Principal component analysis of polar cap convection

We apply a statistical technique called Principal Component Analysis (PCA) for examining underlying patterns of polar cap convection and illustrate potential applications of the PCA-based dimension reduction. Two principal components are identified: the first mode (PC1) is related to "uniform variation" of the flow speed at all MLTs, and is primarily governed by IMF Bz. The second mode (PC2) is related to "dawn-dusk asymmetry", and is predominantly driven by IMF By. PCA gives the relative variance contribution of the two modes: PC1 giving ∼42% of the total variance and PC2 ∼17% of the total variance, which is about 40% of that from PC1. Due to the orthogonality of the principal components, the degree of dawn-dusk asymmetry can be represented by P₂, where P₂ is a component value when the observed data are projected along PC2. We identified P₂ as proportional to IMF By, which leads to stronger dawn flows for By>0 and stronger dusk flows for By<0. The same primary modes are found regardless of the IMF orientation, implying that they are intrinsic properties of the average polar cap convection.