The electrified medium‐scale traveling ionospheric disturbances (EMSTIDs) occurring as geomagnetically conjugate features in the middle latitude F region ionosphere are studied using multi‐instrument ...observations. Airglow imaging of OI 630 nm emission over Sata, Japan, and Darwin, Australia, are used to identify the occurrence of EMSTIDs. Thermospheric wind measurements made with the Fabry‐Perot interferometer observations of OI 630 nm from Shigaraki, Japan, and Darwin, Australia, are used along with ionosonde observations over Yamagawa, Japan, and Darwin, Australia, to study the thermospheric and ionospheric characteristics. These are the first results from such multi‐instrument observations simultaneously made from geomagnetic conjugate locations. Our results show that the amplitudes of the EMSTIDs are often different between the hemispheres. Thermospheric meridional winds appear to control the EMSTID amplitudes in the respective hemisphere. However, EMSTIDs are generated only when there is significant sporadic E activity with foEs often reaching greater than 6 MHz and (foEs − fbEs) reaching above 5 MHz at least for a short duration occurred. Existence of strong sporadic E activity on one of the hemispheres is found to be sufficient enough for generation of EMSTIDs in the conjugate F regions. These results conclusively indicate the importance of sporadic E layers in the generation of EMSTIDs. Further, it shows the significance of interhemispheric coupling between the E and F region ionospheres in the formation of EMSTIDs while their amplitudes in the respective hemispheres appear to have control of thermospheric neutral winds in the same hemisphere.
Key Points
Presence of significant sporadic E activity in at least one of the hemisphere is found to be necessary for formation of EMSTIDs
EMSTIDs form even when thermospheric winds are not favorable in one hemisphere as hemisphere coupled U × B electric fields are important
Amplitudes of EMSTIDs often differ between the hemispheres based on the thermospheric meridional winds
Auroral emission at 427.8‐nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple ...aurora at high altitudes. However, statistical characteristics of auroral 427.8‐nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8‐nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter‐tilting photometer over 14 years (2005–2018) at Athabasca, Canada (magnetic latitude: ~62°). We divided the data set into solar elevation angles (θs) more than and less than −24° (shadow height of sunlight: 600 km) to separate the 427.8‐nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8‐nm emissions of more than 100 R is 10.6% and 7.65% for θs more than and less than −24°, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8‐nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the postmidnight sector and increases with increasing geomagnetic activity, solar wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015–2018, during the declining phase of the 11‐year solar activity. Superposed epoch analysis indicates that the 427.8‐nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large.
Plain Language Summary
Purple auroras can be caused either by energetic electrons at low altitudes (about 100 km), or by scattering of sunlight at much higher altitudes where the auroral curtains are in the Sun although the ground is in darkness. We have studied both types by sorting measurements of the purple emission from ionized nitrogen molecules depending on how far the Sun is below the horizon. We used 14 years' worth of data in order to study the effects over more than a solar cycle (11 years typically). The emission is stronger when general auroral activity increases, which is in turn related to solar wind speed and density. Same result was obtained by binning solar wind and geomagnetic activity data, timing them relative to strong purple aurora. It is also highest in the declining phase of the solar cycle. The emission intensity becomes low in winter. Recently, a type of purple aurora outside of the normal auroral oval has been reported, called “STEVE.” We note that we have studied the “traditional” type of purple aurora.
