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The geomagnetic storm is an intense geomagnetic activity, and one important research aspect is the storm time ultralow frequency (ULF) waves, which can resonate with injected particles and impact particle dynamics in the inner magnetosphere. This study focuses on analyzing ULF wave power during geomagnetic storms on the ground, in the magnetosphere and in the solar wind. During the Van Allen Probe era from year 2012–2019, we identified 94 geomagnetic storm events with 25 as strong storms and 69 as moderate storms based on the variation of SYM‐H index. The analysis revealed that for both satellite and ground‐based measurements during storms with different intensity, the ULF wave power was dramatically enhanced in the main phase. In addition, the ULF wave power of strong storms was significantly stronger than that of moderate storms within the magnetosphere and on the ground. While the finding is opposite in the solar wind. Furthermore, the ULF wave power of strong storms exhibited a higher intensity at higher L shell values and in some cases, it can last up to ∼2 days. Besides, the ULF wave power displayed a linear correlation with the absolute value of the SYM‐H index during the storm main phase while it exhibited an exponential association with the absolute value of the SYM‐H index during the recovery phase. This comprehensive analysis offers valuable insights into the evolution of ULF waves in the inner magnetosphere during geomagnetic storm events, shedding light on plasma waves and wave‐particle interactions in this region. Key Points: We statistically analyzed the storm‐time Pc3‐5 ULF wave power using a combination of ground and satellite measurementsWithin the L shell of 3–6, higher ULF wave power is observed at higher L shell values around the SYM‐H index minimum timeULF wave power is linearly correlated with the SYM‐H index during the main phase and exponentially correlated during the recovery phase [ABSTRACT FROM AUTHOR]

Magnetosonic (MS) waves are common plasma waves in the Earth's magnetosphere. The self‐consistent excitation of MS waves has been studied by 2D particle‐in‐cell simulations in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Therefore, the 3D simulation of MS waves needs to be investigated. In this paper, we investigate the excitation and evolution of MS waves in the Earth's dipole magnetic field based on a 3D general curvilinear particle‐in‐cell simulation. We find that the MS waves are excited primarily within 3° of the equator when the thermal velocity of the ring distribution is much less than the ring velocity of the ring distribution. These waves propagate along both the radial and azimuthal directions nearly perpendicular to the background magnetic field. In the linear stage, the growth rates of MS waves are almost equal in the radial and azimuthal directions. Compared with the waves propagating along the radial direction, the waves propagating along the azimuthal direction can grow for a longer time, resulting in a larger wave amplification in this direction after saturation. The simulation results provide a valuable insight to understand the self‐consistent evolution of MS waves in the Earth's dipole magnetic field, and the findings are useful for understanding the plasma wave‐particle interaction in the Earth's radiation belts. Plain Language Summary: The Earth's magnetosphere is a natural plasma laboratory. Since the plasma in the magnetosphere is collisionless, electromagnetic waves are significant agents for the transport of energy and momentum among different particles. The MS waves are important electromagnetic waves in the magnetosphere, which have a frequency range from several Hertz to several hundred Hertz. Recently, these waves have been receiving an increasing interest due to their potential importance in accelerating radiation belt electrons and heating plasmaspheric ions. The 2D particle‐in‐cell simulations have been used to investigate MS waves in the meridian and equatorial planes of a dipole magnetic field. However, the direction of wave propagation is artificially limited in the previous 2D simulations. Thus, the 3D simulation of MS waves still needs to be studied. In this paper, we perform a 3D particle‐in‐cell simulation to investigate the excitation of the MS waves in a dipole magnetic field. A larger wave amplification is found in the azimuthal direction because they can grow for a longer time in this direction. Our results can contribute to the understanding of the spatial distribution of MS waves in the Earth's magnetosphere. Key Points: Three‐dimensional particle‐in‐cell simulation of magnetosonic (MS) wave excitation in a dipole magnetic field has been studied for the first timeThe linear growth rates of MS waves are almost equal in the radial and azimuthal directionsA larger wave amplification is observed in the azimuthal direction since the waves can grow for a longer time in this direction [ABSTRACT FROM AUTHOR]

