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Ann. Geophys., 23, 1317-1333, 2005
www.ann-geophys.net/23/1317/2005/
© European Geosciences Union 2005
Magnetosheath waves under very low solar wind dynamic pressure: Wind/Geotail observations
C. J. Farrugia1, F. T. Gratton2, G. Gnavi2, H. Matsui1, R. B. Torbert1, D. H. Fairfield3, K. W. Ogilvie3, R. P. Lepping3, T. Terasawa4, T. Mukai5, and Y. Saito5
1Space Science Center and Department of Physics, University of New Hampshire, NH, USA
2Instituto de Física del Plasma, CONICET and FCEyN, University of Buenos Aires, Buenos Aires, Argentina
3NASA Goddard Space Flight Center, Greenbelt, MD, USA
4Department of Earth and Planetary Physics, University of Tokyo, Tokyo, Japan
5Institute of Space and Astronautical Sciences, Kanagawa, Japan
Abstract. The expanded bow shock on and around
"the day the solar wind almost disappeared" (11 May 1999)
allowed the Geotail spacecraft to make a practically uninterrupted 54-h-long
magnetosheath pass near dusk (16:30-21:11 magnetic local time) at a
radial distance of 24 to 30 RE (Earth radii).
During most of this period, interplanetary parameters varied
gradually and in such a way
as to give rise to two extreme magnetosheath
structures, one
dominated by magnetohydrodynamic (MHD) effects and the other by gas dynamic
effects.
We focus attention on unusual features of electromagnetic ion
wave activity in the former magnetosheath state, and compare these features
with those in the
latter.
Magnetic fluctuations in the gas dynamic magnetosheath
were dominated
by compressional mirror mode waves, and left- and right-hand polarized
electromagnetic ion cyclotron (EIC) waves transverse to the
background field.
In contrast, the MHD magnetosheath, lasting for over one day,
was devoid of mirror oscillations and permeated instead by EIC
waves of weak intensity. The weak wave intensity
is related to the prevailing low solar wind dynamic pressures.
Left-hand polarized EIC waves were replaced by bursts of right-hand polarized
waves, which remained for many hours the only ion wave activity present.
This activity occurred
when the magnetosheath proton temperature anisotropy
(=
 )
became negative. This was because the weakened bow shock exposed the
magnetosheath directly to the (negative) temperature anisotropy of the solar
wind.
Unlike the normal case studied in the literature,
these right-hand waves were not by-products of left-hand polarized waves
but derived their energy source directly from the magnetosheath temperature
anisotropy.
Brief entries into the low latitude boundary layer (LLBL) and duskside
magnetosphere
occurred under such inflated conditions
that the magnetospheric magnetic pressure was insufficient to
maintain pressure balance.
In these crossings, the inner edge of the LLBL was flowing
sunward.
The study extends our knowledge of magnetosheath ion wave properties
to the very low
solar wind dynamic pressure regime.
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