Create UTC localized DatetimeIndex for the dataframe.

    Parameters
    ----------
    data: Dataframe
        Must contain columns 'year', 'jday', 'hour' and
        'minute'.

    Return
    ------
    data: Dataframe
        Dataframe with a DatetimeIndex localized to UTC.

def format_index(data):
    """Create UTC localized DatetimeIndex for the dataframe.

    Parameters
    ----------
    data: Dataframe
        Must contain columns 'year', 'jday', 'hour' and
        'minute'.

    Return
    ------
    data: Dataframe
        Dataframe with a DatetimeIndex localized to UTC.
    """
    year = data.year.apply(str)
    jday = data.jday.apply(lambda x: '{:03d}'.format(x))
    hours = data.hour.apply(lambda x: '{:02d}'.format(x))
    minutes = data.minute.apply(lambda x: '{:02d}'.format(x))
    index = pd.to_datetime(year + jday + hours + minutes, format="%Y%j%H%M")
    data.index = index
    data = data.tz_localize('UTC')
    return data