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The top-down crystallisation of Mercury's core

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): A.L. Edgington, L. Vočadlo, L. Stixrude, I.G. Wood, D.P. Dobson, E. Holmström

Abstract

The regime governing the growth of Mercury's core is unknown, but the dynamics of core growth are vital to understanding the origin and properties of the planet's weak magnetic field. Here, we use advanced first-principles methods, which include a magnetic entropy contribution, to investigate the magnetic and thermo-elastic properties of liquid Fe-S-Si and of pure liquid iron at the conditions of Mercury's core. Our results support a ‘top-down’ evolution of the core, whereby solid iron-rich material crystallises at shallow depths and sinks. This process would likely result in a compositionally driven dynamo within a stably stratified uppermost liquid layer, providing an explanation for the observed properties of the weak magnetic field of Mercury.

Multi-stage exhumation history of the West Kunlun orogen and the amalgamation of the Tibetan Plateau

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Guangwei Li, Mike Sandiford, Aimin Fang, Barry Kohn, Dan Sandiford, Bihong Fu, Tongliang Zhang, Yuanyuan Cao, Fei Chen

Abstract

The West Kunlun range, along the northwest margin of the Tibetan Plateau contains an important record of plateau formation and its northwards expansion. However, apart from the well-documented Miocene tectonism, its long-term history of exhumation/uplift remains enigmatic. Here we report an integrated low-temperature thermochronology study (apatite fission track and corresponding zircon (U-Th)/He) across a N-S transect through the West Kunlun range that reveals a prolonged low-temperature thermochronological record, characterized by a complex mosaic of thermal histories from the individual terrane elements during amalgamation to form the Tibetan Plateau. Our new data reveal two prominent cooling episodes during the Cretaceous and the Neogene, as well as several other more subdued and/or localized cooling episodes. Late Permo-Triassic cooling correlates with the accretion of the West Kunlun, Songpan-Ganzi and Tianshuihai terranes. Early Cretaceous cooling is considered as a response to collision between the Qiangtang and Lhasa terranes, while Late Cretaceous cooling relates to collision between the Karakorum terrane and Kohistan-Ladakh Arc. Partially preserved evidence for cooling in the Paleocene-Early Eocene and latest Oligocene-Miocene likely relates to the early stages of Indo-Asian collision. Our work confirms relatively low Neogene denudation rates of about 0.1-0.2 km/myr consistent with its arid, intraplate tectonic setting with deformation resulting from stress propagated via the surface plates and most likely sourced in the buoyancy of the plateau itself.

Graphical abstract

A new perspective for the sediment provenance evolution of the middle Okinawa Trough since the last deglaciation based on integrated methods

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Qian Li, Qiang Zhang, Guangxue Li, Qingsong Liu, Min-Te Chen, Jishang Xu, Jinhua Li

Abstract

The sediment in the Okinawa Trough (OT) preserves considerable information regarding land–ocean interactions and palaeoenvironmental evolution in the East Asian marginal seas. More specifically, the sediment provenance in the OT must be studied to recover this information. However, arguments about this remain, especially regarding the contribution of Yellow River (YR)-derived materials to middle OT depositions. To resolve this problem, we systematically investigated a gravity core (M063-05; 3.87 m in length, since the last deglaciation) collected from the middle OT by integrating environmental magnetism, diffuse reflectance spectroscopy, and transmission electron microscopy, as well as analyses of sediment grain size, clay mineralogy, and major elements. Our results indicated that sediment from the YR has contributed dominantly to this core since the last deglaciation. Limited contributions were also made from the Yangtze River and Taiwan rivers during the Holocene. Terrigenous inputs derived from the YR could be well characterized by the ratio of hematite/(hematite + goethite). Additionally, the Kuroshio Current (KC) activity since the last deglaciation, which is an important influencing factor for sediment provenance changes, was well deciphered using a new proxy RelDM (relative contribution of detrital ferrimagnetic minerals to bulk magnetic properties). Based on comprehensive analyses of our results and previously published studies, we hypothesised that, during the last deglaciation, abundant resuspended sediment from the palaeo-YR mouth was transported to the East China Sea (ECS) continental shelf edge and the northern OT through seaward bottom flow and further spread into the middle OT. Since the Middle to Late Holocene, the central mud area of the South Yellow Sea and the mud area southwest off Cheju Island may have become the main sources to provide YR-derived materials to the middle OT. However, the mud-area-derived sediment could only be restricted to a small range owing to the hindrance of the strengthened KC. This study provides a new perspective for sediment provenance evolution of the middle OT since the last deglaciation and proposes new methods and parameters for future research on palaeoenvironmental evolution in the ECS.