Plasma in the inner magnetosphere is affected by various processes, such as substorms. In this study, we have statistically investigated the ring current properties based on the observations of Van Allen Probes from 2012 to 2019 to examine the substorm effects on the plasma pressure and current system in the inner magnetosphere. The results show that the plasma pressure increases significantly, leading to a ∼3 nT/hr geomagnetic depression during intense substorms. The contribution of <100 keV H+ ions to the plasma pressure increases during substorms, which is more significant at lower L‐shells. Opposite to the H+ ions, the proportion of >10 keV O+ ions contributing to the plasma pressure increases as substorm intensity increases. In addition, the rise of plasma pressure is mainly distributed from dusk to midnight, resulting in the enhancement of asymmetric ring current and the region II field‐aligned currents connecting to the ionosphere. Our results provide a comprehensive view of the variation of plasma pressure and current system in the inner magnetosphere during quiet periods and intense substorms. Key Points: The pressure increase caused by intense substorms can lead to 3 nT/hr magnetic field depressionThe pressure contribution of both <100 keV H+ and >10 keV O+ increase during intense substormsThe ring current densities from dusk to midnight and region II FACs increase dramatically during intense substorms [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shock‐driven sudden compression of the Earth's magnetosphere produces electromagnetic disturbances in the polar ionosphere. Several studies have examined the effects of IP shock on magnetosphere‐ionosphere coupling systems using all‐sky cameras and radars. In this study, we examine responses and drivers of the polar ionosphere following an IP shock compression on 16 June 2012. We observe the vertical drift and concurrent horizontal motion of the plasma. Observations from digisonde located at Antarctic Zhongshan station (ZHO) showed an ionospheric thick E region ionization and associated vertical downward plasma motion at F region. In addition, horizontal ionospheric convection reversals were observed on the Super Dual Auroral Radar Network ZHO and McMurdo radar observations. Findings suggest that the transient convective reversal breaks the original shear equilibrium, it is expected that the IP shock‐induced electric field triggers an enhanced velocity shear mapping to the E region. The horizontal motion of the plasma was attributed to only the dusk‐to‐dawn electric field that existed during the preliminary phase of sudden impulse. We also found that ionospheric convection reversals were driven by a downward field‐aligned current. The results of these observations reveal, for the first time, the immediate and direct cusp ionosphere response to the IP shock, which is critical for understanding the global response of the magnetosphere following an abrupt change in Interplanetory Magnetic Field (IMF) and solar wind conditions. Plain Language Summary: On 16 June 2012, an interplanetary (IP) shock hit near‐Earth space (geospace), resulting in enhanced plasma convection, auroral intensifications, and altering electric current systems in the high‐ and polar‐latitude ionosphere. We present a case study characterizing the effect of IP shock‐driven sudden impulse (SI) impact on geospace systems that changes vertical drift and horizontal convection observed by digital ionosonde and Super Dual Auroral Radar Network high‐frequency radars. We found an SI‐driven immediate vertical downward plasma motion, the sudden appearance of sporadic E‐layer, and an ionospheric convection reversal following the IP shock. We discuss the possible impact of SI on magnetospheric‐ionospheric coupling systems and possible driving mechanisms that are attributed to the observations reported in the study. Key Points: Ionosonde ZHO detected enhanced E region ionization and vertical plasma motion following the arrival of an interplanetary shockEvidence suggests that the strong E layers are the result of sudden impulse (SI)‐driven E‐field drift‐convergence and particle precipitationIP shock‐triggered SI alters the dusk‐to‐dawn electric field that drives a brief downward plasma motion at the dayside polar ionosphere [ABSTRACT FROM AUTHOR]

Interplanetary (IP) shocks have substantial impact on particles and electromagnetic fields when arriving at the Earth's magnetosphere. In this study, we have examined the dynamics of cross‐energy ions and plasma waves observed by Van Allen Probe B near the equator at the noon sector inside the geosynchronous orbit after the impact of an IP shock on 27 February 2014. We found that the Ultra‐Low Frequency (ULF) and electromagnetic ion cyclotron (EMIC) waves are induced, and the differential fluxes of protons of various energies have different responses after the IP shock arrival. The perpendicular flux increased dramatically for low‐energy ions (10–100 eV) due to the electric field drift and betatron acceleration. These adiabatic processes also accounted for the evident proton flux decrease at 30–80 keV energies and increase at energies higher than 100 keV, caused by the positive or negative gradient in phase space density before the IP shock arrival. The short‐lived ULF waves triggered by the IP shock also interacted with the ∼100 keV protons, resulting in the stripes in their pitch angle distribution. The anisotropic distribution of >50 keV protons excited EMIC waves after the shock arrival. This study provides a comprehensive picture of the IP shock effects on the ions of different energies and plasma waves inside the geosynchronous orbit, which shed new lights on the cross‐energy responses of ions and plasma waves in the inner magnetosphere to solar wind discontinuities. Key Points: We analyzed simultaneous cross‐energy ion response through adiabatic and non‐adiabatic processes after the impact of an Interplanetary shockIons with different energies experienced the same adiabatic processes but underwent different evolutions depending on initial distributionsUltra‐Low Frequency and electromagnetic ion cyclotron waves could be generated simultaneously and affect different energy ions [ABSTRACT FROM AUTHOR]

