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NASA NTRSDate not supplied

Intense Equatorial Electrojet and Counter Electrojet caused by the 15 January 2022 Tonga Volcanic Eruption: Space and Ground-based Observations

Abstract Text: We present space and ground-based multi-instrument observations demonstrating the impact of the 2022 Tonga volcanic eruption on dayside equatorial electrodynamics. A strong counter electrojet (CEJ) was observed by Swarm and ground-based magnetometers on 15 January after the Tonga eruption and during the recovery phase of a moderate geomagnetic storm. Swarm also observed an enhanced equatorial electrojet (EEJ) preceding the CEJ in the previous orbit. The observed EEJ and CEJ exhibited complex spatiotemporal variations. We combine them with the Ionospheric Connection Explorer (ICON) neutral wind measurements to disentangle the potential mechanisms. Our analysis indicates that the geomagnetic storm had minimal impact; instead, a large-scale atmospheric disturbance propagating eastward from the Tonga eruption site was the most likely driver for the observed intensification and directional reversal of the equatorial electrojet. The CEJ was associated with strong eastward zonal winds in the E-region ionosphere, as a direct response to the lower atmosphere forcing. Plain-Language Summary: The Earth's E-region ionosphere (~100-150 km altitude) consists of both ionized and neutral gasses, and the two components are coupled through ion-neutral collisions. The state of this region is closely influenced by neutral atmospheric activities from the lower atmosphere and the variability of the solar drivers. On 15 January 2022, the Tonga volcano had a massive eruption and injected an enormous amount of mass and energy into the atmosphere causing disturbances in the E-region ionosphere or even higher. There was also a moderate geomagnetic storm that started one day before the eruption and ended days after. These conditions offer a unique opportunity to understand the different roles they play in controlling the ionosphere. Coordinated observations including the atmosphere, ionosphere and magnetosphere were made from both space and on the ground during this event. We analyzed the magnetic field and neutral wind data and found that a large-scale atmospheric disturbance generated by the volcano eruption was responsible for the observed directional reversal of the dayside equatorial electric field and electric current.

Guan Le↗
NASA NTRS1979

Energetic particle precipitation into the high-latitude ionosphere and the auroral electrojets. I - Definition of electrojet boundaries using energetic electron spectra and ground-based magnetometer data

Magnetometric data are used to define the poleward and equatorward borders of the eastward electrojet in the evening sector. The relationship between central plasma sheet (cps) and boundary plasma sheet (bps) and the auroral electrojet region is explored. This relationship is used to infer the electric field configuration in the cps and bps regions. It is shown that in the evening sector the cps is threaded by field lines carrying current flowing into the ionosphere, while the bps is threaded by field lines carrying current flowing out of the ionosphere.

Winningham, J. D.↗
NASA NTRS2008

Comparative In Situ Measurements of Plasma Instabilities in the Equatorial and Auroral Electrojets

This presentation provides a comparison of in situ measurements of plasma instabilities gathered by rocket-borne probes in the equatorial and auroral electrojets. Specifically, using detailed measurements of the DC electric fields, current density, and plasma number density within the unstable daytime equatorial electrojet from Brazil (Guara Campaign) and in the auroral electrojet from Sweden (ERRIS Campaign), we present comparative observations and general conclusions regarding the observed physical properties of Farley-Buneman two-stream waves and large scale, gradient drift waves. The two stream observations reveal coherent-like waves propagating near the E x B direction but at reduced speeds (nearer to the presumed acoustic velocity) with wavelengths of approximately 5-10m in both the equatorial and auroral electrojet, as measured using the spaced-receiver technique. The auroral electrojet data generally shows extensions to shorter wavelengths, in concert with the fact that these waves are driven harder. With respect to gradient-drift driven waves, observations of this instability are much more pronounced in the equatorial electrojet, given the more favorable geometry for growth provided by the vertical gradient and horizontal magnetic field lines. We present new analysis of Guara rocket observations of electric field and plasma density data that reveal considerable structuring in the middle and lower portion of the electrojet (90-105 km) where the ambient plasma density gradient is unstable. Although the electric field amplitudes are largest (approximately 10-15 mV/m) in the zonal direction, considerable structure (approximately 5-10 mV/m) is also observed in the vertical electric field component as well, implying that the dominant large scale waves involve significant vertical interaction and coupling within the narrow altitude range where they are observed. Furthermore, a detailed examination of the phase of the waveforms show that on some, but not all occasions, locally enhanced eastward fields are associated with locally enhanced upwards (polarization) electric fields. The measurements are discussed in terms of theories involving the non-linear evolution and structuring of plasma waves.

