During its recent pole to pole transit,
Ulysses explored heliographic latitudes from 
S to
N and heliocentric
distances from 1.3 to 2.3 AU, between September 1994 and August 1995,
about 1 year before the 1996
solar activity minimum. This offers an opportunity to study the radial and
latitudinal structure of the solar wind, with minimal variation in the phase
of the solar activity
cycle, during a period when the heliosphere is expected to be in
its simplest state since the heliospheric current sheet is close to the equator.
This paper presents and analyzes in situ solar wind measurements
deduced from the
observations of the plasma
quasi-thermal noise (QTN) with the radio receiver on the Unified Radio
and Plasma Wave (URAP) Experiment [Stone et al., 1992].
The QTN is due to the voltage induced on the electric antenna by the random
motion of the ambient particles which excite plasma
waves near
the plasma frequency 
.
The theoretical interpretation of the QTN spectrum around
yields the density and
temperature of the core and halo electron populations
[Meyer-Vernet & Perche, 1989] (and references therein).
As noted
by [Meyer-Vernet et al., 1997], one of the main advantages of
thermal noise spectroscopy is its relative immunity to the spacecraft potential
and photoelectron perturbations which, in general, affect particle analyzers.
On Ulysses in the ecliptic plane, this method was
applied to study the solar wind plasma
at various
heliocentric distances
[Hoang et al., 1992]; [Maksimovic et al., 1995]
It is currently being extended to measure the solar wind speed
[Issautier et al., 1996]
using the low-frequency part of the thermal spectrum,
which is due to the proton thermal noise Doppler-shifted by
the wind velocity. This extension is necessary
at high latitudes where the solar wind
speed is large, and hence affects significantly the plasma QTN.
In this paper, we concentrate on the pole-to-pole variations of the electron density and core temperature, which are determined with a very good accuracy by this method [Meyer-Vernet et al., 1997], thus completing the preliminary results published by [Hoang et al., 1996] and [Issautier et al., 1997]. It is crucial to measure these parameters in well-defined conditions in order to constrain solar wind models.
Indeed, up to now, theories based on fluid descriptions cannot explain the observed high-speed flow if the wind is only driven by the classical electron heat flux (see, e.g., [Hundhausen, 1972]). Recent kinetic approaches based on the Vlasov evolution of non-Maxwellian velocity distributions yield promising results [Scudder, 1992]; [Maksimovic et al., 1997] but are not fully satisfactory because they neglect collisions, i.e., take the opposite extreme of fluid descriptions although the Knudsen number near the Earth's orbit is of order of unity. Given the present lack of understanding of the transport of thermal energy by the electrons, reliable measurements of the cold electron temperature radial profile and polytropic index are needed.
Previous measurements of temperature gradients were performed near the ecliptic plane and generally referred to several types of flows and/or different phases of the solar cycle. Even a careful separation of slow and fast winds and of stream-stream interaction regions as well as discarding transient events cannot completely sort out a single type of stationary flow and may also create bias. In contrast, during Ulysses fast pole-to-pole transit, the spacecraft was immersed during several months in the steady state fast solar wind near solar minimum, allowing an analysis of the temperature gradients and the north/south asymmetries in a well-defined and stationary type of flow. This is one of the goals of the present paper.
In section 2, we present an overview of URAP observations during the pole-to-pole transit. In section 3, we analyze the radial and latitudinal variations of the electron density and core temperature, and we deduce an empirical polytropic law. Statistical distributions of the above parameters are discussed in section 4. We compare our results with some previous observations and discuss some implications in solar wind physics in section 5. Final remarks are given in section 6.