Fig. 4: Evolution of the L*-coverage of phase space density enhancements during October 2012.

In (a), contours show the L* extent of phase space density values that are within half an order of magnitude of the maximum phase space density for the period (at a specific value of μ ranging from 2000 to 10,000 MeV G⁻¹). To form the contours, the phase space density values (Supplementary Fig. 2) were binned into 5-h intervals and by 0.1 L*. By layering these contours for different values of μ, acceleration regions over a broad energy range are revealed. L* values not sampled by the Van Allen Probes are shown in black. The phase space density profile shows a peaked profile between L* ≈ 4.0–4.5, at which μ = 8000 and 10,000 MeV G⁻¹ correspond to ~5.4–6.5 and  ~6.0–7.3 MeV, respectively (see Fig. 2). The K_p index, shown in (b), provides a measure of geomagnetic activity and is derived from ground magnetometer measurements. K_p < 4 is shown as black bars and K_p ≥ 4 as red bars to highlight periods of geomagnetic disturbance.