Large Doppler Shifts Research Articles

車載用技術を利用した超小型宇宙用GPS受信機の開発と軌道上実証

Miniature space GPS receivers have been developed by means of automobile-navigation technology. We expanded the frequency sweep range in order to cover large Doppler shift on orbit. We tested the performance in low earth orbits by means of a GPS simulator. The range error caused by the receiver is measured to be 0.9m in RMS. The receiver is tolerant for 20krad radiation. Receiver was on-boarded on INDEX (“REIMEI”) satellite, which was launched in 2005. Cold start positioning is confirmed repeatedly to finish within 30min on orbit. The short term random error of GPS positioning is as large as 1.5m for PDDP = 2.7 on orbit. The range error due to the receiver is 0.5m RMS from the flight data. These results on orbit are consistent with the simulation results by means of a GPS simulator. This miniature space GPS receiver is promising as an inexpensive space GPS receiver in commercial market.

Implementation of Joint Pre-FFT Adaptive Array Antenna and Post-FFT Space Diversity Combining for Mobile ISDB-T Receiver

In our application targeted here, four on-glass antenna elements are set in an automobile to improve the reception quality of mobile ISDB-T receiver. With regard to the directional characteristics of each antenna, we propose and implement a joint Pre-FFT adaptive array antenna and Post-FFT space diversity combining (AAA-SDC) scheme for mobile ISDB-T receiver. By applying a joint hardware and software approach, a flexible platform is realized in which several system configuration schemes can be supported; the receiver can be reconfigured on the fly. Simulation results show that the AAA-SDC scheme drastically improves the performance of mobile ISDB-T receiver, especially in the region of large Doppler shift. The experimental results from a field test also confirm that the proposed AAA-SDC scheme successfully achieves an outstanding reception rate up to 100% while moving at the speed of 80km/h.

Fast-ion D-alpha diagnostic for NSTX

A fast-ion D-alpha (FIDA) diagnostic is under development for the National Spherical Torus Experiment (NSTX). The FIDA technique is a charge-exchange recombination spectroscopy measurement that exploits the large Doppler shift of Balmer-alpha light from energetic hydrogenic atoms to infer the fast-ion density. The principal objective of the NSTX installation is to measure the transport of beam ions caused by fast-ion driven instabilities; detection of perpendicular acceleration of fast ions during high harmonic fast wave heating is another important goal. Recent data from a DIII-D FIDA diagnostic guide the design. The planned NSTX diagnostic consists of two separate instruments focusing on different aspects of the measurement. One instrument uses a transmission grating spectrometer to measure the perpendicular energy spectrum and the spatial profile every 10ms; the anticipated resolution is ∼10keV in energy and ∼5cm in radius. A second instrument employs bandpass filters to detect fast-ion redistribution events with millisecond temporal resolution.

Design of the Tore Supra motional Stark effect diagnostic

This article describes the overall design of the motional Stark effect diagnostic on Tore Supra (not water cooled in its first version) and the results obtained. The diagnostic is composed of nine viewing lines measuring the plasma every 8cm with a spatial resolution varying from 3to6cm. A tube placed inside the port adjacent to the neutral beam contains a stainless-steel mirror and SFL6 optic lenses that carry the image of the neutral beam towards the optical fibers. On Tore Supra the diagnostics having components inside the machine have to face thermal load difficulties linked with the long shots, and this will be the case for ITER diagnostics. This is why for safety reasons the insulating window is placed at the rear side of the tube, and consequently the optics is under the machine vacuum. For motional Stark effect, before reaching the thermal limits on the components, a first limitation comes from the polarization modifications induced by the temperature gradients on the lenses (birefringence effect). This limitation is estimated in terms of plasma duration. The associated diagnostic neutral beam (60keV, 400kW, 5s) works in hydrogen for a higher velocity and a better plasma penetration. As a consequence the beam spectrum exhibits a large Doppler shift and a clear separation of the Stark components. The detection uses the classical elements of the polarimetry method, wide aperture photoelastic modulators, linear polarizer, narrow interference filters, and photomultipliers. The signal is processed digitally (250kHz) for the extraction of the Fourier components that allow the calculation of the magnetic field pitch angles. The first measurements obtained during Ohmic shots for various plasma currents are in good agreement with the current diffusion calculations done with the CRONOS code.