We investigated the dawn–dusk asymmetrical distribution of quasi‐dc earthward Poynting flux (EPF) in the dayside polar cap of the Northern Hemisphere based on 3 years of observations at ∼850 km made by the Defense Meteorological Satellite Program (DMSP) F17. The satellite data have been sorted by low (Kp ≤ 2) and high (Kp > 2) geomagnetic activity, as well as the orientation and strength of interplanetary magnetic field (IMF) By and Bz components, binned by magnetic latitude, and magnetic local time. Statistical results show that the enhanced convection electric field favors the asymmetrical EPF distribution. Asymmetrical distribution characteristics are summarized as (a) there are mainly two enhanced EPF regions, near dawn and cusp and the mean EPF is 28% larger on the dawn than on the dusk for all IMF and Kp conditions. (b) The mean EPF for high Kp is 3 times greater than that for low Kp in the dayside polar cap. The enhanced EPF shifted from near the dawn for low Kp to near the cusp for high Kp. The EPF is also larger on the dawn than on the dusk for high Kp. (c) The EPF of the dayside polar cap is significantly enhanced as the IMF strength increases. Under IMF By+, EPF is greater on the dawn than on the dusk. The opposite is true under IMF By−. The IMF Bz− favors enhanced EPF and the EPF on the dawn (dusk) reaches its maximum for IMF By+ and Bz− (IMF By− and Bz−). Key Points: The dawn–dusk asymmetrical distribution of earthward Poynting flux (EPF) in the dayside polar cap of the Northern Hemisphere is presentedDependence of the asymmetrical distribution of EPF on geomagnetic activities and interplanetary magnetic field By/Bz orientations and strengths has been studiedStatistical results indicate that the asymmetrical distribution of EPF is strongly related to the convection electric field [ABSTRACT FROM AUTHOR]

Sporadic E (Es) layers could be composed of metallic ions and formed, modified, or transported by the action of convective electric fields in the high latitude ionosphere. In this paper, by utilizing simultaneous observations from Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radars at Zhongshan Station (ZHS, 69.4°S, 76.4°E), Antarctica, a thin Es layer, which initially formed in the lower F region and descended into the lower E region, with wavelike structures, was recorded by Digisonde on 14 November 2019. The Es layer‐related concurrent ionospheric irregularities were also detected by the SuperDARN ZHS HF radar. By using a global‐scale 2‐D convection map, combined with images from the Special Sensor Ultraviolet Spectrographic Imager instruments onboard Defense Meteorological Satellite Program (DMSP) spacecraft, it is proposed that the flow shears associated with the duskside convective circulation are responsible for the evolution of the Es layer. Moreover, using the HF radar elevation angle data to measure the scatter height, it is strongly suggested that the Es layer was elongated with convection circulation. The electrodynamic processes responsible for the formation and evolution of the Es layer are discussed. Key Points: An Es layer formed in the lower F region and descended to the E region, observed simultaneously by Digisonde and Super Dual Auroral Radar Network (SuperDARN) HF radarsThe formation and evolution of the Es layer related to the afternoon convection reversalSuperDARN HF radar measurements suggest the Es layer is elongated with convection circulation [ABSTRACT FROM AUTHOR]

