Strong Doppler Shifts Research Articles

Large Photospheric Doppler Shift in Solar Active Region 12673. I. Field-aligned Flows

Delta (δ) sunspots sometimes host fast photospheric flows along the central magnetic polarity inversion line (PIL). Here we study the strong Doppler shift signature in the central penumbral light bridge of solar active region NOAA 12673. Observations from the Helioseismic and Magnetic Imager (HMI) indicate highly sheared and strong magnetic fields. Large Doppler shifts up to 3.2 km s−1 appeared during the formation of the light bridge and persisted for about 16 hr. A new velocity estimator, called DAVE4VMwDV, reveals fast converging and shearing motion along the PIL from HMI vector magnetograms, and recovers the observed Doppler signal much better than an old version of the algorithm. The inferred velocity vectors are largely (anti-)parallel to the inclined magnetic fields, suggesting that the observed Doppler shift contains a significant contribution from the projected field-aligned flows. High-resolution observations from the Hinode/Spectro-Polarimeter further exhibit a clear correlation between the Doppler velocity and the cosine of the magnetic inclination, which is in agreement with HMI results and consistent with a field-aligned flow of about 9.6 km s−1. The complex Stokes profiles suggest significant gradients of physical variables along the line of sight. We discuss the implications on the δ-spot magnetic structure and the flow-driving mechanism.

Waveform Design for Joint Sensing and Communications in Millimeter-Wave and Low Terahertz Bands

The convergence of radar sensing and communication applications in the millimeter-wave (mmWave) and low terahertz (THz) bands has been envisioned as a promising technology, since it incorporates high-rate data transmission of hundreds of gigabits per second (Gbps) and mm-level radar sensing in a spectrum- and cost-efficient manner, by sharing both the frequency and hardware resources. However, the joint radar sensing and communication (JRC) system faces considerable challenges in the mmWave and low-THz scale, due to the peculiarities of the propagation channel and radio-frequency (RF) front ends. To this end, the waveform design for the JRC systems in mmWave and low-THz bands with ultra-broad bandwidth is investigated in this paper. Firstly, by considering the JRC design based on the co- existence concept, where both functions operate in a time-domain duplex (TDD) manner, a novel multi-subband quasi-perfect (MS-QP) sequence, composed of multiple perfect subsequences on different subbands, is proposed for target sensing, which achieves accurate target ranging and velocity estimation, whilst only requiring cost-efficient low-rate analog-to-digital converters (A/Ds) for sequence detection. Furthermore, the root index of each perfect subsequence is designed to eliminate the influence of strong Doppler shift on radar sensing. Finally, a data-embedded MS-QP (DE-MS-QP) waveform is constructed through time-domain extension of the MS-QP sequence, generating null frequency points on each subband for data transmission. Unlike the co- existence-based JRC system in TDD manner, the proposed DE-MS-QP waveform enables simultaneous interference-free sensing and communication, whilst inheriting all the merits from MS-QP sequences. Numerical results validate the superiority of the proposed waveforms regarding the communication and sensing performances, hardware cost as well as flexibility of the resource allocation between the dual functions.

Novel Adaptive Distributed Compressed Sensing Algorithm for Estimating Channels in Doubly-Selective Fading OFDM Systems

Doubly-selective (DS) fading channel is often occurred in many orthogonal frequency division multiplexing (OFDM) communication systems, such as high-speed rail communication systems and underwater acoustic (UWA) wireless networks. It is challenging to provide an accurate and fast estimation over the doubly-selective channel, due to the strong Doppler shift. This paper addresses the doubly selective channel estimation problem based on complex exponential basis expansion model (CE-BEM) in OFDM systems from the perspective of distributed compressive sensing (DCS). We propose a novel DCS-based improved sparsity adaptive matching pursuit (DCS-IMSAMP) algorithm. The advantage of the proposed algorithm is that it can exploit the joint channel sparsity information using dynamic threshold, variable step size and tailoring mechanism. Simulation results show that the proposed algorithm achieves 5dB performance gain with faster operation speed, in comparison with traditional DCS-based sparsity adaptive matching pursuit (DCS-SAMP) algorithm.