A new paradigm for plasma transport and zonal flows

Most tokamak experimental results indicate dependence of the ion thermal conductivity on the isotopic mass close to χ⊥∼mi−0.5, i.e., inverse gyro-Bohm. This is in stark contradiction to most present theoretical models predicting Bohm (mi0) or gyro-Bohm (mi0.5) scaling. A basic physics isotopic scaling experiment [V. Sokolov and A. K. Sen, Phys. Rev. Lett. 89, 095001 (2002)] on the anomalous ion thermal conduction due to ion temperature gradient (ITG) instabilities in two different gases (hydrogen and deuterium) closely confirms the tokamak results. Another series of experiments designed to explore the physics basis of this scaling appears to lead to a new model for this scaling based on 3-wave coupling of two ITG radial harmonics and an IA wave. The resulting isotopic scaling of transport is ∼mi−0.5 dictated primarily by the IA damping. This basic physics may be extrapolated to tokamaks resolving the paradox [V. Sokolov and A. K. Sen, Phys. Rev. Lett. 92, 165002 (2004)]. Last, the much discussed theoretical role of zonal flows in transport regulation is critically examined by another set of experiments. A novel diagnostic has been developed on the basis of the observation that the effect of zonal flow can be seen in the FM modulation (at zonal flow frequency) of the carrier frequency of the large equilibrium Doppler shift frequency of ITG modes both in tokamaks and in the Columbia Linear Machine [V. Sokolov, X. Wei, and A. K.Sen, APS DPP meeting, Savannah (2004)]. The present results indicate zonal flow levels close to the theoretical prediction, but its shear is much lower than that predicted by theory for transport regulation.

Chromospheric Evaporation in Solar Flares Revisited

We investigate the initial stage of chromospheric evaporation in flares using soft X-ray spectra obtained by the Bragg Crystal Spectrometer (BCS) experiment on Yohkoh. We find that the centroid wavelength of the Ca XIX line in spectra with the first detectable emission is within about 8.5 × 10-4 A of the rest wavelength, which corresponds to a Doppler shift of no more than 80 km s-1. We also determine the minimum detectable soft X-ray flare volume emission measure from BCS Ca XIX flare spectra. We find that the minimum detectable emission measured by BCS is produced by an X-ray flux that is about equal to the peak intensity of a class A6 flare. These results are difficult to reconcile with one-dimensional hydrodynamic simulations of an impulsively heated flare loop, which predict large Doppler shifts during the initial stage of the heating. Furthermore, inspection of high spatial resolution TRACE images of flare plasma indicate significant differences between the observed morphology and the predictions of hydrodynamic models. The evolution of the intensity and the Doppler shifts are more consistent with models that assume the sequential heating of small-scale threads rather than the heating of an individual loop. However, the bright knots of emission and asymmetrical intensity distributions seen in flare images cannot be explained by current numerical models of chromospheric evaporation.

In-beam measurement of the position resolution of a highly segmented coaxial germanium detector

The position resolution of a highly segmented coaxial germanium detector was determined by analyzing the 2055 keV γ -ray transition of Zr 90 excited in a fusion–evaporation reaction. The high velocity of the Zr 90 nuclei imparted large Doppler shifts. Digital analysis of the detector signals recovered the energy and position of individual γ -ray interactions. The location of the first interaction in the crystal was used to correct the Doppler energy shift. Comparison of the measured energy resolution with simulations implied a position resolution (root mean square) of 2 mm in three-dimensions.

Basic Study on Acoustic Communication Using Direct-Sequence Spread Spectrum for Underwater Acoustic Positioning

Supershort baseline (SSBL) underwater acoustic positioning is commonly utilized in estimating the positions of transponders, observation devices and submersibles. The new SSBL system in which a sea surface station and a sea bottom station communicate using the spectrum spread (SS) technique was studied with simulation. An M-sequence was adopted as pseudonoise (PN) code and PN code length was set for 15, 31, 63 and 127. Random noise and Doppler shift were considered as distortions. It became clear that the system becomes useless due to Doppler shift exponentially, and that under the condition with a large Doppler shift, the system is not available if the PN code length is longer than 31, that if the relative velocity between the transmitter and the receiver is less than 5 m/s, the system has the highest performance with a PN code length of 31 and that in case of a PN code length of 15 the system is stable under the same condition.

