Magnetic Local Time Sector Research Articles

Altitude Distribution of Large and Small‐Scale Equatorial Ionospheric Irregularities Sampled From an Elliptical Low‐Earth Orbit

AbstractWe provide insight into the vertical distribution of multi‐scale scintillation‐inducing irregularities in the low‐latitude ionosphere. In four sets of novel experiments, we sampled altitudes from 330 to 1,280 km in the 18–24 magnetic local time (MLT) sector using the Swarm Echo GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP‐O) GPS receiver with its antenna oriented toward zenith. In order to identify multi‐scale irregularities both above and at the satellite's position, we utilize high‐sample‐rate GAP‐O amplitude and phase measurements along with a measurement of net current on the surface of the imaging and rapid‐scanning ion mass spectrometer sensor on board, which serves as a proxy for density variations. By calculating the rate of change of total electron content index using two sets of GPS parameter choices, we are able to sample irregularities as small as 160 m, which is comparable to or smaller than the Fresnel scale responsible for scintillation‐inducing irregularities. During one campaign, we find that amplitude scintillations on the GPS signal coincide with strong in‐situ small‐scale density irregularities in 32% of cases, indicative of a broad irregularity region extending from the satellite's position to hundreds of kilometers above. Also, we show that large‐scale ionospheric disturbances (larger than 80 km) occur predominantly below 500 km, and down to the 330 km perigee of Swarm Echo in the 18–21 MLT sector. In contrast, small‐scale variations of total electron content are detected at all MLTs between 18 MLT and magnetic midnight and at all altitudes sampled in this experiment. However, they are more frequent in the 22–24 MLT range.

Properties of AKR‐Like Emissions Recorded by the Low Altitude Satellite DEMETER During 6.5 Years

AbstractNormally, auroral kilometric radiation (AKR) which is emitted in the auroral zones escapes from the Earth. But since a few decades very similar radiations are observed by ground‐based receivers and by satellites at altitudes below the AKR generation area. They are called leaked AKR or AKR‐like emissions because it is expected that there are linked to AKR. This paper deals with observations of such AKR‐like emissions observed in the auroral zones (in the North and in the South) by the low‐altitude satellite DEMETER. In total, 2,526 events have been recorded during 6.5 years. These events are not very rare as they occur at least 2% of the time. Although this data set has a severe flaw due to a latitudinal constraint, it was possible to draw interesting properties of these emissions. In fact they are very similar to usual AKR observed at much higher altitudes during auroral activities (the same frequency range, magnetic local time (MLT) sector, and invariant latitude). The main difference concerns a strong asymmetry between the Northern and the Southern hemispheres: (a) the number of AKR‐like emissions in the Northern hemisphere is 32% larger than in the Southern hemisphere but this percentage decreases when the auroral activity increases, and (b) there is an important seasonal effect because the number of events decreases during the winter season both in the North and in the South.

Distribution and Occurrence Frequency of dB/dt Spikes During Magnetic Storms 1980–2020

AbstractThe physical magnetospheric cause for geomagnetically induced currents (GICs) are rapid time‐varying magnetic fields (dB/dt), which occur mainly during magnetic substorms and storms. When, where and why exactly such rapid dB/dt may occur is insufficiently understood. We investigated all storms since 1980 and analyzed the negative and positive dB/dt spikes (>|500| nT/min) in the north and east component using a worldwide coverage (SuperMAG). Our analysis confirmed the existence of two dB/dt spikes “hotspots” located in the pre‐midnight and in the morning magnetic local time sector, independently of the geographic location of the stations. The associated physical phenomena are probably substorm current wedge onsets and westward traveling surges (WTS) in the evening sector, and wave‐ or vortex‐like current flows in the morning sector known as Omega bands. We observed a spatiotemporal evolution of the negative northern dB/dt spikes. The spikes initially occur in the pre‐midnight sector, and then develop in time toward the morning sector. This spatiotemporal sequence is correlated with bursts in the AE index, and can be repeated several times throughout a storm. Finally, we investigated the peak value of Dst and AE during the storm period in comparison with the dB/dt spike occurrence frequency, we did not find any correlation. This result implies that a moderate storm with many spikes can be as (or more) dangerous for ground‐based infrastructures than a major storm with fewer dB/dt spikes. Our findings regarding the physical causes and characteristics of dB/dt spikes may help to improve the GIC forecast for the affected regions.

