Penetration electric fields originating from the solar wind/magnetosphere, shielding electric fields produced by the Region 2 field‐aligned currents, and disturbance dynamo electric fields produced ...by disturbance neutral winds are the major disturbance electric fields in the middle‐ and low‐latitude ionosphere during geomagnetic storms. Understanding the interplay of electric fields from different sources is critical for understanding storm time ionospheric electrodynamics. In this study, we analyze ionospheric data measured by five Defense Meteorological Satellite Program satellites in the dusk‐evening sector during a magnetic storm. A peculiar feature in this case is that the vertical ion drift was enhanced in the upward direction by up to 180 m/s, corresponding to the occurrence of an eastward penetration electric field in the equatorial ionosphere, for 14 hr, including 3 hr of the storm main phase and the first 11 hr of the recovery phase. The variations of other ionospheric parameters (the ion density, ion composition, electron temperature, and zonal drift) were well correlated with the variation of the vertical ion drifts, providing further evidence of prolonged penetration electric fields. Observations of short‐lived penetration electric fields are also presented, and the causes for short‐lived penetration electric fields are discussed. Important issues on the shielding effects are addressed. Scenarios for occurrence of long‐lasting penetration electric fields are proposed. Some outstanding problems associated with penetration electric fields are listed for future studies.
Key Points
Continuous penetration electric fields for 14 hr are reported for the first time
The occurrence and effects of shielding and disturbance dynamo electric fields are discussed
Scenarios for long‐lasting penetration electric fields are proposed
A very challenging task in ionospheric studies is to determine the separate contributions of penetration and disturbance dynamo processes in the generation of equatorial plasma drift during magnetic ...storms. In this study, we analyze the ion drift measured by the Communications/Navigation Outage Forecasting System satellite during the magnetic storm on 15–16 July 2012. A unique feature of this storm is the exceptionally long period of continuous southward interplanetary magnetic field (IMF) for 32 hr. The storm‐induced net change of the meridional/vertical ion drift, the difference drift between the storm time and the quiet time, is derived during the storm main phase and the first 20 hr of the recovery phase with southward IMF. The difference drift during the recovery phase cannot be explained by the disturbance dynamo effect alone. A new method is used to separate the drifts caused by the penetration and disturbance dynamo processes. The penetration drift is represented by an empirical pattern of penetration electric field and depends on the IMF magnitude, and the disturbance dynamo drift is obtained by subtracting the penetration drift from the measured difference drift. The derived disturbance dynamo drift is in good agreement with previous statistical pattern. This is the first effort to identify the separate contributions of the penetration and disturbance dynamo processes to the total drift from observed data. The results have important implications in identifying storm‐time penetration and disturbance dynamo electric fields and their effects on the generation and evolution of plasma bubbles.
Key Points
A new method is proposed to separate the storm‐time penetration and disturbance dynamo electric fields from ion drift measurements
The derived disturbance dynamo drifts in this storm are in reasonable agreement with previous statistical pattern
The combined penetration and dynamo effects provide a good explanation of post‐midnight plasma bubbles during the storm recovery phase
We present the first systematic analysis of global ionospheric disturbance current systems caused by multiple processes of solar and magnetospheric origin, including reorientations of the ...interplanetary magnetic field (IMF), sudden changes in the solar wind dynamic pressure, magnetospheric sawtooth substorms, and ultralow frequency (ULF) waves. Measurements from global magnetometer networks are used to derive the equivalent disturbance currents from the polar cap to the equator. A surprising result is that the equivalent disturbance current systems are very similar, although the driving processes are completely different. The equivalent disturbance current system in response to IMF reorientation or substorm onset is characterized by a large vortex on the dayside and evening sector and a smaller vortex near dawn, and the polarity of the current vortices depends on the IMF direction. The equivalent disturbance current system caused by a sudden change in the solar wind pressure or by ULF waves consists of a single vortex at middle and low latitudes and a very small vortex above ~60° magnetic latitude near dawn. The similar disturbance current systems caused by different solar wind and magnetospheric processes suggest that the global distribution of the ionospheric currents is determined by the intrinsic property of the ionosphere. The global current system takes only ~1 min to completely reconstruct, indicating that the current system can reach a new steady state within 1 min. A scenario is proposed to explain the global distribution and fast reconstruction of the current systems.
