Neutral Perturbations Research Articles

Assessment of Gravity Waves From Tropopause to Thermosphere and Ionosphere in High‐Resolution WACCM‐X Simulations

AbstractA new version of NCAR Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) has been developed. The main feature of this version is the species‐dependent spectral element (SE) dynamical core solved on a cubed sphere grid, eliminating the polar singularity and enabling simulations at high‐resolutions. Molecular viscosity and diffusion in the horizontal direction are also included. The Conservative Semi‐Lagrangian Multi‐Tracer Transport Scheme (CSLAM) is employed for the species transport. An efficient regridding scheme based on the Earth System Modeling Framework is used to map fields between the physics mesh and geomagnetic grid. Simulations have been performed at coarse (∼200 km and 0.25 scale height) and high (∼25 km and 0.1 scale height) resolutions. Spatial distribution of the resolved gravity waves from the high‐resolution simulations compares well with available observations in the middle and upper atmosphere. Analysis of the scale dependence of the gravity wave energy density and momentum flux shows that, while larger scale waves are dominant energetically at most latitudes, smaller scale waves contribute significantly to the total momentum flux, especially at mid‐high latitudes. The waves in the thermosphere are shown to be strongly modulated by the large‐scale wind through Doppler shift and molecular damping, and they cause large neutral atmosphere and plasma perturbations.

Numerical simulation of the equatorial plasma bubble: the effect of seeding by the vertical winds and random background noise perturbations

A wide variety of small-amplitude waves widely exist in the ionosphere and have significant effects on the evolution of equatorial plasma bubbles. In this paper, we simulated equatorial plasma bubbles (EPB) seeded by vertical neutral wind perturbations with wavelengths of 125 km and 250 km, and compared the morphology characteristics of plasma bubble structures with those under random noise perturbations in the background density. The numerical results showed that both vertical winds and random background noise perturbations can contribute to the growth of plasma bubbles, and the perturbations under additional random background noise can promote the growth of the plasma bubble structures faster. Additionally, several processes of the nonlinear behavior of bifurcated EPB structures, including bifurcation, pinching, and small-scale turbulent structures, were successfully obtained. Our simulation captured supersonic flows within the low-density plasma structures characterized by vertical velocities of about 1.5 km/s, which is consistent with experimental studies found in the literature.

Quasinormal modes of phantom Reissner-Nordström-de Sitter black holes

In this paper, we investigate some characteristics of phantom Reissner-Nordström-de Sitter (RN-dS) black holes. The peculiar features of phantom field render this kind of black holes quite different from their counterparts. We can only find at most two horizons in this spacetime, i.e. event horizon and cosmological horizon. For the black hole charge parameter, we find that it is not bounded from below. We calculate quasinormal modes (QNMs) frequencies of massless neutral scalar field perturbation in this black hole spacetime, and some properties related to the large charge parameter are disclosed.

Relative Controllability of ψ-Caputo Fractional Neutral Delay Differential System

The aim of this work is to analyze the relative controllability and Ulamn–Hyers stability of the ψ-Caputo fractional neutral delay differential system. We use neutral ψ-delayed perturbation of the Mitttag–Leffler matrix function and Banach contraction principle to examine the Ulam–Hyers stability of our considered system. We formulate the Grammian matrix to establish the controllability results of the linear fractonal differential system. Further, we employ the fixed-point technique of Krasnoselskii’s type to establish the sufficient conditions for the relative controllability of a semilinear ψ-Caputo neutral fractional system. Finally, the theoretical study is validated by providing an application.

Dynamical behavior of the C -metric: Charged scalar fields, quasinormal modes, and superradiance

