365 research outputs found
Inferring the Evolution of a Large Earthquake from Its Acoustic Impacts on the Ionosphere
We investigate the possibility to constrain the evolution of the 2016 M7.8 Kaikoura earthquake evolution based on Global Positioning System signal-derived ionospheric total electron content (TEC) perturbations, that represent plasma responses to infrasonic acoustic waves (IAWs) generated by surface motion. This earthquake exhibited unusual complexity and some first-order aspects of its evolution remain unclear; for example, how and when the Papatea fault (PF) and the corresponding large surface deformation occurred. For various earthquake models, a seismic wave propagation code is used to simulate time-dependent surface deformations, which then excite IAWs in a 3D compressible nonlinear atmospheric model, coupled with a 2D nonlinear multispecies ionospheric plasma dynamic model. Our preferred finite-fault model reproduces the amplitudes, shapes, and time epochs of appearance of detected TEC perturbations well. Additionally, the incorporation of the PF, ruptured during the earthquake, results in the closest agreement between simulated and observed near-zenith vertical TEC perturbations, whereas its absence shows significant discrepancy. This supports the hypothesis that the PF was ruptured during the Kaikoura earthquake. Furthermore, the IAWs and resulting ionospheric plasma disturbances contain additional information on the PF rupture progression, including the timing of initiation and propagation direction, indicating new opportunities to further constrain the PF rupture with low elevation angle âslantâ TEC data. The results confirm the ability for TEC measurements to constrain evolutions of large crustal earthquakes to provide new insight beyond traditional seismic and geodetic data sets
The Dynamics of Nonlinear Atmospheric Acoustic-Gravity Waves Generated by Tsunamis Over Realistic Bathymetry
The investigation of atmospheric tsunamigenic acoustic and gravity wave (TAGW) dynamics, from the ocean surface to the thermosphere, is performed through the numerical computations of the 3D compressible nonlinear Navier-Stokes equations. Tsunami propagation is first simulated using a nonlinear shallow water model, which incorporates instantaneous or temporal evolutions of initial tsunami distributions (ITD). Ocean surface dynamics are then imposed as a boundary condition to excite TAGWs into the atmosphere from the ground level. We perform a case study of a large tsunami associated with the 2011 M9.1 Tohuku-Oki earthquake and parametric studies with simplified and demonstrative bathymetry and ITD. Our results demonstrate that TAGW propagation, controlled by the atmospheric state, can evolve nonlinearly and lead to wave self-acceleration effects and instabilities, followed by the excitation of secondary acoustic and gravity waves (SAGWs), spanning a broad frequency range. The variations of the ocean depth result in a change of tsunami characteristics and subsequent tilt of the TAGW packet, as the wave\u27s intrinsic frequency spectrum is varied. In addition, focusing of tsunamis and their interactions with seamounts and islands may result in localized enhancements of TAGWs, which further indicates the crucial role of bathymetry variations. Along with SAGWs, leading long-period phases of the TAGW packet propagate ahead of the tsunami wavefront and thus can be observed prior to the tsunami arrival. Our modeling results suggest that TAGWs from large tsunamis can drive detectable and quantifiable perturbations in the upper atmosphere under a wide range of scenarios and uncover new challenges and opportunities for their observations
Measuring the Seeds of Ion Outflow: Auroral Sounding Rocket Observations of Low-Altitude Ion Heating and Circulation
We present an analysis of in situ measurements from the MICA (Magnetosphere-Ionosphere Coupling in the AlfvĂ©n Resonator) nightside auroral sounding rocket with comparisons to a multifluid ionospheric model. MICA made observations at altitudes below 325 km of the thermal ion kinetic particle distributions that are the origins of ion outflow. Late flight, in the vicinity of an auroral arc, we observe frictional processes controlling the ion temperature. Upflow of these cold ions is attributed to either the ambipolar field resulting from the heated electrons or possibly to ion-neutral collisions. We measure convection away from the arc (poleward) and downflows of hundreds of m sâ1 poleward of this arc, indicating small-scale low-altitude plasma circulation. In the early flight we observe DC electromagnetic Poynting flux and associated ELF wave activity influencing the thermal ion temperature in regions of AlfvĂ©nic aurora. We observe enhanced, anisotropic ion temperatures which we conjecture are caused by transverse heating by wave-particle interactions (WPI) even at these low altitudes. Throughout this region we observe several hundred m sâ1 upflow of the bulk thermal ions colocated with WPI; however, the mirror force is negligible at these low energies; thus, the upflow is attributed to ambipolar fields (or possibly neutral upwelling drivers). The low-altitude MICA observations serve to inform future ionospheric modeling and simulations of (a) the need to consider the effects of heating by WPI at altitudes lower than previously considered viable and (b) the occurrence of structured and localized upflows/downflows below where higher-altitude heating rocesses are expected
Preclinical amyloid pathology biomarker positivity: effects on tau pathology and neurodegeneration
