Noctilucent cloud in the western Arctic in 2005: Simultaneous lidar and camera observations and analysis

We report observations of a noctilucent cloud (NLC) over central Alaska by a ground-based lidar and camera on the night of 9–10 August 2005. The lidar at Poker Flat Research Range (PFRR), Chatanika (65°N, 147°W) measured a maximum integrated backscatter coefficient of 2.4×10^?6 sr^?1 with a peak backscatter coefficient of 2.6×10^?9 m^?1 sr^?1 corresponding to an aerosol backscatter ratio of 120 at an altitude of 82.1 km. The camera at Donnelly Dome, 168 km southeast of PFRR, recorded an extensive NLC display across the sky with distinct filamentary features corresponding to wave structures measured by the lidar. The occurrence of the maximum integrated backscatter coefficient corresponded to the passage of a bright cloud band to the southwest over PFRR. The camera observations indicate that the cloud band had a horizontal width of 50 km and a length of 150 km. The horizontal scale of the cloud band was confirmed by medium-frequency radar wind measurements that reported mesopause region winds of 30 m/s to the southwest during the period when the cloud band passed over PFRR. Comparison of these measurements with current NLC microphysical models suggests a lower bound on the water vapor mixing ratio at 83 km of 7–9 ppmv and a cloud ice mass of 1.5–1.8×10³ kg. Satellite measurements show that this NLC display occurred during a burst of cloud activity that began on 5 August and lasted for 10 days. This cloud appeared 10 days after a launch of the space shuttle. We discuss the appearance of NLCs in August over several years at this lower polar latitude site in terms of planetary wave activity and space shuttle launches.     

Polar mesosphere summer echoes (PMSE) studied at Bragg wavelengths of 2.8 m, 67 cm,and 16 cm

We present observations of radar volume reflectivities under conditions of polar mesosphere summer echoes (PMSE) at three frequencies, i.e., 53.5, 224, and 930 MHz corresponding to Bragg wavelengths of 2.8, 0.67, and 0.16 m. These measurements were made with the ALWIN radar in Andenes and the EISCAT VHF and UHF radars in Tromsø. Contributions to the signal at 930 MHz by incoherent scatter are used to estimate electron number densities and their gradient at PMSE altitudes, and spectral width measurements of Doppler spectra recorded at 224 MHz are used to estimate the turbulent energy dissipation rate. We further derive a theoretical expression for the radar volume reflectivity for the case of turbulent scatter aided by a large Schmidt number (i.e., the current standard theory of PMSE) and show that our observations quantitatively agree with this theory if Schmidt numbers between 2500 and 5000 are assumed. We then show that these Schmidt numbers correspond to ice particles with radii in the range 20–30 nm which should frequently occur in the polar summer mesopause region. In addition, we show that for the short period when PMSE was observed at UHF frequencies the volume reflectivity is proportional to a factor determined by the turbulent energy dissipation rate, electron number density, and the electron number density gradient in agreement with theory. We consider our findings as strong support that PMSE at all considered frequencies is indeed created by turbulent scatter in the presence of a large Schmidt number. We finally highlight that ultimate proof of this concept will require the direct measurement of ice particle sizes in a PMSE environment probed by radars covering frequencies between 50 MHz and 1 GHz.     

A Raman lidar to measure water vapor in the atmospheric boundary layer

A new multi-telescope scanning Raman lidar designed to measure the water vapor mixing ratio in the atmospheric boundary layer for a complete diurnal cycle with high resolution spatial (1.25 m) and temporal (1 s) resolutions is presented. The high resolution allows detailed measurements of the lower atmosphere and offers new opportunities for evaporation and boundary layer research, atmospheric profiling and visualization. This lidar utilizes a multi-telescope design that provides for an operational range with a nearly constant signal-to-noise ratio, which allows for statistical investigations of atmospheric turbulence. This new generation ground-based water vapor Raman lidar is described, and first observations from the Turbulent Atmospheric Boundary Layer Experiment (TABLE) are presented. Direct comparison with in-situ point measurements obtained during the field campaign demonstrate the ability of the lidar to reliably measure the water vapor mixing ratio. Horizontal measurements taken with time are used to determine the geometric characteristics of coherent structures. Vertical scans are used to visualize nocturnal jet features, layered structures within a stably stratified atmosphere and the internal boundary layer structure over a lake.     

