Papers by František Němec
International audienceEquatorial noise (EN) emissions are electromagnetic waves at frequencies be... more International audienceEquatorial noise (EN) emissions are electromagnetic waves at frequencies between the proton cyclotron frequency and the lower hybrid frequency routinely observed in the equatorial region of the inner magnetosphere. They propagate in the extraordinary mode nearly perpendicular to the ambient magnetic field, and they exhibit a harmonic structure related to the ion cyclotron frequency in the source region. We analyze more than 2000 EN events observed by the wave instruments on board the Cluster spacecraft, and we find that about 5% of EN events are not continuous in time, but exhibit a quasi-periodic (QP) modulation of the wave intensity. Typical modulation periods are on the order of minutes. The events predominantly occur in the noon-to-dawn local time sector, and their occurrence is related to the periods of increased geomagnetic activity and higher solar wind speeds. We suggest that the QP modulation of EN events may be due to compressional ULF pulsations, whi...
Journal of Geophysical Research: Space Physics, 2015
Equatorial noise (EN) emissions are electromagnetic wave events at frequencies between the proton... more Equatorial noise (EN) emissions are electromagnetic wave events at frequencies between the proton cyclotron frequency and the lower hybrid frequency observed in the equatorial region of the inner magnetosphere. They propagate nearly perpendicular to the ambient magnetic field, and they exhibit a harmonic line structure characteristic of the proton cyclotron frequency in the source region. However, they were generally believed to be continuous in time. We investigate more than 2000 EN events observed by the Spatio-Temporal Analysis of Field Fluctuations and Wide-Band Data Plasma Wave investigation instruments on board the Cluster spacecraft, and we show that this is not always the case. A clear quasiperiodic (QP) time modulation of the wave intensity is present in more than 5% of events. We perform a systematic analysis of these EN events with QP modulation of the wave intensity. Such events occur usually in the noon-to-dawn magnetic local time sector. Their occurrence seems to be related to the increased geomagnetic activity, and it is associated with the time intervals of enhanced solar wind flow speeds. The modulation period of these events is on the order of minutes. Compressional ULF magnetic field pulsations with periods about double the modulation periods of EN wave intensity and magnitudes on the order of a few tenths of nanotesla were identified in about 46% of events. We suggest that these compressional magnetic field pulsations might be responsible for the observed QP modulation of EN wave intensity, in analogy to formerly reported VLF whistler mode QP events.
Journal of Geophysical Research, 2008
We present results of a systematic survey of power line harmonic radiation events observed by the... more We present results of a systematic survey of power line harmonic radiation events observed by the low-altitude DEMETER spacecraft. Altogether, 88 events (45 with frequency spacing 50/100 Hz and 43 with frequency spacing 60/120 Hz) have been found by an automatic identification procedure and confirmed by visual inspection. Frequency-Time intervals of individual lines forming the events have been found by an automated procedure, and the corresponding frequency-time spectrograms have been fitted by a 2d-Gaussian model. It is shown that the mean time duration of the lines forming the events is 20 seconds, with median being 12 seconds (this corresponds to the spatial dimensions of 156/90 km, respectively). The full width at half maximum of the frequency range of the lines is less than 3 Hz in the majority of cases. Moreover, the lines with larger bandwidth and the lines with the largest intensities often occur off exact multiples of base power system frequency. This can be explained either by wave-particle interactions that take place and modify the radiated electromagnetic wave or by the improperly operating radiating power system. Full-wave calculation of the efficiency of coupling of electromagnetic waves through the ionosphere has been done to show that it can explain lower intensity of events observed by satellite during the day as compared with those observed during the night. Estimated radiated peak power on the ground is larger for events observed during the day than for events observed during the night, and more events are observed during the day than during the night.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, May 13, 2015
Annales Geophysicae
We report results of a systematic analysis of a large number of observations of equatorial noise ... more We report results of a systematic analysis of a large number of observations of equatorial noise between the local proton cyclotron frequency and the local lower hybrid frequency. The analysis is based on the data collected by the STAFF-SA instruments on board the four Cluster spacecraft. The data set covers their first two years of measurement in the equatorial magnetosphere at radial distances between 3.9 and 5 Earth radii. Inspection of 781 perigee passages shows that the occurrence rate of equatorial noise is approximately 60%. We identify equatorial noise by selecting data with nearly linearly polarized magnetic field fluctuations. These waves are found within 10 • of the geomagnetic equator, consistent with the published past observations. Our results show that equatorial noise has the most intense magnetic field fluctuations among all the natural emissions in the given interval of frequencies and latitudes. Electric field fluctuations of equatorial noise are also more intense compared to the average of all detected waves. Equatorial noise thus can play a non-negligible role in the dynamics of the internal magnetosphere. Key words. Magnetospheric physics (waves in plasma)-Space plasma physics (waves and instabilities)-Radio science (magnetospheric physics)
Atmosphere
Observations of proton density fluctuations of the solar wind at 1 au have shown the presence of ... more Observations of proton density fluctuations of the solar wind at 1 au have shown the presence of a decade-long transition region of the density spectrum above sub-ion scales, characterized by a flattening of the spectral slope. We use the proton density fluctuations data collected by the BMSW instrument on-board the Spektr-R satellite in order to delimit the plasma parameters under which the transition region can be observed. Under similar plasma conditions to those in observations, we carry out 3D compressible magnetohydrodynamics (MHD) and Hall-MHD numerical simulations and find that Hall physics is necessary to generate the transition region. The analysis of the kω power spectrum in the Hall-MHD simulation indicates that the flattening of the density spectrum is associated with fluctuations having frequencies smaller than the ion cyclotron frequency.
