EFFECTS OF THE GEOMAGNETIC ACTIVITY ON THE IONOSMRIC MONTHLY MEDIAN f₀F₂ AND ON THE IRI PREDICTION PRECISION

Vertical sounding data monthly median f₀F₂ from 1977 to 1986 at 9 Chinese ionospheric observatories in middle and lower latitude region is used in this paper to study the effects of the geomagnetic activity on the average behavior of monthly median f₀F₂ and on the IRI prediction precision. Data is grouped in two cases, one is for monthly averaged magnetic active index Ap < 15, the other for Ap≥ 15. Averaged diumal variations of monthly median f₀F₂ without considering season and solar activity shows that the bigger Ap is, the bigger averaged monthly median f₀F₂ value at all local time, and that the effects of the geomagnetic activity is more important at local noon and in the afternoon, and in lower latitude region. Averaged seasonal variations of monthly median f₀F₂ without considering local time and solar activity shows that averaged monthly median f₀F₂ value at higher geomagnetic activity is bigger than at lower geomagnetic activity during spring and summer. The positive deviation is up to 4MHz in March. And it is smaller than at lower geomagnetic activity during fall and winter. The negative deviation is up to 2MHz in September. Comparative study with the IRI indicates that during 0500-1000LT the IRl results are bigger than observations.The lower geomagnetic activity, the bigger the IRI deviation. During 1600-0400LT in lower latitude region, the IRI results are smaller than observations. The higher geomagnetic activity, the bigger the IRI deviation. During 1100- 1500LT the IRI results are between observations of higher geomagnetic activity and of lower geomagnetic activity. The total averaged IRI errors are bigger in higher geomagnetic activity than in lower. Maximum deviation of the averaged IRI errors from higher geomagnetic activity to lower is about 0.8MHz. The standard deviation of the IRI demonstrates an important latitudinal variation. In the lower latitude region south of Lanzhou to Haiou, The total standard deviation increases with the decrease of latitude.The standard deviation increases with the increase of the magnetic activity. In middle latitude from Lanzhou to Manzhouli, the standard deviation keeps stable when latitude increases, and it increases with the increase of the geomagnetic activity. An important conclusion of this study is that the effects of geomagnetic activity on the monthly median f₀F₂ are considerably important and that ignoring the effects is a main source of IRI errors.