S. Chandra

F-REGION IONIZATION AND HEATING DURING MAGNETIC STORMS.

The continuity and heat conduction equations for electrons, ions and neutral species are solved simultaneously from the viewpoint of studying the F-region magnetic storm behavior. It is shown that many of the observed changes in the F-region during the main phase of a magnetic storm can be successfully explained by assuming a decrease in oxygen concentration at the turbopause level. The specific changes in neutral composition, electron density, and electron, ion, and neutral temperatures during magnetic storms are discussed for different solar conditions.

The influence of varying solar flux on ionospheric temperatures and densities - A theoretical study

Abstract The electron continuity equation and the heat conduction equations for electrons, ions and neutral species are solved simultaneously for specified boundary conditions and values of the solar flux. These solutions have yielded a set of selfconsisterrt steady state profiles for a variety of midday solar conditions that are in agreement with observational data. To produce these profiles only the boundary conditions and solar flux are used as variable parameters of the problem. It is shown that the neutral temperature and the resulting neutral gas composition play a dominant role in determining the charged particle density and temperature profiles. This leads to a picture of the solar cycle variation where composition changes in the neutral gas (introduced at the lower boundary) must be combined with the solar flux variation to produce physically reasonable results. Of the five neutral gases used, (N2, O2, O, He, H) the atomic oxygen density variations are shown to be most effective in producing the observed trends for all the temperatures and densities in the E and F-regions.

Ion and neutral composition changes in the thermospheric region during magnetic storms

Abstract The structural differences of the ion and neutral composition in the thermospheric region are studied by solving a system of basic ionospheric and atmospheric equations. The study shows that the compositional changes during a magnetic storm arise largely as a result of changes in the neutral composition at the turbopause. A decrease in [O]/[N 2 ] in the lower atmosphere triggers a complex chain of events which results in an increase of the neutral gas temperature, depletion of the O + layer and enhancement of NO + . The relative changes in these layers occasionally produce a sequence of electron density profiles giving rise to the so-called G condition. It is shown that, compared to the neutral atmosphere, the ionosphere is much more sensitive to the changes in [O]/[N 2 ] in the lower thernaospheric region. Since the ionospheric parameters can be measured much more accurately than the atmospheric parameters, it is argued that they should form an integral part of the observational data required to construct the atmospheric models.

Energetics and thermal structure of the middle atmosphere

Abstract A study of a large number of temperature measurements in the middle atmosphere shows a much more complex thermal structure of this region than described in the U.S. Standard Atmosphere, 1976. The mesopause height which is generally assumed to be at 80 km varies between 70–100 km, often with two minima in temperature at about 70 and 100 km and a maximum between 80–85 km. By solving the energy balance equation and the equations of continuity, the physical significance of the observed thermal structure is discussed in terms of the energetics of the various regions of the middle atmosphere. It is shown that the solar u.v. radiation plays a major role only in the energy budget of the stratosphere and the lower thermosphere. The energetics of the mesosphere is primarily influenced by the dissipation of eddy energy. The temperature in this region is a good indicator of the eddy diffusivity and can be used in deriving the eddy diffusion coefficient.

Ionospheric warming by neutral winds

Abstract The effect of frictional heating by means of neutral winds on the ion and electron temperature in the undisturbed ionosphere is studied theoretically by solving a system of basic ionospheric and atmospheric equations. The study shows that both the electron and ion temperatures are increased in the night-time ionosphere through friction. In the region between 150 and 200 km Ti may exceed T6 by as much as 130°. The increase of Ti due to friction amounts to about 100–200°, depending on the atmospheric model employed in calculating the neutral wind velocity. It is illustrated that frictional heating may be very important for the determination of the neutral temperature from measured ion temperature values.

The diurnal heat budget of the thermosphere

Abstract Detailed numerical calculations of thermospheric heat sources and sinks are presented and their relative importance is discussed in reference to the energy balance phenomena of the neutral atmosphere. It is shown that the thermal energy available from the absorption in the Schumann-Runge continuum leading to photo-dissociation of O2 is by far the largest energy source in the lower thermosphere. Other sources of varying importance in different altitude ranges are: (1) energy from photoelectrons; (2) energy exchange from thermal plasma; (3) chemical reaction (ion-electron dissociative recombination) energy gain; (4) kinetic and dissipative energy associated with the neutral wind. The energy sinks of importance are (1) thermal conduction at the lower boundary (120km); and (2) radiative cooling of atomic oxygen. It is shown that the combined energy from processes 2–4 constitutes only a small fraction of the total energy available from photoelectrons and is in phase with the latter. These secondary sources (processes 2–4), therefore, do not constitute a significant energy source and their contribution can be simply incorporated into photoelectron energy (process 1) by defining an effective photo-ionization heating efficiency. The heating efficiencies for photo-ionization (including processes 2–4) and photo-dissociation are estimated to be 0.5 and 0.3, respectively. As the important heat input (photo-dissociation) and loss (conduction and radiation) rates are basically governed by the O2 and O densities, any diurnal or seasonal variation in these constituents at the lower boundary would have profound effects on the thermal structure of the overlying atmosphere. For this and other reasons, it is suggested that a choice of lower boundary much below 120km, e.g. near the mesopause level (90 km), should be more appropriate for general thermospheric studies.

