Equatorial noise (EN) emissions are observed inside and outside the plasmapause. EN emissions are referred to as magnetosonic mode waves. Using data from Van Allen Probes and Arase, we found ...conversion from EN emissions to electromagnetic ion cyclotron (EMIC) waves in the plasmasphere and in the topside ionosphere. A low‐frequency part of EN emissions becomes EMIC waves through branch splitting of EN emissions, and the mode conversion from EN to EMIC waves occurs around the frequency of M/Q = 2 (deuteron and/or alpha particles) cyclotron frequency. These processes result in plasmaspheric EMIC waves. We investigated the ion composition ratio by characteristic frequencies of EN emissions and EMIC waves and obtained ion composition ratios. We found that the maximum composition ratio of M/Q = 2 ions is ~10% below 3,000 km. The quantitative estimation of the ion composition will contribute to improving the plasma model of the deep plasmasphere and the topside ionosphere.
Plain Language Summary
Equatorial noise (EN) emissions are whistler mode waves. Using Van Allen Probe and Arase (ERG) plasma wave data, we found that EN emissions propagate toward the Earth and are converted to electromagnetic ion cyclotron (EMIC) waves in the deep plasmasphere and the topside ionosphere. We suggest that minor ions with a mass per charge (M/Q) = 2, that is, deuteron or alpha particles, play an important role in this process. The processes reported here are a new generation process of plasmaspheric EMIC waves. Moreover, we determined the ion composition ratio using characteristics of wave dispersion. We derived the altitude profile of the ion composition ratio and identified the maximum ratio of M/Q = 2 ions of about 10% in the deep plasmasphere.
Key Points
The first measurements of the conversion from equatorial noise to EMIC waves are presented
Existence of M/Q = 2 ions (deuteron or alpha particle) in the deep plasmasphere is essential to cause the conversion
The ion composition ratio is quantitatively estimated in the deep plasmasphere using characteristics of the wave dispersion
Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earth's outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume ...region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large‐amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p‐p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground‐based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA‐POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field‐aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC‐induced loss processes for this event.
Key Points
Compression‐induced EMIC waves were observed across 12 h of local time
EMIC‐triggered emissions appeared during the strongest compression
Intense EMIC waves outside the plasmasphere depleted the radiation belts
A thermospheric wind data set from a Fabry‐Perot interferometer (630 nm) and the ion velocity from a Dynasonde in Tromsø, Norway, was analyzed for nine winter seasons to study the dynamics of the ...thermosphere and F‐region ionosphere at an auroral latitude. This study focused on bifurcation in the zonal component of the neutral wind and ion velocity at midnight and its dependence on the Y component of the interplanetary magnetic field (IMF). Ionospheric plasma convection patterns are evidently imprinted on the thermospheric wind variations as aspects of the westward and eastward accelerations at dusk and late morning, respectively. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may result in the dependence because of anti‐sunward plasma flow distorted in the polar cap.
Plain Language Summary
The ionosphere is partially ionized plasma, but the particle minority of ions plays an important role in controlling dynamics of the thermosphere. Particle collision is the fundamental process for momentum transfer from ionospheric ions to thermospheric neutral particles. The ionospheric plasma flow pattern at high latitudes depends on the direction of the interplanetary magnetic field (IMF), and the pattern may be projected on the thermospheric wind. However, the dependence is not yet well understood. This study derived statistical experimental features regarding the dependence of the thermospheric wind, analyzing data from an optical interferometer (Fabry‐Perot interferometer) and a radio wave technique (Dynasonde) in Tromsø, Norway. The wind pattern around midnight is different from the ionospheric plasma convection, in accordance with the IMF direction. The zonal wind bifurcates immediately before midnight for IMF By < 0, but for By > 0, it inverts gradually into the postmidnight sector. Neutral wind streams, originating from higher latitudes, may cause the dependence because of anti‐sunward plasma flow distortion in the polar cap. In summary, this study concludes that the zonal wind bifurcation at auroral latitudes is caused by the ion velocity bifurcation, and that advection from the polar cap region affects the wind response time to the ion velocity bifurcation.
