Gravity wave and sporadic-E echo signatures on VHF backscatter radar systems

Wave-like features in range seen on the range/time/intensity (RTI) records of VHF backscatter radars operating in the South of New Zealand are interpreted as being the signature of gravity waves propagating in an ionospheric sporadic-E layer. The data show that, during midsummer in particular, sporadic-E ionisation which has been modified by the passage of a gravity wave can produce two distinct echo types : backscatter from field-aligned irregularities within the sporadic-E layer, probably generated by plasma waves, and a second type of echo resulting from energy backscattered from the surface of the sea after specular reflection in the ionosphere. The backscattering and reflecting region can exist at latitudes at least as low as 49° geographic (57° geomagnetic) latitude during quiet magnetic conditions. We confirm the patchiness of dense sporadic-E, and conclude that gravity waves at sporadic-E heights have amplitudes of the order of several tenths of a kilometre.     

Wavelike irregularities in the mid-latitude 6300 Å airglow

The operation of a sky-scanning photometer at mid-latitudes has revealed the presence of elongated irregularities in the 6300 Å airglow, which can have a wavelike nature. Wavefronts are aligned near the magnetic meridian, and the motion is to the east with speeds ～100 m/sec. One airglow event in which the ionosphere appeared to corrugate produced a period of moderate spread-F on a nearby ionosonde.It is not clear from comparison with radio experiments that the airglow disturbances are due to the passage in the ionosphere of gravity waves; some characteristics suggest the irregularities could be due to an instability of the ionospheric plasma.     

Formation of blanketing sporadic E-layers at the magnetic equator due to horizontal wind shears

A careful survey of quarter-hourly ionograms for the year 1969 has shown that blanketing layer type ionization irregularities occur on many occasions in the E-region of the ionosphere over Trivandrum (dip ～ 0.6°S). It is shown that horizontal shears of horizontal neutral winds are the most likely sources of such layer type irregularities at the magnetic equator. The horizontal wind shears of required magnitude are shown to be provided by internal gravity waves of short period. The rarity of E_s-layer occurrence is attributed to the stringent requirements on the amplitude and wave vector orientation of the relevant gravity waves generating the E_s-layers.     

Nonlinear shift of wave parameters of whistlers in the ionosphere

The expression for nonlinear shift of a wave number of a whistler wave propagating through the ionosphere has been derived and the results have been discussed. It is seen that nonlinear shift of a wave number of a whistler is significant in some physical situations. From numerical estimations it is observed that wave number shifts of a whistler for both the LCP and RCP waves become significant when the frequency of the waves are nearly equal to the ion-cyclotron frequency.     

Ionospheric winds in the F-region and their effects on the limiting periods of gravity waves

Spectrum analyses of ionospheric electron density and content fluctuations show periods with a lower limit near 5 min. Interpretation of this cut off in terms of gravity waves in a windless atmosphere leads to unacceptably low thermospheric temperatures near 180°K. It is concluded that neutral winds reduce the apparent cut-off period in the ionosphere. The maximum horizontal wind speed obtained from cut-off data is about 100 m/sec.     

Are Io's Alfvén wings filamented? Galileo observations

The interaction of Io with the Jovian magnetosphere generates auroral and radio emissions. The underlying electron acceleration process is not understood and few observations exist to constrain the theoretical models. The source of energy for the electron acceleration is in all likelihood supplied from the Alfvén wings that stretch out from both poles of Io into the two Jovian hemispheres. The form of the current system associated with the Alfvén wings has been disputed, some suggesting that the greatly slowed flow near Io implies that a steady current loop links Io to Jupiter's ionosphere, others arguing that the return waves appear only downstream of Io and others suggesting that both forms develop. Given the finite inclination of the Alfvén wings implied by the finite value of the Alfvén Mach number and the strong reflection that occurs at the boundary of the Io torus, we argue that no steady current loop can be invoked between Io and Jupiter's ionosphere. However, the energetics of the auroral and radio emissions imply that most of the energy in the Alfvén wings is transformed into electron acceleration at high-latitudes, that is, outside the Io torus. The dilemma then is to understand how a large fraction of the power penetrates the reflecting boundary. We present data from Galileo's multiple flybys of Io that suggest that the coupling with the Jovian ionosphere is mediated by filamentary Alfvén wings associated with electromagnetic waves propagating out of the torus. In particular, we report on the systematic observation, within the cross-section of Io's Alfvén wings and in their immediate vicinity, of intense electromagnetic waves at frequencies up to several times the proton gyrofrequency. We interpret these “high-frequency/small-scale” waves as the signature of a strong filamentation/fragmentation of the Alfvén wings before they reflect off of the sharp boundary gradient of the Io torus. As a consequence, we suggest that most of the primary energy is converted into “high-frequency/small-scale” electromagnetic waves that can propagate out from the torus toward Jupiter's ionosphere. Reaching high-latitudes, these waves are able to accelerate electrons to almost relativistic speeds.     

