Atmospheric Loss Research Articles

Enhanced radiation characteristics of graphene-based patch antenna array employing photonic crystals and dielectric grating for THz applications

The Graphene patch antennas is widely used due to high electrical conductivity, high mobility, and saturation velocity in THz band. In a Terahertz (THz) band, high-gain antennas to be preferred to minimized the atmospheric attenuation and path losses. In this article, the enhanced radiation characteristics graphene-based antenna array design concept is proposed by using homogeneous substrate, photonic band gap crystal (PBG) substrate and superstrated dielectric grating. The projected graphene-based 2×1 patch array antennas works as a resonance patch with complementary split split split ring resonator (CSSSRR) for the frequency range from 0.84 THz to 0.94 THz. The tunable property of graphene material and PBG substrate are investigated by varying the chemical potential (μc) of graphene sheet and different radius of hollow cylinders respectively. The minimum return loss of -57.052 dB, peak gain of 14.69 dB and directivity of 15.5 dBi are achieved with the planned antenna employing with superstrated dielectric grating structures. The suggested 2×1 graphene patch antenna array are operating in low atmospheric attenuation window henceforth suitable for the application of medical imaging, threat detection, and THz wireless communication.

Parasitic‐coupled high‐gain graphene antenna employed on PBG dielectric grating substrate for THz applications

AbstractIn THz frequency region, the high‐gain miniaturized patch antenna is essential to overcome the atmospheric losses. A novel dual‐band miniaturized graphene antenna array with parasitic‐coupled feed and mounted on PBG dielectric grating substrate is proposed in the operating frequency band from 0.85 to 1.04 THz. The analysis of graphene surface conductivity, surface wave polariton (SPP) of photonic band gap (PBG) substrate, and the dielectric grating substrate has been done for the verification of a projected design. Parametric analysis of PBG radius, periodic perturbation of dielectric grating substrate, and mutual coupling distance has been verified to achieve the enhanced radiation characteristics of the antenna. The minimum return loss of −52.58 dB, VSWR of 1.005, impedance bandwidth of 33.34 GHz, the peak gain of 16.4 dB, and peak directivity of 17 dBi has been observed for the projected design. The parasitic‐coupled method is suitable to reduce the complexity of the antenna array and proposed design is useful in imaging, security using spectroscopy, and wireless communication applications.

Measuring the D/H Ratios of Exoplanets and Brown Dwarfs

Abstract The relative abundance of deuterium and hydrogen is a potent tracer of planet formation and evolution. Jupiter and Saturn have protosolar atmospheric D/H ratios, a relic of substantial gas accretion from the nebula, while the atmospheres of Neptune and Uranus are enhanced in D by accretion of ices into their envelopes. For terrestrial planets, D/H ratios are used to determine the mechanisms of volatile delivery and subsequent atmosphere loss over the lifetime of the planet. Planets and brown dwarfs more massive than ∼13 M J quickly fuse their initial D reservoir. Here, we simulate spectra for giant exoplanets and brown dwarfs (2 M Neptune to ∼10 M Jupiter) from T eff = 200–1800 K including both CH3D and HDO to determine the observability of these dominant deuterium isotopologues in mid-infrared thermal emission spectra. Colder objects have stronger molecular features in their spectra, due to the temperature dependence of molecular cross sections. CH3D is easier to observe than HDO at all temperatures considered, due to the strength of its absorption bands and locations of features at wavelengths with few other strong absorption features. We predict that for nearby cool brown dwarfs, the CH3D abundance can be measured at high signal to noise with the James Webb Space Telescope (JWST); for objects from 200 to 800 K closer than 10 pc, a protosolar D/H ratio would be readily observable in 2.5 hr. Moderately young Jupiter-mass planets (100–300 Myr) and young Neptunes (10 Myr) may be discovered with JWST and provide the best targets for detecting deuterium on an exoplanet in the coming decade. Future telescope designs should consider the importance of isotopes for understanding the formation and evolution of planetary atmospheres.

