Yoshiya Kasahara

Propagation characteristics of the ELF emissions observed by the satellite Akebono in the magnetic equatorial region

Emissions with their frequencies below 100 Hz are often observed by the Akebono satellite in the vicinity of the geomagnetic equatorial plane. These ELF emissions are classified into two types: One is an ion cyclotron wave below the local proton cyclotron frequency and the other is assumed to be magnetosonic wave observed not only below but also above the local proton cyclotron frequency. The wave normal directions of the latter type of emissions were estimated by using the wave distribution function method. It is found that the emissions are propagating with their wave normal direction nearly perpendicular to the meridian plane. The propagation characteristics of these emissions are also examined by ray tracing including the effects of ions. The ray tracing study clarified that the wave can propagate around the plasmapause because of the trapping effect of density gradient. In this paper we propose that these emissions are generated just outside the plasmapause and are propagating around the plasmapause with their wave normal nearly perpendicular to the geomagnetic field line and to the meridian plane.

Lunar Radar Sounder Observations of Subsurface Layers Under the Nearside Maria of the Moon

Observations of the subsurface geology of the Moon help advance our understanding of lunar origin and evolution. Radar sounding from the Kaguya spacecraft has revealed subsurface layers at an apparent depth of several hundred meters in nearside maria. Comparison with the surface geology in the Serenitatis basin implies that the prominent echoes are probably from buried regolith layers accumulated during the depositional hiatus of mare basalts. The stratification indicates a tectonic quiescence between 3.55 and 2.84 billion years ago; mare ridges were formed subsequently. The basalts that accumulated during this quiet period have a total thickness of only a few hundred meters. These observations suggest that mascon loading did not produce the tectonics in Serenitatis after 3.55 billion years ago. Global cooling probably dominated the tectonics after 2.84 billion years ago.

Instrumentation and observation target of the Lunar Radar Sounder (LRS) experiment on-board the SELENE spacecraft

The Lunar Radar Sounder (LRS) on-board the SELENE lunar orbiter is currently being equipped to provide the data of subsurface stratification and tectonic features in the shallow part (several km deep) of the lunar crust, by using an FM/CW radar technique in HF (∼5 MHz) frequency range. Knowledge of the subsurface structure is crucial to better understanding, not only of the geologic history of the Moon, but also of the Moon’s regional and global thermal history of the Moon and of the origin of the Earth-Moon system. In addition to the subsurface radar experiment, LRS will provide the spectrum of plasma waves and solar and planetary radio waves in a wide frequency range from 10 Hz to 30 MHz. This paper provides the basic function parameter of the LRS system based on the final function test and proposes observation targets and data analysis that will provide important information leading to a greater understanding of the tectonics and thermal history of the Moon.

Ion cyclotron emissions observed by the satellite Akebono in the vicinity of the magnetic equator

Ion wave observations by a wave measuring instrument named ELF on board the Akebono satellite, interesting ELF emissions in the frequency ranges above the cyclotron frequencies of He(+) and/or O(+) ions have been detected. It is proposed that these emissions are due to electromagnetic ion cyclotron waves, which might have been generated by the ion cyclotron resonant instability due to a temperature anisotropy of hot H(+) ions, trapped along the magnetic field lines around the geomagnetic equatorial plane. In order to confirm the characteristics of these ELF emissions, statistical characteristics, refractive index, and wave normal direction of the emissions are first estimated. The propagation characteristics of these ELF emissions are examined by ray tracing for ion cyclotron waves in the magnetosphere using both cold and hot plasma models. 18 refs.

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

On the sources of energization of molecular ions at ionospheric altitudes

During geomagnetically active times, the suprathermal mass spectrometer on the Akebono satellite frequently observes upflowing molecular ions (NO+, N2+, O2+) in the 2-3 Earth radii geocentric distance regions in the auroral zone. Molecular ions originating at ionospheric altitudes must acquire an energy of the order of 10 eV in order to overcome gravitation and reach altitudes greater than 2 RE. This energy must be acquired in a time short compared with the local dissociative recombination lifetime of the ions; the latter is of the order of minutes in the F region ionosphere (300-500 km altitude). Upflowing molecular ions thus provide a test particle probe into the mechanisms responsible for heavy ion escape from the ionosphere. In this paper we analyze the extensive complement of plasma, field, and wave data obtained on the Akebono satellite in a number of upflowing molecular ion events observed at high altitudes (5000 - 10,000 km). We use these data to investigate the source of energization of the molecular ions at ionospheric altitudes. We show that Joule heating and ion resonance heating do not transfer enough energy or do not transfer it fast enough to account for the observed fluxes of upflowing molecular ions. We found that the observed field-aligned currents were too weak to support large-scale field-aligned current instabilities at ionospheric altitudes. The data suggest but in the absence of high-resolution wave measurements in the 300 to 500 km altitude range cannot ascertain the possibility that a significant fraction of escape energy is transferred to molecular ions in localized regions from intense plasma waves near the lower hybrid frequency. We also compared the energization of molecular ions to that of the geophysically important O+ ions in the 300 to 500 km altitude range, where the energy transfer to O+ is believed to occur via small-scale plasma instabilities, ion resonance, and ion-neutral frictional heating. Direct observation of energy input to the ionosphere from all of these sources in combination with in situ measurements of the density and temperature of neutral and ionized oxygen in the 300 to 500 km range are required to determine the relative importance of these energy sources in providing O+ with sufficient energy to escape the ionosphere.