A Scattered Pilot OFDM Receiver Employing Turbo ICI Cancellation in Fast Fading Environments

This paper proposes a scattered-pilot-OFDM reception scheme employing turbo inter-carrier interference (ICI) cancellation in the fast varying fading environments of mobile communications. In the OFDM transmission, the orthogonality among the subcarriers cannot hold due to large Doppler shift, and the OFDM signal suffers from severe degradation due to ICI. The proposed receiver carries out two modes: (i) a coherent detection (CD) mode, and (ii) a turbo ICI cancellation (TC) mode. Initially, the receiver performs the CD mode. When any decision errors are detected, it shifts from the CD mode to the TC one that carries out both the ICI cancellation and the channel estimation by using the decoder output (the log likelihood ratio). In addition, the iteration of the TC mode can improve the accuracy of the channel estimation and ICI cancellation ability. Computer simulations following specifications for the mobile reception mode in the digital terrestrial television broadcasting demonstrate that the receiver can effectively cancel ICI due to the fast varying fading, and that its average BER performance is much better than that of CD.

Structure determination through measurements of Doppler-split Auger resonances in fixed-in-space molecules

Auger resonances of dissociating atoms in randomly oriented molecules experience large electronic Doppler shifts. We predict that when fixed-in-space molecules are considered there will appear extra Doppler resonances resulting from the diffractional scattering of the Auger electrons by the surrounding atoms. These resonances show sharp maxima in bond directions, something that makes them very promising as probes for local molecular structure using current energy- and angular-resolved electron-ion coincidence experiments.

Laser photodetachment mass spectrometry

We demonstrate that the technique of laser photodetachment spectroscopy on atomic negative ions can be used as a sensitivity enhancement tool in mass spectrometry, useful for suppressing both isotopic as well as molecular isobaric interferences. In the experiment a beam of negative ions and a laser beam are merged in a collinear geometry and the wavelength of the laser is tuned across the photodetachment threshold region. Due to the large differential Doppler shifts associated with the fast moving ions of different masses, it is possible to selectively detach ions of certain isotopes while leaving others unaffected. By choosing co-propagating laser and ion beams, the heavier isotopes of an element can be selectively detached, whereas counter propagating beams can be used to detach the lighter ones. In this paper we demonstrate the feasibility of the method in an experiment that was designed to selectively detach either 32S − or 34S − ions. The isotopic ratio of 34S/ 32S was enhanced by a factor of >50 over the natural value. This factor can be improved by using a laser that is better adapted to the measurement and by improving the vacuum conditions in order to further suppress the collisional background. The applicability of this technique to selectivity enhancement in measurements of the abundances of rare or ultra-rare isotopes is discussed.

Hot coronal loop oscillations observed with SUMER: Examples and statistics

We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly and , with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s-1. The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2-3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8-102 km s-1, together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

Hot loop oscillations seen by SUMER

Recently a new kind of damped oscillations of hot coronal loops was revealed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on SOHO (Kliem et al. 2002; Wang et al. 2002a, b, c). Observations of 17 flarelike events show that the oscillations have periods of 11−31 min, with a decay time of 5.5−29 min, and show an initial large Doppler shift pulse (up to 200 km s−1). The oscillations were interpreted in terms of standing slow or kink magnetosonic modes. An 1D MHD simulation by Ofman & Wang (2002) shows that slow waves can be damped quickly on a time scale comparable to observations due to the large thermal conduction in a hot coronal loop.