Optimal transceiver placement and resource allocation schemes in cooperative dynamic FSO networks

Cooperative dynamic free-space optical (FSO) networks exploit the fact that atmospheric losses are distance dependent to enhance the performance of FSO networks. This enhancement is achieved by sharing the resources of shorter links among different nodes in the network. In this paper, two joint transceiver placement and resource allocation schemes are proposed to optimally place FSO redundant transceivers based on optimal resource allocation in cooperative dynamic FSO networks. Specifically, one scheme increases reliability and capacity, while the other increases reliability and fairness of cooperative dynamic FSO networks during severe weather conditions. The schemes are formulated as multi-objective and bi-level integer linear programming problems and solved using an exhaustive search to obtain optimal solutions. The numerical results reveal that higher reliabilities can be achieved with enhanced capacities and fairness using the first and second schemes, respectively. Furthermore, these improvements are achieved by using many fewer numbers of FSO redundant transceivers than those of random placement.

Role of Planetary Obliquity in Regulating Atmospheric Escape: G-dwarf versus M-dwarf Earth-like Exoplanets

Abstract We present a three-species (H+, O+ and e−) multi-fluid magnetohydrodynamic model, endowed with the requisite upper-atmospheric chemistry, that is capable of accurately quantifying the magnitude of oxygen ion losses from “Earth-like” exoplanets in habitable zones, whose magnetic and rotational axes are roughly coincidental with one another. We apply this model to investigate the role of planetary obliquity in regulating atmospheric losses from a magnetic perspective. For Earth-like exoplanets orbiting solar-type stars, we demonstrate that the dependence of the total atmospheric ion loss rate on the planetary (magnetic) obliquity is relatively weak; the escape rates are found to vary between 2.19 × 1026 s−1 to 2.37 × 1026 s−1. In contrast, the obliquity can influence the atmospheric escape rate (∼1028 s−1) by more than a factor of 2 (or 200%) in the case of Earth-like exoplanets orbiting late-type M-dwarfs. Thus, our simulations indicate that planetary obliquity may play a weak-to-moderate role insofar as the retention of an atmosphere (necessary for surface habitability) is concerned.

VLBI imaging of black holes via second moment regularization

The imaging fidelity of the Event Horizon Telescope (EHT) is currently determined by its sparse baseline coverage. In particular, EHT coverage is dominated by long baselines, and is highly sensitive to atmospheric conditions and loss of sites between experiments. The limited short/mid-range baselines especially affect the imaging process, hindering the recovery of more extended features in the image. We present an algorithmic contingency for the absence of well-constrained short baselines in the imaging of compact sources, such as the supermassive black holes observed with the EHT. This technique enforces a specific second moment on the reconstructed image in the form of a size constraint, which corresponds to the curvature of the measured visibility function at zero baseline. The method enables the recovery of information lost in gaps of the baseline coverage on short baselines and enables corrections of any systematic amplitude offsets for the stations giving short-baseline measurements present in the observation. The regularization can use historical source size measurements to constrain the second moment of the reconstructed image to match the observed size. We additionally show that a characteristic size can be derived from available short-baseline measurements, extrapolated from other wavelengths, or estimated without complementary size constraints with parameter searches. We demonstrate the capabilities of this method for both static and movie reconstructions of variable sources.

The Hubble PanCET program: an extensive search for metallic ions in the exosphere of GJ 436 b

Context. The quiet M2.5 star GJ 436 hosts a warm Neptune that displays an extended atmosphere that dwarfs its own host star. Predictions of atmospheric escape in such planets state that H atoms escape from the upper atmosphere in a collisional regime and that the flow can drag heavier atoms to the upper atmosphere. It is unclear, however, what astrophysical mechanisms drive the process. Aims. Our objective is to leverage the extensive coverage of observations of the far-ultraviolet (FUV) spectrum of GJ 436 obtained with the Cosmic Origins Spectrograph (COS) to search for signals of metallic ions in the upper atmosphere of GJ 436 b, as well as study the activity-induced variability of the star. Methods. We analyzed flux time-series of species present in the FUV spectrum of GJ 436 and successfully performed geocoronal contamination removal in the COS Lyman-α profiles obtained near the Earth’s night-side. Results. GJ 436 displays flaring events with a rate of ~10 d−1. There is evidence for a possibly long-lived active region or longitude that modulates the FUV metallic lines of the star with amplitudes up to 20%. Despite the strong geocoronal contamination in the COS spectra, we detected in-transit excess absorption signals of ~50 and ~30% in the blue and red wings, respectively, of the Lyman-α line. We rule out a wide range of excess absorption levels in the metallic lines of the star during transit. Conclusions. The large atmospheric loss of GJ 436 b observed in Lyman-α transmission spectra is stable over the timescale of a few years, and the red wing signal supports the presence of a variable hydrogen absorption source besides the stable exosphere. The previously claimed in-transit absorption in the Si III line is likely an artifact resulting from the stellar magnetic cycle. The non-detection of metallic ions in absorption could indicate that the escape is not hydrodynamic or that the atmospheric mixing is not efficient in dragging metals high enough for sublimation to produce a detectable escape rate of ions to the exosphere.

