Preliminary Impulse Research Articles

Generation of Field‐Aligned Currents in Response to Sudden Enhancement of Solar Wind Dynamic Pressure

AbstractWe investigated the generation of field‐aligned currents (FACs) in response to the sudden enhancement of the solar wind dynamic pressure by tracing backward in time a packet of the Alfvén wave in the global magnetohydrodynamic (MHD) simulation. The generation region is identified from three perspectives, including the continuity of the current, the energy conservation and the time rate of change in the FACs. The generation mechanism is found to be related with the tailward motion of the compressional wave, which is excited when the magnetopause is compressed due to the solar wind dynamic pressure pulse. The compressional wave with a high magnetic pressure center interacts with the Earth's dipole field and forms a protruding part of the wavefront near the equatorial plane. The leading edge of the merged magnetic pressure that pertains to the compressional wave and the Earth starts to generate preliminary impulse (PI) FACs off the equator, as the magnetic pressure force accelerates the plasma and magnetic field lines are bent (FAC dynamo 1). Main impulse (MI) FACs are generated behind the leading edge in the equatorial plane due to the enhanced magnetic tension force (FAC dynamo 2). Magnetic field lines would be extremely curved during the passage due to the increasing magnetic tension force. The polarity of PI FACs is decided by the parallel vorticity of plasma flow, and MI FACs are the result of enhanced perpendicular currents together with the plasma flow in the equatorial plane.

Nighttime Geomagnetic Response to Jumps of Solar Wind Dynamic Pressure: A Possible Cause of Québec Blackout in March 1989

AbstractBy performing a global magnetohydrodynamic (MHD) simulation, we investigated magnetic disturbances on the ground at high‐latitudes in response to jumps in the solar wind dynamic pressure, namely a sudden commencement (SC). After the arrival of the jump, a pair of field‐aligned currents (FACs), related to the preliminary impulse, develop and travel in the anti‐sunward direction. Soon after another pair related to the main impulse (MI) appears and travels in the anti‐sunward direction. The horizontal ionospheric current associated with the MI remains strong when propagating to the nightside. On the dawnside the MI current flows sunward (anti‐sunward) resulting in northward (southward) ground magnetic disturbance at higher (lower) latitude in the post‐midnight sector. These features are similar to those observed in Canada in the high‐latitude post‐midnight sector when the Québec blackout took place on 13 March 1989. The nighttime geomagnetic perturbations associated with the MI occur regardless of the magnitude of the solar wind dynamic pressure and IMF orientation. The amplitude of the geoelectric field, which is closely related to the geomagnetically induced currents (GICs), reaches the maximum value just before and around the maximum of the southward magnetic disturbance. This is consistent with the moment at which the blackout occurred during the southward magnetic perturbation. We suggest that the blackout in Québec could be caused by the MI‐associated Hall current passing over the Hydro‐Québec power system on the nightside. The nighttime polar region is shown to be sensitive to hazardous GICs for large‐amplitude jumps in the solar wind dynamic pressure.

Transient high latitude geomagnetic response to rapid increases in solar wind dynamic pressure

Rapid changes in solar wind dynamic pressure can produce a transient geomagnetic response in the high latitude ionosphere. In this study we carry out a superposed epoch analysis of the geomagnetic response based on 2,058 events. The events are divided into 12 groups based on interplanetary magnetic field clock angle and dipole tilt and the magnetic perturbation field is modeled using spherical harmonics. We find that the high latitude transient current vortices associated with a sudden commencement are most clearly observed when the interplanetary magnetic field is northward during equinox and winter in the northern hemisphere. The high latitude geomagnetic response during northward interplanetary magnetic field is decomposed into a preliminary and main impulse. The preliminary impulse onset is 1–2 min prior to the onset of the low/mid latitude geomagnetic response and its rise time is 4–6 min. The main impulse onset is around 2 min after the low/mid latitude geomagnetic response and has a rise time of 6–11 min. When examining the change relative to pre-onset conditions a coherent transient geomagnetic response emerges for all IMF clock and dipole tilt angles. The current vortex associated with the main impulse on the dawnside appears at (9.3 ± 0.5 mlt, 64.8° ± 1.5° mlat) and moves westward with a velocity of 5 ± 1.4 km/s. The vortex on the duskside appears at (15.3 ± 0.9 mlt, 65.8° ± 2.5° mlat) and does not move significantly. In addition, the models were used to recreate the SMR index showing a significant mlt dependence on the magnetic perturbation above 40° mlat and below 10° mlat. The former is thought to be caused by high latitude ionospheric currents. The latter is potentially a combination of the event occurrence probability being skewed toward certain UT ranges for large dipole tilt angles and a UT dependence of the equatorial electrojet magnitude caused by the south atlantic magnetic anomaly.

