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The spatial distributions of single-frequency and dual-frequency Electromagnetic ion cyclotron (EMIC) waves in the subauroral ionosphere are investigated under varying geomagnetic activities, using high-resolution magnetic field data from dual Swarm satellites spanning from 2015 to 2017. Single-frequency EMIC waves predominantly occur in the dawn sector, whereas dual-frequency waves exhibit peaks around both dawn and dusk. The occurrence rate of dual-frequency waves shows a more pronounced increase with increasing geomagnetic activity. As magnetic storms evolve, both types of EMIC waves shift from dusk to dawn. The South Atlantic Anomaly (SAA) emerges as a high-incidence region for ionospheric EMIC waves. Dual-frequency EMIC waves display lower frequencies compared to other regions. Additionally, the low-frequency components of dual-frequency waves observed at higher latitudes demonstrate greater power density and longer durations than their high-frequency counterparts. This suggests that higher frequency waves experience more significant damping during propagation. Most dual-frequency EMIC waves observed in the ionosphere belong to the O-band and He-band waves, indicating that magnetospheric bands below the cyclotron frequency of H+ are more likely to propagate into the ionosphere.

On the ground, Pi2 magnetic pulsations are detected at low latitudes (L<2) $(L< 2)$ at all magnetic local times (MLTs), unlike in the inner magnetosphere. To gain insight into the mechanism for the global appearance, we study the MLT dependence of the properties of low-latitude ground Pi2 pulsations detected at four longitudinally separated stations. The pulsation properties are defined with respect to compressional magnetic field Bμ $\left({B}_{\mu }\right)$ oscillations detected by Van Allen Probes at L $L$ = 2.5–6.5 within 2 hr of midnight. Up to two peaks between 6.7 and 40 mHz found in the Bμ ${B}_{\mu }$ spectrum are selected as possible signatures of the source of ground Pi2 pulsations. For each spectral peak, we compute the coherence of the ground horizontal northward (H) $(H)$ component with Bμ ${B}_{\mu }$, and those events exhibiting high coherence are used in statistical analyses. The radial mode structure of the Bμ ${B}_{\mu }$ oscillations indicates they are fundamental or second harmonics of cavity mode oscillations (CMOs). Ground pulsations appear primarily in the H $H$ component with time delays of less than a few seconds and amplitudes comparable relative to the Bμ ${B}_{\mu }$ oscillations in the low-L $L$ region. The observations suggest that, if the dayside ground Pi2 pulsations are driven by ionospheric currents as previously proposed, the current must be coupled to the CMOs, not to the currents flowing on field lines connected to the auroral zone.

The current study explores the dynamic interaction between Interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus, utilizing measurements from the Venus Express (VEX) mission. We have investigated 16 ICME events during the period 2006–2013. The altitude of the inbound bow shock and ionopause at Venus are comprehensively studied during the passage of these ICMEs. The ionosphere is found to be highly magnetized due to the very high magnetic pressure of the induced magnetosphere. Remarkably, the altitude of the ionopause is found to be significantly changed as compared to the previous quiet day due to the increased solar wind dynamic pressure Pdyn $\left({P}_{\mathit{dyn}}\right)$. The ratio of the altitude of ionopause and magnitude of the magnetic field (∣B∣) $(\vert B\vert )$ at ionopause on the event days to the quiet days shows a strong anti-correlation which indicates the ionopause height is inversely related to the magnetic field. Intriguingly, the position of the bow shock exhibited minimal deviations compared to typical quiet days, underscoring that, during ICME events, the ionopause location is more responsive to solar wind pressure fluctuations than the bow shock location. Additionally, the heavy-ion density near and above the ionopause is found to be significantly higher than that observed on previous quiet days. This substantial increase implies that ICMEs can induce atmospheric loss in Venus's atmosphere and also cause a significant reduction in the ionopause location.

