K.-I. Oyama

Seismoionospheric GPS total electron content anomalies observed. before the 12 May 2008 Mw7.9 Wenchuan earthquake

[1]xa0The global ionospheric map (GIM) is used to observe variations in the total electron content (TEC) of the global positioning system (GPS) associated with 35 M ≥ 6.0 earthquakes that occurred in China during the 10-year period of 1 May 1998 to 30 April 2008. The statistical result indicates that the GPS TEC above the epicenter often pronouncedly decreases on day 3–5 before 17 M ≥ 6.3 earthquakes. The GPS TEC of the GIM and electron density profiles probed by six microsatellites of FORMOSAT3/COSMIC (F3/C) are further employed to simultaneously observe seismoionospheric anomalies during an Mw7.9 earthquake near Wenchuan, China, on 12 May 2008. It is found that GPS TEC above the forthcoming epicenter anomalously decreases in the afternoon period of day 6–4 and in the late evening period of day 3 before the earthquake, but enhances in the afternoon of day 3 before the earthquake. The spatial distributions of the anomalous and extreme reductions and enhancements indicate that the earthquake preparation area is about 1650 km and 2850 km from the epicenter in the latitudinal and longitudinal directions, respectively. The F3/C results further show that the ionospheric F2 peak electron density, NmF2, and height, hmF2, significantly decreases approximately 40% and descends about 50–80 km, respectively, when the GPS TEC anomalously reduces.

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.

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.

Akebono observations of electron temperature anisotropy in the polar wind

We report Akebono observations of anisotropic electron velocity distributions in the sunlit polar wind, in which the averaged energy (temperature) of the thermal energy electrons is higher in the upward magnetic field direction than in the downward and perpendicular directions. In the 1500-4000 km altitude region, the observed upward to downward temperature ratio, T{sub eu}/T{sub ed}, was in the range of 1.5 to 2. The observed downward and perpendicular temperatures were similar; T{sub ed}/T(e{perpendicular}){approx_equal}1. The heat flux associated with the observed thermal energy electron velocity distribution was upward and on the order of {approximately}10{sup -2} erg cm{sup -2}s{sup -1}, and was a factor of 5 larger than that of the atmospheric photoelectrons above 10 eV. In this altitude region, the higher-energy (> a few eV) photoelectrons are essentially collisionless. In contrast, the ambient and lower-energy photoelectrons remain collisional because of their larger Coulomb collision cross sections. In the steady state, an ambipolar electric field is required to maintain quasi-neutrality along the field line. It is suggested that the ambipolar electric field and the Coulomb collisions modify the velocity distributions of the thermal energy electrons and the photoelectrons, resulting in the observed temperature anisotropy. The upward heat flux associated with themorexa0» observed temperature anisotropy dominates any downward heat flux due to electron temperature gradients that may be present in the polar wind plasma and demonstrates the important role of the photoelectrons in the dynamics of the polar wind. 49 refs., 6 figs.«xa0less

Ionospheric electron content anomalies detected by a FORMOSAT-3/COSMIC empirical model before and after the Wenchuan Earthquake

An empirical model of ionospheric electron content (IEC), based on FORMOSAT3/COSMIC (F3/C) data, is constructed in order to detect pre-earthquake anomalies. The empirical model provides IEC with four parameters of local time, season, longitude and latitude. For the first time we try to detect anomalies in the F3/C IEC by comparing the model values with observations during the 12 May 2008 Wenchuan Earthquake period. It is found that around the epicentre an IEC enhancement appears on day 3 (9 May) and sequential IEC reductions occur on day 6 to day 1 (6 to 11 May) before the earthquake.

Dynamic model and observation of the equatorial ionosphere

Abstract The Hinotori satellite observed the electron density and temperature around the equatorial anomaly region at ∼600 km altitude during high solar activity. The observed electron temperature in the equatorial F region ionosphere was compared with the International Reference Ionosphere (IRI) model. The results showed the large difference between observation and model. Time dependent three dimensional computer simulations of the equatorial ionospheric F region have been carried out to investigate the electron temperature anomaly. These simulations coincide with the equatorial electron density and temperature structure observed by the Hinotori satellite.

Topside ionospheric electron temperature and density along the Weddell Sea latitude

It has been well known that the ionospheric electron density Ne is greater in the summer nighttime than daytime around the Weddell Sea region, which is named Weddell Sea Anomaly (WSA). This paper for the first time reports unusual increases (decreases) of the daytime (nighttime) electron temperature Te at about 830u2009km altitude over the WSA latitudes probed by Tatiana-2 during December 2009 to January 2010. Concurrent measurements at 660–830u2009km altitude observed by Tatiana-2, Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER), and Formosa Satellite 3/Constellation Observing System for Meteorology, Ionosphere and Climate (F3/C) reveal the anticorrelation between Te and Ne along the WSA latitudes in the daytime and nighttime. Based on F3/C Ne along the WSA latitudes observed at various local times, the associated Te values are computed. The Tatiana-2 and DEMETER observations as well as the computed results show that Te yield the maximum values over the WSA region during daytime and over the Indian and Atlantic Ocean area during nighttime. The maxima or minima in F3/C Ne and the computed Te reveal eastward phase shifts.

Energetics in the plasma bubble

Abstract Energy budget in the plasma bubble is discussed in order to explain the electron temperatures observed at the height of 600 km. From calculations it has been shown that electron temperature profile inside plasma bubble can be basically explained as a function of energy input to the plasma bubble, the ion temperature and on the electron density. These calculations are found to be consistent with our satellite observations. Possible mechanisms by which energy injections in plasma bubble take place under different conditions are suggested.

Dual‐spacecraft radio occultation measurement of the electron density near the lunar surface by the SELENE mission

[1]xa0During the SELENE (Kaguya) mission the dual-spacecraft radio occultation technique was used to investigate the electron population in the vicinity of the lunar surface. One pair of coherent S-band radio signals from one spacecraft was used to probe the possible electron density enhancement near the Moon, and another signal pair from the other spacecraft measured the solar wind and the terrestrial ionosphere plasma fluctuations, which also exist in the measurement by the former signal pair. The results suggest that any stable ionosphere with densities comparable to the ones observed by the Soviet Luna 19 and 22 missions does not exist near the terminator at high latitudes, although the occurrence of temporal or localized density enhancements cannot be ruled out.