def neutrino_to_source_direction(phi, theta, radian=True):
    """Flip the direction.

    Parameters
    ==========
    phi, theta: neutrino direction
    radian: bool [default=True]
        receive + return angles in radian? (if false, use degree)
    """
    phi = np.atleast_1d(phi).copy()
    theta = np.atleast_1d(theta).copy()
    if not radian:
        phi *= np.pi / 180
        theta *= np.pi / 180
    assert np.all(phi <= 2 * np.pi)
    assert np.all(theta <= np.pi)
    azimuth = (phi + np.pi) % (2 * np.pi)
    zenith = np.pi - theta
    if not radian:
        azimuth *= 180 / np.pi
        zenith *= 180 / np.pi
    return azimuth, zenith