SOLAR TERMINATOR AND IONOSPHERIC PROPAGATION OF RADIO WAVES

The paper provides a review of studies devoted to the peculiarities of the propagation of radio waves in the ionosphere in the area of the solar terminator, which is the only global and regular source of acoustic-gravitational waves and other disturbances of the ionosphere. It describes the results of theoretical works devoted to the study of perturbations created by the solar terminator in the area of the difference in the intensity of solar radiation in the atmosphere. The paper gives a review of experimental studies of the effects created by the solar terminator in the entire thickness of the atmosphere. These effects, in particular, include the multipath propagation of radio waves, their phase variations, and variations in amplitudes during the propagation of radio waves in the area of the solar terminator. In the interests of science and practice, a number of problems have been proposed for the further study of wave perturbations arising as a result of the movement of the temperature gradient at sunrise and sunset hours.

Numerical methods for the detection of phase defect structures in excitable media

Electrical waves that rotate in the heart organize dangerous cardiac arrhythmias. Finding the region around which such rotation occurs is one of the most important practical questions for arrhythmia management. For many years, the main method for finding such regions was so-called phase mapping, in which a continuous phase was assigned to points in the heart based on their excitation status and defining the rotation region as a point of phase singularity. Recent analysis, however, showed that in many rotation regimes there exist phase discontinuities and the region of rotation must be defined not as a point of phase singularity, but as a phase defect line. In this paper we use this novel methodology and perform comparative study of three different phase definitions applied to in-silico data and to experimental data obtained from optical voltage mapping experiments on monolayers of human atrial myocytes. We introduce new phase defect detection algorithms and compare them with those that appeared in literature already. We find that the phase definition is more important than the algorithm to identify sudden spatial phase variations. Sharp phase defect lines can be obtained from a phase derived from local activation times observed during one cycle of arrhythmia. Alternatively, similar quality can be obtained from a reparameterization of the classical phase obtained from observation of a single timeframe of transmembrane potential. We found that the phase defect line length was 35.9(62)mm in the Fenton-Karma model and 4.01(55)mm in cardiac human atrial myocyte monolayers. As local activation times are obtained during standard clinical cardiac mapping, the methods are also suitable to be applied to clinical datasets. All studied methods are publicly available and can be downloaded from an institutional web-server.

Phase Variation Measurement in Mach–Zehnder Interferometric Switch

Phase variations of the interrogation field lead to frequency shifts in Ramsey-type atomic clocks. This paper reports the development of a 300 MHz Mach–Zehnder (MZ) switch that effectively suppresses phase-transient effects. Similar to MZ interferometers, this radio-frequency (RF) MZ switch comprises two arms that are power- and phase-matched with each other. By inserting a PIN diode RF switch in one arm, the other arm remains undisturbed, freeing it of the phase transient. Trigger phase fluctuation measurements are implemented by using a lock-in amplifier to extract the in-phase and quadrature (I/Q) demodulation data. The results show that the extinction ratio of the RF MZ switch phase fluctuations is <5 μrad, which is significantly lower than that of a PIN (50 μrad). When applied to a cesium fountain clock, the RF MZ switch produces a frequency shift better than 1.73 × 10−16.

CHEOPS precision phase curve of the Super-Earth 55 Cancri e

Context. 55 Cnc e is a transiting super-Earth (radius 1.88 R⊕ and mass 8 M⊕) orbiting a G8V host star on a 17-h orbit. Spitzer observations of the planet’s phase curve at 4.5 μm revealed a time-varying occultation depth, and MOST optical observations are consistent with a time-varying phase curve amplitude and phase offset of maximum light. Both broadband and high-resolution spectroscopic analyses are consistent with either a high mean molecular weight atmosphere or no atmosphere for planet e. A long-term photometric monitoring campaign on an independent optical telescope is needed to probe the variability in this system. Aims. We seek to measure the phase variations of 55 Cnc e with a broadband optical filter with the 30 cm effective aperture space telescope CHEOPS and explore how the precision photometry narrows down the range of possible scenarios. Methods. We observed 55 Cnc for 1.6 orbital phases in March of 2020. We designed a phase curve detrending toolkit for CHEOPS photometry which allowed us to study the underlying flux variations in the 55 Cnc system. Results. We detected a phase variation with a full-amplitude of 72 ± 7 ppm, but did not detect a significant secondary eclipse of the planet. The shape of the phase variation resembles that of a piecewise-Lambertian; however, the non-detection of the planetary secondary eclipse, and the large amplitude of the variations exclude reflection from the planetary surface as a possible origin of the observed phase variations. They are also likely incompatible with magnetospheric interactions between the star and planet, but may imply that circumplanetary or circumstellar material modulate the flux of the system. Conclusions. This year, further precision photometry of 55 Cnc from CHEOPS will measure variations in the phase curve amplitude and shape over time.

