Nighttime Ionosphere Research Articles

The maintenance of the night-time ionosphere at mid-latitudes. II. The ionosphere above St. Santin

The total rate of recombination in the night-time ionosphere above St. Santin (at L = 1.8) was estimated using a model atmosphere and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was measured but at the latitude of St. Santin this was always small and averaged over the night as a whole the contribution was negligible. Downward diffusion provided the main source of plasma and the flux predicted was compared with the flux measured at 450 km. The comparison was good provided the model atmosphere was modified to use exospheric temperatures based on actual measurements by the incoherent scatter radar. A comparison with the results obtained at Malvern (Paper I) confirmed that the saturation time for the protonosphere at L = 1.8 is far less than at L = 2.6 and that the downward flux from the saturated protonosphere was also less.

The maintenance of the night-time ionosphere at mid-latitudes. I. The ionosphere above Malvern

The total rate of recombination in the night-time ionosphere above Malvern (at L = 2.6) was estimated using a model atmosphere, and the results were compared with the observed rate of change of total electron content to determine the net influx of plasma. Horizontal transport under the influence of electric fields was an important factor on a time-scale of an hour or less but when averaged throughout the night made little contribution. The main influx of plasma was a downward diffusion from the protonosphere, especially before midnight. The average downward flux increased steadily as the protonosphere filled after a magnetic storm, with a saturation time of at least 8 days.

Mapping of ionospheric F-region parameters from atomic oxygen airglow emissions

Simultaneous North-South scanning observations of the OI 7774and 6300nightlow emissions have been carried out at Cachoiera Paulista (geographic coordinates 22.7°S, 45.0°W; geomagnetic latitude 12.0°S), Brazil, to study the tropical F-region dynamics under spread- F and no spread- F conditions. From the observed emission intensities the ionospheric F-region peak electron densities and peak heights can be determined with a good coverage both in latitude and local time. The results are presented in the form of computer generated gray level shade maps showing the variations of the column intensities of the 7774emission ( J 7774) and of the 6300emission ( J 6300), as well as the variations of ( J 7774) 1/2 (related to F-region peak electron density) and of the ratio ( J 7774) 1/2/ J 6300 (related to F-region peak height), as a function of zenith distance (latitude) and local time. Significant features of these observations are discussed. This technique is useful to study dynamical processes and large scale plasma irregularities in the night-time tropical ionosphere.

Characteristics of O+ density depressions in the topside nighttime ionosphere revealed by ISS-b satellite.

The O+ density data resulting from the ISS-b ion mass spectrometer measurements have been analyzed to obtain the global distribution of the nighttime O+ densities at a height of 1, 100km and its seasonal variations. The results of the analysis show the existence of distinct O+ density depression regions in mid-latitudes in the winter hemisphere, two depressions in the northern hemisphere and one depression in the southern hemisphere.The O+ density depressions are coincident with the regions of large downward plasma flow identified by the calculation of the worldwide pattern of vertical plasma motion induced by the neutral winds. And the regions of high or low O+ density are closely connected with the regions with westward or eastward declination of the geomagnetic field in the northern hemisphere, respectively, and vice versa in the southern hemisphere. In addition, the O+ densities in the depression regions in winter season are maintained at rather low levels till just before the equinox, and increase rapidly to the level near the summer values just after the equinox.

The S3?4 ionospheric irregularities satellite experiment: Probe detection of multi-ion component plasmas and associated effects on instability processes

