S. Matsushita

Penetration of high-latitude electric fields into low latitudes

Abstract Simulation studies of ionospheric electric fields with special emphasis placed on the electrical coupling between high and low latitudes are presented by means of the algorithm developed by Kamide and Matsushita ( J. geophys. Res . 84 p. 4083, 1979) to derive the horizontal electric fields in the global ionosphere generated by field-aligned currents in auroral latitudes. The ionospheric electric potential is obtained from a numerical solution of the steady-state current continuity equation assuming anisotropic conductivities as well as upward and downward currents along auroral field lines. These assumptions are based on our current knowledge of auroral phenomena and geomagnetic variations as well as rocket and satellite measurements of field-aligned currents and radar observations of the ionospheric conductivity distribution. We demonstrate the importance of the basic assumptions leading to the main features observed during both quiet and disturbed times. In the simplest model when the conductivity is taken as constant, the values of the electric potential decay very slowly with an increase of the colatitude in middle and low latitudes. On the other hand, when the conductivity gradient is realistically included in terms of both day-night asymmetry of the quiet-time conductivity and slight auroral enhancements in the night side auroral belt for weakly disturbed times, the decay of the electric field toward low latitudes occurs rapidly. Furthermore, the inclusion of the field-aligned currents in the equatorward half of the auroral belt results in extensive reduction of the electric field in low latitudes, and produces a shielding effect against the penetration of the high-latitude electric field into low latitudes. It is also found that there is a notable asymmetry in the decay rate of the electric field toward low latitudes between the morning and evening sectors. In both quiet and disturbed cases, the electric field in the evening sector decays more rapidly with latitudes than that in the morning sector, a result from the difference in the sign of the longitudinal gradient of the ionospheric conductivity. How the electric field of the high latitude origin can (or cannot) penetrate deep into low latitudes is discussed in light of the magnetospheric convection near the so-called Alfven layer.

An analysis of the upper atmospheric wind observed by LOGACS

Abstract Wind velocities at 140–200 km altitude were observed by a Low-G Accelerometer Calibration System (LOGACS) flown on an Agena satellite during a geomagnetic storm. An interesting wind reversal observed by the satellite at auroral latitudes is satisfactorily explained by the neutral air motion caused by the E × B drift deduced from the ground-based geomagnetic data recorded at stations near the meridian of the satellite orbit.

Seasonal differences in the low-latitude F2-region ionization density caused by Ē × B̄ drift and neutral wind

In the low-latitude ionospheric F2-region the electron density distribution is affected by E × B drift and neutral wind as well as by production and loss processes and diffusion. At Jicamarca, Peru (dip angle, 2°), the daily variation of the measured vertical drift velocity undergoes changes in amplitude and phase between the December and June solstices. Incorporating models of the observed vertical drift velocity, the electron density as a function of latitude and local time is theoretically calculated by solving the time dependent plasma continuity equation. In this study the effects of east-west drift are specifically ignored. The results are compared with ionosonde observations at Huancayo, Peru (dip angle, 1°), and Tucuman, Argentina (dip angle, −22°), to determine the extent to which seasonal differences in the drift velocity can account for the seasonal differences in the maximum electron density, NmF2, measured at these two stations. Solving the equations with and without a neutral wind, it is found that at Huancayo the seasonal differences in NmF2 are caused primarily by the differences in E × B drift and only secondarily by neutral wind and production processes. At Tucuman, both E × B drift and neutral wind play important roles in accounting for the seasonal differences. Between the hours of 1200 and 1900 LT for both solstitial periods the meridional neutral wind must be blowing poleward to achieve good agreement between calculated and observed NmF2 values.

LUNAR SEMIDIURNAL VARIATIONS OF THE GEOMAGNETIC FIELD DETERMINED FROM THE 2.5-MIN DATA SCALINGS.

Abstract Most previous investigations of the lunar variations, L, in the geomagnetic field have been conducted using hourly values scaled from magnetograms and hence many years data have been required to obtain reasonable results. Only recently magnetic tapes of scaled field values for every 2.5 min have become available for a number of world stations. These data enable us to examine the L variations fairly accurately even for a small number of days. Our method involved the exclusion of days of high solar-terrestrial activity (Kp > 3 +); removal of the solar quiet, Sq, variation; alignment of the data on a lunar time scale; and, finally, computing the first four Fourier components to fit the averaged 2.5-min values. For an example of determination by this method, the month-to-month, seasonal and solar cycle variations of the lunar semidiurnal field components at Fredericksburg and Huancayo from 1964 through 1966 and at Honolulu from 1964 through 1968 have all been computed. The amplitudes and relative phases of the lunar semidiurnal component between day and nighttime at Huancayo have also been obtained. A comparison with results of older methods was made whenever it was applicable.

