. 1994; Boudjada et al. 1995).

1.2. Simultaneous space and ground observations

Jovian radio emissions have been recorded since a long time by several ground-based observatories and spacecraft. Such simultaneous observations allow to estimate the beaming effect on the emission. The first direct evidence of such effect was reported by Poquerusse & Lecacheux (1978) which attempt to observe an Io-B source at 30 MHz simultaneously from Earth (Nancay observatory) and from the Stereo 5 experiment aboard MARS 3 spacecraft. The authors showed that the emission lobe is smaller than 15o and that a difference of 4o in Io phase makes emissions observable or not. Reyes & May (1981) compared the previous observations with those made at the same time in Chile, Florida, and Colorado and they suggested that Poquerusse & Lecacheux (1978) results may have been applicable only within a relatively narrow frequency range. The arc structures of jovian decameter emissions were analysed by Barrow et al. (1982) using Nançay observations and Voyager 1 and 2 data. They found a linear relationship between the central meridian longitude (CML) of Io-controlled source (Io-A) and the corresponding values of the Jovicentric declination of the Earth. Maeda & Carr (1984, 1988) made comparison of Voyager 1 data and Mizuho-cho Radio Observatory measurements and found a significant correlation with 42 min-lag between non controlled source (non-Io-A) observed from Voyager 1 and the Earth (due to different wave propagation times). This result was supported by Riihimaa (1986) using a jovian non-Io storm observed at the Oulu Observatory (Finland) in 1979 and which was compared with jovian emission observed at the same time from Voyager 1. More recently two groups of authors have compared data acquired from Wind spacecraft launched in 1994 and from Earth. Kaiser & Garcia (1997) compared Wind spacecraft data to Florida data obtained in the range 18-22 MHz during a different period of time (1968-72) when the Jovicentric declination of the Earth (DE) was comparable. They found a greater number of intense controlled emissions (Io-C and Io-D), compared to what is usually reported, and interpreted their result as an effect of the Jovicentric declination of the Earth DE which makes more visible the Southern hemisphere. Lecacheux et al. (1998) combined the Nançay Decameter Array observations with simultaneous Wind/WAVES observations which cover the frequency range from 1 MHz to 13.8 MHz. Using two representative Io-controlled events (Io-B/D and Io-C), they have shown that it is necessary to introduce refraction effects to explain the discrepancy between observations and the apparent emission angle deduced from a hollow conical radiation beam.

In this paper we report on statistical studies of jovian decameter emission observed from space and from ground in the same period when the meridian transit of Jupiter at Nançay (France) is mainly during the night. In Sect. 2 we introduce the characteristics of antenna and receivers used by the NDA and by Wind/WAVES. The data analysis of more than 200 events observed from ground and/or from space is discussed in terms of occurrence probability in Sect. 3. We focus our attention, in Sect. 4, on the morning events (observed from 00 UT to 06 UT) where occurrence probability from the NDA and Wind/WAVES are found to be similar around 03 UT. In Sect. 5, we discuss our results and we emphasize on the source occurrence areas in the diagram ([FORMULA], CML) which is totally dependent on several parameters as frequency of observation, the Jovicentric declination of the Earth and observation conditions. We conclude in the last section on the complementarity of ground and space observations.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000