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

Abstract

Long-term measurements of the high-frequency magnetic field (0.1-100 Hz) have been made at Eskdalemuir Observatory in the United Kingdom since September 2012. We analyze five years of dynamic spectrograms to examine the occurrence and behavior of the Schumann and ionospheric Alfvén resonances (IAR) and Pc1 pulsations. The resonances, observed as diffuse bands, arise from reflections of energy both within the Earth-ionosphere cavity and from the nonlinear conductivity gradient of the ionosphere. Schumann Resonances (SR) occur continuously but IAR are observed to arise at local nighttime in ∼50% of days in the data set. Typically, IAR are found at frequencies of 1-8 Hz, but we find them extending out to 30 Hz and strongly superimposing over the first three Schumann resonances around 9% of the time. These phenomena include constructive and destructive interference, nonlinear frequency changes over the span of several hours, and polarity enhancements. In addition, the magnitude of the IAR does not decline rapidly with frequency as often proposed. We find that the IAR and their superposition with SR are strongly controlled by season and geomagnetic activity. We compare 6 days with the most unusual IAR behavior in the data set to ionosonde measurements of f 0 F 2 , a proxy for ionospheric conductivity but find little correlation. We suggest that, as current theoretical modeling does not account for these observations, further work is needed to understand how they arise. Plain Language Summary Measurements of the very rapid changes of the Earth's magnetic field(changes at a rate of between 1 and 50 times per second) show very weak repeating patterns. They are caused by magnetic fields from lightning strikes in thunderstorms near the equator. The lightning strikes are strong enough to "echo" around the globe repeatedly for a few seconds before fading, similar to the sound from a resonating bell. These patterns repeat at fixed periods of around 8, 14, and 21 times per second and are called the Schumann resonances. At night time, other patterns appear in the measurements caused by magnetic waves temporarily trapped in the upper atmosphere (called the ionosphere) between heights of 100 and 1,000 km. These patterns are labeled the ionospheric Alfvén resonances are a relatively unstudied feature of the Earth's magnetic field. We looked at these patterns in magnetic field data collected from Eskdalemuir Observatory in the United Kingdom over the past 5 years. We checked how often these types of patterns occurred and found a link to the seasons and how active, in general, the magnetic field is. We also found unusual and currently unexplained patterns including interference between the Schumann and ionospheric Alfvén resonances.

Seasonal and Solar Cycle Variation of Schumann Resonance Intensity and Frequency at Eskdalemuir Observatory, UK

“…In 2012, two horizontal induction (search) coil magnetometers were installed at the geophysical observatory in Eskdalemuir (Scottish Borders, UK) operated by the British Geological Survey (BGS). The coils have been recording magnetic field variations over an effective frequency range of 0.1 -50 Hz since September 2012 (Beggan & Musur, 2018. Having collected almost seven years of induction coil data, the primary focus of this study is to confirm the properties and characteristics of the SR presented in some of the above cited studies and to illustrate the complex SR seasonal behaviours in a manner not previously shown in the literature.…”

“…For our analysis, we use seven years of ELF geomagnetic data collected using two refurbished CM11E high-frequency induction coil magnetometers (previously used for MT studies). The equipment was deployed in June 2012 in a geomagnetically quiet location at the Eskdalemuir geophysical observatory in the Scottish Borders (UK, θ geo = 55.3 °N, Φ geo = −3.2 °E) (see Beggan & Musur, 2018). The geomagnetic location of the observatory at 2018.0 was θ gm = 57.5 °N in quasi-dipole coordinates with L ~ 3.5.…”

Solar Cycle-Modulated Deformation of the Earth–Ionosphere Cavity

“…Therefore the intensity of the first SR mode was plotted as a function of UT time for all the days within the processed month and days with unusual daily intensity variation were excluded when calculating the monthly average. SR measurements are carried out by the British Geological Survey at Eskdalemuir Observatory (ESK; 55.3°N, 3.2°W; L 2.7; Figure 2B) near the Scottish Borders of the United Kingdom since September 2012 (Beggan and Musur, 2018;Musur and Beggan, 2019). For the present study processed data are available from June 2012 to December 2020 with a few gaps in the record.…”

A Multi‐Instrument Approach to Determining the Source‐Region Extent of EEP‐Driving EMIC Waves

“…, run by the British Geological Survey (Beggan & Musur, 2018). The ESK magnetometer consists of two coils: one arranged north-south and the other east-west.…”

“…These instruments sample at 64 and 256 Hz, respectively, making them ideal for investigating EMIC activity. We complement these observations with data from the 100 Hz ground‐based search‐coil magnetometer (SCM) located in Eskdalemuir, Scotland (55.3° N, 3.2° E, ), run by the British Geological Survey (Beggan & Musur, ). The ESK magnetometer consists of two coils: one arranged north‐south and the other east‐west.…”