Mars in ice ages for ∼25% of post-Noachian geologic history

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): David K. Weiss

Abstract

Mars is currently a hyperarid, hypothermal desert planet, whose surface inventory of water is primarily confined to the north and south polar ice caps. Previous studies have shown that Mars has undergone massive shifts in its spin-axis obliquity (present-day is 25.2°) due to secular spin orbit resonances. During periods of higher obliquity, water-ice from the polar caps is mobilized to the mid-latitudes (∼35° obliquity) and even the equator (≥45° obliquity), where it is deposited as snow and accumulates over time to form thick regional surface ice sheets. Abundant evidence exists today for the remnants of these ice ages in the form of debris-covered glaciers and ice deposits, but due to the chaotic nature of orbital simulations beyond ∼20 Ma, it has remained unclear to what temporal extent Mars has experienced such ice ages. Recent developments have suggested that impact events which formed in martian surface ice deposits exhibit a distinctive double-layered ejecta morphology. In tandem with cratering statistics, this observation offers the potential to better our understanding of the history of ice ages on Mars. This work explores the timing of ice age events by evaluating the size-frequency distribution of craters forming in surface ice. Using Hartmann isochron model ages, this work shows that Mars has experienced mid-latitude ice ages for up to a cumulative ∼680 Myr out of the past 3.6 Ga, and experienced equatorial ice ages for up to a cumulative ∼250 Myr within the same time period. The results of this study indicate that Mars has experienced mid-latitude/equatorial ice age states for up to approximately 25% of its post-Noachian geologic history, emphasizing that much of the geologic history of Mars is dominated by the presence of widespread non-polar surface ice sheets.

Clustered BSRs: Evidence for gas hydrate-bearing turbidite complexes in folded regions, example from the Perdido Fold Belt, northern Gulf of Mexico

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Alexey Portnov, Ann E. Cook, Derek E. Sawyer, Chen Yang, Jess I.T. Hillman, William F. Waite

Abstract

We describe previously undocumented but extensive gas hydrate accumulations in the mouth of Perdido Canyon in the northern Gulf of Mexico. The accumulations are located within central parts of structural domes (four-way closures) and are characterized by stacked, high-amplitude bottom simulating reflections (BSRs) that we call clustered BSRs. Seismic data from Perdido Canyon show two clustered BSRs associated with turbidite sequences within two dome folds formed from tectonic folding and salt diapir rise. The northwestern (NW) and southeastern (SE) clustered BSRs have aerial extents of ∼25 km2 and 50 km2, respectively. Well log data confirm gas hydrate occurs above the NW clustered BSR, within a 225 m-thick consistently high-resistivity interval that we interpret as gas hydrate in near-vertical fractures and turbidite sands. The SE dome is only drilled at the edge of the BSR; nevertheless, the well log data indicate that a 30 m-thick gas hydrate accumulation is present. Gas chromatographic logs in both domes suggest a gradual transition from predominantly microbial gas below the BSR (500–1000 m below seafloor (mbsf)) to thermogenic gas at 1000–2000 mbsf. Based on the well log data and seismic stratigraphic analysis, we find gas hydrate is concentrated in fractures in marine mud, as well as in the pores of submarine fan turbidities, where saturations reach as high as 75%. An estimate of the total gas hydrate-bound gas volume at standard temperature and pressure is between 0.04 and 0.17 trillion cubic meters (TCM) assuming average hydrate saturation of 5-20% in a ∼45 m thick turbidite sand unit above the Perdido Canyon BSR area. Measured BSR extent and gas volume estimates indicate that the NW and SE reservoirs are among the largest gas hydrate occurrences known in the Gulf of Mexico.

