High Levels Of Solar Activity Research Articles

Current status of solar EUV measurements and modeling

EUV measurements, modeling, and prediction are a part of the body of information needed for detailing our grasp of what anthropogenic changes are now occurring to the upper atmosphere and ionosphere. Since the next set of full spectrum EUV measurements will likely not occur until the late 1990's, several empirical solar EUV models will help bridge the data gap. These models provide solar EUV flux estimates beyond the low, moderate, or high solar activity levels that are given by reference spectra. Each of the models provides important advantages for certain types of calculations and each model contains inherent weaknesses. An overview is presented of 3 of the primary reference spectra and the 5 principal models in use. Model reliability issues, a graphical comparison between models and rocket data, a description of the proxies used by each model, and examples of studies where EUV model fluxes are compared with ionospheric data are discussed.

Parameterized ionospheric model: A global ionospheric parameterization based on first principles models

We describe a parameterized ionospheric model (PIM), a global model of theoretical ionospheric climatology based on diurnally reproducible runs of four physics based numerical models of the ionosphere. The four numerical models, taken together, cover the E and F layers for all latitudes, longitudes, and local times. PIM consists of a semianalytic representation of diurnally reproducible runs of these models for low, moderate, and high levels of both solar and geomagnetic activity and for June and December solstice and March equinox conditions. PIM produces output in several user selectable formats including global or regional latitude/longitude grids (in either geographic or geomagnetic coordinates), a set of user specified points (which could lie along a satellite orbital path), or an altitude/azimuth/elevation grid for a user‐specified location. The user selectable output variables include profile parameters (ƒ0F2, hmF2, total electron content, etc.), electron density profiles, and ion composition (O+, NO+, and O2+).

Night-time enhancements in TEC at equatorial anomaly latitudes

The seasonal and solar activity variations of night-time total electron content (TEC) enhancements and their latitudinal and longitudinal dependencies in the northern equatorial anomaly region (11–23° geomagnetic latitude) are studied by using data from three eastern longitude stations and one western longitude station. Two kinds of enhancement are observed: a postsunset enhancement which occurs most frequently in autumn and a postmidnight enhancement which occurs most frequently in summer. The enhancements, especially the postmidnight enhancements, are more frequent and stronger at the eastern longitude stations than at the western longitude station. The enhancements are weak and least frequent at 23° geomagnetic latitude; the seasonal dependence of the postsunset enhancements at this latitude is different from that at lower latitudes confirming that the effect of the prereversal enhancement of the equatorial fountain, which produces the postsunset enhancements in TEC in the equatorial anomaly region, rarely reaches this latitude even under very high levels of solar activity.

F2 peak electron density at Millstone Hill and Hobart: Comparison of theory and measurement at solar maximum