FSO Cooperative System Analysis for DF based Relay

This paper presents performance and information theoretic analysis for cascaded free space optical (FSO) communication system using decode-and-forward relaying protocol. Optical links experience Gamma-Gamma distributed atmospheric turbulence, path loss, and misalignment losses. Derived closed-form statistical characteristics of the end-to-end SNR of the system are used for outage analysis, bit-error-rate analysis, and information theoretic analysis of the proposed system, for various modulation schemes and adaptive transmission protocols, respectively. In the performance analysis, impact of misalignment losses and scintillations is seen on various performance parameters and the adaptive modulation schemes using capacity of proposed systems.

Laser Ranging to Unknown Objects for Initial Orbit Determination: A Feasibility Study

Satellite laser ranging to objects without corner-cube retroreflectors is explored as a means of providing range data for initial orbit determination. The goal is to couple laser range data with optical angular data and eliminate the need for radar. This study investigates this concept using analysis and simulation. It considers the effects of laser energy per pulse, pulse rate, path loss, telescope diameter, object cross section, atmospheric losses, and noise photons. It develops signal processing algorithms that can successfully determine orbit when 20–40 true-returned photons and 1200–1300 noise photons arrive during a tracking window. These algorithms include initial clustering calculations that distinguish between true and noise photons. They culminate with an optimal estimation algorithm that explicitly accounts for noise/clutter photons. One example case considers a low-Earth-orbit object with an optical cross section of at a distance of 1170 km. The laser transmission/reception telescope has a diameter of 0.18 m, and the laser transmits 100 pulses per second at 0.25 J per pulse. Despite receiving a mere 18 true photon returns during a 20 s tracking interval, the system is able to determine orbit to a 7 m position accuracy and a velocity accuracy.

Feasibility analysis of opto-electronic THz Earth-satellite links in the low- and mid-latitude regions.

Terahertz (THz) communication is considered as a promising technology for the Earth-space application due to its high potential of supporting large data capacity. However, THz wave suffers extreme attenuation from absorption of water vapor (WV) molecules in the propagation path. In this work, we present a theoretical model and analyze the capacities of both inter-satellite and geostationary satellite-to-Earth station (GEO-ES) opto-electronic links operating in the 100-500GHz band within low- and mid-latitude regions. In our work, THz frequency windows in the 140, 220, 340, and 410GHz bands with relatively low atmospheric loss are selectively used, targeting a data capacity of 10Gbps per gigahertz. Our analysis indicates that, in the low-latitude regions, due to high water vapor density (WVD), transmitting and receiving antennas with extremely high gains are required. On the contrary, the mid-latitude regions require less power due to comparatively lower WVD. Moreover, due to seasonal variation in the mid-latitude regions, the requirement of link power budget is tens of decibels less in winter as compared to summer. The results suggest that the establishment of GEO-ES THz links in low- and mid-latitude regions is more realistic in the sub-THz bands, such as 140 and 220GHz, while the potential of using higher carrier frequencies above 300GHz for inter-satellite THz links, due to the absence of WV-induced absorption, is supported.

Engineering equations for the filamentation collapse distance in lossy, turbulent, nonlinear media.