Consequences of glacial cycles for magmatism and carbon transport at mid-ocean ridges

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Nestor G. Cerpa, David W. Rees Jones, Richard F. Katz

Abstract

Magmatism and volcanism transfer carbon from the solid Earth into the climate system. This transfer may be modulated by the glacial/interglacial cycling of water between oceans and continental ice sheets, which alters the surface loading of the solid Earth. The consequent volcanic-carbon fluctuations have been proposed as a pacing mechanism for Pleistocene glacial cycles. This mechanism is dependant on the amplitude and lag of the mid-ocean ridge response to sea-level changes. Here we develop and analyse a new model for that response, eliminating some questionable assumptions made in previous work. Our model calculates the carbon flux, accounting for the thermodynamic effect of mantle carbon: reduction of the solidus temperature and a deeper onset of melting. We analyse models forced by idealised, periodic sea level and conclude that fluctuations in melting rate are the prime control on magma and carbon flux. We also discuss a model forced by a reconstruction of eustatic sea level over the past 800 kyr. It indicates that peak-to-trough variations of magma and carbon flux are up to about 20% and 10% of the mean flux, respectively. Peaks in mid-ocean ridge emissions lag peaks in sea-level forcing by less than about 20 kyr and the lag could well be shorter. The amplitude and lag are sensitive to the rate of melt segregation. The lag is much shorter than the time it takes for melt to travel vertically across the melting region.

Lack of late-accreted material as the origin of <sup>182</sup>W excesses in the Archean mantle: Evidence from the Pilbara Craton, Western Australia

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Gregory J. Archer, Gregory A. Brennecka, Philipp Gleißner, Andreas Stracke, Harry Becker, Thorsten Kleine

Abstract

We report 182W and 142Nd isotopic compositions, 187Re–187Os systematics, and abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, and Au) for a suite of komatiites and basalts from the ∼3.3Ga Ruth Well Formation and the ∼3.45Ga Warrawoona Group of the Pilbara Craton, Western Australia. The 182W compositions from all samples are indistinguishable from each other, and more radiogenic than modern bulk silicate Earth, with a mean μ182W value of +9.1±4.2 (2SD). By contrast, the 142Nd values for all samples are indistinguishable from each other and terrestrial standards, with a mean μ142Nd value of −1.6±3.2 (2SD). The 146Sm–142Nd and 187Re–187Os systematics are consistent with chondritic Sm/Nd and Re/Os ratios in the mantle source during the lifetime of 182Hf, and the observed 182W excesses therefore cannot be accounted for by early Hf–W fractionation by magma ocean processes, neither by silicate liquid-crystal fractionation nor by high P–T metal-silicate equilibration. The estimated abundances of HSE in the mantle source, however, are significantly lower than modern bulk silicate Earth, with only 51±9% (1SD) of modern bulk silicate Earth abundances. These results are consistent with a partial lack of late-accreted material within the Pilbara source at ∼3.3Ga to account for the 182W excesses. Further, widespread 182W excesses of similar magnitude in other Archean mantle-derived rocks worldwide strongly suggests that a common process, most likely incomplete addition of late-accreted material, was responsible. The apparent mismatch between late-accreted 182W–HSE systematics for some other localities likely reflects either the inherent difficulties associated with estimating source HSE abundances, and/or dissociation of W and HSE by mantle processes. Finally, the combined average 182W–HSE systematics of Archean samples indicate that the pre-late accretion BSE likely had a μ182W value similar to that of the lunar mantle, which strongly suggests post-giant impact Earth–Moon equilibration and indicates that the Moon formed after 182Hf extinction.

