Nobuyuki Kaya

On the evolution of ion conics along the field line from EXOS D observations

The altitude dependence of ion conics is investigated by using EXOS D observations on the dayside below 10,000 km altitude. The cone angle of ion conics tends to decrease with increasing altitude, but not so much as expected from a simple adiabatic model. The conic temperature, on the other hand, tends to increase with increasing altitude. The occurrence frequency of ion conics increases with altitude below 6000 km but is approximately constant above 6000 km. The appearance of newly born conics and the extinction of old conics in the statistics at any altitude could make some contribution, if the appearance and the extinction are large enough, to the observation results: less significant change in cone angle and increasing temperature with altitude, but this effect alone hardly provides a full explanation for the differences in the conic characteristics between the observations and the simple adiabatic model. The results rather seem to reflect the real evolution of an ion conic as ions flow up along the field line, suggesting nonconservation of the adiabatic invariant and the height-integrated transverse acceleration of ion conics over a wide range of altitude.

Electric field fluctuations and charged particle precipitation in the cusp

In the cusp region, irregular fluctuations of the electric field are often observed by EXOS D at altitudes of several thousands of kilometers. Amplitudes of the fluctuations sometimes reach 100 mV/m and their spectra are broad. The electric to magnetic field ratios in the frequency range of 0.5-3 Hz agree well with the Alfven velocity at observation point. Hence the electric fluctuations are considered to be Alfven waves. The flux of precipitating ions at 500 eV to 10 keV and that of the electrons at 70-500 eV are enhanced in the cusp and they are well correlated to each other. On the other hand, correlation coefficients between the power spectral density (PSD) of the electric field at 1 Hz and the precipitating particle flux vary from case to case. When the latitude of the cusp is low and IMF is expected to be southward the coefficient is high. This suggests that the waves are generated in association with the injection of particles into the magnetosphere when reconnection occurs. On the other hand, when the latitude of the cusp is high and IMF is expected to be northward, the coefficient is low and the PSD of the electric field is smaller for the same flux of particles than when IMF is southward. In these cases the intensity of the electric fluctuations in the region of the particle injection is possibly not so great as that when reconnection occurs.

Two types of ion energy dispersions observed in the nightside auroral regions during geomagnetically disturbed periods

The Akebono satellite has observed two types of energy dispersion signatures of discrete ion precipitation event in the nightside auroral regions during active geomagnetic conditions. The charged particle experiments and electric and magnetic field detectors on board Akebono provide us with essential clues to characterize the source regions and acceleration and/or injection processes associated with these two types of ion signatures. The magnetic field data obtained simultaneously by the geosynchronous GOES 6 and 7 satellites and the ground magnetograms are useful to examine their relationships with geomagnetic activity. Mass composition data and pitch angle distributions show that different sources and processes should be attributed to two types (Types I and II) of energy dispersion phenomena. Type I consists of multiple bouncing ion clusters constituted by H+. These H+ clusters tend to be detected at the expansion phase of substorms and have characteristic multiple energy-dispersed signatures. Type II consists of O+ energy dispersion(s), which is often observed at the recovery phase. It is reasonable to consider that the H+ clusters of Type I are accelerated by dipolarization at the equator, are injected in the field-aligned direction, and bounce on closed field lines after the substorm onset. We interpret these multiple energy dispersion events as mainly due to the time-of-flight (TOF) effect, although the convection may influence the energy-dispersed traces. Based on the TOF model, we estimate the source distance to be 20–30 RE along the field lines. On the other hand, the O+ energy dispersion of Type II is a consequence of reprecipitation of terrestrial ions ejected as an upward flowing ion (UFI) beam from the upper ionosphere by a parallel electrostatic potential difference. The O+ energy dispersion is induced by the E × B drift during the field-aligned transport from the source region to the observation point.

On the origins of the upward shift of elevated (bimodal) ion conics in velocity space

We present a statistical study of elevated (bimodal) ion conics observed by the Exos D (Akebono) satellite from 0900 to 1500 magnetic local time (MLT). We especially focus on a comparative analysis of elevated conics with standard conics and ion beams. Elevated conics are observed mainly above 6,000 km, while standard conics begin to appear at lower altitudes. The energy of elevated conics is usually higher than that of standard ion conics. This is consistent with the idea that the elevated conics evolve from standard conics through some acceleration process while they flow up to high altitudes. Since the elevated conics studied here received additional energization as they evolve from standard conics, the velocity filter mechanism previously invoked to explain elevated conics is inconsistent with our results. Our observation suggests that two kinds of acceleration processes are responsible for the upward shift in the velocity distribution of elevated conics. The elevated conics with a large cone angle and small upward shift in velocity space are most likely to be caused by the perpendicular energization extended along the field line. The MLT occurrence of this subset of elevated conics is similar to that of standard conics. The ion conics with a cone angle near 60° are found to be the most energetic, which strongly suggests the extended energization. On the other hand, the elevated conics with a small cone angle and large upward shift are mainly due to the parallel acceleration acting in tandem with perpendicular energization. This subset of elevated conics is found more often in the afternoon sector than are standard conics, which evokes rather some similarity to ion beams.