The propagation of high peak-power laser beams in real atmospheres has been an active research area for a couple of decades. Atmospheric turbulence and loss will induce decreases in the filamentation self-focusing collapse distance as the refractive index structure parameter and volume extinction coefficient, respectively, increase. This paper provides a validated analytical method for predicting the filamentation onset distance in lossy, turbulent, nonlinear media. It is based on a modification of Petrishchev's and Marburger theories. It postulates that the ratio of the peak power to critical power at range in turbulence is modified by a multiplicative, rather than additive, gain factor. Specifically, this new approach modifies the Petrishchev's turbulence equation to create the required multiplicative factor. This is necessary to create the shortened filamentation onset distance that occurs when a laser beam propagates through the cited nonlinear medium. This equation then is used with the Marburger distance and the Karr et al loss equations to yield the filamentation onset distance estimate in lossy, turbulent, nonlinear environment. Theory validation is done against two independent sets of computer simulation results. One comes from the NRL's HELCAP software and the other from MZA's Wave Train modeling software package. This paper also shows that once the zero-turbulence onset distance is set based on link loss, the addition of turbulence creates essentially the same PDFs at similar median distances for each loss case. This result had not been previously reported. This is the first quantitative comparison between closed form equations and computer simulation results characterizing filament generation in a lossy, turbulent, nonlinear medium.

Absence of a thick atmosphere on the terrestrial exoplanet LHS3844b.

Most known terrestrial planets orbit small stars with radii less than 60 per cent of that of the Sun1,2. Theoretical models predict that these planets are more vulnerable to atmospheric loss than their counterparts orbiting Sun-like stars3-6. To determine whether a thick atmosphere has survived on a small planet, one approach is to search for signatures of atmospheric heat redistribution in its thermal phase curve7-10. Previous phase curve observations of the super-Earth 55 Cancri e (1.9 Earth radii) showed that its peak brightness is offset from the substellar point(latitude and longitude of 0 degrees)-possibly indicative of atmospheric circulation11. Here we report a phase curve measurement for the smaller, cooler exoplanet LHS3844b, a 1.3-Earth-radii world in an 11-hour orbit around the small nearby star LHS3844. The observed phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1,040±40 kelvin and a nightside temperature consistent with zero kelvin (at one standard deviation). Thick atmospheres with surface pressures above 10 bar are ruled out by the data (at three standard deviations), and less-massive atmospheres are susceptible to erosion by stellar wind. The data are well fitted by a bare-rock model with a low Bond albedo (lower than 0.2 at two standard deviations). These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.

Monitoring Venus and communications relay from Lagrange Points

Orbits around the two close collinear Sun-Earth Lagrange points have been utilized in recent decades for many solar and astronomical missions to exploit the observational advantages. DSCOVR is the first satellite to observe Earth continuously from the vicinity of the L1 point and is filing a crucial gap in Earth observations with its vast collection of polar, geosynchronous and some retrograde low inclination orbits. Spacecraft orbiting Venus in the last four decades have provided us with a wealth of information and many unanswered questions, which cannot be addressed adequately by observing from polar or near equatorial orbits around the planet. The Sun-Venus collinear Lagrange points L1 (sunward) and L2 (behind the planet from the Sun) points are key vantage points located about a million km away from the planet along the direction to the Sun which enable continuous monitoring of the planet's day and night hemispheres. As spacecraft positions at L1 and L2 points are unstable, they can be inserted in orbits around them to observe Venus over a small range of phase angles unlike any Venus orbiter observations which cover 0–180° solar phase angle twice each orbit for a very long time with minimal station keeping costs. To help better understand Venus, we propose that monitoring Venus continuously at nearly the same phase angle from the vicinity of L1 and L2 Lagrange points is critical. Such Lagrange orbiters around Sun-Venus L1 and L2 points can provide crucial information continuously about the evolution and variability of: (i) reflectance of the global cloud cover, (ii) the night side cloud cover opacity, (iii) surface activity, and (iv) interaction of planet's atmosphere with the solar wind, and loss of atmosphere. In addition, the two Lagrange orbiters can provide a crucial continuous communications capability for relaying data from in-situ atmospheric or surface platforms. Well instrumented missions to L1 and L2 points of Venus would significantly improve our understanding Earth's perplexing neighbor by obtaining continuous record of data on the day and night hemispheres not available from Venus orbiting missions.