Key Points
Global ionospheric equivalent disturbance current systems caused by multiple processes are derived and systematically analyzed for the first time
A time scale of 1 min for global ionospheric disturbance current system to reconstruct is identified
A scenario is proposed to explain the global distribution and fast reconstruction of the ionospheric current systems
AZ91 reinforced with four different concentrations (0,0.3,0.6,1)wt% of WS2 micro particles were fabricated using stir casting method. The samples were deformed by equal channel angular pressing ...(ECAP) after homogenization. The scanning electron microscopy and X-ray diffraction were used to investigate the microstructure and phase composition respectively. The results reveal that rod-like precipitates produced during the homogenization process are refined and bounded in the grain boundaries by the combined effect of WS2 enrichment and increasing number of passes of ECAP. The needle precipitates are eminent in 1pass 0.3 wt%WS2/AZ91. The β-Mg17Al12 and extracted Al particles are dissolved and secondary phases are produced by dynamic recrystallization after 2pass 1 wt%WS2/AZ91. The maximum crystallite size and microstrain distribution have been observed for 1pass 0.6 wt%WS2/AZ91. The mechanical properties investigation represents that 2pass 0.6 wt%WS2/AZ91 has optimal yield strength and ultimate tensile strength. The (22.143)%, (44.735)% and (92) % increment in yield strength, ultimate tensile strength and elongation have been observed in 2pass 1 wt%WS2/AZ91 compared with homogenized monolithic AZ91 alloy respectively. The maximum young's modulus (6696.218MPa) has been found for 1pass homogenized WS2/AZ91 and minimum (4108.165MPa) for homogenized 0.6 wt%WS2/AZ91. The ECAP deformed 1pass 0.3 wt%WS2/AZ91 exhibits the maximum strain hardening rate. The YTS, UTS, and elongation are (30.41) %, (47.6) % and (51.9)% increased after 2pass-0.6 wt%WS2/AZ91.
In this work, AZ91 magnesium alloy reinforced with a low fraction (0–1) wt.% tungsten disulfide(WS2) microparticles have been fabricated using stir casting method. Effects of WS2 microparticles on ...mechanical properties, microstructure, and their correlation have been analyzed. The microstructure of composites has been investigated by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffractometer. Results indicate that density, hardness, ultimate tensile strength (UTS), and tensile ductility have a direct relation with reinforcement (WS2) contents and has optimal value for 0.6 wt% WS2/AZ91 composites then slightly decrease afterward. The UTS and hardness of 0.6 wt%WS2/AZ91 were increased by 23.23% and 10.65% respectively with respect to monolithic AZ91. The ultimate compressive strength(UCS) is directly related with WS2 concentration and 10.07% enhancement in UCS for 1 wt%WS2/AZ91 composites have been noted. The trend is interlinked with density and hardness of composites. SEM images depict, WS2 micro-particles suppress β-Mg17Al12 precipitates, refine the microstructure and enhance the grain size resulting in an increase in mechanical properties. The improvement in microstructure and mechanical properties is attributed to the dispersion of WS2 particles which hinder the grain growth. The increase in porosity has causes the decrease in mechanical behavior of 1 wt%WS2/AZ91. Twin boundary fracture has been found major fracture phenomenon in fractured WS2/AZ91 surfaces.
•WS2/AZ91 metal matrix composites have been manufacture by stir casting method.•Density and porosity of WS2/AZ91 composites has been assessed.•Mechanical behavior under compression and tension has been analyzed.•Effect of WS2 on microstructure has been investigated.•Fracture behavior of composites has been studied.
