Abstract
Abstract Waveform of geomagnetic sudden commencement (SC) on the ground depends on both latitude and local time (LT). Using high-resolution (1 s) geomagnetic field measurements obtained at Jicamarca (JIC, MAGLAT = 0.0°) from 1998 to 2005, we examined the waveforms of 188 SC events and found that some of the SCs observed at nighttime (approx. 22-05 LT) and dawn time (approx. 05-07 LT) had a common observational feature, i.e., a positive impulse always appeared at the beginning of the main impulse of the SC, which led to these events show preliminary positive impulse (PPI) or ‘Stepwise’ structure in the waveforms. In particular, the occurrence rate of the PPI/Stepwise events at the dawn time was clearly higher than that at the nighttime. This is the first complete report of this observational feature for the SCs observed at the dip equator. Based on a comparison of our observations with the previously proposed model calculation of Kikuchi et al. (2001), we suggest that the PPI/Stepwise events observed at the dawn-time dip equator reflect the effects of the ionospheric currents (ICs) and field-aligned currents (FACs). We therefore argue that this observational result provides evidence for the validity of the model calculation.
Highlights
A sudden increase in the solar wind dynamic pressure can generally cause a global response in the form of a geomagnetic sudden commencement (SC) (Russell et al, 1992, 1994a, b; Russell and Ginskey, 1995)
We argue that using the nighttime low-latitude observation is better than using the observation at the same local time (LT) sector as JIC, because the ionospheric currents (ICs) have little effects on the nighttime observations of the SC (Han et al, 2007b), and the waveforms of SC at the low latitudes in the morning sector can often be severely deformed from a simple step-like structure, which indicates the effects of the DP field
When we consider that the DPPI field is the first to be excited and that the DPMI field is generated later, as depicted in Araki’s model, the increase-decrease variation is consistent with what is predicted by the model calculation of Kikuchi et al (2001). Such an impulsive DP field superposed on a simple step-like DL field can cause the SC waveforms to show what seems like to be preliminary positive impulse (PPI)/Stepwise events, so we suggest that the frequent occurrence of the PPI/Stepwise events at the dawn time is consistent with the model calculation of Kikuchi et al (2001)
Summary
A sudden increase in the solar wind dynamic pressure can generally cause a global response in the form of a geomagnetic sudden commencement (SC) (Russell et al, 1992, 1994a, b; Russell and Ginskey, 1995). The PRIs appear at the dayside dip equator with enhanced amplitude, but the latter are rarely observed at low latitudes (Araki, 1977, 1994). Kikuchi et al (2001) subsequently examined the PPIs that appeared at mid latitudes and the dip equator in the afternoon by considering the effects of field-aligned currents (FACs) and the existence of differences in the ionospheric conductivities between the winter and summer hemispheres. We note that the δ H shows similar waveform for all of the events—i.e., a positive increase followed by a decrease
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