This study investigates the distribution and formation mechanisms of ionization troughs inside an auroral oval (referred to as high‐latitude troughs) by analyzing Swarm observations from May–August ...2014. Simultaneous measurements of plasma density, 3‐dimensional ion velocity, ionospheric radial current (IRC), and electron temperature are available during this period. Because high‐latitude troughs appear within an auroral oval while mid‐latitude troughs appear at the equatorward edge of the auroral oval, the positioning of troughs relative to the equatorward auroral boundary becomes critical for distinguishing between the two types of troughs. We ascertain the auroral boundary and the orientation of field‐aligned currents using IRC data derived from magnetic field measurements. The principal features of high‐latitude troughs identified from Swarm data include: (a) enhancements in ion velocity and electron temperature, (b) the presence of downward or absent field‐aligned current (FAC), and (c) a more frequent occurrence in the Northern (summer) Hemisphere than in the Southern (winter) Hemisphere and in the dawn and dusk sectors than in the noon and midnight sectors. The alignment of the density minimum with the velocity maximum underscores the role of high‐speed plasma convection in the formation of high‐latitude troughs; atmospheric frictional heating promotes the O+ loss through dissociative recombination. The prevailing appearance of high‐latitude troughs at dawn and dusk sectors, coupled with downward field‐aligned currents, indicates the involvement of outward electron evacuation in trough formation.
Key Points
Measurements of radial current from Swarm satellite provide a tool to distinguish mid‐ and high‐latitude ionization troughs
High‐latitude troughs occur more frequently in summer than winter and in dawn/dusk sectors than the noon/midnight sectors
High‐speed plasma convection is the primary cause of high‐latitude trough, and field‐aligned current is a secondary contributing factor
We have studied the statistical properties of low‐energy proton (H+) and helium (He+) ion flux enhancements associated with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere using ...Van Allen Probes data for 2013–2017. We identified 167 low‐energy ion flux enhancements when the EMIC waves occurred in a He‐band or in a multiple band (H‐band and He‐band) with strong He‐band and weak H‐band wave activity and found that most of them occurred from the noon to the premidnight sector near the magnetic equator just inside the plasmapause. Of 167 flux enhancement events, 68 exhibited only He+ flux enhancements, and 99 exhibited both H+ and He+ flux enhancements. The EMIC wave‐associated flux enhancement events are mostly energized in the direction perpendicular to the background magnetic field. When both H+ and He+ fluxes are simultaneously enhanced, the H+ flux events have a peak energy distributed in the range of 2–100 eV, and the peak energies of the He+ flux events are distributed in the 2–600 eV range, implying that the helium ions are more energized than the protons. The peak energies of only He+ flux enhancement without H+ flux enhancement are mostly distributed in a lower energy range, 2–10 eV. The energization of H+ and He+ ions can be explained by a linear plasma flow associated with EMIC waves. We suggest that the wave‐associated linear plasma motion is a likely mechanism to explain the observations.
Key Points
Cold protons and helium ions are transversely heated by interactions with electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere
Cold helium ions are more energized than cold protons by EMIC waves
EMIC wave‐associated linear plasma motion is a likely mechanism for the cold particle acceleration
Noticeable F‐region electron density (NmF2) depletions were observed in the winter‐nighttime polar cap ionosphere during solar minimum from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) ...with Dynasonde analysis at Jang Bogo Station (JBS) in Antarctica. We focus on the F‐region density depletion events (known as polar holes) following a steady quiet condition that is defined with Kp values ≤ 1+ during 6 hr. Forty‐five polar holes were identified by JBS VIPIR/Dynasonde (JVD) in 2019. All of the events started over a wide range of nightside magnetic local time (22‐05 MLT) with a peak occurrence at 01–03 MLT. JVD measured exponential NmF2 decrease in the nightside MLT (∼19–2.5 hr) zone with e‐fold decay times distributed in the range of ∼0.5–∼3.5 hr before the onset of a polar hole. The e‐folding times decrease along the longitude from dusk toward midnight. The horizontal ion drift velocity (Vhor) estimated from JVD monotonically goes down from ∼190 m/s at 18 MLT to ∼100 m/s near magnetic midnight, and the NmF2 is depleted as Vhor decreases prior to the polar hole formation. The observations of the exponential NmF2 decrease and the positive correlation between NmF2 and Vhor prior to the polar holes are discussed in light of possible formation mechanisms of polar holes, including temporal variations and spatial structure of the polar ionosphere.