Inner belt and slot region electron lifetimes and energization rates based on AKEBONO statistics of whistler waves

Global statistics of the amplitude distributions of hiss, lightning-generated, and other whistler mode waves from terrestrial VLF transmitters have been obtained from the EXOS-D (Akebono) satellite in the Earths plasmasphere and fitted as functions of L and latitude for two geomagnetic activity ranges (Kp 3). In particular, the present study focuses on the inner zone L∈[1.4,2] where reliable in situ measurements were lacking. Such statistics are critically needed for an accurate assessment of the role and relative dominance of each type of wave in the dynamics of the inner radiation belt. While VLF waves seem to propagate mainly in a ducted mode at L∼1.5–3 for Kp 3). Hiss waves are generally the most intense in the inner belt, and lightning-generated and hiss wave intensities increase with geomagnetic activity. Lightning-generated wave amplitudes generally peak within 10° of the equator in the region L<2 where magnetosonic wave amplitudes are weak for Kp<3. Based on this statistics, simplified models of each wave type are presented. Quasi-linear pitch angle and energy diffusion rates of electrons by the full wave model are then calculated. Corresponding electron lifetimes compare well with decay rates of trapped energetic electrons obtained from Solar Anomalous and Magnetospheric Particle Explorer and other satellites at L∈[1.4,2].

Plasma Waves Observed During CUSP Energetic Particle Events and Their Correlation With Polar and Akebono Satellite and Ground Data

We present Polar plasma wave data during cusp energetic particle (CEP) events at 6–9 RE. These data suggest the presence of coherent electrostatic structures that are highly localized and that have typical velocities on the order of hundreds to thousands of km/s along the ambient magnetic field. Some of the wave signatures are solitary waves and some are wave packets. The Polar wave instrument also provides evidence that some of the bursts of electromagnetic waves (with frequencies of a few hundred Hz and just below the electron cyclotron frequency around 800 Hz to 1–2 kHz) that are observed are cohenrent and propagating both up and down the field lines. Electron cyclotron harmonic (ECH) waves are oftern detected but their duration is usually short (< 1 s). Low Frequency (<1 kHz), broadband, bursty electromagnetic waves are also present. The Polar wave data results are used to obtain a better understanding of the macro/microphysics during a CEP event that takes place on September 11, 1996, by correlating various Polar (∼ 7.0 RE) and Akebono (∼ 1.4 RE) data while both spacecraft are in or near the cusp/cleft region and nearly on the same field line, and magnetometer data from the Canadian Intermagnet and Canopus ground stations, which lie near the base of the magnetic pootprint passing through Polar. Solar wind and magnetic field data from the interplanetary medium and magnetosheath are provided by the Geotail and IMP-8 satellites, respectively. Some of the cusp waves may be indicators of the reconnection process taking place through the cusp, the result of mixing of magnetosheath with magnetospheric plasma, and the consequence of an anisotropic electron population in a depressed magnetic field. The low frequency electromagnetic waves are still under study to determine their role, if any, in the heating and acceleration of the MeV He ions during CEP events.

ELF/VLF waves correlated with transversely accelerated ions in the auroral region observed by Akebono

Plasma waves observed by the Akebono satellite in the region of ion heating/acceleration transverse to the geomagnetic field line are studied. Especially, electrostatic, broadband low-frequency noise is closely correlated with transversely accelerated ions. Simultaneous electron precipitation in the energy ranges from a few tens to hundreds eV is also usually observed. Our waveform analysis revealed that the electrostatic broadband noise is classified into two types of noise: one is continuous noise with upper cutoff around a few kilohertz, and the other is an intermittent impulsive waveform extended more than 10 kHz. A possible explanation to the dominant part of continuous broadband noise is that they are ion acoustic waves generated by precipitating electrons (approximately a few hundreds eV) and that the wave is the possible main energy source of ion heating/acceleration. The other mechanisms may, however, be sometimes important for generating the broadband noise. Using long-period observation data sets of Akebono, which is orbiting in the altitude range between 270 and 10,000 km, statistical studies on the spatial and temporal distribution of the continuous broadband noise are made. Several kinds of statistical features are clarified: (1) The occurrence region is distributed in the cusp and along the auroral oval, (2) the region is extended toward the lower latitude while the geomagnetic activity is higher, (3) the intensity is larger in the cusp than it is in the nightside, and (4) it is largest in winter and weakest in summer.

Plasma wave observation using waveform capture in the Lunar Radar Sounder on board the SELENE spacecraft

The waveform capture (WFC) instrument is one of the subsystems of the Lunar Radar Sounder (LRS) on board the SELENE spacecraft. By taking advantage of a moon orbiter, the WFC is expected to measure plasma waves and radio emissions that are generated around the moon and/or that originated from the sun and from the earth and other planets. It is a high-performance and multifunctional software receiver in which most functions are realized by the onboard software implemented in a digital signal processor (DSP). The WFC consists of a fast-sweep frequency analyzer (WFC-H) covering the frequency range from 1 kHz to 1 MHz and a waveform receiver (WFC-L) in the frequency range from 10 Hz to 100 kHz. By introducing the hybrid IC called PDC in the WFC-H, we created a spectral analyzer with a very high time and frequency resolution. In addition, new techniques such as digital filtering, automatic filter selection, and data compression are implemented for data processing of the WFC-L to extract the important data adequately under the severe restriction of total amount of telemetry data. Because of the flexibility of the instruments, various kinds of observation modes can be achieved, and we expect the WFC to generate many interesting data.