A model of ionospheric F-2 region storms in middle latitudes

The proposed ionospheric storm model is based on a heat source located at magnetic noon on Feldstein's auroral oval. The rotation of the Earth produces an apparent motion of the source which is greater than the speed of the disturbance. This gives rise to a wake or front which sweeps over the globe and determines the onset time of the negative phase which results from a change in chemical composition. At the front, focussing will occur which accounts for the sudden drop in electron density (or contents) sometimes observed. The calculated onset times of the negative phase are compared with observations for a number of storms. The local onset times vary from 12 at the latitude of the source to around 24 at 10° geomagnetic latitude. This model predicts that the onset of the negative phase at a given location, for storm which commence between about 2000 LT to about 1000 LT, is independent of the time of storm commencement.     

Measurements of the E region neutral wind field

The neutral E-region wind field was measured at Calgary, Canada (51°N, 114°W) during 75 nights in 1982. Observations of the Doppler shift of the 5577-Å emission line of atomic oxygen using a Fabry-Perot interferometer were converted to horizontal wind vectors. From the analysis of the data, four categories of wind characteristics were identified. In order of increasing magnetic activity these categories are (a) wind field mostly variable in space and time, (b) predominantly equatorward flow throughout the night, (c) predominantly poleward flow throughout the night and (d) north-westward flow before midnight and southward after midnight. The wind magnitude was also variable and on some disturbed days exceeded 200 m s^?1.     

Magnetic disturbance effects in the E region of the ionosphere

Detailed studies of the daytime E-region critical frequency at Aberystwyth (geomagnetic latitude +56°) show clear evidence for changes associated with both the axially-symmetric (Dst) and asymmetric (DS) components of the disturbance magnetic field. Comparison of the sensitivity of the E-region peak density to these two influences shows that the changes cannot entirely (if at all) be ascribed to the influence of electric currents in the region. It is suggested that a major role is played by dynamical influences associated with the neutral air “storm circulation” which distributes the energy fed into the auroral region to lower latitudes.     

A study of blanketing sporadic E in the equatorial region

Ionosonde data, obtained on blanketing sporadic E at some equatorial stations during the I.G.Y. have been analysed to yield temporal and latitude variations. The results are compared with corresponding ones for the middle latitudes and they are also discussed in the light of the wind-shear mechanism. The main features of the occurrence frequency are (i) the absence of a morning peak in the daily variation, (ii) an equinoctial maximum and a June-solstitial minimum, (iii) and a stronger dependence on the dip angle (or geomagnetic latitude) than on geographic latitude. The latitude variation obtained also suggests that blanketing sporadic E would occur over both the dip and geomagnetic equators.     

Photoionization rates in the night-time E- and F-region ionosphere1

Quantitative estimates of ionization sources that maintain the night-time E- and F-region ionosphere are given. Starlight (stellar continuum radiation in the spectral inverval 911–1026 Å) and resonance scattering of solar Ly-β into the night sector are the most important sources in the E-region and are capable of maintaining observable electron densities of order (1–4) × 10³ cm^?3. Starlight ionization rates have substantial variations (factors of 2–4) with latitude and time of year since the brightest stars in the night sky occur in the southern Milky Way and Orion regions. In the lower F-region the major O⁺ source in the equatorial ionosphere is 910 Å radiation from the O⁺ recombination in the F2-region, whereas in the extratropical ionosphere interplanetary 584 Å radiation only exceeds resonance scattering of solar 584 and 304 Å radiation as the dominant O⁺ source during the month of December.     

Comparative studies of E-region ionospheric drifts and meteor winds

Ionospheric drifts using total reflections from the E-region have been compared with neutral winds measured by meteor radar. Close agreement was found when both measurements were made in a common volume of atmosphere. Even with a separation of 700 km between the measuring regions the results were very similar. It is concluded that the drift technique does measure the movement of the neutral atmosphere in the altitude range 95–120 km. The agreement between measurements from widely separated regions indicates the horizontal scale of the wind structure is at least 700 km.     

Rocket observations of middle latitude sporadic E, magnetic fields,winds and ionization

Two Skylark rockets carrying ion and R.F. electron probes, lithium and sodium vapourisers and proton precession magnetometers were launched from Woomera, Australia in November and December 1965 and made at least four encounters with sporadic E ionization. A magnetic field minimum was detected on only one of these encounters, and the minimum was found to be 2–3 km below the observed ion layer. The wind measurements deduced from observations of the vapour trails indicated that the sporadic E layer occurred in a region of ion divergence.     

The role of neutral winds and ionospheric electric field in forming stable sporadic E-layers

Using the combined measurements from a rocket flight through a stable intense sporadic E-layer, we examine the shortcomings of conventional wind shear theory of ion layer formation, principally in underestimating the role of the ambient ionospheric electric field. Our results imply that the ionospheric electric field may control the stability and precise location of such ionisation layers within a region of convergent ion flow.     

An analysis of auroral E region neutral winds based on incoherent scatter radar observations at Chatanika

Auroral E region neutral winds determined from incoherent scatter radar observations at Chatanika, AK, during geomagnetic disturbances (15 May 1974) are compared with detailed theoretical calculations of neutral velocities for these conditions. The theoretical velocities are obtained by numerically solving the ion and neutral momentum equations in the ion drag approximation, including coriolis and viscous forces, using observed electric fields and electron densities. Large vertical gradients are found in the calculated velocities for altitudes below about 130 km. As a consequence of this structure and fluctuations in the electron density profiles, the data analysis procedure of Brekke et al. (1973) for obtaining neutral winds from radar data is found to underestimate the wind speed by up to 40%, but it determines the direction and temporal structure reasonably well. Comparison of observed neutral velocities with calculated values shows that ion drag alone cannot account for the observations. An equation is derived to estimate the pressure gradients required to resolve the discrepancy between calculated and observed neutral winds. Accelerations due to these pressure gradients are of the same order as those due to ion drag, but at least an order of magnitude larger than those due to solar heating. Directions of the horizontal pressure gradients are consistent with expected locations of auroral heating. During geomagnetic disturbances, ion drag and auroral heating both appear to play important roles in the generation and modification of neutral winds.     

