Aoi Nakamizo

Alfvenic‐coupling algorithm for global and dynamical magnetosphere‐ionosphere coupled system

[1] This study proposes a new formula that describes a dynamical magnetosphereionosphere (MI) coupling system through the field-aligned current (FAC) closure and electrostatic potential connectivity. In the past, MI coupling processes were described as either ‘‘inductive’’ or ‘‘static.’’ The inductive coupling scheme is based on the reflection of MHD waves at the ionosphere, whereas in the static coupling scheme the electrostatic potential is determined through the FAC closure. In contrast, in our new formulation these two schemes are combined by the ‘‘Alfvenic-coupling’’ algorithm. The concept of the Alfvenic coupling is as follows. When a distribution of ionospheric current is changed from the background condition through the mapping of magnetospheric disturbances and/ or by the change of ionospheric conductivity, an ionospheric reflection electric filed is instantaneously generated to satisfy the current continuity condition. The electrostatic potential of this reflection field also feeds back to the magnetosphere and excites shear Alfvenic disturbances in the magnetosphere. Therefore, the current continuity condition has to include the FAC of Alfvenic disturbances. In this sense, the reflection potential is not only a modification of the ionospheric potential but also a source of inductive Alfvenic disturbances. Quantitative estimation of feedback components to the magnetosphere reveals that our new formulation is more suitable to global MI simulations than the static MI coupling, which has been the only scheme used in the past.

Pi2 pulsations observed around the dawn terminator

We examined Pi2 pulsations observed simultaneously at low-latitude stations (L = 1.15 − 2.33) around the dawn terminator. Those Pi2 pulsations observed in the sunlit region were polarized in the azimuthal (D, positive eastward) direction. We found that the D component oscillations in the dark and sunlit regions were in antiphase, whereas the H component oscillated in phase. A statistical analysis indicates that these D component phase reversals occurred about 0.5 h sunward of the dawn terminator at 100 km in altitude, corresponding to the highly conducting E layer. The azimuthal polarization and D component phase reversals related to the dawn terminator cannot be explained by the existing models of low-latitude Pi2s (e.g., cavity resonance or substorm current wedge oscillations). Similar D component phase reversals were also found on the dusk side although the amplitude of the D component is smaller than that of the H component. We suggest that the meridional ionospheric current in the sunlit region adjacent to the dawn terminator drives the D component oscillations in antiphase with those D oscillations produced by the oscillatory field-aligned current (FAC) on the postmidnight side. The meridional current is expected to form a part of a current system that extends from the postmidnight FAC to the equatorial Cowling current. The D component oscillations in the Northern and Southern Hemispheres are also in antiphase, indicating that the current system is symmetric with respect to the equator.

Theory, modeling, and integrated studies in the Arase (ERG) project

Understanding of underlying mechanisms of drastic variations of the near-Earth space (geospace) is one of the current focuses of the magnetospheric physics. The science target of the geospace research project Exploration of energization and Radiation in Geospace (ERG) is to understand the geospace variations with a focus on the relativistic electron acceleration and loss processes. In order to achieve the goal, the ERG project consists of the three parts: the Arase (ERG) satellite, ground-based observations, and theory/modeling/integrated studies. The role of theory/modeling/integrated studies part is to promote relevant theoretical and simulation studies as well as integrated data analysis to combine different kinds of observations and modeling. Here we provide technical reports on simulation and empirical models related to the ERG project together with their roles in the integrated studies of dynamic geospace variations. The simulation and empirical models covered include the radial diffusion model of the radiation belt electrons, GEMSIS-RB and RBW models, CIMI model with global MHD simulation REPPU, GEMSIS-RC model, plasmasphere thermosphere model, self-consistent wave–particle interaction simulations (electron hybrid code and ion hybrid code), the ionospheric electric potential (GEMSIS-POT) model, and SuperDARN electric field models with data assimilation. ERG (Arase) science center tools to support integrated studies with various kinds of data are also briefly introduced.

Approximate forms of daytime ionospheric conductance

The solar zenith angle (SZA) dependence of the conductance is studied and a simple theoretical form for the Hall-to-Pedersen conductance ratio is developed, using the peak plasma production height. The European Incoherent Scatter (EISCAT) radar observations at Tromso (67 MLAT) on 30 March 2012 were used to calculate the conductance. The daytime electric conductance is associated with plasma created by solar extreme ultraviolet radiation incident on the neutral atmosphere of the Earth. However, it has been uncertain whether previous conductance models are consistent with the ideal Chapman theory for such plasma productions. We found that the SZA dependence of the conductance is consistent with the Chapman theory after simple modifications. The Pedersen conductance can be understood by approximating the plasma density height profile as being flat in the topside E region and by taking into account the upward gradient of atmospheric temperature. An additional consideration is necessary for the Hall conductance, which decreases with increasing SZA more rapidly than the Pedersen conductance. This rapid decrease is presumably caused by a thinning of the Hall conductivity layer from noon toward nighttime. We expressed this thinning in terms of the peak production height in the Chapman theory.

