The structure of the polar ionosphere during exceptionally quiet periods

Knowledge of the structure of the polar ionosphere during exceptionally quiet periods is basic for studying complicated ionospheric behaviors during disturbances. On the basis of data from an airborne ionosonde as well as a meridian chain of ground-basedionosondes, the circumpolar structure of the E,-and F-regions is elucidated. There are two circumpolar zones of E-region ionization with differing characteristics. The first is an auroral E,-layer and/or retarded type sporadic E-band that has previously (Whalen et al., 1971) been found to be identical with the continuous aurora. The second is a zone of non-retarded type spora die E located poleward of the former band. In general, discrete auroras are co-located with the latter. The main trough, a prominent feature of the night sector F-region, is most pronounced in the early morning. The main trough is bounded on the poleward side by a well defined ‘wall’ of F-region ionization. The night sector poleward trough wall is located approximately three degrees of latitude equatorward of the auroral oval. A ‘plateau’ of F-region ionization extends from the poleward trough wall to the auroral oval.     

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.     

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.     

Ionospheric disturbances: Evidence for the contraction of the plasmasphere during severe geomagnetic storms

Continuous measurements of the ionospheric total electron content (TEC) provide a parameter well suited for the study of F-region disturbance effects. In this investigation, five cases of large and rapid drops in TEC observed near the sunset period are interpreted as being due to the contraction of the plasmasphere to L values less than 3. A model calculation is performed for the specific case of 1 November 1968 using simultaneous Alouette I data to define the position and magnitude of the ionospheric trough. The results indicate that the motion of a deep trough across the ray path from a geostationary satellite to an observing station can cause drastic changes in the measured amounts of Faraday rotation and therefore in the derived values of TEC. All of the TEC data sources available for the five events are then examined in an attempt to describe more completely the latitude dependence of the effects. It is suggested that during severe geomagnetic storms, the large and rapid decays in TEC during the 18–21 LT period to values significantly below normal can be used as a criterion to determine the approximate latitudinal extent of the contracted plasmasphere.     

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.     

Diurnal variation of thermal plasma in the plasmasphere

All of the OGO-5 light ion density measurements (covering the period from March 1968 to May 1969) obtained from the Lockheed Light Ion Mass Spectrometer were used to determine the average global topology of the equatorial plasmasphere density distribution. The variation of the light ion equatorial density at L?3.2 with local time was deduced by determining the average density observed within one hour of a specific local time and within 0.1 of a given L coordinate. The average H⁺ density showed a semidiurnal variation with peaks near noon and midnight. The He⁺ observations also revealed multiple peaks throughout the day but with smaller amplitudes than those of H⁺. At L>3.2 plasma trough conditions increase the scatter of densities. The average variation of the H⁺ density with L within the plasmasphere is found to be steepest near midnight and can be least squares fitted equally well to either an exponential variation exp (?bL) where b is between 0.85 and 1.5 or to a power law L^?a where a varies from 3.2 to 5.     

Activity in the quiet Sun

Macrospicules have been observed in H and He i D₃, on the disk and above the limb. In 1975, a rate of 1400 (A day)^–1 is inferred, and the ratio of equatorial to polar rates 2. D₃ intensities are a few × 10^–3 of the disk center, and do not decrease in coronal holes. The ratio of H to D₃ intensities is 10. The integral number of macrospicules with D₃ intensity I ₀ is proportional to I ₀ ^–1.     

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.     

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).     

High latitude equivalent current systems during extremely quiet times

The magnetic perturbation patterns in the polar cap and auroral zone regions are obtained for extremely quiet days using two different techniques. It is shown that the form of the equivalent current flow pattern is extremely sensitive to the level of quietness, and that even so-called quiet days are at times disturbed by substorm activity. Certain characteristic equivalent flow not typically observed during substorms is noted in the polar cap, and this flow appears to be associated with effects associated with polar cap perturbations discussed by Svalgaard (1973). As well a region of equatorward flow appears at high latitudes near the dawn meridian, which appears to be Hall current driven by an eastward electric field. The dayside sub-auroral zone is dominated by the S_q-current system, while the nightside shows no significant current flow in the absence of substorm activity.     