The diurnal phase anomaly in the upper atmospheric density and temperature

Abstract The heat conduction equation for the neutral atmosphere in the region between 120–1500 km is solved using the concept of dynamic diffusion. It is shown that the present discrepancy in the phase and the diurnal amplitude of the thermospheric temperatures as inferred from incoherent back-scatter and satellite drag measurements can be fully reconciled by such an approach. The numerical solutions show that the phase and the diurnal amplitude of the atmospheric density in the region between 200–500 km are in accord with the satellite drag results; the corresponding neutral temperature, however, follows the diurnal pattern inferred from the back-scatter technique. The characteristics of the density variations above 500 km strongly depend upon the choice of the hydrogen and helium models and their relative concentration. In the region where hydrogen becomes a dominant constituent, the density reaches its maximum value in the night-time and its minimum in the day-time. The physical basis for the dynamic diffusion is discussed, and it is suggested this should replace the conventional static diffusion models in computing temperatures from the atmospheric densities obtained from satellite drag measurements.

The upper atmosphere as a regulator of subauroral red arcs.

Abstract The mechanisms for producing a subauroral red arc (SARARC) are studied by solving a system of basic ionospheric and atmospheric equations. It is shown that many of the observed features of a SARARC can be explained within the framework of the two processes generally responsible for the ionospheric behavior during a magnetic storm: these are (1) energy conduction from the magnetosphere to the ionosphere and (2) the changes in neutral composition of the lower atmosphere caused by the increase in turbulent mixing. Both the processes trigger a complex chain of events which ultimately results in the redistribution of both the charged and neutral particles, an increase in the electron, ion, and neutral temperatures, and a decrease in the electron density in the altitude region near the F2 peak. It is shown that both the occurrence and the emission intensity of a SARARC are regulated by the neutral atmosphere, even though conduction of the thermal energy from the magnetosphere to the ionosphere provides the excitation energy fo the optical remission. Recent satellite measurements of the ionospheric parameters have confirmed the validity of these findings and have provided grounds for rejecting several other theories which have been proposed in the literature.

The role of atomic oxygen in the ionospheric E- and F-region behavior

Abstract The continuity and heat conduction equations for the electron, ion and neutral gases are solved simultaneously to show the effect of using the heat loss arising from the excitation of the fine structure levels of atomic oxygen, and the heat input from a height dependent electron heating efficiency. The solutions are compared to Thomson backscatter data obtained under conditions similar to those used in the theoretical model. A family of solutions is presented for three different values of the atomic oxygen density at the turbopause level, and compares favorably to certain widely observed features of the main phase of a magnetic storm. From the comparison, it is shown that the results of decreasing the atomic oxygen density at the turbopause level are consistent with the observed changes in both the neutral and ionized constituents relative to quiet magnetic conditions.

The response of the upper atmospheric temperature to changes in solar EUV radiation and geomagnetic activity.

The upper atmospheric neutral temperature derived from satellite drag is analyzed with a view to determine the relative importance of solar EUV and magnetic activity on heating of the upper atmosphere. As the indices of solar EUV radiation, the radio fluxes obtained from the ground based observations in the frequency range of 200–9400 Mc/s are used. Also for the period March 7–May 15, 1962, a direct comparison is made with the EUV flux obtained from the OSO 1 Satellite. As the index of magnetic activity the planetary magnetic index Kp, is used. The data have been analyzed by separating the long period components of 27 days and above from the short term components by using a 5 day running mean filter. A cross correlation study between the various parameters shows that the long term variations in temperature are strongly correlated with the similar variations in the radio flux in the frequency range of 1000–3750 Mc/s, the correlation being maximum at 1000 Mc/s. The short term variations in temperature, on the other hand are strongly correlated with those in ∑Kp, the daily sum of Kp indices and not with the radio flux at any of the frequencies considered. The correlation between temperature and EUV flux in the short term components is somewhat better though not as striking as the long term components. An empirical relation between temperature and the changes in radio fluxes and ∑Kp based on the above correlation study shows that TN = 524 + 2.73 S10.7 + 1.77 (∑Kp − ∑Kp) or TN = 470 + 6.53 S30 + 1.77 (∑Kp − ∑Kp) where TN is the nighttime minimum temperature; S10.7, S30and∑Kp are the 5-day running means of 2800 Mc/s radio flux, 1000 Mc/s radio flux and ∑Kp respectively. The present investigation leads to the conclusion that short term variations in temperature are closely associated with the changes in magnetic activity represented by Kp, and all the systematic variations temperature including the so called semi-annual variation can be fully accounted for in terms of the similar variations in solar flux.