This paper compares the observed behavior of the F2 layer of the ionosphere at Millstone Hill and Hobart with calculations from the field line interhemispheric plasma (FLIP) model for solar maximum, solstice conditions in 1990. During the study period the daily F10.7 index varied by more than a factor of 2 (123 to 280), but the 81‐day mean F10.7 (F10.7A) was almost constant near 190. Calculations were performed with and without the effects of vibrationally excited N2 (N2*) which affects the loss rate of atomic oxygen ions. In the case without N2* there is generally good agreement between the model and measurement for the daytime, peak density of the F region (NmF2). Both the model and the measurement show a strong seasonal anomaly with the winter noon densities a factor of 3 to 4 greater than the summer noon densities at Millstone Hill and a factor of 2 greater at Hobart. The seasonal anomaly in the model is caused by changes in the neutral composition as given by the mass spectrometer and incoherent scatter (MSIS) 86 neutral density model. There is generally little or no increase in the observed noon NmF2 as a function of daily F10.7 except at Millstone Hill in winter. In May‐July, where the measured NmF2 shows least dependence on daily F10.7, there is excellent agreement between the model and data. The modeled NmF2 is about 30% less than the measured values at Millstone Hill at the December solstice, but both model and data increase with increasing daily F10.7 index. At Hobart, on the other hand, the model densities are greater than or comparable to the measured densities for the December solstice. This suggests that the differences between model and data are not due to the incorrect solar EUV flux. The effect of including N2* is to worsen the agreement between model and data at Millstone Hill by reducing the summer densities from good agreement to 40% below the data. In winter the N2* effects are much smaller, and the densities are reduced by only 10%. While N2* worsens the model‐data comparison at Millstone Hill, it does bring the model seasonal density ratio into better agreement with the data and also improves the agreement at Hobart. Although the 1990 daytime ionosphere can be well modeled without N2*, it may still be important for high levels of solar and magnetic activity. There is a very close relationship between the height at which peak density occurs hmF2 variation and the NmF2 variation with F10.7 in summer at Millstone Hill. In contrast to the generally good agreement between model and data at noon, the model badly underestimates the density at night at Millstone Hill at all seasons. At Hobart the model reproduces the nighttime density variations well in both winter and summer. The international reference ionosphere (IRI) model generally provides a good representation of the average behavior of noon NmF2 and hmF2 but because the data show a lot of day‐to‐day variability, there are often large differences. The FLIP model is able to reproduce this variability when hmF2 is specified. The IRI model peak densities are better than the FLIP densities at night, but the IRI model does not represent the Millstone Hill summer data very well at night in 1990.

A study of HF field strength variation in India and its prediction

Measured field strength data pertaining to several HF broadcast transmissions monitored in India during 1987-90 has been analysed to study solar activity, local time and seasonal variations in the field-strength. Values tend to saturate at high solar activity levels when sunspot numbers are beyond 120. Variations in field strength have also been studied with varying ratios of operational frequency to maximum usable frequency (f/sub op//MUF) and equivalent vertical frequency of the links. Based on this study a prediction method for HF field strength has been suggested for India. Comparison of values estimated using this technique has been made with actual measured field strength values for different HF transmissions for different months. This method is to be particularly suitable for field operators.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modelling the undisturbed high-latitude E region

Electron density profiles measured by the EISCAT incoherent scatter radar are used to determine the undisturbed behaviour of the E region at 69.6° N, 19.2° E. Results are presented showing the solar zenith dependence for both low (F10.7 < 80) and high (F10.7 > 180) solar activity levels. The results are compared with aeronomical calculations based on the MSIS86 neutral atmosphere model and measurements of solar EUV fluxes, and with the International Reference Ionosphere. Fairly good agreement is found between IRI and the measurements for solar zenith angles less than 80°. The aeronomical calculations show good agreement with the measurements of the bottomside and peak of the E region also for solar zenith angles less than 80°. For zenith angles from 85° to 90° neither model gives a good representation of the true electron density profile, particularly in winter when a strong, narrow layer peaking at about 100 km altitude is observed. A pronounced valley above the E region peak is predicted by the aeronomical calculations at all solar zenith angles but is not observed.

Modelling the undisturbed high-latitude E region

Electron density profiles measured by the EISCAT incoherent scatter radar are used to determine the undisturbed behaviour of the E region at 69.6° N, 19.2° E. Results are presented showing the solar zenith dependence for both low (F10.7 < 80) and high (F10.7 > 180) solar activity levels. The results are compared with aeronomical calculations based on the MSIS86 neutral atmosphere model and measurements of solar EUV fluxes, and with the International Reference Ionosphere. Fairly good agreement is found between IRI and the measurements for solar zenith angles less than 80°. The aeronomical calculations show good agreement with the measurements of the bottomside and peak of the E region also for solar zenith angles less than 80°. For zenith angles from 85° to 90° neither model gives a good representation of the true electron density profile, particularly in winter when a strong, narrow layer peaking at about 100 km altitude is observed. A pronounced valley above the E region peak is predicted by the aeronomical calculations at all solar zenith angles but is not observed.

