Frequency Sweep Rate Research Articles

Observation of discrete VLF emissions at Indian low-latitude ground station Srinagar (L = 1.28)

During the routine analysis of recorded very low-frequency data during the period November, 2012 to February, 2013, at an Indian low-latitude ground station Srinagar (geomag. lat., 24°10′N; L = 1.28), we observed some interesting discrete emissions during quiet period on February 5, 2013, which has been presented in the present study. The spectral analysis of the observed discrete very low-frequency emissions consisting of risers, fallers, hooks and oscillating tones has been presented. To explain the observed dynamic spectra of the discrete chorus emissions, a possible generation mechanism has been presented based on the recent nonlinear theory. On the basis of this theory, frequency sweep rate of discrete chorus emissions has been computed and compared with those of our experimentally observed values.

Frequency sweep rates of rising tone electromagnetic ion cyclotron waves: Comparison between nonlinear theory and Cluster observation

Resonant pitch angle scattering by electromagnetic ion cyclotron (EMIC) waves has been suggested to account for the rapid loss of ring current ions and radiation belt electrons. For the rising tone EMIC wave (classified as triggered EMIC emission), its frequency sweep rate strongly affects the efficiency of pitch-angle scattering. Based on the Cluster observations, we analyze three typical cases of rising tone EMIC waves. Two cases locate at the nightside (22.3 and 22.6 magnetic local time (MLT)) equatorial region and one case locates at the duskside (18MLT) higher magnetic latitude (λ = –9.3°) region. For the three cases, the time-dependent wave amplitude, cold electron density, and cold ion density ratio are derived from satellite data; while the ambient magnetic field, thermal proton perpendicular temperature, and the wave spectral can be directly provided by observation. These parameters are input into the nonlinear wave growth model to simulate the time-frequency evolutions of the rising tones. The simulated results show good agreements with the observations of the rising tones, providing further support for the previous finding that the rising tone EMIC wave is excited through the nonlinear wave growth process.

First observation of rising‐tone magnetosonic waves

AbstractMagnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90° and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be “temporally continuous” in their time‐frequency spectrogram. Here we show for the first time that MS waves actually have discrete wave elements with rising‐tone features in their spectrogram. The frequency sweep rate of MS waves, ~1 Hz/s, is between that of chorus and electromagnetic ion cyclotron (EMIC) waves. For the two events we analyzed, MS waves occur outside the plasmapause and cannot penetrate into the plasmasphere; their power is smaller than that of chorus. We suggest that the rising‐tone feature of MS waves is a consequence of nonlinear wave‐particle interaction, as is the case with chorus and EMIC waves.

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

AbstractWe report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High‐resolution Aurora Imaging Network (VLF‐CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising‐tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling‐tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty‐patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

A numerical study of chorus generation and the related variation of wave intensity using the DAWN code

AbstractChorus waves play an important role in energetic electron dynamics in the inner magnetosphere. In this work, we present a new hybrid code, DAWN, to simulate the generation of chorus waves. The DAWN code is unique in that it models cold electrons using linearized fluid equations and hot electrons using particle‐in‐cell techniques. The simplified fluid equations can be solved with robust and simple algorithms. We demonstrate that discrete chorus elements can be generated using the code. Waveforms of the generated element show amplitude modulation or “subpackets,” and the frequency sweep rate of the generated element is found to be consistent with that of observed chorus waves. Using the DAWN code, we then investigate the variation of wave intensity () with respect to linear growth rates on the equatorial plane. Previous observations showed that the change in linear growth rates of whistler waves modulated by external processes such as density modulations is usually small (), while the variation of the wave intensity is large (). Using a chosen set of background plasma parameters, we demonstrate that a small change () in linear growth rates can lead to significant variation () of wave intensity only in the transition from the broadband whistler wave generation regime to the chorus wave generation regime. Our results demonstrate the importance of including nonlinear dynamics of chorus generation in understanding the whistler wave intensity modulation process in the inner magnetosphere.

