A circuit analogy for magnetosphere-ionosphere current systems has two extremes for drivers of ionospheric currents: the “voltage generator” (ionospheric electric fields/voltages are constant, while current varies) and the “current generator” (current is constant, while the electric field varies). Here we
indicate another aspect of the magnetosphere-ionosphere interaction, which should be taken into account when considering the current/voltage dichotomy. We show that nonsteady field-aligned currents interact with the ionosphere in a different way depending on a forced driving or resonant excitation. A quasi-DC
driving of field-aligned current corresponds to a voltage generator, when the ground magnetic response is proportional to the ionospheric Hall conductance. The excitation of resonant field line oscillations corresponds to the current generator, when the ground magnetic response only weakly depends on the
ionospheric conductance. According to the suggested conception, quasi-DC nonresonant disturbances correspond to a voltage generator. Such ultralow frequency (ULF) phenomena as traveling convection vortices and Pc5 waves should be considered as the resonant response of magnetospheric field lines,
and they correspond to a current generator. However, there are quite a few factors that may obscure the determination of the current/voltage dichotomy.
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Electric Mode Excitation in the Atmosphere by Magnetospheric Impulses and ULF Waves
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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- Award ID(s):
- 1744828
- NSF-PAR ID:
- 10250652
- Date Published:
- Journal Name:
- Frontiers in Earth Science
- Volume:
- 8
- ISSN:
- 2296-6463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract A circuit analogy for magnetosphere‐ionosphere current systems has two extremes for drivers of ionospheric currents: the “voltage generator” (ionospheric electric fields/voltages are constant, while current varies) and the “current generator” (current is constant, while the electric field varies). Here we indicate another aspect of the magnetosphere‐ionosphere interaction, which should be taken into account when considering the current/voltage dichotomy. We show that nonsteady field‐aligned currents interact with the ionosphere in a different way depending on a forced driving or resonant excitation. A quasi‐DC driving of field‐aligned current corresponds to a voltage generator, when the ground magnetic response is proportional to the ionospheric Hall conductance. The excitation of resonant field line oscillations corresponds to the current generator, when the ground magnetic response only weakly depends on the ionospheric conductance. According to the suggested conception, quasi‐DC nonresonant disturbances correspond to a voltage generator. Such ultralow frequency (ULF) phenomena as traveling convection vortices and Pc5 waves should be considered as the resonant response of magnetospheric field lines, and they correspond to a current generator. However, there are quite a few factors that may obscure the determination of the current/voltage dichotomy.
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null (Ed.)Abstract The Electron Loss and Fields Investigation with a Spatio-Temporal Ambiguity-Resolving option (ELFIN-STAR, or heretoforth simply: ELFIN) mission comprises two identical 3-Unit (3U) CubeSats on a polar (∼93 ∘ inclination), nearly circular, low-Earth (∼450 km altitude) orbit. Launched on September 15, 2018, ELFIN is expected to have a >2.5 year lifetime. Its primary science objective is to resolve the mechanism of storm-time relativistic electron precipitation, for which electromagnetic ion cyclotron (EMIC) waves are a prime candidate. From its ionospheric vantage point, ELFIN uses its unique pitch-angle-resolving capability to determine whether measured relativistic electron pitch-angle and energy spectra within the loss cone bear the characteristic signatures of scattering by EMIC waves or whether such scattering may be due to other processes. Pairing identical ELFIN satellites with slowly-variable along-track separation allows disambiguation of spatial and temporal evolution of the precipitation over minutes-to-tens-of-minutes timescales, faster than the orbit period of a single low-altitude satellite (T orbit ∼ 90 min). Each satellite carries an energetic particle detector for electrons (EPDE) that measures 50 keV to 5 MeV electrons with $\Delta $ Δ E/E < 40% and a fluxgate magnetometer (FGM) on a ∼72 cm boom that measures magnetic field waves (e.g., EMIC waves) in the range from DC to 5 Hz Nyquist (nominally) with <0.3 nT/sqrt(Hz) noise at 1 Hz. The spinning satellites (T spin $\,\sim $ ∼ 3 s) are equipped with magnetorquers (air coils) that permit spin-up or -down and reorientation maneuvers. Using those, the spin axis is placed normal to the orbit plane (nominally), allowing full pitch-angle resolution twice per spin. An energetic particle detector for ions (EPDI) measures 250 keV – 5 MeV ions, addressing secondary science. Funded initially by CalSpace and the University Nanosat Program, ELFIN was selected for flight with joint support from NSF and NASA between 2014 and 2018 and launched by the ELaNa XVIII program on a Delta II rocket (with IceSatII as the primary). Mission operations are currently funded by NASA. Working under experienced UCLA mentors, with advice from The Aerospace Corporation and NASA personnel, more than 250 undergraduates have matured the ELFIN implementation strategy; developed the instruments, satellite, and ground systems and operate the two satellites. ELFIN’s already high potential for cutting-edge science return is compounded by concurrent equatorial Heliophysics missions (THEMIS, Arase, Van Allen Probes, MMS) and ground stations. ELFIN’s integrated data analysis approach, rapid dissemination strategies via the SPace Environment Data Analysis System (SPEDAS), and data coordination with the Heliophysics/Geospace System Observatory (H/GSO) optimize science yield, enabling the widest community benefits. Several storm-time events have already been captured and are presented herein to demonstrate ELFIN’s data analysis methods and potential. These form the basis of on-going studies to resolve the primary mission science objective. Broad energy precipitation events, precipitation bands, and microbursts, clearly seen both at dawn and dusk, extend from tens of keV to >1 MeV. This broad energy range of precipitation indicates that multiple waves are providing scattering concurrently. Many observed events show significant backscattered fluxes, which in the past were hard to resolve by equatorial spacecraft or non-pitch-angle-resolving ionospheric missions. These observations suggest that the ionosphere plays a significant role in modifying magnetospheric electron fluxes and wave-particle interactions. Routine data captures starting in February 2020 and lasting for at least another year, approximately the remainder of the mission lifetime, are expected to provide a very rich dataset to address questions even beyond the primary mission science objective.more » « less
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Transient Oscillations Near the Dayside Open‐Closed Boundary: Evidence of Magnetopause Surface Mode?more » « less
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.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/feart.2020.619227