Measurements of ionospheric and thermospheric temperatures and densities with the middle and upper atmosphere radar

The first incoherent scatter radar local time/seasonal/solar cycle averages of F region electron, ion, and neutral temperatures for Asian longitudes are presented. These measurements were made with the four‐pulse experiment by the middle and upper atmosphere (MU) radar in Japan over the period August 1986 to April 1990. The neutral temperature and density results are compared with MSIS‐86 model predictions. We divide our data into low and high solar activity levels, four seasons, and 24 one‐hour times bins. While the seasonal and solar activity behavior of the ionospheric density, and of the anticorrelation of the electron temperature with this density, shows many aspects in common with those at other locations, a few particular differences are evident. At low solar activity the height of the F layer varies from 240 km during the day to 320 km at night, regardless of season. At high solar activity the maximum density of the F layer varies little with season. These facts seem to indicate little seasonal change in F region neutral atmospheric composition at this geographic location. At solar maximum the F layer is established at a higher altitude than at low solar activity, and the summer F layer in particular is formed at such a high altitude that its decay at night is very slow, its diurnal variation is hence weak, and the density remains so high at sunrise that little sunrise effect in the electron temperature is produced. The neutral temperatures in the F region have the same basic diurnal pattern as does the MSIS‐1986 model temperatures, but the radar temperatures are consistently lower by an amount ranging up to 160 K for summer conditions at high solar activity. The measured and model temperatures are closest in the autumn and winter. The neutral density results are of low quality and do not provide any evidence of need to modify the MSIS model densities.

Solar cycle effects on the near‐Earth space environment

Forecasts of the magnitude of solar activity in the 1990s (solar cycle 22) imply that the expected levels of activity might be some of the most extreme ever recorded, and almost certainly the levels of activity will be the highest experienced during the space age. Even as early as 1 year before the expected maximum of solar cycle 22 in 1990, unprecedented levels of solar activity (for example, the solar flares and solar particle events of August–October 1989) and geomagnetic activity (for example, the auroral events and geomagnetic storms of March 1989) have been observed. These solar and geophysical events have stirred scientific interest both in the long‐term behavior of solar activity and in the physics which couples the energy of solar events to the near‐Earth environment. Furthermore, the operational community (including those involved in satellite operations, telephone and radio communication, electric power distribution, aviation, and others) have experienced many adverse effects of these solar and geophysical events. Many more episodes of activity are expected throughout the upcoming 4–5 years. The purpose of this paper is to review the direct and indirect influences of solar activity on the near‐Earth environment and to describe some of the implications of the high levels of solar activity which are expected to occur in the 1990–1994 time period.

Improved world-wide maps of monthly median foF2

New global maps of monthly median values of foF2 have been prepared using over 45,000 station months of foF2 observations, semi-empirical model values of foF2 in the mid-latitude ocean areas and empirical model values for the equatorial and high latitude regions. These observations have been carefully screened and mapped, using the Jones-Galley technique, to produce monthly median maps of foF2 for each hour, each month and for high and low levels of solar activity.

Geophysical model improvement using tranet data

Doppler observations of U.S. Navy Navigation Satellites have been used to strengthen terrestrial networks in many areas of the world and to connect the networks and isolated sites to an earth-centered coordinate system. Parameters of the model used in the computation of the satellite ephemerides used for this purpose result in geodetic positions which are displaced from the center of mass of the earth by about 4 m with respect to the equator, and in longitudes which are rotated from the BIH conventional longitude by about 0.80 seconds of arc. Third order ionospheric effects neglected in the orbit computations in the calculation of site coordinates result in radial displacements of as much as several meters for high levels of solar activity when the right ascension of the node of the satellite is near the right ascension of the sun.