Key Points
We made a statistical study of auroral/resonant‐scattering 427.8‐nm emission observed at subauroral latitudes over 14 years
The resonant‐scattering 427.8‐nm emission indicates upflow of ionospheric nitrogen ions to higher altitudes
The emission increases during magnetic active periods and is associated with increases of solar wind speed and density
An analytical model of the vertical acoustic resonance between the thermosphere and the earth's surface and its geomagnetic effect has been constructed. Resonance with a frequency of several mHz ...occurs when an atmospheric acoustic wave (AW) is excited by vibrations of the earth or ocean surface. The derived analytical relationships give a possibility to examine the dependence of the fundamental resonant frequency on the height of the reflecting atmospheric layer and horizontal AW number. Geomagnetic disturbances caused by the impact of AW on the ionosphere are calculated within the framework of a multi‐layered model of the ground‐atmosphere‐ionosphere system. The E‐layer of the ionosphere is treated in the approximation of a thin layer with an inclined geomagnetic field, while the topside ionosphere is assumed to consist of a cold collisionless plasma. The dependence of the spectral power of magnetic perturbations on the direction of the horizontal AW propagation is examined. The magnetic perturbation spectra are shown to have maxima at frequencies close to the acoustic resonance frequency. The spectral powers of magnetic perturbations and barometric variations measured during the Iwate‐Miyagi Nairik earthquake are in accordance with the model predictions for realistic media and AW parameters. Field‐aligned currents that arise when AWs enter the ionospheric E‐layer is shown to carry measurable electromagnetic disturbances into the geomagnetically conjugated region.
Plain Language Summary
Geophysical phenomena with large energy yield, such as earthquakes and volcano eruptions, can excite specific oscillations of the geomagnetic field and ionospheric plasma with frequencies around a few mHz. These oscillations are spatially localized in the vicinity of earthquake/volcano epicenter and last up to 1–2 hr. This phenomenon was interpreted as the vertical resonance of the acoustic wave (AW) trapped between the ground/ocean surface and thermosphere (∼80 km). It is important to know geomagnetic response to the acoustic resonance because it will give possibility to use data from magnetometer array for in‐depth study of this phenomenon. Presented theoretical model of the acoustic resonance provides analytical relationships to examine the dependence of geomagnetic signature of the acoustic resonance on atmosphere/ionosphere parameters. These simple relationships anybody can use to estimate without sophisticated computer modeling an expected geomagnetic effect of the acoustic resonance for any atmosphere/ionosphere parameters. Recorded spectral powers of magnetic perturbations and variations of atmospheric pressure are found to be in accordance with the model predictions. Currents along the geomagnetic field line that arise when AW interacts with ionospheric plasma can carry electromagnetic disturbances into the opposite hemisphere. This theoretical prediction has been confirmed by observations by Pacific magnetometers.
Key Points
Theory describes analytically magnetic pulsations observed after seismic/volcano events driven by vertical acoustic resonance in atmosphere
A dominant magnetic component of pulsations excited on the ground is directed along the horizontal wave vector of the acoustic wave
Theory predicts that ionospheric field‐aligned currents can cause detectable magnetic response to acoustic resonance in conjugate ionosphere
Two-dimensional structures of medium-scale traveling ionospheric disturbances (MSTIDs) over Europe have been revealed, for the first time, by using maps of the total electron content (TEC) obtained ...from more than 800 GPS receivers of the European GPS receiver networks. From statistical analysis of the TEC maps obtained 2008, we have found that the observed MSTIDs can be categorized into two groups: daytime MSTID and nighttime MSTID. The daytime MSTID frequently occurs in winter. Its maximum occurrence rate in monthly and hourly bin exceeds 70% at lower latitudes over Europe, whereas it is approximately 45% at higher latitudes. Since most of the daytime MSTIDs propagate southward, we speculate that they could be caused by atmospheric gravity waves in the thermosphere. The nighttime MSTIDs also frequently occur in winter but most of them propagate southwestward, in a direction consistent with the theory that polarization electric fields play an important role in generating the nighttime MSTIDs. The nighttime MSTID occurrence rate shows distinct latitudinal difference: The maximum of the occurrence rate in monthly and hourly bin is approximately 50% at lower latitudes in Europe, whereas the nighttime MSTID was rarely observed at higher latitudes. We have performed model calculations of the plasma density perturbations caused by a gravity wave and an oscillating electric field to reproduce the daytime and nighttime MSTIDs, respectively. We find that TEC perturbations caused by gravity waves do not show dip angle dependencies, while those caused by the oscillating electric field have a larger amplitude at lower latitudes. These dip angle dependencies of the TEC perturbation amplitude could contribute to the latitudinal variation of the MSTID occurrence rate. Comparing with previous studies, we discuss the longitudinal difference of the nighttime MSTID occurrence rate, along with the E- and F-region coupling processes. The seasonal variation, of the nighttime MSTID occurrence rate in Europe, is not consistent with the theory that the longitudinal and seasonal variations of the nighttime MSTID occurrence could be attributed to those of the Es layer occurrence.
