Conjugate effects in the generation of travelling ionospheric disturbances (TIDs) in the F-region

A rapid onset of auroral absorptions was simultaneously recorded by a chain of standard riometers, situated in the northern and southern magneto-conjugate areas, during a period of pronounced substorm activity. The first absorption peak was followed by sequential disturbance patterns in the occurrence of the F-region parameters, virtual height (h′F) and spread - F, as deduced from the standard ionosonde data obtained over a wide range of latitudes in both hemispheres. The disturbances were consistent with the simultaneous occurrences of separate trains of large-scale ionospheric disturbances (TIDs), propagating equatorwards from the southern and northern auroral zones. It is suggested that TIDs were generated by an impulse-like increase in the conjugate particle precipitations, inferred from the riometer records. The precipitation pattern was limited to a high-latitude shell whose equatorwards edge was contained between L-values 5.0 and 5.3. The auroral sources of TIDs appeared to have large linear dimensions, extending at least 17 degrees in longitude.     

Atmospheric expansion from Joule heating

An expression for the vertical velocity of the neutral atmosphere in the F-region is derived for Joule heating by the electric field that drives the auroral electrojet. When only vertical expansion is allowed, it is found that the vertical wind must always increase monotonically with altitude. The heating rate is proportional to the F-region ion density, so that appreciable heating, even during high electric fields, requires some production mechanism of ionization such as auroral secondary ionization or solar photoionization, in the lower F-region. Once started at night, when an ionizing source is present in the lower F-region, the expansion of the atmosphere transports ionization upward, thereby increasing the heating rate, and hence the expansion rate, i.e. positive feedback. Electric field strengths and F-region ion densities of 50 mV/m and 2 × 10¹¹e/m³, respectively, will produce vertal neutral wind speeds of several tens of m/sec in the 300–500 km altitude range. During periods of high magnetic activity, i.e. high electric field, Joule heating can produce large increases in the relative N₂ concentration in the upper F-region; computations made with a simple model suggest that tenfold increases can occur at 400 km altitude $\text{1}\text{2?1}\text{hr}$ after the onset of magnetic activity, a result in agreement with satellite observations. When the Joule heating theory is applied to incoherent scatter data taken during one period of high heating, the horizontal electric field in the F-region is found to decrease markedly, possibly approaching zero as the field penetrates a weak, discrete auroral arc; the decrease began 10–20 km from the arc.     

On high-latitude convection field inhomogeneities,parallel electric fields and inverted-V precipitation events

Assuming a certain horizontal distribution of the convection field at a certain altitude above the ionosphere, the associated electric field and current distributions in a vertical plane are calculated using a model with finite current-dependent conductivity along the magnetic field lines. It is seen that given the kind of horizontal distribution of E₆ commonly observed by polar-orbiting satellites at inverted-V electron precipitation events, the calculated distribution of E_∥ is able to reproduce the basic spatial structure of the precipitation. It is also seen that the combined effect of a locally increased ionization within auroral forms and a large potential difference (ΔV)_∥ along the magnetic field lines at higher altitudes is a strong reduction of E₆ within the auroral forms. From the basic features of the electric field, it is concluded that an interpretation of auroral precipitation in terms of a static E_∥ may require a mechanism that can support a large (ΔV)_∥ even at relatively weak current densities and at the same time allow local enhancements of the parallel conductivity within the region of non-zero E_∥. It is suggested that the magnetic mirroring combined with gyro-resonant wave-particle interactions may be a suitable mechanism.     

