Iwane Kimura

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.

VHF Doppler Radar Determination of the Momentum Flux in the Upper Troposphere and Lower Stratosphere: Comparison between the Three- and Four-Beam Methods

Abstract The vertical flux of horizontal momentum in the upper troposphere and lower stratosphere can be measured by VHF Doppler radars using the mesosphere-stratosphere-troposphere (MST) radar technique. Two methods have been used: one using three beams, one vertical and two oblique, and the other using four beams, two pairs of oblique beams symmetrically offset from the vertical. According to theory the four-beam method should be more accurate, but bemuse some radars do not have the capability of using the four-beam method, it is important to assess the accuracy of the three-beam method. In this study the rapid steerability of the Japanese MU radar was used to make three- and four-beam measurements simultaneously. It is found that the three-beam flux agrees with the four-beam flux only for long-period fluctuations. For shorter periods a systematic error is caused by wind fluctuations with wavelengths that are comparable with the separation between the beams (2–7 km in this study). In this study, perform...

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.

Estimation of electron temperature by VLF waves propagating in directions near the resonance cone

Abstract A ray tracing computer program for non-ducted whistler mode waves in a warm plasma in the magnetosphere is developed, where electron temperature effects are taken into account. The refractive index is calculated from the warm-plasma approximation and is used in the ray tracing after its accuracy has been checked by comparison with the hot-plasma solution without approximation. The ray paths do not depend appreciably on electron temperature. However, there are regions where the waves are heavily damped by Landau damping. By paying attention to this damping region, the electron temperature can be estimated from a satellite observation of the Doppler shift and damping of a ground-based VLF signal.

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.

ON OBSERVATIONS AND THEORIES OF THE VLF EMISSIONS.

Abstract Recent observations made on the ground and by satellites have revealed a variety of VLF emissions. In the first part of this paper, the observational results are reviewed. The intensity diurnal and seasonal variations are summarized paying particular attention to chorus and hiss, and characteristics of these two kinds of emissions are compared with other geomagnetic phenomena. Apparent differences are found between these two kinds of emissions suggesting that the generating mechanisms are greatly different. Characteristics of artificially stimulated emissions (ASE), one of which is offset phenomenon, and those of quasiperiodic emissions are described in detail. In the second part, theoretical interpretations for the mechanism of VLF emissions are surveyed. Generally, there are three main theoretical categories; non-coherent radiations, coherent beam wave interactions and radio emissions from a plasma wave. The first are radiations from drifting single particles, the intensity of which is, however, insufficient in comparison with that of the observed emissions. The second deals with instabilities for whistler wave mode which may arise at the gyroresonance condition in a plasma beam system and may be the most promising mechanisms for the generation of intense VLF emissions. The third consists of wide band radioe missions excited by hydromagnetic shock waves, which may be a mechanism of hiss. The frequency versus time spectra of each kind of VLF emissions are discussed, which are the crucial properties for classification of the emissions. It is concluded that the theories of beam wave interaction are not yet complete to the point of interpretation for every fine spectral structure. Theoretical interpretation for the offset phenomenon of ASE is proposed by the present author, which is based on cyclotron instability by an electron beam. The quasiperiodic emissions may be understood as being produced by a sequence of offset effects. Energies of the beam particles (e.g. electrons or protons) required for gyroresonance and the frequency of emissions to be generated are calculated for geocentric distances, under appropriate assumptions. Then the energies of the particles needed for the generation of chorus and hiss are evaluated.

Linear and nonlinear cyclotron instability and VLF emissions in the magnetosphere

Abstract A theoretical study is made of the whistler mode cyclotron instability both in linear and nonlinear regimes in conjunction with the generation of VLF emissions in the magnetosphere. For the nonlinear treatment, a well-established quasilinear method is used and some physical processes of the cyclotron instability viz. energy conservation, mechanism of instability and frequency change of the excited emissions are clarified. The results are applied to some types of the triggered VLF emissions; whistler triggered emissions and artificially stimulated emissions (ASE). It is found that whistler triggered emissions excited around the upper cutoff frequencies of whistlers may be explained by the whistler mode cyclotron instability by a model distribution function inferred from satellite data. In order to see a nonlinear evolution of the whistler mode cyclotron instability, computer simulations were carried out and it is shown that the change of frequency with time of whistler triggered emissions as well as characteristics of ASE are well explained by resonant nonlinear behaviour of whistler mode cyclotron instability considered in the present paper.

Collaborative experiments by Akebono satellite, Tromsø ionospheric heater, and European incoherent scatter radar

Joint experiments using the Akebono satellite and the ionospheric heating facility and European incoherent scatter radar near Tromso were carried out in November 1990 and February 1991. In these experiments, Tromso HF transmissions were amplitude modulated at frequencies of 2.5 and 4.0 kHz. Signals radiated from the polar electrojet (PEJ) antenna in the heated ionosphere at these VLF frequencies were detected for five passes out of seven passes of the semipolar orbiting Akebono satellite. A ground-based observation of the VLF radiation from the PEJ was made in the November campaign at Lycksele in Sweden, 554 km south of the heating facility. Measurements of electron density profiles and dc electric fields in the ionosphere were carried out by the EISCAT incoherent scatter radar [Folkestad et al., 1983] located in the vicinity of the ionospheric heater. The results of the experiments are compared with those obtained by ray tracing and full-wave analyses.

Full wave calculation of ELF/VLF propagation from a dipole source located in the lower ionosphere

A full wave technique has been developed to calculate the field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave is divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground is calculated by the full wave technique. Then the plane waves are summed up to give a horizontal distribution of the radiated wave intensities at any altitudes. This technique is applied to investigate the propagation characteristics of radiation from a polar electrojet (PEJ) antenna modulated at an ELF/VLF frequency, which was created by the Tromso heating facility. Comparison between the results of experiments and the calculation is given in the companion paper [Kimura et al., this issue]. We discuss several propagation characteristics, such as the frequency dependence of the radiated wave propagation up to the upper ionosphere and down to the ground. Under the conditions that the frequency is 2.5 kHz and the ionospheric dc electric field is 5 mV/m, the calculated results are roughly consistent with the experimental results and the radiation efficiency of the PEJ antenna is found to be extremely small, 2.5×10−6 upward and 3×10−8 downward.

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.