Abstract
Abstract. The basic aim of this ‘case study’ is to investigate the variability in the maximum height of the ionospheric F2-layer, hmF2, with periods of planetary waves (2–30 days), and to make an attempt to determine their origin. The hourly data of hmF2 above Millstone Hill (42.6° N, 71.5° W) during 01 September 1998 - 31 March 2000 were used for analysis. Three types of disturbances are studied in detail: (i) the 27- day oscillations observed in the hmF2 above Millstone Hill are generated by the geomagnetic activity and by the global-scale 27-day wave present in the zonal mesosphere/lower thermosphere (MLT) neutral wind. The time delay between the 27-day oscillation in the zonal wind and that in the hmF2 is found to be 5–6 days, while between the 27-day oscillation in the geomagnetic activity and that in the hmF2 is found to be 0.8–1 day; (ii) the 16-day oscillation in the hmF2 observed during summer 1999 is probably generated by the global scale 16-day modulation of the semidiurnal tide observed in the MLT region during PSMOS campaign in June–August. We found that if the modulated semidiurnal tide mediates the planetary wave signature in the ionosphere, this planetary wave oscillation has to be best expressed in the amplitude and in the phase of the 12-h periodicity of the ionosphere; and (iii) the third type of disturbances studied is the quasi-2- day activity in the hmF2 that increases during geomagnetic disturbances. The strong pseudo diurnal periodicities generated during the geomagnetic storms can interact between each other and produce the quasi-2-day oscillations in the ionosphere.Key words. Ionosphere (ionosphere-atmosphere interactions; ionosphere-magnetoshpere interactions; wave propagation)
Highlights
The physics of the ionosphere-thermosphere system is complicated and the reasons responsible for this could be summarised as follows: (i) the variability of the external sources that drive the system; (ii) the internal interactions that occur in this system, and (iii) the interactions with the magnetosphere above and with the middle atmosphere below
Three types of disturbances are studied in detail: (i) the 27day oscillations observed in the hmF2 above Millstone Hill are generated by the geomagnetic activity and by the globalscale 27-day wave present in the zonal mesosphere/lower thermosphere (MLT) neutral wind
The time delay between the 27-day oscillation in the zonal wind and that in the hmF2 is found to be 5–6 days, while between the 27-day oscillation in the geomagnetic activity and that in the hmF2 is found to be 0.8–1 day; (ii) the 16-day oscillation in the hmF2 observed during summer 1999 is probably generated by the global scale 16-day modulation of the semidiurnal tide observed in the MLT region during PSMOS campaign in June–August
Summary
The physics of the ionosphere-thermosphere system is complicated and the reasons responsible for this could be summarised as follows: (i) the variability of the external sources that drive the system; (ii) the internal interactions that occur in this system, and (iii) the interactions with the magnetosphere above and with the middle atmosphere below All these factors set the pattern for the ionospheric variability and can be defined respectively as: solar, magnetospheric and meteorological influences. Rishbeth and Mendillo (2001) used ionosonde data from thirteen stations to study the day-to-day variability of the peak F2-layer electron density, NmF2, versus local time, season and solar cycle They separated day and night variability and found that for years of medium solar activity (solar radio flux approximately 140 units), the daily fluctuations of NmF2 have standard deviation of 20% by day and 33% by night. Geomagnetic activity is accepted by the authors as a major cause of this variability, though ‘meteorological’ causes transmitted from lower levels may make a comparable contribution
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