The pulsed plasma probe technique has been expanded to include simultaneous determinations of absolute electron density, density fluctuations, electron temperature, and mean-ion-mass with resolution limited only by probe geometry, sheath size, and telemetry. The technique has been designed to test for coupling of electron density variations and ion composition irregularities in multi-component plasmas by the comparison of electron density fluctuation power spectraP N(k) and a newly-developed diagnostic parameter, the mean-ion-mass fluctuation spectraδM i/M i→P M(k). In addition, the experiment extends satellite-borne irregularity spectral analyses down to the 5–20 m range while attempting to identify F-region plasma instability processes on the basis of characteristics inN e,T e, ∇N e,P N,M i, andP M. Initial results demonstrate the expanded diagnostic capability for high spatial resolution measurements of mean-ion-mass and provide experimental evidence for the role of ion composition in multi-stepped plasma instability processes. Specific results include a spectral indexX n inP N=A nf−X n of 1.6–2.9 over the wavelength range from 1 km to 6 m under conditions identified with an unstable equatorial nighttime ionosphere. Simultaneous measurements ofδM i/M i(→P M=A M f −X m) andδN e/N e(→P N=A n f −X n) have shown a general behavior tending to lower power (A m<A n) and softer spectra (X m<X n) in ion mass fluctuations when compared with fluctuations in total plasma density. Limited analyses of the two power spectral elements raise hopes for the differentiation between plasma mechanisms that can lead to similar indices inP N.

Volume emission rate profiles of the 6300‐Å tropical nightglow obtained from the AE‐E satellite: Latitudinal and seasonal variations

Surface brightnesses at 6300‐Å measured by the visible airglow experiment on board the AE‐E satellite have been inverted to obtain altitude profiles of volume emission rate. A morphological picture of the emission at low latitudes is developed and qualitatively interpreted in terms of the behavior of electric fields and neutral winds in the nighttime ionosphere. Global pictures of the emission are presented for solstice and equinox conditions between 1800 and 0400 LT. Observed regions of depleted and enhanced emission are in agreement with the emission pattern expected from the effect of E × B drifts on the ionization. Other features observed are interpreted in terms of a transequatorial wind and the presence of a pressure maximum near local midnight. These data suggest that the location of the pressure bulge changes seasonally with respect to the geographic equator. Average bottomside electron density altitude profiles are derived from the 6300‐Å volume emission profiles. The local time variation of the height of the peak electron density deduced from the airglow measurements is in agreement with incoherent scatter measurements made at Arecibo, Puerto Rico, during the same time period.

The electron content and its variations at Natal, Brazil

The Faraday rotation and amplitude data for the period September 1978 through August 1979, recorded from the geostationary satellite SMS 1 (90°W) at Natal (5.9°S, 35.2°W), have been utilized to study the behavior of total electron content (TEC). Apart from diurnal and seasonal behavior of TEC, two characteristic features of the nighttime ionosphere are discussed: (1) a postsunset enhancement in TEC which lasts for several hours following the rapid sunset decrease and (2) the sharp, isolated changes in TEC. In particular, relating to phenomenon 2, the depletions in TEC are usually accompanied by a simultaneous increase in fading rate, scintillation index, and amplitude level, while the enhancements are accompanied by a simultaneous decrease in fading rate, scintillation index, and amplitude level. The statistics of their occurrence and nature are described. The average behavior of the signal amplitude after propagating through either a depleted or an enhanced ionosphere is modeled theoretically and shown to be in agreement with the observed experimental behavior.

Structure and dynamics of the ionosphere

The structure of the Venus ionosphere and the major processes occurring within it are summarized. The daytime ionosphere is created by solar EUV radiation incident on the thermosphere; it is in photochemical equilibrium near its peak at about 142 km, where O2(+) is the major ion, and near diffusive equilibrium in its upper regions, where the major ion is O(+). The day-to-night plasma pressure gradient across the terminator drives a nightward ion flow which, together with electron precipitation, contributes to the formation of the nighttime ionosphere. Large-scale radial holes or plasma depletions extending downwards to nearly the ionization peak in the antisolar region are also observed which are associated with regions of strong radial magnetic fields. The ionopause is a highly dynamic and complex surface, extending from an average altitude of 290 km at the subsolar point to about 1000 km at the terminator and from 200 to over 3000 km on the nightside. A variety of solar wind interaction products are observed in the mantle, a transition region between the ionospheric plasma and the flowing shocked solar wind.

Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.