Whistler mode waves for ring distribution with A.C. electric field in inner magnetosphere of Saturn

Whistler mode waves can propagate upstream without collision impact. They are generated in these areas of vibration. They are known to play a crucial role in thermodynamics and electron acceleration. Sometimes, in some cases, they are seen as waves that strike the wavefront. Mercury, Earth, Venus and Saturn are the planets where whistlers have been recorded in the upstream regions. They are right handed waves and can be left-hand polarized in the frame of spacecraft due to the strong negative Doppler shift. The weaker Doppler shift owes to the large angle between magnetic field vectors at 10 AU (Astronomical unit) and the solar wind velocity. These waves propagate with an angle between 10 to 60 degrees to background magnetic field. In the present paper, we took an advantage of Cassini present in the Saturnian magnetosphere to explore the whistler mode wave’s importance. A dispersion relation for obliquely as well as for whistler waves propagating perpendicular to the magnetic field, has been applied to Saturnian magnetosphere. Using the observations made by Voyager and Cassini, growth rate has been determined for non-relativistic plasma. Whistler waves are excited by temperature anisotropy, where the vertical temperature is higher than the parallel temperature. The effect of electron density, temperature anisotropy, energy density with some other parameters on the growth of whistler mode emission is studied. The result is found to be in good agreement with observations. Whistler mode wave interaction with particles basically emphasizes on the increase (decrease) in the energy of resonant particles and this variation is related to the transfer of energy to (from) other resonant particle group where the wave is the mediator of the energization process. Due to the non-monotonic nature of the ring distribution, at vertical velocities, the magnification produced by this instability is larger than the typical bi-Maxwellian anisotropy distribution because the wave can maintain resonance over a longer portion of its orbit.

Chromospheric counterparts of solar transition region unresolved fine structure loops

Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models.

Asynchronous Detection and Identification of Multiple Users by Multi-Carrier Modulated Complementary Set of Sequences

The correct detection of arrivals is a crucial step in any communication system, in order to achieve frame synchronization. This step is even more relevant in underwater acoustic communications, as the medium is characterized by hazardous phenomena such as long multipath delays and non-uniform Doppler shifts, which hinder the detection process. All these phenomena make the detection of arrivals and identification of multiple users in underwater communication systems a challenging task. In this paper, a multi-user detector is presented. The detection is based on the direct-sequence code-division multiple access technique, by using a combination of two sequences as a preamble: a Zadoff-Chu sequence that provides candidate peaks and frame synchronization, and a complementary set of sequences (CSS) that validates the true Zadoff-Chu arrival while at the same time identifies the user. Both sequences are modulated using multi-carrier techniques, and the users can transmit asynchronously to the channel. In the case of CSS, multi-carrier modulation techniques allow performing channel equalization and demodulation at the receiver in a simple manner, enabling the correlation of the CSS at bit level, thus keeping its ideal correlation properties. Sea trials were conducted in February 2017 at Suruga Bay, Japan, in which the reception of two different users was analyzed under different noise and multipath conditions, as well as uniform and non-uniform Doppler shifts. The results show that, even under strong multipath and non-uniform Doppler shifts caused by roll and pitch motions, the receiver is able to detect and identify the user in 98% of the transmitted packets.

Temporal relation between quiet-Sun transverse fields and the strong flows detected by IMaX/SUNRISE