Pfaff, Robert F.↗
NASA NTRS1972

Convective amplification of Type 1 irregularities in the equatorial electrojet

Wave propagation and refraction of Type 1 irregularities in the equatorial electrojet were investigated. Quantitative calculation of wave refraction in a model electrojet showed that the direction of wave refraction must change sign at one altitude. Waves propagating with the electrons rotate their wave vectors upwards in the upper electrojet and downwards in the lower electrojet during the day, and vice versa at night. Furthermore, the altitude region of largest linear growth rate is also the one with the weakest refraction rate. Consequently, computations of the ray-path integrated wave growth shows that this region would dominate the backscatter spectrum from the electrojet if linear theory were valid, and it is further noted that the maximum amplitude wave should have phase velocities exceeding the ion acoustic speed. It was concluded that propagation alone, without inclusion of nonlinear effects, cannot explain backscatter observations of a constant Doppler frequency shift given by the ion acoustic speed.

Lee, K.↗
NASA NTRS1973

Convective amplification of type I irregularities in the equatorial electrojet.

Wave propagation and refraction of 'type I' irregularities in the equatorial electrojet are investigated. Quantitative calculation of wave refraction in a model electrojet shows that the direction of wave refraction must change sign at one altitude. Waves propagating with the electrons rotate their wave vectors upward in the upper electrojet and downward in the lower electrojet during the day, and vice versa at night. Furthermore, the altitude region of largest linear growth rate is also the one with the weakest refraction rate. Consequently, computations of the ray-path integrated wave growth show that this region would dominate the backscatter spectrum from the electrojet if linear theory were valid, and it is further noted that the maximum amplitude wave should have phase velocities exceeding the ion acoustic speed. It is therefore concluded that propagation alone, without inclusion of nonlinear effects, cannot explain backscatter observations of a constant Doppler frequency shift given by the ion acoustic speed.

Lee, K.↗
NASA NTRS1993

The equatorial electrojet and associated currents as seen in Magsat data

In view of the disagreement between reports of Langel and Estes (1985) and those of other investigators using Nagsat data sets, concerning the evidence for equatorial electrojet and meridional currents in the dawn Magsat data, this paper reexamines the Magsat data for the presence of equatorial electrojet and meridional current in both dawn and dusk local times, using a specially designed method for isolating field variations organized by dip-latitude. It is shown that such fields, if any, at dawn are very weak and do not result in a persistent pattern from longitude to longitude or a pattern consistent with that expected from an electrojet current. On the other hand, fields organized by dip-latitude are clearly present in the dusk data and it is possible to isolate longitudinal and seasonal variations in such fields. Current densities are computed for both the equatorial electrojet and meridional currents and are compared to the current densities and distributions found with recent models of the ionospheric dynamo and equatorial electrojet.

Langel, R. A.↗
NASA NTRS1973

Auroral electrojets and evening sector electron dropouts at synchronous orbit

Evidence is presented in support of the concept that, during magnetospheric substorms, ionospheric auroral electrojet currents are directly coupled to the proton partial ring current in the outer magnetosphere. It has been found that for sufficiently isolated substorms the timing of the start of the electron dropout and of its maximum depression is in good agreement with the start and maximum of electrojet activity as indicated by the auroral electrojet index. This correlation suggests a direct coupling between the electrojet currents and the proton partial ring current.

Erickson, K. N.↗
NASA NTRS2017

First Application of the Zeeman Technique to Remotely Measure Auroral Electrojet Intensity From Space

Using the O2 118 GHz spectral radiance measurements obtained by the Microwave Limb Sounder instrument on board the Aura spacecraft, we demonstrate that the Zeeman effect can be used to remotely measure the magnetic field perturbations produced by the auroral electrojet near the Hall current closure altitudes. Our derived current-induced magnetic field perturbations are found to be highly correlated with those coincidently obtained by ground magnetometers. These perturbations are also found to be linearly correlated with auroral electrojet strength. The statistically derived polar maps of our measured magnetic field perturbation reveal a spatial-temporal morphology consistent with that produced by the Hall current during substorms and storms. With today's technology, a constellation of compact, low-power, high spectral-resolution cubesats would have the capability to provide high precision and spatiotemporal magnetic field samplings needed for auroral electrojet measurements to gain insights into the spatiotemporal behavior of the auroral electrojet system.

spectral radiance measurements; Aur↗
NASA NTRS1974

Westward equatorial electrojet during daytime hours

The phenomenon of the depression of the geomagnetic horizontal field during the daytime hours of magnetically quiet days at equatorial stations is described. These events are generally seen around 0700 and 1600 LT, being more frequent during the evening than the morning hours. The evening events are more frequent during periods of low solar activity and in the longitude region of weak equatorial electrojet currents. The latitudinal extent of the phenomenon is limited to the normal equatorial electrojet region, and on some occasions the phenomenon is not seen at both stations, separated by only a few hours in longitude. During such an event, the latitudinal profile of the geomagnetic vertical field across the equator is reversed, the ionospheric drift near the equator is reversed toward the east, the q type of sporadic E layer is completely absent, and the height of the peak ionization in the F2 region is decreased. It is suggested that these effects are caused by a narrow band of current flowing westward in the E region of the ionosphere and within the latitude region of the normal equatorial electrojet, due to the reversal of the east-west electrostatic field at low latitudes.

