Abstract
Abstract. The ionosonde at the Sodankylä Geophysical Observatory (SOD; 67∘ N, 27∘ E; Finland) routinely performs vertical sounding once per minute which enables the study of fast ionospheric variations at a frequency of the long-period geomagnetic pulsations Pc5–6/Pi3 (1–5 mHz). Using the ionosonde data from April 2014–December 2015 and colocated geomagnetic measurements, we have investigated a correspondence between the magnetic field pulsations and variations of the critical frequency of radio waves reflected from the ionospheric F2 layer (foF2). For this study, we have developed a technique for automated retrieval of the critical frequency of the F2 layer from ionograms. As a rule, the Pc5–6/Pi3 frequency band fluctuations in foF2 were observed at daytime during quiet or moderately disturbed space weather conditions. In most cases (about 80 %), the coherence between the foF2 variations and geomagnetic pulsations was low. However in some cases (specified as “coherent”) the coherence was as large as γ2≥0.5. The following conditions are favorable for the occurrence of coherent cases: enhanced auroral activity (6 h maximal auroral electrojet (AE) ≥800 nT), high solar wind speed (V>600 km/s), fluctuating solar wind pressure and northward interplanetary magnetic field. In the cases when the coherence was higher at shorter periods of oscillations, the magnetic pulsations demonstrated features typical for the Alfvén field line resonance.
Highlights
Ionospheric variations at the frequency corresponding to simultaneous geomagnetic pulsations were observed in the total electron content (TEC) and have been interpreted as a modulation of the ionosphere by magnetospheric processes (e.g., Davies and Hartmann, 1976; Okuzawa and Davies, 1981)
We present two examples of the from the ionospheric F2 layer (foF2) frequency and geomagnetic field variations simultaneously recorded at Sodankylä Geophysical Observatory (SOD)
They are characterized by a high 1foF2-bX coherence; the amplitudes of geomagnetic pulsations were essentially different in these two cases, 4 and 40 nT, respectively
Summary
Ionospheric variations at the frequency corresponding to simultaneous geomagnetic pulsations were observed in the total electron content (TEC) and have been interpreted as a modulation of the ionosphere by magnetospheric processes (e.g., Davies and Hartmann, 1976; Okuzawa and Davies, 1981). Davies and Hartmann (1976) reported on pulsations in TEC, recorded under undisturbed conditions and associated with Pc3–4s on the ground. Okuzawa and Davies (1981) confirmed the correspondence between ground and TEC variations in the Pc3–4 frequency range, but they obtained the maximal probability under disturbed conditions. Particle precipitation at auroral latitudes is one of the most intensive processes of the magnetosphere–ionosphere coupling. It is modulated by intensive ultralow-frequency (ULF)
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