On the nature of response of dayside equatorial geomagnetic H-field to sudden magnetospheric compressions

Abstract

A step-like increase in the solar wind ram pressure causes a sudden compression of the magnetosphere which manifests as a sudden commencement (SC) in the geomagnetic field. SC manifests in two basic forms in the geomagnetic H-field of the dayside dip equatorial region, namely, as a positive impulse termed SC(+), and as a positive impulse preceded by a sharp negative impulse termed, SC(‑, +) or SC*. Unlike at high latitudes, the factor(s) that determine this bi-modal response of the equatorial geomagnetic H-field to sudden magnetospheric compressions are not known. As an exploratory step in identifying the causative factor(s), we have performed a statistical study of the characteristics of the daytime (06-18 IST) SCs recorded at the equatorial station, Kodaikanal (10.25N, 77.5E, dipole latitude 0.6N) and the low latitude station, Alibag (18.6N, 72.9E, dipole latitude 9.5N) over the period 1957 2002. A total of 304 SCs have been analyzed out of which 99 (32.6 per cent) were of SC* type. We find that the average value of the amplitude of the positive main impulse (MI) in H-component of SC* is higher than that of the conventional SC(+). This difference in the MI amplitude is statistically significant and persists even when the analysis is restricted to the interval of SC* occurrence, namely, 0730 1730 IST. Such a behavior is not seen in the amplitude of the MI at Alibag, away from the influence of the equatorial electrojet. It is found for the first time that a linear relationship exists between the amplitudes of the preliminary reverse impulse (PRI) and MI in SC* events. Our study of the H-field response at dayside dip equatorial stations of the Circum-pan Pacific Magnetometer Network (CPMN) to sudden magnetospheric compressions under different IMF orientation (northward and southward), showed that the impact of an impulse in solar wind dynamic pressure under southward IMF is associated with SC* in both the events considered. On the other hand, under northward IMF the dynamic pressure increase consistently led to SC(+) in the two events studied. The statistical and case study results imply that, when compared to SC(+), the contribution of ionospheric currents of polar origin gain prominence when SC* is excited at dayside dip equator, and the orientation of IMF Bz may be one of the (if not the only one) deterministic factors underlying the bi-modal response of the dayside equatorial H-field to a sudden increase in solar wind dynamic pressure.