The Geotail spacecraft carries a high-resolution Magnetic Field Experiment to provide magnetic field data in the frequency range below 50 Hz. This experiment includes dual fluxgate magnetometers and a search coil magnetometer. Fluxgate sensors are mounted at distances of 4 m and 6 m from the spacecraft on a deployable mast to reduce spacecraft-generated noises. Both outboard and inboard fluxgate magnetometers have 7 automatically switchable ranges from ±16 nT to ±65536 nT (full scale) and resolutions equivalent to a 15-bit A/D conversion in each range. The basic sampling rate for the A/D conversion is 128 Hz for both magnetometers. Sampled signals are averaged to 16 vectors/s for the outboard magnetometer and 4 vectors/s for the inboard magnetometer for telemetry. Time-derivatives of magnetic field in the frequency range of 1-50 Hz (128 vector-samples/s) are acquired by the three-component search coil magnetometer (located on another mast), separated by 4 m from the spacecraft. Fluxgate data are continuously obtained at the same rate for both real-time and recorded modes of operation, while search coil data are only acquired in the real-time telemetry operation.The instruments were operated after the time of mast deployment on September 4, 1992, and are presently working in all modes as designed. The details of this experiment and initial observations are presented.
[1] We present ground-based and in situ observations from March 13, 2007. The THEMIS satellites were in the evening sector conjugate to THEMIS ground-based imagers. At $0507 UT there was an optical onset on inner CPS field lines. This involved near-simultaneous brightening of 1 MLT hour longitudinal segment of the onset arc. The part of the arc that brightened was that closest to the equatorward boundary of the diffuse (proton) aurora. Within one minute, a dipolarization front moved across four THEMIS satellites. Based on their locations, the order in which they detected the dipolarization front, and the auroral evolution, we assert that the expansion phase began earthward of the four satellites and evolved radially outwards. We conclude that this onset occurred in an azimuthally localized region of highly stretched field lines.
In our wave observations by a wave measuring instrument named ELF on board the Akebono satellite, interesting ELF emissions in the frequency ranges above the cyclotron frequencies of He+ and/or O+ ions have been detected. This paper is to propose that these emissions are due to electromagnetic ion cyclotron waves, which might have been generated by the ion cyclotron resonant instability due to a temperature anisotropy of hot H+ ions, trapped along the magnetic field lines around the geomagnetic equatorial plane. In order to confirm the characteristics of these ELF emissions, statistical characteristics, refractive index, and wave normal direction of the emissions are first estimated. The propagation characteristics of these ELF emissions are examined by ray tracing for ion cyclotron waves in the magnetosphere using both cold and hot plasma models.
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