Peak proton flux differential spectrum at the interplanetary space measured by PSP/EPI-Hi/HETA (magenta points), STA/HET (red triangles), SolO/EPD (blue triangles), GOES/SEISS (open circles), SOHO/ERNE (green diamonds), and SOHO/EPHIN (black squares). The dotted black and cyan lines indicates the inverse power-law spectra for the higher energy part for Earth (i.e. GOES and SOHO) and SolO, respectively. A slope of (a) γ ∼ 2.41 ± 0.15 was obtained from GOES/SEISS and SOHO/EPHIN measurements from 86–610 MeV (black dotted line) and (b) γ ∼ 2.43 ± 0.08 from SolO/EPD from 108–896 MeV (cyan dotted line). The dashed orange line depicts the obtained E-R fit from Eq. (1).