Fig. 4: Simultaneous observation of AKR precursors and wave-like auroral forms: Case 2.

a Wave electric field spectrogram measured by the Geotail Plasma Wave instrument from 2002-03-30 20:28:30 to 20:42:20 UT. AKR precursor emissions are indicated by white, yellow, and pink arrows. Negatively drifting features are instrumental artefacts related to the frequency-sweeping logic of the receiver. b Ground-based auroral images captured at 557.7 nm by the all-sky camera at the Abisko (ABK) station. c Global proton auroral images from the Spectrographic Imager (S12) onboard the IMAGE satellite. The black circle marks the field of view of the Abisko station. The horizontal black dashed line denotes the dawn–dusk meridian. d Normalized integrated intensities of: AKR emissions (yellow; 100–800 kHz from a ~8 s resolution), ground-based aurora (green; integrated over the entire image in (b), 20 s resolution), global nightside aurora (dark blue; integrated below the dashed line in (c), and local aurora within the Abisko field of view in (c) (orange; 2 min 3 s resolution). Normalization follows: \(\frac{{I}_{{\mbox{integrated}}}-{I}_{\min }}{{I}_{\max }-{I}_{\min }}\). For saturated ground-based images after onset, a denominator of 0.6 × (I_max−I_min) is applied empirically. e Radio visibility test for the interval shown in (a). Visible AKR source field lines from the Northern hemisphere are shown in red; those from the Southern hemisphere in orange. f Footprints of visible AKR sources projected onto magnetic polar coordinates. The background is the same IMAGE auroral frame as in the 2nd bottom row of (c). Boundaries of northern visible sources are shown as solid colored lines; southern hemisphere sources as dotted colored lines. Source data are provided as a Source Data file.