Localized strongly Doppler-shifted Stokes V signals were detected by IMaX/SUNRISE. These signals are related to newly emerged magnetic loops that are observed as linear polarization features. We aim to set constraints on the physical nature and causes of these highly Doppler-shifted signals. In particular, the temporal relation between the appearance of transverse fields and the strong Doppler shifts is analyzed in some detail. We calculated the time difference between the appearance of the strong flows and the linear polarization. We also obtained the distances from the center of various features to the nearest neutral lines and whether they overlap or not. These distances were compared with those obtained from randomly distributed points on observed magnetograms. Various cases of strong flows are described in some detail. The linear polarization signals precede the appearance of the strong flows by on average 84+-11 seconds. The strongly Doppler-shifted signals are closer (0.19") to magnetic neutral lines than randomly distributed points (0.5"). Eighty percent of the strongly Doppler-shifted signals are close to a neutral line that is located between the emerging field and pre-existing fields. That the remaining 20% do not show a close-by pre-existing field could be explained by a lack of sensitivity or an unfavorable geometry of the pre-existing field, for instance, a canopy-like structure. Transverse fields occurred before the observation of the strong Doppler shifts. The process is most naturally explained as the emergence of a granular-scale loop that first gives rise to the linear polarization signals, interacts with pre-existing fields (generating new neutral line configurations), and produces the observed strong flows. This explanation is indicative of frequent small-scale reconnection events in the quiet Sun.

MEASUREMENTS OF ANISOTROPIC ION TEMPERATURES, NON-THERMAL VELOCITIES, AND DOPPLER SHIFTS IN A CORONAL HOLE

We present a new diagnostic allowing one to measure the anisotropy of ion temperatures and non-thermal velocities as well as Doppler shifts with respect to the ambient magnetic field. This method provides new results, as well as independent test for previous measurements obtained with other techniques. Our spectral data come from observations of a low latitude, on-disk coronal hole. A potential field source surface model was used to calculate the angle between the magnetic field lines and the line of sight for each spatial bin of the observation. A fit was performed to determine the line widths and Doppler shifts parallel and perpendicular to the magnetic field. For each line width component we derived parallel and perpendicular ion temperatures and non-thermal velocities. The perpendicular ion temperature was cooler than off-limb polar coronal hole measurements. The parallel ion temperature was consistent with a uniform temperature of 1.8 +/- 0.2 x 10^{6} K for each ion. Since parallel ion heating is expected to be weak, this ion temperature should reflect the proton temperature. A comparison between our results and others implies a large proton temperature gradient around 1.02 R_sun. The non-thermal velocities are thought to be proportional to the amplitudes of various waves. Our results for the perpendicular non-thermal velocity agree with Alfv\'en wave amplitudes inferred from off-limb polar coronal hole line width measurements. Our parallel non-thermal velocity results are consistent with slow magnetosonic wave amplitudes inferred from Fourier analysis of time varying intensity fluctuations. Doppler shift measurements yield outflows of ~5 km s^-1 for ions formed over a broad temperature range. This differs from other studies which found a strong Doppler shift dependence on formation temperature.

Mit castor satellite: Design, implementation, and testing of the communication system

Cathode Anode Satellite Thruster for Orbital Reposition (CASTOR) is an orbital manoeuvre and transfer micro-satellite bus developed at MIT Space System Laboratory. The technical objective of the mission is achieving 1km/s of delta-V over a 1 year mission in Low Earth Orbit (LEO). This will be accomplished using a novel electric propulsion system, the Diverging Cusped Field Thruster (DCFT), which enables high efficiency orbital changes of the ESPA-ring class satellite. CASTOR is capable of improving rapid access to space capabilities by providing an orbital transfer platform with a very high performance to mass ratio, thus greatly reducing launch costs and allowing for highly efficient orbital manoeuvre. Furthermore, CASTOR is highly scalable and modular, allowing it to be adapted to a wide range of scales and applications. CASTOR is developed as part of the University Nanosatellite Program (UNP) funded by Air Force Research Laboratory (AFRL).In order to accomplish CASTOR mission objective, a highly optimized, scalable, light weight, and low cost communication system needed to be developed. These constraints imply the development of trade studies to select the final communication system architecture able to maximize the amount of data transmitted, while guaranteeing reliability, redundancy and limited mass, power consumption, and cost. A special attention is also required to guarantee a reliable communication system in cases of tumbling, or in case of strong Doppler shift which is inevitable due to the high delta-V capabilities of the vehicle. In order to accomplish all the mission requirements, different features have been introduced in the design of the communication system for this mission. Specifically, customized patch antennas have been realized, and a customized communication protocol has been designed and implemented. The communication subsystem has been validated through an intense testing campaign which included software tests in the laboratory, hardware tests in anechoic chamber, and in flight tests through a balloon experiment.The article presents an overview of CASTOR mission, a presentation of the trade studies analysis and of the final communication architecture selected, a description of the customized antenna developed, of the customized protocol designed, and a presentation of the results of the tests performed.