Determination of physical and radiant meteor properties using PFISR interferometry measurements of head echoes

Over the past decade, High Power and Large Aperture (HPLA) radars have been widely utilized for the study of sub-millimeter extraterrestrial particles via the detection of the meteor head-echo. These observations have been a successful tool in the study of the sporadic meteor background, however, they have been limited by the lack of precise knowledge of the particle's location within the radar beam and its absolute trajectory and velocity. This limitation prevents for example the accurate determination of the meteors radiant and orbit. Interferometry measurements of the head-echo has been proven to be a detection technique that satisfies this need. Unfortunately very few radars are capable of performing them. We have developed a methodology which takes advantage of the multi-receiving capabilities of the 450 MHz Poker Flat Incoherent Scatter Radar (PFISR) enabling us to utilize the phased array of crossed-dipoles as an interferometer. This new PFISR capability allows us to determine the instantaneous position of meteors within the radar beam. This enables us to determine absolute velocities and ultimately meteor radiant and orbit around the Sun. In this work, we present initial results from 9 h of observations during which 142 particles were individually detected by the three different receiving channels simultaneously. For these meteors absolute velocities were obtained and meteor dynamical, physical and radiant properties were derived.     

Inter-hemispheric asymmetry in polar mesosphere summer echoes and temperature at 69° latitude

An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (～69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1 km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ～80–87 km of the Southern Hemisphere (SH) to be up to 7.5 K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.     

Seasonal and diurnal variation of electron and iron concentrations at the meteor heights above Arecibo

We report observations of seasonal and local time variation of the averaged electron and iron concentrations, as well as simultaneous measurements of the two species, above the Arecibo Observatory (18.35°N, 66.75°N), Puerto Rico. The average Fe profile between 21:00 and 24:00 LT has a single peak at about 85 km with the exception of the summer when an additional peak exists at about 95 km. The higher Fe peak in the summer is correlated with higher electron concentrations in this season. The three nights of simultaneous measurements of electron and iron concentrations show that narrow layers of Fe and electrons are well correlated. Comparison of the climatological and simultaneous Fe and electron data suggests that recombination of Fe⁺ plays an important role in determining the Fe profile in the upper part of the Fe layer. Above 93 km, the Fe concentration appears to increase after sunset if the electron concentration exceeds about 4000 electrons cm⁻³. The average rate of Fe production is about 0.1 atom cm⁻³ s⁻¹ for all seasons at 100 km in the early evening hours. A chemical model reveals that the concentration of Fe⁺ must be 50–80% of the total ionization over Arecibo for typical equinox conditions to explain the observed rate of Fe production. These high relative Fe⁺ concentrations are consistent with in situ observations that Fe⁺ is usually the dominant ion in sporadic E layers in the nighttime lower E region. This suggests that the source of Fe⁺ is provided by sporadic E layers descending over Arecibo after sunset. The Fe density between 80 and 85 km decreases during the night, for all seasons. This is attributed to the formation of stable molecular Fe species, such as FeOH, due to the increase in O₃ and decrease in atomic O and H during the night at these altitudes.     

Analysis of observations backing up the existence of VLF and ionospheric TEC anomalies before the Mw6.1 earthquake in Greece,January 26, 2014

The present work integrates ground-based ionosphere measurements using very-low-frequency radio transmissions with satellite measurements of the total electron content to draw common conclusions about the possible impact that the Mw6.1 earthquake that took place in Greece on January 26, 2014, had on the ionosphere.Very-low-frequency radio signals reveal the existence of an ∼4-day anomaly in the wavelet spectra of the signals received inside the earthquake preparation zone and a significant increase in the normalized variance of the signals prior to the earthquake (approximately 1 day before).Through total electron content analysis, it was possible to identify a clear anomaly from 15:00 until 20:00 UT on the day before the earthquake that appears again on the day of the earthquake between 07:00 UT and 08:00 UT. The anomalous values reach TEC1Sigma ∼4.36 and 3.11, respectively. Their spatial and temporal distributions give grounds to assume a possible link with the earthquake preparation. The geomagnetic, solar and weather conditions during the considered period are presented and taken into account.This work is an initial and original step towards a multi-parameter approach to the problem of the possible earthquake-related effects on the ionosphere joining observations made from both ground stations and satellites. A well-founded knowledge of these phenomena is clearly necessary before dealing with their application to earthquake prediction purposes.     

