Giant pulsations in the plasmasphere

It has been generally accepted up to now that giant pulsations (Pg) are auroral zone phenomena but here we present observations of a sequence of three Pg events on successive days at three stations well within the plasmasphere. Field line resonance behaviour is exhibited with one of the events clearly resonating at L ? 2.8. From the resonant frequency (10.4 mHz) equatorial mass densities are calculated and from these, and the measured azimuthai polarization at resonance, the inference is drawn that Pgs are oscillations in the fundamental guided poloidal mode. We suggest that the drift wave instability of the compressional Alfvén wave may be the source mechanism for Pgs and speculate how conditions for the instability may have arisen.     

Diurnal and seasonal variation of the surface ozone in Spain

In this study we present the diurnal and monthly variations in the surface ozone concentrations of five Spanish remote stations belonging to the EMEP network. In two of the stations, a maximum in the afternoon is presented because of the turbulent mixing producing appreciable downward ozone flux. In other two stations no maxima are produced. In one of them this result is due to the strong westerly winds that dominate the air flow and the low solar insolation. In the other station the reason are two phenomena: the mountain induced flow regime that produce high concentrations during the night and the photochemical production that results in high concentrations during the afternoon.Four of the five considered stations show the typical natural spring maximum. This maximum extends into summer in two of the stations. In one of the stations the maximum is reached in summer and it is due to the proximity of the station to strong nitrogen oxide emission areas.     

Diurnal variations in the spectrum of lower-ionosphere plasma irregularities

We examined the variations caused in the spectrum of lower-ionosphere plasma irregularities by diurnal changes of ionospheric parameters. We demonstrate that at a height of 95 km, where the irregularities arise due to the turbulence of the neutral atmosphere, the expected level of plasma irregularity fluctuations at a constant turbulent mixing intensity is smaller in the daytime than at night. The one-dimensional spectrum which can be measured in experiments also changes, namely, the spectrum slope is greater at night.     

Diurnal latitude variation of the location of the dayside cusp

The properties of specific high-latitude pulsations (ipcl) reveal the existence of a significant diurnal variation in latitude of the position of the day side cusp (Δφ 6°). This systematic change of the position of the cusp during 24 hr must be taken into account when the rapid shirtings of the cusp connected with the changes of magnetic activity are studied.

A method of determination of the position of the cusp, using a limited number of ground stations is suggested.     

Helium ions in the mid-latitude plasmasphere

An initial study of the behaviour of He⁺ ions in the mid-latitude plasmasphere is carried out by solving the time-dependent equations of continuity and momentum. Starting with a low He⁺ tube content, results are obtained for a period of 8 days. In the topside ionosphere there is an upflow of He⁺ during the day and downflow at night, for the sunspot maximum conditions considered. The downflow at night differs from the behaviour of H⁺ for these atmospheric conditions. However, little of the He⁺ produced in the daytime is lost by recombination at night; it is suggested that the supply of He⁺ to the mid-latitude plasmasphere is, in effect, an escape process for neutral helium.     

Diurnal thermal tides in a non-synchronized hot Jupiter

We perform a linear analysis to investigate the dynamical response of a non-synchronized hot Jupiter to stellar irradiation. In this work, we consider the diurnal Fourier harmonic of the stellar irradiation acting at the top of a radiative layer of a hot Jupiter with no clouds and winds. In the absence of the Coriolis force, the diurnal thermal forcing can excite internal waves propagating into the planet's interior when the thermal forcing period is longer than the sound crossing time of the planet's surface. When the Coriolis effect is taken into consideration, the latitude-dependent stellar heating can excite weak internal waves (g modes) and/or strong baroclinic Rossby waves (buoyant r modes) depending on the asynchrony of the planet. When the planet spins faster than its orbital motion (i.e. retrograde thermal forcing), these waves carry negative angular momentum and are damped by radiative loss as they propagate downwards from the upper layer of the radiative zone. As a result, angular momentum is transferred from the lower layer of the radiative zone to the upper layer and generates a vertical shear. We estimate the resulting internal torques for different rotation periods based on the parameters of HD 209458b.     

Diurnal variation of sodium and potassium at Mercury

Abstract— A summary is given of our published observations showing a large (3 to 4) morning/afternoon ratio of the abundances of sodium and potassium. The proposed mechanism is deposition of ions and atoms on the cold night side, followed by their outward diffusion and evaporation as the Sun rises. Published criticisms of this mechanism are discussed and answered. The rate at which Na atoms can evaporate from the surfaces of the Moon and Mercury is uncertain, but, after a review of laboratory measurements, we propose that it is substantial at temperatures of 400 K and higher. Possible reasons are discussed why another group does not find the diurnal variation. There are differences in observing geometry, but the matter remains unclear.     

