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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.

Using Exploration of energization and Radiation in Geospace (ERG or Arase) spacecraft data, we studied low-frequency magnetic field and energetic particle flux oscillations and high-frequency waves deep in the inner magnetosphere at a radial distance of ~4–5 during substorm dipolarization. The magnetic field oscillated alternately between dipole-like and taillike configuration at a period of 1 min during dipolarization. When the magnetic field was dipole-like, the parallel magnetic component of the Pi2 waves was at trough. Both energetic ion and electron fluxes with a few to tens of kiloelectronvolts enhanced out of phase, indicating that magnetosonic waves were in slow mode. Field-aligned currents also oscillated. These observations are consistent with signatures of ballooning instability. In addition, we found that broadband waves from the Pi1 range to above the electron cyclotron frequency tended to appear intermittently in the central plasma sheet near dipole-like configuration.

著者人数: 20名, Accepted: 2018-11-13, 資料番号: SA1180262000

We examined a fortuitous case of an omega band pulsating aurora observed simultaneously on the ground at Sanikiluaq in Canada and onboard the Time History of Events and Macroscale Interactions during Substorm (THEMIS) spacecraft on 1 March 2011. We observed almost the entire process of the generation of the omega band aurora from the initial growth to the declining through expansion period. The omega band aurora grew from a faint seed, not via distortion of a preexisting east-west band aurora. The size scale of the omega band aurora during the maximum period was ~500 km in the north-south direction and ~200 km in the east-west direction. The mesoscale omega band aurora consisted of more than 15 patches of complex-shaped small-scale auroras. Each patch contained an intense pulsating aurora with a recurrent period of ~9–12 s and a poleward moving form. The footprints of the THEMIS D and THEMIS E spacecraft crossed the poleward part of the omega band aurora. THEMIS D observed significant signatures in the electromagnetic fields and particles associated with the time at which the spacecraft crossed the omega band aurora. In particular, it was found that the Y and Z components of the DC electric field intensity, especially the Z component, modulated with almost the same period as that of the optical pulsating auroras. The electrostatic low-frequency waves of less than 30 Hz showed quasiperiodic intensity variations similar to those of the DC electric field. These observations suggest that DC electric field variation and low-frequency electrostatic waves may play important roles in the driving mechanism of omega band pulsating auroras.

We studied the coherence and phase structure of Pc 3 pulsations in the H and D components of the magnetic field observed at latitudinally distributed stations along the 210° magnetic meridian by using the H component at GUA (L = 1.01) as the reference. At GUA, which is located near the magnetic equator, field line resonance does not occur because of the small vertical component of the ambient magnetic field, so pulsations observed there are likely produced by fast mode waves propagating from sources external to the magnetosphere. We found that the D-component Pc 3's in the prenoon sector 0900-1200 LT at L = 1.2-5.5 show a coherent, in-phase (out-of-phase) oscillation in the northern (southern) hemisphere with respect to the H component Pc 3's at GUA. This result indicates that the D-component Pc 3's in the prenoon sector at L = 1.2-5.5 are closely related to the upstream source waves.

[1] Ionospheric Sq current systems during unusually strong and prolonged stratospheric sudden warming (SSW) events in January 2006 and January 2009 are examined by analyzing ground-magnetometer data for the American and Asian longitude sectors. During these SSW events, a significant decrease and increase of the Sq equivalent current intensity are observed in the Northern and Southern Hemispheres, respectively, along with a reduction in the longitudinal separation between the northern and southern current vortices. Numerical experiments using the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General-Circulation Model show that changes in the solar anti-symmetric (2,3) semidiurnal tide can bring about similar changes in the Sq current system.

[1] We investigate the characteristics of poleward moving auroral arcs (PMAAs) observed with the all-sky monochromatic imager at the South Pole Station (74.3°CGLAT) in the interval 08:00–11:00 MLT on July 8, 2004. During this period, Pc 5 geomagnetic pulsations are also observed as well as quasi-stationary auroral patches (QSAPs), which keep their forms, luminosities, and locations for up to several hours. It is demonstrated that the PMAAs, Pc 5 pulsations in the north-south component, and oscillations in the drift of the QSAPs in the east-west direction have similar dominant periods and are well correlated with each other. The oscillatory ionospheric plasma drifts accompanying the PMAAs are extracted from the horizontal motion of the QSAPs and are found to propagate poleward with the PMAAs. The phase relations of the PMAAs and Pc 5 pulsations in the northward component with respect to the westward drift velocity of the QSAPs are close to anti-phase and in-phase, respectively. To examine whether these phenomena can be interpreted by field line resonances (FLRs), we perform a numerical simulation using a horizontally two-dimensional model that consists of the auroral acceleration region and the ionosphere with time-evolution conductivity. The results indicate that the observed data can be well explained by the FLR model with realistic physical parameters, except for the phase relation of the PMAAs with respect to the Pc 5 pulsations and the QSAPs.