The chirp-control of frequency-tunable narrowband terahertz pulses by nonlinearly chirped laser pulse beating

We demonstrate a method for controlling the chirp of the frequency-tunable narrowband terahertz pulses that are generated by photomixing with nonlinearly chirped laser pulse pairs. We find that in a grating-based laser-pulse stretcher, the frequency sweep rates of the generated terahertz pulses can be controlled by simply changing the incident angle. This method is also applicable to other mechanisms of terahertz pulse generation.

Frequency sweep rate dependence on the dielectrophoretic response of polystyrene beads and red blood cells

Alternating current (AC) dielectrophoresis (DEP) experiments for biological particles in microdevices are typically done at a fixed frequency. Reconstructing the DEP response curve from static frequency experiments is laborious, but essential to ascertain differences in dielectric properties of biological particles. Our lab explored the concept of sweeping the frequency as a function of time to rapidly determine the DEP response curve from fewer experiments. For the purpose of determining an ideal sweep rate, homogeneous 6.08 μm polystyrene (PS) beads were used as a model system. Translatability of the sweep rate approach to ∼7 μm red blood cells (RBC) was then verified. An Au/Ti quadrapole electrode microfluidic device was used to separately subject particles and cells to 10Vpp AC electric fields at frequencies ranging from 0.010 to 2.0 MHz over sweep rates from 0.00080 to 0.17 MHz/s. PS beads exhibited negative DEP assembly over the frequencies explored due to Maxwell-Wagner interfacial polarizations. Results demonstrate that frequency sweep rates must be slower than particle polarization timescales to achieve reliable incremental polarizations; sweep rates near 0.00080 MHz/s yielded DEP behaviors very consistent with static frequency DEP responses for both PS beads and RBCs.

Frequency drift of Saturn chorus emission compared to nonlinear theory

The fine structure of nonlinear drifting‐frequency chorus is observed at Saturn by the Cassini Radio and Plasma Wave Investigation. During a high‐inclination orbit in which Cassini is at near‐constant L‐shell within about 10° of the magnetic equator, moderately intense nonlinear chorus is observed. Cassini observed a region of intense chorus and large bandwidth a few degrees on either side of the magnetic equator, with lower intensities and bandwidths observed nearest the magnetic equator. Using the observed plasma wave spectra and electron phase space distribution, we have measured plasma parameters within or near the chorus generation region and evaluated the theoretical value of the frequency sweep rate, ∂f/∂t, based on the nonlinear wave growth theory of Omura et al. and the backward wave oscillator theory of Trakhtengerts. Both theories produce rates that are within a factor of 2 of the observed values, but the nonlinear wave growth theory values are closer to the observations for the cases examined. The work presented is consistent with nonlinear theory in the generation of chorus, but also reveals a distinct region of weaker or linear chorus growth nearest to the magnetic equator at Saturn.

Reversal of magnetization of a single-domain magnetic particle by the ac field of time-dependent frequency

We report numerical and analytical studies of the reversal of the magnetic moment of a single-domain magnetic particle by a circularly polarized ac field of time-dependent frequency. For the time-linear frequency sweep, the phase diagrams are computed that illustrate the dependence of the reversal on the frequency sweep rate v, the amplitude of the ac field h, the magnetic anisotropy field d, and the damping parameter alpha. It is shown that the most efficient magnetization reversal requires a non-linear time dependence of the frequency, omega(t), for which an exact analytical formula is derived with account of damping. The necessary condition of the reversal is h > alpha d. Implementation of a small-scale magnetization reversal is proposed in which a nanomagnet is electromagnetically coupled to two weak superconducting links controlled by the voltage. Dynamics of such a system is analyzed with account of the back effect of the magnet on the superconducting links.

THEMIS observation of chorus elements without a gap at half the gyrofrequency

Using waveform data obtained by one of the THEMIS satellites, we report properties of rising tone chorus elements without a gap at half the gyrofrequency in a region close to the magnetic equator. The wave normal angle of the chorus elements is typically field‐aligned in the entire frequency range of both upper‐band and lower‐band chorus emissions. We find that the observed frequency sweep rates are consistent with the estimation based on the nonlinear wave growth theory of Omura et al. (2008). In addition, we compare the frequency profiles of the chorus wave amplitudes with those of the optimum and threshold wave amplitudes derived from the nonlinear wave growth theory for triggering rising tone chorus emissions. The results of the comparison show a reasonable agreement, indicating that rising tone chorus elements are continually generated through a triggering process which generates elements with the optimum amplitudes for nonlinear growth.