Analysis of the orbit of the satellite Cosmos 482 in its last 15 days

The satellite Cosmos 482 (1972-23A) was launched on 31 March 1972 into an orbit of eccentricity 0.42 inclined at 52° to the equator, and decayed on 5 May 1981. Orbits have been determined daily in the last 15 days of its life from NORAD and U. S. Navy observations, with the aid of the R. A. E. orbit determination program PROP 6. The orbital accuracy was about 40 m radially and cross-track. Various properties of the upper atmosphere were determined by analysing four of the orbital elements. The decrease in orbital inclination gave two values for atmospheric rotation rate at heights near 200 km, a morning value of 0.90 ± 0.05 rev/day and an average value of 1.05 ± 0.10 rev/day. The variation in the right ascension of the node, after removal of perturbations due to gravity and atmospheric rotation, gave a value for the meridional wind at a similar height: the wind was weak, 20 ± 20 m/s north to south at 08 h local time. From the decrease in perigee distance, density scale heights were determined at heights of 180-220 km accurate to 3%, and were found to be significantly higher (by 6%) than those indicated by the COSPAR International Reference Atomsphere; at the high levels of solar activity prevailing in 1981, CIRA gives too low a scale height. Finally, the angular motion of perigee was analysed, and indicated a day-to-night variation in density 0.7 ± 0.1 of that given by CIRA , implying a value of 1.14 ± 0.02 for the ratio of maximum day-time to minimum night-time density at 180 km height in April-May 1981.

Upper-atmosphere features revealed by the orbit of 1980-43A

The satellite NOAA-B (1980-43A) was launched in May 1980 into an orbit with perigee height near 260 km and apogee height 1440 km, at an inclination of 92.2°.The lifetime was 11 months. The orbit has been determined at 40 epochs between October 1980 and May 1981 from about 3000 radar and optical observations. The average orbital accuracy, radial and cross-track, was about 100 m, with rather better accuracy in the final 14 days. The variation of orbital inclination has been analysed to determine two good values of atmospheric rotation rate, namely 1.10 ± 0.10 rev day −1 at 300 km (average local time) and 1.15 ± 0.06 rev day −1 at 225 km (evening). The effect of atmospheric rotation on the precession of the orbital plane of an actual satellite has never previously been detected; it is clearly apparent for 1980-43A in its last days and conforms to the expected theoretical change. The variation of perigee height has been analysed to determine ten values of atmospheric density scale height, for heights of 280–370 km. These values, accurate to about 3%, exceed by 15% the values indicated by the COSPAR International Reference Atmosphere. Solar activity was higher in the years 1980–1981 than at any time since early 1958 and it appears that the CIRA model underestimates the density and density scale height at high levels of solar activity.

Prediction of total electron content using the International Reference Ionosphere

The International Reference Ionosphere (IRI) is a model of the ionosphere, based on experimental data, which has been proposed as a standard ionospheric model. As such, it should be tested extensively to determine its range of validity. One of the ways in which the electron denisty profile given by the IRI, especially above the peak of the F layer, can be tested is to compare calculated and observed values of total electron content (TEC). We have therefore studied the discrepancies between calculated and observed values of TEC recorded at 15 stations covering a wide range of longitudes and latitudes, mainly in the northern hemisphere, and mainly for high levels of solar activity. W have found that the IRI produces reasonably accurate values of TEC at mid and high latitudes, but that it greatly underestimates the daytime values of TEC at low latitudes. We conclude therefore that the daytime electron density profile given by the IRI is reasonably accurate at mid and high latitudes, at least above the peak of the F2 layer. The situation at low latitudes clearly requires more work, and we have suggested two possible lines of study. The generally low discrepancies at night indicate that the night-time electron density profiles given by the IRI correspond fairly closely to the actual profiles.