We report the first statistical study of stable auroral red (SAR) arcs detached from the main auroral oval during non‐storm time, using multi‐event conjugate measurements by the Defense ...Meteorological Satellite Program (DMSP) satellites (F13–F19) and a ground‐based all‐sky imager at Athabasca (Canada) (54.6°W, 246.36°Е, MLAT = 61.5°, MLON = 308.3°, L = 4.4). We found 63 events of detached SAR arc conjunctions with the DMSP satellites in the northern hemisphere and 18 events in the opposite southern hemisphere from 2006 to 2018. Measurements aboard DMSP satellites show that detached SAR arcs are in general associated with enhancements of electron temperature (60 cases) and electron density troughs (58 cases). Only 14 cases show strong horizontal flow associated with the detached SAR arcs, indicating that the strong plasma flow is not a necessary condition to cause the detached SAR arcs. The electron temperature measured by DMSP associated with detached SAR arcs positively correlates with F10.7 solar activity index. The measured emission intensities at 630.0 nm in the SAR arcs show a good correlation with the electron temperature. These results indicate that the detached SAR arcs during non‐storm time are caused by heat flux from the magnetosphere associated with substorms, and their intensity depends on the background plasma condition in the ionosphere.
Key Points
We show the first statistical study of stable auroral red (SAR) arcs detached from the auroral oval during non‐storm times using the Defense Meteorological Satellite Program satellites
Detached SAR arcs are mainly associated with electron temperature enhancement and electron density trough
In the pre‐midnight and midnight sectors, in some cases, detached SAR arcs are co‐located with horizontal flows
The equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors ...during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand‐Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM‐A and SWARM‐C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.
Key Points
Unusually large equatorial zonal electric field at dusk sector due to eastward PPEF
Enhanced electric field limited to a narrow longitudinal sector centered on sunset terminator
Unique electrodynamic conditions at sunset terminator are responsible for large zonal electric field
In recent years, experimental results have consistently shown evidence of electromagnetic ion cyclotron (EMIC) wave‐driven electron precipitation down to energies as low as hundreds of keV. However, ...this is at odds with the limits expected from quasi‐linear theory. Recent analysis using nonlinear theory has suggested energy limits as low as hundreds of keV, consistent with the experimental results, although to date this has not been experimentally verified. In this study, we present concurrent observations from Polar‐orbiting Operational Environmental Satellite, Radiation Belt Storm Probes, Global Positioning System, and ground‐based instruments, showing concurrent EMIC waves and sub–MeV electron precipitation, and a global dropout in electron flux. We show through test particle simulation that the observed waves are capable of scattering electrons as low as hundreds of keV into the loss cone through nonlinear trapping, consistent with the experimentally observed electron precipitation.
Key Points
Closely correlated EMIC wave activity and sub–MeV electron precipitation is observed across multiple satellites
Both local and global dropouts in trapped electron flux are observed, down to hundreds of keV
Nonlinear test particle simulations show consistency between the observed electron precipitation and the observed wave
The characteristics of nighttime medium‐scale traveling ionospheric disturbance (MSTID) features observed over Yonaguni (24.5°N, 123.0°E; 19.3°N dip latitude), Japan are studied using all‐sky imaging ...of OI 630.0 nm airglow emission. The uniqueness of these observations is that the area observed by the imager covers the transition region between low to middle latitudes in the ionosphere. Typical low‐latitude limit of midlatitude‐type nighttime MSTIDs possessing phase front alignments along the northwest to the southeast occurs in this region. These MSTID features are rarely sighted at dip latitudes below 15°. We selected 2 year period for analysis in which 1 year corresponded to the solar minimum conditions and another year to the solar maximum conditions. The MSTIDs were observed to extend to farther lower latitudes during the solar minimum conditions than during the solar maximum periods. Their observed range of wavelengths, phase velocities, phase front alignment, and propagation directions are similar to those observed at typical midlatitude sites. However, on many occasions the phase fronts of the observed MSTIDs did not extend over the whole field of view of the imager indicating that some process inhibits their extension to further lower latitudes. Detailed investigation suggests that the poleward propagating enhancement of airglow intensity, probably associated with the midnight pressure bulge, causes the MSTID features to disappear when they reach lower latitudes later in the night. When the MSTIDs reach lower latitudes well before midnight, they are found to be inhibited by the equatorial ionization anomaly crest region.
