Eiichi Sagawa

Control of equatorial ionospheric morphology by atmospheric tides

[1] A newly discovered 1000-km scale longitudinal variation in ionospheric densities is an unexpected and heretofore unexplained phenomenon. Here we show that ionospheric densities vary with the strength of nonmigrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics. A strong connection between tropospheric and ionospheric conditions is unexpected, as these upward propagating tides are damped far below the peak in ionospheric density. The observations can be explained by consideration of the dynamo interaction of the tides with the lower ionosphere (E-layer) in daytime. The influence of persistent tropical rainstorms is therefore an important new consideration for space weather. Citation: Immel, T. J., E. Sagawa, S. L. England, S. B. Henderson, M. E. Hagan, S. B. Mende, H. U. Frey, C. M. Swenson, and L. J. Paxton (2006), Control of equatorial ionospheric morphology by atmospheric tides, Geophys. Res. Lett., 33, L15108, doi:10.1029/2006GL026161. [2] The ionosphere is the region of highest plasma density in Earth’s space environment. It is a dynamic environment supporting a host of plasma instability processes, with important implications for global communications and geo-location applications. Produced by the ionization of the neutral atmosphere by solar x-ray and UV radiation, the uppermost ionospheric layer has the highest plasma density with a peak around 350–400 km altitude and primarily consists of O + ions. This is called the F-layer and it is considered to be a collisionless environment such that the charged particles interact only weakly with the neutral atmosphere, lingering long after sunset. The E-layer is composed of molecular ions and is located between 100–150 km where collisions between ions and neutrals are much more frequent, with the result that the layer recombines and is reduced in density a hundredfold soon after sunset [Rees ,1 989;Heelis, 2004]. The respective altitude regimes of these two layers are commonly called the E- and F-regions. [3] The ionosphere glows as O + ions recombine to an excited state of atomic oxygen (O I) at a rate proportional to

EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° invariant latitude, ILAT) above the collision-dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low-energy (a few eV) upward ion flows in the high-altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field-aligned and cold (Ti 80° ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s, respectively. Both the ion velocity and temperature distributions exhibited a day-to-night asymmetry, with higher average values on the dayside than on the nightside.

Effect of atmospheric tides on the morphology of the quiet time, postsunset equatorial ionospheric anomaly

longitudinal wave number four pattern in the magnetic latitude and concentration of the F region peak ion density when measured at a fixed local time. In a new comparison of two data sets with observations made by the OGO 4 satellite, this pattern is seen to be persistent over many days around equinox during magnetically quiet conditions close to solar maximum but can be dominated by other processes such as cross-equator winds during other periods. It is found that the longitudinal variability is created by a processes occurring in the dayside ionosphere. A longitudinal modulation of the dayside equatorial fountainisthemostlikelydrivingmechanism.ThroughcomparisonwithGWSM-02model,it isshownthatthepredictedmodulationofthedaysidethermosphericwindsandtemperaturesat E region altitudes created by non-migrating diurnal tides can explain the modulation in the dayside equatorial fountain. This result highlights the importance of understanding the temporal variability of tropospheric weather systems on our understanding and possible predictability of the development of the F region ionosphere. It may also provide a possible further means of testing our understanding of atmospheric tides on a global scale.

Altitude profile of the polar wind velocity and its relationship to ionospheric conditions

The authors report recent results from the Akebono satellite. They present data on polar wind velocities, examined in conjunction with electron properties, as a function of altitude in the ionosphere. This data came from the Suprathermal ion Mass Spectrometer and the Thermal Electron energy Distribution instruments. The measurements show a vertical component to the polar wind, consistent with model results, when measured in terms of H[sup +] ions. There was a definite altitude dependence of the velocity of the hydrogen ions, and there was also a positive correlation of this velocity with the measured electron temperature.

Vertical wind observations with two Fabry‐Perot interferometers at Poker Flat, Alaska

Characteristics of vertical winds in the polar thermospheric region were examined using data sets generated with two types of Fabry-Perot interferometers at Poker Flat, Alaska (65.11°N, 147.42°W). The Communications Research Laboratory Fabry-Perot Interferometer (CRLFPI) simultaneously observed the O I 557.7 nm and O I 630.0 nm emissions, whereas the Geophysical Institute Scanning Doppler-Imaging Interferometer (GI-SDI) observed the O I 630.0 nm emission. The height of the O I 557.7 nm and O I 630.0 nm emissions were 100–140 and 200–240 km, respectively. The data were obtained from October 1998 to February 1999, and our findings were as follows: (1) Observations of the O I 630.0 nm emission showed that upward (downward) vertical winds were often present when bright aurora existed equatorward (poleward) of the observatory. This is consistent with previous studies [Crickmore et al., 1991; Innis et al., 1996, 1997]. (2) Comparison of vertical winds estimated from two different wavelengths (557.7 and 630.0 nm) showed that vertical winds were often observed simultaneously at both wavelengths, as reported by Price et al. [1995]. However, the vertical winds observed at different heights sometimes had different features when thin but bright aurora passed over the observatory. A similar observation was reported by Ishii et al. [1999]. (3) Vertical winds were often observed along with divergence and rotation of the horizontal wind field. Some vertical winds not associated with active aurora may be driven by the divergence in the horizontal wind.

