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Abstract

Real-time time transfer is the basis for the time synchronization and establishment and maintenance of time scales. In this contribution, we present a regional multi-station real-time time transfer approach, in which observations from multi-global navigation satellite system (GNSS) received at regional multi-station are integrated to conduct an undifferenced network solution. Consequently, the simultaneous estimation of receiver clock offsets for multiple stations becomes possible, enabling the execution of real-time time transfer. One week of observation data from five multi-GNSS Experiment (MGEX) stations located in Europe are selected to generate four links (distances from 919.7 to 2153.4 km), and four schemes are designed, i.e., GPS-only, BDS-3-only, Galileo-only, and GPS/BDS-3/Galileo (GCE) solutions, respectively. Experiment results show that the mean number of observations, time dilution of precision (TDOP) values, and standard deviation (STD) of clock offset difference time series between the Center for Orbit Determination in Europe (CODE) and the estimated solutions for four time links are 90, 76, 72, 239 and 0.34, 0.38, 0.39, 0.21 and 0.039, 0.050, 0.043, 0.036 ns for GPS-only, BDS-3-only, Galileo-only, and GCE solutions, respectively. When the averaging time is shorter than 7680 s, the frequency stability of GCE solutions shows the best performance. The mean frequency stability of four time links at 7680 s is 6.59 × 10–15,7.25 × 10–15, 6.88 × 10–15 and 5.96 × 10–15 for GPS-only, BDS-3-only, Galileo-only, and GCE solutions, respectively. The GCE solution shows the best performance among the four schemes in terms of the number of observations, TDOP, STD, and frequency stability. The experiment results demonstrate that this approach is suitable for regional multi-station real-time time transfer.

Abstract

Equatorial plasma irregularities in the ionospheric F-region proliferate after sunset, causing the most apparent radio scintillation “hot-spot” in geospace. These irregularities are caused by plasma instabilities, and appear mostly in the form of under-densities that rise up from the F-region's bottomside. After an irregularity production peak at sunset, the amplitude of the resulting turbulence decays with time. Analyzing a large database of irregularity spectra observed by one of the European Space Agency's Swarm satellites, we have applied a novel but conceptually simple statistical analysis to the data, finding that turbulence in the F-region tends to decay with a uniform, scale-independent rate, thereby confirming and extending the results from an earlier case study. We find evidence for two regimes, one valid post-sunset (1.4 hr decay rate) and one valid post-midnight (2.6 hr). Our results should be of utility for large-scale space weather modeling efforts that are unable to resolve turbulent effects.

Abstract

We analyze the gravity waves (GWs) from the ground to the thermosphere during 11–14 January 2016 using the nudged HI Altitude Mechanistic general Circulation Model. We find that the entrance, core and exit regions of the polar vortex jet are important for generating primary GWs and amplifying GWs from below. These primary GWs dissipate in the upper stratosphere/lower mesosphere and deposit momentum there; the atmosphere responds by generating secondary GWs. This process is repeated, resulting in medium to large-scale higher-order, thermospheric GWs. We find that the amplitudes of the secondary/higher-order GWs from sources below the polar vortex jet are exponentially magnified. The higher-order, thermospheric GWs have concentric ring, arc-like and planar structures, and spread out latitudinally to 10 − 90°N. Those GWs with the largest amplitudes propagate against the background wind. Some of the higher-order GWs generated over Europe propagate over the Arctic region then southward over the US to ∼15–20°N daily at ∼14 − 24 UT (∼9 − 16 LT) due to the favorable background wind. These GWs have horizontal wavelengths λ H  ∼ 200 − 2,200 km, horizontal phase speeds c H  ∼ 165 − 260 m/s, and periods τ r  ∼ 0.3 − 2.4 hr. Such GWs could be misidentified as being generated by auroral activity. The large-scale, higher-order GWs are generated in the lower thermosphere and propagate southwestward daily across the northern mid-thermosphere at ∼8–16 LT with λ H  ∼ 3,000 km and c H  ∼ 650 m/s. We compare the simulated GWs with those observed by AIRS, VIIRS/DNB, lidar and meteor radars and find reasonable to good agreement. Thus the polar vortex jet is important for facilitating the global generation of medium to large-scale, higher-order thermospheric GWs via multi-step vertical coupling.