Key Points
Disappearance of MSTID phase fronts associated with airglow enhancementsDetailed statistical study of MSTIDs over YonaguniMSTID occurrences are high during solar minimum conditions
Prompt penetration of convection/overshielding electric fields to equatorial and low latitudes during the southward/northward turnings of interplanetary magnetic field (IMF Bz) have been widely ...studied in the literature. The other types of penetration electric fields due to sudden changes in the solar wind dynamic pressure, IMF By and during the onset of substorms have also been recently reported. In this paper, we present the exclusive role of solar wind density changes on the prompt equatorial electric field disturbances using the long‐term observations of equatorial electrojet (EEJ) from the Indian sector. In response to the sharp increases in the solar wind density, prompt increases/decreases in the EEJ indicating the eastward/westward prompt penetration electric field (PPEF) of ~20 min periods have been consistently observed on the dayside/nightside. The prompt equatorial electric field disturbances of the opposite polarity have also been observed when the density decreases sharply. Further, the polarity of these PPEF disturbances does not show any clear dependency on the direction of IMF Bz and By. This paper is the first report with a statistically significant number of observations on the characteristics of equatorial electric field disturbances in response to the sudden enhancements/decreases in the solar wind density alone on both dayside and nightside. The underlying physical mechanisms for the prompt equatorial electric field disturbances have been discussed in light of enhanced high‐latitude convection and additional field‐aligned currents due to sudden enhancement of solar wind density.
Key Points
Prompt eastward/westward electric field disturbances of ~20 min are observed on dayside/nightside in response to sharp density enhancements
Similar PPEF disturbances of opposite polarity are observed in response to sharp decreases in the solar wind density
The polarity of PPEF/EEJ disturbances depends mainly on the local time without any dependency on the polarity of IMF Bz and By
A ground‐based network of Global Navigation Satellite Systems receivers has been used to monitor medium‐scale traveling ionospheric disturbances (MSTIDs). MSTIDs were studied using total electron ...content perturbation maps and keograms over south‐southeast of Brazil during the period from December 2012 to February 2016. In total, 826 MSTIDs were observed mainly in daytime, thus presenting median values of horizontal wavelength, period, and horizontal phase velocity of 452 ± 107 km, 24 ± 4 min. and 323 ± 81 m/s, respectively. The direction of propagation varies on the season: during the winter (June–August), the waves preferentially propagated to north‐northeast, while in the other seasons the waves propagated to other directions. The anisotropy observed in the MSTID propagation direction could be associated with the region of the gravity wave generation that takes place in the troposphere. We also found that the MSTIDs were observed most frequently during the daytime, between 11 and 15 local time in winter and near to dusk solar terminator (17–19 local time) in the other seasons. Furthermore, the occurrence of MSTIDs was higher in winter. We suggest that atmospheric gravity waves in the thermosphere, mesosphere, and troposphere could play an important role in generating the MSTIDs and the propagation direction may depend on location of the wave sources.
Key Points
The paper presents the statistical study of MSTIDs in equatorial latitudes over south‐southeast of Brazil
The propagation direction of MSTIDs changes according to the seasons
The MSTIDs can be originated by gravity waves that propagate from the lower atmosphere
PDF
