Ionospheric Alfvén resonator observed at low‐latitude ground station, Muroto

Abstract

The ionospheric Alfvén resonator (IAR) is found in a dynamic power spectrum of the geomagnetic field variations as spectral resonance structures in the frequency range of 0.1–10 Hz. To date, observations of IAR at low latitude have only been made by a few studies employing a single ground observatory and are not sufficient enough to reveal its general characteristics. We analyze data measured with an induction magnetometer installed at Muroto, Japan (24.40° geomagnetic latitude) for the period from 28 December 2013 to 13 August 2016. A statistical analysis reveals that the occurrence probability of the low‐latitude IAR is (1) dominant during nighttime with a gradual increase from dusk to midnight and a broad maximum at 00–05 LT followed by a sudden decrease at dawn, (2) slightly higher during May through September, and (3) independent of the Kp index for Kp ≤ 5. We also find that (4) the frequency separation between the harmonics (Δf) of the low‐latitude IAR is 0.1–0.5 Hz with a peak at 0.2–0.275 Hz. These observational results are compared with the model calculation results along with previous studies about possible energy sources. It is found that the global thunderstorm activity is a likely source for IAR at Muroto; that is, it produces electromagnetic disturbances that are converted into the Alfvén waves and that are then trapped in the ionospheric cavity bounded by the conductive E layer and a steep gradient of Alfvén velocity above the F2 layer to form the resonant Alfvén waves.

Observation of Ionospheric Alfvén Resonances at 1–30 Hz and Their Superposition With the Schumann Resonances

“…IAR, in general, can arise in a slightly wider geomagnetic activity range (mode of Kp ≤4). This does contradict the findings of Nosé et al () who found no IAR dependence on Kp. However, they examined a shorter data set of around 2.8 years.…”

“…Work by Yahnin et al (), Hebden et al (), and Parent et al () found that the IAR were anticorrelated with geomagnetic activity, for example. Further research by Nosé et al () with magnetometer data from Japan again confirmed that the IAR are a local nighttime phenomena, with peak occurrence before midnight and have a strong seasonal dependence, being most common in winter months. In addition, the peak‐to‐peak difference between the IAR fringes also changes with season and is largest in winter at 0.5–0.7 Hz but smaller in summer and the equinoxes (cf.…”

“…Consequently, the IAR are usually visible during the quiescent period of the day (22:00–06:00 UT) when the magnitude of the SR declines overall. Other sites where IAR have been observed (e.g., Molchanov et al, ; Nosé et al, , in Asia) make their measurements at local nighttime around 12:00–18:00 UT, which is during the hours of peak SR magnitude.…”

“…The elliptogram plots are computed using the formulation in Sentman (). Positive ellipticity is defined as being left‐handed and negative ellipticity is right‐handed, following the convention of Nosé et al (), for example. As can be observed, the SR are always positive, showing a peak between 12:00 and 18:00 UT.…”

“…The ionospheric Alfvén resonances are typically detectable in the 0.5–10 Hz region of the magnetic field spectrum (Belyaev et al, ). They arise primarily from the partial reflection of energy in the ionosphere excited along magnetic field lines by the leakage of electric fields, most likely associated with terrestrial lightning activity (Füllekrug & Reising, ; Greifinger & Greifinger, ; Nosé et al, ). The waves reflect off the “walls” of the E region and F 2 region where the gradients of electron density reach a maximum (Belyaev et al, ).…”