Statistical Survey of Arase Satellite Data Sets in Conjunction With the Finnish Riometer Network

AbstractDuring disturbed geomagnetic conditions, the energetic particles in the inner magnetosphere are known to undergo precipitation loss due to interaction with various plasma waves. This study, investigates the energetic particle precipitation events statistically using coordinate observations from the ground riometer network and the inner‐magnetospheric satellite mission, Arase. We have compared cosmic noise absorption (CNA) data obtained from the Finnish ground riometer network located in the auroral/sub‐auroral latitudes with the comprehensive data set of omnidirectional electron/proton flux and plasma waves in ELF/VLF frequency range from the Arase satellite during the overpass intervals. The study period includes one and a half years of data between March 2017 and September 2018 covering Arase conjunctions with the riometer stations from all magnetic local time sectors. The relation between the plasma flux/waves observed at the satellite with the riometer absorptions are investigated statistically for CNA (absorption >0.5 dB) and non‐CNA (absorption <0.5 dB) cases separately. During CNA events, Arase observed elevated electron flux in the medium energy range (2–100 keV), and plasma wave activity in the whistler‐mode frequency range (0.5–3 kHz) of the spectra. Our study provides an estimate of the statistical dependence of the electron flux and plasma wave observations at Arase with the ground reality of actual precipitation.

Statistical Study of Electron Density Enhancements in the Ionospheric F Region Associated With Pulsating Auroras

AbstractPulsating auroras (PsAs) are considered to be caused by energetic (>a few keV) electron precipitation. Additionally, soft electron precipitation (<a few keV) has often been observed in PsAs. This soft electron precipitation enhances the electron density in the ionospheric F region. However, to date, the relationship between PsAs and soft electron precipitation has not been well understood. In this study, using the data taken by the European incoherent scatter radar and the auroral all‐sky imager at Tromsø, we conducted two case studies to investigate, in detail, the relationship between the electron density height profile and the type of aurora. Additionally, we conducted statistical studies for 14 events to elucidate how often F region electron density enhancement occurs with a PsA. We consequently found that 76% of electron density height profiles showed a local peak in the F region, with electron temperature enhancements. It was also found that 89% of the F region peak altitudes were above the peak altitude of the ionization rate produced by electrons of characteristic energy below 100 eV. The occurrence rate of these profiles in the hourly magnetic local time (MLT) exceeded 80% in the 22–3 MLT sectors. We suggest that the electron density enhancement in the F region would have been caused by electrostatic electron cyclotron harmonic waves in the magnetosphere. Another candidate would have been polar patches that had traveled from the dayside ionosphere.

Observational Properties of 15MLT‐PCA in the Southern Hemisphere and the Switching Effects of IMF By on 15MLT‐PCA Occurrence

AbstractBased on Special Sensor Ultraviolet Spectrographic Imager on Defense Meteorological Satellite Program spacecraft (DMSP/SSUSI) observations in the Northern Hemisphere, a particular polar cap arc observed at ∼1,500 magnetic local time (MLT) sector was identified and was named 15MLT‐PCA. A 15MLT‐PCA model proposed for the Northern Hemisphere suggests that the 15MLT‐PCA is caused by a lobe reconnection related to the tilt angle and lobe field line twisting due to a long‐lasting positive interplanetary magnetic field (IMF) By condition. However, it is unclear if the 15MLT‐PCA can be observed in the Southern Hemisphere and how the IMF By influences its occurrence and location. Using the long‐term DMSP/SSUSI observations in the Southern Hemisphere, we confirmed that the 15MLT‐PCA could be observed in the Southern Hemisphere in summer under long‐lasting negative IMF By and predominantly positive IMF Bx. The ion energy dispersion and some other statistical properties are also investigated. Further, we showed that the IMF By has a switching effect on the occurrence of the 15MLT‐PCA using multiple cases. We found that the 15MLT‐PCA disappear as soon as the IMF By changed to an unfavorable condition, but it seems that the favorable conditions need to be maintained for ∼1 hr for the occurrence of 15MLT‐PCA. We also found that the IMF By magnitude is well correlated to the MLT location of the 15MLT‐PCA. We suggest that all the results support the 15MLT‐PCA model and imply that a magnetotail lobe reconnection should be common in the summer hemisphere because the well‐known Parker Spiral favors meeting the model's IMF requirements.