Magnesium alloys are attractive for the production of lightweight parts in modern automobile and aerospace industries due to their advanced properties. Their mechanical properties are usually ...enhanced by the incorporation with reinforcement particles. In the current study, reinforced AZ31 magnesium alloy was fabricated through the addition of bulk Al and the incorporation of SiC nanoparticles using a stir casting process to obtain AZ31-SiC nanocomposites. Scanning electron microscope (SEM) investigations revealed the formation of Mg
Al
lamellar intermetallic structures and SiC clusters in the nanocomposites. Energy dispersive spectroscopy (EDS) detected the uniform distribution of SiC nanoparticles in the AZ31-SiC nanocomposites. Enhancements in hardness and yield strength (YS) were detected in the fabricated nanocomposites. This behavior was referred to a joint strengthening mechanisms which showed matrix-reinforcement coefficient of thermal expansion (CTE) and elastic modulus mismatches, Orowan strengthening, and load transfer mechanism. The mechanical properties and wear resistance were gradually increased with an increase in SiC content in the nanocomposite. The maximum values were obtained from nanocomposites containing 1 wt% of SiC (AZ31-1SiC). AZ31-1SiC nanocomposite YS and hardness were improved by 27% and 30%, respectively, compared to AZ31 alloy. This nanocomposite also exhibited the highest wear resistance; its wear mass loss and depth of the worn surface decreased by 26% and 15%, respectively, compared to AZ31 alloy.
Penetration electric fields and disturbance dynamo electric fields are the two major disturbance electric fields in the equatorial ionosphere during geomagnetic storms. In this study, we analyze ...equatorial ion drifts measured by radar and satellites during the geomagnetic storm on November 7–11, 2004. The magnetic storm had two main phases, with minimum Dst values of −373 and −289 nT, respectively. The equatorial vertical ion drifts in the dusk‐evening sector were continuously enhanced in the upward direction for 13 h on November 7–8, including 7 h of the first main phase and 6 h of the early stage of the recovery phase. On November 10, the equatorial vertical ion drifts were continuously enhanced in the upward direction for ∼10 h, including ∼9 h of the second main phase and ∼1.5 h of the early stage of the recovery phase. Enhanced thermospheric nitric oxide 5.3 μm emission and westward disturbance winds were measured in the equatorial region during the two main phases of the magnetic storm, indicating occurrence of disturbance dynamo electric fields. The observations show that penetration electric fields are the dominant electric fields in the equatorial ionosphere for 13 and 10 h with continuous southward IMF even if disturbance winds already occur. Very large westward ion drifts are generated in the dusk‐evening equatorial region during the magnetic storm. It is suggested that the westward drifts are caused by penetration of electric fields associated with subauroral polarization streams.
Key Points
Equatorial ion drifts, neutral winds, and thermospheric nitric oxide emission are measured simultaneously with multiple satellites
Relative importance of penetration and disturbance dynamo electric fields is identified from ion drift and neutral wind data
Dominance of penetration electric fields over dynamo electric fields in the equatorial ionosphere for 10–13 h is verified
Penetration electric fields during geomagnetic storms have important effects on the equatorial ionospheric electrodynamics. Previous studies focus on the vertical ion drifts, corresponding to ...electric fields in the zonal direction, in the equatorial ionosphere. In this study, we analyze the characteristics of the zonal ion drifts in the equatorial region measured by the Defense Meteorological Satellite Program (DMSP) satellites during 10 severe magnetic storms with a minimum Dst < −200 nT in the solar maximum years of 2000–2003. It is found that the net change of the equatorial zonal ion drifts caused by magnetic storms is westward and can reach 200–300 m s−1 in the evening sector and that the westward ion drifts remain in the same direction during both the storm main and recovery phases. The storm‐induced zonal ion drifts are highly correlated with the Dst index, and the correlation coefficient between the average zonal drift and the average Dst value during the 10 storms is 0.89. The magnitude of the storm‐induced westward ion drifts is approximately linearly proportional to the Dst index in the evening sector, and on average, a change of −100 nT in Dst causes a change of −52 m s−1 in the zonal drift. The westward ion drifts caused by magnetic storms become small when Dst recovers to the range of −55 to −123 nT. A new mechanism is proposed to explain the generation of the westward plasma drifts in the nighttime equatorial ionosphere. This new mechanism is that the poleward electric fields associated with the subauroral polarization stream (SAPS) penetrate or extend to low latitudes, producing the westward plasma drifts in the equatorial region. The results of this study provide new insight into equatorial ionospheric electrodynamics during geomagnetic storms.
