| Vol. 27, Issue 7 |
Lui et al. [911] use more than 9,000 satellite images of auroras taken in January 1997 to monitor the energy output over the entire magnetospheric system. They obtain, for the first time, statistics of size and energy dissipated by the magnetospheric system on a world-wide scale. They demonstrate that the dynamic magnetosphere resembles a simple avalanche system, implying that the coupled solar-wind-magnetosphere system is out of equilibrium, dissipating energy via avalanches which have no intrinsic scale. This result has implications for the predictability of space weather at Earth. Following a coronal mass ejection from the sun, the magnetosphere underwent a sudden compression on 24 September 1998. This event was observed by the POLAR spacecraft which was near apogee over the northern polar cap and traveling toward the magnetopause. From the data, Cladis et al. [915] show that the parallel velocity components of four ions increased by about 75 km/s during the compression due to centrifugal acceleration, the first definitive observation of this process, a key signature being the ions which appear to gain their energies from the acceleration not only during the compression but also all along their trajectories. Using a computer simulation, the authors relate the centrifugal acceleration process to the local magnetic field components and to the perpendicular drift and parallel velocity of the ions. Ion velocities in the lower thermosphere display strong tidal modes with periods of 24 hours, 12 hours and 8 hours. van Eyken et al. [931] measure the velocities using the EISCAT Radar at Longyearbyen, Svalbard, from which it is possible to measure ion velocity throughout a 24-hour period. The radar enables high-quality ionospheric measurements at 93 km where the semidiurnal mode is strongest. The authors find a terdiurnal mode but no diurnal mode and, further, evidence for a planetary wave with a period of ~ 2.5 days. |
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