References
J. Geophys. Res., Vol.107, No.A7, 10.1029/2001JA900152, 2002
Geomagnetic Negative Sudden Impulses:
Interplanetary Causes and Polarization Distribution
T. Takeuchi, T. Araki, A. Viljanen, and J. Watermann
Abstract
We made a study of the characteristics of geomagnetic negative sudden
impulses (SI-s) identified in the mid-latitude geomagnetic SYM indices
and the causative structures in the solar wind
using data from the Wind and ACE spacecraft.
A total of 28 SI-s with an amplitude larger than 20 nT in the
H component SYM index were found over the period 1995 through 1999,
with 50 per cent of them occurring in conjunction with a positive
sudden impulse, SI+ (i.e., SI pair).
In the SI pairs, the amplitude of SI- was almost always larger than
that of the preceding SI+.
We attempted for the first time a classification of structures in the
solar wind associated with SI-s. It is found that reverse shocks are
not responsible for SI-s. Instead, SI-s are associated with
varied structures such as tangential discontinuities at
high-low speed stream interfaces, front boundaries of interplanetary
magnetic clouds and trailing edges of heliospheric plasma sheets.
There is no preferential association of SI-s in our sample with
any particular type of solar wind structure.
We investigated statistically the polarization characteristics of SI-s
at high-latitude.
The sense of the polarization in the auroral zone tended to be
clockwise in the afternoon and counterclockwise in the morning.
The rotational sense reversed in the polar cap. The latitudinal reversal
occurred in the range from 65 to 80.
Thus, the polarization distribution of SI- is not opposite to
but is consistent with that of SI+.
We suggest that the contribution from the longitudinal movement of
a twin vortex ionospheric current system is dominant to produce
the polarization of SC and SI-.