PC index as a proxy of the solar wind energy that entered into the magnetosphere: 2. Relation to the interplanetary electric field E KL before substorm onset

Oa Troshichev,Da Sormakov

doi:10.1186/s40623-015-0338-4

Abstract

This paper (the second of a series) presents the results of statistical investigation of relationship between the interplanetary electric field EKL and the Polar Cap (PC) index in case of magnetic substorms (1998–2001), which have been analyzed in Troshichev et al. (J. Geophys. Res. Space Physics, 119, 2014). The PC index is directly related to the EKL field variations on interval preceding the substorm sudden onset (SO): correlation R > 0.5 is typical of more than 90 % of isolated substorms, 80 % of expanded substorms, and 99 % of events with coordinated EKL and PC jumps. The low or negative correlation observing in ~10 % of examined substorms suggests that the solar wind flow measured by the Advanced Composition Explorer (ACE) spacecraft in the Lagrange point L1 did not encounter the magnetosphere in these cases. Examination of the delay times ΔT in the response of PC index to EKL variations provides the following results: (1) delay times do not depend on separate solar wind parameters, such as solar wind speed VX and interplanetary magnetic field (IMF) BZ component, contrary to general conviction, (2) the ΔT value is best controlled by the EKL field growth rate (dEKL/dt), (3) the lower ΔT limit (5–7 min is attained under conditions of the higher EKL growth rate, and (4) the PC index provides the possibility to verify the solar wind flow transportation time from ACE position (where the solar wind speed is estimated) to magnetosphere. These results, in combination with data testifying that the substorm onsets are related to the PC precursors, demonstrate that the PC index is an adequate ground-based indicator of the solar wind energy incoming into the magnetosphere.

Highlights

In the first paper of a series (Troshichev et al 2014), the statistical relationships between the Polar Cap (PC) and AL indices for the isolated and expanded magnetic substorms have been studied
These results distinctly demonstrate that the PC index growth, preceding the substorm onset, is closely related to the appropriate rise in the EKL field, the close linkage between EKL and PC in case of expanded substorms is essentially deformed by the high magnetic activity, serving as a background of the expanded substorm onsets
It implies that the ground-based PC index provides more reliable evidence of the solar wind coupling with the magnetosphere, than the EKL field derived from the solar wind measurements in the Lagrange point L1

Summary

Introduction

In the first paper of a series (Troshichev et al 2014), the statistical relationships between the Polar Cap (PC) and AL indices for the isolated and expanded magnetic substorms have been studied. The substorm sudden onsets (SO) are related to sharp increases in the PC growth rate, and the linear dependency of the AL value on the PC index is typical of the periods before and after the sudden onsets, irrespective of the substorm type and intensity. Basing on these results, the conclusion was made that the PC index. The PC index has been introduced (Troshichev and Andrezen 1985; Troshichev et al 1988) as a magnitude of the polar cap magnetic disturbance δF normalized by the geoeffective interplanetary electric field value expressed by the formula of Kan and Lee (1979). The EKL field can be regarded as a parameter related to the solar wind energy input into the magnetosphere

Methods

Results

Discussion

Conclusion