TY  - RPRT
TI  - Variations in frequency of occurrence of sporadic E, 1949-1959 
AU  - Chadwick, W B
PY  - 1961
PB  - National Bureau of Standards
CY  - Gaithersburg, MD
SN  - NBS TN 117
DO  - 10.6028/NBS.TN.117
ER  -