Supplementary materials

Figure SF1: Value in the near-Earth magnetotail (Z = 0 plane, at t = 1100 s) of the κ parameter used as a criterion for proton pitch-angle scattering due to the magnetic field curvature in the magnetotail, defined by Sergeev et al. (1983). The κ parameter is defined as the square root of the ratio between the magnetic field curvature radius and the gyroradius of the considered charged particles. Here, the proton thermal velocity is used to infer the proton gyroradius. Pitch-angle scattering occurs when , which corresponds to the orange and red hues delimited with the black isocontour at .

Figure SF2: Schematic view of a flux transfer event (FTE) being convected into the northern-hemisphere cusp after being formed near the subsolar point of the magnetopause. As the leading edge of the FTE reaches a virtual spacecraft placed in the high-altitude cusp (red dot), it produces a positive deflection in the radial component of the measured magnetic field, B_r. If the FTE were entering the southern-hemisphere cusp, its leading edge would in turn produce a negative deflection in Br.

Figure SF3: (a–l) Precipitating proton energy spectrum (i.e. differential number flux) at selected locations in the southern-hemisphere cusp region as a function of time in the simulation. (m–o) Temporal variations in the radial magnetic field component at three virtual spacecraft placed in the high-altitude cusp at 10, 12 and 14 MLT, at (X_GSE, Y_GSE, Z_GSE) = (4.67, −2.70, −7.50) RE, (5.50, 0.00, −7.50) R_E, and (4.67, 2.70, −7.50) R_E, respectively. Black arrows indicate times preceding signatures of the leading edge of incoming FTEs and are reproduced above panels d–l. The thin dashed lines in panels g–i indicate the theoretical time of detection of enhanced precipitating proton flux, as a function of proton energy, following the arrival of selected FTEs in the high-altitude cusp.

Figure SF4: Integrated energy flux of proton precipitation in the Y = 0 plane (magnetic noon on the dayside) at three selected time steps (1175, 1200 and 1225 s), emphasising the transit of a FTE in the high-altitude northern cusp and its appending to lobe field lines. In each panel, the streamlines represent magnetic field lines, and the red dot indicates the location of the virtual spacecraft where magnetic field data were sampled to produce Figure 5n. A FTE can be seen approaching the cusp (left, magnetic island indicated with a white arrow), starting being appended to lobe field lines (middle) and leading to enhanced precipitating proton flux in the cusp (right).

Animation SA1 : Animation of the proton precipitation in the polar regions during the simulation, from t = 900 to t = 1506 s. The four panels on top show the precipitating proton integrated energy flux and mean energy in the northern and southern hemispheres, in the same format as Figure 4 in the manuscript. The radial coordinate is geomagnetic latitude, and the angular coordinate corresponds to magnetic local time (noon on top). The red circles indicate selected geomagnetic latitudes. The bottom panel shows the Sunward component of the proton bulk velocity, V_x, in the near-Earth magnetotail, in the Z = 0 plane. The fast Earthward plasma flows discussed in the article mainly occur in the upper half of the panel (X > –11 RE).

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© M. Grandin et al., Published by EDP Sciences 2023