Abstract
The non-negative Polar Cap PCC index built from PCN (North) and PCS (South) indices correlates 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 represented by the Kp index and ring current indices 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 from 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 has 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. With current available PC indices, detailed and accurate SYM-H or Dst index values could be derived up to nearly one hour ahead of actual time by integration of the PCC-based source function from any previous quiet state. Thus, the PCC indices enabling accurate estimates of the energy input from the solar wind are powerful tools for space weather monitoring and for solar-terrestrial research.
Highlights
In the early Space Age, Dungey (1961) formulated the concept of magnetic merging processes taking place at the front of the magnetosphere between the Interplanetary Magnetic Field (IMF), when southward oriented, and the geomagnetic field, followed by the draping of the combined field over the pole and reconnection processes in the tail region, where the solar wind magnetic fields as well as the geomagnetic fields were restored.The model implies a two-cell convection system, where the high-latitude antisunward ionospheric and magnetospheric plasma drift across the polar cap and the return flow in a sunward motion along auroral latitudes generate the two-cell “forward convection” patterns, termed DP2
The coefficients for the correlation between PCN and electric field parameter (EM) are displayed in blue line, The PCS – EM correlation in red, while the PCC – EM correlation coefficients are shown in heavy magenta line
It is seen that the correlation between PCC and EM is lowest during the northern winter months (November–January)
Summary
In the early Space Age, Dungey (1961) formulated the concept of magnetic merging processes taking place at the front of the magnetosphere between the Interplanetary Magnetic Field (IMF), when southward oriented, and the geomagnetic field, followed by the draping of the combined field over the pole and reconnection processes in the tail region, where the solar wind magnetic fields as well as the geomagnetic fields were restored. A prime objective for the present work has been to systematically compare the performances of the individual (unipolar) PC indices and combinations such as non-negative or simple averages or seasonal selections used in correlation studies involving the solar wind merging electric field, EM, the mid-latitude Kp index, and ring current indices. Such comparisons have not yet been published. A important application is the use of strongly enhanced PC index levels (Stauning, 2013c, 2020a) to predict violent substorm events that could threaten important subauroral power grids (Kappenman, 2010)
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