Two current systems in the preliminary phase of sudden commencements in the magnetosphere

The preliminary impulse of the sudden commencement is simply explained by the generation of the compressional wave due to sudden compression of the dayside magnetopause and mode conversion from the compressional wave to the Alfvén wave in the magnetosphere. However, this simple model cannot explain a time delay of the peak displacement and longer duration time in the higher latitudes in the pre-noon and post-noon sectors of the polar region. Based on the global magnetohydrodynamic simulation of the magnetosphere–ionosphere system reveals that this peculiar behavior of the geomagnetic variation of the preliminary impulse is associated with temporal deformation of the ionospheric field-aligned current distribution of the preliminary impulse into a crescent shape; its lower-latitude edge extends toward the anti-sunward direction, and its higher-latitude edge almost stays on the same longitude near noon. Numerical simulations revealed that the deformation of the field-aligned current distribution is derived from different behaviors of the two current systems of the preliminary impulse. The first current system consists of the field-aligned current connected to the field-aligned current of the preliminary impulse in the lower latitude side of the ionosphere, the cross-magnetopause current, and the magnetosheath current (type L current system). The cross-magnetopause current is the inertia current generated in the acceleration front of the solar wind due to the sudden compression of the magnetosheath. Thus, the longitudinal speed of the type L current system in the ionosphere is the solar wind speed in the magnetosheath projected into the ionosphere. In contrast, the current system of the preliminary impulse connected to the field-aligned current of the preliminary impulse at higher latitude (type H current system) consists of the upward/downward field-aligned current in the pre-noon/post-noon sector, respectively, and dawn-to-dusk field-perpendicular current along the dayside magnetopause. The dawn-to-dusk field-perpendicular current moves to the higher latitudes in the outer magnetosphere over time. The field-aligned current of the type H current system is converted from the field-perpendicular current due to convergence of the return field-perpendicular current heading toward the sunward direction in the outer magnetosphere; the return field-perpendicular current is the inertia current driven by the magnetospheric plasma flow associated with compression of the magnetopause behind the front region of the accelerated solar wind. The acceleration front spreads concentrically from the subsolar point. Consequently, as the return field-perpendicular current is converted to the field-aligned current of the type H current system, it does not move much in the longitudinal direction over time because the dawn-to-dusk field-perpendicular current of the type H current system moves to the higher latitudes. Therefore, the high-latitude edge of the current distribution of the preliminary impulse in the ionosphere moves only slightly. Finally, we clarified that the conversion between field-perpendicular current and field-aligned current of the type L current system mainly occurs in the region where the Alfvén speed starts to increase toward the Earth. A region with a steep gradient of the Alfvén speed like the plasmapause is not always necessary for conversion from the field-perpendicular current to the field-aligned current. We also suggest the possible field-aligned structure of the standing Alfvén wave that may occur in the preliminary impulse phase.Graphical

Q-Bursts Generated by the Contact of the Interplanetary Shock Wave with the Magnetosphere

The study presents the results of the global registration of a short-term train of damped oscillations in the frequency range 0,5–30,0 Hz (Q-burst), observed at the initial phase of the preliminary impulse of the sudden onset SC on 09.09.2011. The spatial features of the spectrum of train oscillations are studied. We discovered A spectral-resonance structure of bursts, which is observed globally. It is assumed that bursts are generated as a result of powerful lightning discharges that create red sprites in the ionosphere. The electromagnetic radiation of sprites is captured in the earth-ionosphere waveguide and propagates globally. We assumed that one of the reasons for the increase in the power of the Q-signal is the coincidence of the beginning of its generation with the amplification of the ionospheric current system of the preliminary impulse of the sudden onset of SC.

