Abstract
We investigated evolution of ionospheric currents during sudden commencements using a ground magnetometer network in conjunction with an all-sky imager, which has the advantage of locating field-aligned currents much more accurately than ground magnetometers. Preliminary (PI) and main (MI) impulse currents showed two-cell patterns propagating antisunward, particularly during a southward interplanetary magnetic field (IMF). Although this overall pattern is consistent with the Araki (solar wind sources of magnetospheric ultra-low-frequency waves. Geophysical monograph series, vol 81. AGU, Washington, DC, pp 183–200, 1994. doi: 10.1029/GM081p0183 ) model, we found several interesting features. The PI and MI currents in some events were highly asymmetric with respect to the noon–midnight meridian; the post-noon sector did not show any notable PI signal, but only had an MI starting earlier than the pre-noon MI. Not only equivalent currents but also aurora and equatorial magnetometer data supported the much weaker PI response. We suggest that interplanetary shocks impacting away from the subsolar point caused the asymmetric current pattern. Additionally, even when PI currents form in both pre- and post-noon sectors, they can initiate and disappear at different timings. The PI currents did not immediately disappear but coexisted with the MI currents for the first few minutes of the MI. During a southward IMF, the MI currents formed equatorward of a preexisting DP-2, indicating that the MI currents are a separate structure from a preexisting DP-2. In contrast, the MI currents under a northward IMF were essentially an intensification of a preexisting DP-2. The magnetometer and imager combination has been shown to be a powerful means for tracing evolution of ionospheric currents, and we showed various types of ionospheric responses under different upstream conditions.
Highlights
Sudden commencements (SCs) are abrupt changes in the magnetic field that occur on a global scale and are caused by solar wind dynamic pressure jumps
Event 1 The event in Fig. 1 has a large SYM-H amplitude of 42 nT associated with a solar wind dynamic pressure pulse preceded by a steady SYM-H level (Fig. 1b, c)
By comparing the equivalent currents with the aurora, we identified auroral features that correspond to FACs of the PI and MI current systems, and the high spatial resolution and multiwavelength measurements by imaging can provide more details of the SC-related current evolution
Summary
Sudden commencements (SCs) are abrupt changes in the magnetic field that occur on a global scale and are caused by solar wind dynamic pressure jumps. Because of their distinct nature in magnetometer records, SCs have been used to study the response of the magnetosphere– ionosphere coupling system to solar wind disturbances. Nishimura et al Earth, Planets and Space (2016) 68:144 to occur simultaneously across the entire latitude range including the magnetic equator, and this is attributed to rapid changes in large-scale electric fields (Boudouridis et al 2011) and their instantaneous penetration to lower latitudes (Araki 1977; Kikuchi 1986). The interplanetary magnetic field (IMF) By, Bz, and Alfvén Mach number are known to alter FAC locations (Yu and Ridley 2011; Peng et al 2011)
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