Electron precipitation induced by VLF noise bursts at the plasmapause and detected at conjugate ground stations

A new type of wave‐induced electron precipitation event has been identified. During observations at conjugate stations Siple, Antarctica, and Roberval, Canada (L ∼4.2), VLF noise bursts were found to be associated on a one‐to‐one basis with amplitude perturbations of subionospheric radio propagation. The amplitude perturbations are attributed to patches of enhanced ionization that extended below ∼80 km in the nighttime ionosphere and that were produced by precipitating electron bursts. Similar amplitude perturbations seen previously were correlated with whistlers that propagated within the plasmasphere. For the new events the driving waves were structured collections of rising elements that propagated just beyond the plasmapause at roughly 5‐min intervals over a several‐hour period. These noise bursts were of relatively long duration (∼10 s) and strong intensity (inferred to be &gt;30 pT at the equator). Triggering of the noise bursts appears to have been mostly by whistlers but changed in character with time. Some later bursts had narrowband precursors at constant frequencies possibly locked to power line harmonic radiation. The burst initiation characteristics suggest the existence of a variable threshold for rapid temporal growth in the magnetosphere controlled by the trapped electron dynamics. The temporal signatures of the amplitude perturbations show that precipitation was maintained over multiple bounces of the trapped magnetospheric electrons. In some cases these signatures included a new undershoot effect during the recovery phase lasting 2–5 min. This effect may have been related to cutoff of background drizzle precipitation. Precipitation effects were observed on both long (∼10 Mm) and short (∼½ Mm) subionospheric paths and were monitored simultaneously at the conjugate stations. Similarities in the perturbation signatures on long and short paths suggest that the form of the signatures was governed by ionospheric changes and was not distorted by the subionospheric propagation mechanism. Some differences in the conjugate perturbation signatures are believed to be caused by the difference in loss cone widths for precipitation in the northern and southern hemispheres. Existence of this loss cone gap produced an estimated ∼30‐pT noise amplitude threshold for significant northern precipitation and possibly caused saturation of southern precipitation.

Experimental testing of “corpuscular” hypothesis of night-time mid-latitude ionosphere—results of simultaneous rocket-satellite investigations

In June of 1978 a joint Soviet-American experiment (JASPIC) was carried out; its objective was to investigate nighttime corpuscular fluxes in the mid-latitude ionosphere. Simultaneous satellite measurements are in a good agreement with rocket results.

Ion temperature troughs induced by a meridional neutral air wind in the night-time equatorial topside ionosphere

A mathematical model has been developed to calculate consistent values for the O + and H + concentrations and field-aligned velocities and for the O +, H + and electron temperatures in the night-time equatorial topside ionosphere. Using the results of the model calculations a study is made to establish the ability of F-region neutral air winds to produce observed ion temperature distributions and to investigate the characteristics of ion temperature troughs as functions of altitude, latitude and ionospheric composition. Solar activity conditions that give exospheric neutral gas temperatures 600 K, 800 K and 1000 K are considered. It is shown that the O +-H + transition height represents an altitude limit above which ion cooling due to adiabatic expansion of the plasma is extremely small. The neutral atmosphere imposes a lower altitude limit since the neutral atmosphere quenches any ion cooling which field-aligned transport tends to produce. The northern and southern edges of the ion temperature troughs are shown to be restricted to a range of dip latitudes, the limiting dip latitudes being determined by the magnetic field line geometry and by the functional form of the F-region neutral air wind velocity. Both these parameters considerably influence the interaction between the neutral air and the plasma within magnetic flux tubes.

D-region electron densities observed by incoherent-scatter radar during auroral-absorption spike events

Electron density profiles in the night-time auroral ionosphere were obtained with the incoherent-scatter radar at Chatanika, Alaska, during short duration precipitation events characterized by riometer data as spike events. The measurements show exceptionally large electron densities in the D-region during spike events, the electron density typically exceeding 10 6 cm 3 at 90 km altitude for a short time. The existence of a steep horizontal gradient, particularly on the poleward edge of the event, is inferred. The altitude and thickness of the absorbing layer are deduced. It is shown that 20–40 keV electrons make the greatest contribution to an absorption spike and that the spectrum of electrons producing such an event is probably softer than that producing a more slowly varying absorption peak. These absorption layers are too high for their altitudes to be measured by the technique of multi-frequency riometry.

