CENTRIFUGAL ACCELERATION OF IONS OBSERVED ON POLAR SPACECRAFT NEAR
9 RE OVER POLAR CAP UPON ARRIVAL OF CME-DRIVEN SHOCK

John B. Cladis, Harry L. Collin, and William K. Peterson (all at Lockheed Martin Advanced Technology Center, 3251 Hanover St., B255, Palo Alto, CA 94304, USA, emai:cladis@spasci.com) Thomas E. Moore (NASA Goddard Space Flight Center, Code 692, Bldg. 2, Greenbelt, MD 20771-1000, USA, email thomas.e.moore@gsfc.gov), Christopher T. Russell (Institute of Geophysics and Planetary Physics, University of California, Los Angeles. CA 90095-1567, USA. Email: ctrussel@igpp.ucla.edu)

 

A shock driven by a coronal mass ejection struck the magnetosphere about 2344 UT on September 24, 1998. The POLAR spacecraft was near apogee over the northern polar cap, at (X Y Z)GSM = (-2.3803, -2.7148, 8.1450) in RE. During the following 144 s, from 2345:18 UT a 2347:42 UT, (1) the magnetic field measured on-board with the UCLA magnetometer increased sharply from 126.16 nT to 182.00 nT, (2) the field components changed from (BX BY BZ) GSM = (84.91, 32.55 -87.45) to (151.11, 50.655, -87.867); (3) the full rotation of B was 19.87o; and (iv] the velocity components of both the H+ and 0+ ions increased by about 75 km/s antiparallel to B. We suggest the ions were flung along -B by the "motion" of the magnetic field according to the centrifugal-acceleration process first described by Cladis (GRL 13, 893, 1986) A parallel-velocity increase of 35 km/s was computed by integrating the term dvll/dt=vE.de/dt, using the local values of vE (the drift velocity) implied by the ion-velocity components perpendicular to the magnetic field and the measured values of e(t) = B(t)/B(t) The higher acceleration of the ions was confirmed by a computer simulation of the process along the ion paths