Akimasa Yoshikawa

Reflection of shear Alfvén waves at the ionosphere and the divergent Hall current

A general expression for the reflection and mode conversion of MHD waves at the ionosphere is derived. On the basis of the expression, the effect of ionospheric divergent Hall current (Hall part of the induction current associated with the inductive electric field of fast magnetosonic wave) on localized toroidal oscillation is examined. When the horizontal scale of localized oscillation is of the order of several times of the height of ionosphere, the inductive electric field can play a significant role in the reflection of shear Alfven waves with longer periods (e.g., ∼100 s) in the high-latitude region, especially, in the auroral zone. Then, the contribution of the divergent Hall current to the field-aligned one can be no longer neglected and so the eigenfrequency of localized toroidal oscillation is effectively controlled by the height-integrated Hall conductivity in the ionosphere.

The nature of reflection and mode conversion of MHD waves in the inductive ionosphere: Multistep mode conversion between divergent and rotational electric fields

The nature of reflection and mode conversion of MHD waves at the high-latitudinal inductive ionosphere is analyzed, based on the current conservation law of wave modes. The term “inductive ionosphere” refers to the nonzero rotational electric field or nonzero compressional magnetic field in the reflection process of shear Alfven waves on the ionosphere. The finite rotational electric field causes mutual induction between the divergent and rotational current systems at the ionosphere. The one-step Hall effect for the divergent electric field of the shear Alfven wave produces a rotational Hall current and excites the ionospheric surface compressional wave. The Hall effect for the rotational electric field of an ionospheric surface compressional wave produces a divergent Hall current (two-step Hall effect), which feeds back the compressional magnetic energy to the reflected field-aligned current. We find that the renormalization of the ionospheric rotational electric field to the reflection process of the shear Alfven wave causes some peculiarities in the distribution of ionospheric currents and mode-converted wave magnetic fields. Such peculiarities become particularly obvious in the high-conducting ionosphere. For example, in the ionospheric current distributions, a considerable component of the ionospheric divergent current is accounted for by the divergent Hall current. The rotational Hall and Pedersen currents cancel each other out and lead to zero total ionospheric rotational current. The amplitude of the poloidal magnetic field transmitted from the toroidal magnetic field of the incident shear Alfven wave shows a nonlinear dependence on ΣH/Σp. It also shows a new type of effective ionospheric shielding effect in the Σp/ΣA parameter space for a fixed ΣH/Σp condition. We assert that the inductive response of the ionosphere should become an indispensable concept for reflection, mode conversion, transmission, and generation of various phenomena relating to the field-aligned current system.

Wave characteristics of daytime and nighttime Pi 2 pulsations at the equatorial and low latitudes

Peculiarities of daytime and nighttime Pi 2 pulsations at the dip equator are examined by using multipoint measurements from the 210° magnetic meridian (MM) magnetometer network. We found that during daytime the amplitude of Pi 2 pulsations at the dip equator is enhanced, and the phase lags ∼ 34° behind those at low-latitude (magnetic latitude Φ = 19.5-46.2°) stations. On the other hand, during nighttime the amplitude of Pi 2 pulsations at the dip equator is depressed, and the phase lags ∼ 18° behind those at the lower latitudes. Because the zonal ionospheric conductivity at the dip equator is much higher than that at the off-dip equator region, Pi 2 signals are expected to be distorted more effectively at the dip equator. The observations imply that the daytime and nighttime Pi 2 pulsations in the equatorial and low-latitude regions can be explained by invoking an instantaneous penetration of electric field variations from the nightside polar ionosphere to the dayside equatorial ionosphere, and a direct incidence of compressional oscillations from the nightside inner magnetosphere, respectively.