Interference Effects of Radiation Emitted from Nuclear Excitons

Interference effects in nuclear forward scattering of synchrotron radiation (NFSSR) from two spatially separated stainless-steel foils A and B mounted downstream behind each other have been investigated. Target A can be sinusoidally vibrated by high-frequency (MHz) ultrasound (US), target B is moved at a constant Doppler velocity which is large compared to the natural width of the nuclear transition. Due to this large Doppler shift radiative coupling between both targets is disrupted and the nuclear excitons in A and B develop independently in space and time after the SR pulse. As a consequence, the emission from the whole system (A&B) is dominated by the interference of the emissions from A and B. The application of US to target A is a powerful method to change the relative phasing of the emissions and thus to investigate interference effects originating from the two nuclear excitons in detail. Four distinct cases were studied: (a) If target A is kept stationary and only B is moved at large constant velocityv, the interference pattern exhibits a Quantum Beat (QB) whose period is determined byv. (b) If, in addition, target A is sinusoidally vibrated in a piston-like motion by US and the initial US phase Φ0 is locked to the SR pulse, the QB is frequency modulated by the US. The variation of the QB frequency increases with the US modulation indexm. (c) In the case that Φ0 is not synchronized to the SR pulse (phase averaging over Φ0) drastic changes of the amplitude and phase reversals of the QB pattern occur in the time regions around odd multiples of half of the US period. (d) If Φ0 is not synchronized to the SR pulse and the US motion is no longer pistonlike, the NFSSR intensity has to be averaged over both Φ0 andm (amplitude) of the US motion. Surprisingly the QB interference pattern does not vanish completely but a short QB signal remains at times of the full US period even at high values ofm. All NFSSR patterns investigated are interpreted and quantitatively described by the dynamical theory.

Advantages of a new modulation scheme in an optical self-mixing frequency-modulated continuous-wave system

A new frequency-modulated continuous wave modulation scheme, which gives correct results even when the Doppler shift is larger than the frequency difference associated with the range, is presented and tested with a tunable laser diode and fiber-based system. By insert- ing a constant frequency region in the modulation scheme, both the magnitude and the sign of all beat frequencies can be determined. When they are known, the correct frequency difference as a result of the range can be calculated. This new scheme gives more freedom when choosing the modulation parameters of the laser, because increasing the modula- tion frequency, and/or the frequency sweep, to avoid ambiguities result- ing from a large Doppler shift no longer becomes necessary. This is especially useful when using a somewhat cheaper laser diode source, since the maximum obtainable modulation frequency and frequency sweep can be somewhat limited. The suggested modulation scheme makes it possible to use some laser diodes in an application they other- wise would not be suited for. © 2002 Society of Photo-Optical Instrumentation En- gineers. (DOI: 10.1117/1.1467063)

Properties and Origin of Asymmetric and Unusual StokesVProfiles Observed in Solar Magnetic Fields

Between 5% and 10% of Stokes V profiles observed outside of sunspots with the High Altitude Observatory/National Solar Observatory Advanced Stokes Polarimeter are of unusual shape. We categorize them as follows. Normal profiles have two wings of opposite sign that typically show an asymmetry between both lobes. Single-lobed profiles have an amplitude above the noise level only for one wing. Single-lobed profiles occur because of a strong asymmetry between the red and blue wings in combination with low signal amplitude and the limited sensitivity of the measurement. The strongest verifiable amplitude asymmetry found is 90%. Mixed-polarity profiles are the most common type of unusual profiles. These profiles can occur because of the unresolved mixture of polarities in the solar atmosphere on spatial scales ≤600 km or because of the spreading of V signals due to seeing and diffraction. Single-lobed profiles and mixed polarities are mainly found outside high magnetic flux areas. Dynamic profiles consist of two or more superposed components with large Doppler shifts relative to each other. They are mainly observed in an emerging flux region. The measured relative Doppler shifts range up to 18 km s-1, indicating upward and downward moving shocks in magnetic fields.