The $C$-metric is a boost-symmetric spacetime solution to the vacuum Einstein field equations which describes black holes that are uniformly accelerated under the tension of a cosmic string. Only recently the thermodynamics of accelerating black holes and their modal stability against neutral scalar perturbations were concisely established. The generalization of accelerating black holes to incorporate an electric charge, namely the charged $C$-metric, possess three distinct families of quasinormal mode frequencies; the complex photon surface quasinormal modes associated with unstable null particles at the equatorial plane of the photon surface, the purely imaginary acceleration modes whose existence solely depends on the acceleration of spacetime and the purely imaginary near-extremal modes which dominate the dynamics of the ringdown at late times when the event and Cauchy horizon approach each other. We extend the quasinormal mode analysis to charged scalar fluctuations and find that the photon surface modes are continuously deformed with respect to their neutral counterpart as the scalar charge is increased. We further find that the acceleration and near-extremal families acquire an oscillation frequency when the scalar charge is introduced. Finally, we study the superradiant amplification of charged scalar monochromatic waves impinging charged accelerating black holes. We find that even though the frequency range of superradiant amplification is lessened due to the acceleration, the amplification factors are considerably elevated with respect to those transpiring in Reissner-Nordstr\"om black holes and are maximized when the scalar charge is significantly large.

Atmospheric Lunar Tide in the Low Latitude Thermosphere-Ionosphere.

We present simultaneous, independent measurements of the atmospheric semidiurnal lunar tide in neutral winds and plasma velocities from NASA's Ionospheric Connection Explorer, and in atomic oxygen 135.6 nm airglow measured by the Global‐scale Observations of the Limb and Disk. Westward tidal winds near 115 km at the magnetic equator occur during part of the upward phase of the in‐situ plasma drift. Vertical motions associated with the field‐aligned plasma velocity occur away from the magnetic equator. The morphology of the lunar tide, and the phasing between the airglow and plasma velocities are consistent with E × B drift as a mechanism for linking neutral wind and plasma perturbations. This work provides the first observational quantification of global‐scale E‐ and F‐region coupling through E × B and field‐aligned vertical drifts.

Thermospheric Density Perturbations Produced by Traveling Atmospheric Disturbances During August 2005 Storm

AbstractThermospheric mass density perturbations are commonly observed during geomagnetic storms and fundamental to upper atmosphere dynamics, but the sources of these perturbations are not well understood. Large neutral density perturbations during storms greatly affect the drag experienced by low Earth orbit. We investigated the thermospheric density perturbations at all latitudes observed along the CHAMP and GRACE satellite trajectories during the August 24–25, 2005 geomagnetic storm. Observations show that large neutral density enhancements occurred not only at high latitudes, but also globally. Large density perturbations were seen in the equatorial regions away from the high‐latitude, magnetospheric energy sources. We used the high‐resolution Multiscale Atmosphere Geospace Environment (MAGE) model to simulate consecutive neutral density changes observed by satellites during the storm. The MAGE simulation, which resolved mesoscale high‐latitude convection electric fields and field‐aligned currents, and included physics‐based specification of auroral precipitation, was contrasted with a standalone ionosphere‐thermosphere simulation driven by a high‐latitude electrodynamics empirical model. The comparison demonstrates that first‐principles representations of highly dynamic and localized Joule heating events in a fully coupled whole geospace model is critical to accurately capture both generation and propagation of traveling atmospheric disturbances (TADs) that produce neutral density perturbations globally. The MAGE simulation shows that larger density peaks in the equatorial region observed by CHAMP and GRACE are the result of TADs generated at high‐latitudes in both hemispheres, and intersect at low‐latitudes. This study reveals the importance of investigating thermospheric density variations at all latitudes in a fully coupled geospace model with sufficiently high resolving power.

Evolution of anti–de Sitter black holes in Einstein-Maxwell-dilaton theory

We study the nonlinear evolution of the spherical symmetric black holes under a small neutral scalar field perturbation in Einstein-Maxwell-dilaton theory with coupling function $f(\phi)=e^{-b\phi}$ in asymptotic anti-de Sitter spacetime. The non-minimal coupling between scalar and Maxwell fields allows the transmission of the energy from the Maxwell field to the scalar field, but also behaves as a repulsive force for the scalar. The scalar field oscillates with damping amplitude and converges to a final value by a power law. The irreducible mass of the black hole increases abruptly at initial times and then saturates to the final value exponentially. The saturating rate is twice the decaying rate of the dominant mode of the scalar. The effects of the black hole charge, the cosmological constant and the coupling parameter on the evolution are studied in detail. When the initial configuration is a naked singularity spacetime with a large charge to mass ratio, a horizon will form soon and hide the singularity.