Key Points
Zonal plasma drifts in the nighttime equatorial ionosphere during 10 severe magnetic storms are analyzed systematically
High correlation between the Dst index and the storm time zonal plasma drift in the nighttime equatorial ionosphere is identified
A new mechanism is proposed to explain the generation of equatorial westward plasma drifts during magnetic storms
In this paper, solution and ageing heat treatment processes were used to improve microplastic deformation behaviors of as-cast SiCp/AZ61 magnesium metal matrix composites (Mg MMCs) fabricated by stir ...casting method. Higher percentages of SiC particle reinforcements showed higher microhardness values. Ageing heat treatment process was seen significant on the 12h aged 2wt% SiCp/AZ61 Mg MMC which induced lower microhardness value. At the 12h ageing of 2wt% SiCp/AZ61 Mg MMC the formations of particle free regions and discontinuous secondary phases were observed. For a higher ageing time, the secondary phases distribution became continuous and laminar structure. The addition of 5wt% SiC particles resulted in the formation of Mg2Si phase throughout the whole heat treatment processes. The addition of SiC particles reinforcements increased the phase heterogeneity during ageing heat treatment processes. XRD patterns revealed the presence of nanocrystalline MgSiO3 phase on the 12h aged 2wt% SiCp/AZ61 Mg MMC. Using reference intensity ration (RIR) method a 51.6% of MgSiO3 phase was determined which can cause the formation of microplastic deformation behavior. And also, the maximum average crystallite size, compressive microstrain and microcrack-free phase boundaries were observed on the 12h aged 2wt% SiCp/AZ61 Mg MMC.
We analyze horizontal plasma drifts measured by the Defense Meteorological Satellite Program satellites during two intense magnetic storms. It is found, for the first time, that westward plasma flows ...associated with subauroral polarization streams (SAPS) in the dusk‐evening sector penetrate continuously to equatorial latitudes. The westward ion drifts between subauroral and equatorial latitudes occur nearly simultaneously. The latitudinal profile of the westward ion drifts at low latitudes (approximately within ±30° magnetic latitude MLat) is relatively flat, and the westward ion drifts at the magnetic equator reach 200–300 m s−1. In the dawn‐morning sector, eastward ion drifts at subauroral latitudes are also SAPS. The storm‐time dawnside auroral boundary moves to ∼±55° MLat, and the dawnside SAPS penetrate to ∼±20° MLat at 0930 local time. A dawnside SAPS flow channel appears to exist, although it is not as well defined as the duskside SAPS flow channel. Thermospheric wind data measured by the Challenging Minisatellite Payload satellite are analyzed, and zonal disturbance winds are derived. Disturbance winds can reach equatorial latitudes rapidly near midnight but are limited to ±40° geographic latitude or higher near noon. The effects of disturbance winds on the zonal ion drifts at middle and low latitudes are discussed. It is suggested that both the westward ion drifts at middle and low latitudes in the dusk‐evening sector and the eastward ion drifts at middle and lower latitudes in the dawn‐morning sector are caused primarily by penetration of the SAPS and auroral electric fields.
Key Points
Horizontal ion drifts from auroral to equatorial latitudes and thermospheric winds are analyzed during two intense magnetic storms
Penetration of westward plasma flows of subauroral polarization streams to equatorial latitudes near dusk is identified for the first time
Penetration of auroral and subauroral electric fields to low latitudes near dawn is identified for the first time
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