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We use conjugate observations of magnetospheric whistler‐mode waves at frequencies up to 16 kHz by the DEMETER spacecraft (at an altitude of approximately 660 km) and the ground‐based Kannuslehto station in Finland (L≈5.38) $(L\approx 5.38)$ to investigate the wave propagation to the ground and their characteristic spatial scales. For this purpose, we evaluate correlations between the wave intensities measured by the spacecraft and the ground‐based station at various frequencies as a function of their longitudinal and L‐shell separations. Two different approaches are used: (a) direct correlation of wave intensities measured at the same times and (b) correlation of wave intensities within corresponding frequency‐time windows, focusing on the identification of the same frequency‐time wave signatures. We show that the characteristic longitudinal scales of the investigated waves are between about 60° $60{}^{\circ}$ and 90° $90{}^{\circ}$. We further demonstrate that, while the wave intensities measured by DEMETER are generally larger during periods of enhanced geomagnetic activity, wave intensities measured on the ground during increased activity are only slightly larger during the daytime and decrease during the nighttime. Key Points: Overall correlations of whistler‐mode wave intensities at low altitudes (660 km) and on the ground are evaluated for the first timeCharacteristic longitudinal scales of observed waves are approximately between 60 and 90°Nighttime wave intensities measured on the ground are lower during geomagnetically active periods [ABSTRACT FROM AUTHOR]

We evaluate average wave intensities at frequencies up to 10 kHz measured by two ground stations in Canada and two others in Finland at auroral and subauroral latitudes over a full year, as well as by the low‐altitude DEMETER spacecraft during the years 2004–2010. Lightning location and energy data obtained by the World Wide Lightning Location Network, along with geomagnetic activity characterized by the Kp index, are further used. Latitudinal, diurnal, and annual variations are analyzed, and the global intensities measured on the ground and by the spacecraft are systematically compared for the first time. We show that lightning‐generated waves often dominate the measured wave intensities, particularly during the night, in summer, and at higher frequencies. DEMETER observations, supported by ray‐tracing analysis, reveal a significant role of nonducted lightning‐generated whistler propagation between the hemispheres. Finally, the wave intensity response to geomagnetic activity variations is quite different on the ground compared to in space. While spacecraft‐measured wave intensities are considerably larger during periods of enhanced geomagnetic activity, the ground‐based intensities are only sporadically enhanced during geomagnetically active periods. Key Points: A global comparison of wave intensities measured by the DEMETER satellite and the Kannuslehto/PWING ground stations is performedLightning‐generated waves often dominate both DEMETER and ground observations, especially at night, in summer, and at higher frequenciesDaytime intensities measured by DEMETER are larger during active periods, while on the ground they are only sporadically enhanced [ABSTRACT FROM AUTHOR]

High energy resolution DEMETER satellite observations from the Instrument for the Detection of Particle (IDP) are analyzed during an electromagnetic ion cyclotron (EMIC)‐induced electron precipitation event. Analysis of an Interval Pulsation with Diminishing Periods (IPDP)‐type EMIC wave event, using combined satellite observations to correct for incident proton contamination, detected an energy precipitation spectrum ranging from ∼150 keV to ∼1.5 MeV. While inconsistent with many theoretical predictions of >1 MeV EMIC‐induced electron precipitation, the finding is consistent with an increasing number of experimentally observed events detected using lower resolution integral channel measurements on the POES, FIREBIRD, and ELFIN satellites. Revised and improved DEMETER differential energy fluxes, after correction for incident proton contamination shows that they agree to within 40% in peak flux magnitude, and 85 keV (within 40%) for the energy at which the peak occurred as calculated from POES integral channel electron precipitation measurements. This work shows that a subset of EMIC waves found close to the plasmapause, that is, IPDP‐type rising tone events, can produce electron precipitation with peak energies substantially below 1 MeV. The rising tone features of IPDP EMIC waves, along with the association with the high cold plasma density regime, and the rapidly varying electron density gradients of the plasmapause may be an important factor in the generation of such low energy precipitation, co‐incident with a high energy tail. Our work highlights the importance of undertaking proton contamination correction when using the high‐resolution DEMETER particle measurements to investigate EMIC‐driven electron precipitation. Plain Language Summary: Energetic electrons are lost rapidly from the outer radiation belt. Several processes are thought to drive the electron losses. One process is through interactions with electromagnetic ion cyclotron (EMIC) waves. Theoretical studies suggest that electrons primarily with energy >1 MeV are lost through this process, however, previous experimental satellite observations indicate that precipitation bursts with much lower electron energies are more common. One issue is that the previous satellite observations were made with poor energy resolution and are challenging to interpret due to coincident proton precipitation, which contaminate the electron measurements. Here we use observations from the DEMETER satellite which we have corrected for proton contamination. The measurements, made with higher energy resolution than before, confirm that indeed, low energy electron precipitation can happen when EMIC waves drive electron losses. The study finds that this lower energy characteristic is likely to be driven by a small subset of rising tone EMIC waves, known as Interval Pulsation with Diminishing Periods (IPDP), typically confined to the magnetic local time evening sector. Key Points: An electromagnetic ion cyclotron wave event (an interval of pulsations with diminishing period, IPDP) was studied from Low Earth OrbitCo‐incident satellite observations detected IPDP‐induced energetic electron precipitation, starting at 150 keV, peaking at 215 keVHigh‐resolution measurements from the DEMETER satellite show enhanced fluxes from 215 keV to 1.5 MeV exhibiting a "hard" power‐law spectrum [ABSTRACT FROM AUTHOR]

