Explore the vast range of titles available.

The postperovskite transition in MgSiO₃ at conditions similar to those expected at the D\" discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D\" region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are (i) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D\" discontinuity and (ii) the anticorrelation between shear and bulk velocity anomalies as detected within the D\" region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.

This paper presents a new method for probing ionospheric D‐layer fluctuations with time‐domain very‐low and low‐frequency (VLF/LF) lightning waveforms detected several hundred kilometers away from lightning storms. The technique compares the amplitude and the time delay between the direct ground wave and the first‐hop ionospheric reflection of the lightning signal to measure the apparent D‐layer reflectivity and height. This time‐domain technique allows a higher time and spatial resolution measurement of the D‐layer fluctuations compared to previously reported frequency‐domain techniques. For a region near a nighttime thunderstorm, results demonstrate that the apparent reflectivity and height exhibit significant variation on spatial scales of tens of kilometers and over time periods of hours. The range of the reflectivity variation was observed as large as 100% away from the averaged reflectivity for some localized regions, and the height varies by as much as 5% (4 km). The time scales and propagation velocities of the fluctuations appear to be consistent with signatures of atmospheric gravity waves at D‐layer altitudes, and the direction of the fluctuation propagation suggests that the gravity waves are originated from the storm. Superimposed on the fluctuations, a general decreasing trend (by ∼4–8 km) in reflection height over the nighttime is observed. In some localized ionosphere regions, apparent splitting of the D‐layer by 2–4 km is observed to last a short time period of about 10 min.

This study uses multi‐station time‐domain very‐low and low‐frequency (VLF/LF) lightning waveforms detected from a range of several hundred kilometers to probe fluctuations in D‐layer ionospheric height and peak reflection ratio in three regions near a large thunderstorm on the night of 17 June 2005. These measurements show propagation of the fluctuations away from the thunderstorm in addition to a background eastward‐propagating fluctuation over the entire region. Measured speeds of propagation range from ∼45 m/s to ∼85 m/s, consistent with horizontal propagation speeds of atmospheric gravity waves. The fluctuation propagation is seen in both the ionospheric height measurement and the peak reflection ratio measurement with similar periods and speeds. Ionospheric height perturbations in the measured regions can be as large as 6 km from average, and perturbations in peak reflection ratio can be as large at 100%. Key Points D‐layer disturbances are detected propagating outward from thunderstorm region A new VLF probing technique is used with high temporal and spatial‐resolution Background atmospheric gravity waves cause large scale disturbances in D‐layer

We show evidences for anomalous ionospheric behaviour in the signal of Indian navy VLF transmitting station named VTX due to earthquakes in the South Asian region. We concentrate on the variation of the D-layer preparation time (DLPT) and D-layer disappearance time (DLDT) in a period of sixteen months and study their average behaviors. We identify those days in which DLPT and DLDT exhibit significant deviations. Separately, we compute the energy release by earthquakes during this period and show that \"anomalous VLF\" days are associated with anomalous energy release. We find that the anomaly and the deviation of DLPT and DLDTs from the mean are linearly correlated. We discuss the predictability in this approach and compare with the terminator shift approach using the same set of data.

We investigate the shear response of possible slip systems activated in pure and Fe-bearing MgSiO3 post-perovskite (PPv) through ab initio generalized stacking fault (GSF) energy calculations. Here we show that the [100](001) slip system has the easiest response to plastic shear among ten possible slip systems investigated. Incorporation of Fe2+ decreases the strength of all slip systems but does not change the plastic anisotropy style. Therefore, pure and Fe-bearing MgSiO3 PPv should demonstrate similar LPO patterns with a strong signature of the [100](001) slip system. An aggregate with this deformation texture is expected to produce a VSH > VSV type polarization anisotropy, being consistent with seismological observations. (Communicated by Ikuo KUSHIRO, M.J.A.)