Potentiating antibiotic efficacy via perturbation of non-essential gene expression

Proliferation of multidrug-resistant (MDR) bacteria poses a threat to human health, requiring new strategies. Here we propose using fitness neutral gene expression perturbations to potentiate antibiotics. We systematically explored 270 gene knockout-antibiotic combinations in Escherichia coli, identifying 90 synergistic interactions. Identified gene targets were subsequently tested for antibiotic synergy on the transcriptomic level via multiplexed CRISPR-dCas9 and showed successful sensitization of E. coli without a separate fitness cost. These fitness neutral gene perturbations worked as co-therapies in reducing a Salmonella enterica intracellular infection in HeLa. Finally, these results informed the design of four antisense peptide nucleic acid (PNA) co-therapies, csgD, fnr, recA and acrA, against four MDR, clinically isolated bacteria. PNA combined with sub-minimal inhibitory concentrations of trimethoprim against two isolates of Klebsiella pneumoniae and E. coli showed three cases of re-sensitization with minimal fitness impacts. Our results highlight a promising approach for extending the utility of current antibiotics.

Strong cosmic censorship in near-extremal Kerr-Sen-de Sitter spacetime

In this work, we first calculate equations of motion for particles in the Kerr-Sen-de Sitter black hole spacetime. Then, in the eikonal regime, we analytically obtain the quasi-normal resonant modes of massless neutral scalar field perturbation and find its imaginary part to be characterized by the surface gravity of a near-extremal Kerr-Sen-de Sitter black hole with the Cauchy horizon approaching the event horizon. We further show that the Penrose strong cosmic censorship conjecture is thus respected in this spacetime with dilaton scalar field and axion pseudoscalar field.

Martian nonmigrating atmospheric tides in the thermosphere and ionosphere at solar minimum

The response of Martian nonmigrating tidal perturbations in the thermospheric and ionospheric densities to solar minimum, as contrasted with their behavior in the solar declining phase, is investigated using five years of data collected by the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite. We find that the average tidal amplitudes, expressed as the percent variation of the neutral and electron densities from their respective average background values, are larger (by a factor of ~1.3 above 170 km) during solar minimum compared to the declining phase of the solar cycle. These larger amplitudes, on the dayside, may be due to a decrease in the atmospheric scale height, which for neutral argon averaged over a Martian year, goes from 12.5 km during declining phase, to 10.5 km for solar minimum. An additional controlling factor is that the average vertical dissipation length scale becomes shorter, going from ~34 km during the declining phase to ~29 km at solar minimum. On the nightside, the amplitude of the neutral atmospheric tides is similar between the declining phase and solar minimum. There are changes in the length scales, with the average night side scale height becoming somewhat greater during solar minimum, as well as an increase in the dissipation length scale. Dayside tidal perturbations in the ionosphere at ~145 km to ~170 km exhibit the expected amplitude relation predicted by simplified photochemistry for the observed neutral perturbation amplitudes as well as strong correlation with the longitudinal neutral density perturbations. Interpretation of the data are faced with the complication of limited coverage between season, latitude, and local times together with long temporal baselines that may mix different tidal modes; discussion on the possible role of the latitude effect on amplitude are discussed. The results of the study are put into context of a hypothesis on how tidal amplitudes might be expected to change in response to solar minimum.

Sensitivity of Upper Atmosphere to Different Characteristics of Flow Bursts in the Auroral Zone.

Meso‐scale plasma convection and particle precipitation could be significant momentum and energy sources for the ionosphere‐thermosphere (I‐T) system. Following our previous work on the I‐T response to a typical midnight flow burst, flow bursts with different characteristics (lifetime, size, and speed) have been examined systematically with Global Ionosphere‐Thermosphere Model (GITM) simulations in this study. Differences between simulations with and without additional flow bursts are used to illustrate the impact of flow bursts on the I‐T system. The neutral density perturbation due to a flow burst increases with the lifetime, size, and flow speed of the flow burst. It was found that the neutral density perturbation is most sensitive to the size of a flow burst, increasing from ∼0.3% to ∼1.3% when the size changes from 80 to 200 km. A westward‐eastward asymmetry has been identified in neutral density, wind, and temperature perturbations, which may be due to the changing of the forcing location in geographic coordinates and the asymmetrical background state of the I‐T system. In addition to midnight flow bursts, simulations with flow bursts centered at noon, dawn, and dusk have also been carried out. A flow burst centered at noon (12.0 Local Time [LT], 73°N) produces the weakest perturbation, and a flow burst centered at dusk (18.0 LT, 71°N) produces the strongest. Single‐cell and two‐cell flow bursts induce very similar neutral density perturbation patterns.