Complex demodulation applied to Pi2 geomagnetic pulsations

The spectral technique of complex demodulation is applied to Pi2 pulsations recorded along a meridional profile. The technique provides instantaneous values of amplitude and phase and allows frequency dispersion to be taken into account. The variations of magnetospheric wave polarization parameters are observed as a function of both space and time. The results are directly compared with recent theories of the resonance of geomagnetic field lines and the effects of the ionosphere on ground based observations. The theoretical predictions are tested and the experimental results indicate that the night-time ionosphere is capable of a controlling influence on the source characteristics of these magnetospheric waves in the plasmapause region.

A new concept for the daytime magnetosphere of Venus

The Pioneer Venus magnetic field measurements have suggested the possibility of a different type of interaction between the solar wind and the ionosphere of a non‐magnetized planet than has been envisioned in the past. An unexpected aspect of the observations is that the magnetic field of the shocked solar wind does not penetrate the ionosphere; thus it appears that the ionosphere acts approximately as a superconductor, excluding any substantial penetration of interplanetary magnetic field. The ionosphere on the day side is terminated sharply at an ionopause and is magnetized weakly or not at all. Just beyond the ionopause is a magnetized region with relatively little plasma, and whose magnetic pressure is approximately equal to the impact pressure of the solar wind. This suggests to us the possibility of an interaction where the current system associated with the region of enhanced magnetic field flows along the ionopause and is closed by currents in the shocked solar wind plasma, where the forces act to slow down the plasma approaching the stagnation region and accelerate the plasma flowing away from it. With this concept, the lack of magnetic constraint in the ionosphere would allow ionospheric plasma to flow freely from the day to the night side and this flow, whose magnitude should depend inversely on the solar wind pressure, could probably maintain the nighttime ionosphere.

The interaction of electrons in the optical umbra of Venus with the planetary atmosphere—The origin of the nighttime ionosphere

The ionization produced by electrons having energies of several tens of electron volts in the nighttime Venusian atmosphere is investigated on the basis of plasma measurements made on board Venera 9 and 10. We conclude that electron fluxes are sufficient to produce a nighttime ionosphere with a maximal electron density of 104 cm−3. Calculated density profiles of the Venusian nighttime ionosphere show shapes closely resembling measured ones with closely agreeing values of maximal density and corresponding altitudes as well.

Modeling the effects of an H2 gas release on the equatorial ionosphere

Release of a chemically reactive gas such as H2 into the ionospheric F region results in a depletion of the electron density. This is due to the high charge exchange rate between the dominant O+ ion and the H2 gas, with subsequent dissociative recombination of the product ions and electrons. We investigate the simulated response of the low‐latitude nighttime (dusk) ionosphere to a point release of H2 gas by solving the coupled set of time‐dependent continuity equations for O+ and the product ions OH+, H2O+, and H3O+, including the effects of production, loss, and transport of ionization. We find that a 5‐kg release at 300 km over the magnetic equator at 1930 LT produces a depleted electron density region 40 km wide 52 s after release. At the center the electron density is reduced 37% from its prerelease value. Releasing 20 kg at the same altitude produces a hole 60 km wide and a density reduction of 54%. Corresponding values for a 10‐kg release at 350 km are 65 km and a 51% reduction. We also calculate the flux tube electron content reduction and the Pedersen conductivity reduction capable of being produced by the chemical releases. A large depletion will produce a ‘bubble’ which will buoyantly rise through the ionosphere owing to a Rayleigh‐Taylor instability mechanism. For the three simulated releases of 5 kg at 300 km, 10 kg at 350 km, and 20 kg at 300 km the depletions in the electron content 276 s after release are 5.8%, 7.4%, and 11.8%, respectively. The corresponding reductions in the flux tube integrated Pedersen conductivity are 2.1%, 5.1%, and 4.3%. In addition, the H2 releases produce optical emissions at 6300 Å from O(¹D) and 3060 Å from OH(²Σ+) with intensities greater than 1 kR.

