F2-region Equatorial Anamoly Modeling

Synopsis of the Ph.D.

Thesis Entitled :

A THEORETICAL STUDY OF THE Fp - REGION EQUATORIAL ANOMALY IN SOLAR MAXIMUM AND MINIMUM Submitted by

K.S.R. NARAYANA MURTHY; MSc., Department of Physics, Andhra University, Visakhapatnam-530 003, INDIA

The thesis is basically concerned with a study of the terrestrial ionospheric F2 - region equatorial anomaly (Appleton anomaly) features in solar cycle maximum and minimum conditions based on the theoretically developed models of the anomaly by the author. The off-set between geographic and geomagnetic equators is ignored in the models. The model derived features are compared with experimental observations of the seasonal and solar cycle variations of the anomaly growth, decay and latitudinal extent.

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Sections 1 to 5 of Chapter-I briefly introduce the terrestrial atmospheric structure in relation to the variation of temperature with altitude. the origin of the thermospheric wind system. the competing processes of photoionisation of the neutral species by solar XEUV radiation fluxes, chemical loss of ions arising from a variety of ionchemical reactions and loss of ions by transport processes that lead to the Formation of the various ionospheric layers. the number density, momentum and energy conservation equations as well as the thermodynamic equation besides. the role of transport and chemistry in the formation of the F!; - region peak.

A brief introduction to the origin of electric fields in the ionosphere is given in sector 6. In section 7, expressions for the longitudinal, Pedersen and Hall conductivities are arrived at starting from the ion and electron momentum transfer equations. The variation of these conductivities with height is described. A brief introduction to the E-region dynamo driven by global scale tidal winds of the atmosphere is given in section 8. The transmission of the dynamo electric fields via the highly conducting magnetic field lines to the F-region is mentioned. Maintenance of the F-region ionization during the night and the F2-region anomalies in the middle and high latitudes are dealt with in section 9. The role of thermospheric winds, energetic particles and ionosphere-magnetosphere coupling processes in maintaining the ionization during the jong polar nights and anomalous behavbur of the F-region at middle and high latitudes is explained. Earlier model and experimental studies of the F2-region equatorial anomaly, its morphological features and attempts made to explain the various features of the anomaly are described in section 10. The justification for the present investigation, its limitations and scope are addressed in section 1 1 . Chapter II mainly deals with the transformation of the O+-ion balance equation to a suitable form for numerical computation of the O+-ion densities along

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dipolar geomagnetic field lines in the anomaly region. The sources of O+-ions, their production and chemical loss mechanisms are discussed in section 2 of this chapter. The successive transformations of the coefficients and variables of the @-ion diffusion equation required to impt‘ove the numerical stabitity of the solution are addressed in section 4. The last transformation leads to a parabolic type partial differential equation suitable for obtaining stable numerical solutions for the ion densities along a number of flux tubes by using the Crank-Nicolson finite differencing scheme. The equations for determining the variable coefficients of the parabolic partial differential equation are developed. Expressions for estimating the components of ionospheric currents, parallel and perpendicular to the geomagnetic field, are deduced in section 5 of Chapter II. Section I of Chapter Ill deals with the variety of atmospheric and equatorial plasma transport parameters needed to determine the coefficients at every step of the numerical integration process. Neutral species concentrations as functions of local time, latitude and altitude are taken from MSIS-86 therrmospheric mod41 of Hedin (1987). Photoionization probability of atomic Oxygen at the top of the atmosphere in solar maximum and minimum is computed from the latest values of photon fluxes and photoionizing cross-sections at wavelengths below the H Lyman series limit. Most recent chemical loss coefficients are employed. The assumed exospheric, ion and electron temperatures as well as assumed height variation of the electron and ion temperatures are explained. An expression for determining the ambipolar diffusion coefficient as a function of the atomic Oxygen concentration and exospheric temperature is developed. Models of the thermospheric meridional winds, assumed to be height-independent, based on NCAR TGCM and HWM-87 horizontal wind models at 300 km are described. Dip equator measurements of vertical and zonal plasma drifts and their daily variations used in the numerical models are described. Section 2 of the chapter deals with the Crank-Nicotson finite differencing scheme employed besides a discussion of the determination of the dipole parameter req as the function of local time as the field line drifts in space and time by using Runge-Kutta scheme and ths dipole coordinate q for a given value of its hyperbolic transformation X. The boundary conditions required to close the tridiagonal system of linear equations in the finite differencing scheme are mentioned. The model derived features of the equatorial anomaly in solar maximum equinox, June and December solstices are discussed in the context of earlier model results and observational characteristics of the anomaly in Chapter IV. In the absence of equatorial E X B, zonal plasma drifts and neutral winds, the peak plasma densities are observed on the equator at 1600 LT in all seasons. The equatortal E X 6 drift together ambipolar diffusion of the plasma from the equator give rise to the anomaly. Thermospheric winds do influence the plasma distribution in the anomaly region. The zonal plasma drift on the equator, too, affects the anomaly evolution in space and time. The latitudinal extent of the anomaly maximizes at 2000 LT in all seasons. The maximum anomaly width is greater (= 35”) in the equinox than in the solstices (r 30’). The anomaly onset time is between 09001000 LT in all seasons. The developing anomaly crests move poleward until about 1600 LT and thereafter, the crests decay while continuing their poleward motion till about 2000 LT. At 2000 LT, the decaying crests reverse their direction of motion and by early hours, the anomaly disappears. Post-sunset enhancement of equatorial E X B plasma drift present in all seasons does not lead to an enhancement of the crests in the present model as the development of the anomaly crest ceases by about 1600 LT in the equinox and a little earlier in the solstices. The evolution of the anomaly crests in space and time is symmetrical about the equator in the equinox. In solstices, the evolution of crests is asymmetric due to the

