Abstract. The accurate measurement of the magnetic field along the orbits of the four Cluster spacecraft is a primary objective of the mission. The magnetic field is a key constituent of the plasma in and around the magnetosphere, and it plays an active role in all physical processes that define the structure and dynamics of magnetospheric phenomena on all scales. With the four-point measurements on Cluster, it has become possible to study the three-dimensional aspects of space plasma phenomena on scales commeasurable with the size of the spacecraft constellation, and to distinguish temporal and spatial dependences of small-scale processes. We present an overview of the instrumentation used to measure the magnetic field on the four Cluster spacecraft and an overview the performance of the operational modes used in flight. We also report on the results of the preliminary in-orbit calibration of the magnetometers; these results show that all components of the magnetic field are measured with an accuracy approaching 0.1 nT. Further data analysis is expected to bring an even more accurate determination of the calibration parameters. Several examples of the capabilities of the investigation are presented from the commissioning phase of the mission, and from the different regions visited by the spacecraft to date: the tail current sheet, the dusk side magnetopause and magnetosheath, the bow shock and the cusp. We also describe the data processing flow and the implementation of data distribution to other Cluster investigations and to the scientific community in general.

The biggest halo coronal mass ejection (CME) since the Halloween storm in 2003, which occurred on 2006 December 13, is studied in terms of its solar source and heliospheric consequences. The CME was accompanied by an X3.4 flare, EUV dimmings, and coronal waves. It generated significant space weather effects such as an interplanetary shock, radio bursts, major solar energetic particle (SEP) events, and a magnetic cloud (MC) that were detected by a fleet of spacecraft including STEREO, ACE, WIND, and Ulysses. Reconstruction of the MC with the Grad-Shafranov (GS) method yields an axis orientation oblique to the flare ribbons. Observations of the SEP intensities and anisotropies show that the particles can be trapped, deflected, and reaccelerated by the large-scale transient structures. The CME-driven shock was observed at both the Earth and Ulysses when they were separated by 74 in latitude and 117 in longitude, which is the largest shock extent ever detected. The ejecta seem to have been missed at Ulysses. The shock arrival time at Ulysses is well predicted by an MHD model that can propagate the 1 AU data outward. The CME /shock is tracked remarkably well from the Sun all the way to Ulysses by coronagraph images, type II frequency drift, in situ measurements, and the MHD model. These results reveal a technique that combines MHD propagation of the solar wind and type II emissions to predict the shock arrival time at the Earth, which is a significant advance for space weather forecasting, especially when in situ data become available from the Solar Orbiter and Solar Sentinels.

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This paper describes the principal features of 24 hr averages of the magnetic field strength variations B(t) and their relationships to the plasma and energetic particles observed prior to and after the crossing of the termination shock (TS) by Voyager 2 (V2). The solar wind (pre-TS crossing) and heliosheath (post-TS crossing) data extend from day of year (DOY) 1 through 241, 2007 and from 2007 DOY 245 through 2008 DOY 80, respectively. In the solar wind, two merged interaction regions (MIRs) were observed in which the ratio of plasma pressure to magnetic pressure in the solar wind was relatively low. Strong magnetic fields and low values of beta were also observed just prior to its crossing of the TS. The predicted correlation between peaks in the intensity of energetic particles in the solar wind when V2 crossed the heliospheric current sheet from positive to negative magnetic polarity in the solar wind was not observed. In the heliosheath, V2 observed a feature characterized by large enhancements of the density N and the proton temperature T, a small increase in speed V, and a depression in B. The distributions of 24 hr averages of B and beta were approximately log-normal in both the solar wind and the heliosheath. A unipolar region was observed for 73 days in the heliosheath, as the heliospheric current sheet moved toward the equatorial plane to latitudes lower than V2.

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After its successful encounter with comet P/Halley and a four‐years hibernation period ESA's Giotto spacecraft has been reactivated in February 1990 and performed the first‐ever Earth gravity‐assisted maneuver on July 2, 1990 to be retargeted for comet P/Grigg‐Skjellerup. This swing‐by is of unique scientific interest due to Giotto's hyperbolic, high‐inclination orbit. Here, we shall report on scientific results of the Giotto magnetic field experiment. Due to the high fly‐by velocity and the relative quietness of the magnetosphere during the swingby period these measurements present a snapshot view of the Earth magnetosphere with clearly identified inbound and outbound bow shock and magnetopause crossings. The outbound crossings are of particular interest as surface waves at the polar magnetopause at a distance of 28 RE as well as a strong quasi‐perpendicular bow shock at a distance of about 64 RE are observed