Climatological Statistics of Extreme Geomagnetic Fluctuations With Periods From 1 s to 60 min

AbstractUsing a global database of 125 magnetometers covering several decades, we present occurrence statistics for fluctuations of the horizontal geomagnetic field (dBh/dt) exceeding the 99.97th percentile (P99.97) for both ramp changes (Rn) and the root‐mean‐square (Sn) of fluctuations over periods,τ, from 1 to 60 min and describe their variation with geomagnetic latitude and magnetic local time (MLT). Rates of exceedance are explained by reference to the magneto‐ionospheric processes dominant in different latitude and MLT sectors, including ULF waves, interplanetary shocks, auroral substorm currents, and traveling convection vortices. By fitting generalized Pareto tail distributions aboveP99.97, we predict return levels (RLs) forRnandSnover return periods of between 5 and 500 years. P99.97and RLs increase monotonically with frequency (1/τ) (with a few exceptions at auroral latitudes) and this is well modeled by quadratic functions whose coefficients vary smoothly with latitude. For UK magnetometers providing 1‐s cadence measurements, the analysis is extended to cover periods from 1 to 60 s and empirical Magnetotelluric Transfer functions are used to predict percentiles and return levels of the geoelectric field over a wide frequency range (2 × 10−4to 4 × 10−2 Hz) assuming a sinusoidal field fluctuation. These results help identify the principal causes of field fluctuations leading to extreme geomagnetically induced currents (GIC) in ground infrastructure over a range of timescales and they inform the choice of frequency dependence to use withdBh/dtas a GIC proxy.

An Investigation of Auroral E Region Energy Exchange Using Poker Flat Incoherent Scatter Radar Observations During Fall Equinox Conditions

AbstractWe present new results using data collected by the Poker Flat Incoherent Scatter Radar (PFISR) of energy transfer rates, which include the effects from neutral winds in the high latitude E‐region ionosphere‐thermosphere (IT) during Fall 2015. The purpose of our investigation is to understand the magnetic local time (MLT) dependence of the peak energy transfer, which occurs asymmetrically in the morning‐evening (dawn‐dusk) MLT sector. The statistical characteristics of both altitude‐resolved and altitude‐integrated energy transfer rates in the auroral E region local to PFISR during different geomagnetic conditions are quantified. Our analysis shows that the geomagnetic activity level has large impacts on the energy transfer rates. In contrast with previous investigations, we find both the altitude integrated electromagnetic (EM) energy transfer rate and Joule heating rate are larger in the evening sector than in the morning sector during all geomagnetic activity conditions. We also observe a non‐negligible negative EM energy transfer rates below 110 km in the morning sector during active conditions, which is associated with neutral winds during this MLT interval. The statistical results show that the neutral winds tend to increase the Joule heating rate in a narrow altitude range in the morning sector and impact a broader region with respect to altitude and time in the evening sector in the E region under moderate and active conditions. We find that during quiet conditions that the neutral winds have a significant contribution to the Joule heating and contribute up to 75% of the Joule heating. However, during active conditions the enhanced electric fields are a dominant driver of Joule heating, while the neutral wind effects can reduce the Joule heating rates by 25% or more relative to the passive heating rates.

Ion Kappa Distribution Parameters in the Magnetosphere of the Earth at Geocentric Distances Smaller Than 20 RE During Quiet Geomagnetic Conditions

AbstractThe Earth's magnetosphere is mainly a collisionless plasma system with non‐Maxwellian particle distributions, which are often fitted by the kappa function. While the Maxwell distribution function is described by two parameters (density and temperature), the kappa distribution function has three parameters: density, core energy, and the kappa index that characterizes the slope at high energies. In this study, we fitted an ion flux measured by the five Time History of Events and Macroscale Interactions during Substorms satellites in the Earth's magnetosphere during quiet geomagnetic conditions by a single kappa distribution function. The data were constrained to the following regions: from seven Earth Radii (7 ) to the Earth's magnetopause at the dayside, and up to 20 for other magnetic local time sectors. The two‐dimensional spatial distribution of the fitted parameters and their radial dependencies were analyzed. Regions of high and low ‐parameter are selected and their formation is discussed.

The Scalable Plasma Ion Composition and Electron Density (SPICED) Model for Earth's Inner Magnetosphere

AbstractThe plasma mass loading of the terrestrial equatorial inner magnetosphere is a key determinant of the characteristics and propagation of ultra low frequency waves. Electron number density is also an important factor for other types of waves such as chorus, hiss and EMIC waves. In this paper, we use Van Allen Probe data from September 2012 to February 2019 to create average models of electron densities and average ion mass in the plasmasphere and plasmatrough, near the Earth's magnetic equator. These models are combined to provide an estimate of the most probable plasma mass density in the equatorial region. We then use machine learning to form a set of models which are parameterized by the SuperMAG ring current index based on the design of the average models. The resulting set of models are capable of predicting the average ion mass, electron density and plasma mass density in the range and over all magnetic local time sectors during a range of conditions where nT.