Optimizing frequency and pulse shape for ultrasound current source density imaging.

Electric field mapping is commonly used to identify irregular conduction pathways in the heart (e.g., arrhythmia) and brain (e.g., epilepsy). Ultrasound current source density imaging (UCSDI), based on the acoustoelectric (AE) effect, is a promising new technique for mapping electrical current in four dimensions with enhanced resolution. The frequency and pulse shape of the ultrasound beam affect the sensitivity and spatial resolution of UCSDI. In this study, we explore the effects of ultrasound transducer frequency bandwidth and coded excitation pulses for UCSDI and the inherent tradeoff between sensitivity and spatial resolution. We used both simulations and bench-top experiments to image a time-varying electrical dipole in 0.9% NaCl solution. To study the effects of ultrasound bandwidth, we chose two ultrasound transducers with different center frequencies (1.0 and 2.25 MHz). For coded excitation, we measured the AE voltage signal with different chirp excitations. As expected, higher bandwidth correlated with improved spatial resolution at the cost of sensitivity. On the other hand, chirp excitation significantly improved sensitivity (3.5 μV/mA) compared with conventional square pulse excitation (1.6 μV/mA) at 1 MHz. Pulse compression achieved spatial resolution similar to that obtained using square pulse excitation, demonstrating enhanced detection sensitivity without loss of resolution. Optimization of the time duration of the chirp pulse and frequency sweep rate can be further used to improve the quality of UCSDI.

Precise beam energy measurement using resonant spin depolarization in the SOLEIL storage ring

The average electron beam energy for the operational mode of 400mA in 416 bunches in the SOLEIL storage ring was measured to be 2.73724±0.00016GeV with an accuracy of 5.9×10−5 using the method of resonant spin depolarization (RSD). A Touschek-dominated electron beam was excited using a shaker magnet, and the beam polarization and depolarization were monitored using the change in beam lifetime and beam loss rate. To establish the primary condition that is required to perform energy measurement using the RSD method, the radiative beam polarization was first simulated using the SLIM beam dynamics code and then measured using the relative increase of beam lifetime for a Touschek-dominated electron beam. With a fast frequency sweep rate, the main depolarization resonance to be used to extract the beam energy, along with sidebands within the range of frequency sweep, was identified during our first trials. Sweeping the frequency of the excitation field around the main resonance with a slower frequency sweep rate, the beam energy measurement accuracy was increased from 1.7×10−4 to 5.9×10−5. Finally, the effects of closed orbit distortions on the radiative polarization and measured energy accuracy are discussed.

Nonlinear pitch angle scattering of relativistic electrons by EMIC waves in the inner magnetosphere

We derive the second‐order resonance condition for interaction between a relativistic electron and a coherent Electromagnetic Ion Cyclotron (EMIC) wave with a variable frequency. We perform test particle simulations of relativistic electrons interacting with EMIC waves with a fixed frequency and a rising‐tone frequency such as EMIC triggered emissions observed in the inner magnetosphere. Trapping of resonant electrons leads to rapid and efficient pitch angle scattering of relativistic electrons, resulting in bursty precipitation of relativistic electrons. The efficiency of the pitch angle scattering depends on the gradient of the magnetic field, the frequency sweep rate, and the wave amplitude. The effective wave trapping occurs for a wide range of pitch angles from 10 to 60 degrees. The most effective pitch angle scattering takes place for the case of a rising‐tone emission with an enhanced magnetic field gradient. Since the efficiency of pitch angle scattering also depends on the wave amplitude, resonant electrons may not be scattered into the loss cone in a single passage through the wave packet. However, repeated interactions with a series of wave packets result in scattering of relativistic electrons into the loss cone.

