*Tomohiko Takeuchi[1]
,Tohru Araki [1],Ari Viljanen [2]
Jurgen Watermann [3]
Department of Earth and Planetary Science, Kyoto University[1]
Finnish Meteorological Institute, Finland[2]
Danish Meteorological Institute, Copenhagen[3]
We made a study of the characteristics of geomagnetic negative
sudden impulses (SI-s). 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). We investigated statistically the polarization characteristics
of SI-s at high-latitude. The dependence of the sense of polarization
of SI- on local time and latitude is found to be similar to that
of sudden commencement (SC) or SI+. The SI- is therefore not
a mirror image of SI+ in polarization. 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-.
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 dependence of the sense of polarization of SI- on local time and latitude is found to be similar to that of sudden commencement (SC) or positive sudden impulse (SI+). The SI- is therefore not a mirror image of SC or SI+ in polarization. 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-.