A New Ionospheric Index to Investigate Electron Temperature Small-Scale Variations in the Topside Ionosphere

The electron temperature (Te) behavior at small scales (both spatial and temporal) in the topside ionosphere is investigated through in situ observations collected by Langmuir Probes on-board the European Space Agency Swarm satellites from the beginning of 2014 to the end of 2020. Te observations are employed to calculate the Rate Of change of electron TEmperature Index (ROTEI), which represents the standard deviation of the Te time derivative calculated over a window of fixed width. As a consequence, ROTEI provides a description of the small-scale variations of Te along the Swarm satellites orbit. The extension of the dataset and the orbital configuration of the Swarm satellites allowed us to perform a statistical analysis of ROTEI to unveil its mean spatial, diurnal, seasonal, and solar activity variations. The main ROTEI statistical trends are presented and discussed in the light of the current knowledge of the phenomena affecting the distribution and dynamics of the ionospheric plasma, which play a key role in triggering Te small-scale variations. The appearance of unexpected high values of ROTEI at mid and low latitudes for specific magnetic local time sectors is revealed and discussed in association with the presence of Te spikes recorded by Swarm satellites under very specific conditions.

Observational Evidence of the Excitation of Magnetosonic Waves by an He++ Ion Ring Distribution

AbstractWe report plasma wave observations of equatorial magnetosonic waves at integer harmonics of the local gyrofrequency of doubly ionized helium (). The waves were observed by Van Allen Probe A on 08 Feb 2014 when the spacecraft was in the afternoon magnetic local time sector near inside of the plasmasphere. Analysis of the complementary in‐situ energetic ion measurements (1–300 keV) reveals the presence of a helium ion ring distribution centered near 30 keV. Theoretical linear growth rate calculations suggest that the local plasma and field conditions can support the excitation of the magnetosonic waves from the unstable ring distribution. This represents the first report of the generation of magnetosonic equatorial noise via a ring distribution in energetic ions in the near‐Earth space plasma environment.

Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Statistics During Coronal Mass Ejections and Seeding of EMIC Waves

AbstractHere we perform a statistical analysis of low frequency ultra‐low‐frequency (ULF) waves ( mHz ‐ Hz) in the Earth's inner magnetosphere excluding electromagnetic ion cyclotron waves concurrently observed. We use the magnetic field data from the two Van Allen Probes during their first magnetic local time (MLT) revolution that cover the periods of coronal mass ejections. The major results of our analysis are as follows. (a) Spectra of both the transverse and compressional ULF waves are well approximated by the power‐laws in the mHz ‐ Hz frequency range. (b) There are two sources of the low frequency ULF waves: an internal magnetospheric source and an external source outside of the magnetosphere. (c) The average transverse power in the 6–24 h MLT sector dominates that in the 0–6 h sector, whereas the compressional power in the 12–24 h sector dominates that in the 0–12 h sector. (d) The average powers of transverse and compressional ULF waves in the plasmasphere dominate the average powers in the high L shell region of , and there is a deep power minimum in the intermediary region of . (e) The compressional ULF wave power has a maximum in the near equatorial region, whereas the transverse power has a minimum there. (f) A wave energy cascade from low frequency ULF waves into the higher frequency range of EMIC waves ( Hz) supplies the nonthermal seed fluctuations from which EMIC waves can then grow due to instabilities of the energetic magnetospheric ions.

Statistics of Magnetosonic Waves in the Slot Region Observed by Van Allen Probes

AbstractWe perform a statistical analysis of magnetosonic (MS) waves in the slot region based on Van Allen Probes observations from September 2012 to February 2018. Our results demonstrate that the wave occurrence rate increases with enhanced geomagnetic activity and decreasing magnetic latitude, with the presence of strongest slot region MS waves near the geomagnetic equator within the 08–20 magnetic local time sector. Power spectral densities of slot region MS waves also intensify during geomagnetically active times, with the occurrence of the major wave power (>∼10−5 nT2/Hz) below ∼25fcp (where fcp is the proton gyrofrequency) and the peak wave intensity (∼10−3 nT2/Hz) below ∼5fcp at L > ∼2.6. A remarkable gap in the MS wave frequency spectrum is also revealed at <∼15fcp during weak substorm activities (AE 300 nT).

