def METHOD_NAME(C, mag):
    """
    Returns the magnitude dependent intra-event standard deviation (phi)
    (equation 15)
    """
    phi = C["phi1"] + (C["phi2"] - C["phi1"]) * ((mag - 5.5) / 0.25)
    phi[mag < 5.5] = C["phi1"]
    phi[mag >= 5.75] = C["phi2"]
    return phi