Tadatoshi Takahashi

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.

Longitudinal variations of the topside ionosphere at low latitudes: Satellite measurements and mathematical modelings

The longitudinal variations of the topside ionosphere at low latitudes observed by the Hinotori satellite during equinoctial periods at high solar activity are studied using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model values show that both the neutral wind and E × B drift velocities make important contributions to the observed longitudinal variations in the topside ionosphere. The displacement of the geographic and magnetic equators and the magnetic declination angle, which give rise to conjugate-hemisphere differences in the neutral wind in the magnetic meridian, are the principal causes of the observed north-south asymmetries in the electron density about the magnetic equator. A comparison of the modeled and observed electron densities shows that the modeled longitudinal variations are, in general, in qualitative agreement with the observations when the neutral winds are taken from the HWM90 thermospheric wind model. Improved agreement in the magnitudes is achieved if HWM90 is modified so that at low latitudes (1) the eastward component of the zonal wind is increased during the day and decreased at night and (2) the diurnal variations of the meridional wind in the northern hemisphere at eastern longitudes and the equatorward wind at around midnight at western longitudes are reduced. The model reproduces the observed longitudinal variations in the development of the equatorial peak electron density during the day and in the equatorial trough and associated crests during the afternoon and postsunset periods. The trough and crests are most prominent at around 2000 LT, where the crest-to-trough ratio varies from about 1.15 at eastern longitudes to about 2.0 at western longitudes. Model calculations show that the longitudinal differences of these features can arise from a longitudinal variation in the vertical E × B drift velocity.

High electron temperature associated with the prereversal enhancement in the equatorial ionosphere

High electron temperature in the equatorial ionization anomaly region detected first by Kyokko satellite and later observed frequently by Hinotori satellite are found to be closely associated with the ionization crests of the anomaly. This phenomenon, called the equatorial electron temperature anomaly, is found to occur predominantly in the equinoctial months and to become enhanced with the increase in solar activity. It is mainly a nighttime phenomenon and shows maximum temperature enhancement at around 2100 LT. Using a theoretical model, a mechanism for its occurrence is presented. The mechanism is based on the plasma transport in the evening equatorial ionosphere resulting from the sunset electrodynamical processes.

The solar coronal sheet during the period of sunspot maximum

It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.

Spatial and temporal variations of the electron temperature at equatorial anomaly latitudes

Abstract The latitudinal, altitude and diurnal variations of the electron temperature at equatorial anomaly latitudes in December are investigated using the Sheffield University Plasmasphere-Ionosphere Model. The information from the electron temperature and density measurements at 600 km altitude made by the Hinotori satellite is used to find the appropriate E×B drift pattern and neutral wind pattern for the model input. It is found that the modelled altitude variations of the temperature are in good agreement with incoherent scatter radar observations at several fixed locations. The model results extend our knowledge of the spatial and temporal variations of the electron temperature which are unable to be obtained from experimental studies at the present time. The present study suggests the possibility of improving the descriptions of the electron temperature in the IRI by combining theoretical model results and observations.

Disturbances of cometary and Earth's magnetospheres by single solar flares

Comets P/Brorsen-Metcalf and C/Okazaki-Levy-Rudenko appeared successively in 1989 and displayed various disturbances of their magnetospheres. We observed two cases of a phenomenon in which a disconnection event of the cometary plasma tail (on August 13 and November 16, 1989) was followed by a terrestrial magnetic storm (on August 14 and November 17). A survey of solar flares suggests that an identical solar flare excited successively the cometary magnetosphere and the Earths magnetosphere in each case. The average velocities of the shock front in interplanetary space were estimated by using the magnetospheric disturbances of both the comets and the Earth together with the assumed responsible solar flare. It is speculated from the results that the propagation of the shock front associated with the flare is not symmetric with respect to the radial axis from the flare region of the Sun.

SOLAR CYCLE VARIATION OF LOCAL TIME DEPENDENCE IN FREQUENCY OF OCCURRENCE OF Pi2

Geomagnetic Pi2 pulsations are observed simultaneously with the onset of magnetic substorms. Individual Pi2 pulsations generally have a one—to—one correspondence with individual substorms (Saito, 1969). Therefore, the frequency of occurrence of the individual substorm can be estimated from the frequency of occurrence of the individual Pi2. In order to find some solar cycle variations in the frequency of occurrence of Pi2, Saito and Matsushita (1968) analyzed the data of Pi2 pulsations observed at Onagawa from 1956 to 1964. Together with the report by Afanasieva (1961), they pointed out that the local time of the maximum frequency of occurrence of Pi2 shows some solar cycle variation so that it tends to have a peak in pre—midnight hours during declining and minimum phases, and around midnight during maximum phases. Their analyses were limited in the data for two solar cycles. The purpose of the present paper is to examine the validity of their conclusion for solar cycle Nos. 20, 21 and the former half of 22.