Mesospheric bore formation from large-scale gravity wave perturbations observed by collocated all-sky OH imager and sodium lidar

On 9 October 2007, long-horizontal-wavelength gravity waves were observed for the first time to steepen and form mesospheric bores at the altitude of ～87 km, by an all-sky OH imager located at Fort Collins (41°N, 105°W), Colorado. The collocated sodium lidar simultaneously observed the presence of a temperature inversion layer as the ducting region. One mesospheric bore uniquely later evolved into a large-amplitude soliton-like perturbation. When the gravity wave and the associated soliton-like perturbation passed through the lidar beams, the lidar detected strong vertical disturbance at 90 km, indicating convective instability. A large cold front system recorded several hours before in the troposphere was aligned to phase fronts of these large gravity waves. For all of the 7 mesospheric bores observed over a 5 year period, we found a similar alignment with a cold front 1000–1500 km away as the likely source of these large-scale gravity waves.     

Behavior of sporadic Na layers on small time scale

Seventeen sporadic Na (Na_s) layers were observed from ~150 h of the 8-s Na density profiles obtained by lidar measurements at Wuhan, China. Each of them consists of a sequence of small-timescale density enhancement bursts. The burst intensity ranges from 200 to 10,400 cm^?3. The burst duration and interval vary between 16 and 112 s. The instantaneous growth and decay rates often have an order of 100 cm^?3 s^?1. This suggests that there exists a very rapid atom removal process corresponding to the dramatic burst density enhancement if the advection effect by large-scale wind could be ignored. These results provide a new clue for explaining the formation mechanism of sporadic metal layers.     

Latitudinal and seasonal variability of the micrometeor input function: A study using model predictions and observations from Arecibo and PFISR

In this work, we use a semi-empirical model of the micrometeor input function (MIF) together with meteor head-echo observations obtained with two high power and large aperture (HPLA) radars, the 430 MHz Arecibo Observatory (AO) radar in Puerto Rico (18°N, 67°W) and the 450 MHz Poker flat incoherent scatter radar (PFISR) in Alaska (65°N, 147°W), to study the seasonal and geographical dependence of the meteoric flux in the upper atmosphere. The model, recently developed by Janches et al. [2006a. Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars. Journal of Geophysical Research 111] and Fentzke and Janches [2008. A semi-empirical model of the contribution from sporadic meteoroid sources on the meteor input function observed at arecibo. Journal of Geophysical Research (Space Physics) 113 (A03304)], includes an initial mass flux that is provided by the six known meteor sources (i.e. orbital families of dust) as well as detailed modeling of meteoroid atmospheric entry and ablation physics. In addition, we use a simple ionization model to treat radar sensitivity issues by defining minimum electron volume density production thresholds required in the meteor head-echo plasma for detection. This simplified approach works well because we use observations from two radars with similar frequencies, but different sensitivities and locations. This methodology allows us to explore the initial input of particles and how it manifests in different parts of the MLT as observed by these instruments without the need to invoke more sophisticated plasma models, which are under current development. The comparisons between model predictions and radar observations show excellent agreement between diurnal, seasonal, and latitudinal variability of the detected meteor rate and radial velocity distributions, allowing us to understand how individual meteoroid populations contribute to the overall flux at a particular location and season.     

Detection of large scale TIDs associated with the dayside cusp using SuperDARN data