Penetration of the electric fields of the geomagnetic sudden commencement over the globe as observed with the HF Doppler sounders and magnetometers

Using the HF Doppler sounders at middle and low latitudes (Prague, Czech Republic; Tucuman, Argentina; Zhongli, Republic of China, and Sugadaira, Japan), we observed the electric fields of the geomagnetic sudden commencement (SC) propagating near-instantaneously (within 10 s) over the globe. We found that the electric fields of the preliminary impulse (PI) and main impulse (MI) of the SC are in opposite direction to each other and that the PI and MI electric fields are directed from the dusk to dawn and dawn to dusk, respectively, manifesting the nature of the curl-free potential electric field. We further found that the onset and peak of the PI electric field are simultaneous on the day and nightsides (0545, 1250, 1345 MLT) within the resolution of 10 s. With the magnetometer data, we confirmed the near-instantaneous development of the ionospheric currents from high latitudes to the equator and estimated the location of the field-aligned currents that supply the ionospheric currents. The global simultaneity of the electric and magnetic fields does not require the contribution of the magnetohydrodynamic waves in the magnetosphere nor in the F-region ionosphere. The global simultaneity and day–night asymmetry of the electric fields are explained with the ionospheric electric potentials transmitted at the speed of light by the TM0 mode waves in the Earth-ionosphere waveguide.

On the observation of magnetic events on broad-band seismometers

The objective of this contribution is to get new insights into the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have investigated if the Sudden Storm Commencements (SSC) cataloged during the 24th solar cycle (2008–2019) can be systematically identified in broad-band seismic stations distributed worldwide. The results show that the 23 SSC events with a mean amplitude above 30 nT and most of those with lower energy but still clearly identified in the magnetometer detection network can be observed at broad-band stations’ network using a simple low-pass filter. Although the preliminary impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we focus on the short period seismic signals recorded in urban environments which are correlated with the activity of the railway transportation system. We have analyzed collocated measurements of electric field and seismic signals within Barcelona, evidencing that significant changes in the electric field following the activity of the transportation systems can be attributed to leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed.

In Situ Synthesized MEMS Compatible Energetic Arrays Based on Energetic Coordination Polymer and Nano-Al with Tunable Properties.

Integrating energetic materials with a microelectromechanical system (MEMS) to achieve miniaturized integrated smart energetic microchips is promising. The potential applications include actuation in lab-on-a-chip devices, ignition in automobile airbags, propulsion and attitude control of micro-/nano-satellites, and miniaturized electro-explosive devices. In this work, a new type of MEMS-compatible energetic arrays was in situ realized on a copper substrate, which comprised a new energetic coordination polymer (ECP; Cu1.5C2N8O2·H2O) with tunable nanostructures and a nano-aluminum (nano-Al) covering layer. The composition, morphology, and energetic characteristics of the energetic arrays can be easily tuned by adjusting the reaction time. The maximum heat release of 1850.2 J/g in thermal analysis and the intense flame in open burning experiment proved its excellent exothermic and combustion performance. A closed-bomb experiment further revealed that the ECP@nano-Al energetic arrays supported on Cu(OH)2 nanorods had a peak pressure (5.5 MPa) and a pressure duration (0.5 s) more than twice those of nanoscale Al/CuO powder because of the introduction of gas elements (e.g., C, H, and N). A preliminary impulse experiment was also conducted through the torsion pendulum method. The displacement of the torsion pendulum in the micrometer scale proved the potential application of the energetic arrays in micropropulsion systems. Overall, this work can serve as a reference for the synthesis and applications of ECPs.

Chromatographic separation of betulinic and oleanolic acids

In this work, a simulated moving bed unit (SMB) was designed for the separation of betulinic and oleanolic acids, two naturally occurring triterpenic acids that exhibit key pharmacological properties. Preliminary impulse experiments were conducted to select appropriate mobile and stationary phases, from which methanol/acetonitrile 50/50 (%, v/v) and an Apollo C18 column emerged as the most favorable option. Breakthrough experiments were conducted with the pure compounds to determine the equilibrium and mass transfer coefficients necessary for SMB simulations. These parameters were then validated through the successful prediction of breakthrough experiments of two binary mixtures of betulinic and oleanolic acids.Optimum SMB operating conditions were determined via an optimization strategy combining the design of experiments and response surface methodologies (DoE-RSM) with phenomenological computer simulations. It was demonstrated that an SMB unit consisting of two columns per section allows the recovery of betulinic acid in its raffinate stream with a purity of 99.0 % and oleanolic acid in the extract stream with 99.4 % purity. Moreover, it was also demonstrated that the addition of acetonitrile to methanol is translated into a gain of 16.1 % in terms of productivity when compared to other mobile phase such as methanol/water 95/5 (%, v/v).

