Electric field screening of the polar cap according to data on the magnetospheric superstorm of 20 November 2003

We studied interval 03:00-08:02 UT of the 20 November 2003 superstorm. The first hours of this interval were characterised by the weak short-term substorm preceding the SMC/GAI mode (stationary magnetospheric convection with superposition of short-term unloading) [Sergeev, 1977; Lin et al., 2009]. In this mode, the Pointing flux ε' entering the magnetosphere and the potential difference on the polar cap boundary U pc showed good correlation. At the end of the interval, oscillations and the following increase in the solar wind dynamic pressure (P d) were observed. Thus, three disturbance modes were observed: normal substorm (at P d ~ 2 nPa), SMC/GAI, and disturbance at P d≥3 nPa. Together with change and development of disturbance modes, we describe changes in the variable magnetic flux Ψ 2=βΨ 02, where 002 is the constant magnetic flux through the “old” polar cap (PC). Boundaries and area of the old PC are determined for the quiet period before the superstorm commencement with the map of distribution of the field-aligned current density in the polar ionosphere. The old PC is the base of the field tube, containing the Earth’s “old” magnetotail. New PC and magnetotail appear around old PC and magnetotail when a substorm starts. At the beginning of the SMC/GAI interval, values β are slowly (with the scale of about 2 hours) reduced several times; when passing to the P d-disturbance mode, they regain the initial value β=1. These changes describe the new event in magnetospheric physics, referred to as the β-effect. It involves activation of a part of the old magnetotail that is totally passive before the disturbance onset. The activation is caused by plasma convection with open field lines of the new magnetotail - transfer of the electromagnetic energy flux of the solar wind (SW) to the magnetosphere. At the beginning, the transfer involves the new expanding magnetotail; then, the central part of magnetotail lobes (the old magnetotail). Being physically distinct, processes in the old and new parts of each magnetotail lobe are interrelated as cause and effect: convection of the new flux results in convection of the old one. Due to the β-effect, transfer efficiency of the SW pulse and energy increases many-fold. In contrast to the existing conception, the tail lobe in the new scenario is not thus a whole: it consists of two open magnetic fluxes interrelated as cause and effect. This paper first presents description of β-effects based on data on three different disturbance modes.