Author Index

Publication date: 1 November 2019

Source: Advances in Space Research, Volume 64, Issue 9

Author(s):

List of Referees

Publication date: 1 November 2019

Source: Advances in Space Research, Volume 64, Issue 9

Author(s):

Revisiting erosion rate estimates from luminescence profiles in exposed bedrock surfaces using stochastic erosion simulations

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): N.D. Brown, S. Moon

Abstract

Exposed bedrock is ubiquitous on terrestrial and planetary landscapes, yet little is known about the rate of granular-scale bedrock erosion on timescales longer than the instrumental record. As recently suggested, using the bleaching depth of luminescence signals as a measure of bedrock erosion may fit these scales. Yet this approach assumes constant erosion through time, a condition likely violated by the stochastic nature of erosional events. Here we simulate bedrock luminescence bleaching in response to power-law distributions of removal lengths and hiatus durations. We compare simulation results with previously measured luminescence profiles from boulder surfaces (Sohbati et al., 2018) to illustrate that prolonged hiatuses are unlikely and that typical erosion scales are sub-granular with occasional loss at mm scales, consistent with ideas about microflaws governing bedrock detachment. For a wide range of erosion rates, measurements are integrated over many removal events, producing reasonably accurate estimates despite the stochastic nature of the simulated process. We hypothesize that the greater or equal erosion rates atop large boulders compared to rates at ground level suggest that subcritical cracking may be more influential than aeolian abrasion for boulder degradation in the Eastern Pamirs, China.

Lunar deep materials observed by Chang'e-4 rover

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Sheng Gou, Kaichang Di, Zongyu Yue, Zhaoqin Liu, Zhiping He, Rui Xu, Honglei Lin, Bin Liu, Man Peng, Wenhui Wan, Yexin Wang, Jianzhong Liu

Abstract

China's Chang'e-4 spacecraft achieved the first ever soft-landing within the South Pole-Aitken (SPA) basin on the farside of the Moon. The Chang'e-4 rover, named Yutu-2, made in-situ spectral observations on lunar regolith and a rock fragment at 11 locations during a nominal three-month mission period. The lunar regolith has a relative high olivine/pyroxene ratio, with the pyroxene being chiefly Mg-rich Low-Ca pyroxene (LCP). The rock fragment has a similar Mg-rich composition to that of the regolith. According to the surrounding topographic and geologic context, though originating from the lower base of a differentiated melt pool cannot be excluded here, the rover observed regolith and rock fragment are very likely to be lunar mantle materials excavated from nearby Finsen crater.

Extreme geomagnetic reversal frequency during the Middle Cambrian as revealed by the magnetostratigraphy of the Khorbusuonka section (northeastern Siberia)

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Yves Gallet, Vladimir Pavlov, Igor Korovnikov

Abstract

We present new magnetostratigraphic results obtained for the Drumian stage (504.5–500.5 Ma; Epoch 3/Middle Cambrian) from the Khorbusuonka sedimentary section in northeastern Siberia. They complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time. Magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions. Thermal demagnetization makes it possible to distinguish two magnetization components. The low unblocking temperature (<350 °C) component has a steep inclination and likely originates from remagnetization in a recent field. At higher temperatures, the magnetization isolated possesses the two polarities. Its direction is usually well determined; however, for a noticeable set of samples, a strong overlap between the demagnetization spectra of the two components prevents the determination of reliable directions, although their polarities are well established. The directions from 437 samples define a sequence of 78 magnetic polarity intervals, 22 of which are observed in a single sample. Biostratigraphic data available from the Khorbusuonka section indicate that the duration of the studied section is ∼3 Myr. A geomagnetic reversal frequency of 26 reversals per Myr is therefore estimated for the Drumian, reduced to 15 reversals per Myr if only the polarity intervals defined by at least two consecutive samples are retained. This is an extreme reversal rate, similar to that reported for the Late Ediacaran (late Precambrian), ∼50 Myr earlier, and proposed to be potentially linked to a late nucleation of the inner core. The reversal frequency appears to have drastically dropped for ∼3–4 Myr from a value probably >20 reversals per Myr during the Drumian to ∼1.5 reversals per Myr during the Furongian/Upper Cambrian. Such a sharp decrease is consistent with a transition at a ∼1-Myr timescale, probably caused by threshold effects in core processes, between two geodynamo modes, one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per Myr, and the other marked by hyperactivity of the reversing process, with reversal rates >15 reversals per Myr.