Recent discovery of a phase transition from perovskite to post-perovskite suggests that the physical properties of Earth's lowermost mantle, called the D'' layer, may be different from those of the overlying mantle. We report that the electrical conductivity of (Mg₀.₉Fe₀.₁)SiO₃ post-perovskite is >10² siemens per meter and does not vary greatly with temperature at the conditions of the D'' layer. A post-perovskite layer above the core-mantle boundary would, by electromagnetic coupling, enhance the exchange of angular momentum between the fluid core and the solid mantle, which can explain the observed changes in the length of a day on decadal time scales. Heterogeneity in the conductivity of the lowermost mantle is likely to depend on changes in chemistry of the boundary region, not fluctuations in temperature.

Natural olivine with 12 mol % Fe2 SiO4 and synthetic orthopyroxenes with 20% and 40% FeSiO3 were studied beyond the pressure-temperature conditions of the core-mantle boundary. All samples were found to convert entirely or partially into the CalrO3 postperovskite structure, which was recently reported for pure MgSiO3. The incorporation of Fe greatly reduces the pressure needed for the transition and establishes the new phase as the major component of the D″ layer. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D″ layer.

Using ab initio simulations and high-pressure experiments in a diamond anvil cell, we show that alumina ( Al2O3) adopts the CaIrO3-type structure above 130 GPa. This finding substantially changes the picture of high-pressure behavior of alumina; in particular, we find that perovskite structure is never stable for Al2O3at zero Kelvin. The CaIrO3-type phase suggests a reinterpretation of previous shock-wave experiments and has important implications for the use of alumina as a window material in shock-wave experiments. In particular, the conditions of the stability of this phase correspond to those at which shock-wave experiments indicated an increase of the electrical conductivity. If this increase is caused by high ionic mobility in the CaIrO3-type phase of Al2O3, similar effect can be expected in the isostructural postperovskite phase of MgSiO3(which is the dominant mineral phase in the Earth's D″ layer). The effect of the incorporation of Al on the perovskite/postperovskite transition of MgSiO3is discussed.

High-pressure experiments and theoretical calculations demonstrate that an iron-rich ferromagnesian silicate phase can be synthesized at the pressure-temperature conditions near the core-mantle boundary. The iron-rich phase is up to 20% denser than any known silicate at the core-mantle boundary. The high mean atomic number of the silicate greatly reduces the seismic velocity and provides an explanation to the low-velocity and ultra-low-velocity zones. Formation of this previously undescribed phase from reaction between the silicate mantle and the iron core may be responsible for the unusual geophysical and geochemical signatures observed at the base of the lower mantle.

Solar flares significantly impact the conditions of the Earth’s ionosphere. In particular, the sudden increase in X-ray flux during a flare penetrates down to the lowest-lying D-region and dominates ionization at these altitudes ( ≈ 60  – 100 km). Measurements of very low frequency (VLF: 3 – 30 kHz) radio waves that reflect at D-region altitudes provide a unique remote-sensing probe to investigate the D-region response to solar-flare emissions. Here, using a combination of VLF amplitude measurements at 24 kHz together with X-ray observations from the Geostationary Operational Environment Satellite (GOES) X-ray sensor, we present a large-scale statistical study of 334 solar-flare events and their impacts on the D-region over the past solar cycle. Focusing on both GOES broadband X-ray channels, we investigate how the flare peak fluxes and position on the solar disk dictate an ionospheric response and extend this to investigate the characteristic time delay between incident X-ray flux and the D-region response. We show that the VLF amplitude linearly correlates with both the 1 – 8 Å and 0.5 – 4 Å channels, with correlation coefficients of 0.80 and 0.79, respectively. For the two X-class flares in our sample, however, there appears to be a turnover in the linear relationship, similar to previous works. Unlike higher altitude ionospheric regions for which the location of the flare on the solar disk affects the ionospheric response, we find that the D-region response to solar flares does not depend on the flare location. By comparing the time delays between the peak X-ray fluxes in both GOES channels and VLF amplitudes, we find that there is an important difference between the D-region response and the X-ray spectral band. We also demonstrate for several flare events that show a negative time delay, the peak VLF amplitude matches with the impulsive 25 – 50 keV hard X-ray fluxes measured by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). These results highlight the importance of performing full spectral analysis when studying the ionospheric responses to solar flares.