Concerning the electron temperature discrepancy between in situ and remote probes in the E-region

Rocket borne Langmuir probe measurements of electron temperature in the E-region are examined in relation to recent laboratory investigations of surface drift effects which can lead to erroneously high and time-dependent electron temperature measurements. The rocket data is consistent with the laboratory expectations thus supporting the suggested importance of surface effects in rocket measurements and in relation to the E-region discrepancy with simultaneous incoherent radar scatter measurements.     

Determination of electric fields and neutral wind velocities in the ionospheric E- and F-regions from observations of artificial ion clouds

A new method of interpreting the behaviour of artificial ion clouds released in the Earth's ionosphere is presented. It is shown that values for the ionospheric electric field, neutral wind velocities and, in some circumstances, ion collision frequency, can be deduced from a study of the motion and deformation of the ion clouds, including those released in the E-region.     

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.     

Ion composition in the E- and lower F- region above kiruna during sunset and sunrise

A magnetic type mass spectrometer has been flown on two ESRO sounding rockets from ESRANGE (Kiruna 68°N) on February 25 and 26, 1970. The first launch was at sunset (16:33 UT) and the second the next morning, during sunrise (04:47 UT). For both flights the solar zenith angle was approximately 98°. The instrument was measuring simultaneously the neutral gas and positive ion composition and the total ion density. In this paper the results of the ion composition measurements are presented. For both flights the main ion constituents measured between approximately 110–220 km were O⁺, NO⁺ and O₂⁺. Only at sunset were N⁺ and N₂⁺ detected above 200 km. In spite of the identical solar UV-radiation, pronounced sunset/sunrise variations in the positive ion composition were found. The total ion densities at sunrise were between 5×10³ and 5 × 10⁴ ions cm^?3 and therefore too high to be explained without a night-time ionization by precipitated particles. At sunrise the NO⁺ and O₂⁺ profiles show a correlated wavelike structure with three pronounced almost equally spaced layers in the E-region. Only the highest layer is present in the O⁺ profile. Locally enhanced field aligned ionization originated by particle precipitation and an E × B instability are the most likely source for this structure. In the E- and lower F-regions the $\text{NO}{}^{\mathrm{+}}\text{O}{}_2{}^{\mathrm{+}}$ ration increased overnight from values around 7 at sunset to 15 at sunrise, correlated with an increase of the local magnetic activity index K from 0⁺ to 2°. This could be explained if the NO density and magnetic activity are correlated.     

On the u.v. source of the night-time E region

The photo-ionization produced in the night-time E region by the total stellar u.v. continuum is considered. It is shown to be approximately equal to that produced by the scattered Lyman β above about 115 km.     

A study of F2 region night-time vertical ionization fluxes at millstone hill

Vertical fluxes of ionization in the F2 region have been measured by the incoherent scatter technique over Millstone Hill in 1969. The results obtained near midnight for the region above h_maxF2 have been examined to determine whether there is a significant flux of ionization from the magnetosphere to the ionosphere that serves to maintain the F-layer. It is found that H⁺ ions are a minor constituent over the altitude range in which useful measurements can be made, so that any conclusion must rest upon properly interpreting the observed O⁺ fluxes. By selecting periods when the layer did not appear to be decaying rapidly it was hoped to find cases where the O⁺ flux did not vary with altitude in the range 500 h 800 km (i.e. where losses are unimportant), since this would imply that the flux is of magnetospheric origin.

While three cases exhibited this behaviour, the majority exhibited a decrease in the O⁺ flux with height, indicating that the layer was descending. Attempts to correct for this were made, and the average flux from the magnetosphere was estimated as 3 × 10⁷ el/cm²/sec. This is in fair agreement with other recent estimates, and implies that at this latitude the ionosphere is not maintained solely by the magnetospheric flux. Moreover, large increases in flux that could give rise to nocturnal increases in the total content of the layer do not appear to have been seen.     

Dominant acceleration processes of ambient energetic protons (E ⩾ 50 keV) at the Bow Shock: Conditions and limitations

Energetic proton (E_p ? 50 keV) and magnetic field observations during crossings of the Earth's Bow Shock by the IMP-7 and 8 spacecraft are incorporated in this work in order to examine the effect of the Bow Shock on a pre-existing proton population under different “interplanetary magnetic field-Bow Shock” configurations, as well as the conditions for the presence of the Bow Shock associated energetic proton intensity enhancements. The presented observations indicate that the dominant process for the efficient acceleration of ambient energetic particles to energies exceeding ~ 50 keV is by “gradient-B” drifting parallel to the induced electric field at quasi-perpendicular Bow Shocks under certain well defined limitations deriving from the finite and curved Bow Shock surface. It is shown that the proton acceleration at the Bow Shock is most efficient for high values of the upstream magnetic field (in general B₁ > 8γ), high upstream plasma speed and expanded Bow Shock fronts, as well as for directions of the induced electric field oriented almost parallel to the flanks of the Bow Shock, i.e. when the drift distance of protons parallel to the electric field at the shock front is considerably smaller than the local radius of curvature of the Bow Shock. The implications of the presented observations of Bow Shock crossings as to the source of the energetic proton intensity enhancements are discussed.