1 Introduction

The study we propose will allow us to analyse the same plasma sample at three different observation points. In fact, at the end of August 2007, a lineup of Venus, Earth and Ulysses suggested to perform a coordinated study on the radial evolution of solar wind turbulence and coronal transients like CMEs between 0.7 and 1.4 AU.

Being around the minimum of solar cycle 23, the solar wind profile is charaterized by a bi-modal distribution in the ecliptic, due to the fact that coronal holes widen up reaching the equatorial regions and producing an alternation of fast and slow wind streams. This will allow us to study Alfvénic turbulence typical of fast coronal wind, within corotating fast streams.

Most of what we know today about the radial evolution of interplanetary Alfvénic turbulence is due to Helios observations of fast wind between 0.3 and 1.0 AU performed during 1976 (Tu and Marsch 1995; Bruno and Carbone 2005).

In particular, during its first solar mission, Helios 2 was able to sample, at three different heliocentric distances, fast wind plasma coming from the same solar source region during three consecutive solar rotations, as shown in Fig. 1.

Fig. 1
figure 1

High velocity streams and slow wind as seen in the ecliptic during solar minimum as function of time and radial distance from the sun, as observed by Helios 2 in 1976.

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Power density spectra of magnetic field fluctuations observed by Helios 2 between 0.3 and 1 AU and by Ulysses at 4.8 AU in the ecliptic for fast and slow solar wind stream are shown in Fig. 2. While slow wind spectra do not change their spectral index during the wind expansion, the shift of the knee of the power spectrum towards lower frequencies, observed in fast wind, has been interpreted has an evidence that non-linear interactions are at work during the wind expansion (Tu and Marsch 1995). Obviously, the underlying hypothesis is that physical conditions at the source region have not changed for three consecutive solar rotations. The strength of the present study is that of allowing to release the assumption of stationarity at the source region, offering the unique opportunity of a true study of the radial evolution of solar wind turbulence and, if lucky enough, of CMEs.

Fig. 2
figure 2

Power density spectra of magnetic field fluctuations observed by Helios 2 between 0.3 and 1 AU and by Ulysses at 4.8 AU for fast and slow solar wind stream. The spectral break shown by each spectrum of the fast wind, moves to lower and lower frequency as the heliocentric distance increases.

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2 s/c Lineup

Interplanetary magnetic field data and moments of proton velocity distribution function such as density, speed and temperature are required for this programme. ACE at Earth (at L1 libration point), Venus Express at Venus (0.7 AU) and Ulysses (1.4 AU) will provide the magnetic field and plasma measurements during the alignment of August 2007.

While ACE and Ulysses are constantly immersed in the solar wind, Venus Express will be in the solar wind only during part of its highly elliptical polar orbit around Venus since the s/c will routinely visit the induced magnetosphere of the planet (pericenter 250 km and apocenter 66,000 km, orbital period 24 h). However, the spacecraft will spend the majority of its orbital time in the solar wind and could be used as a solar wind monitor.

During the alignment, the angle between Ulysses and the Earth–Sun direction will vary between 14.22° and 8.34° from August 17 to 28, respectively. The ephemerides of Earth and Venus are shown in Table 1.

Table 1 Earth’s and Venus’ SE ephemerides
Full size table

Magnetic field and plasma data from ACE and Ulysses will be freely accessible on the web. Magnetic field data from Vex will be provided by the Vex-Mag magnetometer (P.I.T. Zhang, SRI, Graz, Austria) while solar wind plasma data by the Vex ASPERA-4 (P.I.S. Barabash, IRF, Kiruna, Sweden) IMA analyser (H. Nilsson, IRF, Kiruna, Sweden).

For this study it is not required a time resolution particularly high since the time duration of the averages should be enough longer than the local ion gyro-period (Vex 6s, ACE 11s, Ulysses 20s) to stay within the MHD regime.