Variations in the dayside ionosphere parameters as a result of a large-scale acoustic gravity wave (LS AGW) were studied for the 17 February 1998 substorm using the super dual auroral radar network (SuperDARN) measurements. This event was characterised by a sharp rise in the AE index with a maximum of ～900 nT. The source of the disturbance responsible for the LS AGW appears to have been located within the plasma convection throat and in the dayside cusp region. The location of the source was obtained from studies of a number of datasets including high-latitude convection maps, data from 4 DMSP satellites and networks of ground-based magnetometers. The propagation of the LS AGWs caused quasi-periodic variations in the skip distance (with an amplitude up to 220–260 km) of the ground backscatter measured by up to 6 SuperDARN radars, including Goose Bay and Kapuskasing, resulting in two large-scale travelling ionospheric disturbances (LS TIDs). The LS TIDs had wave periods of 1.5 and 2 h, a velocity of ～400 m/s for both, and wavelengths of 2200 and 2900 km, respectively. These quasi-periodic variations were also present in the peak electron density and height of the F2 layer measured by the Goose Bay ionosonde. The numerical simulation of the inverse problem show good agreement between Goose Bay radar and Goose Bay ionosonde measurements. But these LS TIDs would be difficult to deduce from the ground based ionospheric station data alone, because h_mF2 variations were 10–40 km only and f_OF2 variations between 10% and 20%. The results demonstrate how important SuperDARN radars can be, and that this is a more powerful technique than routine ground-based sounding for studies of weak quasi-periodic variations in the dayside subauroral ionosphere related to LS AGW.     

The mesospheric metal layer topside: Examples of simultaneous metal observations

We show examples of common volume observations of three metals by lidar focusing on the altitude of the topside of the meteoric metal layer as described by Höffner and Friedman (H&F) [The mesospheric metal layer topside: a possible connection to meteoroids, Atmos. Chem. Phys. 4 (2004) 801–808]. In contrast to H&F, we will focus on time scales of a few hours and less whereas the previous study examined the seasonally averaged climatological state on time scales of several days or weeks, and we examine the entire topside, whereas H&F focused on data at 113 km. The examples, taken under different observation conditions in 1997 and 1998 at Kühlungsborn, Germany (54°N, 15°E), show that the metal layers can often be observed at altitudes as high as 130 km if the signal is integrated over a period of several hours. Under such conditions it is possible to derive reasonably good metal abundance ratios from nocturnally averaged data, which, in turn, allow the discussion of metal abundance ratios to broaden from a single altitude as discussed in H&F to an altitude range extending as high as 130 km. The examples herein show, for the first time, that it is possible to track the transition in the metal abundance ratios from the main layer to an altitude region that has not been studied in the past by lidar. On shorter time scales, small structures are detectable and observable, sometimes above 120 km, resulting in, on average, a broad but weak topside layer above 105 km. In particular, the example of 26–27 October 1997, obtained during enhanced meteor activity, is an indication that this broad layer may result from meteor ablation occurring in this altitude range during the observation. Ratios of metal densities for Ca, Fe, K, and Na are remarkably consistent above about 110 km and in close agreement with the results of H&F. They are less consistent with ratios measured in individual meteor trails and appear to have little relation to the ratios measured in CI meteorites. Finally, it is the temporal smoothing of descending sporadic metal atom layers on top of an undisturbed background metal layer that is the basis of the summer topside extension as described by H&F.     

Nocturnal temperature changes over tropics during CAWSES-III campaign: Comparison with numerical models and satellite data

In the frame of the third CAWSES tidal campaign in June–August 2007, lidar and satellite data were collected and compared with numerical models. Continuous nocturnal middle atmospheric temperature measurements performed with a Rayleigh lidar located at La Reunion Island (20.8°S–55.5°E) were obtained for three subsequent nights. The results clearly show the presence of tidal components with a downward phase propagation. Comparisons with SABER satellite data show good agreement on tidal amplitude; however, some differences on the structures are reported probably due to the zonal nature of the retrieval provided by the SABER data. The observed tidal components are compared with two different numerical models such as the 2D global scale wave model and the 3D-GCM LMDz-REPROBUS. Both models reveal good agreement with temperature lidar patterns, while simulated tidal amplitudes are smaller by a factor of around 2–2.5 K.     

Evidence of mesospheric bore formation from a breaking gravity wave event: simultaneous imaging and lidar measurements

A large wave event was observed in the three upper-mesospheric (80–105 km) airglow emissions of O(¹S), Na and OH by the Boston University all-sky imager, at the Arecibo Observatory, during the night of 3 May 2003. The airglow structures appeared to be due to a large upward propagating internal gravity wave, which subsequently became unstable near the 95 km height level and produced large-scale vertical motions and mixing. Simultaneous density and temperature lidar measurements indicated the presence of a large temperature inversion of 80 K valley-to-peak between 88 and 96 km during the time of the event. Near-simultaneous temperature profiles, made by the TIMED SABER instrument, provided evidence that the horizontal extent of the inversion was localized to within 500 km of Arecibo during the wave event. As the gravity wave dissipated, an internal bore was generated, apparently due to the deposition of momentum and energy into the region by the original wave. Although mesospheric gravity wave breaking has been reported previously (Swenson and Mende, 21(1994); Hecht et al., 102(1997); Yamada et al., 28(2001), for example), this was the first time that the phenomenon has been associated with the generation of an internal mesospheric bore. The event suggested that the breaking of a large mesospheric gravity wave can lead to the generation of an internal bore, as suggested by Dewan and Picard 106(2001). Such behavior is of particular interest since little is known of their origins.     

