On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.

Abstract. Highlights are presented from studies of the electric field data from various regions along the CLUS-TER orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from 1 mHz to 9 kHz. This presents completely new opportunities to study spatialtemporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.

The electrical properties of the medium around a probe in the outer magnetosphere and in interplanetary space are modified by photoemission. Information on the energy distribution of the photoemitted electrons is important for the evaluation of the physical parameters of the surrounding photosheath and for the interpretation of the probe measurements. This energy distribution has been determined for various materials exposed to sunlight, at the earth's orbit, by combining laboratory measurements on photoemission with solar spectrum data collected in space. It is found that the photoelectron saturation current density is a function of the material and can vary from a few microamperes per square meter up to several tens of microamperes per square meter; the photoelectron density is of the order of 10²–10³/cm³ in the vicinity of the probe. The shape of the distribution is approximately Maxwellian, and the mean kinetic energy is of the order of 1.5 ev. Current voltage characteristics and conductance of surfaces emitting photoelectrons are also derived. A number of photosheath parameters, such as shielding distance and surface electric field, are tabulated. Finally, the consequences of photoemission for the interpretation of space measurements are discussed.

The main objective of the Mutual Impedance Probe (MIP), part of the Rosetta Plasma Consortium (RPC), is to measure the electron density and temperature of Comet 67P/ChuryumovGerasimenko's coma, in particular inside the contact surface. Furthermore, MIP will determine the bulk velocity of the ionised outflowing atmosphere, define the spectral distribution of natural plasma waves, and monitor dust and gas activities around the nucleus. The MIP instrumentation consists of an electronics board for signal processing in the 7 kHz to 3.5 MHz range and a sensor unit of two receiving and two transmitting electrodes mounted on a 1-m long bar. In addition, the Langmuir probe of the RPC/LAP instrument that is at about 4 m from the MIP sensor can be used as a transmitter (in place of the MIP ones) and MIP as a receiver in order to have access to the density and temperature of plasmas at higher Debye lengths than those for which the MIP is originally designed.

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