A number of interdependent conditions and processes contribute to ionospheric‐origin energetic (
Near‐ and far‐field ionospheric responses to atmospheric acoustic and gravity waves (AGWs) generated by surface displacements during the 2015 Nepal
- NSF-PAR ID:
- 10374949
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 125
- Issue:
- 4
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract 10 eV to several keV) ion outflows. Due to these interdependences and the associated observational challenges, energetic ion outflows remain a poorly understood facet of atmosphere‐ionosphere‐magnetosphere coupling. Here we demonstrate the relationship between east‐west magnetic field fluctuations ( ) and energetic outflows in the magnetosphere‐ionosphere transition region. We use dayside cusp region FAST satellite observations made near apogee ( 4,180‐km altitude) near fall equinox and solstices in both hemispheres to derive statistical relationships between ion upflow and spectral power as a function of spacecraft frame frequency bands between 0 and 4 Hz. Identification of ionospheric‐origin energetic ion upflows is automated, and the spectral power in each frequency band is obtained via integration of power spectral density. Derived relationships are of the form for upward ion flux at 130‐km altitude, with the mapped upward ion flux for a nominal spectral power nT . The highest correlation coefficients are obtained for spacecraft frame frequencies 0.1–0.5 Hz. Summer solstice and fall equinox observations yield power law indices 0.9–1.3 and correlation coefficients , while winter solstice observations yield 0.4–0.8 with . Mass spectrometer observations reveal that the oxygen/hydrogen ion composition ratio near summer solstice is much greater than the corresponding ratio near winter. These results reinforce the importance of ion composition in outflow models. If observed perturbations result from Doppler‐shifted wave structures with near‐zero frequencies, we show that spacecraft frame frequencies 0.1–0.5 Hz correspond to perpendicular spatial scales of several to tens of kilometers. -
Abstract Aquatic vegetation protects the shoreline by dissipating the wave energy and reducing the mean water level. For the latter, the phase‐averaged depth‐integrated drag force induced by vegetation (
) plays an essential role. For linear waves, the exerted by submerged vegetation ( ) and by the submerged part of emergent vegetation ( ) equal 0. As the wave nonlinearity increases, the profile of the horizontal velocity ( u ) becomes skewed and non–cosine shaped, and thus, bothand are nonzero (phase average of u |u |≠0) and their significance increases. This study examines the effects of wave nonlinearity and vegetation submergence onbased on stream function wave theory. In deep water, it is found that the wave nonlinearity slightly affects due to the negligible weight of in the overall . Both the wave nonlinearity and vegetation submergence have negligible effects on as well. In shallow water, takes up a large percentage in the overall for emergent vegetation, and a linear relationship between and vegetation submergence exists for waves with relatively small wave heights. The applicable range of the linear wave theory based is determined using from stream function wave theory as a reference solution. Moreover, a parametric model is developed for evaluating for random waves. The mean water level changes, or wave setup, on a vegetated sloping beach are validated and quantified using experimental data obtained from literature. -
Abstract Estimates of turbulence kinetic energy (TKE) dissipation rate (
ε ) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates ofε with concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates ofε at a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scaleand the newer Langmuir stability length scale , suggesting that ocean‐side friction velocity implicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes drift and that is likely facilitated by the steady Southeast trade winds regime. In certain regimes, , where is the von Kármán constant and is instrument depth, and surface buoyancy flux capture our estimates of well, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based on and , scales ε well at times but is overall less consistent than. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes. -
Array‐Based Iterative Measurements of Travel Times and Their Constraints on Outermost Core Structure
Abstract Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate a
wave velocity, , at the top of the outer core (the layer), which is lower than that in the Preliminary Reference Earth Model. However, these low models do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuring arrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure ‐ differential travel times. This approach extracts individual signal from mixed waveforms of the series, allowing us to reliably measure differential travel times. We successfully use this method to measure time delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones. time delays are measured by waveform cross correlation between and , and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to synthetic seismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias the estimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of , , and , supporting a low at the top of Earth's outer core. -
Abstract A new azimuthal anisotropy model for the North American and Caribbean Plates, namely,
, is constructed based on full waveform inversion and records from the USArray and other temporary/permanent networks deployed in the study region. A total of 180 earthquakes and 4,516 seismographic stations are employed in the inversion to simultaneously constrain radially and azimuthally anisotropic model parameters: , , , and , within the crust and mantle. Thirty‐two preconditioned conjugate gradient iterations have been utilized to minimize frequency‐dependent phase discrepancies between observed and predicted seismograms for three‐component short‐period (15–40 s) body waves and long‐period (25–100 s) surface waves. Model exhibits complicated variations in anisotropic fabrics underneath the western and eastern United States, especially at depths shallower than 100 km. For instance, the fast axis orientations in model suggest the presence of trench‐perpendicular mantle flows underneath the Cascadia Subduction Zone and also follow the strikes of the Snake River Plain, the Ouachita Orogenic Front, and the Grenville and Appalachian Orogenic Belts. The amplitudes of azimuthal anisotropy reduce to around 1% at depths greater than 200 km, and the orientations are subparallel to the global plate motion directions to the east of the Rocky Mountain, except for large discrepancies in central and eastern Canada. At a depth of 700 km, the fast axes change along the trajectory of the Farallon slab underneath the Great Lakes region and Gulf of Mexico, which might indicate the development of 2‐D poloidal‐mode mantle flows perpendicular to the strike of the sinking slab within the uppermost lower mantle. Comparisons between model with a western U.S. model from ambient noise tomography and SKS splitting measurements demonstrate a relatively good agreement for the fast axis orientations, considering the usage of different data sets and imaging techniques. However, the absolute magnitude of azimuthal anisotropy in model might be underestimated, especially at greater depths, given the poor agreement on the amplitudes of predicted and observed SKS splitting times. At the current stage, the agreement among different azimuthal anisotropy models at global and continental scales is still poor even for the United States with a dense station coverage.