Electromagnetic waves observed in the inner magnetosphere at frequencies between about 0.5 and 4 kHz sometimes exhibit a quasiperiodic (QP) time modulation of the wave intensity with modulation periods from tens of seconds up to a few minutes. Such waves are typically termed "QP emissions" and their origin is still not fully understood. We use a large set of more than 2,000 of these events identified in the low‐altitude DEMETER spacecraft data to check for energetic electron flux variations matching the individual QP wave elements. Altogether, seven such events are identified and their detailed analysis is performed. Energetic electron fluxes are found to be modulated primarily at energies lower than about 250 keV. While the waves may propagate unducted across L‐shells, the energetic particles follow magnetic field lines from the interaction region down to the observation point. This is used to estimate the locations of anticipated generation regions to L‐shells between about 4 and 6, and the respective source radial dimensions to about 0.6–1.2 Earth radii. The frequencies of the events are confined below half of the equatorial electron gyrofrequency in the determined source regions. Finally, it is shown that individual QP elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres. Key Points: Energetic electron precipitation bursts corresponding to quasiperiodic emission peaks are identifiedInteraction regions occur at L‐shells between about 4 and 6 and have dimensions of about 0.6–1.2 Earth radiiIndividual wave elements exhibit a fine inner structure corresponding to the wave bouncing between the hemispheres [ABSTRACT FROM AUTHOR]

We constructed a program, we call "Iriatten," to calculate using parameters from the International Reference Ionosphere, attenuation of Very Low Frequency (VLF) radiation transiting the ionosphere. The calculations are performed as function of latitude, longitude, year, month, day, and hour and based upon the quasi‐longitudinal approximation put forth by Helliwell, where the wave vector k, is in the direction of propagation, and follows the Earth's magnetic field lines. The best values of various parameters are taken from the literature. Attenuation occurs through electron neutral collisions. In his seminal 1965 text, Helliwell presented atmospheric attenuation curves for day and night, 2 and 20 kHz, as a function of latitude. Program Iriatten is used to re‐calculate Helliwell's curves and then to fully exercise access to seasonality. High power VLF transmitters are utilized as a reference signal and two physical models are used to represent them. In the Vertically Inclined model, the attenuation is assumed to occur "straight up" from the location of the VLF transmitter. In the Crary model waves are launched from a dipole source and ionospheric attenuation is applied where the wave is turned up in the Earth‐ionospheric waveguide. Both representations reveal a seasonality that is unmistakable. Using the two models the ionospheric attenuation is calculated for four VLF transmitters and compared to observations by the DEMETER. Results from the Crary transmitter model more realistically portray seasonal variations exemplified by DEMETER satellite data. Crary model errors, which are within 11 dB, are lower than or comparable to Vertically Inclined errors. Key Points: There is a seasonality in VLF attenuation for waves transiting the ionosphereCrary model most accurately portrays the seasonality, as verified by DEMETER DataCrary model errors, within 11 dB, are lower than, or comparable to Vertically Inclined model errors [ABSTRACT FROM AUTHOR]