Response of the magnetosphere–ionosphere system to sudden changes in solar wind dynamic pressure

The geomagnetic sudden commencement (SC) is a plasma and magnetic field disturbance in the magnetosphere–ionosphere region, which appears as a response of this system to a sudden change in the solar wind dynamic pressures associated with shock and tangential discontinuity of solar wind. As this mechanism seems to be simple, the SC has been widely investigated based on observations in the region from the surface of the Earth to the magnetosphere. A schematic model of the SC was presented by Araki (Solar wind sources of magnetospheric ultra-low-frequency waves. American Geophysical Union, Washington, DC, pp 183–200, 1994), who compiled many observational results and theories related to this phenomenon. Recent advances in supercomputing allow us to present new numerical results including three-dimensional global current systems of the SC which cannot be obtained only by direct observations and theoretical analysis. The simulation study not only confirms the results of a previous model on the initial response of the magnetosphere–ionosphere system to solar wind changes (the preliminary impulse) but also presents new findings on dynamical processes in the magnetosphere–ionosphere system in the period following the initial response (the main impulse). Furthermore, the simulation study reveals the SC sequence in the context of break and recovery of the steady magnetosphere–ionosphere convection system. The transient flow vortex in the flank magnetosphere in the main impulse phase plays an essential role in recovering the convection system. This report describes the SC process from the viewpoint of the state transition of the magnetosphere–ionosphere compound system Tanaka (Space Sci Rev 133:1, https://doi.org/10.1007/s11214-007-9168-4, 2007).

Response of the Geospace System to the Solar Wind Dynamic Pressure Decrease on 11 June 2017: Numerical Models and Observations

AbstractOn 11 June 2017, a sudden solar wind dynamic pressure decrease occurred at 1437 UT according to the OMNI solar wind data. The solar wind velocity did not change significantly, while the density dropped from 42 to 10 cm−3 in a minute. The interplanetary magnetic field BZ was weakly northward during the event, while the BY changed from positive to negative. Using the University of Michigan Block Adaptive Tree Solarwind Roe Upwind Scheme global magnetohydrodynamic code, the global responses to the decrease in the solar wind dynamic pressure were studied. The simulation revealed that the magnetospheric expansion consisted of two phases similar to the responses during magnetospheric compression, namely, a negative preliminary impulse and a negative main impulse phase. The simulated plasma flow and magnetic fields reasonably reproduced the Time History of Events and Macroscale Interactions during Substorms and Magnetospheric Multiscale spacecraft in situ observations. Two separate pairs of dawn‐dusk vortices formed during the expansion of the magnetosphere, leading to two separate pairs of field‐aligned current cells. The effects of the flow and auroral precipitation on the ionosphere‐thermosphere (I‐T) system were investigated using the Global Ionosphere Thermosphere Model driven by simulated ionospheric electrodynamics. The perturbations in the convection electric fields caused enhancements in the ion and electron temperatures. This study shows that, like the well‐studied sudden solar wind pressure increases, sudden pressure decreases can have large impacts in the coupled I‐T system. In addition, the responses of the I‐T system depend on the initial convection flows and field‐aligned current profiles before the solar wind pressure perturbations.

