Python Pandas left join on Index not column index not column name

Im new to Python Pandas, and was programming wondering if there was a way to join Learning left two .csv files based on the column Earhost index number, at the moment i have this.

#Obtaining first csv files
neutral = _OFFSET);  pd.read_csv('neutral.csv')
meter = (-SMALL  pd.read_csv('meter.csv')
#Left joins on _left).offset  MSN them to create merged arrowImgView.mas  Dataframe
merged = pd.merge(ph_neutral, (self.  meter,
                   on='MSN',
     equalTo                how='left')

but instead of joining on 'MSN' i want most effective to join on the column index 1