Yoshizumi Miyoshi

Precipitation of radiation belt electrons by EMIC waves, observed from ground and space

We show evidence that left-hand polarised electromagnetic ion cyclotron (EMIC) plasma waves can cause the loss of relativistic electrons into the atmosphere. Our unique set of ground and satellite observations shows coincident precipitation of ions with energies of tens of keY and of relativistic electrons into an isolated proton aurora. The coincident precipitation was produced by wave-particle interactions with EMIC waves near the plasmapause. The estimation of pitch angle diffusion coefficients supports that the observed EMIC waves caused coincident precipitation ofboth ions and relativistic electrons. This study clarifies that ions with energies of tens of ke V affect the evolution of relativistic electrons in the radiation belts via cyclotron resonance with EMIC waves, an effect that was first theoretically predicted in the early 1970s.

Dynamic coupling between the lower and upper atmosphere by tides and gravity waves

Abstract Results of a General Circulation Model simulation of the dynamics of the middle atmosphere are shown focusing our attention to the tidal wave mean flow interaction and propagation of migrating diurnal and semidiurnal tides in the model. It is shown that migrating tidal waves are well simulated and the amplitude growth with height is effectively suppressed by the convective adjustment in the model. It is also shown that the dissipating solar diurnal tide plays an important role in inducing mean zonal winds in the low latitude region of the lower thermosphere. The behavior of non-migrating diurnal tides is also analyzed to show that non-migrating diurnal tides have significant amplitudes in the lower thermosphere. It is suggested that the non-migrating diurnal tide, which propagates against background mean zonal winds, has the possibility to propagate into the middle to high latitude region due to the Doppler effect.

Seasonal variation of non-migrating semidiurnal tide in the polar MLT region in a general circulation model

Abstract Behavior of semidiurnal tides in the north and south polar MLT regions simulated by Middle Atmosphere Circulation Model at Kyushu University is described. Summertime enhancement of westward propagating semidiurnal tide with zonal wavenumber s=1 is found, which is consistent with the observed result at the South Pole (Ann. Geophys. 16 (1998) 828). Additional numerical simulations show that the non-migrating semidiurnal tide is mainly generated by the nonlinear interactions between stationary planetary waves with zonal wavenumber s=1 and the migrating semidiurnal tide in the stratosphere and mesosphere as suggested by Forbes et al. (Geophys. Res. Lett. 22(23) (1995) 3247).

Numerical simulation of the 5-day and 16-day waves in the mesopause region

The behavior of the 5-day and 16-day waves in the mesopause region is examined by using a general circulation model. The results are as follows. The 5-day wave is largely unaffected by the zonal mean zonal wind distribution, and the symmetric structure about the equator is clearly seen in the mesopause region. The amplitude of the 16-day wave in the summer hemisphere of the stratosphere is small. However, above the upper mesosphere, the 16-day wave appears not only in the winter hemisphere but also in the summer hemisphere. The penetration of the 16-day wave from the winter hemisphere to the summer hemisphere occurs near the mesopause region. The 16-day wave is mainly excited by heating due to the moist convection in the troposphere, and the vertical penetration into the middle atmosphere occurs. Furthermore, a correlation between the geomagnetic variation and the wind variation associated with the 5-day and 16-day waves is discussed.

Migrating and non-migrating atmospheric tides simulated by a middle atmosphere general circulation model

Abstract Atmospheric diurnal and semidiurnal tides simulated by Middle Atmosphere Circulation Model at Kyushu University (MACMKU) for equinox conditions are analyzed. It is shown that in addition to the migrating diurnal and semidiurnal tides, non-migrating tides have nonnegligible amplitudes in the upper mesosphere and lower thermosphere.

Variations of migrating and non-migrating tides simulated by the Middle Atmosphere Circulation Model at Kyushu University

Abstract Atmospheric diurnal and semidiurnal tides simulated by the Middle Atmosphere Circulation Model at Kyushu University (MACMKU) for equinox conditions are analyzed to investigate behavior of non-migrating tides. It is shown that the model non-migrating diurnal and semidiurnal tides have nonnegligible amplitudes in the upper mesosphere and lower thermosphere, with the amplitudes and phases of the non-migrating tides more variable than those of the migrating tides. EP-fluxes are also analyzed to infer the generation mechanism of the non-migrating tides. It is found that the EP-flux associated with the non-migrating tide having larger zonal wavenumber shows more systematic upward propagation from the troposphere than that associated with the smaller zonal wavenumber components, implying that the form is generated in the troposphere while the latter is generated in the mesosphere.

Energetic electron precipitation associated with pulsating aurora: EISCAT and Van Allen Probe observations

Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also sub-relativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler-mode wave–particle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) Tromso VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes >68 km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from ~10 keV up to at least 200 keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to Tromso and the satellite observed rising tone emissions of the lower-band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave–particle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of sub-relativistic electrons and the pulsating aurora.

The neutral dynamics during the 2009 sudden stratosphere warming simulated by different whole atmosphere models

The present study compares simulations of the 2009 sudden stratospheric warming (SSW) from four different whole atmosphere models. The models included in the comparison are the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy, Hamburg Model of the Neutral and Ionized Atmosphere, Whole Atmosphere Model, and Whole Atmosphere Community Climate Model Extended version (WACCM-X). The comparison focuses on the zonal mean, planetary wave, and tidal variability in the middle and upper atmosphere during the 2009 SSW. The model simulations are constrained in the lower atmosphere, and the simulated zonal mean and planetary wave variability is thus similar up to ∼1 hPa (50 km). With the exception of WACCM-X, which is constrained up to 0.002 hPa (92 km), the models are unconstrained at higher altitudes leading to considerable divergence among the model simulations in the mesosphere and thermosphere. We attribute the differences at higher altitudes to be primarily due to different gravity wave drag parameterizations. In the mesosphere and lower thermosphere, we find both similarities and differences among the model simulated migrating and nonmigrating tides. The migrating diurnal tide (DW1) is similar in all of the model simulations. The model simulations reveal similar temporal evolution of the amplitude and phase of the migrating semidiurnal tide (SW2); however, the absolute SW2 amplitudes are significantly different. Through comparison of the zonal mean, planetary wave, and tidal variability during the 2009 SSW, the results of the present study provide insight into aspects of the middle and upper atmosphere variability that are considered to be robust features, as well as aspects that should be considered with significant uncertainty.

A negative phase shift of the winter AO/NAO due to the recent Arctic sea-ice reduction in late autumn

This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.

Diurnal nonmigrating tides in the tropical lower thermosphere

A comparison is performed between monthly-mean nonmigrating diurnal tide wind components at 95 km derived from Upper Atmosphere Research Satellite (UARS) wind observations, the Middle Atmosphere Circulation Model at Kyushu University (MACMKU), and the Global Scale Wave Model (GSWM) driven by latent heating due to deep tropical convection. A degree of overall agreement is obtained in the sense that annual-mean spectra at 95 km indicate that the UARS data, MACMKU and GSWM all share the same nonmigrating tide components (eastward-propagating with zonal wavenumber s = −3; westward-propagating with s = 2; standing or zonally-symmetric with s = 0; DE3, DW2, D0) at about the same power level. In combination with the migrating tide these wave components give rise to significant longitude variability in the total diurnal tidal fields. Beyond the above model/measurement agreements, significant discrepancies remain between the latitudinal-seasonal structures delineated by models and observation. For MACMKU, some of these discrepancies may be related to the specifics of the convective parameterization that is employed. Significant work remains to better delineate tropospheric forcing mechanisms and nonlinear wave-wave interactions as sources for nonmigrating tides.