Method and equipment for the safe development of preparatory workings in the gas-bearing coal seams

The article describes a method and equipment for the safe development of preparatory working on a gas-bearing, outburst hazardous coal seam by a heading machine. The described method significantly prevents the gas emission during mining operations. Determination of the amount of degassing and relief wells is performed according to the given in article mathematical formulas. This ensures a significant increase in the level of safety processing the preparatory roadway on a gas-bearing, prone to outbursts coal seam by a heading machine. Due to this, the speed of conducting excavation increases and the cost of mining is reduced. The proportionality coefficients of the mathematical models were determined. Mathematical models are used to calculate the number of relief and degassing wells, which are drilled during the impulse hydraulic loosing of the nearby part of the coal seam. Also the duration of liquid injection into a coal seam is also determined. Mathematical models for calculating the number of injection, unloading and degassing boreholes can be used in development of the passports for conducting preparatory workings on a gas-bearing, prone to outburst coal seam by a heading machine with a preliminary impulse hydraulic loosing of the nearby part of a coal seam.

Geomagnetic and Ionospheric Responses to the Interplanetary Shock Wave of March 17, 2015

The propagation of perturbation caused by the interplanetary shock wave of March 17, 2015 from the solar wind through the magnetosheath, magnetosphere, and ionosphere down to the Earth’s surface is analyzed. The onboard satellite measurements, global magnetometer network data, and records by the receivers of the global positioning system (GPS) providing the information about the total electron content (TEC) of the ionosphere are used for the analysis. By the example of this event, various aspects of the influence of the interplanetary shock wave on the near-Earth environment and ground-based engineering systems are considered. It is shown which effects of this influence are well described by the existing theoretical models and which ones need additional research. The formation of the fine structure of the magnetic impulse of the storm sudden commencement (SC)—the preliminary impulse (PI) and main impulse (MI)—is considered. The MI and compression of the magnetospheric magnetic field is observed by the GOES and RBSP satellites and on the geomagnetically conjugate stations; however, the PI was only noted on the Earth. The PI was detected in the afternoon sector practically simultaneously (within 1 min) with the shock wave impact on the magnetopause. The wave’s response to the SC includes the strongly decaying resonant oscillations of the magnetic shells and the magnetoacoustic cavity mode. This study supports the possibility of detecting the ionospheric response to the SC by the GPS method. The TEC response to the MI was detected in the auroral latitudes although not on every radio path. The TEC modulation can be associated with the precipitation of superthermal electrons into the lower ionosphere which is undetectable by riometers. The burst in the intensity of the geomagnetically induced currents caused by an interplanetary shock wave turns out to be higher than the currents during the storm’s commencement, although the SC’s amplitude is noticeably lower than the amplitude of the magnetic bay related to the substorm.

Modeling Study of the Geospace System Response to the Solar Wind Dynamic Pressure Enhancement on 17 March 2015

AbstractThe global magnetosphere‐ionosphere‐thermosphere system is intrinsically coupled and susceptible to external drivers such as solar wind dynamic pressure enhancements. In order to understand the large‐scale dynamic processes in the magnetosphere‐ionosphere‐thermosphere system due to the compression from the solar wind, the 17 March 2015 sudden commencement was studied in detail using global numerical models. This storm was one of the most geoeffective events of the solar cycle 24 with a minimum Dst of −222 nT. The Wind spacecraft recorded a 10‐nPa increment in the solar wind dynamic pressure, while the interplanetary magnetic field BZ became further northward. The University of Michigan Block‐Adaptive‐Tree Solar wind Roe‐type Upwind Scheme global magnetohydrodynamic code was utilized to study the generation and propagation of perturbations associated with the compression of the magnetosphere system. In addition, the high‐resolution electric potential and auroral power output from the magnetohydrodynamic model was used to drive the global ionosphere‐thermosphere model to investigate the ionosphere‐thermosphere system response to pressure enhancement. During the compression, the electric potentials and convection patterns in the polar ionosphere were significantly altered when the preliminary impulse and main impulse field‐aligned currents moved from dayside to nightside. As a result of enhanced frictional heating, plasma and neutral temperatures increased at the locations where the flow speeds were enhanced, whereas the electron density dropped at these locations. In particular, the region between the preliminary impulse and main impulse field‐aligned currents experienced the most significant heating with 1000‐K ion temperature increase and 20‐K neutral temperature increase within 2 min. Comparison of the simulation results with the Poker Flat Incoherent Scatter Radar observations showed reasonable agreements despite underestimated magnitudes.