Mode Conversion of Solar p Modes in non-Vertical Magnetic Fields – i. two-Dimensional Model

Sunspots absorb incident p modes. The responsible mechanism is uncertain. One possibility is mode conversion to slow magnetoacoustic–gravity waves. In vertical field mode conversion can adequately explain the observed f-mode absorption, but is too inefficient to explain the absorption of p modes. In this investigation we calculate the efficiency of fast-to-slow magnetoacoustic–gravity wave conversion in non-vertical field. We assume two-dimensional propagation where the Alfvén waves decouple. It is found that resultant p-mode absorption is significantly enhanced for moderate inclinations at higher frequencies, whereas for p modes at lower frequencies, and the f mode in general, there is no useful enhancement. However, the enhancement is insufficient to explain the observed p-mode absorption by sunspots. Paper II considers the efficiency of mode conversion in non-vertical field with three-dimensional propagation, where fast and slow magnetoacoustic–gravity waves and Alfvén waves are coupled.     

Observations of ELF electromagnetic waves associated with equatorial spread F

Extreme low frequency electromagnetic waves have been observed below the F peak in the equatorial ionosphere by instruments onboard OGO-6. Electrostatic wave observations indicate that the steep gradient was unstable to the process which causes equatorial spread F above the region where the electromagnetic waves were observed. The data are very similar to observations near the polar cusp and give further evidence that ELF waves are excluded from regions of rapid and irregular density increases. Low level electromagnetic waves with similar properties were occasionally observed on the nightside by the OVI-17 electric field sensor and may be plasmaspheric hiss which has propagated to low altitude.     

Propagation characteristics of hydromagnetic waves in various layers of the ionosphere

This paper derives the basic propagation characteristics of hydromagnetic waves in various layers of the ionosphere. It is shown that propagation in the upper ionosphere and the F₂ layer is largely isotropic. In the lower region of the ionosphere there are two possible modes of propagation, both being anisotropic. Propagation characteristics of waves in this lower region, however, are relatively independent of the direction of horizontal propagation. Calculations of intrinsic wave attenuation show that ducted propagation of Pc 1 signals over appreciable horizontal distances may only take place in the upper layers of the ionosphere.     

Generation and propagation of the ULF planetary-scale electromagnetic wavy structures in the ionosphere

In the present article, the results of theoretical investigation of the dynamics of generation and propagation of planetary (with wavelength 10³ km and more) ultra-low frequency (ULF) electromagnetic wave structures in the dissipative ionosphere are given. The physical mechanism of generation of the planetary electromagnetic waves is proposed. It is established, that the global factor, acting permanently in the ionosphere—inhomogeneity (latitude variation) of the geomagnetic field and angular velocity of the earth's rotation—generates the fast and slow planetary ULF electromagnetic waves. The waves propagate along the parallels to the east as well as to the west. In E-region the fast waves have phase velocities (2-20) km s⁻¹and frequencies (10⁻¹-10⁻⁴) s⁻¹; the slow waves propagate with local winds velocities and have frequencies (10⁻⁴-10⁻⁶) s⁻¹. In F-region the fast ULF electromagnetic waves propagate with phase velocities tens-hundreds km s⁻¹ and their frequencies are in the range of (10-10⁻³) s⁻¹. The slow mode is produced by the dynamoelectric field, it represents a generalization of the ordinary Rossby-type waves in the rotating ionosphere and is caused by the Hall effect in the E-layer. The fast disturbances are the new modes, which are associated with oscillations of the ionospheric electrons frozen in the geomagnetic field and are connected with the large-scale internal vortical electric field generation in the ionosphere. The large-scale waves are weakly damped. The features and the parameters of the theoretically investigated electromagnetic wave structures agree with those of large-scale ULF midlatitude long-period oscillations (MLO) and magnetoionospheric wave perturbations (MIWP), observed experimentally in the ionosphere. It is established, that because of relevance of Coriolis and electromagnetic forces, generation of slow planetary electromagnetic waves at the fixed latitude in the ionosphere can give rise to the reverse of local wind structures and to the direction change of general ionospheric circulation. It is considered one more class of the waves, called as the slow magnetohydrodinamic (MHD) waves, on which inhomogeneity of the Coriolis and Ampere forces do not influence. These waves appear as an admixture of the slow Alfven- and whistler-type perturbations. The waves generate the geomagnetic field from several tens to several hundreds nT and more. Nonlinear interaction of the considered waves with the local ionospheric zonal shear winds is studied. It is established, that planetary ULF electromagnetic waves, at their interaction with the local shear winds, can self-localize in the form of nonlinear solitary vortices, moving along the latitude circles westward as well as eastward with velocity, different from phase velocity of corresponding linear waves. The vortices are weakly damped and long lived. They cause the geomagnetic pulsations stronger than the linear waves by one order. The vortex structures transfer the trapped particles of medium and also energy and heat. That is why such nonlinear vortex structures can be the structural elements of strong macroturbulence of the ionosphere.     