Brain autopsy and biomarker studies indicate that the pathology of Alzheimer's disease (AD) is initiated at least 10-20 years before clinical symptoms. This provides a window of opportunity to initiate preventive treatment. However, this emphasizes the necessity for biomarkers that identify individuals at risk for developing AD later in life. In this cross-sectional study, originating from three epidemiologic studies in Sweden (n=1428), the objective was to examine whether amyloid pathology, as determined by low cerebrospinal fluid (CSF) concentration of the 42 amino acid form of ÎČ-amyloid (AÎČ42), is associated with biomarker evidence of other pathological changes in cognitively healthy elderly. A total of 129 patients were included and CSF levels of AÎČ42, total tau, tau phosphorylated at threonine 181 (p-tau), neurogranin, VILIP-1, VEGF, FABP3, AÎČ40, neurofilament light, MBP, orexin A, BDNF and YKL-40 were measured. Among these healthy elderly, 35.6% (N=46) had CSF AÎČ42 levels below 530âpgâml(-1). These individuals displayed significantly higher CSF concentrations of t-tau (P<0.001), p-tau (181) (P<0.001), neurogranin (P=0.009) and FABP3 (P=0.044) compared with amyloid-negative individuals. Our study indicates that there is a subpopulation among healthy older individuals who have amyloid pathology along with signs of ongoing neuronal and synaptic degeneration, as well as tangle pathology. Previous studies have demonstrated that increase in CSF tau and p-tau is a specific sign of AD progression that occurs downstream of the deposition of AÎČ. On the basis of this, our data suggest that these subjects are at risk for developing AD. We also confirm the association between APOE É4 and amyloid pathology in healthy older individuals
Mesopause Airglow Disturbances Driven by Nonlinear Infrasonic Acoustic Waves Generated by Large Earthquakes
Near-epicentral mesopause airglow perturbations, driven by infrasonic acoustic waves (AWs) during a nighttime analog of the 2011 M9.1 Tohoku-Oki earthquake, are simulated through the direct numerical computation of the 3D nonlinear Navier-Stokes equations. Surface dynamics from a forward seismic wave propagation simulation, initialized with a kinematic slip model and performed with the SPECFEM3D_GLOBE model, are used to excite AWs into the atmosphere from ground level. Simulated mesopause airglow perturbations include steep oscillations and persistent nonlinear depletions up to 50% and 70% from the background state, respectively, for the hydroxyl OH(3,1) and oxygen O(1S) 557.7-nm emissions. Results suggest that AWs excited near a large earthquake\u27s epicenter may be strong enough to drive fluctuations in mesopause airglow, some which may persist after the AWs have passed, that could be readily detectable with ground- and/or satellite-based imagers. Synthetic data demonstrate that future airglow observations may be used for the characterization of earthquake mechanisms and surface seismic waves propagation, potentially complementing tsunami early-warning systems based on total electron content (TEC) observations
Modeling of Ionospheric Responses to Atmospheric Acoustic and Gravity Waves Driven by the 2015 Nepal M w 7.8 Gorkha Earthquake
Near- and far-field ionospheric responses to atmospheric acoustic and gravity waves (AGWs) generated by surface displacements during the 2015 Nepal 7.8 Gorkha earthquake are simulated. Realistic surface displacements driven by the earthquake are calculated in three-dimensional forward seismic waves propagation simulation, based on kinematic slip model. They are used to excite AGWs at ground level in the direct numerical simulation of three-dimensional nonlinear compressible Navier-Stokes equations with neutral atmosphere model, which is coupled with a two-dimensional nonlinear multifluid electrodynamic ionospheric model. The importance of incorporating earthquake rupture kinematics for the simulation of realistic coseismic ionospheric disturbances (CIDs) is demonstrated and the possibility of describing faulting mechanisms and surface deformations based on ionospheric observations is discussed in details. Simulation results at the near-epicentral region are comparable with total electron content (TEC) observations in periods ( 3.3 and 6-10 min for acoustic and gravity waves, respectively), propagation velocities ( 0.92 km/s for acoustic waves) and amplitudes (up to 2 TECu). Simulated far-field CIDs correspond to long-period ( 4 mHz) Rayleigh waves (RWs), propagating with the same phase velocity of 4 km/s. The characteristics of modeled RW-related ionospheric disturbances differ from previously-reported observations based on TEC data; possible reasons for these differences are discussed
Auroral Ionospheric F Region Density Cavity Formation and Evolution: MICA Campaign Results
Auroral ionospheric F region density depletions observed by PFISR (Poker Flat Incoherent Scatter Radar) during the MICA (MagnetosphereâIonosphere Coupling in the AlfvĂ©n Resonator) sounding rocket campaign are critically examined alongside complementary numerical simulations. Particular processes of interest include cavity formation due to intense frictional heating and Pedersen drifts, evolution in the presence of structured precipitation, and refilling due to impact ionization and downflows. Our analysis uses an ionospheric fluid model which solves conservation of mass, momentum, and energy equations for all major ionospheric species. These fluid equations are coupled to an electrostatic current continuity equation to selfâconsistently describe auroral electric fields. Energetic electron precipitation inputs for the model are specified by inverting optical data, and electric field boundary conditions are obtained from direct PFISR measurements. Thus, the model is driven in as realistic a manner as possible. Both incoherent scatter radar (ISR) data and simulations indicate that the conversion of the F region plasma to molecular ions and subsequent recombination is the dominant process contributing to the formation of the observed cavities, all of which occur in conjunction with electric fields exceeding âŒ90âmV/m. Furthermore, the cavities often persist several minutes past the point when the frictional heating stops. Impact ionization and fieldâaligned plasma flows modulate the cavity depth in a significant way but are of secondary importance to the molecular generation process. Informal comparisons of the ISR density and temperature fits to the model verify that the simulations reproduce most of the observed cavity features to a reasonable level of detail
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