Geoelectrical constraints on radar probing of shallow water-saturated zones within karstified carbonates in semi-arid environments

Shallow carbonates are of utmost importance as potential sources of groundwater in karstified semi-arid terrains. Ground-Penetrating Radar (GPR) is being increasingly used as a prominent mapping tool in such environments. However, its potential in exploring and identifying shallow water-saturated zones (WSZs) in carbonates is constrained by the geoelectrical properties of carbonate soils as a function of moisture content. We report results of a case study that includes laboratory geoelectrical characterization and their comparison to in situ GPR attenuation measurements performed on Cretaceous Edwards Formation rudist mounds in central Texas, which we hypothesize as analogs for water-bearing formations in semi-arid karstified carbonate terrains. Dielectric measurements on field-collected rock samples carried out in the laboratory under controlled conditions of moisture saturation suggest that real and imaginary parts of dielectric constants of rocks with higher porosity and/or permeability have steeper dependence on pore moisture content; they produce better dielectric contrasts but allow shallower penetration. Our analyses suggest that within carbonates, dielectric contrasts improve with decrease in sounding frequency and/or increase in moisture content; and the relationship between dielectric permittivity and moisture content may be represented by 3rd order polynomial equations. GPR surveys using a wide-band 400 MHz antenna reveal subsurface mound morphologies with heights of ～ 1–2 m and basal diameters of ～ 8–10 m resembling outcrop analogs. Each mound appears to be composed of smaller amalgamated lithounits that seem geoelectrically similar. Amplitudes decays of the backscattered radar signal correlate to moisture distribution. Measuring the differences in signal attenuation allows differentiation between saturated and non-saturated zones. Velocity analyses of GPR profiles enable estimation of moisture distribution in the vicinity of the mounds. Optimal delineation and production of high-resolution GPR data up to a depth of ～ 10 m were observed for a sounding frequency of ～ 250 MHz with moisture content of ～ 5% by weight. Below this moisture level, the dielectric contrast is insufficient to uniquely identify water-saturated zones from the surrounding geoelectrical context, and above it, the radar signal is substantially attenuated leading to a total inefficiency of the method.     

High-altitude sporadic metal atom layers observed with Na and Fe lidars at 30°N

The seasonal/annual characteristics of the high-altitude sporadic metal atom layers are presented on the basis of extensive Na and Fe lidar measurements at 30°N during the past several years. It is found that the extremely high sporadic Na (Na_s) and Fe (Fe_s) layers above 105 km occurred mostly during summer. They had long durations (a few hours) and broad layer widths (much larger than 2 km). Their absolute peak densities could be comparable to or even larger than those of the corresponding main layers on a few nights. By using all the raw data profiles including sporadic layers, we have constructed the contour plots of Na and Fe densities versus month and altitude at 30°N. The Na and Fe layers both exhibit evidence for summer topside extension, which is consistent with the earlier observations for K and Ca at different latitudes. The summer topside extension of mean metal atom layers might represent a universal phenomenon that is alike for different atom species, different geographic locations and different measurement years. The extremely high sporadic metal atom layers above 105 km occurring during summer give rise to the phenomenon.     

