Solar Wind-Magnetosphere-Ionosphere Interactions During Passage at Earth of Interplanetary CMEs

Abstract

In a series of case studies we investigate the ionospheric convection response to enhanced magnetopause reconnection rate driven by interplanetary (IP) CMEs. In this response we distinguish between two stages of evolution, i.e., an initial transient phase, and a persistent phase, and different convection regimes in different sectors of the polar cap (center versus periphery). Plasma convection in these two regimes may have different boundary layer sources, i.e., the high-latitude boundary layer (HBL) versus the low-latitude boundary layer (LLBL). The temporal evolution of flows in the center and periphery of the PC is monitored by the ground magnetic signatures in the form of the PCN - index and magnetograms from the IMAGE chain of magnetometers in Svalbard - Scandinavia - Finland, respectively. In order to determine the temporal structure of the boundary flow channels we study during steady IP conditions and a south-west (By < 0) directed magnetic field in ICMEs we selected the observation interval from 1200 to 1800 UT, when the IMAGE chain of magnetometers traverses the post-noon and dusk sectors. Flow channels (FCs) along the periphery of the PC and in the dusk-sector of the auroral oval are placed in the context of the Dungey flux circulation cycle which includes the following stages of field line evolution: newly open (NOFL; FC 1 flow) and old open field lines (OOFLs; FC 2 flow) and field lines connected to the tail lobe (FC 3 flow) and plasma sheet (FC 4 flow). We find that the polar cap (PC) boundary flow in the postnoon sector is characterized by the persistence of flux tubes excited by magnetopause reconnection events (see prediction by Southwood (1987)) and enhanced flow speeds associated with conductivity gradients at the boundary against the auroral oval, particularly during winter conditions (our FC 2 flows). The temporal variability of the cross-polar cap potential (CPCP) is estimated by a combination of the direct, but low-resolution measurements of the CPCP from satellite ion drift data with high-resolution (continuous) ground observations of equivalent convection in the central polar cap (PCN - index). The inferred CPCP variability is explained in terms of the expansion - contraction model of polar cap convection. This is summarized in the following empirical formula where we split the CPCP in contributions from (i) enhanced flows at the PC boundary, mapping to the LLBL (first term), and (ii), the potential in the rest of the PC, with contributions from the HBL dynamo source and the magnetotail source (second term): CPCP (V) = P_LLBL (V) + kPCN(mV/m)L_PC (km). k is an empirical constant which is conductivity (season) - dependent and LPC (km) is the cross-polar cap distance. We find that the persistent phase of solar wind - magnetosphere coupling (steady, strong IP driving) is characterized by a repetitive substorm activity and associated convection enhancements in the contracting (ΔL_PC < 0) phases of the polar cap oscillations.