SM51C-0671
Variation of the cold plasma density structure above the polar ionosphere associated with geomagnetic storms
Plasma outflow from the polar ionosphere into the magnetosphere is one of the most important processes in the magnetosphere-ionosphere coupling in the polar region. Recent satellite observations have clarified that plasma outflow takes an important role for plasma transport into the magnetosphere, abrupt changes of the ring current ion composition, and the disappearance of the auroral acceleration region during geomagnetic storms. In the present study, we analyzed the electron density data observed by the Akebono satellite in an altitude range from 300 to 10500 km, in order to clarify the formation process of the plasma density enhancement above the polar ionosphere. The electron density along the satellite path was derived using the upper-hybrid resonance (UHR) frequency and maximum frequency of whistler-mode waves observed by the PWS instrument onboard the Akebono satellite with the time resolution of 2 seconds. In the present data analysis, we used the electron density data from March, 1989 to July, 1990 for statistical analyses. First, we investigate the statistically averaged density distributions during the quiet time in summer, equinox, and winter seasons. The data are sorted by day and night in magnetic local time, 5 degrees in invariant latitude and 100 km in altitude. The logarithmically averaged data in each bin are fitted by using the non-linear least square fitting method in altitude direction, using the equation of sum of the exponential and power law functions. Then, the fitted profiles are interpolated in ILAT direction by exponential functions. Finally, we obtain electron density distribution on the meridian plane. From comparing these distributions, it is identified that electron density in summer is 5 to 50 times larger than that in winter below 5000 km altitude in the polar cap and auroral zone. Next, we perform case studies for the geomagnetic storm events which occurred on June 6, June 9, 1989 and March 30, 1990. In these events, enhancements of the plasma density are identified in the entire polar cap being associated with the period of storm main phase. In June 7, the electron density enhanced up to 100 times larger than the quiet-time level. In March 30, the SMS instrument onboard the Akebono satellite observed upward flow of oxygen ions, in the electron density enhancement region. These results indicate that a large amount of the ionospheric plasma flows upward to at least about 10000 km in the polar magnetosphere during geomagnetic storms.
SM51C-0672
Observations of ELF Signals Over HAARP HF Ionospheric Heater and its Conjugate Region Aboard DEMETER Satellite
Generation of ELF waves by ionospheric HF heaters via modulation of the auroral electrojet current is well studied technique [Stubbe and Kopka, 1977]. The main knowledge about physical processes connected with ELF generation up to now has been acquired from theoretical treatment and ground-based observations. On the other hand, there were relatively few space observations [Platino et al., 2006 and references therein] that are important for understanding of ELF waves injection into the magnetosphere and their interaction with energetic particles. In this work we present observations made on the low-earth orbit DEMETER satellite (660 km) that recorded waveforms of E and B fields up to 20 kHz and the flux of energetic electrons in 256 channels from 70 keV to 2.5 MeV while the HAARP HF heater was operating at the full power level of 3600 kW. Observations over the HAARP revealed three regions of ELF field classified by signal intensity and the distance d between the satellite and the magnetic field line of the HAARP. The first big region within d~900 km is where the waves propagate in the Earth-ionosphere waveguide. The second is the region of waves propagating directly from the source in the ionosphere and within 20° cone (d~250 km) around the field line. Also not explained before the third small region of high field (E = 350 μV/m, B = 25 pT) within d≤50 km was observed. In addition, the first space observations of one-hop ELF signal over the HAARP conjugate point were accomplished. They show that the size of the region of detected signal can reach d~600 km while the field intensity is up to E = 4.5 μV/m. Besides the signal directly generated by the HAARP, triggered emissions with E up to 20 μV/m were simultaneously observed.
SM51C-0673
2-dimensional FDTD simulations of plasma wave propagations in the ionosphere
We developed a 2-dimensional FDTD simulation code which can treat wave propagations in magnetized plasma. Though we need to perform full particle simulations in order to recognize accurate characteristics of waves propagating in space plasma, FDTD simulations can be performed with much less computer resources than those necessary for full particle simulations, in memories as well as cpu times. Since space plasma is magnetized, it is necessary to incorporate the dielectric tensor with anisotropy and dispersibility in FDTD simulation code, in order to calculate the electromagnetic field in space plasma. We use PLRC method to formulization FDTD scheme to reduce numerical errors. In FDTD simulations, it is essential that how to realize an effective absorbing boundary. We developed PML absorbing boundary condition with anisotropy and dispersibility, and succeeded to realize very effective absorbind boundary. According to the rocket observations, we can receive MF radio wave above the dense ionospheric layer whose density is larger than those corresponding to cutoff frequency of MF radio wave. We consider that this is because the thickness of the ionopheric layer is smaller than the wavelength of MF radio wave, the density of ionospheric layer is not constant in the horizontal plane. We have been analyzing the characteristics of MF wave propagation in the ionospher with Full-wave method. In the Full-wave method, since the electron density profile is assumed to change in one-dimensional corrsponding to the alititude, we can only treat one-dimensional electron density profiles. In this study, therefore, we performed a series of FDTD simulations of MF wave propagations in ionospheres with several types of electron density distributions in the horizontal plane, such as electron dense cloud, sporadic layer, etc., and studied the relation between spatial scale of ionospheric layer and MF radio wavelength. In addition, we performed a FDTD simulation of MF radio wave propagations with the ionospheric layer model which is estimated by Full-wave analysis of S-310-37 sounding rocket observations. S-310-37 sonding rocket was launched at USC (Uchinoura Space Center, Kagoshima) in Jan. 2007. We are going to compare FDTD simulation results, Full-wave analysis and rocket observations, and study the influence of electron density profile on the propagation characterictics of MF radio wave in the ionosphere.
SM51C-0674
Linear Theory of Waves in Space Plasmas Modeled by Lorentzian (Kappa) Distribution
Resonant wave-particle interaction plays an essential role in the growth or damping of many plasma waves in collisionless plasma. It also dominates the collective processes that produce anomalous transport of density, momentum and energy due to plasma turbulence. The resonant interaction involves particles that are moving with the speed of the plasma wave. Space plasmas, e.g., plasma in the Earth's magnetosphere and the solar wind, as well as astrophysical plasmas are characterized by particle distribution function, such as the Lorentzian (or kappa) distribution function, which has power-law energy dependence above the thermal energy. The presence of a substantially larger number of suprathermal particles in such a distribution, in contrast with a Maxwellian distribution, can significantly change the rate of growth or damping of plasma waves, as well as the rate of anomalous transport processes that rely on resonant wave-particle interaction. In this paper we present the linear theory of waves in current-carrying, inhomogeneous magnetoplasma in which the equilibrium state is modeled by the anisotropic kappa distribution function. The dispersion relation including the effects of the spatial gradients of density, temperature and flow velocity as well as those of the temperature anisotropy is derived. Such a comprehensive dispersion relation is useful for the study of the various plasma waves that can be excited in space plasma situation. The dispersion relation is used to obtain the excitation conditions and the growth rates of a few interesting plasma waves, and the results are compared and contrasted with those in Maxwellian plasma.