Evaluating the planktic foraminiferal B/Ca proxy for application to deep time paleoceanography

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Laura L. Haynes, Bärbel Hönisch, Kate Holland, Yair Rosenthal, Stephen M. Eggins

Abstract

The Cenozoic Era has been characterized by large perturbations to the oceanic carbon cycle and global climatic changes, but quantifying the magnitude and cause of these shifts is still subject to considerable uncertainty. The boron/calcium (B/Ca) ratio of fossil planktic foraminifera shells is a promising tool for reconstructing surface ocean carbonate chemistry during such events. Previous studies indicate that symbiont-bearing, planktic foraminiferal B/Ca depends on the [B(OH)4−/DIC] ratio of seawater and potentially, when combined with foraminiferal δ11B proxy reconstructions of B(OH)4−, an opportunity to reconstruct surface ocean DIC in the geologic past. There are, however, two barriers towards interpreting records from the pre-Pleistocene era: (1) changes in seawater major ion chemistry in the past might have affected foraminiferal B/Ca; and (2) modern foraminifera species show variable B/Ca calibration sensitivities that cannot be constrained in now-extinct species. Here we address these challenges with new experiments in which we have cultured modern, symbiont-bearing foraminifera Globigerinoides ruber (pink) and Trilobatus sacculifer in seawater with simulated early Cenozoic seawater chemistry (high [Ca], low [Mg], and low [B]T). We explore mechanisms that can account for the inter-species trends that are observed in foraminiferal B/Ca, and propose a framework that can be used to apply B/Ca calibrations to now-extinct species for reconstructing climate perturbations under varying seawater chemistries.

Seismicity and outgassing dynamics of Nyiragongo volcano

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Julien Barrière, Nicolas d'Oreye, Adrien Oth, Nicolas Theys, Niche Mashagiro, Josué Subira, François Kervyn, Benoît Smets

Abstract

Active lava lakes at volcanoes can be regarded as open windows to their magmatic systems. The dynamics of such lakes may therefore provide decisive insights into deeper magmatic processes, potentially leading to fundamental theoretical implications and volcano monitoring improvements. Among the rare volcanoes on Earth hosting a persistent lava lake, Nyiragongo in D.R. Congo directly threatens a massive population of roughly 1 million inhabitants. Here we analyze close-range (i.e., summit) and distant (around 17 km) seismic measurements acquired at this African volcano between 2011 and 2018 in order to better understand the seismic signature of the lava lake activity and how it relates to the deeper volcanic processes. Both summit and distant seismic records contain a similar continuous tremor pattern attributable to the lava lake activity. Combining this information with time-lapse camera images and lava lake level measurements confirms the mechanism of gas pistoning at Nyiragongo, which is characterized by short-duration (a few minutes long) and meter-scale level variations during the period of observation. We also characterize the dominant periodicity of this shallow tremor signature of about a few tens of minutes. Because this marked periodic pattern varies during a significant one-month fluctuation of SO2 emissions (estimated from space), we suggest that this particular seismic periodicity corresponds to the convective lake movement driven by the persistent degassing typical of active open-vent volcanoes. Finally, new seismic evidence reveals the effect of deep magmatic intrusion and consequent major pressure changes in the plumbing system, resulting in sudden and large drops of the lava lake level associated with strong degassing. Such transient episodes have similar characteristics to total lava lake drainage associated with flank eruptions already observed at this volcano in 1977 and 2002, or at Kīlauea volcano in 2018.