Unexpected features of the ion precipitation in the so‐called cleft/low‐latitude boundary layer region: Association with sunward convection and occurrence on open field lines

Ion precipitations along the boundary of the polar cap are studied using the Akebono (EXOS D) observations of plasma, electric field and magnetic field during the dawn-dusk crossings across the dayside polar cap at the altitudes of 7500 km to 10,000 km. Only the cases where the convection streamlines consisted of two cells rather than four have been chosen for analysis, and hence the Kp index was moderate or high and the IMF polarity was southward in most of the intervals studied. It is found that the ion precipitations in morning and evening sectors along the polar cap boundary are associated with the sunward convection at the above heights, and judging from the presence of the polar rain the high-latitude part of these precipitations occur in the region of open field lines. Association with the sunward convection and occurrence on open field lines contradict the view that these ions originate from the low-latitude boundary layer (LLBL) on closed field lines. We propose to designate these ions as circumpolar ion precipitation (CPIP) in order to avoid questionable presumptions on their source region. The sunward convection in the morning and evening sectors of the CPIP is observed systematically, has speeds of about 1 km/s, and often extends over a few degrees or more in invariant latitude. The CPIP ions may have leaked from the boundary region of the plasma sheet, may have been generated in the distant neutral sheet, or may have entered from the magnetosheath. Only within a few hours of the noon meridian the CPIP ions are associated with the antisunward flow at the height range of the Akebono observations.

Distribution function of precipitating ion beams with velocity dispersion observed near the poleward edge of the nightside auroral oval

Using low energy ion data obtained by Akebono satellite, we have calculated distribution functions of velocity-dispersed ion beams observed at the poleward edge of the auroral electron precipitation region. The calculated distribution functions can well be fitted by one-dimensional shifted-Maxwellians, whose bulk energy and temperature are several keV and several hundreds of eV, respectively. The bulk energy and temperature show a positive correlation, which may indicate that when the ions are accelerated to higher energy, they are heated to higher temperature simultaneously. We have also found a relation between the invariant latitude width of the observed ion beams divided by the square root of the temperature and their bulk velocity, which indicates that the source region of the ion beam is compact. These ion beams are obtained with high occurrence probability, suggesting that they are supplied from a steady X-type neutral line in the earths magnetotail.

Monoenergetic ion drop‐off in the inner magnetosphere

A monoenergetic drop-off of ions around 10 keV, which we term as “ion drop-off band” (IDB) in this paper, has been observed by Akebono. The IDB is identified as a sharp and deep dip at about 10 keV in ion spectra, which is usually observed at latitudes below the discrete auroral region over several degrees or more. As the ion motion of this energy in this inner part of the magnetosphere is basically described by the adiabatic theory, we have numerically traced the ion drift trajectories. From the results, it is proposed that the lower-energy boundary of the drop-off demarcates the open/closed character of the drift orbits, only below which continuous supply from the magnetotail is present. This model explains the energy, local time, and latitudinal extent of IDB as well as the formation of its poleward edge very well.

Meridional structures of electric potentials relevant to premidnight discrete auroras: A case study from Akebono measurements

