Open AccessObservations of long‐term field‐aligned flow of O + ions near the equator during summer and winter
Author(s) -
Cladis J. B.,
Collin H. L.
Publication year - 1997
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/97ja01500
Subject(s) - magnetosphere , physics , ionosphere , solar wind , pitch angle , atmospheric sciences , field line , ring current , magnetic dip , geophysics , equator , electric field , magnetic field , ion , local time , flux (metallurgy) , latitude , astronomy , materials science , statistics , mathematics , quantum mechanics , metallurgy
In the equatorial region of the outer magnetosphere ( R > 4 R E ), highly asymmetric storm time pitch angle distributions (0° versus 180°) of O + ions (<20 keV) have been observed near dawn and dusk during winter and summer seasons with ion spectrometers on board the CRRES and AMPTE/CCE satellites. These ion asymmetries, although variable in magnitude and ion energy, persist in the same sense for periods of up to 3 hours. The observed O + pitch angle distributions suggest a net summer‐to‐winter ion flow in the dawn sector and a net winter‐to‐summer ion flow in the dusk sector. This is consistent with the direction of an induced parallel (to magnetic field B ) electric field that accompanies the decay of the tail current or the growth of the Birkeland currents when the Earth's magnetic dipole is tilted toward or away from the Sun. A net magnetic flux due to the tail current (mainly along the Z GSM axis) threads the areas enclosed by these field lines only when the areas are inclined to the ( X , Y ) GSM plane. The net flux due to the Birkeland current arises because this current system is asymmetric during summer and winter, being stronger in the hemisphere of the sunlit polar ionosphere. To date, one of the 17 observed events has been examined in detail. IMP 8 data in the solar wind and onboard magnetic field measurements were used to estimate the field‐aligned electromotive force (EMF) induced by each of these current systems. The EMFs were found to be appropriate to account for the asymmetry of the ions. Furthermore, the general features of the observed pitch angle distributions were confirmed with a computer program that simulated the response of initially trapped ions to a parallel electric field.