The effect of radial interplanetary magnetic field (IMF BX ${\mathrm{B}}_{\mathrm{X}}$) has been ignored in the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling. We present a statistical study of IMF Bx effects on the Heppner‐Maynard Boundary (HMB) midnight latitude calculated from SuperDARN measurements between January 2002 and December 2017. The HMB represents the equatorward extent of the ionospheric convection pattern and can be used as a proxy for the equatorward boundary of the auroral oval. That is, the HMB expands toward lower latitude when the level of S‐M‐I coupling is enhanced. It is found that the averaged HMB midnight latitudes during both sunward and antisunward radial IMFs are around 64–65° magnetic latitude (MLAT). HMB midnight latitude is negatively correlated with the magnitude of IMF Bx as well as solar wind speed, while no significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. For antisunward radial IMF, the relationship between upstream parameters and HMB is more dependent than in sunward radial direction. As the first long‐term statistical study focused on HMB during radial IMF conditions, this work provides observational evidence that the IMF Bx plays an important role in the coupling process and the traditional coupling function could not be applicable in the case of radial IMF. Plain Language Summary: The magnetic reconnection between solar wind and magnetosphere can drive a convection structure in the high‐latitude ionosphere. Heppner and Maynard developed a method to calculate the equatorward boundary of the ionospheric convection, which is called Heppner‐Maynard Boundary (HMB). HMB could be used as the monitor of the interaction process between the solar wind and geomagnetic field. In this paper, we report the effects of the radial (sunward or antisunward) interplanetary magnetic field (IMF) on HMB in the northern hemisphere. For normal cases, the Bz component o IMF is the most important. But our statistical results indicate that there is a decreasing trend between the strength of IMF Bx and HMB latitudes at midnight. No significant trend is found between HMB midnight latitude and IMF Bz or By during radial IMF. Moreover, the results provide observational evidence the Bx term should be considered in the coupling function during radial IMF. Key Points: The HMB midnight latitude has a negative trend on the IMF Bx, especially for negative IMF BxThe averaged values of HMB during radial IMFs are larger than those for substorm periods and a long‐term period of 17 yearsThe traditional coupling function may not be applicable in the case of radial IMF [ABSTRACT FROM AUTHOR]

A sodium Doppler lidar system with three‐directional measurements of sodium density, atmospheric wind field, and temperature was established at Zhongshan (69.4°S, 76.4°E), Antarctica. On November 14, 2019, a sporadic sodium layer (SSL) was observed at an altitude range of 93–103 km. The temporal/spatial sodium density variations of this SSL are associated with a strong sporadic E (Es) layer at nearly the same height, which is modulated by the convective electric field. By considering the structures and the time lags of the SSL's growth at three positions, the SSL appears to have a horizontal advection in an approximately westward direction with a velocity of the order of 80 m/s. This is consistent with the zonal wind velocity derived from the lidar system itself. The temporal/spatial sodium density variations strongly indicate that the formation and perturbation of SSLs are related to the evolution of ES layers due to varied electric fields and atmospheric gravity waves, while it is advected by the horizontal wind. Plain Language Summary: A sporadic E layer (Es) could be formed by metallic ions and then modified or transported by the action of magnetospheric electric fields in the high latitude ionosphere. It has been widely proposed that Es layers play an important role in the formation of sporadic sodium layers (SSL), but detailed studies of their dynamic process and evolution are still lacking. A three‐frequency Sodium (Na) resonance fluorescence Doppler lidar has been recently deployed by the Polar Research Institute of China, which could measure the sodium density, temperature, and wind profiles simultaneously in three directions. To clarify the dynamic properties of Es/SSL, we have performed observations of an event at Zhongshan Station (69.4°S, 76.4°E), Antarctica, which includes sodium density profiles and wind velocity measured by the multidirectional lidar system, detection of the Es layer by a collocated Digisonde radar, F region ion velocity, that is, electric field, derived by the SuperDARN HF radar network, as well as gravity wave perturbations determined from the Davis medium frequency radar. Key Points: Separation of temporal/spatial variations in sporadic sodium layer (SSLs) is observed by a three‐directional lidar system at Zhongshan, AntarcticaThe formation and perturbance of SSLs are associated with a sporadic E layer at the same heightThe SSLs are advected by the background wind [ABSTRACT FROM AUTHOR]

