The file tha_l1_esa_cal.txt contains time-dependent gain factors for
the ESA instrument. This file is used for calibration of all probe
data, not only THEMIS A. The text file has each time followed by an
array of 12 gain factors, 6 for electrons for each probe, plus probe
'f', the flatsat, and then six for ions. The relative gain factor for
a given time is found via interpolation.