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Some effects of a mean zonal thermocline gradient on planetary equatorial waves

Planetary equatorial waves are studied with the shallow water equations in the presence of a mean zonal thermocline gradient. The interactions between this gradient and waves are represented by three non-linear terms in the equations: one in the wind-forcing formulation in the x-momentum equation, and two for the advection of mass and divergence of the velocity field in the continuity equation. When the mean gradient is imposed but small, these three (linearized) terms will perturb the behaviour of the equatorial waves. This paper gives a simple analytic treatment of this problem. The equatorial Kelvin mode is first solved with all three contributions, using a Wentzel-Kramers-Brillouin method. The Kelvin mode shows a spatial or/and temporal growth when the thermocline gradient is negative which is the usual situation in the equatorial Pacific ocean (deep thermocline in the west and shallow in the east). The more robust and efficient contribution comes from the advection term. The single effect of the advection of the mean zonal thermocline gradient is then studied for the Kelvin and planetary Rossby modes. The Kelvin mode remains unstable (damped), while the Rossby modes appear damped (unstable) for a negative (positive) thermocline gradient.
Radar observations of auroral zone flows during a multiple-onset substorm

We present an analysis of ground magnetic field, ionospheric flow, geosynchronous particle, and interplanetary data during a multiple-onset substorm on 12 April 1988. Our principal results concern the modulations of the ionospheric flow which occur during the impulsive electrojet activations associated with each onset. During the first hour of the disturbance these take place every ~12.5 min and involve the formation of a new intense westward current filament in the premidnight sector, just poleward of the pre-existing extended current system driven by the large-scale flow. These filaments are ~1 h MLT wide (~600 km), and initially expand poleward to a width of ~300 km before contracting equatorward and coalescing with the pre-existing current, generally leaving the latter enhanced in magnitude and/or expanded in latitude. Within the impulsive electrojets the flow is found to be suppressed to values 50–100 m s–1 or less during the first few minutes, before surging equatorward at 0.5–1.0 km s–1 during the phase of rapid coalescence. The implication is that the precipitation-induced Hall conductivity within the impulsive electrojet initially rises to exceed ~100 mho, before decaying over a few minutes. This value compares with Hall conductivities of ~20 mho in the quasi-steady current regions, and a few mho or less in the regions poleward of the electrojets and in the preonset ionosphere. Preliminary evidence has also been found that the flow surges propagate from midnight to the morning sector where they are associated with arrested equatorward motion or poleward contractions of the current system. These observations are discussed in terms of present theoretical paradigms of the global behaviour of fields and flows which occur during substorms.
Response of the ionosphere to natural and man-made acoustic sources

A review is presented of the effects influencing the ionosphere which are caused by acoustic emission from different sources (chemical and nuclear explosions, bolides, meteorites, earthquakes, volcanic eruptions, hurricanes, launches of spacecrafts and flights of supersonic jets). A terse statement is given of the basic theoretical principles and simplified theoretical models underlying the physics of propagation of infrasonic pulses and gravity waves in the upper atmosphere. The observations of "quick" response by the ionosphere are pointed out. The problem of magnetic disturbances and magnetohydrodynamic (MHD) wave generation in the ionosphere is investigated. In particular, the supersonic propagation of ionospheric disturbances, and the conversion of the acoustic energy into the so-called gyrotropic waves in the ionospheric E-layer are considered.
A satellite study of dayside auroral conjugacy

A study of dayside auroral conjugacy has been done using the cleft/boundary layer auroral particle boundaries observed by the DMSP-F7 satellite in the southern hemisphere and the global UV auroral images taken by the Viking spacecraft in the northern hemisphere. The 22 events have been studied on the basis of an internal IGRF 1985 magnetic field; it is shown that there is a displacement of up to 4° in latitude from the conjugate points with the northern aurora appearing to be located poleward of the conjugate point. No local time dependence of the north-south auroral location difference was seen. The use of a more realistic magnetic field model for tracing field lines which incorporates the dipole tilt angle and Kp index, the Tsyganenko 1987 long model plus the IGRF 1985 internal magnetic field model, appears to organize the data better. Although with this external plus internal model some tracings did not close in the opposite hemisphere, 70% of those that did indicated satisfactory conjugacy. The study shows that the degree of auroral conjugacy is dependent upon the accuracy of the magnetic field model used to trace to the conjugate point, especially in the dayside region where the field lines can either go to the dayside magnetopause near the subsolar point or sweep all the way back to the flanks of the magnetotail. Also the discrepancy in the latitude of northern and southern aurora can be partially explained by the displacement of the neutral sheet (source region of the aurora) by the dipole tilt effect.
The role of upstream ULF waves in the generation of quasi-periodic ELF-VLF emissions