Universe
Magnetic fields in the inner magnetosphere can be obtained as vector sums of the Earth’s own inte... more Magnetic fields in the inner magnetosphere can be obtained as vector sums of the Earth’s own internal magnetic field and magnetic fields stemming from currents flowing in the space plasma. While the Earth’s internal magnetic field is accurately described by the International Geomagnetic Reference Field (IGRF) model, the characterization of the external magnetic fields is significantly more complicated, as they are highly variable and dependent on the actual level of the geomagnetic activity. Tsyganenko family magnetic field models (T89, T96, T01, TA15B, TA15N), parameterized by the geomagnetic activity level and solar wind parameters, are often used by the involved community to describe these fields. In the present paper, we use a large dataset (2001–2018) of magnetospheric magnetic field measurements obtained by the four Cluster spacecraft to assess the accuracy of these models. We show that, while the newer models (T01, TA15B, TA15N) perform significantly better than the old ones ...
<p>The solar wind is a unique laboratory to study the turbulent processes occurring... more <p>The solar wind is a unique laboratory to study the turbulent processes occurring in a collisionless plasma with high Reynolds numbers. A turbulent cascade—the process that transfers the free energy contained within the large scale fluctuations into the smaller ones—is believed to be one of the most important mechanisms responsible for heating of the solar corona and solar wind. The paper analyzes power spectra of solar wind velocity, density and magnetic field fluctuations that are computed in the frequency range around the break between inertial and kinetic scales. The study uses measurements of the Bright Monitor of the Solar Wind (BMSW) on board the Spektr-R spacecraft with a time resolution of 32 ms complemented with 10 Hz magnetic field observations from the Wind spacecraft propagated to the Spektr-R location. The statistics based on more than 42,000 individual spectra show that: (1) the spectra of both quantities can be fitted by two (three in the case of the density) power-law segments; (2) the median slopes of parallel and perpendicular fluctuation velocity and magnetic field components are different; (3) the break between MHD and kinetic scales as well as the slopes are mainly controlled by the ion beta parameter. These experimental results are compared with high-resolution 2D hybrid particle-in-cell simulations, where the electrons are considered to be a massless, charge-neutralizing fluid with a constant temperature, whereas the ions are described as macroparticles representing portions of their distribution function. In spite of several limitations (lack of the electron kinetics, lower dimensionality), the model results agree well with the experimental findings. Finally, we discuss differences between observations and simulations in relation to the role of important physical parameters in determining the properties of the turbulent cascade.</p>
<p>Turbulent cascade transferring the free energy contained... more <p>Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind and thus the turbulent behavior of these quantities is intensively studied. However, the temperature is also highly fluctuating quantity but behavior of its variations is studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of the temperature variations requires knowledge of the full velocity distribution with a time resolution and such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board the Spektr-R spacecraft uses the Maxwellian approximation and provides the thermal velocity with 32 ms time resolution. We use these measurements and complement them with 10 Hz magnetic field observations from the Wind spacecraft propagated to the Spektr-R location and analyze factors influencing the shape of the temperature power spectral density. A special attention is devoted to mutual relations of power spectral densities of different quantities like parallel and perpendicular temperature, magnetic field and velocity fluctuations and their evolution in course of solar wind expansion.</p>
<p>We analyze variations of energetic particle fluxes measured by low altit... more <p>We analyze variations of energetic particle fluxes measured by low altitude spacecraft after interplanetary shock arrivals and around the times of significant geomagnetic storms. Data from two different spacecraft and energetic particle detectors are used and compared. First, we use data measured by the energetic particle detector (IDP) onboard the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) spacecraft. The spacecraft operated between 2004 and 2010 on a circular Sun-synchronous orbit at an altitude of initially 710 km, which was decreased to 660 km in December 2005. The IDP instrument measured electron flux close to the loss cone at energies between about 70 keV and 2.3 MeV (128 energy channels). Second, we use data measured by the Space Application of Timepix Radiation Monitor (SATRAM) onboard the Proba-V satellite operating since May 2013 on a circular Sun-synchronous orbit at an altitude of 820 km. The semi-conductor based pixelated radiation detector called Timepix is capable of detecting all charged particles and X-rays with sufficiently high energies. Electron energies higher than about 2 MeV and proton energies higher than about 20 MeV are detected. We identify the times of interplanetary shock arrivals and significant (Dst < –100 nT) geomagnetic storms during the mission durations. Then we perform a superposed epoch analysis to reveal characteristic particle flux variations around these times at different energies and L-shells. Although the used satellite missions do not overlap in time, we aim to compare the revealed flux variation signatures between these two independent data sets.</p>
