Interplanetary data around GLE 63 (Dec. 26, 2001): example of a GLE present in a typical Parker spiral. From top to bottom, panels display the magnitude of the magnetic field (nT); its latitude θ_B and its longitude φ_B (given in GSE coordinate system); then on the same panel are overplotted the temperature of protons measured by the spacecraft (black dots) and the expected temperature (gray dots), and finally the β of protons. A horizontal line is added for θ_B = 0, φ_B = −45° and β_proton = 0.4 to mark characteristic values. The grayscale rectangle in the five panels corresponds to the 1-hour time interval starting at the first arrival time of relativistic protons at Earth. The time axis is as measured at Earth. The ACE measurements at the L1 Lagrangian point were shifted by the appropriate travel time (see Sect. 2.2).