Spectral Diagnostics of Chromospheres—A Signature of Emergent Magnetic Flux

The chromospheres of the Sun and solar-like stars respond to the underlying magnetic activity. The spectral lines of singly ionized calcium (e.g., Ca II K at 3933 Å) exhibit single or double reversals and serve as diagnostic tools for the magnetic activity. We present observations showing that the Ca II K spectral line may exhibit reversals in addition to the well-known K1, K2, and K3 components, when observed with high angular and spectral resolution. These Ca II K spectral line anomalies occur when small-scale, subarcsecond magnetic fields emerge through the chromosphere. The anomalous profiles originate at locations that are cospatial with Hα brightening at footpoints of dark active region fibrils, or active region filaments. The photospheric magnetic field is bipolar (small opposite polarity magnetic nodules) or has neutral lines at cospatial locations where the spectral lines are anomalous. Small-scale reconnective processes can cause these anomalous profiles. From a simultaneous time series of magnetic field data, we find that the emerging magnetic flux is associated with these profiles. The additional reversals primarily occur redward of the familiar K3 absorption trough, implying chromospheric downflow velocities that lead to the formation of shocks. The magnitude of the Doppler shift is of the order 40-50 km s-1. We present evidence to suggest that a combination of subresolution elements, each of which has regular profiles with large relative Doppler shifts could explain the formation of some of these profiles. A heuristic model that combines buoyant magnetic fields, convective collapse, gas evacuation, shock formation, heating, and a multicomponent model atmosphere can reproduce these profiles.

Sea trial results of a robust and spectral-efficient OFDM underwater communication system

In single-carrier, high data-rate systems symbol durations are much smaller than channel delay spreads. Reliable equalization in such systems is extremely difficult to attain because the channel is time varying and highly frequency selective. One way to circumvent the need of using costly equalization schemes, while maintaining high data rates, is to use multicarrier modulation in which broadband data is transmitted in parallel on multiple narrowband subcarriers. Symbol duration T in such systems is usually larger than delay spreads such that each narrowband subcarrier experiences only nonfrequency-selective flat fading. To improve spectral efficiency and eliminate the use of guard bands between subcarriers, each subcarrier can be chosen to be orthogonal to the others. Such a multicarrier system is referred to as orthogonal frequency division multiplexing (OFDM). Unfortunately, the orthogonality is destroyed in typical underwater channels due to large Doppler shifts and spreads (compared to the inverse of symbol durations). Therefore, previous work in UWA required the spacing of adjacent subcarriers to be 2/T, which is twice the minimum spacing 1/T. In this paper, time-rescaling and pilot-symbol aided channel estimation techniques are employed to combat Doppler shift and spreads. Reliable results are obtained with 1/T subcarrier spacing in sea trials.

Coherent and incoherent scatter radar observations during intense mid‐latitude spread F

An intense mid‐latitude spread‐F event occurred over Puerto Rico during the night of February 17, 1998. Simultaneous observations were made with the Cornell University Portable Radar Interferometer (CUPRI) located near Isabela, PR, the University of Illinois VHF radar located at Salinas, PR, GPS receivers at Isabela and St. Croix, measuring total electron content, the Arecibo incoherent scatter radar, and the Cornell All‐Sky imager located at the Arecibo Observatory. This was the first time that such a broad range of complementary instrumentation captured a mid‐latitude spread‐F space weather event. It was the first (and still only) time that a spread‐F event over the Caribbean exhibited large Doppler shifts in the VHF spectra. This event was characterized with multiple filaments that initially produced receding Doppler velocities exceeding 300 m/s as seen by CUPRI and the Illinois radar. The Arecibo incoherent scatter radar recorded line‐of‐sight velocities exceeding 100 m/s that moved the F‐layer peak to over 400‐km altitude. Airglow images of 630.0 nm emissions from F‐region heights showed depleted structures oriented southeast to northwest. The large velocities observed with the radars suggest that we caught this event in a stage of explosive development. It is interesting that the first fully documented Caribbean event occurred during a magnetically active period.

Laser cooling of metastable He atoms in an inhomogeneous electric field

We have investigated laser cooling of metastable helium atoms by laser light at 389 nm using inhomogeneous electric fields for compensating the varying Doppler shift. As a result of the high polarizability of the upper cooling level, we were able to compensate for the large Doppler shift associated with the UV transition frequency and the high beam velocity with moderate field strengths. The method represents an alternate way to the standard Zeeman slowing technique. It is expected to be superior in applications, where high magnetic fields cannot be tolerated.