The Effect of Variable Directions of Viewing on the In- terpretation of Diurnal Variations Observed by Neutron Monitors

Conventional interpretations of the Cosmic Ray Diurnal Variation gener-ally assume that the mean viewing direction of the stations concerned remains constant over each 24-hour period. At the lower end of the energy range to which ground-based neutron monitors respond, two effects combine to question the va-lidity of this assumption. Firstly, distortion of the geomagnetic field by the solar wind causes the direction of viewing of a ground-based station at a specified en-ergy to vary according to the local time-of-day. Secondly, there is a seasonal effect due to the tilt of the earth’s rotation axis. At lower energies and some stations both these effects can be significant. Furthermore, the directions of viewing are also affected by the degree of disturbance in the geomagnetic field. We report here the results of calculating the directions of viewing for Mawson at representative neutron monitor energies for each three-hour period throughout 1999, using the Tsyganenko model of the geomagnetic field and the appropriate value of Kp for each period, and discuss how these results affect the interpretation of the observed diurnal variation. 1