Ondřej Santolík

Propagation of lower‐band whistler‐mode waves in the outer Van Allen belt: Systematic analysis of 11 years of multi‐component data from the Cluster spacecraft

Lower-band whistler-mode emissions can influence the dynamics of the outer Van Allen radiation belts. We use 11 years of measurements of the STAFF-SA instruments onboard the four Cluster spacecraft to systematically build maps of wave propagation parameters as a function of position. We determine probability distributions of wave vector angle weighted by the wave intensity. The results show that wave vector directions of intense waves are close to a Gaussian-shaped peak centered on the local magnetic field line. The width of this peak is between 10 and 20 degrees. The cumulative percentage of oblique waves is below 10–15%. This result is especially significant for an important class of whistler-mode emissions of lower-band chorus at higher latitudes, well outside their source region, where a simple ray tracing model fails and another mechanism is necessary to keep the wave vectors close to the field-aligned direction.

Case studies on the wave propagation and polarization of ELF emissions observed by Freja around the local proton gyrofrequency

Using a multicomponent measurement of electric and magnetic fields in the ELF range, we investigate the polarization and propagation characteristics of several types of electromagnetic emissions around the local proton gyrofrequency. The data of the low-orbiting Freja satellite are examined in the auroral and subauroral regions as well as inside the plasmasphere. We confirm previous results on the properties of the high-latitude hiss with a sharp lower cutoff just below the local proton gyrofrequency. The waves propagate downward from higher altitudes outside the plasmasphere and reflect at the local two-ion cutoff frequency. With an enhanced data representation we are able to characterize the reflected upgoing waves on both poleward and equatorward edges of the hiss emission. The high-latitude emissions in a large band below the local proton gyrofrequency contain right-hand circularly polarized waves propagating nearly parallel to the terrestrial magnetic field. We believe these emissions can originate by tunnelling of the right-hand-polarized hiss below the two-ion crossover frequency. We report high-latitude “frequency drift phenomena” below the local proton gyrofrequency. The frequency of these faint emissions increases by several tens of hertz per latitude degree. The electromagnetic noise at low frequencies is always found with a high degree of a nearly linear polarization. Upgoing ion whistlers are observed to follow each other with time delays as low as about 100 ms.

Application of wave distribution function methods to an ELF hiss event at high latitudes

Multicomponent waveform data are used to calculate wave polarization and propagation characteristics, with both simple plane wave methods and more general wave distribution function (WDF) techniques. The WDF techniques allow us to estimate a continuous distribution of wave energy with respect to the wave vector directions. We describe several newly developed analysis methods, and we compare their results with existing techniques. This comparison allows us to reach a common interpretation of propagation properties and to exclude possible artifacts. The methods are applied to an electromagnetic ELF hiss event observed by the low-altitude satellite Freja. A part of the emission can be described by simple plane wave methods. WDF techniques are, however, needed to fully characterize the wave propagation when the components of the wave magnetic field vector are not mutually coherent and the polarization degree is low. In this case we observe large peaks on the WDF shape or simultaneous propagation of downgoing and reflected upgoing waves. The WDF techniques also confirm that experimental polarization properties are consistent with their predictions by the cold plasma theory.

Propagation analysis of electromagnetic waves between the helium and proton gyrofrequencies in the low-altitude auroral zone

We present a statistical study of electromagnetic emissions with a lower frequency limitation observed in a broadband below the local proton gyrofrequency. An extensive data set recorded by the low-orbiting Freja satellite is examined. The emissions occur outside the plasmasphere, mainly at auroral latitudes. The maximum wave power is observed before the local noon, and no events have been detected during the local night. In the majority of cases we can assume the presence of a single plane wave. Further analysis shows a nearly circular and right-handed polarization. The statistics of wave vector directions reveal a downward and nearly field aligned propagation. An experimental estimation of the phase velocity modulus is in rough agreement with the cold plasma theory. The origin of the emissions could be possibly explained by the tunnelling of the downgoing ELF hiss below the two-ion cross-over frequency.

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.

Properties of the unusually short pulse sequences occurring prior to the first strokes of negative cloud‐to‐ground lightning flashes

We analyze broadband magnetic and electric field measurements of pulse sequences occurring prior to first return strokes of negative cloud-to-ground lightning flashes. Our observations take place a few tens of kilometers from the thunderstorm but we also report the first simultaneous observations of preliminary breakdown pulses from a distance of 400 km. Their amplitudes reach up to 50% of the corresponding return stroke peak and typically decrease by ~40% during the sequences. A typical time interval between neighboring pulses was several tens of microseconds. We observe an unusually short duration of the prestroke activity (1–7 ms) reported for the first time during a summer thunderstorm, with a low height of initiation (3-4 km). A very fast propagation speed (~106 m/s) is probably maintained for the entire prestroke process. A possible explanation can be based on a hypothesis of unusually strong negative charge sources in the observed thundercloud.

Propagation properties of quasiperiodic VLF emissions observed by the DEMETER spacecraft

Quasiperiodic (QP) emissions are electromagnetic waves in the frequency range of about 0.5–4 kHz observed in the inner magnetosphere that exhibit a periodic time modulation of the wave intensity, with modulation periods from a few seconds up to 10 min. We present results of a detailed wave analysis of nearly 200 events measured by the low-altitude Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft. Upper frequency range of studied emissions was limited to 1 kHz due to the sampling rate of the analyzed data. It is found that QP emissions propagate nearly field aligned at larger geomagnetic latitudes; they become more oblique at midlatitudes and eventually perpendicular to the ambient magnetic field at the geomagnetic equator and thus perpendicular to the Earths surface, allowing their downward propagation through the ionosphere. The observed propagation pattern is consistent with the source of emissions located in the equatorial region at larger radial distances.

Statistical analysis of VLF radio emissions triggered by power line harmonic radiation and observed by the low-altitude satellite DEMETER

DEMETER was a low-altitude satellite in operation between 2004 and 2010 in a circular polar orbit. One of its main scientific objectives was to study ionospheric perturbations related to man-made activity. This paper investigates electromagnetic emissions triggered by Power Line Harmonic Radiation (PLHR), the man-made waves emitted at harmonics of 50 or 60 Hz. They look like rising tones or hooks with a starting frequency associated to a parent line with the frequency equal to a multiple of 50 or 60 Hz. They occur preferentially during daytime in a frequency band between 1 and 4 kHz. It is shown that these emissions are rather frequent at high latitudes (3 < L <6) above industrialized areas during periods of moderate magnetic activity. Their average intensity is of the order of 10 μV 2 m A2 Hz A1. PLHR propagates in the magnetosphere and triggers emissions due to wave-particle interactions in the equatorial region.

VESPA: A community-driven Virtual Observatory in Planetary Science

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science.

Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud‐to‐ground lightning discharges

We analyze broadband electromagnetic measurements of pulse sequences occurring prior to first return strokes of negative cloud-to-ground lightning flashes. Signals generated by lightning discharges were recorded close to the thunderstorm by a magnetic field receiver and traveled up to 600 km to three distant electric field receivers. We found that amplitudes of observed preliminary breakdown pulses, as well as amplitudes of the corresponding return strokes, are attenuated approximately by 2 dB/100 km when propagating in the Earth-ionosphere waveguide over mountainous terrain. Propagation simulations show that there is a significant contribution of the sky wave signals in the waveforms observed beyond 500 km from their source. The estimated peak currents of the largest preliminary breakdown pulses reach over 60 kA. Such current pulses propagating through in-cloud lightning leader channels in a strong electric field may be able to initiate terrestrial gamma ray flashes.