EXOS D (Akebono) observations of molecular NO+ and N2 + upflowing ions in the high‐altitude auroral ionosphere

The authors report on observation of molecular ion drift upward in the ionosphere in regions near to the cleft or auroral oval. In particular they observed NO{sup +}, N{sub 2}{sup +}, and even O{sub 2}{sup +} molecular ion flow upward. These fluxes were typically 5 to 15% of the total ion flux. Molecular ion drift is not observed in all passes through the polar region, and seems to correlate with periods of more intense activity. Their observations are compared with, and correlated with other observations at lower altitudes.

Low-energy charged particle measurement by MAP-PACE onboard SELENE

MAP-PACE (MAgnetic field and Plasma experiment-Plasma energy Angle and Composition Experiment) is one of the scientific instruments onboard the SELENE (SELenological and ENgineering Explorer) satellite. PACE consists of four sensors: ESA (Electron Spectrum Analyzer)-S1, ESA-S2, IMA (Ion Mass Analyzer), and IEA (Ion Energy Analyzer). ESA-S1 and S2 measure the distribution function of low-energy electrons below 15 keV, while IMA and IEA measure the distribution function of low energy ions below 28 keV/q. Each sensor has a hemispherical field of view. Since SELENE is a three-axis stabilized spacecraft, a pair of electron sensors (ESA-S1 and S2) and a pair of ion sensors (IMA and IEA) are necessary for obtaining a three-dimensional distribution function of electrons and ions. The scientific objectives of PACE are (1) to measure the ions sputtered from the lunar surface and the lunar atmosphere, (2) to measure the magnetic anomaly on the lunar surface using two ESAs and a magnetometer onboard SELENE simultaneously as an electron reflectometer, (3) to resolve the Moon-solar wind interaction, (4) to resolve the Moon-Earth’s magnetosphere interaction, and (5) to observe the Earth’s magnetotail.

Minor ion composition in the polar ionosphere

Ion composition measurements from the EXOSD Suprathermal Ion Mass Spectrometer (SMS) are presented. Ions other than H+, notably O+, He+, O++ N+ and N++, are found to constitute a significant (>0.1) and at times dominant (>0.5) component of the thermal ion population in the high-altitude polar ionosphere. Their relative abundance and occurrence are highly variable. Ion flux ratios in the range of 0.1–0.5 are typical for 0+/H+, 0.1–0.3 for He+/H+, 0.1–0.3 for 0++/O+, and 0.05–0.1 for N++/N+. Our observations show that (1) ions other than H+, notably He+, O+, O++ and N+, often constitute a significant component (>0.1) of the thermal ion population in the high-altitude polar ionosphere; (2) doubly charged oxygen and nitrogen (O++ and N++) are sometimes present with fluxes up to 0.1 of the singly-charged (O+ and N+) ion fluxes; and (3) the He+ and O++ fluxes are sometimes comparable to the H+ and O+ fluxes.

Simultaneous ground- and satellite-based airglow observations of geomagnetic conjugate plasma bubbles in the equatorial anomaly

We compare, for the first time, geomagnetically-conjugate plasma bubbles observed by ground-based OI 630.0-nm all-sky imagers at Shigaraki, Japan (34.8°N, 136.1°E; magnetic latitude 25.4°N) and Darwin, Australia (12.4°S, 131.0°E; magnetic latitude 22.0°S), with global-scale plasma structures (≈10,000 km in longitude) in the equatorial anomaly simultaneously detected with an OI 135.6-nm imager on the IMAGE satellite at ≈7 earth radii. As found previously, global-scale plasma structures in both hemisphere imaged by IMAGE consist of an array of geomagnetically-conjugate small- to medium-scale (a few hundreds to 1000 km in longitude) wavy structures that move to the east at ≈100 ms−1. We find the following: 1) plasma bubbles detected with the allsky imagers reach an apex altitude of ≈1800 km over the geomagnetic equator while moving to the east at ≈100 m s−1 with spacings of 200–250 km. 2) Bubbles observed with the all-sky imagers and IMAGE are embedded within the small- to medium-scale wavy structures, and some of them are located near the crest of an enhanced electron density region associated with the wavy structures. 3) The bubbles and wavy structures that are generated near sunset slant to the west with increasing latitude in both hemispheres, and tilts do not change with longitude (i.e., local time). The results suggest that the generation and evolution of plasma bubbles are closely related to those of the small- to medium-scale plasma structures.

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.