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.     

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.     

Absence of the plasma sheet at lunar distance during geomagnetically quiet times

Plasma data from the Apollo XIV Charged Particle Lunar Environment Experiment (CPLEE) are presented to show that contrary to previously published analyses the plasma sheet does not extend to the lunar orbit with a thickness of 8 R_H. Two electron spectral types are observed: (1) low energy photoelectrons with no statistically significant medium and high energy fluxes, and (2) double peaked medium and high energy electrons. The second type is observed either coincident with auroral substorms or at the center of the tail during quiet times. These spectra are one to several orders of magnitude less intense than plasma sheet spectra measured near 20 R_E.     

Standing slow magnetosonic waves in a dipole-like plasmasphere

A problem of the structure and spectrum of standing slow magnetosonic waves in a dipole plasmasphere is solved. Both an analytical (in WKB approximation) and numerical solutions are found to the problem, for a distribution of the plasma parameters typical of the Earth's plasmasphere. The solutions allow us to treat the total electronic content oscillations registered above Japan as oscillations of one of the first harmonics of standing slow magnetosonic waves. Near the ionosphere the main components of the field of registered standing SMS waves are the plasma oscillations along magnetic field lines, plasma concentration oscillation and the related oscillations of the gas-kinetic pressure. The velocity of the plasma oscillations increases dramatically near the ionospheric conductive layer, which should result in precipitation of the background plasma particles. This may be accompanied by ionospheric F2 region airglows modulated with the periods of standing slow magnetosonic waves.     

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.     

Probability distribution functions of microscale magnetic fluctuations during quiet conditions

A statistical study of microscale magnetic fluctuations in the interplanetary and magnetosheath region during quiet conditions is approached from the concept of probability distribution function. Magnetic field data from Explorer 34 were used to reconstruct the distribution functions and to calculate some of their moments. The distribution functions are found to be nearly tri-Maxwellian as the background field is relatively quiet. The direction of maximum fluctuations is found to be nearly perpendicular to that of the background magnetic field, but the fluctuations are rarely circularly polarized. Across the Earth's bow shock, the degree of fluctuation anisotropy increases, but no noticeable change in relative fluctuation intensity has been observed.     

Mid-latitude horizontal electric fields in the stratosphere during magnetically disturbed periods

The horizontal electric field has been measured with balloons over the Pacific Ocean near the Sanriku Coast in Japan. By comparing the electric-field data obtained during magnetically disturbed periods, 16–17 October 1973, 6–7 October 1975 and 3–4 October 1977, with IMF B_z, auroral zone AU and AL, equatorial Dst and $\text{Δ(Dst)}\text{Δt}$, mid-latitude magnetic fields (H, D, Z at Kakioka), and the ionospheric electron density (?₀F2 at Kokubunji), it is found that the observed electric fields of about 9 mVm^?1 made the clockwise rotation during the growth and recovery stages of the magnetospheric substorms. Relations between high and middle latitude ionospheres and between the magnetosphere and the ionosphere are discussed in relation to the origin and propagation of these electric fields.     

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.     

Wave propagation in the quiet solar chromosphere

In order to precise previous results about wave propagation in the quiet chromosphere (N. Mein and P. Mein, 1976), we study the behaviour of Doppler shifts and intensity fluctuations in 3 lines of Ca ii. We use the same observation as in our previous work, that is to say a sequence of spectra lasting 27 mn, taken at Sacramento Peak Observatory solar tower. Results can be summarized as follows:

1. Phase-lag between intensity fluctuations and dopplershifts is always near 90° in the Ca ii lines, even for frequencies as high as 15 mHz, and whatever is the location in the chromospheric network.
2. Magneto-acoustic waves propagating vertically in a vertical or horizontal magnetic field could account for the observations only if they were, on one hand reflected in the upper atmosphere, on the other hand propagating with a very high sound or Alfvén speed. The lower limit for the speed (70 km s^-1) does not seem to be realistic. Oblique waves could be investigated for better agreement.