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The day-night dependence of mesospheric response to variations in geomagnetic activity has been studied by a correlation analysis between neutral temperature in the 61-to 90-km altitude region, measured from daytime and nighttime rocket grenade soundings at an auroral station (Fort Churchill, Canada), and the 3-hourly Kp index over the period 1956-1969. A significant positive correlation between mesospheric neutral temperature and Kp, with a time lag of 12-15 hours between the Kp enhancement and the resultant temperature increase, is observed in the daytime soundings. The magnitude of the heating effect, represented by the linear regressipn coefficient AT/AKp, is large in tt•e day sector of the auroral region but is not significant in the night sector. The maximum value of AT/AKp was 8.9øK in the daytime 61-to 90, km region. The results are discussed in terms of current theories and observations of upper atmospheric response to variations in geomagnetic activity and the possible linkages with observed effects in the lower atmosphere. ß ß ß DAY (16) ß ß ß i i i

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