“Blob” analysis of auroral substorm dynamics

Abstract

One month's worth of Polar ultraviolet imager (UVI) data were subjected to a “blob” analysis to determine the statistical dynamics of substorm features observed in the Lyman‐Birge‐Hopfield long (LBHL) band (152–188 nm). Adapted from similar DoD analyses of target images, the analysis consists of finding, on a frame‐by‐frame basis beginning at substorm onset, the following aspects of an individual auroral feature: peak power (i.e., power of precipitating electrons), total power, centroid location (magnetic local time (MLT) and magnetic latitude (MLAT), and speed of centroid. Over 120 individual auroral features were successfully acquired at onset and tracked until dissipation during January 1997. The power in the peak pixel and total power were random in time but displayed transient spikes that lasted 5–10 min. Over the course of a substorm, the total energy of blobs averaged ∼2.0 − 104 GJ. A histogram of these energies suggests no preferred energy but that lower energies were more common than higher energies. Analysis of the blob positional dynamics generally supports a poleward and westward movement. During the course of a substorm, 90% of the blobs moved poleward, while over 60% moved westward. However, these movements were not steady and displayed random components. Furthermore, a sizable minority (∼35%) of the blobs moved eastward, which does not agree with the conventional picture of auroral surges. Blob speeds varied from essentially zero up to several kilometers per second. However, during the January substorms the blobs did appear to have a preferred speed of 0.84 ± 0.34 km s−1.