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- Title
Climatology of the Harmonic Frequency Separation of Ionospheric Alfvén Resonances at Eskdalemuir Observatory, UK.
- Authors
Hodnett, R. M.; Yeoman, T. K.; Beggan, C. D.; Wright, D. M.
- Abstract
We extracted the harmonic frequency separation (Δf) of Ionospheric Alfvén Resonances (IAR) observed in the Eskdalemuir induction coil magnetometer data for the 9 year data set of 2013–2021. To obtain Δf values, we used a machine learning technique that identifies the harmonics and from this we calculated the average separation. To investigate the climatology of the IAR, we have modeled the Δf of the IAR for the data set using a time of flight calculation with model Alfvén velocity profiles. When analyzing Δf from the model and data, we found that in general they follow the same trends. The modeled Δf and Δf from the data both show an inverse correlation with foF2, which confirms that the frequencies of the IAR are controlled by electron density. It follows that Δf is greater around midnight and during the winter months, due to the decrease in plasma mass density. Variability is also reflected when comparing yearly trends in Δf with the sunspot number; higher frequencies are observed and modeled at low sunspot number. It is difficult to examine trends with instantaneous geomagnetic activity as IAR are not visible in spectrograms when geomagnetic activity is high. We find cases where the difference in measured and modeled Δf is significant, suggesting that the model does not capture short term variations in plasma mass density that influence the IAR during these days. We plan to undertake further modeling of Δf on shorter timescales. Plain Language Summary: Ionospheric Alfvén Resonances occur when waves traveling along magnetic field lines in the ionosphere are reflected at boundaries where the change in the velocity of the wave reaches a maximum. These waves are observed in data from Eskdalemuir Geophysical Observatory. We have modeled the frequency of these waves and compared this with a 9‐year data set we obtained from Eskdalemuir, enabling us to examine trends and increase our understanding of their behavior. We found that the data and the model both reach higher frequencies during the night, winter and during years when the Sun is less active. We compared the frequencies of the Ionospheric Alfvén Resonances with models of electron density in the ionosphere and found that the frequencies are higher when the density of the electrons is lower. We found cases where the frequencies in the data do not match with the frequencies in our model and so we plan to investigate these in further studies. Key Points: Machine learning was used to identify harmonics of Ionospheric Alfvén Resonances and harmonic frequency separation for 9 years of dataThe harmonic frequency separation was modeled for this data set and comparisons were made between model and dataThe model and data follow the same general trends, with higher frequencies corresponding to lower plasma mass density
- Subjects
UNITED Kingdom; OBSERVATORIES; CLIMATOLOGY; RESONANCE; ELECTRON density; INDUCTION coils; SUNSPOTS; MACHINE learning; WINTER
- Publication
Journal of Geophysical Research. Space Physics, 2023, Vol 128, Issue 6, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2023JA031541