Un nuovo indice di misurazione dell'attività ionosferica

Nell'ambito del progetto europeo Digital upper Atmosphere Server (DIAS) è stato sviluppato un nuovo indice dell'attività ioniosferica puramente basato sull'osservazione di alcuni valori forniti dalle ionosonde (tipicamente la frequenza f0 dello strato f2). L'indice, chiamato AI, mostra una forte dipendenza dalle condizioni geomagnetiche e rappresenta una novità rispetto agli indici tradizionalmente ricavati dalla misurazione dell'attività solare. Un recente articolo pubblicato su Annales Geophysicae, descrive alcuni aggiustamenti statistici dell'indice AI che riducono la variabilità rispetto a fattori stagionali. E' possibile che la conoscenza delle condizioni ionosferiche attraverso il nuovo indice possa fornire utili indicazioni sull'andamento della propagazione delle onde corte. Ecco l'abstract e le conclusioni dello studio.

A modified index for the description of the ionospheric short- and long-term activity

J. Mielich and J. Bremer

Leibniz-Institute of Atmospheric Physics at the Rostock University, 18225 Kühlungsborn, Schloss-Str. 6, Germany

Abstract. A modified ionospheric activity index AI has been developed on the basis of ionospheric foF2 observations. Such index can be helpful for an interested user to get information about the current state of the ionosphere. Using ionosonde data of the station Juliusruh (54.6° N; 13.4° E) this index has been tested for the time interval from January 1996 until December 2008. This index has no diurnal and seasonal variations, only a small positive dependence on the solar activity could be found. The variability of this index has, however, a marked seasonal variability with maxima during the equinoxes, a clear minimum in summer, and enhanced values in winter. The observed variability of AI is strongly correlated with the geomagnetic activity, most markedly during the equinoxes, whereas the influence of the solar activity is markedly smaller and mostly insignificant. Strong geomagnetic disturbances cause in middle latitudes in general negative disturbances in AI, mostly pronounced during equinoxes and summer and only partly during winter, thus in agreement with the current physical knowledge about ionospheric storms.

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5. Conclusions and outlook

The main results can be concluded with the following points:

– A modified ionospheric index AI has been derived by use of a new reference value calculated by a regression analysis of the foF2 values of the preceding 27 days at the same hour as the current foF2 value.

– The new index has no distinct diurnal and seasonal variation. This is the necessary precondition for the creation of homogeneous AI data series.

– AI data can be estimated for other stations, other time intervals, or other ionosonde parameters (e.g. M(3000)F2) without any problems.

– Monthly median and yearly mean values of AI do not depend on geomagnetic activity but depend on the solar activity level. However, the solar influence on the AI values is only small (not more than about ±1%).

– The variability of the AI values significantly depends on the geomagnetic activity, most clearly during equinoxes but also significant during summer and winter.

– Ionospheric storms in mid-latitudes are characterized during summer and equinox conditions by markedly reduced AI values. During winter the mean negative response is markedly smaller. These results are in general agreement with current storm theories.