Key Points
Polar holes were mostly observed at ∼22−06 MLT under extremely quiet geomagnetic conditions
Exponential electron density decrease occurred ∼1.0−4.5 hr prior to the onset of the polar holes
The exponential declination of NmF2 due to recombination loss is the reason for the formation of the polar holes
We analyze data acquired by the Kaguya satellite on 14 October 2008 when the Moon was in the terrestrial magnetotail lobe to gain new insight into the energization of ions originating from the Moon. ...The Moon‐originating ions were detected over a broad range of latitudes from −80° to 50° above the Moon's dayside at ∼100 km altitude. The fluxes of the Moon‐originating ions were observed at energies from ∼50 to ∼1,000 eV. Additionally, these ions exhibited a wide distribution pitch angle spanning from ∼45 to 90°. The energy levels of ions originating from the Moon show rapid changes, either increasing or decreasing by a factor of ∼10 within 8 min without the solar zenith angle dependence. Such rapid energy changes were observed over the highland regions. These observations are discussed in light of possible acceleration mechanisms of Moon‐originating ions, including temporal and spatial effects.
Key Points
The Moon‐originating ions are distributed over a broad range of latitudes from −80° to 50°
The median peak energy of Moon‐originating ions does not show a significant dependence on the solar zenith angle between 0° and 60°
The energy of the Moon‐originating ions shows a sudden rise or drop by a factor of ∼10 in 8 min over the highland regions
Electromagnetic ion cyclotron (EMIC) waves generated by hot anisotropic (T⊥ > T∥) protons (∼10–100 keV), play an important role in accelerating cold (<1 eV) protons (H+) and helium (He+) ions in the ...magnetosphere. Using a hybrid code with parameters found in the inner magnetosphere, we examine when and how cold H+ and He+ ions are energized by EMIC waves. Hybrid simulations show that the energization of the cold particles occurs in two steps. In the first step, EMIC waves, which are linearly excited in the early stage of the simulation, interact with cold H+ and He+ ions, resulting in energization mostly in the direction perpendicular to the background magnetic field. The energization in this step is mainly contributed by enhanced bulk motion of these ions as a result of the linear response, consistent with recent observations in the inner magnetosphere. In the second step, nonlinear evolution of energized cold H+ and He+ ions are confirmed in the parallel direction, which is seen after about 200 proton gyroperiods (∼8.5 s). Throughout the simulation run, cold He+ ions are much more energized in the perpendicular direction than in the parallel direction. However, the cold protons are more energized in the parallel direction than in the perpendicular direction after 500 proton gyroperiods (∼21.3 s). By comparing recent observations and the present simulation results, we suggest that the cold particle energization by EMIC waves occurs at an early stage of wave generation when the nonlinear evolution of EMIC waves is not dominant in the inner magnetosphere.
Key Points
Hybrid simulations are performed to examine energizations of cold H+ and He+ ions by EMIC waves in the inner magnetosphere
Cold He+ ions are more energized than cold protons in the perpendicular direction by EMIC waves
The H+ and He+ perpendicular energizations are mainly due to the bulk motions of the cold ions in the early stage of the simulation
The Korean Pathfinder Lunar Orbiter (KPLO)-MAGnetometer (KMAG) consists of three triaxial fluxgate sensors (MAG1, MAG2, and MAG3) that measure the magnetic field around the Moon. The three sensors ...are mounted in the order MAG3, MAG2, and MAG1 inside a 1.2 m long boom, away from the satellite body. Before it arrived on the Moon, we compared the magnetic field measurements taken by DSCOVR and KPLO in solar wind to verify the measurement performance of the KMAG instrument. We found that there were artificial disturbances in the KMAG measurement data, such as step-like and spike-like disturbances, which were produced by the spacecraft body. To remove spacecraft-generated disturbances, we applied a multi-sensor method, employing the gradiometer technique and principal component analysis, using KMAG magnetic field data, and confirmed the successful elimination of spacecraft-generated disturbances. In the future, the proposed multi-sensor method is expected to clean the magnetic field data measured onboard the KPLO from the lunar orbit.
