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activity and red-violet analysis August auroral activity auroral display auroral features auroral form bays or displacements brightness BRITISH ANTARCTIC SURVEY commenced conﬁrmed decreasing bays deﬁnite diffuse rays diffuse surfaces double arc activity easterly exposure faint auroral light Falkland Islands ﬁeld ﬁgs ﬁlm ﬁrst ﬂaming frequency of occurrence Halley Bay homogeneous arcs increasing displacement increasing to decreasing INSENSITIVE interference ring interquartile ranges ionosphere July June K-index large bay magnetic activity magnetic bay magnetic disturbance magnetic effect magnetic zenith Magnetogram mainly mean bearing median latitudes midnight moonlight morning nautical twilight NORMAL northwards observing conditions occurred oscillatory overhead activity overhead position Percentage frequency phase quiet arcs quiet-day curve rayed bands RB red-violet coloration September sharp movements shows signiﬁcant small bay southern horizon sunspot surfaces and diffuse Table three elements throughout the night Total number U.T. approximately U.T. RV undulating activity UNIVERSAL TIME FIGURE University of Edinburgh violet coloration westerly