Properties of the magnetospheric backward wave oscillator inferred from CLUSTER measurements of VLF chorus elements

According to the backward wave oscillator (BWO) model, a sharp gradient (or step‐like deformation) on the electron distribution function is the most important factor in chorus generation, but such a feature is very difficult to observe directly. The properties of the step in the BWO model determine the dimensionless parameterqquantifying the excess of the energetic electron flux above the absolute‐instability threshold. This parameter, in turn, is related to the frequency sweep rate of chorus elements, which we obtained by using data from the WBD instrument onboard the CLUSTER satellites in the equatorial region for more than 7000 chorus elements. Then, using the CLUSTER data for the plasma density and magnetic field, we calculatedq assuming the validity of the BWO theory and found that the q values depend only weakly on the density; the average values of q ≈ 7 for the lower band chorus (f/fce < 0.5) and q ≈ 13 for the upper band (f/fce > 0.5). These q values constitute a large excess over the generation threshold (q > 3) resulting from numerical simulation of discrete elements with rising frequency and are thus consistent with the simulations. Another important feature of the q parameter is the significant scatter of its values during each Cluster passage of the generation region. Using the obtained q values we estimate the relative height of the step in the electron distribution function to lie in the range from 0.01 to 0.3.

Rheological Investigation of Wood-Polypropylene Composites in Rotational Plate Rheometer

In this research work, the rheological properties of Wood-Plastic Composites (WPC) with some selected compositions are investigated. WPC is being recognized as a green composite that, in the past 20 years, has emerged to a commercial product. A study on rheological properties of these materials can give insight into the proper selection of composition and processing condition. Two grades of polypropylene (PP) with two different melt flow indexes (MFI) were selected to prepare WPCs with three different wood contents (50, 60 and 70 % wt.). Four types of rheological experiments were performed utilizing a rotational plate rheometer: (1) strain sweep, (2) frequency sweep, (3) temperature sweep and (4) steady shear rate sweep. The independent variables were chosen as wood content, MFI of polymer (two types), melt temperature, frequency or shear rate, the gap between the plates, and strain percentage. The strain sweep tests specified the linear and non-linear viscoelastic zones of each experiment. The results of frequency sweep experiments indicated that increasing the wood content and frequency and also decreasing the strain percentage and the gap distance, lead to an increase in the storage modulus. Regarding the loss modulus, wood percentage and the gap distance presented positive effects and strain percentage showed a negative effect. The behavior of complex viscosity was almost similar to that of the storage modulus but increasing the frequency caused a decrease in the complex viscosity. In case of temperature sweep experiments, it was observed that the rheological properties exhibit a rapid change near to a temperature of 160 °C. The results also showed that beyond this point, increasing the wood content and also MFI of polypropylene caused an increase in the storage modulus. The results of steady shear rate sweep experiments specified that increasing wood content and also decreasing the MFI of PP, the gap distance and shear rate lead to an increase in both viscosity and shear stress.

The ac Stark effect in nitric oxide induced by rapidly swept continuous wave quantum cascade lasers

A large ac Stark effect has been observed when nitric oxide, at low pressure in a long optical path (100 m) Herriot cell, is subjected to infrared radiation from a rapidly swept, continuous wave infrared quantum cascade laser. As the frequency sweep rate of the laser is increased, an emission signal induced by rapid passage occurs after the laser frequency has passed through the resonance of 1-0 R(11.5)(3/2 /)molecular absorption line. At very high sweep rates a laser field-induced splitting of the absorptive part of the signal is observed, due to the ac Stark effect. This splitting is related to the Autler-Townes mixing of the e, f lambda doublet components of the 1-0 R(11.5)(3/2) transition, which lie under the Doppler broadened envelope.