3 trans-equatorial meridional winds. The sharp fall in the equatorial E X B plasma drift in the equinox and June solstice gives rise to night-time enhancements of equatorial region foF2 which persists until early hours. In the December solstice the night-time enhancement of equatorial region foF2 is delayed by about 4 hours and persists through the night. Noon bite-out inequatorial foF2 is projected only in December solstice model. Barring larger magnitudes of foF2 and the stoppage of anomaly growth 4 hours earlier to 2000 LT attributed to the neglect of the offset between geographic and geomagnetic equators, the rest of the model results are in excellent agreement with observed features of the solar maximum equatorial anomaly as well as Anderson (1973b) model for equinox. The model derived features of the equatorial anomaly in solar minimum equinox, June and December solstices are discussed in the context of the Sterling et al. (1969) model for equinox and observed characteristics of the anomaly in chapter V. In the absence of E X B and zonal plasma drifts and thermospheric neutral winds, peak plasma densities occur at about 1500 LT on either side of equator in all seasons. Besides ambipolar diffusion of the equatorial plasma, E K B plasma drift at the equator determine the anomaly development as in solar maximum. It has been observed that Fejer et al. (197'9) measurements of E X B plasma drift daily profiles at Jicamarca give rise to unreasonably low f o b values in the equatorial region due to the large magnitudes of the upward plasma drift in the pre-noon period. Consequently, the E X B plasma drift magnitudes are halved at every instant of time. This modification leads to comparable plasma densities with experimental observations in the solstices. But, the plasma densities are still smaller in the equinox. As in solar maximum. thermosphwic neutral winds influence the plasma distribution in the anomaly zone; but, to a lesser extent in the equinox and June solstice. The evolution of the anomhly crests in space and time is more or less symmetrical in the equinox. In the solstices, the trans-equatorial winds bring about an asymmetrical evolution of the crests with respect to the equator. The anomaly onset is again between 0900-1000 LT. In the equinox, the developing crests move poleward until 1600 LT, at which time full development of crests occurs. This time is in agreement with the observed time. beginning 1600 LT, the crests decay while moving poleward and the anomaly disappears in the early hours and not earlier as inferred from some observed data. The maximum width of the anomaly region is equinox is 26" in agreement with experimental observations. In the solstices, the development and decay of the crests follow different courses in the two hemispheres. The maximum anomaly region width is less than that observed for the equinox. On the equator, foF2 peaks at about 0700 LT in all seasons with a mild enhancement of foF2 at 16OD LT. Other than this. the model does not project equatorial region foF2 n o m bite-out. The descent of the Flayer after sunset gives rise to delayed night-time enhancement of equatorial f o b in the equinox and June solstice. In the December solstice. fob rises sharply at about 0400 LT with no night-time enhancement. The need to consider lighter ions in addition to O+ during the extreme solar minimum conditions is emphasized in the light of observational evidence for the existence of such ions at concentration levels comparable or greater than that of O+ at helghts as low as 600 km.