Strong turbulence and atmospheric waves in stellar occultations

Many of the problems of stellar occultation observations stem from the difficulty of determining the effects of realistic atmospheric structure on the lightcurves. General techniques for producing model lightcurves for a variety of realistic atmospheric irregularities, including turbulence and inertia-gravity waves, are presented and applied. Using numerical simulations which model the propagation of a wave through a phase-changing screen, the limit of strong scintillations for one-dimensional, Kolmogorov-like turbulence, both for a point source and for extended sources, is investigated in some detail, and significant departures from the behavior in the weak scintillation regime are found. The results are compared with published analytical results and recent occultation data. The effects of large-scale atmospheric waves with realistic horizontal structure are examined, and the reliability of the numerical inversion method of retrieving the true atmospheric vertical structure under circumstances of strong ray crossing and horizontal inhomogeneities is assessed. The simulations confirm that large-scale layered features of the atmosphere are accurately recovered; horizontally inhomogeneous structures (including turbulence) with coherence scale $\text{L ? (2πRH)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ (where R = planetary radius and H = scale height) have little effect on the derived temperature profiles. It is concluded that analysis of occultations may eventually allow us to determine both the quasiglobal atmospheric structure and the statistical characteristics of small-scale refractivity variations.     

Electron heating resulting from interhemispherical transport of photoelectrons

Ambient electron heating rates along several magnetic field lines have been determined for subsequent studies of electron and ion temperatures. Use is made of the modified diffusion method for computing the heating of the ambient plasma, and the escape fluxes from both hemispheres are coupled by self-consistent upper boundary conditions supplied by interhemispheric fluxes degraded in energy along the magnetic field tubes. Heating rates and fluxes are presented for several low L-shells appropriate for noon solstice conditions when both hemispheres are illuminated. The opacity of the field tubes as a function of L is expected to go through a minimum due to the transition from large collective effects of coulomb small angle scattering and energy loss for high L-shells, to a domination by neutral scattering all along the field lines of low L-shells.     

Time-dependent studies of the aurora: Effects of particle precipitation on the dynamic morphology of ionospheric and atmospheric properties

A self-consistent, time-dependent numerical model of the aurora and high-latitude ionos-phere has been developed. It is used to study the response of ionospheric and atmospheric properties in regions subjected to electron bombardment. The time history of precipitation events is arbitrarily specified and computations are made for a variety of electron spectral energy distributions and flux magnitudes. These include soft electron precipitation, such as might occur on the poleward edge of the auroral oval and within the magnetospheric cleft, and harder spectra representative of particle precipitation commonly observed within and on the equatorward edge of the auroral oval. Both daytime and night-time aurorae are considered. The results of the calculations show that the response of various ionospheric and atmospheric parameters depends upon the spectral energy distribution and flux magnitudes of the precipitating electrons during the auroral event. Various properties respond with different time constants that are influenced by coupling processes described by the interactive model. The soft spectrum aurora affects mainly the ionospheric F region, where it causes increases in the electron density, electron temperature and the 6300 Å red line intensity from normal quiet background levels during both daytime and night-time aurora. The fractional variation is greater for the night-time aurora. The hard spectrum aurorae, in general, do not greatly affect the F-2 region of the ionosphere; however, in the F-1 and E regions, large increases from background conditions are shown to occur in the electron and ion temperatures, electron and ion densities, airglow emission rates and minor neutral constituent densities during the build-up phase of the auroral event. During the decay phase of the aurora, most of these properties decrease at nearly the same rate as the specified particle precipitation flux. However, some ionospheric and atmospheric species have a long memory of the auroral event. The odd nitrogen species N(⁴S) and NO probably do not ever reach steady-state densities between auroral storms.     

On the dynamics of the jovian ionosphere and thermosphere.: IV. Ion-neutral coupling

We use the fully coupled, three-dimensional, global circulation Jovian Ionospheric Model (JIM) to calculate the coupling between ions in the jovian auroral ovals and the co-existing neutral atmosphere. The model shows that ions subject to drift motion around the auroral oval, as a result of the E×B coupling between a meridional, equatorward electric field and the jovian magnetic field, generate neutral winds in the planetary frame of reference. Unconstrained by the magnetic field, these neutral winds have a greater latitudinal extent than the corresponding ion drifts. Values of the coupling coefficient, k(h), are presented as a function of altitude and cross-auroral electric field strength, for different incoming electron fluxes and energies. The results show that, with ion velocities of several hundred metres per second to over 1 km s⁻¹, k(h) can attain values greater than 0.5 at the ion production peak. This parameter is key to calculating the effective conductivities required to model magnetosphere-ionosphere coupling correctly. The extent to which angular momentum (and therefore energy) is transported vertically in JIM is much more limited than earlier, one-dimensional, studies have predicted.     