Statistics on Omega Band Properties and Related Geomagnetic Variations

AbstractUsing the list of the omega structures based on the Magnetometers‐Ionospheric Radars‐All‐sky Cameras Large Experiment network (Partamies et al., 2017, https://doi.org/10.5194/angeo-35-1069-2017), we obtained a number of important statistical characteristics describing the surface magnetic field. Based on 438 events, typical magnetic variations associated with the passage of the single omega were obtained. The typical variation, obtained using superposed epoch analysis, is associated with a local bending of the westward electrojet and statistically confirms the distribution of equivalent ionospheric currents obtained in earlier observations of single omegas. It was found that during low and moderate geomagnetic activity, the appearance of the omega structures in the dark morning magnetic local time (MLT) sector results in two times higher than average dB/dt on the ground surface. Also, the velocity, direction of movement, and area of omega structures were calculated. It is shown that faster and bigger omegas produce larger time derivatives of the ground magnetic field. Furthermore, we demonstrate that in the 03–08 MLT sector, superposed magnetic variations for the arbitrary events of very high time derivatives |dB/dt| > 10 nT/s, reveal magnetic signatures similar to omegas. Our findings, together with the results described by Apatenkov et al. (2020, https://doi.org/10.1029/2019gl086677), emphasize the important role of omega structures in the formation of large geomagnetically induced currents.

Ionospheric Electron Density and Conductance Changes in the Auroral Zone During Substorms

AbstractEnhanced precipitation of magnetospheric energetic electrons during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the existing ionospheric models. We use the linear prediction filter (LPF) method to reconstruct the substorm‐related response of electron densities and integral conductances from long‐term ionospheric observations made by the European Incoherent SCATer radar located at Tromsø. To characterize the intensity of substorm dipolarization at a 5 min time step, we use the midlatitude positive bay index. We build response functions (LP filters) as a function of substorm time between T0−1 h and T0 + 4 h (T0 is a substorm onset time) in different magnetic local time (MLT) sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. Systematic and large relative changes are mostly observed in the D‐ and E regions. The duration of the response is about 3 h. It starts and reaches maximum magnitude near midnight, propagating from there toward the east and decaying after passing into the noon‐evening sector. The reliability of LPF results is confirmed by the consistency of D‐region response with independently derived response of the auroral absorption. Whereas strong ionization increases are seen in both E‐ and D‐regions on the nightside, the D‐region response is stronger in the morning‐dayside sector. Such MLT variation corresponds to the drift motion and precipitation of the high‐energy electrons injected in the nightside magnetosphere during substorm dipolarization. The inferred ionization changes result in strong enhancements of integral Hall (and Pedersen) conductance in the nightside auroral zone, where intense auroral currents are known to occur during substorms

FTA: A Feature Tracking Empirical Model of Auroral Precipitation

AbstractThe Feature Tracking of Aurora (FTA) model was constructed using 1.5 years of Polar Ultraviolet Imager data and is based on tracking a cumulative energy grid in 96 magnetic local time (MLT) sectors. The equatorward boundary, poleward boundary, and 19 cumulative energy bins are tracked with the energy flux and the latitudinal position. With AE increasing, the equatorward boundary moves to lower latitudes everywhere, while the poleward boundary moves poleward in the 2300–0300 MLT region and equatorward in other MLT sectors. This results in the aurora getting wider on the nightside and becoming narrower on the dayside. The peak intensity of the aurora in each MLT sector is almost linearly related to AE, with the global peak moving from pre‐midnight to post‐midnight as geomagnetic activity increases. Ratios between the Lyman‐Birge‐Hopfield‐long and ‐short models allow the average energy to be calculated. Predictions from the FTA and two other auroral models were compared to the measurements by the Defense Meteorological Satellite Program Special Sensor Ultraviolet Spectrographic Imagers (SSUSI) on March 17, 2013. Among the three models, the FTA model specified the most confined patterns with the highest energy flux, agreeing with the spatial and temporal evolution of SSUSI measurements better and predicted auroral power (AP) better during higher activity levels (SSUSI AP > 20 GW). The Fuller‐Rowell and Evans (1987) and FTA models specified very similar average energy compared with SSUSI measurements, doing slightly better by ∼1 keV than the OVATION Prime model.