Angular momentum transport by thermal emission in black hole accretion disks

AbstractWe calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disk, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disk material. While thermal emission can not drive accretion on its own, we show that along with disk heating it does nonetheless result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity‐driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi‐color disk is ∼0.4rs/r, where rs is the Schwarzschild radius of the black hole. We briefly highlight the key similarties between this effect and the closely related Poynting‐Robertson effect (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

STELLAR-MASS BLACK HOLE SPIN CONSTRAINTS FROM DISK REFLECTION AND CONTINUUM MODELING

Accretion disk reflection spectra, including broad iron emission lines, bear the imprints of the strong Doppler shifts and gravitational red-shifts close to black holes. The extremity of these shifts depends on the proximity of the innermost stable circular orbit to the black hole, and that orbit is determined by the black hole spin parameter. Modeling relativistic spectral features, then, gives a means of estimating black hole spin. We report on the results of fits made to archival X-ray spectra of stellar-mass black holes and black hole candidates, selected for strong disk reflection features. Following recent work, these spectra were fit with reflection models and disk continuum emission models (where required) in which black hole spin is a free parameter. Although our results must be regarded as preliminary, we find evidence for a broad range of black hole spin parameters in our sample. The black holes with the most relativistic radio jets are found to have high spin parameters, though jets are observed in a black hole with a low spin parameter. For those sources with constrained binary system parameters, we examine the distribution of spin parameters versus black hole mass, binary mass ratio, and orbital period. We discuss the results within the context of black hole creation events, relativistic jet production, and efforts to probe the innermost relativistic regime around black holes.

Magnetic field emergence in quiet Sun granules

We describe a new form of small-scale magnetic flux emergence in the quiet Sun. This process seems to take vertical magnetic fields from the solar interior to the photosphere, where they appear above granular convection cells. High-cadence time series of spectropolarimetric measurements obtained by Hinode in a quiet region near disk center are analyzed. We extract line parameters from the observed Stokes profiles and study their evolution with time. The circular polarization maps derived from the observed \ion{Fe}{i} 630 nm lines show clear magnetic signals emerging at the center of granular cells. We do not find any evidence for linear polarization signals associated with these events. The magnetic flux patches grow with time, occupying a significant fraction of the granular area. The signals then fade until they disappear completely. The typical lifetime of these events is of the order of 20 minutes. No significant changes in the chromosphere seem to occur in response to the emergence, as revealed by co-spatial \ion{Ca}{ii} H filtergrams. The Stokes I and V profiles measured in the emerging flux concentrations show strong asymmetries and Doppler shifts. The origin of these events is unclear at present, but we suggest that they may represent the emergence of vertical fields lines from the bottom of the photosphere, possibly dragged by the convective upflows of granules. Preliminary inversions of the Stokes spectra indicate that this scenario is compatible with the observations. The emergence of vertical field lines is not free from conceptual problems, though.

Doppler Shift Correlations in the Solar Transition Region

I investigate dynamical correlations between the lower transition and the upper transition using spectra from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The measured quantity is the Doppler shift (mostly solar radial) along the line of sight. The purpose is to shed light on the fraction of the lower transition that resides in cool structures not physically associated with higher temperature regions and the fraction that can be described by a classical transition with a physical connection from the chromosphere into the corona. Transition region in this context means the transition sufficiently bright in ultraviolet spectral lines to be observed. I find strong Doppler shift correlations between two different lower lines from C IV and S V, but much weaker correlations between these lines and a line of Ne VIII formed well into the upper transition region. I conclude that most of the lower transition that is observable because of its brightness arises in cool loop structures. The results also favor a scenario in which the lower transition is heated and cooled transiently on timescales less than or perhaps comparable to the SUMER exposure times of 21 s for the data analyzed in this work.