Upper atmospheric temperatures from Doppler line widths—V. Auroral electron energy spectra and fluxes deduced from the 5577 and 6300 Å atomic oxygen emissions

This is a report upon further data obtained from the auroral OI 5577 Å emission with a Wide Angle Michelson Interferometer (WAMI), and upon our first observations made with it on the 6300 Å emission. The method used for converting emission intensities and temperatures to auroral electron fluxes and energy spectra is described. Data for the 5577 Å emission are presented for the (lack of) heating in auroral forms, vertical temperature profiles in aurora, electron flux and energy spectrum variations in pulsating aurora, and a ‘cold’ subvisual auroral arc. Data from the OI 6300 Å emission are presented for the diurnal variation of exospheric temperature and for the thermalization of $\text{O}\text{(}{}^1\text{D)}$ in the F-region.     

Modelling of thermospheric composition changes caused by a severe magnetic storm

The UCL 3-dimensional time-dependent thermospheric model, with atomic and molecular components, is used to study composition changes in the neutral gas at F-layer heights produced by a severe magnetic storm. The computations give the mean molecular weight (MW), temperature and winds as functions of latitude, longitude, height and time for a period of 30 h.Starting from quiet-day conditions, the simulation starts with a 6-h “substorm” period in which strong electric fields are imposed in the auroral ovals, accompanied by particle input. Weaker electric fields are imposed for the remaining 24 h of the simulation. The energy input causes upwelling of air in the northern and southern auroral ovals, accompanied by localized composition changes (increases of MW), which spread no more than a few hundred kilometres from the energy sources. There is a corresponding downward settling of air at winter midlatitudes and low latitudes, producing widespread decreases of MW at a fixed pressure-level. These storm effects are superimposed on the quiet-day summer-to-winter circulation, in which upwelling occurs in the summer hemisphere and down welling in the winter hemisphere. The composition changes seen at a fixed height differ somewhat from those at a fixed pressure-level, because of the expansion resulting from the storm heating.The results can be related to the well-known prevalence of “negative” F-layer storms (with decreases of F2-layer electron density) in summer, and “positive” F-layer storms in winter and at low latitudes. However, the modelled composition changes are not propagated far enough to account for the observed occurrence of negative storms at some distance from the auroral ovals. This difficulty might be overcome if particle heating occurs well equatorward of the auroral ovals during magnetic storms, producing composition changes and negative storm effects at midlatitudes. Winds do not seem a likely cause of negative storm effects, but other factors (such as increases of vibrationally-excited N₂) are possibly important.     

A numerical computer investigation of the polar F-region ionosphere

Results of a numerical computer investigation of the geomagnetically quiet, high latitude F-region ionosphere are presented. A mathematical model of the steady state polar convective electric field pattern is used in conjunction with production and loss processes to solve the continuity equation for the ionization density in a unit volume as it moves across the polar cap and through the auroral zones.Contours of electron density (～ 300 km altitude) over the polar region are computed for various geophysical conditions. Results show changes in the F-region morphology within the polar cap in response to varying the asymmetry of the global convective electric fields but no corresponding change in the morphology of the mid-latitude ionospheric trough. The U.T. response of the ionosphere produces large diurnal changes in both the polar cap densities and trough morphology. In agreement with observations, the model shows diurnal variations of the polar cap density by a factor of about 10 at midwinter and a negligible diurnal variation at midsummer. The phase of the polar cap diurnal variation is such that the maximum polar cap densities occur approximately when the geomagnetic pole is nearest to the Sun (i.e. when the polar cap photo-ionization is a maximum).Within the accuracy of this model, the results suggest that transport of ionization from the dayside of the auroral zone can numerically account for the maintenance of the polar cap ionosphere during winter when no other sources of ionization are present. In addition, east-west transport of ionization, in conjunction with chemical recombination is responsible for the major features of the main trough morphology.There is little seasonal variation in the depth or latitude of the ionization trough, the predominant seasonal change being the longitudinal extent of the trough.The polar wind loss of ionization is of secondary importance compared to chemical recombination.     

