def _compute_linear_magnitude_term(self, C, mag):
        """
        Compute and return second term in equations (1a)
        and (1b), pages 2981 and 2982, respectively.
        """
        if mag <= self.c1:
            # this is the second term in eq. (1a), p. 2981
            return C['a2'] * (mag - self.c1)
        else:
            # this is the second term in eq. (1b), p. 2982
            return C['a3'] * (mag - self.c1)