John S. Nisbet

Photoelectron escape from the ionosphere

Abstract Photoelectron fluxes escaping from the ionosphere into the magnetosphere are calculated including the effect of photoelectron diffusion for a range of solar activity. The method employs a two dimensional matrix of continuity equations. In the region where the mean free path of the photoelectrons is small compared with the scale height the predominant term in the transport equation is the upward diffusion flux. As the altitude increases three components of the flux can be considered, an upward escaping component, an upward non-escaping component and a downward component. The relation between these fluxes, the heating of the protonosphere, and the photoelectron fluxes at the far end of the field line are examined.

Global exospheric temperatures and densities under active solar conditions

Temperatures measured by the OGO-6 satellite using the 6300 A airglow spectrum are compared with temperatures derived from total densities and N2 densities. It is shown that while the variation of the total densities with latitude and magnetic activity agree well with values used for CIRA (1972), the temperature behavior is very different. While the temperatures derived from the N2 density were in much better agreement there were several important differences which radically affect the pressure gradients. The variation of temperature with magnetic activity showed seasonal and local time variations. Neutral temperature, density, pressure and boundary oxygen variations for the storm of 8 March 1970 are presented.

Relations between the Birkeland currents, the auroral electrojet indices and high latitude Joule heating

Abstract A simplified model of the auroral ionosphere, the Birkeland currents, and the auroral electrojets is used to derive theoretical relationships between the field-aligned currents, the auroral electrojet indices, and the energy deposited in high latitude Joule heating. Experimental observations and values estimated from computer models of the current system are used to establish typical values for the parameters in the equations. Statistical relations between the magnetic indices are used to provide estimates also in terms of the planetary Kp index. It is shown that the auroral electrojet indices AL, AU and AE are related to the total Birkeland current in amps, Ip by the approximate expressions AL = −(1.4 × 10 −6 −1.6 × 10 −8 δ)I t 1.26 [nT] , AU = (6.7 × 10 −7 −2.1 × 10 −9 δ)I t 1.28 [nT] , where δ is the solar declination angle in degrees. The total power dissipated in Joule heating in the auroral electrojets is estimated to be, for equinox conditions P = I t 2 8.4 × 10 −7 I t 1.27 + 120 [W] . where I/t is the total Birkeland current out of the hemisphere in amps.

Simulations of Electric Fields within a Thunderstorm

Abstract Numerical simulations based on a three-dimensional model for the electric fields in a thunderstorm are presented. In some of the simulations, we solve problems with known analytical solutions in order to determine the relevant physical properties that must be incorporated in a thunderstorm model. We then examine the inverse problem: Given measurements for the electric fields in a thunderstorm what are the associated current generators? Fits based on an analytic formula that neglects conduction currents give approximations to the current generators while simulations based on the thunderstorm model yield refinements to the generators. As a specific illustration, we obtain estimates for current generators associated with a storm observed at the Kennedy Space Center on 11 July 1978. Finally, we explore qualitative properties of our method used to simulate lightning. It is observed that as the charged particles associated with the thunderstorm are spread over a larger and larger volume, the flesh rate...

Effects of tidal oscillations in the neutral atmosphere on electron densities in the E-region

Abstract Asymmetries of the electron density about local noon in the E -region are examined. A model study has been conducted in which the effects of the large changes in neutral density and temperatures on calculated electron densities have been compared with incoherent scatter measurements in the height range 95–120 km at St. Santin, France. It is shown that the large differences in the electron density at fixed altitudes between morning and afternoon are produced by the tidal temperature and density changes in the neutral atmosphere. The significance of these results to the development of ionospheric and thermospheric models and to the analysis of electron densities on a global basis to study tidal modes is discussed.

A thermospheric model from satellite orbital decay densities and incoherent scatter temperatures.

Abstract Although Thomson scatter measurements of the neutral temperature have shown that the timing of the maximum is later than the 1400 hr density maximum, current reference atmospheres have built-in restrictions forcing the temperature parameter to be in phase with the densities. Measurements at Arecibo have shown that temperature gradient changes in the region of 125 km are consistent with this phase difference. This fact is exploited in the development of an analytic model which is compatible with both the satellite measurements and the available incoherent-scatter measurements. A main feature of this model is that day-length is included as a major model parameter.

