Shigeto Watanabe

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.

Zonal winds in the equatorial upper thermosphere: Decomposing the solar flux, geomagnetic activity, and seasonal dependencies

Using 3 years (2002–2004), over 16,400 orbits of measurements from the accelerometer on board the CHAMP satellite, we have studied the climatology of the equatorial zonal wind in the upper thermosphere. Several main features are noticed. The most prominent one is that the solar flux significantly influences both the daytime and nighttime winds. It overrides the geomagnetic activity effect, which is found to be rather limited to the nightside. An elevation of the solar flux level from F10.7 ? 100 × 10?22 W m?2 Hz?1 to F10.7 ? 190 × 10?22 W m?2 Hz?1 produces an eastward disturbance wind up to ?110 m s?1. This consequently enhances the nighttime eastward wind but suppresses the daytime westward wind. A seasonal variation with weaker wind (by over 50 m s?1 at night) around June solstice than in other seasons has been observed regardless of solar flux and geomagnetic activity levels. The zonal wind is eastward throughout the night except around June solstice, where it ebbs to almost zero or turns even westward in the postmidnight sector at low solar flux level. The daytime wind is found to be generally more stable than the nighttime wind, particularly unresponsive to geomagnetic activities. Predictions from the Horizontal Wind Model find good agreement with the CHAMP?observed wind at high solar flux levels during nighttime. At low solar flux levels, however, the model strongly underestimates the westward wind during morning hours by 50–120 m s?1 depending on season. The major difference between the HWM?predicted and the CHAMP?observed wind is seen in the phase of its diurnal variation. The CHAMP?observed wind turns eastward around 1200–1300 MLT instead of 1600–1700 MLT predicted by the model. Comparisons with ground FPI observations and the NCAR Thermosphere?Ionosphere?Electrodynamics General Circulation Model (TIEGCM) predictions show that the solar flux effect obtained from CHAMP is consistent with that modeled by TIEGCM. The solar flux dependence of zonal wind found here together with that of the zonal ion drift found in previous studies reflect the relative importance of the E? and F?region wind dynamo in the thermosphere?ionosphere coupling process. Furthermore, these wind measurements indicate that the Earths atmosphere superrotates. The average superrotation speed amounts to about 22 m s?1 for a solar flux level of F10.7 ? 100 × 10?22 W m?2 Hz?1 but increases to 63 m s?1 for F10.7 ? 190 × 10?22 W m?2 Hz?1. Finally, the wind behavior presented in this study is longitudinally averaged and may differ from wind measurements at a certain longitude.

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.

Computer simulation of electron and ion densities and temperatures in the equatorial F region and comparison with Hinotori results

A time-dependent three-dimensional computer simulation of equatorial F region ionosphere has been carried out to understand the electron temperature structure observed by Hinotori satellite in the low and middle ionosphere. This model provides three-dimensional distributions of ion densities, electron temperature, and ion temperatures. The simulations showed the electron temperature enhancements around the equator in the morning, in the midlatitude in the afternoon, and around the equatorial anomaly region from afternoon to midnight. The enhancements in the morning are due to photoelectron heating. The afternoon enhancements in the midlatitude come from the balance of heating and cooling. When no meridional neutral wind is included in the simulation, the electron temperature did not show remarkable enhancements in the midlatitude in the afternoon because of strong cooling by the dense electron density. Around the equatorial anomaly region the electron temperature increased at high altitude in the evening because of the competing effects of plasma cooling and the plasma movements. Since the ionospheric plasma zonal E×B drift is eastward near the sunset (where E is ionospheric electric field and B is magnetic field) and the vertical drift is downward, the high-altitude dayside hot plasma can enter into the topside F region in the premidnight. The computer simulations were directly compared with the Hinotori satellite data. The simulation results were consistent with the equatorial electron density and temperature observed by the Hinotori satellite.

Simultaneous EISCAT Svalbard and VHF radar observations of ion upflows at different aspect angles

A simultaneous EISCAT Svalbard and VHF radar experiment has shown that field-aligned (FA) ion upflows observed at an altitude of 665 km in the dayside cusp are associated with significant anisotropy of ion temperature, isotropic increases of electron temperature and enhancements of electron density. There is no clear correspondence between the enhancements of the electric field strength and the occurrence of the ion upflows. This suggests that the upflow is driven primarily by precipitation. The data support that in addition to “direct” precipitation effects, namely enhanced ambipolar diffusion and heat flux, also wave-particle interaction, like wave-induced transverse ion heating, which causes a hydrodynamic mirror force, may play a role.