A theoretical study of lunar variations in ƒ0F2 at low latitude

Abstract In low latitudes ƒ 0 F2 undergoes a regular oscillation in lunar time with amplitude ranges of 0.1–1 MHz and phases such that maxima occur about 4 h after lunar transit in the zone where magnetic dip is less than 10° and 10 h after transit outside this zone. Numerical solutions of the time-dependent electron continuity equation with a lunar electric field included give amplitudes and phases of the lunar oscillation in ƒ 0 F2 consistent with observational results. The phase shift in L(ƒ 0 F2 ) is a consequence of the ‘fountain effect’ which produces the F2 equatorial anomaly.

ROCKET OBSERVATIONS OF ELECTRON DENSITIES IN THE NIGHT-TIME AURORAL E REGION AT FORT CHURCHILL, CANADA.

Abstract Seven rockets were launched into the night-time auroral ionosphere over Fort Churchill, Canada, at different levels of magnetic activity. The rocket-measured electron densities in the 70–170 km height range are presented for all the seven cases. Based on these rocket results and the simultaneous ionograms and magnetograms, a consistent model of the ionization changes in the auroral E -region in relation to the level of local magnetic disturbance is presented. It is shown that the increase in magnetic activity is accompanied by (1) a substantial increase of ionization in the entire height range of about 90–140 km and (2) by the appearance and rapid variations of small-scale ionization irregularities over a large part of the E -region. The semi-transparent portion of the auroral ( a -type) E -region trace in ionograms seems to arise from strong scatter-type reflections from these irregularities which are generally ‘weak’. It is shown further that the blanketing frequency ( fbEs ) gives the (average) maximum plasma density in the auroral E -region, while fEs is more sensitive to changes in the size and structure of the ionization irregularities.

Propagation characteristics of hydromagnetic waves in a cold plasma mixed with a hot plasma and right-hand polarized Pc1 and Pc5

Abstract Propagation characteristics of hydromagnetic waves in a cold plasma mixed with a hot plasma under a uniform static magnetic field are investigated. The existence of cold plasma seriously affects the polarization properties of the waves. The results are applied to the interpretation of Pcl and Pc5 with righthand polarizations guided along the geomagnetic field line.

Ionospheric storm of 4–5 August 1972 in the Asia-Australia-Pacific sector

Abstract The ionospheric storm of 4–5 August 1972 is analyzed using data from 35 middle and low latitude ionospheric stations and 7 magnetic stations in the Asia-Australia-Pacific longitude sector. The largest ionospheric and geomagnetic disturbances were both observed in the mid-Pacific. Strong upward electrodynamic drifts occurred in the western Pacific during the initial phase of the storm. True-height profiles of electron density in the Pacific area show that the ionosphere was raised during the first several hours after the geomagnetic storm sudden commencement, that large variations later occurred in the mid-Pacific, possibly caused by wave-like thermospheric winds, and that the F -layer was reduced in density and raised in height during the main phase of the storm on the following day. Synoptic pictures of the variations in foF 2 are presented, showing that electron density enhancements occurred during the storm initial phase in the western Pacific at middle latitudes, peaking in the 20°–45° geomagnetic latitude range, and becoming density deficits during the storm main phase. The observed variations are interpreted in terms of possible physical mechanisms.

Magnetospheric convections and damped-type geomagnetic pulsations associated with storms

Abstract It is shown that the steady magnetospheric motion generated by the solar wind is current free in the first approximation, regardless of the driving mechanism. Also the sufficient condition for the hydromagnetic stability of cold plasma in the presence of an external magnetic field and a steady motion is obtained. Based on these theoretical estimations the damped-type geomagnetic pulsations associated with geomagnetic storms are interpreted as an interaction between hydromagnetic oscillations and magnetospheric motions caused by solar winds.

Space charge waves and ionospheric irregularities

Abstract The dispersion of space charge waves in a partially ionized plasma is investigated mainly for transverse propagation to both applied electro- and magneto-static fields, E0 and B0 respectively. The waves consist of six modes of which five are damped and only one increases with time. The wave of this mode in low frequency travels approximately as fast as the electron drift and causes a two stream instability which may explain the equatorial sporadic-E. Generally, the movement of irregularities in the ionosphere may also be interpreted in terms of this drift wave at the low frequency limit. Further, in the presence of a static charge-density gradient orthogonal to B0, a drift instability is induced by the space charge wave of this mode. The simple relation of the velocity of this drift wave to E0 suggests a fascinating possibility for finding E0 by tracking the movement of irregularities such as meteor trails or artificial Ba+ clouds released from rockets.