Electrified Postsunrise Ionospheric Perturbations at Millstone Hill

AbstractWe provide evidence that midlatitude postsunrise traveling ionospheric disturbances (TIDs) are comprised of electrified waves with an eastward propagation component. The post‐sunrise gravity wave (GW) wind‐induced dynamo action effectively generated periodic meridional polarization electric fields (PEFs), facilitating TID zonal propagation in a similar fashion as GW‐driven neutral perturbations. A combination of near‐simultaneous eastward and upward observations using the Millstone Hill incoherent scatter radar along with 2‐dimensional total electron content maps allowed resolution of TID vertical and horizontal propagation as well as zonal ion drifts (meridional PEFs). In multiple observations, oscillated in the early morning during periods when TIDs exhibited downward phase progression, 30–60 min period, 140 m/s eastward speed, and 70 km vertical wavelength. Inside these TIDs, multiple flow vortexes occurred in a vertical‐zonal plane spanning the ionospheric topside and bottomside. Subsequently, PEFs weakened after a few hours as TID horizontal wavefronts rotated clockwise.

The Relationship Between Large Scale Thermospheric Density Enhancements and the Spatial Distribution of Poynting Flux

AbstractLarge thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured.

Interannual Variability of Atmospheric Gravity Waves in the Martian Thermosphere: Effects of the 2018 Planet‐Encircling Dust Event

AbstractWe study the interannual variability of atmospheric gravity waves (GWs) in the Martian thermosphere and their relation to the background temperatures using Ar densities measured by Neutral Gas and Ion Mass Spectrometer/Mars Atmosphere and Volatile Evolution (MAVEN). The amplitudes and wavelengths of the GWs are extracted from the neutral density perturbations in the inbound segment of the MAVEN orbit. The GW amplitudes predominantly lie between 5% and 10% and are mostly less than 30% of the background densities. The horizontal wavelengths are between 50 km and 300 km with peak wavelengths of 100–130 km. GW amplitudes show a significant diurnal variation with larger amplitudes during nighttime and smaller amplitudes during daytime. In addition, the GW amplitudes negatively correlate with the background temperatures indicating the role of convective instability in the saturation of the GWs. Most importantly, the GW amplitudes are larger and the wavelengths are smaller when there was a planet‐encircling dust event in the Mars’ lower atmosphere during 2018. As a result, the typical diurnal variation of GW amplitudes is not apparent. During the lower atmospheric dust event, the GW amplitudes do not show any significant correlation with the background temperatures. It is proposed that the PEDE‐2018 led to changes in the circulation of the mesosphere and lower thermosphere, so that the modified circulation enhanced the upward propagation/reduced the filtering of GWs leading to their enhancement in the upper thermosphere.

Strong cosmic censorship for a scalar field in an Einstein-Maxwell-Gauss-Bonnet-de Sitter black hole * *This work is supported in part by NSFC (11005016, 11875196, 11375121)

It has been shown that the Christodoulou version of the strong cosmic censorship (SCC) conjecture can be violated for a scalar field in a near-extremal Reissner-Nordstrom-de Sitter black hole. In this paper, we investigate the effects of higher derivative corrections to the Einstein-Hilbert action on the validity of SCC, by considering a neutral massless scalar perturbation in - and -dimensional Einstein-Maxwell-Gauss-Bonnet-de Sitter black holes. Our numerical results show that the higher derivative term plays a different role in the case than it does in the case. For , the SCC violation region increases as the strength of the higher derivative term increases. For , the SCC violation region first increases and then decreases as the higher derivative correction becomes stronger, and SCC can always be restored for a black hole with a fixed charge ratio when the higher derivative correction is strong enough. Finally, we find that the version of SCC is respected in the case, but can be violated in some near-extremal regimes in the case.