Seismic tomography of the uppermost inner core

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Scott Burdick, Lauren Waszek, Vedran Lekić

Abstract

Seismic body wave and normal mode analyses have revealed that the inner core is solid, strongly anisotropic, and characterized by dramatic quasi-hemispherical differences in elastic structure and attenuation. Yet, despite these discoveries, the highly heterogeneous and incomplete data coverage of the inner core has impeded the development of tomographic models even at the longest wavelengths. Here, we use a probabilistic and transdimensional tomographic approach (TBI) on a newly expanded dataset of P-wave travel-times sensitive to the upper 120 km of the inner core. The TBI approach yields a ensemble of parsimonious models that simultaneously capture both the dominant hemispheric dichotomy and laterally abrupt velocity variations. Analysis of the model ensemble allows us to determine the locations of the hemisphere boundaries and rule out the presence of hemispheric dichotomy in anisotropy. Instead, we robustly map regional variations in anisotropy beneath Africa and the eastern Pacific, and detect variations at high latitudes suggesting that cylindrical anisotropy may not be adequate for describing the uppermost inner core.

Two billion years of mantle evolution in sync with global tectonic cycles

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Jamie A. Cutts, Matthijs A. Smit, Dirk Spengler, Ellen Kooijman, Herman L.M. van Roermund

Abstract

The continental crust and sub-continental lithospheric mantle (SCLM) are co-dependent reservoirs in terms of their geochemistry, tectonics, and long-term evolution. Obtaining insight into the mechanisms of lithosphere formation and differentiation requires robust constraint on the complex petrological history of mantle rocks. This has proven difficult as samples from the deep mantle are rare and, although many may have formed in the Archean, no such age has been obtained directly from mantle-derived silicate minerals. Lutetium-hafnium geochronology of garnet has the potential of overcoming this limitation. In this study, this technique was applied on fragments of the SCLM exposed in the Norwegian Caledonides. The chronologic record of these rocks is rich and extensive, yet it is difficult to interpret and is, in part, inconsistent. Our Lu–Hf results from supersilicic pyrope in dunite provide the first Archean internal isochron ages for mantle rocks. These ages are consistent with a period of juvenile crust formation worldwide and provide a record of deeply sourced mantle upwellings from >350 km depth. Results from fertile rock types indicate that melting and isotope re-equilibration occurred in sync with two Proterozoic supercontinent break-up events that are recorded in the Laurentian and Baltic lithospheres. Together, the results indicate that since its extraction during a period of rapid Archean crustal growth, the SCLM appears to have largely been at petro-physical and chemical stasis, with the exception of major episodes of continental break-up. The evolution of the SCLM is thus, highly punctuated and ultimately controlled by the Wilson cycle.

Preface: Variability and coupling of the equatorial, low- and mid-latitude mesosphere, thermosphere and ionosphere: Latest developments of monitoring and modeling techniques

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): Michael Pezzopane, Venkatesh Kavutarapu

Evaluation of ionospheric models for Central and South Americas

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): Telmo dos Santos Klipp, Adriano Petry, Jonas Rodrigues de Souza, Gabriel Sandim Falcão, Haroldo Fraga de Campos Velho, Eurico Rodrigues de Paula, Felix Antreich, Mainul Hoque, Martin Kriegel, Jens Berdermann, Norbert Jakowski, Isabel Fernandez-Gomez, Claudia Borries, Hiroatsu Sato, Volker Wilken