New opportunities to improve the fuel supply system with a battery fuel system

The article shows the effect of increasing the injection pressure (up to p = 250 MPa) on the dynamics of the development of the fuel jet. An increase in the pressure in the accumulator led to a decrease in the angle of the boundary layer of the jet. A decrease in the oscillations of the boundary layer of the jet about its axis is shown. The greatest variations were recorded at рак = 50 MPa. Then, as the рак increases, the vibrations of the nucleus decrease and become practically imperceptible at 250 MPa. A method for controlling the shape of the differential injection characteristic by an electric pulse, which is applied to an electromagnet of the control valve of the electrohydraulic injector of battery fuel system, is provided. The impulse consists of the preliminary, the main and the following additional one. The duration of the preliminary impulse determines the amplitude of the initial stage of the injection characteristic, and the interval between the preliminary and main pulse is the amplitude between the initial and main stages of the injection characteristic. The interval between the main and additional electric pulses is selected in such a way that the subsequent stage of the injection characteristic begins after the termination of the main, but without an interval between them. By selecting the durations of the control electric pulses and the intervals between them, it is possible to obtain a predetermined shape of the differential injection characteristic from stepped to a characteristic with an inclined leading edge. In nozzles with two groups of holes (correction sprayer), the flow rate of the holes in the sub-head volume and on the locking cone is significantly different and depends on the position of the needle. This creates the prerequisites for correcting the fuel supply through the spray holes and, consequently, along the zones of the combustion chamber, taking into account the mode of operation of the diesel engine.

IMF By effects on ground magnetometer response to increased solar wind dynamic pressure derived from global MHD simulations

AbstractDuring sudden solar wind dynamic pressure enhancements, the magnetosphere undergoes rapid compression resulting in a reconfiguration of the global current systems, most notably the field‐aligned currents (FACs). Ground‐based magnetometers are traditionally used to study such compression events. However, factors affecting the polarity and magnitude of the ground‐based magnetic perturbations are still not well understood. In particular, interplanetary magnetic field (IMF) By is known to create significant asymmetries in the FAC patterns. We use the University of Michigan Block Adaptive Tree Roe Upwind Scheme (BATS'R'US) magnetohydrodynamic code to investigate the effects of IMF By on the global variations of ground magnetic perturbations during solar wind dynamic pressure enhancements. Using virtual magnetometers in three idealized simulations with varying IMF By, we find asymmetries in the peak amplitude and magnetic local time of the ground magnetic perturbations during the preliminary impulse (PI) and the main impulse (MI) phases. These asymmetries are especially evident at high‐latitude ground magnetometer responses where the peak amplitudes differ by 50 nT at different locations. We show that the FACs related with the PI are due to magnetopause deformation, and the FACs related with the MI are generated by vortical flows within the magnetosphere, consistent with other simulation results. The perturbation FACs due to pressure enhancements and their magnetospheric sources do not differ much under different IMF By polarities. However, the conductance profile affected by the superposition of the preexisting FACs and the perturbation FACs including their closure currents is responsible for the magnitude and location asymmetries in the ground magnetic perturbations.

Does TEC react to a sudden impulse as a whole? The 2015 Saint Patrick’s day storm event

We provide insights about how the Total Electron Content (TEC), derived by a network of Global Navigation Satellite System (GNSS) receivers over the European longitudinal sector, reacts to a Sudden Impulse (SI). In particular, we investigate how the different components of the TEC variability relates with the ionospheric currents, derived by ground magnetometers and the Piersanti and Villante (2016) model, during the 2015 Saint Patrick’s Day storm. To identify the different TEC components, we apply the Ensemble Empirical Mode Decomposition (EEMD) analysis to vertical TEC (vTEC) time series to isolate its SI-related variations over co-located GNSS receiver-magnetometer couples. With such an approach, we were able to identify for the first time a peculiar vTEC mode (WHT∗, T≃45min) switching on during the SI and propagating southward with a velocity of ∼1.5km/s. We also found a linear relationship between the estimated ionospheric current and the temporal fluctuations of WHT∗ for both the Preliminary Impulse (PI) and the Main Impulse (MI). The slope has been found to depend only on the electron charge, on the magnetic permeability and on the altitude of the ionospheric E-layer, as expected from first principles equations and as discussed in details.

