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1. Electric Mode Excitation in the Atmosphere by Magnetospheric Impulses and ULF Waves

   https://doi.org/10.3389/feart.2020.619227  

   Pilipenko, V. A.; Fedorov, E. N.; Martines-Bedenko, V. A.; Bering, E. A. (January 2021, Frontiers in Earth Science)

   null (Ed.)

   Variations of vertical atmospheric electric field E z have been attributed mainly to meteorological processes. On the other hand, the theory of electromagnetic waves in the atmosphere, between the bottom ionosphere and earth’s surface, predicts two modes, magnetic H (TE) and electric E (TH) modes, where the E-mode has a vertical electric field component, E z . Past attempts to find signatures of ULF (periods from fractions to tens of minutes) disturbances in E z gave contradictory results. Recently, study of ULF disturbances of atmospheric electric field became feasible thanks to project GLOCAEM, which united stations with 1 sec measurements of potential gradient. These data enable us to address the long-standing problem of the coupling between atmospheric electricity and space weather disturbances at ULF time scales. Also, we have reexamined results of earlier balloon-born electric field and ground magnetic field measurements in Antarctica. Transmission of storm sudden commencement (SSC) impulses to lower latitudes was often interpreted as excitation of the electric TH 0 mode, instantly propagating along the ionosphere–ground waveguide. According to this theoretical estimate, even a weak magnetic signature of the E-mode ∼1 nT must be accompanied by a burst of E z well exceeding the atmospheric potential gradient. We have examined simultaneous records of magnetometers and electric field-mills during >50 SSC events in 2007–2019 in search for signatures of E-mode. However, the observed E z disturbance never exceeded background fluctuations ∼10 V/m, much less than expected for the TH 0 mode. We constructed a model of the electromagnetic ULF response to an oscillating magnetospheric field-aligned current incident onto the realistic ionosphere and atmosphere. The model is based on numerical solution of the full-wave equations in the atmospheric-ionospheric collisional plasma, using parameters that were reconstructed using the IRI model. We have calculated the vertical and horizontal distributions of magnetic and electric fields of both H- and E-modes excited by magnetospheric field-aligned currents. The model predicts that the excitation rate of the E-mode by magnetospheric disturbances is low, so only a weak E z response with a magnitude of ∼several V/m will be produced by ∼100 nT geomagnetic disturbance. However, at balloon heights (∼30 km), electric field of the E-mode becomes dominating. Predicted amplitudes of horizontal electric field in the atmosphere induced by Pc5 pulsations and travelling convection vortices, about tens of mV/m, are in good agreement with balloon electric field and ground magnetometer observations. 

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2. Characterization of High‐m ULF Wave Signatures in GPS TEC Data

   https://doi.org/10.1029/2021GL094282  

   Zhai, Changzhi; Shi, Xueling; Wang, Wenbin; Hartinger, Michael D.; Yao, Yibin; Peng, Wenjie; Lin, Dong; Ruohoniemi, J. Michael; Baker, Joseph B. H. (July 2021, Geophysical Research Letters)

   Abstract GPS total electron content (TEC) measurements were used to investigate high‐m ultralow frequency (ULF) waves during the recovery phase of a geomagnetic storm. ULF wave signals in TEC data show high coherence and significant common power in the wavelet coherence and cross wavelet transform analyses with magnetic field radial component data from GOES‐15. They did not cause significant ionospheric scintillation or ground magnetic signatures due to ionospheric screening effects. An automatic identification procedure is developed to identify ULF wave signature in TEC data from 10 GPS receivers on January 25, 2016. The waves were mainly distributed on the dayside and post dusk sector from ∼64° to ∼71° magnetic latitude. This is the first time that the large‐scale 2D spatial structure and temporal evolution of high‐m ULF waves are revealed, which demonstrates TEC measurements as an effective high‐m ULF wave remote sensing tool. 

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3. Daytime Pc5 Diffuse Auroral Pulsations and Their Association With Outer Magnetospheric ULF Waves

   https://doi.org/10.1029/2021JA029218  

   Motoba, T.; Ogawa, Y.; Ebihara, Y.; Kadokura, A.; Gerrard, A. J.; Weatherwax, A. T. (August 2021, Journal of Geophysical Research: Space Physics)

   Abstract South Pole Station, Antarctica (SPA, magnetic latitude = −74.5°, magnetic local time (MLT) = UT–3.5 h), is a unique observatory which can capture daytime auroral forms throughout austral winter season. We have studied the properties and origin of ultralow‐frequency (ULF) range modulation of daytime diffuse aurora, using data acquired on June 23, 2017 by multi‐instrument measurements at SPA and in situ measurements in the dayside outer magnetosphere. At 1500–1600 UT, monochromatic Pc5‐range pulsations (period ∼10 min) emerged in the midday diffuse auroral region. The sequential 2‐D images reveal that the auroral pulsations result from the repetitive formation of faint, diffuse auroral patches, propagating poleward at a speed of ∼1.5 km s⁻¹. Interestingly, no obviously similar magnetic pulsations were found at SPA. The results differ fundamentally from the ground optical and magnetic signatures expected for a standing field line resonance. On the other hand, the co‐located riometer and VLF receiver observed clearly synchronized pulsations, suggesting that tens‐of‐keV electrons interact with modulated chorus waves and then are scattered down to the auroral pulsation region. During the same interval, the THEMIS‐D spacecraft detected corresponding Pc5 oscillations in the dayside outer magnetosphere (9–10R_Eand ∼15 MLT). The compressional component of the magnetospheric Pc5 waves, presumably driven by an external source, exhibited a good correspondence to the daytime Pc5 auroral pulsations. The simultaneous SPA–THEMIS observations highlight the role of compressional Pc5 pulsations in the dayside outer magnetosphere in determining the periodicity of daytime high‐latitude diffuse auroral pulsations. 

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4. Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?

   https://doi.org/10.1029/2018JA025684  

   Kozyreva, O.; Pilipenko, V.; Lorentzen, D.; Baddeley, L.; Hartinger, M. (November 2019, Journal of Geophysical Research: Space Physics)

   Abstract Geomagnetic pulsations in Pc5‐6 band (~3–20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open‐closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive “intrusion” of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of ~1.7–2.0‐mHz geomagnetic pulsations. Near‐cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error ~1°–2° of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large‐scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them. 

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5. Low-Power ULF Transmitter Design Using Static Permanent Magnet

   https://doi.org/10.23919/USNC-URSINRSM66067.2025.10907169  

   Chinnakkagari, Shashank; Manteghi, Majid (January 2025, IEEE)

   This paper presents a low-power Ultra Low Frequency (ULF) transmitter design that uses a static, permanent magnet to bring the surrounding magnetic shield near saturation. A low-power control coil modulates this static magnetic field by toggling sections of the shield between saturated and unsaturated states. The transmitter, operating at 400 Hz and tested using a 3D magnetometer, demonstrated an increase of 22.3 dB along the pole at 800 Hz and a 17.7 dB increase at 400 Hz perpendicular to the pole, with a further 33 dB enhancement than the control coil’s leakage at 800 Hz. These results highlight a novel method for efficient magnetic field modulation with significantly lower power requirements than traditional approaches. This novel approach reduces power consumption and opens new possibilities for designing scalable, energy-efficient ULF communication systems, with potential applications in underwater communication, remote sensing, and long-range wireless networks. 

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