Eigenmode analysis of field line oscillations interacting with the ionosphere‐atmosphere‐solid earth electromagnetic coupled system

In order to understand the effect of ionosphere on the localized toroidal magnetic shell oscillation (field line oscillation), we have constructed a rectangular box model for the magnetosphere-ionosphere-solid earth system including the anisotropically conducting ionosphere and performed the eigenmode analysis of the magnetohydrodynamic (MHD) waves in the model. We have found that the eigenmodes of field line oscillation are effectively controlled by the height-integrated Pedersen and Hall conductivities, meaning that the field-aligned current (FAC) in the magnetosphere is closed to both the Pedersen current and the Hall current. The divergent Hall current, that is, the Hall current, which closes the FAC, is a newfound current that is driven by the Hall effect of the ionospheric inductive (rotational) electric field. The divergent Hall current brings into sharp relief by considering the ionosphere to be inductive, and it inevitably requires the electromagnetic coupling between the magnetosphere, ionosphere, atmosphere, and solid earth. The nature of the ionospheric current associated with the standing shear Alfven mode is also analyzed in detail by using the concept of wave reflection and mode conversion at the inductive ionosphere. In particular, we emphasize that the magnetic flux sustained by the rotational Hall current, that is, accompanied by the radiation of fast magnetosonic wave to the magnetosphere and atmospheric poloidal magnetic wave to the atmosphere, should be taken into account for magnetosphere-ionosphere coupling of FAC.

A new index to monitor temporal and long-term variations of the equatorial electrojet by MAGDAS/CPMN real-time data: EE -Index

A new index, EE-index (EDst, EU, and EL), is proposed to monitor temporal and long-term variations of the equatorial electrojet by using theMAGDAS/CPMN real-time data. The mean value of the H component magnetic variations observed at the nightside (LT = 18–06) MAGDAS/CPMN stations along the magnetic equatorial region is found to show variations similar to those of Dst; we defined this quantity as EDst. The EDst can be used as a proxy of Dst for the real-time and long-term geospace monitoring. By subtracting EDst from the H component data of each equatorial station, it is possible to extract the Equatorial Electrojet and Counter Electrojet components, which are defined as EU and EL, respectively.

Wave characteristics of geomagnetic pulsations across the dip equator

In order to clarify the wave characteristics of Pi2 and Pc 4–5 magnetic pulsations around the dip equator, we analyzed magnetic data from the latitudinally dense magnetometer array in Brazil. We found that the phase difference between Pi2 pulsations observed at globally separated low-latitude stations is small, whereas Pi2 pulsations observed within the dayside dip equator region of ±2° latitude show phase lags of about 30° ∼ 50° behind those in the off-dip equator region. Pc 4–5 magnetic pulsations at the dip equator also show the same phase character. Pi2 amplitudes are enhanced in the equatorial region, where the phase lags of pulsations must be associated with the enhancement of ionospheric conductivity. The equatorial phase lags can be explained by invoking the induction effect of the equatorial enhanced ionospheric current above the good conductor Earth.

Ground magnetic effects of the equatorial electrojet simulated by the TIE-GCM driven by TIMED satellite data

Quiet-time daily variations of the geomagnetic field near the magnetic equator due to the equatorial electrojet are simulated using the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere Electro- dynamics General Circulation Model (TIE-GCM), and compared to those observed by ground-based magnetometers. Simulations are run both with and without tidal forcing at the height of the model lower boundary (∼97 km). When the lower-boundary forcing is off, the wind that generates an electro- motive force in the model is primarily the vertically non-propagating diurnal tide, which is excited in the thermosphere due to daytime solar ultra-violet heating. The lower-boundary tidal forcing adds the effect of upward-propagating tides, which are excited in the lower atmosphere and propagate vertically to the thermosphere. The main objective of this study is to evaluate the relative importance of these thermospherically-generated tides and upward-propagating tides in the generation of the equatorial electrojet. Fairly good agreement is obtained between model and observations when the model is forced by realistic lower-boundary tides based on temperature and wind measurements from the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite, as determined by Wu et al. [2012]. The simulation results show that the effect of upward-propagating tides increases the range of the geomagnetic daily variation in the magnetic-northward component at the magnetic equator approximately by 100%. It is also shown that the well-known semiannual change in the daily variation is mostly due to upward-propagating tides, especially the migrating semidiurnal tide. These results indicate that upward-propagating tides play a substantial role in producing the equatorial electrojet and its seasonal variability.

