Fig. 3From: A possible case of sporadic aurora observed at Rio de JaneiroBritish Geological Survey 3-h aa index (solid orange line) and the 24-h Aa index (solid green line/shaded green area) for the interval 1–24 February 1875. The dashed blue line indicates the onset of the aurora observation (15 February 1875 at 19:45 LMT/16:45 GMT) reported by Emmanuel Liais to the Jornal do Commercio (1875)Back to article page