Distance d from the flare to the magnetic field line as a function of distance s along the field line for two values of the footpoint longitude ϕ_B. The flare and field-line footpoints are both located on the solar equator: θ_f = 0° and θ_B = 0°. “1 au” denotes the distance s where the field line crosses 1 au (1 au = 215 ﹩{R}_{\odot }﹩).