Key Points
The thermospheric wind from a Fabry‐Perot interferometer (630 nm) and the ionospheric plasma velocity from a Dynasonde were compared
The zonal wind bifurcates immediately before midnight for interplanetary magnetic field By < 0, but for By > 0, it inverts gradually into the postmidnight sector
The wind bifurcation signature is different from the ion velocity bifurcation, probably due to advection from the polar cap region
The present study aims to investigate the influence of the zonal wind velocity on equatorial plasma bubble (EPB) occurrences over Southeast Asia. The observation of the EPB occurrence is obtained ...from the GPS Rate of TEC change index. Meanwhile, the zonal winds were measured using a Fabry‐Perot interferometer located at Kototabang and Chiang Mai stations, and the height of F layer was acquired using an ionosonde at Chumphon station near the magnetic equator. This is the first study to report the influence of zonal wind velocity variation on EPB occurrences with the presence and absence of EPB using GPS data in the Southeast Asian sector. The results illustrated that the average magnitude of zonal wind velocity during the presence of EPB (78 ± 23 m/s) was higher than that of its absence (68 ± 21 m/s). It was observed using long‐term data analyses which led to in‐depth analyses. The analysis of temporal variation of zonal wind variation demonstrated that the zonal winds during EPB were higher in the evening compared to midnight and postmidnight periods from medium to high solar activities. The dependence of zonal wind velocity on EPB over local time was obtained based on the analysis which utilized the data collected during equinox in high solar activity. Besides that, a positive correlation was obtained between the zonal wind velocity and EPB occurrences during pre‐reversal enhancement (PRE) corroborated the effects of zonal wind influence on PRE, and thus EPB occurrences.
Key Points
The zonal wind velocity was continuously monitored using the Fabry‐Perot interferometer during medium to high solar activity levels
Zonal wind velocity tends to be higher during equatorial plasma bubble (EPB) existence
The range of zonal wind velocity during EPB is higher in the evening compared to midnight and postmidnight
Three different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward ...interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high‐latitude field‐aligned currents and reconfiguration of high‐latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high‐frequency radar observations.
Key Points
Three sharp and intense PPEF disturbances under steady southward IMF Bz phase of St. Patricks's Day geomagnetic storm
A sharp and strong westward PPEF disturbance due to sharp duskward to dawnward reversal of IMF By barring any significant change in Bz
SW‐Pdyn and Substorm induced a strong eastward PPEF, equatorial super fountain and quick rejuvenation and symmetric redistribution of EIA
The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a ...superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.
A physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi‐instrument observations, theoretical modeling, and basic principles. According to ...the mechanism, an equatorward neutral wind is required to produce positive ionospheric storms. The mechanical effects of the wind (1) reduce (or stop) the downward diffusion of plasma along the geomagnetic field lines, (2) raise the ionosphere to high altitudes of reduced chemical loss, and hence (3) accumulate the plasma at altitudes near and above the ionospheric peak centered at around ±30° magnetic latitudes. Daytime eastward prompt penetration electric field (PPEF), if it occurs, also shifts the equatorial ionization anomaly crests to higher than normal latitudes, up to approximately ±30° latitudes. The positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning‐noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind. The mechanism agrees with the multi‐instrument observations made during the supergeomagnetic storm of 7–8 November 2004, with 18 h long initial phase (IP) and 10 h long main phase (MP). The observations, which are mainly in the Japanese‐Australian longitudes where the MP onset was in the morning (0600 LT, 2100 UT), show (1) strong positive ionospheric storms (in Ne, Nmax, hmax, Global Positioning System–total electron content (GPS‐TEC), and 630 nm airglow intensity) in both Northern and Southern hemispheres started at the morning (0600 LT) MP onset and lasted for a day, (2) repeated occurrence of strong eastward PPEF events penetrated after the MP onset and superposed with westward electric field started before the MP onset, and (3) storm time equatorward neutral winds (inferred from 1 and 2). Repeated occurrence of an unusually strong F3 layer with large density depletions around the equator was also observed during the morning‐noon MP.