Abstract

Several borehole cores intersecting faults related to coseismic slip display high-temperature features, including thermal decomposition of fault gouge. We present evidence that these features may be related to fluid drainage of the slip zone during seismic slip. We sheared water-saturated kaolinite powders under both fluid drained and undrained conditions, expected for seismic slip at shallow crustal depths. Our results show typical dynamic weakening behavior regardless of conditions. Under fluid drained condition, restrengthening accompanied by the thermal decomposition of kaolinite occurs. In addition, thermal decomposition of kaolinite tends to be initiated at high normal stresses (>5 MPa) with short displacement (<5 m). We propose that thermal pressurization acts as a weakening mechanism but ceases because of fluid drainage, triggering kaolinite thermal decomposition. This finding explains seismic-slip-related clay anomalies at depth rather than at the surface, as observed in the borehole after the 1999 Mw 7.6 Chi-Chi earthquake, Taiwan.

Abstract

A negative ionospheric storm occurred over the North American sector on 4 November 2021. By the integration of hemispheric power (HP), Ionospheric Connection Explorer (ICON), Global-scale Observations of the Limb and Disk and global navigation satellite system total electron content (TEC) observations, a complete observation chain is obtained for the first time. At 07:11 UT, prominent energy was input into high latitudes. From 11:51 to 15:49 UT, strong southwestward neutral wind was observed by ICON over the south of North America, followed by significant ∑O/N2 depletion (60%) and temperature enhancement (∼300 K at ∼150 km). TEC depletion started at 12:00 UT in the northeastern North America and expanded to most of the United States. The TEC reduction showed the latitudinally tilted structure similar to that of ∑O/N2 depletion and temperature enhancement, which was shaped by the southwestward neutral wind. The thermosphere-ionosphere-electrodynamics general circulation model (TIEGCM) reproduced the observations qualitatively and was utilized to investigate the details of the negative storm generation and evolution processes. Strong equatorward wind and ∑O/N2 depletion reached the North America at midnight, much earlier than the TEC depletion, which began at sunrise. At 90°W, the time delay between TEC and ∑O/N2 depletions increased from ∼3.3 hr at 30°N to ∼11 hr at 60°N. Several factors contributed to the time delay including the time difference between midnight and sunrise, ∑O/N2 transport direction, westward wind and the variation of solar elevation angle with latitude.

Abstract

This work describes the instrumental error budget for space-based measurements of the absolute flux of the sky synchrotron spectrum at frequencies below the ionospheric cutoff (≲ $\lesssim $20 MHz). We focus on an architecture using electrically short dipoles onboard a small satellite. The error budget combines the contributions of the dipole dimensions, plasma noise, stray capacitance, and front-end amplifier input impedance. We treat the errors using both a Monte Carlo error propagation model and an analytical method. This error budget can be applied to a variety of experiments and used to ultimately improve the sensing capabilities of space-based electrically short dipole instruments. The impact of individual uncertainty components, particularly stray capacitance, is explored in more detail.

Abstract

Pulsating proton auroras are often attributed to periodic proton precipitation. However, how the proton precipitation is periodically generated in the magnetosphere remains an open issue. Utilizing multi-point space-borne and ground-based observations, this study proposed a potential mechanism responsible for pulsating proton precipitation and intermittent ionospheric electron density disturbances. On 8 September 2017, Pc4 ULF waves and electromagnetic ion cyclotron (EMIC) wave packets were simultaneously observed by Van Allen Probes (RBSP) in the inner magnetosphere. The EMIC wave packets were quasi-periodically excited at the same frequency as the ULF waves, which resulted in 30–100 keV proton precipitation detected by Low-Earth-Orbit (LEO) POES satellites. Meanwhile, conjugate European Incoherent Scatter (EISCAT) radar on the ground observed E-region electron density enhancements that intermittently appeared nearly at the same frequency as the EMIC wave packets in space. These observations together suggest that ULF waves in the magnetosphere are the ultimate driver that modulates quasi-periodic EMIC waves to induce proton precipitation and pulsating disturbances in the ionosphere.