Quasiperiodic emissions are magnetospheric whistler mode waves at frequencies between about 0.5 and 4 kHz which exhibit a nearly periodic time modulation of the wave intensity. We use large data sets of events observed by the Van Allen Probes in the equatorial region at larger radial distances and by the low‐altitude DEMETER spacecraft. While Van Allen Probes observe the events at all local times and longitudes, DEMETER observations are limited nearly exclusively to the daytime and significantly less frequent at the longitudes of the South Atlantic Anomaly. Further, while the events observed by Van Allen Probes are smoothly distributed over seasons with only mild maxima in spring/autumn, DEMETER occurrence rate has a single pronounced minimum in July. The apparent inconsistency is explained by considering a nondipolar Earth's magnetic field and significant background wave intensities which in these cases prevent the quasiperiodic events from being identified in DEMETER data. Key Points: Van Allen Probes observe the quasiperiodic events at all local times and longitudes, but more often during spring and autumn seasonsDEMETER observes the events nearly exclusively during the day, with a significant longitudinal variation and minimum occurrence in JulyThe difference is explained by nondipolar Earth's magnetic field and by background wave intensities complicating DEMETER observations [ABSTRACT FROM AUTHOR]

The feasibility of detection of electromagnetic response in the upper ionosphere to ground large‐scale extremely low frequency (ELF) transmitters (e.g., submarine communication systems) by low‐orbiting satellites is discussed. Several times when the DEMETER satellite (660 km) was in the vicinity of the ELF transmitter on the Kola Peninsula, the electric and magnetic sensors operating in a burst mode detected a narrowband 82‐Hz emission. The same emission associated with the ELF transmitter was observed by a ground‐based magnetometer. We modeled the rate of the ELF wave energy leakage into the upper ionosphere from an oscillating 82‐Hz linear current with an infinite length suspended above a high‐resistive ground. A realistic altitudinal profile of the plasma parameters has been reconstructed with the use of the IRI ionospheric model. The modeled amplitudes and polarization of electromagnetic response of the upper ionosphere are in reasonable agreement with the properties of emission recorded by the satellite. Key Points: An 82‐Hz emission from ground ELF transmitter was recorded in the upper ionosphere by satellite DEMETERSpectral power of electric and magnetic components in the nightside ionosphere is about several nT2/Hz and tens of (μV/m)2/HzModeling the electromagnetic response to oscillating line current above a high‐resistive crust matches the observational result [ABSTRACT FROM AUTHOR]

We present results of numerical simulation of quasiperiodic (QP) extra low frequency/very low frequency emissions performed by using a theoretical model of flow cyclotron maser based on a self‐consistent set of equations of the quasi‐linear plasma theory averaged over oscillations of waves and particles in a geomagnetic flux tube. Calculations were made for a wide range of plasma parameters (i.e., cold plasma density, L‐shell, and energetic electron flux) in order to obtain a statistical relationship between various properties of QP emissions, such as the repetition period, the frequency bandwidth, the frequency drift rate, and the characteristic wave spectral energy density. The theoretical results are compared with the results of a statistical study of QP emissions measured by the DEMETER spacecraft (Hayosh et al., 2014, https://doi.org/10.1002/2013JA019731). The simulation results are in a good agreement with the observation data in the case of reasonable choice of cold plasma density value and its dependence on the QP‐source location (L‐shell). In particular, an increase in the frequency bandwidth of QP very low frequency waves with increasing central frequency of QP emissions, a decrease in the frequency drift rate of QP elements with increasing repetition period, and a decrease in the characteristic wave spectral energy density with increasing repetition period are confirmed. Key Points: Numerical simulation of QP emissions for a wide range of plasma parameters is performedTheoretical results are in a good agreement with DEMETER spacecraft observationsA realistic choice of radial profile of cold plasma density is important for this agreement [ABSTRACT FROM AUTHOR]