Abstract

This work shows a 20-month statistical evaluation of different Total Electron Content (TEC) estimators for the Central and South America regions. The TEC provided by the International GNSS Service (IGS) in the area covered around the monitoring GNSS stations are used as reference values, and they are compared to TEC estimates from the physics-based (Sheffield University Plasmasphere Ionosphere Model—PIM) and the empirical (Neustrelitz TEC Model-Global—NTCM-GL) models. The mean TEC values show strong dependence on both solar activity and seasonal variation. A clear response was noticed for a period close to 27 days due to the mean solar rotation, as seen in the solar flux measurements. Consistently, the mean TEC values present an annual variation with maxima during December solstices for southern stations with geographic latitudes greater than 25° S. Semi-annual dependence has been observed in TEC for the sector between ±25° of geographical latitude but with modulations caused by fluctuation in the solar radiation. We observed a high correlation between solar radio flux F10.7 and NTCM-GL outputs. The fast increases in F10.7 index have caused significant differences between IGS data and NTCM-GL results mainly for equatorial and low latitudes. For the initial months of the evaluated period (January–April, 2016), the errors of the physics-based model were considerably larger, mainly near the equatorial ionization anomaly. The discrepancies observed in SUPIM results are mainly due to inputs of solar EUV flux. The EUVAC model has underestimated EUV flux between January and April, 2016, when the solar activity was moderated and Solar2000 model has overestimated such flux during low solar cycle period between May and August, 2017. In relation to IGS data, the two assessed models presented smaller differences during the June solstice season of 2016.

Variations in the ionosphere-thermosphere system from tides, ultra-fast Kelvin waves, and their interactions

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): Colin C. Triplett, Thomas J. Immel, Yen-Jung Wu, Chihoko Cullens

Abstract

Large scale waves, such as the atmospheric tides and ultra-fast Kelvin waves (UFKW), have direct effects on the neutral wind and temperature fields of the ionosphere-thermosphere (I-T) system. In this study we examine the response of the I-T system to the atmospheric tides, one UFKW, and the secondary waves generated from their interactions using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM). We find that forcing an UFKW at the lower boundary of the TIEGCM is all that is required for it to setup in the model. We see variations around 10% in the zonal winds that lead to similar variations in the total electron content (TEC) depending on the phase of the UFKW. From these simulations, we expect the Ionospheric Connection Explorer (ICON) mission will be able to fully capture these wave interactions by observing winds and temperatures at the mesopause and above.

Modeling of GPS-TEC using QR-decomposition over the low latitude sector during disturbed geomagnetic conditions

Publication date: 15 November 2019

Source: Advances in Space Research, Volume 64, Issue 10

Author(s): J.R.K. Kumar Dabbakuti, Y. Mallika, M. Venugopala Rao, K. Raghava Rao, D. Venkata Ratnam

Abstract

Given the continuous operation of satellite-based navigation applications, modeling of Total Electron Content (TEC) during magnetic disturbed periods is a significant challenge. The Global Positioning System (GPS)-TEC observations of the Bangalore station (13.02°N, 77.57°E) has been considered and covers the period (2009–2016) of the solar cycle 24. The study emphases on the analysis of TEC variations in eight geomagnetic storms of different intensity: (−223 nT < Dst < −80 nT). The QR decomposition is computed using the Gram-Schmidt (GS) process and is based on observational data from low latitude sectors. For interpolation, the QR model was evaluated on storms that occurred during different periods of solar activity (2009–2016), while for extrapolation the assessment was conducted for the intense storm of March 17, 2015 (St. Patrick's Day storm: Dst −223 nT) in different latitudes, covering the Asian sector between 10°N and 26°N. The R1 and Q1 modes patterns are consistent with changes in the solar proxy index (F10.7) and with regular daily variations and the correlation coefficient is 0.80 and 0.99. The post-residue between the QR model TEC and the GPS- TEC values is ±3 TECU. The QR model captured the TEC responses in consecutive storm cases (18–24, February 2014).

The spatial variation of the TEC deviations increases as it moves towards the Equatorial Ionization Anomaly (EIA) crest from the magnetic equator and decreases beyond the crest. The proposed work could be useful for the further study of the Global Navigation Satellite System (GNSS) performance during geomagnetic magnetic disturbed periods.

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