Extremely clean air on the ground, warm air intrusions and sulfate aerosol at high altitudes—a Leipzig research project has gained new insights into clouds in Antarctica. From January to December 2023, the vertical distribution of aerosol particles and clouds in the atmosphere above the German Neumayer Station III of the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI) was investigated from the ground for the first time.

Climate change is accelerating the melting of ice in Greenland at an alarming rate, with serious implications not only for the Arctic, but also for the global climate, including Europe. According to a study led by researchers at the University of Barcelona, extreme melting episodes—periods when large areas of snow and ice melt rapidly—have been about twice as frequent during summers in recent decades compared to the period 1950–1990.

Researchers at University of Tsukuba have shown that the 1995 Kobe (Hyogo-ken Nanbu) earthquake, which struck southern Hyogo Prefecture, may have been triggered by deep underground flooding beneath Arima Hot Springs. By analyzing the stable isotope ratios of hydrogen and oxygen as well as chloride ions in Arima hot spring water over several decades, the researchers have uncovered a likely connection between the earthquake and water originating from the subducting Philippine Sea Plate.

Abstract

Aqueous fluids are extensively present in the middle to lower crust, as revealed by seismic and magnetotelluric soundings. The α−β quartz phase transition significantly affects many physical properties and leads to substantial microcracks that can provide pathways for the migration of crustal fluids. A systematic investigation of macroscopic physical properties and microstructure of quartz is crucial to elucidate their correlation. In the present study, the effects of water content, trace elements, orientations, and phase transition on the electrical conductivity of quartz were thoroughly evaluated at 400−900°C and 1 GPa. Individual annealing experiments were simultaneously conducted on quartz single crystals at different peak temperatures and 1 GPa to investigate the evolution and spatial distribution of microcracks using X-ray microtomography (CT) and backscattered electron imaging. We found that trace element content and orientations, rather than H2O, are the dominant factors controlling the conductivity of quartz. The distinct changes in conductivity of single crystals at around α−β phase transition temperature are attributed to the transformation of microcracks from isolated to interconnected networks, as confirmed by two-dimensional (2-D) and three-dimensional (3-D) microstructure images. Based on the variation in electrical conductivity and microstructure across the transition, it thus is proposed that the intragranular microcracks caused by quartz phase transition can serve as fluid or melt pathways within highly conductive zones present in the middle to lower crust, while α-quartz acts as an impermeable cap.

Abstract

Elevated surface ozone levels are often detected in the New York metropolitan area during summertime. Moreover, surface ozone in this region exhibits sharp spatial gradients and distinctive diurnal cycles under the influence of complex boundary layer circulations induced by the intricate coastal geometry. This study examines how surface ozone is impacted by local circulations spatially and temporally under different temperature scenarios (all summer days, hot summer days, and extreme heat days) with the help of cluster-based meteorological conditions during the summertime of 2017–2019. The most polluted days are found to be highly associated with hot sea breeze days with weak background flow. When sea breeze development in the New York Bight is delayed and its penetration north is intercepted by the dominant westerlies during hot summer days, daily maximum 8-hr average ozone (DMA8) in some ozone hot spots of New York City (NYC) and the south shore of Connecticut (CT) typically drops 9–10 ppb under comparable temperature levels. The average regional decrease of DMA8 for NYC and coastal CT is 6.7 and 8.3 ppb, respectively. Furthermore, we conclude that a change in early morning meridional wind direction is the most critical meteorological characteristic in controlling sea breeze onset type and helping modulate ozone exceedances in the region during extreme hot days when ozone exceedances are expected to be very common. The conclusion is further demonstrated with two case studies during the Long Island Sound Tropospheric Ozone Study 2018 field campaign.