Very low frequency electromagnetic waves in the inner magnetosphere sometimes exhibit either frequency or time modulation. These phenomena are called, respectively, magnetospheric line radiation (MLR) and quasiperiodic (QP) emissions. Data from Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft were used to analyze their properties, such as MLR frequency spacing, QP modulation period, and QP intensity as functions of geomagnetic activity and solar wind parameters. Altogether, 1,152 MLR events and 2,172 QP emissions were analyzed. It is shown that the influence of the analyzed parameters on QP emissions is different for QP events with modulation periods shorter/longer than 20 s. While the properties of QP events with long modulation periods are significantly related to the geomagnetic activity and solar wind parameters, no such dependences are observed for events with short modulation periods. This suggests that there might be two types of QP emissions generated by two different mechanisms. It is further shown that there seems to be no relation between the properties of QP and MLR events observed at the same times. Finally, the event properties do not seem to be related to the whistler occurrence rate. Key Points: Dependences obtained for quasiperiodic emissions with short/long modulation periods are differentProperties of quasiperiodic emissions and magnetospheric line radiation observed at the same times are not relatedEvent properties are not affected by the whistler occurrence [ABSTRACT FROM AUTHOR]

Frequency‐time spectrograms measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft (2004–2010, altitude about 700 km) at frequencies below 20 kHz sometimes contain lightning‐generated whistlers whose intensity is significantly reduced at specific frequencies. The frequencies of the reduced intensity vary smoothly over the event duration, forming apparent curves in frequency‐time spectrograms. Events at frequencies higher than the Earth‐ionosphere waveguide cutoff frequency were explained by the interference of the first few waveguide modes of lightning‐generated spherics propagating therein (Záhlava et al., 2015, https://doi.org/10.1002/2015JA021607. Here we present an analysis of events observed at frequencies lower than about 1 kHz. Altogether, we analyze 263 events identified at the times when DEMETER operated in the burst mode. The vast majority of the events (95%) took place during the nighttime, and they occurred more frequently during spring/autumn than during winter/summer. We present an overview of event properties. Moreover, measurements of all six electromagnetic field components performed by DEMETER allow us to perform a detailed wave analysis. It is shown that the emissions propagate with high wave normal angles inclined toward the Earth. We suggest that the events might be due to the wave propagation in the ionospheric waveguide formed around the refractive index maximum at the altitude of about 105 km. Key Points: We analyze ELF events formed by lightning‐generated whistlers with intensity significantly reduced at specific frequenciesWe perform a study of 263 events identified in the DEMETER spacecraft data, including their detailed wave analysisWe suggest that the events might be created in the waveguide formed around the refractive index maximum in the lower ionosphere [ABSTRACT FROM AUTHOR]

U‐shaped spectrograms observed by the DEMETER satellite in the equatorial region of the upper ionosphere are presented and explained for the first time. These spectra are characterized by the latitude‐dependent upper cutoff frequency that increases with the latitude during nighttime. The observations suggest that this very low frequency wave phenomenon is closely connected with the anomalous increase in cold plasma density at and below the satellite. We show that restricting the analysis to propagation effects is insufficient to explain U‐shaped spectra, and other effects, in particular, the collisional wave damping should be taken into consideration. To calculate the wave damping, we use the method of successive approximations. In the first approximation, we calculate the ray trajectory and the wave normal vector along it, using the equations of geometrical optics neglecting collisions. In the second approximation we calculate the collisional wave damping along the precalculated trajectory. We show that the effects of very low frequency wave propagation and attenuation in the equatorial region of the upper ionosphere explain the main features of the phenomenon under discussion. Key Points: U‐shaped spectrum registered in the equatorial region of the upper ionosphere during nighttime is formed by lightning‐induced whistler wavesLatitudinal dependence of the upper cutoff frequency is determined by the peculiarities of oblique wave propagation and attenuationThe wave attenuation can be calculated along the trajectories found from the equations of geometrical optics that disregard collisions [ABSTRACT FROM AUTHOR]

