Winter auroral morphology and substorm electrodynamics.

Abstract

Global morphology and electrodynamics of the aurora are examined using images from the Polar spacecraft Ultraviolet Imager (UVI) and ground-based magnetometer measurements inverted using the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique. The primary aim is to understand the temporal and spatial details of how the aurora is influenced by conditions in the solar wind and interplanetary magnetic field (IMF). It is determined that the northern hemisphere aurora is brighter when: (1) the IMF is southward and lagged by 60 minutes, (2) The <italic>B_x</italic> component is negative during northward IMF, and (3) the IMF is aligned along or counter to the Parker spiral. The Method of Natural Orthogonal Components (MNOC) is applied to a month of UVI images and the highest order eigenvectors interpreted in terms of common auroral processes. The dominant modes are: (1) the main auroral oval brightness, (2) polar cap contraction, (3) dawn sector brightening, and (4) midnight sector brightening/quenching. The MNOC technique allows the dominant influence of the <italic>B_z</italic> component to be removed, thereby isolating the more subtle effects due to the solar wind density and IMF <italic>B_y</italic> and <italic> B_x</italic>. Double auroral ovals are investigated and found to occur during: (1) northward IMF, (2) high solar wind pressure, (3) strong IMF <italic>B_y</italic>, and (4) substorms when the auroral electrojets are overlapped. During positive (negative) <italic>B_y</italic>, double ovals form on the evening (morning) side. An examination of substorms using UVI and ground magnetic measurements yields the following results: (1) substorms are inhibited if the solar wind density and velocity are low, (2) the substorm growth phase can evolve on time scales much longer than previously considered, (3) the substorm current wedge can form prior to the main auroral onset, and (4) strong substorm expansions are triggered by the IMF and associated with reconfiguration of the ring current and field-aligned currents.