Jaroslav Chum

Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss

We interpret observations of low-altitude electromagnetic ELF hiss observed on the dayside at subauroral latitudes. A divergent propagation pattern has been reported between 50° and 75° of geomagnetic latitude. The waves propagate with downward directed wave vectors which are slightly equatorward inclined at lower magnetic latitudes and slightly poleward inclined at higher latitudes. Reverse ray tracing using different plasma density models indicates a possible source region near the geomagnetic equator at a radial distance between 5 and 7 Earth radii by a mechanism acting on highly oblique wave vectors near the local Gendrin angle. We analyze waveforms received at altitudes of 700–1200 km by the Freja and DEMETER spacecraft and we find that low-altitude ELF hiss contains discrete time-frequency structures resembling wave packets of whistler mode chorus. Emissions of chorus also predominantly occur on the dawnside and dayside and have recently been considered as a possible source of highly accelerated electrons in the outer Van Allen radiation belt. Detailed measurements of the Cluster spacecraft at radial distances of 4–5 Earth radii show chorus propagating downward from the source region localized close to the equator. The time-frequency structure and frequencies of chorus observed by Cluster along the reverse raypaths of ELF hiss are consistent with the hypothesis that the frequently observed dayside ELF hiss is just the low-altitude manifestation of natural magnetospheric emissions of whistler mode chorus.

Statistics of multispacecraft observations of chorus dispersion and source location

We report emission characteristics of 52 chorus events on 23 August 2003 and10 events on three other days, modeled with a ray tracing technique. Chorus waves have acharacteristic frequency/time variation that is a combination of frequency separation bypropagation dispersion and a time-dependent source frequency emission drift. A cross-correlation technique comparing data from multiple Cluster spacecraft quantifies thefrequency variation owing to propagation dispersion. The comparison of the datacross correlations with the simulated cross correlations allows the identification of acorrelation region which has at least one common point with the chorus source region.Any remaining frequency/time variation in the single-spacecraft spectrograms notaccounted for by the cross correlations is then used to determine the time-dependentsource frequency emission drift. The final modeled correlation region and sourcefrequency emission drift for each chorus event is consistent with both the cross-correlationand single-spacecraft data. The modeled correlation regions are located near the magneticequator and are, in general, more extended parallel to the Earth’s magnetic field thanperpendicular to it. It is found that waves with frequencies above and below 1/2 theequatorial electron cyclotron frequency on the magnetic field line of the spacecraft (lowerand upper band, respectively) are emitted in a broad spectrum of wave normal angles.There is also some preference for lower band waves observed at the spacecraft tohave been emitted near the Gendrin angle and at earthward-pointing wave normal anglesof between 20 and 30 . The latter result is close to the range of wave normal anglesshown recently to be connected with chorus that propagates into the plasmasphereand evolves into the incoherent plasmaspheric hiss spectrum, known to be connectedto pitch angle scattering and loss of electrons in the electron slot region. Finally, thetime-dependent source frequency emission drift for these eventsranges from 1to20 kHz/s.For most events these rates account for at least 2/3 of the chorus frequency/time variationwith the rest being due to propagation dispersion.

Propagation of unducted whistlers from their source lightning: A case study

(1) We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3-D electric and magnetic field waveforms of fractional-hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground-based station located in Nancay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray-tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100-900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional-hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.

The Origin of Plasmaspheric Hiss

Observations by the THEMIS spacecraft are providing a better picture of the electromagnetic environment surrounding Earth. The role of electromagnetic waves in shaping the space environment around our planet has been studied since the early 1960s (1, 2). Initial analysis of these waves at audible frequencies consisted of playing the recorded data through a loudspeaker. The historical terminology in this field thus resembles an experimental musical score where we can encounter whistlers, noise, hiss, and chorus. On page 775 of this issue, Bortnik et al. (3) invite us to this world of “space sound.” On the basis of measurements by NASAs THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft mission, the authors describe two types of natural electromagnetic waves: chorus and plasmaspheric hiss. They show that plasmaspheric hiss can be interpreted as arising from transformed chorus waves, thus providing important clues as to its origin.