<p>Wave intensity measured in the very low frequency (VLF) range (up to 20 ... more <p>Wave intensity measured in the very low frequency (VLF) range (up to 20 kHz) is typically represented using frequency-time spectrograms. Since the characterization of spectrogram main features and/or their direct comparison is a challenging task, we transform the measurements of the low-altitude DEMETER spacecraft using the principal component analysis (PCA). The present study is focused on both the physical interpretation of the first two principal components and their application to real physical problems. To understand the physical meaning of the first principal components, their scatter plot is constructed and discussed. Moreover, the dependence of the first principal component (PC1) coefficients on the geomagnetic activity and their seasonal/longitudinal variations are analyzed. The obtained distributions are well comparable with those obtained by previous studies for average wave intensities, indicating that the PC1 coefficients are directly related to the overall wave intensity. Furthermore, the variations of PC1 coefficients around interplanetary (IP) shock arrivals are analyzed, suggesting that the fast forward shock occurrence has the most significant effect. It is shown that the wave intensity variations depend on the wave intensity detected before the shock arrival. The shock strength and interplanetary magnetic field orientation are also important. To further demonstrate the adaptability of PCA, we use a similar method to analyze also ground-based VLF measurements performed by the Kannuslehto station located in northern Finland.</p>
Soil and Tillage Research
<p>... more <p>The Space Application of Timepix Radiation Monitor (SATRAM) on board the Proba-V satellite of the European Space Agency (ESA) was launched in May 2013 into a sun-synchronous orbit with an altitude of about 820 km. This technology demonstration payload is based on the Timepix technology developed by the CERN-based Medipix2 Collaboration. It is equipped with a 300 um thick silicon sensor with a pixel pitch of 55 um in a 256 x 256 pixel matrix. The device is sensitive to X-rays and all charged particles. A Monte Carlo simulation was conducted to determine the detector response to electrons (0.5–7 MeV) and protons (10–400 MeV) taking into account the shielding of the detector housing and the satellite. With the help of the simulation, a strategy was developed to estimate omnidirectional electron, proton, and ion fluxes around Earth using stopping power, maximum energy deposition per pixel of the particle track, and the shape of the particle tracks in the sensor. Presented are typical overall dose rates as well as fluxes of individual particle species. A superposed epoch analysis is used to analyze variations of particle fluxes related to geomagnetic storms and interplanetary shock arrivals as a function of time and L-shell.</p>
The Astrophysical Journal
Geophysical Research Letters
The Astrophysical Journal
We study the polarization properties of the velocity fluctuations in solar wind turbulence using ... more We study the polarization properties of the velocity fluctuations in solar wind turbulence using high-resolution data from the Spektr-R spacecraft. The ratio of perpendicular to parallel velocity fluctuations in the inertial range is smaller than the equivalent ratio for magnetic fluctuations, but gradually increases throughout this range. In the kinetic range, there is alarge decrease in the ratio, similar to the magnetic fluctuations. We compare the measurements to numerical solutions for acombination of kinetic Alfvén waves and slow waves, finding that both the slow increase and sharp decrease in the ratio are consistent with amajority population of Alfvén waves and minority population of slow waves in critical balance. Furthermore, the beta-dependence of this scale-dependent ratio can be successfully captured in the model when incorporating abeta-dependent Alfvén to slow wave ratio similar to that observed in the solar wind.
Journal of Geophysical Research: Space Physics
Journal of Geophysical Research: Space Physics
ABSTRACT We present a detailed analysis of three electromagnetic wave events observed by the Clus... more ABSTRACT We present a detailed analysis of three electromagnetic wave events observed by the Cluster Wide-Band Data (WBD) instruments at frequencies of a few kHz, which exhibit a periodic time modulation of the wave intensity (quasi-periodic emissions). The events were measured when crossing the geomagnetic equator at radial distances of about 3–4 Earth radii during the close separation campaign in July-October, 2013. Coincident compressional magnetic field pulsations with the magnitude on the order of a few tenths of nT were detected in two events. Their period corresponded to the modulation period of the quasi-periodic emissions in one event, while it was about double in the other. A possible explanation for this discrepancy between the two periods is given. Although Cluster 1, Cluster 3, and Cluster 4 were close to each other, Cluster 2 was located by more than 0.5 Earth radii closer to the Earth. The same quasi-periodic modulation was observed by all Cluster spacecraft, but the emissions detected by Cluster 2 had a noticeable time delay. This can be explained by the wave propagation from the generation region located at larger radial distances, close to the plasmapause. The large range of L-shells where the emissions are detected thus appears to be due to the unducted propagation from a spatially limited source. The frequency-time structure of individual elements forming the events is evaluated, and it is shown that the time dependence of the intensity modulation can be approximated by a simple model.
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Papers by František Němec