Quasi-normal modes of near-extremal black holes in generalized spherically symmetric spacetime and strong cosmic censorship conjecture

A number of near-extremal conditions are utilized to simplify the equation of motion of the neutral scalar perturbations in generalized spherically symmetric black hole background into a differential equation with the Pöschl–Teller potential. An analytic formula for quasinormal frequencies is obtained. The analytic formula is then used to investigate strong cosmic censorship conjectures (SCC) of the generalized black hole spacetime for the smooth initial data. The Christodoulou version of the SCC is found to be violated for certain regions of the black hole parameter space including the black holes in general relativity while the C^{1} version of the SCC is always valid.

Relative Contributions of Ion Convection and Particle Precipitation to Exciting Large‐Scale Traveling Atmospheric and Ionospheric Disturbances

AbstractDuring geomagnetically active times, the enhanced ion convection and particle precipitation at high latitudes cause substantial disturbances in the ionosphere and thermosphere. Large‐scale traveling ionospheric disturbances (LSTIDs) were identified from Global Positioning System (GPS) total electron content (TEC) measurements from 06:30 to 08:30 UT on 26 March 2014 as a result of southward turning of the interplanetary magnetic field (IMF) Bz and enhanced particle precipitation during a substorm. The comparison of LSTIDs from the global ionosphere‐thermosphere model (GITM) simulations with GPS TEC measurements shows a general agreement. Further theoretical analyses with GITM were conducted to sperate the influence of ion convection and particle precipitation on the total Joule heating as well as on the resulting large‐scale traveling atmospheric disturbances (LSTADs) and LSTIDs. It was found that ion convection and particle precipitation have comparable contributions to the total Joule heating, although the changes of height‐integrated Joule heating due to these two forcing terms may display different distributions. In addition, the magnitudes of neutral density and TEC perturbations due to these two forcing terms were found to be comparable. Using the total energy flux versus time derived from all‐sky imager measurements for this event to drive GITM improves the data‐model comparison of LSTIDs. However, data‐model discrepancies still exist in the timing of LSTIDs and the magnitude of TEC perturbations, which calls for further investigation and realistic event‐specific specifications.

Understanding Inter-Hemispheric Traveling Ionospheric Disturbances and Their Mechanisms

Traveling ionospheric disturbances (TIDs) are wave-like disturbances in ionospheric plasma density. They are often observed during both quiet (medium-scale TID) and geomagnetically disturbed (large-scale TID) conditions. Their amplitudes can reach double-digit percentages of the background plasma density, and their existence presents a challenge for accurate ionosphere specification. In this study, we examine TID properties using observations obtained during two geomagnetically disturbed periods using multiple ground and space-borne instruments, such as magnetometers, Global Navigation Satellite System (GNSS) receivers, and the SWARM satellite. Reference quiet time observations are also provided for both storms. We use a thermosphere–ionosphere–electrodynamics general circulation model (TIEGCM) results to properly interpret TID features and their drivers. This combination of observations and modeling allows the investigation of variations of TID generation mechanisms and subsequent wave propagation, particularly as a function of different plasma background densities during various geophysical conditions. The trans-equatorial coupling of TIDs in the northern and southern hemispheres is also investigated with respect to attenuation and propagation characteristics. We show that TID properties during trans-equatorial events may be substantially affected by storm time background neutral wind perturbation.

Prediction of the thermospheric and ionospheric responses to the 21 June 2020 annular solar eclipse

On 21 June 2020, an annular solar eclipse will traverse the low latitudes from Africa to Southeast Asia. The highest latitude of the maximum eclipse obscuration is approximately 30°. This low-latitude solar eclipse provides a unique and unprecedented opportunity to explore the impact of the eclipse on the low-latitude ionosphere–thermosphere (I–T) system, especially in the equatorial ionization anomaly region. In this study, we describe a quantitative prediction of the impact of this upcoming solar eclipse on the I–T system by using Thermosphere–Ionosphere–Electrodynamics General Circulation Model simulations. A prominent total electron content (TEC) enhancement of around 2 TEC units occurs in the equatorial ionization anomaly region even when this region is still in the shadow of the eclipse. This TEC enhancement lasts for nearly 4.5 hours, long after the solar eclipse has ended. Further model control simulations indicate that the TEC increase is mainly caused by the eclipse-induced transequatorial plasma transport associated with northward neutral wind perturbations, which result from eclipse-induced pressure gradient changes. The results illustrate that the effect of the solar eclipse on the I–T system is not transient and linear but should be considered a dynamically and energetically coupled system.