Different types of medium‐scale traveling ionospheric disturbances (MSTIDs) have been observed at Cachoeira Paulista (22.4°S; 45.0°W), Brazil, from June 2013 to December 2015, using airglow OI ...630.0‐nm images. During the period, 58 MSTIDs were identified and classified as follows: dark band MSTIDs (around 10 events) and periodic MSTIDs (48 events). Dark band MSTIDs present phase velocity between 50 and 200 m/s and propagation direction to northwestward. On the other hand, periodic MSTIDs have phase velocity of 50 to 200 m/s, horizontal wavelengths from 80 to 160 km, periods ranging between 5 and 45 min, and propagation directions are mainly north‐northeastward. The wave parameters indicate that periodic MSTIDs have different characteristics when compared to dark band MSTIDs, suggesting that periodic MSTIDs are not generated through the well‐known Perkins and E‐F coupling instability. In addition to it, the present study indicates that the spectral characteristics found in Brazil are different from other regions such as Japan and Indonesia. Therefore, we intend to do the statistics of the wave parameters (wavelength, phase velocity, period, propagation direction, and time occurrence) and investigate the generation mechanisms of periodic MSTIDs at low to middle latitude for the first time. Furthermore, the anisotropy observed in periodic MSTID propagation direction can be explained by different mechanisms.
Key Points
Two distinct types of nighttime MSTIDs have been identified and classified in relation to their morphology and wave characteristics
The anisotropy observed in the dark band and periodic MSTID propagation direction can be explained by different physical mechanisms
We have shown that the gravity waves resulting in MSTIDs can propagate from the troposphere without reaching the critical level
We report on nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) observed at Kototabang, Indonesia (geographic longitude: 100.3°E; geographic latitude: 0.2°S; and geomagnetic latitude: ...10.6°S) during a 7‐year period from October 2002 to October 2009. MSTIDs were observed in 630‐nm nighttime airglow images by using a highly sensitive all‐sky airglow imager at Kototabang. The averages and standard deviations of horizontal phase velocity, period, and horizontal wavelength of MSTIDs observed during the 7 years were 320 ± 170 m/s, 42 ± 11 min, and 790 ± 440 km, respectively. The occurrence rate of the observed MSTIDs decreased with decreasing solar activity. The average horizontal wavelength of MSTIDs increased with decreasing solar activity. Southward MSTIDs were dominant throughout the 7 years of observations. These facts are consistent with the hypothesis that the observed MSTIDs are caused by gravity waves in the thermosphere. Moreover, we compared the propagation directions of the observed MSTIDs with the locations of tropospheric convection activity for the events where gravity waves producing the observed MSTIDs could have existed in the lower atmosphere. Strong tropospheric convection was found within ±30 degrees from the source directions of MSTIDs in 81% of the MSTID events. In such events, gravity waves were possibly generated from deep convection in the troposphere and directly propagated into the thermosphere.
Key Points
7‐year airglow observations at equatorial latitudes
Observed MSTIDs seem to be caused by gravity waves
MSTID propagation directions were correlated to tropospheric convection
Abstract
We present a comprehensive investigation on the propagation characteristics of duskside medium‐scale traveling ionospheric disturbances (MSTIDs) using 630.0‐nm airglow emissions over Tromsø ...(69.6°N, 19.2°E; magnetic latitude: 66.7°N). The unique points of our observation are (1) duskside MSTIDs primarily exhibited eastward motion under quiet conditions but turned to the westward direction associated with geomagnetic disturbances, (2) the westward moving MSTIDs again turned to the eastward direction when the geomagnetic disturbance ceased, (3) the turning of MSTIDs to the westward direction was invariably associated with an increase of the northward component of the magnetic field observed by the local ground‐based magnetometers and with the equatorward expansion of the auroral oval, and (4) the Super Dual Auroral Radar Network convection maps revealed that the location of Tromsø was inside (outside) the duskside convection cell during the time of appearance of westward (eastward) moving MSTIDs. The average eastward and westward velocities of MSTIDs were ~25–80 and ~40–140 m/s, respectively. The Doppler shift measurement of the 630‐nm airglow by a Fabry‐Perot interferometer at Tromsø showed that northeastward winds were predominant during the appearance of eastward moving MSTIDs. These experimental evidences suggest that the oscillatory motion of MSTIDs over high latitudes is driven by the convection electric field. The MSTIDs tend to move eastward under geomagnetically quiet conditions but show westward motion under the influence of convection electric field associated with auroral activities in the duskside of two‐cell convection pattern.
Key Points
Oscillatory motion of MSTIDs over Tromsø in the duskside is driven by high‐latitude convection electric field
The duskside MSTIDs over Tromsø propagate in the eastward direction during quiet geomagnetic conditions
The westward turning of duskside MSTIDs over Tromsø invariably coincides with increase in the local X component of the magnetic field
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