Rastogi, R. G.↗
NASA NTRS1975

A relationship between synchronous altitude electron fluxes and the auroral electrojet

Simultaneous observations during four substorms are reported from the Lockheed auroral particle spectrometer on ATS 5 and the University of Alberta meridian magnetometer chain. During the four events studied there was a good correlation between the magnitude of the trapped electron fluxes in the energy range from 1.8 to 53 keV and the magnitude of the electrojet current as measured by a station in the magnetometer chain at a latitude close to that expected for the ATS conjugate point. A model electrojet is constructed based on the work of Coroniti and Kennel (1972) which gives a good absolute agreement between the two measured quantities. The results are consistent with the convection electric field's remaining approximately constant during a substantial portion of each of the substorms studied. The temporal variations of the electrojet were apparently controlled by conductivity changes in the ionosphere as determined by the precipitating auroral electrons.

Sharp, R. D.↗
NASA NTRS1975

A relationship between synchronous-altitude electron fluxes and the auroral electrojet, appendix A

Simultaneous observations during four substorms are reported from the Lockheed auroral particle spectrometer on ATS-5 and the University of Alberta meridian magnetometer chain (Canada). During the four events studied, there was a good correlation between the magnitude of the trapped electron fluxes in the energy range from 1.8 to 53 keV and the magnitude of the electrojet current as measured by a station in the magnetometer chain at a latitude close to that expected for the ATS conjugate point. The Hall effect was studied and a model electrojet was constructed which gave a good absolute agreement between the two measured quantities. The results are consistent with the convection electric field remaining approximately constant during a substantial portion of each of the substorms studied. The temporal variations of the electrojet were apparently controlled by conductivity changes in the ionosphere as determined by the precipitating auroral electrons.

Sharp, R. D.↗
NASA NTRS1984

A case-study of the evolution of polar-cap currents and auroral electrojets during polar geomagnetic disturbances with IMS magnetometer data

The development of the polar cap current and the relationship of that development to the evolution of auroral electrojets during individual polar geomagnetic disturbances is studied using 1 min average data from US-Canada IMS network stations and standard magnetograms from sites on the polar cap and in the auroral zone. It is found that even when the auroral electrojet activity is weak, polar cap currents producing fields of magnitude approximately 100-200 nT almost always exist. A normal convection current system exists quasi-persistently in the polar cap during extended quiet or weakly disturbed periods of auroral electrojet activity. After one such period, some drastic changes occur in the polar cap currents, which are followed by phases of growth, expansion, and recovery. Polar cap currents cannot all be completely ascribed to a single source mechanism.

Iijima, T.↗
NASA NTRS1987

Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet. I - The unstable layer and gradient drift waves. II - Two-stream waves

The results of electric field and plasma density measurements in the strongly driven daytime equatorial electrojet over Peru, made during the March 1983 Condor electrojet experiment from Punta Lobos, Peru, are discussed together with the rocket instrumentation used for the measurements and the pertinent payload dynamics. The overall characteristics of the irregularity layer observed in situ in the electrojet are described. Special consideration is given to the waves generated by the gradient drift instability (observed between 90 and 106.5 km) and to primary and secondary two-stream waves detected by the two probes on the topside between 103 and 111 km, where the electron current was considered to be strongest.

Pfaff, R. F.↗
NASA NTRS1988

Thermal filamentation instability driven by the auroral electrojet current

A thermal instability leading to the filamentation of auroral electrojet currents and giving rise to purely growing magnetic field-aligned density irregularities in the E region of the high-latitude ionosphere is investigated. The physical process of the instability is through the modification of the electron-neutral collision frequency due to the electron temperature perturbation in the electrojet. A dispersion relation of the instability is derived, from which the threshold electrojet current and the growth rate of the instability are determined. It is found that they become independent of the scale sizes of the irregularities for scale sizes larger than about 13 m. The proposed instability can thus be considered to be one of the mechanisms responsible for observed relatively large-scale E region irregularities (Pfaff et al., 1984).

Kuo, S. P.↗
NASA NTRS1991

Rocket observations in the equatorial electrojet - Current status and critical problems

The current status of in situ investigations in the equatorial electrojet is reviewed. Emphasis is placed on: (1) the relation of the vertical polarization field to the electrojet current and the electron number density; (2) the puzzling square shapes of the large amplitude kilometer-scale horizontal electric field structures; (3) the intense vertical, meter-scale waves observed on the topside of the electrojet associated with horizontal laminar-like primary two-stream waves; (4) measurements of upgoing and downgoing secondary two-stream and gradient drift wave packets driven by delta E x B drifts; (5) the nonlinear meter-scale 'turbulence' with small mean phase velocities observed by radars at altitudes outside the regions of high Cowling conductivity, and wave-particle heating by the plasma instabilities.

Pfaff, R. F., Jr.↗