Resonant Cavities and Waveguides in the Ionosphere and Atmosphere

The strong inhomogeneities in plasma parameters in the ionosphere and adjacent regions can trap waves in the upper end of the ULF range (Pcl/Pil). The topside ionosphere is characterized by a rapidly increasing Alfven speed with a scale height on the order of 1000 km. Shear-mode Alfven waves in this region can be partially trapped at frequencies in the 0.1-1.0 Hz range. The same structure can trap fast-mode compressional waves in this frequency band. Since these waves can propagate across magnetic field lines, this structure constitutes a waveguide in which energy can propagate at speeds comparable to the Alfven speed, typically on the order of 1000 km/s. Hall effects in the ionosphere couple these two wave modes, so that the introduction of a field-aligned current by means of a shear-mode Alfven wave can excite compressional waves that can propagate in the waveguide. In the limit of infinite ionospheric conductivity, these waves are isolated from the atmospheric fields; however, for finite conductivity, ionospheric and atmospheric waves are coupled. Transverse magnetic modes in the atmosphere can propagate at ULF frequencies and form global Schumann resonances with the fundamental at 8 Hz. It has been suggested that signals that propagate at the speed of light through this atmospheric waveguide can rapidly transmit signals from the polar region to lower latitudes during sudden storm commencements.

Ground magnetic perturbations associated with the standing toroidal mode oscillations in the magnetosphere-ionosphere system

The behavior of toroidal mode oscillations of standing Alfvén waves (refer to as standing Alfvén oscillations) in the coupled magnetosphere-ionosphere system is investigated using a trapezoid-shape magnetosphere model. It is found that the magnetic perturbation is transmitted across the ionosphere differently in the two cases where the ionospheric electric field perturbation is static (Pedersen conductivity > Hall conductivity) and where it is inductive (Pedersen conductivity < Hall conductivity). It is noted that the ionospheric Hall current for the inductive condition shields the magnetic field perturbation. The north-south asymmetry of the conjugate ground magnetic perturbations is calculated by using a trapezoid model with the ionospheric and magnetospheric parameters based on the IGRF and IRI. It is revealed that the ionospheric electric field is almost static for the fundamental mode oscillation, whereas inductive for the higher harmonic ones. It is also found that the north-south asymmetry of the ground magnetic perturbations depends not only on the L-value but also on magnetic longitude; this is because the ionosphere and magnetic field conditions are not uniform as a function of longitude.

A case study of ionospheric storm effects during long‐lasting southward IMF Bz‐driven geomagnetic storm

Multiple instrumental observations including GPS total electron content (TEC), foF2 and hmF2 from ionosondes, vertical ion drift measurements from Communication/Navigation Outage Forecasting System, magnetometer data, and far ultraviolet airglow measured by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) are used to investigate the profound ionospheric disturbances at midlatitude and low latitude during the 14–17 July 2012 geomagnetic storm event, which was featured by prolonged southward interplanetary geomagnetic field component for about 30 h below −10 nT. In the East Asian/Australian sector, latitudinal profile of TEC variations in the main phase were characterized by three bands of increments and separated by weak depressions in the equatorial ionospheric anomaly (EIA) crest regions, which were caused by the combined effects of disturbance dynamo electric fields (DDEF) and equatorward neutral winds. In the recovery phase, strong inhibition of EIA occurred and the summer crest of EIA disappeared on 16 July due to the combined effects of intrusion of neutral composition disturbance zone as shown by the TIMED/GUVI O/N2 measurements and long-lasting daytime westward DDEF inferred from the equatorial electrojet observations. The transit time of DDEF over the dip equator from westward to eastward is around 2200 LT. In the American longitude, the salient ionospheric disturbances in the summer hemisphere were characterized by daytime periodical intrusion of negative phase for three consecutive days in the recovery phase, preceded by storm-enhanced density plume in the initial phase. In addition, multiple short-lived prompt penetration electric fields appeared during stable southward interplanetary magnetic field (IMF) Bz in the recovery phase and were responsible for enhanced the EIA and equatorial ionospheric uplift around sunset.