Abstract

This study derives polarimetric radar vertical profiles and microphysical retrievals for 25 Synoptic Snow (SS) and 23 Lake Effect Snow (LES) cases using the Range-Defined Quasi-Vertical Profiles (RD-QVP), Columnar Vertical Profiles (CVP), and Process-oriented Vertical Profiles (POVP) methods. For all vertical profile techniques, SS cases exhibit a near-linear increase in reflectivity from −30 to 0°C whereas Z DR and K dp locally peak in the dendritic growth layer. LES cases universally exhibit negative Z DR , rather high Z, negligible K dp , and near-unity ρ hv . Ground measurements from the past OWLeS campaign provide direct evidence that conical graupel may strongly affect these polarimetric measurements in LES bands. Aggregation efficiencies for SS cases are estimated by optimizing the theoretical number concentration (N t ) and mean volume diameter (D m ) steady-state vertical profiles against radar-retrieved profiles derived from 20 of the 25 synoptic storm RD-QVPs. The median estimated aggregation efficiency is approximately 0.15 with a relatively narrow interquartile range that spans from 0.1 to just over 0.2. Values of optimized aggregation efficiencies are nearly independent of the assumed gamma distribution shape parameter. These results are used to derive temperature-dependent, climatological steady-state relations for vertical profiles of N t , D m , and liquid-equivalent snowfall rates. These results can be used in numerical weather prediction model aggregation parameterizations and can also provide climatologically representative vertical profiles of radar and microphysical quantities.

Abstract

The Makassar Strait throughflow (MST) is the major component of the Indonesian Throughflow (ITF), transferring Pacific water into the Indian Ocean. In our previous study, we identified a new zonal pathway, a. k.a. the North Equatorial Subsurface Current (NESC), which carried equatorial water into the MST sub-thermocline (>300 m) in the summer 2016 following the 2015/16 El Niño. We now show continued strong southward MST in the sub-thermocline during the winter of 2016–2017, with salinity higher than that in the summer 2016, due to direct South Pacific water intrusion into the Sulawesi Sea. The origin of the intrusion is identified from the New Guinea Coastal Undercurrent (NGCUC) and from an anomalous westward flow along 3°N in the western equatorial Pacific. The identified interannual variability of the western Pacific Ocean circulation is particularly strong in the winter following super El Niño events.

Abstract

The balance between particulate inorganic carbon (PIC) and particulate organic carbon (POC) holds significant importance in carbon storage within the ocean. A recent investigation delved into the spatial distribution of phytoplankton and the physiological mechanisms governing their growth. Employing random forests, a machine learning technique, this study unveiled apparent relationships between POC and 10 environmental fields. In this work, we extend the use of random forests to compare how observed PIC and POC respond to environmental conditions. PIC and POC exhibit similar responses to certain environmental drivers, suggesting that these do not explain differences in their distribution. However, PIC is less sensitive to iron and more sensitive to light and mixed layer depth. Intriguingly, both PIC and POC display weak sensitivity to CO2, contrary to previous studies, possibly due to the elevated pCO2 in our data set. This research sheds light on the underlying processes influencing carbon sequestration and ocean productivity.

Abstract

Climate models struggle to produce sea surface temperature (SST) gradient trends in the tropical Pacific comparable to those seen recently in nature. Here, we find that the magnitude of the cloud-SST feedback in the subtropical Southeast Pacific is correlated across models with the magnitude of Eastern Pacific multi-decadal SST variability. A heat-budget analysis reveals coupling between cloud-radiative effects, circulation, and SST gradients in driving multi-decadal variability in the Eastern Pacific. Using this relationship and observed feedback estimates, we find that internal Eastern Pacific SST variability is underestimated in most models. Adjusting for model bias increases the likelihood of generating a cooling trend at least as large as observations in preindustrial control simulations by ∼ ${\sim} $56% on average. If models underestimate climate “noise,” as our results suggest, this bias should be accounted for when attributing the relative importance of forced versus unforced changes in the climate.

Abstract

The depth of the Los Angeles (LA) basin is a critical factor for seismic hazard assessment and active tectonic studies. By analyzing S-waves generated by earthquakes below the basin that convert to P-waves at the sediment-bedrock interface, we estimate the maximum depth of the LA basin to be 9 km. This estimate depends on the velocities within and below the basin, and the depth presented here is based on the latest community velocity model. To map the basin depth, we use two dense arrays: the Community Seismic Network, a dense network of low-cost accelerometers in schools across LA region, and a basin-wide node survey conducted in 2022, consisting of about 300 geophones deployed for a month. Utilizing differential travel times between direct S and Sp converted phases of local earthquakes, we produce a detailed depth map of the LA basin.