Total current of the auroral electrojet estimated from the IMS Alaska meridian chain of magnetic observatories

Based on magnetic data from the IMS Alaska meridian chain of observatories, the total current of the westward auroral electrojet flowing across the meridian is estimated by using two independent methods. The first one is a simple integration of the north-south component of magnetic perturbations along the meridian, providing the quantity F in units of nT·km. The other is to use the forward method, providing the total current I in units of A. It is shown that F and I have nearly identical time variations. Thus, by normalizing the two quantities and taking the numerical value of F in units of nT·km, it is possible to estimate the total electrojet current flowing across a magnetic meridian, with an accuracy of 90%, by using the latitudinal profile of the H component, namely I (A) = 2.0 F (nT·km).     

Interhemispheric flow of thermal plasma in a closed magnetic flux tube at mid-latitudes under sunspot minimum conditions

The coupled H⁺ and O⁺ time-dependent continuity and momentum equations are solved within a region of the L = 3 magnetic flux tube lying between (and including) the F2-layers of conjugate hemispheres. The method of solution is an extended and modified version of the Murphy et al. (1976) method. The model is used to study the coupling between the F2-layers of conjugate hemispheres during magnetically quiet periods.The results of the calculations strongly indicate that the protonosphere acts as a reservoir, with variable H⁺ content, which prevents direct coupling between the F2-layers of conjugate hemispheres. However there is generally a significant interhemispheric flow of plasma. This flow is caused by conditions in the summer and winter topside ionospheres and it maintains continuity in the plasma concentration within the protonosphere. There are times when the direction of flow is from the winter hemisphere to the summer hemisphere. It is suggested that maintenance of the winter F2-layer at night is not assisted directly by the F2-layer of the conjugate summer hemisphere.It is shown that during the first few days of protonosphere replenishment after a magnetic storm there is an upflow of H⁺ in the topside ionosphere at all times in the summer hemisphere. There is also an upflow of H⁺ during the daytime in both hemispheres. A comparison with the results obtained when the interhemispheric H⁺ flux is held permanently at zero shows that both F2-layers are little affected by the interhemispheric H⁺ flux. Nevertheless both F2-layers are affected by the H⁺ tube content of the protonosphere. When the H⁺ flux at 1000 km in one hemisphere is much greater than the H⁺ flux at 1000 km in the conjugate hemisphere, there is a corresponding signature in the interhemispheric H⁺ flux.The results suggest that there is insufficient time between magnetic storms for complete replenishment of the protonosphere to occur.     

A theoretical evaluation of the lunar tidal variations in the ionospheric F2-layer