Performance Analysis of FSO DF Relays with Log-Normal Fading Channel

Abstract This paper studies the bit error performance analysis of decode-and-forward (DF) relays-based free-space optical (FSO) communication systems using avalanche photodiode (APD). The system uses subcarrier intensity binary phase shift keying (BPSK) modulated signals and subjects to scintillation due to optical turbulence. A log-normal random process is considered to model the received signal intensity fluctuation for a clear-air condition (weak atmospheric turbulence) scenario. Mathematical expressions of the average bit error probability and bit error rate (BER) are derived by taking into account the impact of atmospheric loss, thermal-noise and shot-noise limited conditions. It can be inferred from the numerical results that using DF relay nodes can extend the transmission distance and minimize BER performance of FSO system significantly compared with the direct transmission. Moreover, the selection of APD’s gain values is essential to minimize the system’s error rate performance. Therefore, the system could achieve the minimal BER by selecting an optimal average APD gain value appropriately. Moreover, the optimal average gain values of APD significantly depend on various conditions, such as the bit rate, total transmitted power, and the number of relay nodes.

Extremely High Plasma Densities in the Mars Ionosphere Associated With Cusp‐Like Magnetic Fields

AbstractThe Martian crustal magnetic field modifies the structure of the ionosphere, the loss of atmosphere to space, and the interactions of the upper atmosphere with the solar wind. The Mars Advanced Radar for Subsurface and Ionosphere Sounding topside sounder on Mars Express has regularly observed oblique echoes from magnetically controlled plasma structures in regions of strong and vertical crustal magnetic fields. Here we present 11 observations of oblique echoes with unusually high plasma densities and compare them to more common oblique echoes with typical ionospheric densities. In two cases, the density is at least 3.6×105 cm−3, perhaps the highest recorded in the Mars ionosphere. We find that high‐density features occur only where the magnetic inclination angle is nearly vertical and the magnetic field is sufficiently strong (>200 nT at 150 km). Additionally, the more frequently the magnetic field is open at a given location, the greater the probability that a high‐density echo is found there. Oblique echoes with typical densities, in contrast, are found at a wider range of magnetic geometries and strengths and are coherent and stable across as much as 45° in longitude (−53° latitude and 165–210°E longitude) and 5.5 years of observation. Based upon these findings, we suggest that these high‐density features occur where the magnetic field is open and interactions with the solar wind, such as plasma instabilities arising from heating by the solar wind electric field, have increased ionospheric plasma densities. The more common oblique echoes with typical densities are more likely to be created by transport along closed field lines in response to pressure gradients and winds.

Observing Isotopologue Bands in Terrestrial Exoplanet Atmospheres with the James Webb Space Telescope: Implications for Identifying Past Atmospheric and Ocean Loss

Abstract Terrestrial planets orbiting M dwarfs may soon be observed with the James Webb Space Telescope (JWST) to characterize their atmospheric composition and search for signs of habitability or life. These planets may undergo significant atmospheric and ocean loss due to the superluminous pre-main-sequence phase of their host stars, which may leave behind abiotically generated oxygen, a false positive for the detection of life. Determining if ocean loss has occurred will help assess potential habitability and whether or not any O2 detected is biogenic. In the solar system, differences in isotopic abundances have been used to infer the history of ocean loss and atmospheric escape (e.g., Venus, Mars). We find that isotopologue measurements using transit transmission spectra of terrestrial planets around late-type M dwarfs like TRAPPIST-1 may be possible with JWST, if the escape mechanisms and resulting isotopic fractionation were similar to Venus. We present analyses of post-ocean-loss O2- and CO2-dominated atmospheres containing a range of trace gas abundances. Isotopologue bands are likely detectable throughout the near-infrared (1–8 μm), especially 3–4 μm, although not in CO2-dominated atmospheres. For Venus-like D/H ratios 100 times that of Earth, TRAPPIST-1b transit signals of up to 79 ppm are possible by observing HDO. Similarly, 18O/16O ratios 100 times that of Earth produce signals at up to 94 ppm. Detection at signal-to-noise ratio = 5 may be attained on these bands with as few as four to 11 transits, with optimal use of JWST’s NIRSpec Prism. Consequently, H2O and CO2 isotopologues could be considered as indicators of past ocean loss and atmospheric escape for JWST observations of terrestrial planets around M dwarfs.

