Fig. 2: Examples of AKR precursor emissions.
From: Radio emissions reveal Alfvénic activity and electron acceleration prior to substorm onset
a Dynamic spectrum of AKR precursor emission observed by the Polar Plasma Wave Investigation instrument. The spectrogram shows the radio wave electric field intensity as a function of frequency and time, with color indicating signal strength. The white dashed line marks the onset of the AKR precursor, while the white dot-dashed line indicates the AKR breakup time, corresponding to the substorm onset identified from AKR observations. The blue dot-dashed line denotes the substorm onset time derived independently from ground-based magnetometer data or global auroral imaging, typically coinciding with the AKR breakup time. b–d Additional examples of AKR precursor emissions observed in separate substorm events. e A representative case showing two distinct groups of AKR precursors associated with both a pseudo-storm and a regular substorm. f, g Geomagnetic indices (AE, AU, AL) during the same interval as shown in (e), illustrating the magnetospheric activity associated with the AKR observation. Source data are provided as a Source Data file.