Abstract

Surface fluxes and states can recur and remain consistent across various spatial and temporal scales, forming space-time patterns. Quantifying and understanding the observed patterns is desirable, as they provide information about the dynamics of the processes involved. This study introduces the empirical spatio-temporal covariance function and a corresponding parametric covariance function as tools to identify and characterize spatio-temporal patterns in remotely sensed fields. The method is demonstrated using 2 km hourly GOES-16/17 land surface temperature (LST) data over the Contiguous United States by splitting the area into 1.0° × 1.0° domains. The summer day-time LST ESTCFs for 2018 to 2022 are derived for each domain, and a parametric covariance model is fitted. Clustering analysis is applied to detect areas with similar spatio-temporal LST patterns. Six main zones within CONUS are identified and characterized based on their variance and temporal and spatial characteristic length scales (i.e., scales for which the temperature variations are temporally and spatially related), respectively: (a) Eastern plains with 3 K2, ∼6 hr, and 0.15°, (b) Gulf of California with 60 K2, ∼8 hr, and 0.34°, (c) mountains and coasts transition 1 with 16 K2, ∼11 hr, and 0.25°, (d) central US, Midwest, and South cities with 5.5 K2, ∼8 hr, and ∼0.2°, (e) mountains and coasts transition 2 with ∼10 K2, ∼8 hr, and 0.2°, and (f) largest mountains and coastlines with ∼19 K2, ∼13 hr, and 0.3°. The tools introduced provide a pathway to formally identify and summarize the spatio-temporal patterns observed in remotely sensed fields and relate those to more complex processes within the Soil-Vegetation-Atmosphere System.

Abstract

On 18 December 2022, Hawaiian Airlines flight HA35 encountered severe turbulence without warning in a cloud-free height. We reproduced this incident using the Weather Research and Forecasting Model (WRF) at a convection-permitting resolution. We found that this case of tropical upper-level turbulence occurred primarily due to the fast-growing convective tower in the unstable layer created by gravity wave breaking. At lower altitudes, a mesoscale convective system (MCS) caused a decrease in wind speed in both upstream and downstream regions. At upper levels, a large-scale jet descended and accelerated after flowing over the top of the MCS, which acted like a barrier and produced a situation similar to a downslope windstorm due to mountain terrain. Upper-level turbulence is 2–3 km higher than the top of the MCS. The critical level above the jet and the locally self-induced critical level created the locally enhanced descending jet stream, which destabilized the flow through Kelvin–Helmholtz instabilities. The convective tower existed near the flight route and played an important role in triggering turbulence in the unstable environment through its convective gravity waves.

Abstract

Low latitude ionosphere experiences complex dynamical and electrodynamical processes, which make the spatiotemporal variations of the corresponding electron density complicated and therefore influence trans-ionosphere radio communications. The monitoring of low latitude dynamical drivers, such as neutral wind and ionospheric electric field, is essential for both dynamic mechanism investigations and applications. The Sanya Incoherent Scatter Radar Tristatic System (SYISR-TS) was proposed with the main objective of low latitude ionospheric monitoring and investigation and has been successfully developed over the past decade. The system consists of the Sanya (18.3°N, 109.6°E) trans-receiving main station with key parameters of ∼1,600 m2 antenna aperture, >4 MW peak power, <120 K system noise temperature, and ∼46 dBi normal gain, and Danzhou (19.5°N, 109.1°E) and Wenchang (19.6°N, 110.8°E) receiving only stations with key parameters of ∼790 m2 antenna aperture, <130 K system noise temperature, and ∼43 dBi normal gain. Three stations form a quasi-equilateral triangle at Hainan Island and use Global Navigation Satellite System satellite common view technique to achieve the time synchronization with the uncertainty of the timing and time synchronization less than 50 and 10 ns, respectively. Initial collaborative satellite tracking and ionospheric common volume experiments among three stations have confirmed the detection ability of SYISR-TS and the feasibility of achieving its scientific goals in the future.