The Polar Cap (PC) index combination, PCC: relations to solar wind properties and global magnetic disturbances

Abstract

The non-negative Polar Cap PCC index built from PCN (North) and PCS (South) correlates considerably better with the solar wind merging electric field and is more representative for the total energy input from the solar wind to the magnetosphere and for the development of geomagnetic disturbances than either of the hemispheric indices. The present work shows that the ring current index, Dst, to a high degree of accuracy can be derived from a source function built on the PCC indices. The integration of the PCC-based source function throughout the interval from 1992 to 2018 without attachment to the real Dst indices based on low latitude magnetic observations has generated equivalent Dst values that correlate very well (R=0.86) with the real Dst index values, which are represented with a mean deviation less than 1 nT and an overall rms deviation less than 13 nT. The precise correlation between the real and equivalent Dst values have been used to correct the PCC indices for saturation effects at high intensity disturbance conditions where the Dst index may take values beyond -100 nT. The relations between PCC and the ring current indices, Dst and ASY-H have been used, in addition, to derive the precise timing between polar cap convection processes reflected in the polar cap indices and the formation of the partial and total ring current systems. Building the ring current is considered to represent the energy input from the solar wind, which also powers auroral disturbance processes such as substorms and upper atmosphere heating. Thus, the PCC indices measuring accurately the energy input from the solar wind are powerful tools for space weather monitoring and for solar-terrestrial research.

Transpolar Convection and Magnetospheric Ring Current Relations: Real‐Time Applications of the Polar Cap (PC) Indices

“…A comparison of the correlations between EM and PCC, PCN, and PCS values throughout 2009 to 2018 is presented in Fig. 4a here (see Stauning, 2021). In this figure, the PC indices are based on post-event derivation.…”

“…From the investigations in Stauning (2021), the source term was defined to become Q(nT/h) = -4.5 (nT/h)/(mV/m)•PCC(mV/m) in order to provide the best agreement between real and equivalent Dst values for an integration starting from Dst=0 on 1 January 1992 and proceeding to 31 December 2018 without attachment to the real Dst values. From the same processing, the decay time constants were redefined to become τ = 5.5 h for large disturbances where Dst < -52 nT, and τ = 7.0 h for small disturbances where Dst >-52 nT.…”

“…Here we apply the simulated real-time PCC indices. Compared to the initial (post-event) version in Stauning (2021), the use of simulated real-time PC indices has generated very little change in correlation coefficients and other parameters resulting from the calculations such as the mean and rms differences between real and equivalent Dst values.…”

“…Both the asymmetrical (partial) ring currents scaled by ASY-H indices and symmetrical (total) ring current intensities scaled by Dst or SYM-H involve geomagnetic activity at both polar caps conveniently scaled by the PCC indices. The relations between PC indices and the ring current ASY-H and Dst (or SYM-H) indices have been investigated previously by Stauning et al (2008), Stauning (2012), and in a recent work by Stauning (2021). The target in the present work is to examine the validity of the established relations based on post-event data for their use in real-time applications.…”

“…The PCC indices are derived as the average of positive values of the two hemispherical polar cap PC indices disregarding (zero filling) negative values. It has been demonstrated that the PCC indices have a higher degree of correlation with the merging electric fields than either of the individual polar cap indices or their averages (Stauning et al, 2008;Stauning, 2012Stauning, , 2021. Basic models of the magnetospheric convection of plasma and embedded magnetic fields are based on the two-cell convection patterns (DP2) introduced by Dungey (1961).…”