Small‐scale irregularities of electron density in the F region from ƒN resonances observed by a rocket‐borne relaxation sounding experiment

The plasma resonance phenomena observed near the plasma frequency ƒN through active experiments of relaxation sounding have been experimentally investigated within the range 100‐500 km in the ionosphere by means of the three rocket launches of the EIDI payloads (Experiment on the Impedance of a Dipole in the Ionosphere). The design of the EIDI relaxation sounders has been improved with respect to that of previous topside sounding experiments (Alouette and ISIS programs) in that it records both the envelope and the waveform of the resonance signals in a large dynamic range. This allows the study of spectral features of the plasma resonance phenomena. The data of EIDI 3 are first used to point out the accuracy and the reliability of relaxation sounding experiments for the measurement of the ionospheric electron density; the electron density profile obtained is typical of a midlatitude, winter, nighttime ionosphere. Secondly, the spectral features of the ƒN resonance signals are investigated. Their comparison with those predicted by the theory of oblique echoes for the ƒN resonances leads to a disagreement, although the waves received are shown to be the electrostatic slow ones used in this theory. However, a correlation processing between the Fourier spectra of the ƒN resonance signals observed in sequence allows us to point out the existence of small‐scale irregularities of the electron density. This result is inconsistent with the theoretical hypothesis of a linear density variation versus altitude. By taking these irregularities into account, the features of the resonance signals can then be qualitatively explained. The properties deduced from the data regarding the size and the amplitude of the density irregularities are in agreement with those obtained under comparable ionospheric conditions through another kind of experiment for measuring the irregularities (Retarding Potential Analyzer on the OGO 6 satellite). Moreover, the unexplained phenomena of beat frequency oscillations observed on the Alouette and ISIS ƒN resonance data are tentatively interpreted in a similar way through the existence of sinusoidal irregularities of the electron density. Finally, it is pointed out that the relaxation sounding experiment on‐board the GEOS satellite has been designed to provide an effective measurement of the electron density irregularities in the magnetosphere.

Artificially created holes in the ionosphere

The artificial creation of ionospheric holes by the release of highly reactive molecules (e.g., H2 or H2O) into the F region is investigated. Through ion‐atom interchange or charge transfer reactions, H2 or H2O reacts with O+ to form OH+ or H2O+, respectively, which subsequently dissociatively recombines with electrons at a very rapid rate. The diffusion of H2 is also modified by chemical loss to the ambient atomic oxygen atmosphere. The limited spatial and temporal extent of the hole‐making process allows several approximations to be made which permit three‐dimensional analytic solutions of the continuity equations for the released particles, the O+ and e− densities, and the intermediary molecular ions. A versatile formalism is developed whereby the hole‐making capability of virtually any spatial‐temporal configuration of released particles can be determined by convolving a set of ‘destruction operators’ which can be viewed as Green's functions for the problem. Sample calculations are presented which demonstrate the competing effects of differences in the molecular weight of the released particles, the altitude of the release, the reactivity of the released molecules with the ambient atomic oxygen atmosphere, and the quantity and distribution of the released molecules. As a specific application of the techniques developed, the modification of a winter nighttime ionosphere over Millstone Hill is described by simulating the release (for example, from the space shuttle) of 1000 kg of water vapor (≃3 × 1028 molecules) near a height of 300 km.

The effect of sporadic- E on the nocturnal propagation of ELF radio waves

Calculations have been made of the parameters of ELF propagation under typical nocturnal ionospheres in the presence of sporadic- E. The presence of nocturnal sporadic- E is shown to produce marked maxima and minima in the propagation characteristics of ELF radio waves and these features are found to move to lower frequencies as the height of the sporadic layer is increased. It is considered that these maxima and minima are produced by interference between the ELF waves reflected from the lower night-time ionosphere and the ELF waves which penetrate this lower region to be reflected from the layer of sporadic ionization above. These results provide an explanation for the marked variability of nocturnal ELF propagation obtained by other workers ( Bannister, 1975; Davis, 1974). Atmospheric spectra recorded by the author at an earlier date are shown to exhibit spectral maxima and minima which can be reasonably explained in terms of propagation in the presence of sporadic ionization in the E-region.