Viewing and illumination geometry during coma observations. Lines correspond to solar direction (in yellow), nadir direction from the spacecraft (in green), and line of sight (LOS) through the coma (in white). The white dash line taken perpendicular to the LOS and passing through the comet nucleus centre defines the tangent altitude of a given point of the coma above the surface. Points P, SUBSOL, and SUBSPC are the tangent, subsolar, and subspacecraft points, respectively. Red and blue arrows indicate the x and z (rotation axis) directions of the comet.