We present observations of electromagnetic ion cyclotron (EMIC) waves associated with a sudden commencement (SC) on 19 November 2007. In our study, we clearly showed that there was a time delay of ...∼10–15 min between the SC onset and the occurrence of EMIC waves at GOES 12 and GOES 10 in the afternoon sector, while the SC‐associated EMIC wave activity observed by GOES 11 in the morning sector started almost immediately after the onset of the SC. This indicates that the EMIC wave source drifts eastward and that magnetospheric compression alone cannot generate EMIC waves without source particles. We suggest that the wave source motion drifting eastward is attributed to the SC‐associated convection electric field, which is slower than the speed of the initial SC disturbance propagating as a fast mode wave. The spectral broadening of EMIC waves for nonlinear wave growth is examined by changing a spatial gradient of the background magnetic field. We have shown that the spectral properties of the observed SC‐associated EMIC waves are in good agreement with the nonlinear theory. In addition, we observed substorm‐associated EMIC wave activity accompanied by a sudden decrease in magnetic field intensity when the satellites were near dusk. By comparing the wave onset times, we confirmed that the substorm‐associated wave source drifted westward.
Key Points
SC‐associated EMIC wave source drifting eastward
The magnetospheric compression alone cannot generate EMIC waves
Spectral properties of SC‐associated EMIC waves are in good agreement with the nonlinear theory
In this paper, we provide statistical evidence that the level of solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling is weaker under radial (Sun‐Earth component dominant) interplanetary magnetic ...field (IMF) conditions than non‐radial IMF conditions. This is performed by analyzing auroral electrojet activity (using SuperMAG auroral electrojet indices) in the sunlit and dark ionospheres for long‐duration (at least 4 hr) radial IMF events and comparing against the same for long‐duration azimuthal (dusk‐dawn component dominant) IMF events. We show that the north‐south IMF component (IMF Bz) plays a crucial role in controlling the level of auroral electrojet activity as a negative half‐wave rectifier even for both IMF orientation categories. However, it is found that the magnitudes of the auroral electrojet indices are generally lower for radial IMF than for azimuthal IMF under similar sets of solar wind (radial bulk velocity and number density) and IMF Bz conditions, regardless of whether these indices are derived in the sunlit or dark regions. Moreover, the efficiency of coupling functions is lower for radial IMF than for azimuthal IMF, implying that increased coupling strength due to the azimuthal IMF component alone cannot well explain weaker auroral electrojets during radial IMF periods. Lastly, the contribution of the radial IMF component itself to auroral electrojet activity is also lower compared to the azimuthal IMF component. Our results suggest that the level of SW‐M‐I coupling characterized by auroral electrojet activity can be modulated by the radial IMF component, although the effect of this component is weaker than the other two IMF components.
Plain Language Summary
The ionospheric electric current that flows along the auroral oval, called the auroral electrojet, is one of the manifestations of solar wind energy transfer into the magnetosphere‐ionosphere system via magnetic reconnection processes between the interplanetary and terrestrial magnetic field lines. Previous studies mainly focused on the role of interplanetary magnetic field (IMF) orientation in the plane perpendicular to the Sun‐Earth line (approximately solar wind flow direction) in controlling auroral electrojet activity, based on the dayside magnetopause magnetic field topology viewed from the Sun. In this study, we turn our attention to auroral electrojet activity under radial (nearly parallel to the Sun‐Earth line) IMF conditions by statistically examining the SuperMAG auroral electrojet indices for radial IMF intervals and comparing them with those for non‐radial IMF intervals under similar sets of interplanetary conditions. Our statistical results reveal that both the auroral electrojet intensity and the coupling efficiency between interplanetary and auroral electrojet parameters are lower under radial IMF conditions than non‐radial IMF conditions. These findings indicate that the radial component of the IMF can modulate the level of solar wind‐magnetosphere‐ionosphere coupling, although its effect is weaker than the other IMF components.