PFISR observations of strong azimuthal flow bursts in the ionosphere and their relation to nightside aurora

Flow bursts within the ionosphere are the ionospheric signatures of flow bursts in the plasma sheet and have been associated with poleward boundary intensifications (PBIs). Some PBIs extend equatorward from the polar cap boundary, where they can be roughly divided into north–south-aligned and east–west-aligned structures. In this paper, we present two flow burst events observed by the new Poker Flat Advanced Modular Incoherent Scatter Radar (PFISR) in the pre-midnight auroral zone on 28 April 2007, one towards the west and the other towards the east. In both cases, enhanced flows lasted for about 8–10 min with peak velocities exceeding 1500 m/s. The concurrently measured electron density showed that the flow bursts occurred in low conductivity regions. However, near the poleward (equatorward) edge of the westward (eastward) flow burst, strong electron density enhancements were observed in the E region, indicating the presence of discrete auroral arcs. Auroral images from the Polar spacecraft were available at the time of the eastward flow burst and they indicate that this burst was associated with an east–west-aligned auroral structure that connected at later MLT to a north–south structure. In addition, simultaneous precipitating particle energy spectrum measured by the the Defense Meteorological Satellites Program (DMSP) F13 satellite reveals that this auroral structure resulted from mono-energetic electron precipitation associated with a significant field-aligned potential drop. These observations show direct evidence of the relationship between flow bursts, field-aligned currents and auroral intensifications, and suggest that eastward/westward flow bursts are associated with east–west-oriented PBI structures that have extended well within the plasma sheet. This is in contrast to the equatorward-directed flow that has been previously inferred for PBIs near the polar cap boundary and for north–south auroral structures. This paper illustrates the use of the PFISR radar for studying the magnetosphere–ionosphere coupling of flow bursts.     

A LiDAR study of the effective size of cirrus ice crystals over Chung-Li,Taiwan

In this paper, we estimated the effective size of ice crystals in cirrus clouds using fall velocity derived from LiDAR (light detection and ranging) measurements at Chung-Li (24.58°N, 121.1°E), Taiwan. Nine shapes of the ice crystals, viz. hexagonal plates, hexagonal columns, rimed long columns, crystals with sector-like branches, broad-branched crystals, stellar crystal with broad arms, side planes, bullet rosettes and assemblages of planar poly-crystals of specific dimensions have been analyzed. The results show that the lidar derived most probable mean effective size of ice crystals is 340±180 μm with a dominant size range of 200–300 μm. The lidar derived mean effective size of cirrus crystals are parameterized in terms of cloud mid-height temperature as well as optical depth. The discussed method will be useful to study the most probable effective size distribution of ice crystals in cirrus cloud.     

A case study of the evolution of a Kelvin–Helmholtz wave and turbulence in noctilucent clouds

Bright and extensive noctilucent clouds (NLC) were observed in Århus (Denmark) on 3/4 July of 2008 with an automatic digital camera taking images every minute. This event was unique in the sense that bright NLC were seen at high elevation angles (more than 30°) that allowed observing the evolution of a Kelvin–Helmholtz (KH) wave, resulted in well-developed turbulence. In particular, coherent vortex structures of a horseshoe-shaped form were observed for the first time in noctilucent clouds. The turbulent diffusion coefficient and turbulent energy dissipation rate around the mesopause are estimated in the range 162–667 m²/s and 300–1235 mW/kg, respectively, representing a case of strong neutral air turbulence in noctilucent clouds. Turbulent structures were observed to be in the vicinity of breaking small-scale gravity waves that seems to be responsible for a high level of turbulence.At the same time, it has been demonstrated that it is of importance to take into account non-turbulent process such as the gravity wave motion that is always present in NLC layers. Unless non-turbulent process is taken into account, this certainly leads to overestimating of the value of the turbulent diffusion coefficient. More accurate characteristics of turbulence in NLC can be obtained by analyzing a sequence of high-resolution images with a high frame-rate high-resolution digital camera.     

Determination of post-perovskite phase transition boundary up to 4400 K and implications for thermal structure in D″ layer

We have determined the post-perovskite phase transition boundary in MgSiO₃ in a wide temperature range from 1640 to 4380 K at 119–171 GPa on the basis of synchrotron X-ray diffraction measurements in-situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). The results show a considerably high positive Clapeyron slope of + 13.3 ± 1.0 MPa/K and a transition temperature of about 3520 ± 70 K at the core–mantle boundary (CMB) pressure. The thermal structure in D″ layer can be tightly constrained from precisely determined post-perovskite phase transition boundary and the depths of paired seismic discontinuities. These suggest that temperature at the CMB may be around 3700 K, somewhat lower than previously thought. A minimum bound on the global heat flow from the core is estimated to be 6.6 ± 0.5 TW.