Magnetotail Dipolarizations and Ion Flux Variations During the Main Phase of Magnetic Storms

AbstractNear‐Earth tail dipolarizations are usually associated with fast plasma flows and an increase in energetic particle fluxes. Yet, the role of these dipolarizations in energetic ion flux transport toward the inner magnetosphere and in ring current (including partial ring current) development during the main phase of a geomagnetic storm is not fully understood. We investigated observations from Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes A, D, and E with apogees at X ≈ −13 RE and Los Alamos National Laboratory (LANL) geosynchronous spacecraft in the midnight magnetic local time (MLT) sector (21.5–1.5 h MLT) during the main phases of storms with integrated Dst exceeding 600 nT*hr from the 2010 to 2016 THEMIS tail seasons. We selected 10 storms during which at least one of the THEMIS probes was in the midnight sector during a storm’s main phase and identified 39 dipolarization events with a Bz increase exceeding 10 nT within 500 s during these storms’ main phases. In 21 of the 39 events, an increase in the magnetic field elevation angle Θ estimated from LANL magnetospheric plasma analyzer (MPA) data was detected at geosynchronous orbit (GEO) within ±15‐min‐long time window around dipolarization onset detected by THEMIS. In only 10 of those 21 events, however, did ion fluxes at energies from 50 to 500 keV increase in three or more consecutive energy bins at LANL. Comparisons of ion spectra collected by THEMIS and LANL in these 10 events revealed a similar increase in fluxes at energies E > 20 keV. Our results indicate that a dipolarization that occupies a large portion of the nightside magnetosphere from GEO to ∼12 RE is a necessary but not sufficient condition for energetic ion flux enhancements at GEO. As suggested in previous studies, an increase in the azimuthal electric field (i.e., enhanced earthward magnetic flux transport) is most likely required for an ∼100 keV ion flux increase at GEO.

Relationship Between the Locations of the Midlatitude Trough and Plasmapause Using GNSS‐TEC and Arase Satellite Observation Data

AbstractRelationship between the locations of the midlatitude trough minimum in the ionosphere and plasmapause in the inner magnetosphere has been statistically investigated using global navigation satellite system (GNSS)‐total electron content (TEC) and electron density data obtained from the Arase satellite from March 23, 2017 to May 31, 2020. In this analysis, we identify the midlatitude trough minimum as a minimum value of GNSS‐TEC at subauroral and midlatitude regions, and determine the plasmapause as an electron density decrease by a factor of 5 or more within ΔL < 0.5 in the inner magnetosphere. As a result, the plasmapause does not always coincide with the midlatitude trough minimum in all magnetic local time (MLT) sectors under all geomagnetic conditions. During the geomagnetically quiet periods, the midlatitude trough minimum is located at higher and lower geomagnetic latitudes (GMLATs) of the plasmapause in the MLT ranges of 5–21 and 21–5 h, respectively. This implies that both the features could not be on the same magnetic field line. On the other hand, during the storm main phase, the midlatitude trough minimum and plasmapause move toward a low‐latitude region with day‐night and dawn‐dusk asymmetries and the correlation becomes highest, compared with that under other geomagnetic conditions. Especially, both the features mapped on the ionosphere at a height of 300 km exist near GMLAT in the afternoon‐midnight sectors. This suggests that the midlatitude trough and plasmapause are formed at almost the same location due to an enhanced subauroral polarization stream during the storm main phase.

Statistical Investigation of the Frequency Dependence of the Chorus Source Mechanism of Plasmaspheric Hiss

AbstractWe use data from eight satellites to statistically examine the role of chorus as a potential source of plasmaspheric hiss. We find that the strong equatorial (|λm| < 6°) chorus wave power in the frequency range 50 < f < 200 Hz does not extend to high latitudes in any magnetic local time sector and is unlikely to be the source of the low frequency plasmaspheric hiss in this frequency range. In contrast, strong equatorial chorus wave power in the medium frequency range 200 < f < 2,000 Hz is observed to extend to high latitudes and low altitudes in the pre‐noon sector, consistent with ray tracing modeling from a chorus source and supporting the chorus to hiss generation mechanism. At higher frequencies, chorus may contribute to the weak plasmaspheric hiss seen on the dayside in the frequency range 2,000 < f < 3,000 Hz band but is not responsible for the weak plasmaspheric hiss on the nightside in the frequency range 3,000 < f < 4,000 Hz.