Thermal structure of the Jovian plasmasphere

An equation of heat transport in the Jovian magnetosphere is formulated and solved in the L range between 2 and 7. Sources of thermal energy include the heating associated with inward radial diffusion and a hypothetical heat supply originating from Io's dynamo action. The principal sink of the thermal energy is charge exchange in Io's hydrogen torus. In order to explain the density and temperature profile reported by Frank et al. (1976), the presence of the heat source at Io is essential and the density of the torus hydrogen has to be considerably lower than the value inferred from L_α observations by Carlson and Judge (1975). Radial diffusion represents the principal heating mechanism for plasma at very low L values.     

Particle density in the plasmasphere during quiet periods

Density distribution of plasmaspheric particles in the equatorial plane is derived from a model of plasmaspheric streaming, which may produce S_q current system in the lower ionosphere, and from one integral of motion, which seems to be generally valid for steady-state magnetospheric convection. The results satisfy not only the observed features of S_q variation but also the observed pattern of the density distribution in the magnetospheric equatorial plane during quiet periods.     

Asymmetric eigenmodes in a simple model plasmasphere

A theoretical plasma is supposed to be contained between two perfectly conducting cylinders. A magnetic field from a hypothetical line pole is postulated because the metric coefficients have similar properties to those of a dipole field. It is found, as expected, that the asymmetric hydromagnetic eigenmodes involve coupling of torsion and compression. Nevertheless the behaviour is found to be still roughly like that of a uniform field model. The modes are found to be either roughly torsional or roughly compressional, and the roughly torsional eigenfrequencies are found to be insensitive to the eigenmode structure across the field lines. It is therefore suggested that estimates of micropulsation spectra from the axisymmetric toroidal mode in the dipole field may be fairly valid whatever the true longitude distribution of the micropulsations.     

Dynamical interpretation of observed plasmasphere deformations

Based on all of the OGO-5 light ion density measurements (covering the period from March, 1968 to May, 1969), a definition of “isolated plasma regions” was employed to locate the most prominent patches of enhanced light ion densities in the midst of the depleted region, outside of the main plasmasphere. On the dayside, the distribution of these isolated plasma in L.T. vs. L coordinates was quite similar to that of the “detached plasma regions” by Chappell (1974a). On the nightside, however, the new distribution revealed more frequent occurrence of these regions. Elongated thick plasmatails produced during periods of sudden enhancement of convection electric fields and subsequentially thinning and corotating of the plasmatails during quieting periods, in general, could account for the statistical distribution as well as the individual events, such as those between March 27 and April 2, 1968 and Oct. 21 and Oct. 24, 1968. As demonstrated by Kivelson (1976), wave-particle interactions could produce tremendously complicated structures observed in the near vicinity of the plasmapause and far away from the plasmasphere. Examination of H⁺ and He⁺ density measurements for period of Aug. 12–Aug. 20, 1968 indicated that the density reduction of the plasmasphere during a magnetic storm was on the same order of magnitude as that obtained from whistler techniques during a magnetospheric substorm.     

Ring current protons in the upper atmosphere within the plasmasphere

keV protons observed by the ESRO 1A satellite in the upper atmosphere equatorward of the main precipitation zone are described and discussed. The protons are highly anisotropic (empty loss cone) between the low-latitude boundary of the main precipitation zone and approximately L=4 during quiet and moderately disturbed conditions (K_p=0?4). Between L=4 and L=2.7 the proton flux is generally enhanced compared to that at L values somewhat above 4 and only moderately anisotropic. Substorms push the outer main precipitation zone equatorwards, but the boundaries of the innermost, moderately anisotropic region (at L=2.7 and L=4) move only when strong magnetic storms compress the plasmasphere to within L=4. It is suggested that the moderately anisotropic zone is caused by the ion-cyclotron instability for which the growth rate may have a broad maximum between about L=2.7 and L=4. For proton energies in the keV range the instability is excited only in regions with cold plasma densities above several hundred ions per cubic centimeter. It is finally concluded that the observations of low-latitude proton precipitation lend further support to the mechanism of ion-cyclotron instability as the cause of proton pitch angle diffusion, as proposed by Cornwall et al. (1970).     

Effects on the plasmasphere of irregular electric fields

A conservative convection electric field model developed by Volland (1973) to describe the solar wind induced plasma flow within the inner magnetosphere is modified to include a noisy spatial component. Under steady state conditions such a random component will result in spatial irregularities in the thermal plasma density distribution in the vicinity of the plasmapause—particularly near dusk. Spatial irregularities in the convection can produce longitudinally restricted perturbations near the plasmapause some of which are detached from the main body of the plasmasphere. Temporal variations in the midnight to noon flow intensity are shown to produce elongated extensions of the plasmasphere known as plasmatails but even short period variations of the overall magnitude of the convection cannot produce longitudinally localized perturbations in the thermal plasma distribution. Convection models based on the 3 hr magnetic index K_p yield plasmasphere structures which are qualitatively similar to those based on shorter period variations, but the exact location at any given time of the plasmapause is dependent upon the characteristic time scale employed.     