The vertical propagation of disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake over Taiwan

In this paper, concurrent/colocated measurements of seismometers, infrasonic systems, magnetometers, HF-CW (high frequency-continuous wave) Doppler sounding systems, and GPS receivers are employed to detect disturbances triggered by seismic waves of the 11 March 2011 M9.0 Tohoku earthquake. No time delay between colocated infrasonic (i.e., super long acoustic) waves and seismic waves indicates that the triggered acoustic and/or gravity waves in the atmosphere (or seismo-traveling atmospheric disturbances, STADs) near the Earths surface can be immediately activated by vertical ground motions. The circle method is used to find the origin and compute the observed horizontal traveling speed of the triggered infrasonic waves. The speed of about 3.3 km/s computed from the arrival time versus the epicentral distance suggests that the infrasonic waves (i.e., STADs) are mainly induced by the Rayleigh waves. The agreements in the travel time at various heights between the observation and theoretical calculation suggest that the STADs triggered by the vertical motion of ground surface caused by the Tohoku earthquake traveled vertically from the ground to the ionosphere with speed of the sound in the atmosphere over Taiwan.

Propagation of gravity waves and spread F in the low-latitude ionosphere over Tucumán, Argentina, by continuous Doppler sounding: First results

Results of systematic analysis of propagation directions and horizontal velocities of gravity waves (GWs) and spread F structures in low-latitude ionosphere (magnetic inclination ~27°) in Tucuman region, Argentina, are presented. Measurements were carried out by multipoint continuous Doppler system during 1 year from December 2012 to November 2013. It was found that meridian propagation of GWs dominated and that southward propagation prevailed in the local summer. Oblique spread structures observed in Doppler shift spectrograms and associated with spread F propagated roughly eastward at velocities from ~70 to ~180 m/s and were observed at night from ~ September to ~ March. The velocities were computed for 182 events and the azimuths for 64 events. Continuous Doppler sounding makes it possible to analyze more events compared to optical observations often used for propagation studies since the measurements do not depend on weather.

Spectral features of lightning‐induced ion cyclotron waves at low latitudes: DEMETER observations and simulation

[1] We use a comprehensive analysis of 6-component ELF wave data from the DEMETER satellite to study proton whistlers, placing emphasis on low-latitude events originating from lightning strokes in the hemisphere opposite to the hemisphere of observation. In this case, the formation of proton whistlers does not involve mode conversion caused by a strong mode coupling at a crossover frequency, although a polarization reversal remains an important element in formation of the phenomenon. DEMETER measurements of the six electromagnetic field components in the frequency band below 1000 Hz make it possible to determine not only the dynamic spectrum, but also the wave polarization, the wave normal angle, and the normalized parallel component of the Poynting vector. This permits us to address fine features of proton whistlers, in particular, we show that the deviation of the upper cutoff frequency from the equatorial cyclotron frequency is related to the Doppler shift. Experimental study of proton whistlers is supplemented by an investigation of ion cyclotron wave propagation in a multicomponent magnetoplasma and by numerical modeling of spectrograms, both in the frame of geometrical optics.

Propagation Spectrograms of Whistler-Mode Radiation from Lightning

We show images of propagation spectrograms based on spacecraft observations of electromagnetic radiation of lightning strokes. We use multicomponent data of the DEMETER spacecraft, and we match observations of fractional-hop whistler signals with the source lightning strokes in the Mediterranean region. We perform multidimensional analysis of the spacecraft data, and we obtain detailed information on wave polarization characteristics and propagation directions.

Toward an Improved Representation of Middle Atmospheric Dynamics Thanks to the ARISE Project

This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions.

Nonlinear acoustic waves in the viscous thermosphere and ionosphere above earthquake

The nonlinear behavior of acoustic waves and their dissipation in the upper atmosphere is studied on the example of infrasound waves generated by vertical motion of the ground surface during the Mw 8.3 earthquake that occurred about 46 km from Illapel, Chile on September 16, 2015. To conserve energy, the amplitude of infrasound waves initially increased as the waves propagated upwards to the rarefied air. When the velocities of air particles became comparable with the local sound speed, the nonlinear effects started to play an important role. Consequently, the shape of waveform changed significantly with increasing height, and the original wave packet transformed to the “N-shaped” pulse resembling a shock wave. A unique observation by the continuous Doppler sounder at the altitude of about 195 km is in a good agreement with full wave numerical simulation that uses as an input the measured vertical motion of the ground surface.