SIMULATIONS OF HIGH-LATITUDE IONOSPHERIC CLIMATOLOGY

Historically, the high latitude ionosphere has been viewed as the most complex of the ionospheric regions because it is driven by both magne tospheric and solar inputs. At lower latitudes the direct, and highly variable, magnetospheric input is relatively unimportant, which makes these other regions amenable to empirical modeling. To date, however, no empirical model of the high latitude ionosphere is available which includes these complex dependencies. On the other hand, numerical mode ls that include the physics of this region have been developed and hav e proven to be successful at the climatology lever. In this study we p resent the climatological results of one of these models, namely the U tah State University (USU) time-dependent ionospheric model (TDIM). A total of 108 separate TDIM simulations for different ionospheric condi tions were used to elucidate the high latitude ionospheric trends. The se trends depend on solar cycle, season, universal time (UT), magnetic activity, interplanetary magnetic held (IMF) orientation, and hemisph ere. The ionospheric climatology is not dominated by any one of these parameters. The solar cycle (F-10.7 index), season (day), and magnetic activity (K-p index) compete on an even footing for control of the hi gh latitude ionosphere. Mean Variations of over an order of magnitude in NmF2, of over 150 km in h(m)F(2), and of over 50 km in the transiti on height are present in the high latitude ionospheric climatology. Th e 108 simulations quantify the trends and show the UT dependence and s patial variability of the ionosphere. Some aspects of these UT trends are compared successfully with observations. Many of the simulation re sults are predictions that can be verified as more complete observatio nal databases become available. The UT dependence, which at times can be a factor of two modulation of the F region densities, is a key reas on for the failure of statistical models at high latitudes. At lower l atitudes, Statistical models based mainly on local time rather than UT have been very successful. At high latitudes, this is not so and, the refore, local time and UT (longitude) must be treated as independent v ariables. This fact alone explains why data sets based on a fixed grou nd location or satellite orbital plane cannot unravel the LT and UT de pendencies at high latitudes. Also, the high latitude ionosphere is no t spatially uniform; morphological features on latitudinal scales of 1 -2 degrees are present. These structures play a key role in identifyin g the ionospheric climatology. Copyright (C) 1996 Published by Elsevie r Science Ltd.