Fluctuations of electron density in the daytime F-region

Abstract Further work on the large electron density and electron temperature fluctuations observed on quiet summer and winter days is reported. The data are examined in relation to explanations based on electrodynamic effects, neutral wind variations, gravity waves, and movement of ionospherie irregularities over the site. The characteristics of the fluctuations, as measured at Arecibo, have been supplemented by ionosonde data at other sites. Vertical transport velocities have been calculated based on the assumption that the fluctuations represent a time term variation by the solution of the F -region continuity equation from 250 km to 700 km. CIRA 1965 neutral atmosphere models together with current estimates of the solar flux and loss rate coefficients have been employed. The vertical velocities calculated in this way have been compared with estimates based on magnetograms from San Juan, neutral wind velocities, E - and F -region drifts measurements by the method of similar fades in Puerto Rico, and with measurements made at Arecibo by Thome using antenna rotation.

Coupling of mid-latitude spread-F between conjugate stations

Abstract It appears that spread-F is associated with electric field perturbations and that above a certain scale length transverse electric fields are coupled between hemispheres. Two stations, Puerto Rico and Port Stanley, have been examined which though similar in geomagnetic latitude and longitude have very different geographical latitudes. This has allowed the separation of effects due to the season, the daylength and the post midnight collapse. The scale length of the coupling has been estimated and related to the degree to which the spread-F effects appeared to be coupled. It is shown that at times when the conditions are different in the two hemispheres marked differences occur in the spread-F incidence. It is concluded that dominant control of midlatitude spread-F does not involve interhemispheric coupling. This seems to imply that dominant control of midlatitude spread-F is played by irregularities of scales shorter than 2 km and consequently that coupling between the E-and F-regions is also unlikely to be of dominant importance. At times the marked coincidence of spread-F in the two hemispheres makes it appear that when conditions at both ends of the field line are suitable for the initiation of spread-F, coupling takes place. During the decrease in spread-F during the post midnight collapse at Arecibo there is some evidence that the incidence of spread-F increases in the conjugate hemisphere and that this coupling may be related to interhemispheric plasma flow.

The evolution and discharge of electric fields within a thunderstorm

A 3-dimensional electrical model for a thunderstorm is developed and finite difference approximations to the model are analyzed. If the spatial derivatives are approximated by a method akin to the ☐ scheme and if the temporal derivative is approximated by either a backward difference or the Crank-Nicholson scheme, we show that the resulting discretization is unconditionally stable. The forward difference approximation to the time derivative is stable when the time step is sufficiently small relative to the ratio between the permittivity and the conductivity. Max-norm error estimates for the discrete approximations are established. To handle the propagation of lightning, special numerical techniques are devised based on the Inverse Matrix Modification Formula and Cholesky updates. Numerical comparisons between the model and theoretical results of Wilson and Holzer-Saxon are presented. We also apply our model to a storm observed at the Kennedy Space Center on July 11, 1978.

Effect of excited states of atomic oxygen ions on the reaction rates and thermal balance in the F-region

Abstract A recent paper by Rutherford and Vroom (1971) showed that the charge transfer reaction of the metastable O+(2D) with N2 was significantly faster than the ground state, O+(4S), reaction. Similarly, Stebbings et al. (1966) showed that the charge transfer with O2 was at least an order of magnitude greater for the excited O+(2D) than the ground state atomic oxygen ion. Thus a substantial portion of the atomic oxygen ions in the O+(2D) state will provide an important source of N2+ and O2+ which will have the effect of increasing the electron-ion recombination rate. The excited states of atomic oxygen ions can also undergo de-excitation by means of radiative transitions or collisional electron deactivation. The electron deactivation provides the electrons with several eV of energy for each collision, and thus provides a highly efficient heating source for the ambient electrons. This paper presents the reaction rates for the metastable states of O+ and other relevant data needed to calculate the electron-ion recombination rate and the ambient electron heating rate. A comparison is made to indicate how much effect the excited states of O+ have on these two processes in the F-region.