Time-varying solutions of the full continuity equation for electrons in the F2-region are obtained. The effects of production, loss, diffusion and electrodynamic ‘E × B’ drift are taken into account. The ‘E × B’ drift term consists of a solar and a lunar component. The solar component of drift is assumed diurnal with 14.6m/sec maximum upward speed at mid-day. The lunar component is assumed sinusoidal with period of half lunar day and amplitude one tenth of the solar drift; the phase is assumed to remain constant in lunar time, in accordance with Chapman's phase law.The results show that the lunar variations in the F2-region are markedly dependent on solar time and latitude. It is also shown that the average semi-diurnal lunar variations in N_mF2 and h_mF2 at any particular lunar time are almost opposite in phase to each other (i.e. out of phase by 6 hr) in the magnetic equatorial zone, and out of phase by 2 hr at moderate latitudes. The phase of δh_mF2 is 10 hr at low latitudes and 9 hr at moderate latitudes. The phase of δN_mF2 is 4 hr at low latitudes and 11 lunar hr at moderate latitudes.The results also show that the phase of the lunar semi-monthly oscillations in N_mF2 undergoes a rapid shift of about 5 lunar hr in going from 8 to 12° and the so called phase reversal occurs at about 10° lat at which the amplitude of N_mF2. becomes extremely small.These and other results are in good agreement with observations. Thus it is shown that the main features of the observed lunar tidal variations of the F2-region within 20° of the magnetic equator can be explained satisfactorily by the superposition of a small lunar drift on a large solar drift.     

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.     

Relationships between lobe structure and total luminosity of radio sources

We made a statistical analysis using the Cambridge 5km, 5 GHz data on double radio sources and found highly significant correlations between the following pairs of parameters; the energies in the hot spots and the extended regions, E_h and E_e; their luminosities, L_h and L_e; energy density in the hot spots K and the total luminosity L_t and the ratio of energy densities $\text{K}\text{A}$ and L_t. These correlations exemplify the role of the hot spots in the radio sources.     

A snapshot of the polar ionosphere

This paper presents a picture of the north polar F layer and topside ionosphere obtained primarily from three satellites (Alouette 2, ISIS 1, ISIS 2), that passed over the region within a time interval of ca. 50 min on 25 April 1971, a magnetically quiet day. The horizontal distribution of electron densities at the peak of the F layer is found to be similar to synoptic results from the IGY. Energetic particle and ionospheric plasma data are also presented, and the F layer data are discussed in terms of these measurements, and also in terms of electric field and neutral N₂ density measurements made by other satellites on other occasions. The major features observed are as follows: A tongue of F region ionization extends from the dayside across the polar cap, which is accounted for by antisunward drift due to magnetospheric convection. In the F layer and topside ionosphere, the main effect of auroral precipitation appears to be heating and expansion of the topside. A region of low F layer density appears on the morning side of the polar cap, which may be due to convection and possibly also to enhanced N₂ densities.     

A spherical harmonic analysis of the austral auroral oval

Computerized spherical harmonic analysis is applied to the morphology of the southern auroral oval. Records from 23 All Sky Camera stations together with visual observation reports for the period 1957–1959 constitute the raw data set. The mode of the derived auroral occurrence distribution function F(K_p, θ, φ) is regarded as the maximum probability contour and yields a set of auroral ovals. These 10 contours, one for each K_p level, are expressed in the invariant magnetic co-ordinates of Bond (1968).     

Short-term critical frequency variations and their predictions in the midlatitude ionospheric F2 region

Short-term variations δf₀F₂ in the values of the critical frequency of the ionospheric F2 region in middle latitudes due to solar and geomagnetic activities have been investigated. Diurnal and seasonal features of the energy flow from the auroral into midlatitude ionosphere are revealed. It is shown that they could be taken into account if instead of the 3-hour geomagnetic indices or their daily averages a new index is employed which characterizes the average level of geomagnetic activity over intervals of time no less than nine hours usually during the evening and night hours. A technique for short-term predicting δf₀F₂ in the midlatitude ionosphere is developed which employs the indices of solar and geomagnetic activities, and errors in the predictions are estimated.     