Improving robustness of 3D multi-shot EPI by structured low-rank reconstruction of segmented CAIPI sampling for fMRI at 7T

Three-dimensional (3D) encoding methods are increasingly being explored as alternatives to multi-slice two-dimensional (2D) acquisitions in fMRI, particularly in cases where high isotropic resolution is needed. 3D multi-shot EPI is the most popular 3D fMRI acquisition method, but is susceptible to physiological fluctuations which can induce inter-shot phase variations, and thus reducing the achievable tSNR, negating some of the benefit of 3D encoding. This issue can be particularly problematic at ultra-high fields like 7T, which have more severe off-resonance effects. In this work, we aim to improve the temporal stability of 3D multi-shot EPI at 7T by improving its robustness to inter-shot phase variations. We presented a 3D segmented CAIPI sampling trajectory ("seg-CAIPI") and an improved reconstruction method based on Hankel structured low-rank matrix recovery. Simulation and in-vivo results demonstrate that the combination of the seg-CAIPI sampling scheme and the proposed structured low-rank reconstruction is a promising way to effectively reduce the unwanted temporal variance induced by inter-shot physiological fluctuations, and thus improve the robustness of 3D multi-shot EPI for fMRI.

Longitudinally Resolved Phase-curve Retrievals of WASP-43b

&lt;p&gt;Thermal phase variations of exoplanets are a patent testimony of their multidimensional nature: day-to-night temperature contrasts range from hundreds to thousands of degrees. &amp;#160;Nonetheless, the spectra of these planets have typically been fit using 1D retrieval codes that only account for vertical temperature gradients. &amp;#160;Recent multi-dimensional retrieval schemes are generally based on linear combinations of 1D models, which are more liable to degeneracies and more computationally demanding. &amp;#160;Here we present an alternative: phase-dependent spectral observations are inverted to produce longitudinally resolved spectra that can then be fitted using standard 1D spectral retrieval codes. We test this scheme on the phase-resolved spectra of WASP-43b and on simulated JWST observations using the open-source Pyrat Bay retrieval framework. &amp;#160;We show that 1D spectral retrievals on longitudinally resolved spectra are more accurate than applying 1D spectral retrieval codes to disk-integrated emission spectra, highlighting the impact of longitudinal variations in composition in addition to temperature. &amp;#160;In particular, we find that JWST phase measurements of WASP-43b should be treated with longitudinally resolved spectral retrieval.&lt;/p&gt;

Single-shot ranging and velocimetry far beyond the coherence length of CW lasers

The spectral linewidth of the continuous-wave (CW) lasers is one of the key limitations on the coherent lidar systems, which defines the maximum detection range. Furthermore, precise phase or frequency sweeping requirements is a deterrent in many applications. Here, we present the Phase-Based Multi-Tone Continuous Wave (PB-MTCW) lidar measurement technique that eliminates the necessity of using high coherence laser sources as well as any form of phase or frequency sweeping while employing coherent detection. In particular, we modulate a CW laser source with multiple radio-frequency (RF) tones to generate optical sidebands. Then we utilize the relative phase variations between the sidebands that are free from laser phase noises to calculate the target distance via post-processing and triangulation algorithms. We prove that the PB-MTCW technique is capable of performing single-shot ranging and velocimetry measurements at more than 500× the coherence length of a CW laser in a benchtop experimental demonstration.

Seasonal mode-1 M2 internal tides from satellite altimetry

The seasonal variability of mode-1 M2 internal tides is investigated using 25 years of multi-satellite altimeter data from 1992–2017. Four seasonal internal tide models are constructed using seasonally-subsetted altimeter data and World Ocean Atlas seasonal climatologies. This work is made possible by a newly-developed mapping procedure that can significantly suppress model errors. Seasonal-mean and seasonally-variable internal tide models are derived from the four seasonal models. All the models are inter-compared and evaluated using independent CryoSat-2 data. The seasonal-mean model is overall the best model because averaging the four seasonal models further reduces model errors. The seasonally-variable models are better in the tropical zone, where large seasonal signals may overcome model errors. Each seasonal model works best in its own season and worst in its opposite season. These internal tide models reveal that mode-1 M2 internal tides are subject to significant seasonal variability and their seasonal variations are a function of location. Large seasonal variations dominantly occur in the tropical zone, where the World Ocean Atlas climatology shows strong seasonal variations in ocean stratification. Seasonal phase variations are obtained from the directionally-decomposed internal tide components. They are dominantly ±60° at the equator and up to ±120° in the central Arabian Sea. Incoherence caused by seasonal phase variations is usually <10%, but may be up to 40–50% in the tropical zone.