The relation between latitudinal structures in particles and electric/magnetic field is studied along the premidnight meridian from the polar cap to low latitudes. Observations were made by the Akebono (EXOS D) satellite at altitudes of about 10,000 km on several successive orbits on February 7, 1990, when the magnetic activity was high (Kp = ∼4) and a series of substorms took place. Combining the particle and the electric field measurements, height-latitude structures of electric potentials relevant to premidnight discrete auroras are derived. The UV images of aurora are used to monitor the structure of precipitating electrons at the ionospheric heights. A large-scale minimum of electric potential, where the convection velocity rotates clockwise, corresponded to a discrete aurora that delineated the poleward limit of auroras recorded by the Akebono UV imager. The poleward boundary of the plasma sheet, identified by the particle measurements, did not coincide with this clockwise rotation, and was located at a few degrees further poleward. A weaker but distinct potential maximum associated with a counterclockwise rotation of the flow was observed in the boundary region of the plasma sheet where accelerated electrons with intense lower-energy (E<1keV) populations were present, but their energy flux at the ionospheric heights is too low (<1erg s−1cm−2 sr−1) to produce bright auroras. Upward flowing ion (UFI) conics and sometimes ∼10-keV ions with a clear dispersion (where energy fell down toward lower latitudes) were observed in this boundary region of the plasma sheet. Thus the upward field-aligned electric field should exist above the satellite level, and no significant acceleration took place below it. In the region of the potential minimum and the clockwise rotation, accelerated electrons were observed which had a “hole” distribution (characterized by the absence in the low - energy component), and UFI beams were also seen. This indicates that the upward field-aligned electric field existed both above and below the satellite. Because of this electric field the energy flux of the electrons became high enough to produce the discrete aurora. These observational facts suggest that electric potentials in the premidnight auroral zone are more structured in heights and latitudes during substorms. That is, in the boundary region of the plasma sheet (the region of potential maximum) the field-aligned acceleration takes place at the higher altitudes. On the lower-latitude side (the region of potential minimum), the acceleration occurs at both the higher and the lower altitudes. The latter corresponds to the region of maximum auroral intensity. In the boundary region of the plasma sheet, the magnetospheric electrons are accelerated by the higher-altitude electric field only, and would be mostly reflected back by the magnetic mirror force. On the other hand, the electrons in the lower latitudes are further accelerated at the lower altitudes and could sufficiently account for bright auroral emissions. Thus the most poleward discrete aurora does not necessarily project to the plasma sheet boundary layer (PSBL), and could be situated significantly closer to Earth than previously expected.

Multiple energy‐dispersed ion precipitations in the low‐latitude auroral oval: Evidence of E × B drift effect and upward flowing ion contribution

The polar-orbiting satellites, Akebono and DMSP F8, have frequently observed energy-dispersed ion precipitation events in the low-latitude auroral oval. The general properties of these precipitations are the following: (1) the characteristic energy decreases with decreasing latitude, (2) the signatures appear simultaneously with the diffuse components of electrons and ions on closed field lines, and (3) multiple (overlapped) signatures of the energy-dispersed ion precipitations are sometimes observed simultaneously in the northern and the southern hemispheres. These precipitation events are classified into two types: type A and type B according to their energy ratios and mass composition. The ratios between the energies of the type A multiple energy-dispersed signatures are constants 1:32:52:72 or 1:22 (“odd” or “natural” integer cases). These odd or natural integers indicate the ratios of the flight distances from the source region to the observation point of ion clusters consisting mainly of a single ion species. There are two possible sources: upward flowing ions (UFIs) from the ionosphere and bidirectional ion injection in the equatorial region. The latter model, however, is consistent only with the odd-integer case. The triple energy-dispersed signatures (type B) consist of three ion species (H+, He+, O+), and the ratios of the energies of these signatures always correspond to the mass ratios; that is, E(H+):E(He+):E(O+) = M(H+);M(He+):M(O+) = 1:4:16. This relationship indicates that these multicomposition ion clusters of ionospheric origin have the same flow velocity and the source distance. We conclude that the source of these two types of the energy-dispersed ion precipitations are UFIs ejected from the ionosphere, The ions of UFI origin are velocity-filtered (energy-dispersed), dominated by the E × B drift on closed field lines, and reenter the ionosphere as downward flowing ions. The pitch angle distributions indicate that parallel acceleration through an electrostatic potential producing the UFI beams is the most probable acceleration process.

Characteristics of downward flowing ion energy dispersions observed in the low-altitude central plasma sheet by Akebono and DMSP

We present characteristics of downward flowing ion (DFI) energy dispersions observed in the low-altitude central plasma sheet by two polar-orbiting satellites: Akebono and DMSP F8. In general, the typical energy decreases with decreasing latitude. Their main ion composition frequently consists of singly charged oxygen, suggesting that the energy dispersing ions are of ionospheric origin. We conclude that the energy-dispersed signature represents a spatial structure, produced mainly by the global plasma convection driven by the E × B drift on the closed field lines. A probable source of these DFI energy dispersions is the upward flowing ion (UFI) beam accelerated by parallel electrostatic potential at low altitudes. Some case studies show that the energy ratios of the multiple (overlapped) energy dispersions and the pitch angle distributions have some important characteristics consistent with features expected from this model. Clear DFI energy dispersions are observed frequently in stable eastward convection in the postmidnight region, rather than in the premidnight sector. Although the occurrence frequency is different from the local time dependence of the UFIs as reported previously, this discrepancy is due probably to some differences of the convection patterns in the premidnight and postmidnight sectors. The clear energy dispersions might be smeared out by the distorted or complicated convection pattern, irregularities, and disturbances frequently seen in the premidnight sector.