Earth atmospheric loss through the plasma mantle and its dependence on solar wind parameters

Atmospheric loss and ion outflow play an important role in the magnetospheric dynamics and in the evolution of the atmosphere on geological timescales—an evolution which is also dependent on the solar activity. In this paper, we investigate the total hbox{O}^+ outflow [hbox{s}^{-1}] through the plasma mantle and its dependency on several solar wind parameters. The oxygen ion data come from the CODIF instrument on board the spacecraft Cluster 4 and solar wind data from the OMNIWeb database for a period of 5 years (2001–2005). We study the distribution of the dynamic pressure and the interplanetary magnetic field for time periods with available hbox{O}^+ observations in the plasma mantle. We then divided the data into suitably sized intervals. Additionally, we analyse the extreme ultraviolet radiation (EUV) data from the TIMED mission. We estimate the hbox{O}^+ escape rate [ions/s] as a function of the solar wind dynamic pressure, the interplanetary magnetic field (IMF) and EUV. Our analysis shows that the hbox{O}^+ escape rate in the plasma mantle increases with increased solar wind dynamic pressure. Consistently, it was found that the southward IMF also plays an important role in the hbox{O}^+ escape rate in contrast to the EUV flux which does not have a significant influence for the plasma mantle region. Finally, the relation between the hbox{O}^+ escape rate and the solar wind energy transferred into the magnetosphere shows a nonlinear response. The hbox{O}^+ escape rate starts increasing with an energy input of approximately 10^{11} {hbox{W}}.

Same masses, different sizes

Exoplanets Exoplanets with masses between those of Earth and Neptune come in two types that display a bimodal distribution of densities: “super Earths,” with rocky bodies and small (or no) atmospheres, and “mini Neptunes,” with extended atmospheres of hydrogen and helium surrounding rocky cores. That difference might reflect the removal of light gasses from some of their atmospheres by stellar activity. Gandolfi et al. have discovered two exoplanets orbiting the star HD15337. Planet c is a mini Neptune with a 50% larger radius than planet b, a super Earth. As both planets have identical masses (within the uncertainties) and orbit the same star, studying them will help explain the atmospheric loss processes that lead to these planet types. Astrophys. J. Lett. 876 , L24 (2019).

Wavenumber Analysis of EMIC Waves

AbstractIt has been highly controversial as to whether and how efficient electromagnetic ion cyclotron (EMIC) waves can scatter sub–megaelectronvolt (MeV) to multi–megaelectron volt electrons into the atmospheric loss cone. The capability of EMIC wave scattering essentially depends on the wavenumber spectra, instead of readily available frequency spectra. We present a method to obtain the EMIC wavenumbers directly from the wave electromagnetic field measurements, from which we further evaluate the characteristics of electron minimum resonant energy and electron scattering rate due to EMIC waves. Based on the analysis of EMIC waves identified from 5.5 years of Van Allen Probes field measurement, we find that only a small fraction of H+ band wave events can resonate with electrons down to MeV, while none of He+ band waves at the maximum wave power can. The electron scattering rate increases with increasing levels of geomagnetic activities. The scattering rate for H+ band is sufficient to cause significant precipitation of electrons for a broad energy range (including MeV electrons), especially during times with high geomagnetic activity.

The Compositional Diversity of Low-Mass Exoplanets

Low-mass planets have an extraordinarily diverse range of bulk compositions, from primarily rocky worlds to those with deep gaseous atmospheres. As techniques for measuring the masses of exoplanets advance the field toward the regime of rocky planets, from ultrashort orbital periods to Venus-like distances, we identify the bounds on planet compositions, where sizes and incident fluxes inform bulk planet properties. In some cases, the precision of measurement of planet masses and sizes is approaching the theoretical uncertainties in planet models. An emerging picture explains aspects of the diversity of low-mass planets, although some problems remain: Do extreme low-density, low-mass planets challenge models of atmospheric mass loss? Are planet sizes strictly separated by bulk composition? Why do some stellar characterizations differ between observational techniques? With the Transiting Exoplanet Survey Satellite ( TESS) mission, low-mass exoplanets around the nearest stars will soon be discovered and characterized with unprecedented precision, permitting more detailed planetary modeling and atmospheric characterization of low-mass exoplanets than ever before. ▪ Following the Kepler mission, studies of exoplanetary compositions have entered the terrestrial regime. ▪ Low-mass planets have an extraordinary range of compositions, from Earth-like mixtures of rock and metal to mostly tenuous gas. ▪ The TESS mission will discover low-mass planets that can be studied in more detail than ever before.