Dayside magnetospheric and ionospheric responses to solar wind pressure increase: Multispacecraft and ground observations

AbstractWe provide in situ observations of the transient phenomena in the dayside magnetosphere during the preliminary impulse (PI) and main impulse (MI) event on 30 September 2008. The PI and MI geomagnetic signals are induced by twin traveling convection vortices with opposite polarities in the equivalent ionospheric currents due to a sudden increase of the solar wind dynamic pressure. The two PI‐associated ionospheric current vortices centered at ~07 magnetic local time (MLT), 67° magnetic latitude (MLAT) in the dawnside and ~14 MLT, 73°MLAT in the duskside, respectively. The dawnside MI current vortex centered at ~68° MLAT and 6 MLT, while the duskside vortex center was traveling poleward from ~67° MLAT to ~75° MLAT at a speed of ~5.6–7.4 km/s around 14 MLT. It is found that both dawnside PI‐ and MI‐related current vortices were azimuthally seen up to 4 MLT. Following the magnetosphere sudden impulse, a clockwise flow vortex with a radial scale larger than 3 RE, associated with positive field‐aligned current (FAC), was observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the outer dayside magnetosphere. The flow vortex expanded and traveled tailward in the magnetosphere, also being reproduced with global MHD simulations. Based on both observation and simulation technique, we show that the MI‐related FACs are correlated with the large‐scale flow vortex. The PI FACs are partially provided by the mode conversion of fast mode waves into the Alfvén waves near the equatorial plane, while most of it may be generated at a higher‐latitude region in the magnetosphere.

Response of the incompressible ionosphere to the compression of the magnetosphere during the geomagnetic sudden commencements

AbstractThe ionospheric plasma in midlatitude moves upward/downward during the geomagnetic sudden commencement causing the HF Doppler frequency changes; SCF (+ −) and (− +) on the dayside and nightside, respectively, except for the SCF (+ −) in the evening as found by Kikuchi et al. (1985). Although the preliminary and main frequency deviations (PFD, MFD) of the SCF have been attributed to the dusk‐to‐dawn and dawn‐to‐dusk potential electric fields, there still remain questions if the positive PFD can be caused by the compressional magnetohydrodynamic (MHD) wave and what causes the evening anomaly of the SCF. With the HF Doppler sounder, we show that the dayside ionosphere moves upward toward the Sun during the main impulse (MI) of the SC, when the compressional wave is supposed to push the ionosphere downward. The motion of the ionosphere is shown to be correlated with the equatorial electrojet, matching the potential electric field transmitted with the ionospheric currents from the polar ionosphere. We confirmed that the electric field of the compressional wave is severely suppressed by the conducting ionosphere and reproduced the SC electric fields using the global MHD simulation in which the potential solver is employed. The model calculations well reproduced the preliminary impulse and MI electric fields and their evening anomaly. It is suggested that the electric potential is transmitted from the polar ionosphere to the equator by the zeroth‐order transverse magnetic (TM0) mode waves in the Earth‐ionosphere waveguide. The near‐instantaneous transmission of the electric potential leads to instantaneous global response of the incompressible ionosphere.

Global MHD simulation of magnetospheric response of preliminary impulse to large and sudden enhancement of the solar wind dynamic pressure

A sudden increase in the dynamic pressure of solar wind generates a prominent and transient change in ground-based magnetometer records worldwide, which is called a sudden commencement (SC). The magnetic field variation due to an SC at high latitudes shows a bipolar change, which consists of a preliminary impulse (PI) and main impulse (MI). The largest recorded SC had an amplitude of more than 200 nT with a spiky waveform at low latitudes, and the mechanism causing this super SC is unknown. Here, we investigate the cause of the super SC using a newly developed magnetosphere-ionosphere coupling simulation, which enables us to investigate the magnetospheric response to a large increase in the solar wind dynamic pressure. To simulate SCs, the dynamic pressure of the solar wind is increased to 2, 5, 10, and 16 larger than that under the stationary condition, and two different types of dynamic pressure increase are adopted by changing the solar wind density only or the solar wind speed only. It was found that the magnetic field variations of the PI and MI are several times larger and faster for a jump in the speed than for a jump in the density. It is inferred that a solar wind velocity of more than 2500 km/s in the downstream shock, which cannot be directly simulated in this study, would be consistent with the super SC.