On existence conditions of dust-acoustic solitary waves in a plasma with positively charged dust

Dust acoustic (DA) solitary wave existence conditions are investigated for positively charged dust particles in the presence of nonthermal electrons. Once Sagdeev pseudo-potential derived through fluid equations, for large amplitude DA waves, the lower limit on Mach number is calculated analytically using the necessary condition for the solitary waves existence. The double layers conditions provides the upper limit on Mach number. This allowed us to numerically investigate the effect of the temperature, density and nonthermal parameters on the solitary waves’ characteristics. The present study is devoted to a complex plasma subject to ultraviolet radiations such as the one in the lower earth’s ionosphere.     

Response of an optically thin,isothermal atmosphere to a convective overshoot

Radiation is believed to be hostile to the generation of gravity waves by granulation at the base of photosphere where the radiation is effective. A convective overshoot from subphotosphere seems able to penetrate to a height where the solar temperature is minimum and to excite the gravity waves in a stable region there.The response of the solar atmosphere to a Gaussian disturbance characterizing such a convective overshoot is studied in an unbounded isothermal atmosphere. Radiative effects are included, but only in regions which are optically thin. The response is measured in terms of mean vertical kinetic energy density (E _z) and mean vertical external energy flux (Q _z). E _z and Q _z were calculated for a wide range of frequencies centered at the observed 5-min velocity oscillation period. The computed sharp and broad power spectra at the lower chromosphere and the upper photosphere, respectively, are attributed to the combined effects of space damping and source function. Low-frequency waves (2000 s or longer) are found to be not responsible for depositing energy in the upper solar atmosphere.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Small-Amplitude Disturbances in a Radiating and Scattering Grey MediumIII. Gravitational Effects on the Solutions of Given Real Wave Number k

We study the fundamental modes of radiation hydrodynamic linear waves that arise from one-dimensional small-amplitude initial fluctuations with wave number k in a radiating and scattering grey medium by taking into account the gravitational effects. The equation of radiative acoustics is derived from three hydrodynamic equations, Poisson’s equation, and two moment equations of radiation, by assuming a spherical symmetry for the matter and radiation and by using the Eddington approximation. We solve the dispersion relation as a quintic function of angular frequency ω, the wave number k being a real parameter. Numerical results reveal that wave patterns of five solutions are distinguished into three types: the radiation-dominated, type 1, and type 2 matter-dominated cases. In the case of no gravitaional effects (Kaneko et al., 2005), the following wave modes appear: radiation wave, conservative radiation wave, entropy wave, Newtonian-cooling wave, opacity-damped and cooling-damped waves, constant-volume and constant-pressure diffusions, adiabatic sound wave, cooling-damped and drag-force-damped isothermal sound waves, isentropic radiation-acoustic wave, and gap mode. Meanwhile, the gravitaional effects being taken into account, the growing gravo-diffusion mode newly arises from the constant-pressure diffusion at the point that k agrees with Jeans’ wave number specified by the isothermal sound speed. This mode changes to the growing radiation-acoustic gravity mode near the point that k becomes Jeans’ wave number specified by the isentropic radiation-acoustic speed. In step with a transition between them, the isentropic radiation-acoustic wave splits into the damping radiation-acoustic gravity mode and constant-volume diffusion. The constant-volume diffusion emerges twice if the gravitational effects are taken into account. Since analytic solutions are derived for all wave modes, we discuss their physical significance. The critical conditions are given which distinguish between radiation-dominated and type 1 matter-dominated cases, and between type 1 and type 2 matter-dominated cases. Waves in a self-gravitating scattering grey medium are also analyzed, which provides us some hints for the effects of energy and momentum exchange between matter and radiation.     

A comparison study between the newly-born ion effect and the thermal effect on the whistlers in the ionosphere

We have considered an ionospheric plasma model that includes the thermal effect along with the newly born ionic effect and derived a group travel time for the low-frequency whistlers with a view to employing it as a diagnostic tool in the ionosphere. The mathematical development shows that the thermal effect contribution varies with ( _i – )_–7/2 whereas that of the newly born ionic effect varies with _i – )_–5/2. Both the effects are discussed separately. It is concluded that the effects are reasonably countable in the ionosphere. The investigations finally conclude that both the effects should be taken into the whistler waves, otherwise the method might cause a discrepancy in the results, which could affect their accuracy.     