Recent work suggests that the quasi-periodic (QP) modulation ~10–50 s of naturally occurring ELF-VLF radio emissions (~0.5–5 kHz) is produced by the compressional action of Pc3 magnetic pulsations on the source of the emissions. Whilst it is generally accepted that these magnetic pulsations have an exogenic source, it is not clear what the mechanism of their generation is. A study of QP emissions observed during 1988 at Halley, Antarctica, in conjunction with IMP-8 satellite solar wind data, shows that the occurrence and modulation frequency of the emissions are strongly dependent upon the direction and strength of the IMF, respectively. The observed relationships are very similar to those previously reported for Pc3 pulsations associated with upstream ion-cyclotron resonance, involving proton beams reflected at the bowshock. In comparing the observed QP modulation frequencies with upstream wave theory, agreement was found by considering wave excitation exclusively associated with a proton beam reflected from a position on the bowshock at which the shock normal is parallel to the ambient IMF direction. Other geometries were found to be either impropitious or uncertain. The work indicates the useful diagnostic role QP emissions could play in the study of compressional ULF waves in the upstream solar wind and in monitoring the IMF conditions responsible for their generation.
ELF-VLF atmospheric waveforms under night-time ionospheric conditions

Tweek atmospherics generated by lightning discharges and propagated in the night-time Earth-ionosphere waveguide, have often very pronounced dispersive features near the first few waveguide cut-off frequencies ( fcm~mdot{s} fc1, fc 1~1.6–1.9 kHz, m=1, 2, . . . , ), being very extended in time, and have rather large amplitudes of oscillations with periods corresponding to the narrow vicinity of the cut-off frequencies. In this paper an analytical approach is developed to describe the waveform of distant tweeks. It is based on the solving of the Maxwell equations in two qualitatively different regions, whose changes are related in the first instance to the changes in the relative magnitudes of the displacement current and components of the conduction currents, and the following asymptotic matching of the solutions in the transitional region. The analytical night-time waveguide model accounts for both anisotropy and vertical inhomogeneity of the low ionosphere. The model is valid for upper ELF – lower VLF range and is well suitable for the analysis of the QTEm modes in the cut-off frequency regions, which determine the most important part of the tweek spectra and tweek amplitudes. The influence of the different ionospheric heights on the tweek characteristics is determined. The efficiency of the tweek generation by cloud-to-cloud discharge is also evaluated.
Excitation of VLF quasi-electrostatic oscillations in the ionospheric plasma

A numerical solution of the dispersion equation for electromagnetic waves in a hot magnetized collisionless plasma has shown that, in a current-free ionospheric plasma, the distortion of the electron distribution function reproducing the downward flow of a thermal electron component and the compensating upward flow of the suprathermal electrons, which are responsible for the resulting heat flux, can destabilize quasi-electrostatic ion sound waves. The numerical analysis, performed with ion densities and electron temperature taken from the data recorded by the Interkosmos-24 (IK-24, Aktivny) satellite, is compared with a VLF spectrum registered at the same time on board. This spectrum shows a wide frequency band emission below the local ion plasma frequency. The direction of the electron heat flux inherent to the assumed model of VLF emission generation is discussed
On the prediction of solar activity using different neural network models

Accurate prediction of ionospheric parameters is crucial for telecommunication companies. These parameters rely strongly on solar activity. In this paper, we analyze the use of neural networks for sunspot time series prediction. Three types of models are tested and experimental results are reported for a particular sunspot time series: the IR5 index.
Test of GPS for permanent ionospheric TEC monitoring at high latitudes

The Global Positioning System (GPS) observables are affected by the ionosphere. The dispersive nature of this effect and the use of two frequencies in the GPS observations make possible to measure the ionospheric total electron content (TEC) from dual frequency GPS data. In this work we test the concept of permanent monitoring of TEC using a network of GPS receivers at high latitudes. We have used GPS data from five permanent receivers in Scandinavia, from 1-30 January 1994, with geographic latitudes ranging from 57.4°N to 78.9°N. The results show the capability of the method to monitor the evolution of TEC as a function of time and geographical location. We have detected night-time enhancements almost every night for some of the stations, and we have also been able to produce maps of the instantaneous TEC as a function of both latitude and longitude around the GPS network. We also present some of the current limitations in the use of GPS for estimating TEC at high latitudes such as the difficulties in solving for cycle-slips, and the necessity of reliable values for the receiver and satellite differential instrumental biases.
Electromagnetic proton cyclotron instability: heating of cool magnetospheric helium ions

The electromagnetic proton cyclotron anisotropy instability is excited if the hot proton temperature anisotropy, T⊥h/Tmidmid h, is sufficiently large compared to unity, where the subscript h denotes the hot protons and the perpendicular and parallel symbols denote directions relative to the background magnetic field. This instability is important in the outer magnetosphere because it has been shown to lead to an upper bound on T⊥h/Tmidmid h and to cool iron heating. Here one-dimensional initial-value hybrid simulations with spatial variations in the direction of the background magnetic field are used to study this instability in a homogeneous plasma model which represents three ionic constituents of the outer magnetosphere: hot anisotropic protons, cool, initially isotropic protons, and cool, initially isotropic singly ionized helium. These simulations show that the presence of a tenuous helium component does not significantly change the scalings of either the hot proton anisotropy upper bound or the heating of the cool protons. The simulations also show that the helium ion heating rate increases with βmidmidh in contrast to the cool proton energization which decreases with this parameter. The prediction of this homogeneous plasma model, therefore, for cool ions subject to heating by the proton cyclotron instability is that the observed ratio of cool helium temperature to cool proton temperature should increase as βmidmidh increases.