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Using digital ionosonde observations at a low mid-latitude\udstation, New Delhi (28.6°N, 77.2°E, dip 42.4°N), we have derived hourly\udmonthly values of hmF2 (the real height corresponding to the peak\udelectron density in the F2-region), employing both the Dudeney (1983) and\udBilitza (1990) empirical formulations for the period from January 2001 to\udAugust 2002. The diurnal and seasonal variations of hmF2 are analyzed.\udFurther, to assess the predictability of the latest available model,\udInternational Reference Ionosphere, (IRI-2001), we have obtained the\udmedian values of hmF2 derived from M(3000)F2 for each hour during\uddifferent seasons and compare these with the model. Our results show that\udboth the Dudeney (1983) and Bilitza (1990) formulations reveal more or\udless a similar diurnal trend of hmF2, with higher values around midnight\udand lower during sunrise, in all the seasons. It is also noted that the hmF2\udshows a larger variability around midnight than by daytime, in all the\udseasons. Further, the study shows that median values of observed hmF2,\udusing both formulations, are somewhat larger than those predicted by\udthe IRI, in all seasons and at all local times. During summer, the IRI values\udagree comparatively well with the observations, especially during daytime.\udMajor discrepancies occur when the IRI underestimates observed hmF2\udfor local times from about 14:00 LT to 18:00 LT and 04:00 LT to 05:00 LT\udduring winter and equinox, where the percentage deviation of the observed\ud\udhmF2 values with respect to the IRI model varies from 15 to 25%. The\uddifference between the model and observations, outside this time period,\udremains less than 20% during all the seasons. \udKey words. Ionosphere (modelling and forecasting; equatorial\udionosphere

annales geophysicae

Diurnal and seasonal variations of &amp;lt;i&amp;gt;hm&amp;lt;/i&amp;gt;F2 deduced from digitalionosonde over New Delhi and its comparison with IRI 2001