Comparison of satellite electron density and temperature measurements at low latitudes with a plasmasphere-ionosphere model

Observations made by the Hinotori satellite of the latitude and diurnal variations of electron density and temperature near 600 km altitude in the low-latitude region are studied by comparison with values from the Sheffield University plasmasphere-ionosphere model (SUPIM). The model results show that the observed features of higher electron density in the summer hemisphere and higher electron temperature in the winter hemisphere are caused principally by the difference in the summer and winter hemisphere values of the meridional neutral wind. Closer agreement between the modeled and observed values is obtained when the interhemisphere difference in the meridional wind, as given by the horizontal wind model (HWM) 90, is reduced and when the peak value of the daytime poleward wind is moved to the afternoon sector in the winter hemisphere and to the morning sector in the summer hemisphere. The model results also show that the altitude variation of the vertical E×B drift velocity plays an important role in the development of the ionospheric equatorial anomaly. The latitude and diurnal variations of the modeled electron density and temperature are in good agreement with the observations when the E×B drift velocity used by the model is in accord with the observations made by the AE-E satellite for magnetic field lines with apex altitude less than 400 km and at Arecibo for magnetic field lines with apex altitude greater than 2000 km; linear interpolation of the observed values is used for the intermediate magnetic field lines.

Season, local time, and longitude variations of electron temperature at the height of ∼600 km in the low latitude region

Abstract Electron temperature observed at the height of ∼600 km by the low inclination satellite Hinotori was studied in terms of local time, season, latitude, magnetic declination and solar flux intensity. The electron temperature shows a steep rise in the early morning (well known as “morning overshoot”), a decrease after that and again an increase at ∼18 hours (hereafter named “evening overshoot”). Generally the morning overshoot becomes more enhanced in the winter hemisphere and during higher solar flux. The evening overshoot becomes more pronounced in the higher latitudes in all seasons and more enhanced in the winter hemisphere as similar to the morning overshoot. These two overshoots show a slight difference in the 210° – 285° and 285° – 360° longitude sectors. This is most likely due to the difference in magnetic declination of these two zones and the resulting difference in the effect of the zonal neutral wind on the thermal structure in the low latitude ionosphere. Significant difference exists between IRI and the observation during daytime.

Variability of an additional layer in the equatorial ionosphere over Fortaleza

The day-to-day variations (or the weather) of an additional layer, called the F 3 layer, that has been predicted to exist at altitudes above the F 2 peak in the equatorial ionosphere are studied through ionosonde observations and theoretical modeling. The ionograms recorded in 1995 at the equatorial station Fortaleza (4°S, 38°W; dip angle 9°S) in Brazil show the occurrence of the F 3 layer during daytime from 0800 to 1630 LT, with the duration of occurrence ranging from 15 min to 6 hours. Although the layer occurs most frequently (75% of the days) in local summer as previously predicted, there are consecutive and individual magnetically quiet and disturbed days when the layer does not occur. There are also days when the layer reoccurs. The model results, obtained using the Sheffield University plasmasphere-ionosphere model, show that the day-to-day variations of the F 3 layer arise from the corresponding variations of the vertical plasma velocity. The layer occurs when the time-cumulative vertical velocity displaces the daytime F 2 peak to high altitudes, to form the F 3 layer, while the normal F 2 layer develops at low altitudes. Sudden displacements result in more distinct F 3 layers than gradual displacements. Model results also show that the plasma temperature within the F 3 layer decreases as the plasma density increases, and, like the plasma density, the plasma temperature also undergoes large day-to-day variations.

A plasma temperature anomaly in the equatorial topside ionosphere

A study of the thermal structure of the low-latitude (30°N to 30°S) ionosphere under equinoctial conditions at low, medium, and high solar activity has been carried out using the Sheffield University plasmasphere-ionosphere model (SUPIM) and Hinotori satellite observations. The study reveals the existence of an anomaly in the plasma (electron and ion) temperature in the topside ionosphere during the evening-midnight period. The anomaly, called the equatorial plasma temperature anomaly (EPTA), is characterized by a trough around the magnetic equator with crests on either side. The trough develops before the crests. The model results show that the anomaly occurs between 1900 and 0100 LT at altitudes between 450 and 1250 km; the strongest anomaly occurs around 2130 LT at 950 km altitude during high solar activity. The temperature trough of the anomaly arises from the adiabatic expansion of the plasma and an increase in plasma density caused by the prereversal strengthening of the upward vertical E × B drift. The temperature crests arise from the combined effect of the reverse plasma fountain and nighttime plasma cooling. The electron temperature measured by the Hinotori satellite near 600 km altitude during medium and high solar activity periods shows the existence of the EPTA with characteristics in close agreement with those obtained by the model. The model also reproduces the occurrence of a daytime temperature bulge in the electron temperature in the bottomside ionosphere; the ion temperature shows no bulge.

Temperature structure of plasma bubbles in the low latitude ionosphere around 600 km altitude

Abstract The electron temperature inside plasma bubbles at a height of 600 km was first measured by means of Japans seventh scientific satellite Hinotori which is an equator orbiting satellite with an inclination of 31°. During the period between June 1981 and February 1982, 724 plasma bubbles were detected and studied. The electron temperature inside the plasma bubbles is either higher or lower than that outside and can also be equal to the electron temperature outside, depending on the local time and on the place where the data were taken. Heating of electrons inside plasma bubbles sometimes occurs over the South Atlantic geomagnetic anomaly and over the Hawaiian anomaly where particle precipitation can frequently be observed.