The effect of the lower ionosphere on hydromagnetic modes in the plasmasphere

Dimensionless resonant frequencies of hydromagnetic modes have been calculated for a simple model plasmasphere including a lower ionosphere. Results for the Alfvén mode are broadly consistent with those obtained by Hughes and Southwood [1976]. It is further concluded that the lower ionosphere, despite its strong damping effect for part of the day, does not provide much dissipative coupling between adjacent magnetic field shells in the Alfvén mode. The fast mode is found to be only slightly damped for horizontal wavelengths of global extent.     

Meridional characteristics of a Pc 4 micropulsation event in the plasmasphere

An analysis was made of a complex large amplitude Pc 4 micropulsation, of four hours duration around local noon, observed at five ground stations in the United Kingdom (2.4? L ?3.8). The final pulsation waveform was shown to be the results of the superposition of wave packets of different periods. The meridional variation of the amplitude of the different period wavepackets was consistent with their being fundamental “toroidal” field line resonances within the plasmasphere, rotated through 90° in their transmission through the ionosphere in accordance with recent theoretical predictions. Other predicted ionospheric effects, such as the loss of the sense-of-polarization reversal across the amplitude maximum, were apparent in the meridional variation of the polarization characteristics.     

Thermal proton flow in the plasmasphere: The morning sector

Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-II topside sounder have been analyzed to assess the presence of H⁺ flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause; i.e. there is a sharp boundary separating the poleward regions of polar wind H⁺ flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upwards H⁺ flow is deduced to be present to invariant latitudes as low as 48° (L = 2·2) in the regions normally known to be well inside the plasmasphere. The upwards H⁺ flux is sufficiently large (3 × 10⁸ ions cm^?2 sec^?1) that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere. The cause for this flow remains unknown but it may be a result of a diurnal refilling process.     

Temperature variation of the plasma sheet during substorms

The temperature and density of the plasma in the Earth's distant plasma sheet at the downstream distances of about 20–25 R_e are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ (adiabatic process) in some instances, while by T ∝ N^?1 (isobaric process) in other cases. The total plasma and magnetic pressure decreases when $\text{T ∝ N}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ and increases when T ∝ N^?1. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward flow component near the central plasma sheet during substorms.     

Diffusive equilibrium distributions of He+ in the plasmasphere

Recent satellite observations of thermal ion composition in the near-equatorial plasmasphere have shown that He⁺ comprises 5–10% typically and occasionally 25% or more of the total thermal ion density. A steady state diffusive equilibrium model for the distributions of H⁺, He⁺ and O⁺ along a plasmaspheric flux tube is used to elicit effects that may help explain these observed high He⁺ fractional concentrations. The model indicates that both the ionospheric composition and the temperature distribution along the flux tubes are important factors controlling the equatorial He⁺ composition, through the plasma scale height and thermal diffusion effects. Direct comparison of the model results with thermal ion observations by ISEE-1 indicates that the effects incorporated into the model may explain some of the elevated He⁺ concentrations. In some instances, however, effects not included in the model may also be of importance.     

The behaviour of the geomagnetic pulsations near the boundary of the plasmasphere

Based on the observational data obtained at eleven stations along a geomagnetic meridian (Φm = 45–63°), the characteristics of pc 3, 4 pulsations are investigated. It has been shown that pc 3, 4 pulsations possess two amplitude maxima: one in the high latitudes and the other in middle latitudes. Consequently, the amplitude minimum between the two maxima is observed in subauroral latitudes (Φm ≈ 60°). Examining the peculiarities of the polarization behaviour of pc 3, 4 pulsations along the meridian array, two different regions, where the pulsations are generated, are noticed. One is situated in the middle latitudes of about 55–60°, and the other in the auroral area of about 65–70° in geomagnetic latitude. The former region corresponds to a projection of an area inside the plasmapause and the latter of an area of the outer radiation belt in the magnetosphere. The dependence of the pc 3, 4 periods on the position of the plasmapause is clarified. It is also shown that both the position of the pc 3 amplitude maximum in the middle latitudes and the position of pc 4 minimum in the subauroral area shift according to the variation in the magnetic activity and the position of plasmapause.The dynamic spectra of the simultaneous wave-packets of Pc-pulsations are investigated along the meridional profile. The maximum time delay of the Pc-signals is found at a latitude of about 57°, corresponding to the region of low values of Alfvén velocity inside the plasmasphere. On the other hand, a sharp decrease in the time delay is observed at a latitude of about 60°, the region of the rapid increase of Alfvén velocity at the plasmaspheric boundary in the magnetosphere.