[1] We formulate the evolution of ionospheric conductivity in the framework of 3-D M-I coupling. Two important physical processes are taken into account. One is the ionization process by precipitating mono-energetic particles, which are accelerated by parallel-potential drops in the auroral acceleration region. The other process reflects the fact that part of field-aligned current (FAC) carried by electrons is closed with a perpendicular ionic current. Here, whereas the electric current is divergence-free, the divergence of electron current is finite. Therefore, the ionospheric electron density changes, and so does the conductivity. If the energy of electron precipitation is below ∼10 eV, this second process plays an important role in plasma transportation, production, and evacuation processes. In this case the density variation does not extend in space at the perpendicular electron velocity, but it rather moves at the ion perpendicular velocity. If the energy of electron precipitation is above ∼1 keV, in contrast, the precipitation has a nonlinear effect on plasma evolution. That is, the propagation speed of the density variation increases with increasing upward-FAC density, and the propagation takes place in the direction of the converging current into the upward FAC region. The Cowling effect on the plasma evolution process is crucially important. Our formulation is more general than the previous studies and is not limited to certain geometries, current component or interaction modes between the ionosphere and magnetosphere. It is therefore better-suited for describing the self-organized M-I coupling system, which evolves with current systems, conductivity, and magnetospheric processes interacting with each other.

[1] The horizontal spatial structure of Pc 3 pulsations observed at low geomagnetic latitude (22–46°) around dawn is studied statistically using data acquired by the Circum-pan Pacific Magnetometer Network (CPMN). It is found that while the phase of the H component of low-latitude Pc 3 pulsations remains largely unchanged with the passing of dawn, the D component undergoes a phase shift of ca. 180°. This phase variation across dawn is related to the abrupt change in the major axis orientation of polarization ellipses observed in previous studies. Both the H and D components have higher amplitude after dawn than before dawn. This horizontal amplitude and phase structure is well explained by the response of a nonuniform ionosphere around dawn to incident Alfven waves, where the secondary electric field caused by charge accumulation at the dawn terminator plays an important role in deformation of the current system. Enhancement of the D/H ratio is also observed just after dawn at very low latitudes (22°). As Alfven waves are not excited efficiently at very low geomagnetic latitudes, including the magnetic equator, the observed horizontal structure in such regions may be due to a large-scale current system originating at higher latitudes.

[1] We present quasi-stationary auroral patches (QSAPs) observed by an all-sky imager at the South Pole Station (−74.3° CGLAT) on the dayside between ∼0900 and ∼1400 MLT. QSAPs appeared in a closed field line region where I(557.7 nm) ≫ I(630.0 nm) and each patch tends to preserve its form, luminosity, and location for up to several hours. The quasi-stationary nature of the QSAPs cannot be explained by the traditional role of the corotation electric field because the South Pole is located on the axis of Earth's rotation. The most plausible scenario is that a corotation electric potential, which dominates the convection potential, results in a trapping region of the magnetospheric cold plasmas. On the analogy of the formation of the plasmasphere, the trapping region would capture cold plasmas originated from the ionosphere and result in a locally enhanced plasma density (miniplasmasphere) that may lead to electron scattering through cyclotron wave-particle interactions. This circumstance may occur when the South Pole is located on a closed field line and the local electric potential is dominated by the corotation potential. QSAPs were also accompanied with periodic fluctuations in the Pc 5 range, which coincide closely with ground magnetic fluctuations. Assuming that the pulsation is associated with field line resonances, we could estimate the equatorial mass density of thermal plasmas, which gradually increased from 0.13 to 0.44 amu cm−3. The increase in the thermal plasma density would support the hypothesis that a miniplasmasphere exists, though further investigation is necessary to confirm it.

[1] Ground observations of magnetospheric field line eigenfrequency are useful in continuously monitoring the magnetospheric plasma density. The amplitude-phase gradient method (APGM) [Pilipenko and Fedorov, 1994] is a technique which is applied to H-component magnetometer data from two ground stations aligned along a meridian; as a result, it yields the eigenfrequency and the resonance width as continuous functions of latitude. This feature (yielding the two quantities at any latitudes from two stations' data) is unique to APGM: Classic gradient methods yield the two quantities only at the midpoint of the two stations. This paper presents the first time APGM is applied to actual data: We apply it to two pairs of stations, independently, to test if the two results agree. The two pairs are made from three stations (one overlapped) almost aligned along a meridian, at L = 1.32, 1.36, and 1.39 (latitudinal separation: 1.51° and 1.06°). The data were taken when Pc3 pulsations took place. The resultant two latitude profiles of the eigenfrequency (resonance width) are very close to each other, demonstrating the consistency and usefulness of APGM. We also present a new technique to obtain one latitude profile of the eigenfrequency and that of the resonance width from any number of stations; we apply it to our three stations' data and show its consistency and usefulness. The application result unambiguously shows that the eigenfrequency at low latitudes increases with increasing latitude, which is ascribed to mass loading on field lines of ionospheric heavy ions. The result also indicates a large damping rate of the eigen-oscillations.