Beamforming and Synchronization Algorithms Integration for OFDM HAP-Based Communications

High Altitude Platforms (HAPs) are currently experiencing a great interest as potential solutions for the growing demand of wireless broadband applications, but their rapid motion and the characteristics of the stratospheric channel, which may cause strong attenuation, multipath fading and Doppler shift, can strongly impact their performances. The described scenario requires the use of beamforming techniques, able to provide medium to high gains in a mobile environment. Given these premises, this paper discusses the conjugation of OFDM techniques with smart antennas for HAP-based communications, stressing the need for integration of the different functional blocks of an OFDM receiver. In fact, in order to face a time varying scenario, adaptive techniques are needed, and only a careful joint design of time and frequency synchronization with beamforming algorithms can guarantee reliable communication. The detailed and integrated receiver design described in the paper is based on the OFDM version of the IEEE 802.16a transmission standard.

Stimulated acoustic emission imaging ("sono-scintigraphy") with the ultrasound contrast agent Levovist: a reproducible Doppler ultrasound effect with potential clinical utility.

Levovist (Schering AG, Berlin, Germany) is a microbubble agent which improves Doppler ultrasound examinations by increasing the intensity of the Doppler signal. It is licensed for the improvement of suboptimal Doppler examinations in several countries, including the UK. Levovist is constituted from galactose microaggregates and water, with a small admixture of palmitic acid (0.1%). When made up, multiple small stabilized air microbubbles are produced with a mean diameter of 2-3 mm. Extensive clinical trials with Levovist have demonstrated excellent tolerance and a very good safety profile (1). This increase in backscatter is generally regarded as the major clinically important mechanism of most microbubble agents, but there is increasing evidence that ultrasound at diagnostic energies can also disrupt microbubbles to produce transient intense Doppler signals. In color Doppler ultrasound mode, these are displayed as strong random-velocity Doppler shifts, producing a mosaiclike display. A novel microbubble agent, SHU563A (Schering AG), is known to be particularly effective at producing

Comparison of properties of upstream whistlers at different planets

Whistler mode waves have been recorded in the upstream region of Mercury, Venus, Earth and Saturn. They are elliptically polarized and observed typically at frequencies between 0.1 to 4 Hz. These intrinsically right handed waves can be left-hand polarized in the spacecraft frame as a result of strong negative Doppler shift. The waves propagate at an angle between 10 and 60 deg to the background magnetic field, with ΔB B rarely exceeding 0.1. Comprehensive studies of these waves at Earth and Venus indicate that upstream whistlers are generated at the shock rather than locally in the foreshock. In this paper we compare properties of upstream whistlers at all these planets. We also discuss the utilization of selected properties of these waves to evaluate the effective Alfvenic Mach number and the shock thickness at Mercury where solar wind measurements are not available.

In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Capillary‐gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary‐gravity waves is in reasonable agreement with measurements by Jähne and Riemer (1990) obtained from measurements in a large wind‐wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope‐frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary‐gravity waves. Close examination of combined wavenumber‐frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary‐gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.

Chromospheric jets - Possible extreme-ultraviolet observations of spicules

Stigmatic EUV spectra of chromospheric lines obtained by the Naval Research Laboratory High Resolution Telescope and Spectrograph (HRTS) show small arc second regions with strong (10-20 km s/sup -1/) Doppler shifts which we call ''chromospheric jets''. Because of the strong resemblance of their physical and statistical properties, such as temperature, density, velocity, size, and birthrate, to those of visible light spicules, it is highly likely that they are different manifestations of the same phenomena, although a one-to-one identification has not yet been made. The main difference is the 40 s lifetime of the chromophseric jets which is shorter than the 5 minute lifetime of visible light spicules. The chromospheric jets are found in the supergranular cell interiors where chromospheric and transition zone line intensities are weak. Data H..cap alpha.. mottles found in the cell boundaries are well correlated with intense, symmetric chromospheric EUV line profiles and are only weakly related to the chromospheric jets.