The generation of vertical thermospheric winds and gravity waves at auroral latitudes—I. Observations of vertical winds

Observations of vertical and horizontal thermospheric winds, using the OI (3P-1D) 630 nm emission line, by ground-based Fabry-Perot interferometers in Northern Scandinavia and in Svalbard (Spitzbergen) have identified sources of strong vertical winds in the high latitude thermosphere. Observations from Svalbard (78.2N 15.6E) indicate a systematic diurnal pattern of strong downward winds in the period 06.00 U.T. to about 18.00 U.T., with strong upward winds between 20.00 U.T. and 05.00 U.T. Typical velocities of 30 m s^?1 downward and 50 m s^?1 upward occur, and there is day to day variability in the magnitude (30–80 m s^?1) and phase (+/- 3 h) in the basically diurnal variation. Strong and persistent downward winds may also occur for periods of several hours in the afternoon and evening parts of the auroral oval, associated with the eastward auroral electrojet (northward electric fields and westward ion drifts and winds), during periods of strong geomagnetic disturbances. Average downward values of 30–50 m s^?1 have been observed for periods of 4–6 h at times of large and long-lasting positive bay disturbances in this region. It would appear that the strong vertical winds of the polar cap and disturbed dusk auroral oval are not in the main associated with propagating wave-like features of the wind field. A further identified source is strongly time-dependent and generates very rapid upward vertical motions for periods of 15–30 min as a result of intense local heating in the magnetic midnight region of the auroral oval during the expansion phase of geomagnetic disturbances, and accompanying intense magnetic and auroral disturbances. In the last events, the height-integrated vertical wind (associated with a mean altitude of about 240 km) may exceed 100–150 m s^?1. These disturbances also invariably cause major time-dependent changes of the horizontal wind field with, for example, horizontal wind changes exceeding 500 m s^?1 within 30 min. The changes of vertical winds and the horizontal wind field are highly correlated, and respond directly to the local geomagnetic energy input. In contrast to the behaviour observed in the polar cap or in the disturbed afternoon auroral oval, the ‘expansion phase’ source, which corresponds to the classical ‘auroral substorm’, generates strong time-dependent wind features which may propagate globally. This source thus directly generates one class of thermospheric gravity waves. In this first paper we will consider the experimental evidence for vertical winds. In a second paper we will use a three-dimensional time-dependent model to identify the respective roles of geomagnetic energy and momentum in the creation of both classes of vertical wind sources, and consider their propagation and effects on global thermospheric dynamics.     

Effects of interplanetary magnetic field and magnetospheric substorm variations on the dayside aurora

Photometric observations of dayside auroras are compared with simultaneous measurements of geomagnetic disturbances from meridian chains of stations on the dayside and on the nightside to document the dynamics of dayside auroras in relation to local and global disturbances. These observations are related to measurements of the interplanetary magnetic field (IMF) from the satellites ISEE-1 and 3. It is shown that the dayside auroral zone shifts equatorward and poleward with the growth and decay of the circum-oval/polar cap geomagnetic disturbance and with negative and positive changes in the north-south component of the interplanetary magnetic field (B_z). The geomagnetic disturbance associated with the auroral shift is identified as the DP2 mode. In the post-noon sector the horizontal disturbance vector of the geomagnetic field changes from southward to northward with decreasing latitude, thereby changing sign near the center of the oval precipitation region. Discrete auroral forms are observed close to or equatorward of the ΔH = 0 line which separates positive and negative H-component deflections. This reversal moves in latitude with the aurora and it probably reflects a transition of the electric field direction at the polar cap boundary. Thus, the discrete auroral forms observed on the dayside are in the region of sunward-convecting field lines. A model is proposed to explain the equatorward and poleward movement of the dayside oval in terms of a dayside current system which is intensified by a southward movement of the IMF vector. According to this model, the Pedersen component of the ionospheric current is connected with the magnetopause boundary layer via field-aligned current (FAC) sheets. Enhanced current intensity, corresponding to southward auroral shift, is consistent with increased energy extraction from the solar wind. In this way the observed association of DP2 current system variations and auroral oval expansion/contraction is explained as an effect of a global, ‘direct’ response of the electromagnetic state of the magnetosphere due to the influence of the solar wind magnetic field. Estimates of electric field, current, and the rate of Joule heat dissipation in the polar cap ionosphere are obtained from the model.