We use the measurements performed by the DEMETER (2004–010) and the Van Allen Probes (2012–016, still operating) spacecraft to investigate the longitudinal dependence of the intensity of whistler mode waves in the Earth's inner magnetosphere. We show that a significant longitudinal dependence is observed inside the plasmasphere on the nightside, primarily in the frequency range 400 Hz to 2 kHz. On the other hand, almost no longitudinal dependence is observed on the dayside. The obtained results are compared to the lightning occurrence rate provided by the Optical Transient Detector/Lightning Imaging Sensor mission normalized by a factor accounting for the ionospheric attenuation. The agreement between the two dependencies indicates that lightning‐generated electromagnetic waves may be responsible for of the observed effect, thus substantially affecting the overall wave intensity in the given frequency range. Finally, we show that the longitudinal dependence is most pronounced for waves with oblique wave of normal angles. Key Points: Longitudinal dependence of whistler mode waves in the inner magnetosphere is investigatedSignificant longitudinal dependence at frequencies between about 0.4 and 2 kHz is found inside the plasmasphereThe dependence is primarily observed on the nightside and is likely related to lightning activity [ABSTRACT FROM AUTHOR]

Abstract: We investigate the influence of lightning‐generated whistlers on the overall intensity of electromagnetic waves measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft (2004–2010, quasi Sun‐synchronous polar orbit with an altitude of about 700 km) at frequencies below 18 kHz. Whistler occurrence rate evaluated using an onboard neural network designed for automated whistler detection is used to distinguish periods of high and low whistler occurrence rates. It is shown that especially during the night and particularly in the frequency‐geomagnetic latitude intervals with a low average wave intensity, contribution of lightning‐generated whistlers to the overall wave intensity is significant. At frequencies below 1 kHz, where all six electromagnetic wave components were measured during specific intervals, the study is accompanied by analysis of wave propagation directions. When we limit the analysis only to fractional‐hop whistlers, which propagate away from the Earth, we find a reasonable agreement with results obtained from the whole data set. This also confirms the validity of the whistler occurrence rate analysis at higher frequencies. [ABSTRACT FROM AUTHOR]

Abstract: This paper concurrently investigates the midlatitude trough and the plasmapause positions in the ionosphere by colocated measurements of the electron density, electron temperature, and whistler count probed by DEMETER (Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions) satellite in the nighttime at 2230 LT (local time; 1900–0100 MLT, magnetic local time, mainly in the premidnight period) during the 4‐year period of 2006–2009. More than 13,000 Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions orbits of the electron density and the electron temperature are used to search the trough position, while the same amount of the whistler count is employed to determine the plasmapause position at the satellite altitude. The plasmapause is more sensitive to solar activity, which moves equatorward 1.0–1.2° form the low to high solar activity of the study period. On the other hand, the midlatitude trough is more sensitive to seasonal variation, which shifts poleward 1.7–2.5° from the winter to summer month in the study period. The midlatitude trough usually appears in the poleward side of the plasmapause during the study period. Both of the midlatitude trough and the plasmapause move equatorward during the magnetic disturbed condition. For the magnetic disturbed Kp ≥ 6−, the midlatitude trough can appear in the equatorward side of the plasmapause. [ABSTRACT FROM AUTHOR]

Electromagnetic wave data measured by the low-altitude Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft sometimes exhibit intense emissions at discrete, harmonically spaced frequencies. We present a comprehensive analysis of this type of phenomenon, and we demonstrate that it is caused by very low frequency (VLF) transmitters in Europe. All available data were investigated for a possible presence of the events. Altogether, 87 events were identified, occurring exclusively during the nighttime. The occurrence of the events does not show any clear relation to the A E and D s t geomagnetic activity indices. The events are clearly localized, occurring either close to Great Britain or in the vicinity of its geomagnetically conjugated region. The frequency spectra of the events typically consist of up to four spectral peaks at multiples of about 1.3 kHz. Additional weaker spectral peaks at frequencies of about 1.9 kHz and 3.2 kHz are occasionally present. The events observed in the Northern Hemisphere tend to have larger-frequency bandwidths than the events observed in the Southern Hemisphere. The occurrence of these events is correlated with the simultaneous detection of signals from VLF transmitters. A bicoherence analysis is employed to demonstrate that wave-wave coupling takes place. Finally, it is shown that the occurrence of the events is associated with a significantly increased precipitation of energetic electrons in a wide range of energies. [ABSTRACT FROM AUTHOR]