Comparison between theory and observation of the frequency sweep rates of equatorial rising tone chorus

Theoretical predictions for chorus frequency sweep rates by Helliwell and Trakhtengerts are compared with observations from the THEMIS satellites and a previously published dataset from the Cluster satellites. We first extend the theories to use a general magnetic field model to include the effects of magnetic local time and geomagnetic activity, and then show that both theories give the same dependence of the frequency sweep rate on background plasma parameters. The theoretical scaling of frequency sweep rates are shown to agree very well with observations. We demonstrate that for a given equatorial magnetic field strength, nightside and dawnside chorus waves have higher frequency sweep rates because of the stretching of the magnetic field, while dayside chorus waves have lower frequency sweep rates because of the compression of the field. Increasing geomagnetic activity will enhance the asymmetry by increasing the day‐night asymmetry of the background field. The results are important for understanding the generation mechanism of chorus waves.

Particle simulations of whistler‐mode rising‐tone emissions triggered by waves with different amplitudes

We perform self‐consistent electromagnetic particle simulations to analyze whistler‐mode triggered emissions in the magnetosphere. The whistler‐mode triggering waves with different wave amplitudes and a constant frequency are injected at the equator. With triggering wave amplitudes greater than the threshold for the nonlinear wave growth, rising‐tone emissions are successfully excited near the equator. The detailed time evolutions of amplitudes and frequencies of the rising‐tone emissions show similar development. A recent theoretical study found the optimum amplitude of triggering waves for rising‐tone emissions. The optimum amplitude condition is confirmed by the simulations. A triggering wave with an amplitude much greater than the optimum amplitude cannot trigger a rising emission. In a process where triggered emissions develop, phase‐organized resonant electrons clearly appear in the velocity‐phase space, contributing to the nonlinear wave growth. The simulation study shows that amplitudes and frequency sweep rates of triggered emissions do not depend on the amplitude of a triggering wave.

Amplitude dependence of frequency sweep rates of whistler mode chorus emissions

[1] The frequency and amplitude characteristics of chorus emissions are studied by performing electron hybrid code simulations with different initial number densities of energetic electrons. Chorus emissions with rising tones are generated in all simulation runs except for the simulation assuming the lowest number density. The frequency sweep rates of reproduced chorus vary depending on the variation of the wave amplitude of respective chorus elements. We find that the theoretically estimated frequency sweep rates are consistent with the simulation results. The simulation results reveal that the characteristic frequency variation of chorus elements showing rising tones are formed at the region very close to the magnetic equator while the wave amplitudes of chorus elements are significantly amplified through their propagation away from the equator. The spatial scale of the region where the observed explosive wave growth varies in each simulation run, corresponding to the difference of the wave amplitudes of reproduced chorus elements. We estimate the spatial scale hc, a measure of the spatial extent of the generation region derived from the nonlinear wave growth theory, using parameters of the simulation runs and find its consistency with the simulation results. We also analyze the energy spectra of reproduced chorus elements in the simulation results and find that the spectra of chorus are essentially different from those predicted by the linear growth rates in the frequency domain. These results clearly demonstrate that the nonlinear wave-particle interaction governs the chorus generation mechanism.

Triggering process of whistler mode chorus emissions in the magnetosphere

[1] Chorus emissions are triggered from the linear cyclotron instability driven by the temperature anisotropy of energetic electrons (10–100 keV) in the magnetosphere. Chorus emissions grow as an absolute nonlinear instability near the magnetic equator because of the presence of an electromagnetic electron hole in velocity space. The transition process from the linear wave growth at a constant frequency to the nonlinear wave growth with a rising tone frequency is due to formation of a resonant current −JB antiparallel to the wave magnetic field. The rising-tone frequency introduces a phase shift to the electron hole at the equator and results in a resonant current component antiparallel to the wave electric field −JE, which causes the nonlinear wave growth. To confirm this triggering mechanism, we perform Vlasov hybrid simulations with JB and without JB. The run without JB does not reproduce chorus emissions, while the run with JB does successfully reproduce chorus emissions. The nonlinear frequency shift ω1 due to JB plays a critical role in the triggering process. The nonlinear transition time TN for the frequency shift is found to be of the same order as the nonlinear trapping period, which is confirmed by simulations and observation. The established frequency sweep rate is ω1/TN, which gives an optimum wave amplitude of chorus emissions.