In general, lobe reconnection occurs under northward interplanetary magnetic field (IMF) Bz. However, what process can trigger lobe reconnection has not been fully understood. Using observations from SuperDARN radar and Defense Meteorological Satellite Program (DMSP) spacecraft, we found that (a) short‐term sunward ionospheric flow bursts observed by SuperDARN poleward of the cusp showed one‐to‐one correspondent with the sudden increase of the IMF Bz, (b) the sunward flow bursts observed by SurperDARN were associated with sunward ion drifts identified from the DMSP satellite, and (c) a cusp auroral spot, together with an inverse ion energy dispersion, was observed in association with one of the flow bursts. We suggest that these observations provide evidence that the lobe reconnections can be triggered by a sudden increase of the IMF Bz component. What's more, we found that the reconnection signatures disappeared quickly after the IMF Bz enhancement being over, which indicates that the lobe reconnection triggered by the sudden increase of the IMF Bz is a transient process. Plain Language Summary: The lobe reconnection between solar wind and magnetosphere occurs under a strong northward interplanetary magnetic field (IMF). And it will produce sunward ionospheric convection flow in the dayside polar cap. In this study, we find that short‐term sunward ionospheric convection flows in the dayside polar cap show one‐to‐one correspondence with the sudden increase of the IMF Bz under strong IMF Bx condition. By combining SuperDARN and Defense Meteorological Satellite Program data, we can identify that it is caused by transient lobe reconnection. The transient lobe reconnection may take place even if the increasing magnitude of the IMF Bz is not substantially large, and then disappears quickly after the finish of IMF Bz enhancement. Thus, we believe that the transient lobe reconnection is caused by sudden positive enhancement of IMF Bz. Key Points: Sunward ionospheric convection flow bursts at the dayside polar cap showed one‐to‐one correspondent with the sudden increase of the interplanetary magnetic field (IMF) BzCusp auroral spot and inverse ion energy dispersion were confirmed to associate with one of the flow burstsEvidence of transient lobe reconnection triggered by sudden increase of the IMF Bz is presented [ABSTRACT FROM AUTHOR]

Using the full relativistic test particle (TP) simulation code, we investigate the parametric dependence of electron scattering and phase space density evolution driven by magnetosonic (MS) waves at L = 4.5 both inside and outside the plasmapause. The scattering effects caused by Landau resonance, bounce resonance, and the transit‐time effect are all involved in the study. The net scattering effects are evaluated in the form of diffusion coefficients with different combinations of MS wave parameters, such as frequency bandwidth and wave normal angle, and ambient plasma density. The results demonstrate that (1) Landau resonance and the transit‐time effect dominate the electron scattering inside and outside the plasmapause, respectively, while both are modulated by bounce resonant scattering; (2) bounce resonant scattering becomes more important with narrowband MS waves; (3) electron scattering induced by MS waves is highly sensitive to wave normal angle. The temporal phase space density (PSD) evolution obtained from 2‐D kinetic Fokker‐Planck simulations shows that MS waves with larger wave normal angles are more likely to generate electron butterfly pitch angle distributions (PADs) for hundreds of keV electrons outside the plasmapause. Our study suggests that the electron butterfly distribution has important implications for revealing the combined scattering of MS wave‐particle interactions, and the combination of the multiple scattering mechanisms should be carefully incorporated in future global modeling of radiation belt dynamics. Key Points: The transit‐time effect with bounce resonance by MS waves dominates the electron scattering outside the plasmapauseElectron scattering by MS waves is sensitive to frequency bandwidth, wave normal angle, and plasma densityMS waves generate electron butterfly pitch angle distributions more efficiently outside the plasmapause [ABSTRACT FROM AUTHOR]

Abstract: We report results from the analysis of a case of conjugate polar cap arcs (PCAs) observed on 5 February 2006 in the Northern Hemisphere by the ground‐based Yellow River Station all‐sky imager (Svalbard) and in both hemispheres by the space‐based DMSP/SSUSI and TIMED/GUVI instruments. The PCA's motion in dawn‐dusk direction shows a clear dependence on the interplanetary magnetic field (IMF) By component and presents a clear asymmetry between Southern and Northern Hemispheres, that is, formed on the duskside and moving from dusk to dawn in the Northern Hemisphere and vice versa in the other hemisphere. The already existing PCAs' motion is influenced by the changes in the IMF By with a time delay of ~70 min. We also observed strong flow shears/reversals around the PCAs in both hemispheres. The precipitating particles observed in the ionosphere associated with PCAs showed properties of boundary layer plasma. Based on these observations, we might reasonably expect that the topological changes in the magnetotail can produce a strip of closed field lines and local processes would set up conditions for the formation and evolution of PCAs. [ABSTRACT FROM AUTHOR]