The effect of storms driven by solar wind high-speed streams (HSSs) on the high-latitude ionosphere is inadequately understood. We study the ionospheric F-region during a moderate magnetic storm on 14 March 2016 using the EISCAT Tromsø and Svalbard radar latitude scans. AMPERE field-aligned current (FAC) measurements are also utilized. Long-duration 5-day electron density depletions (20%–80%) are the dominant feature outside of precipitation-dominated midnight and morning sectors. Depletions are found in two major regions. In the afternoon to evening sector (12–21 magnetic local time, MLT) the depleted region is 10° ${}^{\circ}$–18° ${}^{\circ}$ magnetic latitude (MLAT) in width, with the largest latitudinal extent 62° ${}^{\circ}$–80° ${}^{\circ}$ MLAT in the afternoon. The second region is in the morning to pre-noon sector (04–10 MLT), where the depletion region occurs at 72° ${}^{\circ}$–80° ${}^{\circ}$ MLAT within the auroral oval and extends to the polar cap. Using EISCAT ion temperature and ion velocity data, we show that local ion-frictional heating is observed roughly in 50% of the depleted regions with ion temperature increase by 200 K or more. For the rest of the depletions, we suggest that the mechanism is composition changes due to ion-neutral frictional heating transported by neutral winds. Even though depleted F-regions may occur within any of the large-scale FAC regions or outside of them, the downward FAC regions (R2 in the afternoon and evening, R0 in the afternoon, and R1 in the morning) are favored, suggesting that downward currents carried by upward moving ionospheric electrons may provide a small additional effect for depletion.

Arctic permafrost thaw holds the potential to drastically alter the Earth's surface in Northern high latitudes. We utilize high-resolution large eddy simulations to investigate the impact of the changing surfaces onto the neutrally stratified atmospheric boundary layer (ABL). A stochastic surface model based on Gaussian Random Fields modeling typical permafrost landscapes is established in terms of two land cover classes: grass land and open water bodies, which exhibit different surface roughness length and surface sensible heat flux. A set of experiments is conducted where two parameters, the lake areal fraction and the surface correlation length, are varied to study the sensitivity of the boundary layer with respect to surface heterogeneity. Our key findings from the simulations are the following: The lake areal fraction has a substantial impact on the aggregated sensible heat flux at the blending height where surface heterogeneities become horizontally homogenized. The larger the lake areal fraction, the smaller the sensible heat flux. This result gives rise to a potential feedback mechanism. When the Arctic dries due to climate heating, the interaction with the ABL may accelerate permafrost thaw. Furthermore, the blending height shows significant dependency on the correlation length of the surface features. A longer surface correlation length causes an increased blending height. This finding is of relevance for land surface models concerned with Arctic permafrost as they usually do not consider a heterogeneity metric comparable to the surface correlation length.

To better understand how sharp changes in the solar wind and interplanetary magnetic field conditions affect the ionosphere outflows at high latitudes, we analyze an event observed on 17 July 2002 showing suprathermal (tens to hundreds of eV) outflowing H+ ions in the lobe driven by the impact of an interplanetary (IP) shock. A spacecraft in the lobe at altitudes of ∼6.5 R E first observed enhanced downward DC Poynting fluxes ∼2 min after the shock impact and then, another 8 min later, the appearance of suprathermal outflowing H+ ions as ion beams and ion conics. The increasing downward DC Poynting fluxes and the increasing outflowing H+ fluxes that appeared later were highly correlated because they shared a similar increasing trend with a time scale of ∼5 min. To explain such time delay and correlation, we conclude that a plausible scenario was that the enhanced DC Poynting fluxes reached down to lower altitudes, drove processes to accelerate the pre-existing polar wind ions to ion beams and ion conics, and then these newly generated suprathermal ions flowed upward to the spacecraft altitudes. This event indicates that an IP shock can drive a significant amount of suprathermal H+ outflows from the polar cap.

Biogenic volatile organic compounds (BVOCs) are regarded as important precursors for ozone and secondary organic aerosol, mainly from vegetation emissions. In the context of the expanding trend of vegetation greening, the development of high-precision vegetation data and accurate BVOC emission estimates are essential to develop effective air pollution control measures. In this study, by integrating the multi-source vegetation cover data, we established a high-resolution vegetation distribution (HRVD) data set to develop a high spatio-temporal resolution emission inventory and investigated the impact of different land cover data sets on emission simulation and impact of land cover change on BVOC emissions during 2001–2020. The annual total BVOC emissions in China for 2020 was 15.66 Tg, which were mainly from trees. The emissions simulated by CNLUCC and MODIS data sets were 1.53% and 1.72% higher than those simulated by HRVD data sets, respectively. The spatial distribution of emission differences was consistent with that of land cover differences. The simulated BVOC emissions by the HRVD data set had the best accuracy as they improved the bias between modeling and observation from 69.06% to 65.35% and decreased the underprediction of observations by a factor of 2.13 compared with simulation by MEGAN default vegetation data. The annual BVOC emissions caused by changing vegetation distribution and LAIv (LAI of vegetation covered surfaces) enhanced at a rate of 72.06 Gg yr−1 during 2001–2020. LAIv was the main driver of emission variations. The total OH reactivity of the resulted BVOC emissions increased at a rate of 1.59 s−1 yr−1, with isoprene contributed the most.

