A balloon payload instrumented with a double-probe electric field detector and an X ray scintillation counter was launched from Roberval, Quebec, Canada (L=4.1) at 0828 UT (0328 LT) on July 9, 1975. A magnetospheric substorm was observed locally between 0815 and 1100 UT, which produced a maximum ΔB of ~500 nT at ~0930 UT. A single-cell atmospheric thunderstorm developed northeast of Roberval beginning around 0925 UT which was most intense from ~1000 to 1035 UT. Detailed study of the electrical properties of the thunderstorm, the X ray precipitation data, and VLF spheric data leads to three conclusions. First, the electrical coupling from the thunderstorm to the magnetosphere increases with frequency from dc to the VLF; for the observed storm the amplitude at the ionosphere of thunderstorm produced electric fields was not significant at frequencies below 0.1 Hz. Second, the atmospheric conductivity above the thunderstorm was observed to be about one-half the fair weather value prior to 1000 UT; decreased to about one-quarter the fair weather value at about 1000 UT; and remained depressed after the end of the thunderstorm. This result was contrary to that expected on the basis of previous work and is one which merits considerably more investigation. Third, the data show a high probability that half-hop whistlers initiated by sferics from the thunderstorm triggered energetic electron precipitation from the magnetosphere.