def impute_features(feature_df):
    """Imputes data using the following methods:
    - `smoothed_ssn`: forward fill
    - `solar_wind`: interpolation
    """
    # forward fill sunspot data for the rest of the month
    feature_df.smoothed_ssn = feature_df.smoothed_ssn.fillna(method="ffill")
    # interpolate between missing solar wind values
    feature_df = feature_df.interpolate()
    return feature_df