2.1 Cluster Wave Instruments
The four CLUSTER satellites carry eleven identical instruments to measure the electric field, the magnetic field and the electron and ion distribution functions (Escoubet et al. 1997). Three of them are particularly suited to study wave phenomena within or in the vicinity of the plasmasphere (see Sects. 4, 8, 9 and 10):
- The Spatio-Temporal Analysis of Field Fluctuations (STAFF) instrument measures the magnetic field between 8 Hz and 4 kHz with a three axis search coil magnetometer. Its spectrum analyzer performs auto- and cross-correlations between the three magnetic components estimated by the search coil and the two electric components measured by the Electric Field and Wave (EFW) experiment (Gustafsson et al. 2001). From autocorrelations, the energy densities of electric and magnetic components are inferred, together with the electrostatic/electromagnetic nature of the observed waves. The cross-power spectra are needed to estimate the polarization characteristics of electromagnetic waves. The time resolution varies between 0.125 s and 4 s. For a complete description of STAFF, see Cornilleau-Wehrlin et al. (2003).
- At higher frequencies (2-80 kHz), radio wave signals are continuously monitored by the active soundings and passive measurements of the Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) instrument. The hardware of WHISPER mainly consists of a pulse transmitter, a wave receiver and a wave spectrum analyzer. Electric signals are acquired by the EFW electric antennas and only the onboard calculated fast fourier transform of the digital electric waveforms acquired are transmitted to the ground. A passive spectrum is recorded every 2.2 s and an active one every 52 s in normal mode for a frequency resolution of 162 Hz. Unlike a passive receiver, such a relaxation sounder enables to trigger plasma resonances when the medium does not show them naturally. For a detailed description of WHISPER, see Decreau et al. (2001).
- The Wide-Band Data (WBD) experiment consists of a wide-band passive receiver, which provides electric waveforms with high time resolution in three possible frequency bands: 100 Hz to 9.5 kHz, 100 Hz to 19 kHz and 700 Hz to 77 kHz. The first frequency band is the one mostly operated to study plasmaspheric wave phenomena. It provides continuous waveforms with a 27.4 kHz sampling rate. When no soundings are performed, WBD electric data may be seen as high resolution zooms of WHISPER spectra. For a complete description of WBD, see Gurnett et al. (2001).
IMAGE (Imager for Magnetopause to Aurora Global Exploration) was the first satellite dedicated to imaging the Earth's inner magnetosphere (Burch 2000). It was equipped with six instruments, which use neutral atom, ultraviolet and radio imaging techniques. Two of these instruments have been particularly used to study wave phenomena in the plasmasphere (see Sects. 3,5, 6, 7 and 11):
- The Extreme UltraViolet (EUV) imager was able to picture the entire plasmasphere in a single "snapshot". It captured the helium ion (He+) distribution outside Earth's shadow by measuring their emission line at 30.4 nm. He+ is the second most abundant ion species in the plasmasphere accounting for roughly 20% of the plasma population while hydrogen ion (H+), the most abundant one, has no optical emission. Because the plasmaspheric
He+ emission is optically thin, the integrated column density of He+ along the line-of-sight through the plasmasphere is directly proportional to the intensity of the emission. Moreover, the 30.4 nm emission line is the brightest ion emission from the plasmasphere and is spectrally isolated with a negligible background. For a full description of EUV, see Sandel et al. (2000).
- The Radio Plasma Imager (RPI) was a low-power radar with three dipole antennas. The two spin plane antennas were of lengths 370 m and 470 m tip-to-tip (Benson et al. 2003) while the one along the spin axis was 20 m long (spin rate: 0.5 rpm). The spin plane antennae are so far the longest ever deployed in space for such an instrument. RPI was able to locate regions of various plasma densities by observing radar echoes from the plasma. These echoes were reflected when the radio frequency was equal to the plasma frequency. By stepping the transmitted signal frequency through a wide frequency range (3 kHz-3 MHz), features of various plasma densities were observed. Derived densities, from those locations returning radio sounding echoes, were combined with line-of-sight images captured by EUV to infer quantitative, global distributions of plasmaspheric plasma. For a full description of RPI, see Reinisch et al. (2000).
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