Key Points
Auroral electrojet intensity is typically lower for radial interplanetary magnetic field (IMF) than for azimuthal IMF under similar IMF Bz and solar wind conditions
Increased coupling strength due to IMF By alone cannot well explain weaker auroral electrojets during radial IMF compared to azimuthal IMF
The level of solar wind‐magnetosphere‐ionosphere coupling can be modulated by the radial IMF component as well as the other two IMF components
Intervals of pulsations of diminishing periods (IPDPs) are a subtype of electromagnetic ion cyclotron (EMIC) waves that can be triggered by substorm onset. Pi1B waves are ultralow frequency (ULF) ...broadband bursts that are well correlated with substorm onset. IPDPs are associated with increased fluxes of 40–60 keV substorm‐injected protons which undergo gradient‐curvature drifting and interact with the cold plasmasphere population. While particle trajectories and the generation of IPDPs have been modeled in the past, those models neglect the role that drift shell splitting plays in the process. This research investigates the different pathways that Pi1B and IPDPs take from their shared origin in substorm onset to their distinct observations on the ground, including the effects of drift shell splitting en route. This paper presents two case studies using data from an array of four ground‐based Antarctic magnetometers that cover the evening sector, as well as in situ magnetometer data, proton fluxes, and proton pitch angles from the Van Allen Probes spacecraft. These observations identify a separation in geomagnetic latitude between Pi1Bs and IPDPs, and pinpoint a separation in magnetic local time (MLT). From these observations we model the drift shell splitting which injected particles undergo post‐onset. This study shows that simulations that incorporate drift shell splitting across a full injection front are dominated by injection boundary effects, and that the inclusion of drift shell splitting introduces a slight horizontal component to the time axis of the time–frequency dependence of the IPDPs.
Key Points
Pi1Bs and IPDPs observed from Antarctic ground stations occur simultaneously with particle injections from substorm onset
We simulate the trajectories of particle injections from substorm onset as subject to drift shell splitting across a full injection front
Particle trajectories are dominated by injection boundary effects, but drift shell splitting affects the time‐frequency features of IPDPs
Although electromagnetic ion cyclotron (EMIC) waves are commonly observed in the magnetosphere and are believed to energize background cold ions, it is not clear whether EMIC waves play a significant ...role in determining spacecraft potential change. In this paper, we present two strong He‐band EMIC wave events observed by the Van Allen Probe‐B spacecraft inside the plasmasphere. One event occurred on 11 March 2016 when the spacecraft was on the dayside, and the other occurred on 9 October 2016 when the spacecraft was in the postmidnight sector. When a strong He‐band EMIC wave activity was detected, low‐energy ion flux enhancements occurred nearly simultaneously with the EMIC wave power enhancements. Both events presented in this study are clearly unique in that He‐band wave power and enhanced proton flux are extremely high. During the wave activity interval, we found that the spacecraft charged more positively without a significant change in the ambient electron density. We discuss whether low‐energy ions energized by EMIC waves can contribute to the spacecraft potential change.
Plain Language Summary
In the Earth's magnetosphere the spacecraft potential is determined by the current balance between escaping photoelectrons and incoming ambient electrons. Since the ions are considerably more massive than the electrons, implying that electrons are mobile than ions, the ion currents from the ambient plasma have been neglected for the current balance. Spacecraft interactions with the ambient plasma give rise to spacecraft charging. During the interval we focus on in this study, the spacecraft charged positively, indicating that the photoelectron current is larger than the electron current from the ambient plasma. When the spacecraft observed strong electromagnetic ion cyclotron (EMIC) wave activity, low‐energy proton, and helium ion flux enhancements occurred nearly simultaneously with the EMIC power enhancements. At the time of EMIC waves, the spacecraft charged more positively. From these observations, we suggest that wave‐associated low‐energy ion flux enhancements, corresponding to the accumulation of positive ions from the surrounding plasma on spacecraft surface, lead to more positive surface charging.
Key Points
Low‐energy proton and helium ion flux enhancements occur nearly simultaneously with electromagnetic ion cyclotron (EMIC) waves
Cold protons and helium ions below 1 eV are accelerated by EMIC waves
More positive surface charging occurs during the interval of EMIC wave activity