The return stroke of cloud-to-ground (CG) lightning is an impulsive radiator of very low-frequency/low-frequency (VLF/LF) electromagnetic signals allowing for the remote sensing of lower ionosphere over large spatial coverage. In this study, we examined the LF magnetic fields measured in Malacca, Malaysia, to probe reflection heights of the lower ionosphere near the equator on three different nights in 2021. The results show that the virtual ionospheric height at nighttime typically ranged from 82.0 to 90.0 km, with a mean value of 85.3 km. Our measurements also revealed significant variations in the virtual ionospheric height across different regions over a spatial scale of about 800 km. The maximum height difference was about 5.0 km. Moreover, the fluctuation characteristics are observed in both estimated ionospheric height and calculated peak reflection ratio during similar periods. This fluctuation may be related to atmospheric gravity waves in the nighttime ionosphere. In addition, we compared the virtual ionospheric height estimated from CG strokes of different polarities, and the results showed that the virtual reflection height for positive CG strokes is lower than that for negative ones.

A persistent shortwave radiative bias of Southern Ocean (SO) clouds in climate models is strongly associated with incorrect cloud phase representation, which impacts precipitation. Measurements characterizing precipitation in low-level mixed-phase clouds, which frequently form over the SO, are rare, and our understanding of precipitation efficacy within these clouds remains limited. The simulated surface precipitation bias has an indirect effect on determining global climate sensitivity and a direct impact on the hydrological cycle. This study investigates the representation of low clouds, cloud variability, and precipitation statistics over the SO in real-case Icosahedral Nonhydrostatic (ICON) simulations at the kilometer scale. The simulations are contrasted with 48 hr of continuous shipborne observations of open and closed-cell stratocumuli, south of Tasmania. Our simulations show the significance of heavily rimed particle formation, their in-cloud growth, and subcloud melting to capture the observed cloud-precipitation vertical structure. In addition, supercooled drizzle formation impacts the vertical structure and precipitation statistics. ICON captures the observed intermittency of precipitation even at a standard vertical resolution of 200 m in the boundary layer but only captures the observed sparse distribution of intense precipitation (>1 mm hr−1) when the maximum vertical resolution is reduced to 100 m. However, the simulations of the 2-day accumulated precipitation and the radiative effect are largely insensitive to the vertical resolution. The cloud reflectivity of the broken cloud deck is underestimated due to negative biases in cloud optical depth.

Air pollution caused by various anthropogenic activities and biomass burning continues to be a major problem in India. To assess the effectiveness of current air pollution mitigation measures, we used a 3D global chemical transport model to analyze the projected optical depth of carbonaceous aerosol (AOD) in India under representative concentration pathways (RCP) 4.5 and 8.5 over the period 2000–2100. Our results show a decrease in future emissions, leading to a decrease in modeled AOD under both RCPs after 2030. The RCP4.5 scenario shows a 48%–65% decrease in AOD by the end of the century, with the Indo-Gangetic Plain (IGP) experiencing a maximum change of ∼ ${\sim} $25% by 2030 compared to 2010. Conversely, RCP8.5 showed an increase in AOD of ∼ ${\sim} $29% by 2050 and did not indicate a significant decrease by the end of the century. Our study also highlights that it is likely to take three decades for current policies to be effective for regions heavily polluted by exposure to carbonaceous aerosols, such as the IGP and eastern India. We emphasize the importance of assessing the effectiveness of current policies and highlight the need for continued efforts to address the problem of air pollution from carbonaceous aerosols, both from anthropogenic sources and biomass burning, in India.

Using peak electron density data from the Global-scale Observations of the Limb and Disk (GOLD) imager, equatorial plasma bubbles (EPBs) from October 2018 to December 2022 are identified in this paper. The occurrence characteristics of EPBs is statistically analyzed. The results show that EPBs have strong seasonal and longitudinal variations in the range of longitude −60°–0° and magnetic latitude 25° to −20°: (a) The occurrence of EPBs is highest during the spring and autumn equinoxes and lowest during the summer. (b) Equinox asymmetry is found, that the occurrence of EPBs is much higher in autumn than in spring. (c) A peak in the longitudinal distribution of EPBs is observed, with the highest occurrence occurring between −10° and 0° longitude. Additionally, a second peak is evident at −50° longitude in autumn. The GOLD imager is capable of conducting prolonged observations of EPBs in the same region from space, thereby offering a novel perspective on EPBs.

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.

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.

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.

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.

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.

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.

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.

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.

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.