Calculate density from THEMIS spacecraft potential

[xandrd]

Transfer IDL routine which calculate density from the spacecraft potential.

There are 3 available routines (in themis/common):

• thm_scpot_density.pro (is an updated (from 2017) version of the suite of "spacecraft potential to density" programs)
• thm_scpot2dens.pro
• thm_scpot2dens_opt_n.pro (originally favorite, but seems outdated)

Examples can be found in cribshet:

• /themis/common/thm_gen_overplot.pro
• /themis/examples/advanced/thm_crib_scpot2dens.pro

The PySPEDAS version should be located in /themis/analysis