Solar terminator effects on middle- to low-latitude Pi2 pulsations

Abstract To clarify the effect of the dawn and dusk terminators on Pi2 pulsations, we statistically analyzed the longitudinal phase and amplitude structures of Pi2 pulsations at middle- to low-latitude stations (GMLat = 5.30°–46.18°) around both the dawn and dusk terminators. Although the H (north–south) component Pi2s were affected by neither the local time (LT) nor the terminator location (at 100 km altitude in the highly conducting E region), some features of the D (east–west) component Pi2s depended on the location of the terminator rather than the LT. The phase reversal of the D component occurred 0.5–1 h after sunrise and 1–2 h before sunset. These phase reversals can be attributed to a change in the contributing currents from field-aligned currents (FACs) on the nightside to the meridional ionospheric currents on the sunlit side of the terminator, and vice versa. The phase reversal of the dawn terminator was more frequent than that of the dusk terminator. The D-to-H amplitude ratio on the dawn side began to increase at sunrise, reaching a peak approximately 2 h after sunrise (the sunward side of the phase reversal region), whereas the ratio on the dusk side reached a peak at sunset (the antisunward side). The dawn–dusk asymmetric features suggest that the magnetic contribution of the nightside FAC relative to the meridional ionospheric current on the dusk side is stronger than that on the dawn side, indicating that the center of Pi2-associated FACs, which probably corresponds to the Pi2 energy source, tends to be shifted duskward on average. Different features and weak sunrise/sunset dependences at the middle-latitude station (Paratunka, GMLat = 46.18°) can be attributed to the larger annual variation in the sunrise/sunset time and a stronger magnetic effect because of closeness from FACs. The D-to-H amplitude ratio decreased with decreasing latitude, suggesting that the azimuthal magnetic field produced by the FACs in darkness and the meridional ionospheric current in sunlight also decreased with decreasing latitude.

Application of a global magnetospheric-ionospheric current model for dayside and terminator Pi2 pulsations†

Pi2 magnetic oscillations on the dayside are considered to be produced by the ionospheric current that is driven by Pi2-associated electric fields from the high-latitude region, but this idea has not been quantitatively tested. The present study numerically tested the magnetospheric-ionospheric current system for Pi2 consisting of field-aligned currents (FACs) localized in the nightside auroral region, the perpendicular magnetospheric current flowing in the azimuthal direction, and horizontal ionospheric currents driven by the FACs. We calculated the spatial distribution of the ground magnetic field produced by these currents using the Biot-Savart law in a stationary state. The calculated magnetic field reproduced the observational features reported by previous studies: (1) the sense of the H component does not change a wide range of local time sectors at low latitudes, (2) the amplitude of the H component on the dayside is enhanced at the equator, (3) the D component reverses its phase near the dawn and dusk terminators, (4) the meridian of the D component phase reversal near the dusk terminator is shifted more sunward than that near the dawn terminator, and (5) the amplitude of the D component in the morning is larger than that in the early evening. We also derived the global distributions of observed equivalent currents for two Pi2 events. The spatial patterns of dayside equivalent currents were similar to the spatial pattern of numerically derived equivalent currents. The results indicate that the oscillation of the magnetospheric-ionospheric current system is a plausible explanation of Pi2s on the dayside and near the terminator.

Development of the global simulation model of the heliosphere

The heliospheric structure ranging from the solar surface to the earth’s orbit is self-consistently reproduced from a time-stationary three-dimensional (3D) magnetohydrodynamic (MHD) simulation. The simulation model incorporates gravity, Coriolis, and centrifugal forces into the momentum equation, and coronal heating and field-aligned thermal conduction into the energy equation. The heating term in the present model has its peak at 2.8 solar radius (Rs) and exponentially falls to zero at greater distance from the solar surface. The absolute value of heating depends on the topology of the solar magnetic field so as to be in inverse proportion with the magnetic expansion factor. The results of the simulation simultaneously reproduce the plasma-exit structure on the solar surface, the high-temperature region in the corona, the open- and closed-magnetic-field structures in the corona, the fast and slow streams of the solar wind, and the sector structure in the heliosphere.