Method on Using Shadow Altitude to Remove Geocoronal H$\alpha$

Abstract

Spectroscopic surveys allow spatially resolved spectroscopy of galaxies to study their interstellar medium (ISM). However, observations of Galactic H$\alpha$ emission are contaminated by geocoronal H$\alpha$ emission. The latter is known to depend on the shadow altitude, a geometric parameter relating the line of sight to Earth's shadow cone. Using fibres on blank skys from the SDSS-IV/MaStar survey, we established an empirical relation between the geocoronal H$\alpha$ emission and the shadow altitude, with a root mean square fractional scatter of 23.52$\%$. This relation can be used to predict geocoronal H$\alpha$ emission so that it can be removed from observed spectra. This removal method is advantageous when the observed targets are extensive in the sky, and it does not require a large velocity separation between the observed target and the local standard of rest. This will enable reliable studies of Galactic H$\alpha$ in intermediate spectral resolution integral field spectroscopic surveys. We also find tentative evidences for the dependences of geocoronal emission on solar activity and the distance between the Earth and the Sun.