The effect of finite ionosphere conductivities on axisymmetric toroidal Alfvén wave resonances

Previously developed solutions for pure toroidal mode Alfvén waves with finite ionosphere conductivities are modified to apply both inside and outside the plasmapause.Detailed diagrams are provided to illustrate the effect of realistic ionosphere conductances on the wave-forms. As well as graphs of wave-period, these include: (a) half-wave solutions showing the effect of dipole field distortion and consequent enhancement of ionosphere electric fields; (b) half-wave solutions with low damping that are symmetric and asymmetric about the equatorial plane; (c) highly-damped half-wave and quarter-wave solutions with wave admittance at the ionosphere nearly equal to the ionosphere conductance; (d) a quarter-wave solution with low damping that has a “near-node” of electric field at one ionosphere and an antinode of electric field at the other.     

The propagation of Alfven waves along Io's flux tube

The transmission of Alfvén waves from Io to the Jovian ionosphere is discussed. Various simplified laws for the variation of plasma density are analyzed and juxtaposed to simulate a realistic density variation along the Io-Jupiter flux line. Apparently the ionosphere is previously only to frequencies in excess of 1 Hz. The ionosphere participates in tilting the flux tube trapped by Io leading to the formation of a neutral point in the vicinity of the satellite.     

Ion-acoustic solitons in pair-ion plasma with non-thermal electrons

Properties of fully nonlinear ion-acoustic solitary waves in an unmagnetized and collisionless pair-ion (PI) plasma containing superthermal electrons obeying Cairns distribution have been analyzed. A linear biquadratic dispersion relation has been derived, which yields the fast (supersonic) and slow (subsonic) modes in a pair-ion-electron plasma with nonthermal electrons. For nonlinear analysis, Korteweg-de Vries equation is obtained using the reductive perturbation technique. It is found that in case of slow mode, both electrostatic hump and dip type structures are formed depending on the temperature difference between positively and negatively charged ions, whereas, only dip type solitary structures have been observed for fast mode. The present work may be employed to explore and to understand the formation of solitary structures in the space (especially, the Earth’s ionosphere where two distinct pair ion species (H ^±) are present) and laboratory produced pair-ion plasmas with nonthermal electrons.     

Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances

This study results from a coordinated experiment involving ionospheric observations of Faraday rotation between a geostationary satellite and three ground based receivers at Aberystwyth and Bournemouth in the U.K. and Lannion, France, together with incoherent scatter observations at St. Santin-Nancay, France.Quasi-periodic variations of electron content observed simultaneously at the three stations are interpreted in terms of medium scale gravity waves travelling in the ionospheric F-region. Characteristics of these waves are derived by means of a cross-correlation technique.A reverse ray tracing computation, using data on the neutral atmosphere and neutral wind stratification from the incoherent scatter observations, has been used in an attempt to locate the sources of these waves.The results show that some of the waves are almost certainly generated above 100 km altitude, probably by auroral phenomena, while the others could be produced near ground level by meteorological sources. The reverse ray tracing indicates that the latter sources are in general located in a geographic area in the vicinity of a weather disturbance. A production mechanism for these waves is proposed involving ageostrophic perturbations of the neutral wind in a jet stream.     

Rapid changes of the outer high-latitude ionosphere combined with a penetration of intensive ULF (Pcl) signals

Variable values of the Pcl signal transmissivity through the ionosphere, experimentally obtained from the conjugated ULF experiment GEOS-1-Husafell during a roughly 1-h interval of the micropulsation disturbance on 13 July 1977, have been compared with the results of numerical simulation. Variations of the basic physical parameters of the high-latitude outer ionosphere have been considered. Quantitative estimates of the rapid changes of the ionized particle concentration in connection with the considered changes of the ionospheric plasma temperature have been made. It is assumed that the ioncyclotron waves themselves propagating along the plasmapause give rise to the non-stationary conditions in the outer high-latitude ionosphere (Φ≈ 70°).     

Wave-particle interaction around the lower hybrid resonance

Wave-particle interaction in the ionosphere is studied theoretically for wave frequencies around the lower hybrid resonance (LHR) frequency. An expression is derived for the growth rate of whistler-mode waves propagating in a magneto-active plasma penetrated by a tenuous beam of nonthermal particles. This expression is an extension of that derived in a previous paper by employing the electrostatic dispersion equation; here, the full-wave dispersion equation is used which reduces to the electrostatic one for large values of refractive index.