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Cluster II WBD Science Overview


The primary purpose of the WBD investigation is to support the Cluster II Wave Experiment Consortium (WEC) science objectives by providing high-resolution spectral analysis, particularly at boundaries and regions with steep spatial gradients. During these times, WBD provides high-time resolution, single-spacecraft measurements for comparison with data from other instruments, such as the magnetometer and plasma instruments. From these data, waves produced by current-driven instabilities and other mechanisms involving spatial inhomogeneities can be clearly identified. In addition to single-point measurements, wideband data from two or more spacecraft can also be used to resolve space-time ambiguities as the spacecraft pass through complex spatial structures. The study of such structures is one of the primary objectives of the Cluster mission. In cases where the upper hybrid frequency or elecron plasma frequency can be identified, WBD can also provide very high resolution measurements of the electron density. Also, multiple-point comparisons of electron densities can be used to analyze the motion and evolution of plasma structures in the auroral zone and polar cap.

In addition to providing measurements that support the overall objectives of the WEC, the WBD instrument can be used to provide multi-spacecraft, long-baseline radio interferometry measurements. The basic principles of a radio interferometer are that an electromagnetic wave of wavelength lamda is detected by two antennas separated by a baseline distance L. The signals from the two antennas, E1 and E2, are multiplied and averaged to produce a cross-correlation {E1,E2}. The sign and amplitude of the cross-correlation is determined by the angle of arrival, theta, relative to the symmetry axis of the interferometer. If the wavelength of the radiation is less than the separation distance between the antennas, then the cross-correlation arises as the cosine of the angle of arrival. The antenna pattern of the interferometer then has a series of lobes, each with a beamwidth alpha. If a source moves through this fan-shaped antenna pattern, a sinusoidal "fringe" pattern is produced in the cross-correlation. With Cluster a total of six baselines can be achieved between the four spacecraft. Multi-spacecraft interferometer measurements have the potential of giving important new information on the angular motion and size of various terrestrial and astronomical radio sources.