A global view of F‐region electron density and temperature at solar maximum

Dynamics Explorer‐2 is permitting the first measurements of the global structure of the F‐region at very high levels of solar activity ( &gt; 200). Selected full orbits of Langmuir probe measurements of electron temperature, Te, and density, Ne, are shown to illustrate this global structure and some of the ionospheric features that are the topic of other papers in this issue. The ionospheric thermal structure is of particular interest because Te is a sensitive indicator of the coupling of magnetospheric energy into the upper atmosphere. A comparison of these heating effects with those observed at solar minimum shows that the magnetospheric sources are more important at solar maximum, as might have been expected. Heating at the cusp, the auroral oval and the plasmapause is generally both greater and more variable. Electron cooling rate calculations employing low latitude measurements indicate that solar extreme ultraviolet heating of the F region at solar maximum is enhanced by a factor that is greater than the increase in solar flux. Some of this enhanced electron heating arises from the increase in electron heating efficiency at the higher Ne of solar maximum, but this appears insufficient to completely resolve the discrepancy.

Diurnal and seasonal variations in atomic and molecular oxygen inferred from Atmosphere Explorer‐C

Ion mass spectrometer data collected onboard the Atmosphere Explorer‐C satellite during the period December 1974‐January 1975 have been used to infer, via the ion chemistry, the molecular oxygen concentration near 250km. To filter out temperature induced variations the O2 densities and in situ neutral composition measurements of atomic oxygen were extrapolated down to 120km. At this level the latitudinal and diurnal variations of O are found to be consistent with the ones inferred from OGO‐6 during high levels of solar activity. The results also show that at mid to high northern (winter) latitudes the O2 concentration is about a factor of two higher than at low southern (summer) latitudes, thus revealing an apparent reversal of the winter to summer increase in O2 deduced from optical and incoherent scatter measurements during higher levels of solar activity. Considering that the winter oxygen bulge in the lower thermosphere apparently prevails throughout the solar cycle the latitudinal (annual) variations in O2 can play a decisive role in explaining the reversal of the winter anomaly during minimum solar activity. The diurnal variations in O2 reveal a pronounced minimum at 18 LT thus suggesting that photo dissociation of O2 is important.

Electron density profiles from ionograms: Underlying ionization corrections and their comparison with rocket results

Electron density profiles from nine daytime rocket flights at Wallops Island, Va., conducted by the University of Illinois at high and low levels of solar activity, are compared with profiles calculated by inversion of ionograms obtained at the same times and location. Sources of error and uncertainty in the ionogram inversion are discussed, as are means for their amelioration. In most cases, agreement between the two kinds of measurement within a few percent in electron density and within a few percent of a scale height can be achieved.

Intensity variation of atomic oxygen red line in the day airglow with solar activity

Rates of production of O(1D) atoms in the upper atmosphere by photodissociation of O2, dissociative recombination of O2+, NO+ and electron impact excitation of O(3P) have been calculated for low, medium and high levels of solar activity. Variations with solar activity, of neutral and ionic composition, electron and neutral temperatures of the upper atmosphere and solar extreme ultraviolet fluxes incident on it have been taken into consideration.

Change of the main characteristics of cosmic-ray variations and interplanetary space with the cycle of solar activity

Changes of the characteristics of the solar-diurnal variation and Forbush decreases of cosmic rays with the solar activity cycle are analyzed. It is shown that the energy spectrum of the solar-diurnal variation is well described by the Krymsky diffusion mechanism spectrum with b parameters of 59, 40, and 30 BeV for high, middle, and low solar activity levels, respectively. The Forbush-decrease spectrum is described by a power law with exponents −0.8 and −0.6 during high and low solar activity. These changes are interpreted to be the result of a decrease of solar wind velocity and a decrease of the mean strength of the interplanetary magnetic field with a decrease in solar activity.

Variations in exospheric density at heights near 1100km, derived from satellite orbits

In an earlier paper, values of exospheric density were obtained from the orbit of Echo 2 for the years 1964–1965. The results indicated a semi-annual variation in density by a factor of between 2 and 3, considerably larger than predicted by existing atmospheric models. These studies have now been extended to the beginning of 1967, using both Echo 2 and Calsphere 1, to show how the density is responding to increasing solar activity. Variations in density during 1964 have been analysed in more detail. The long-term variation associated with the solar cycle and the short-term variations associated with magnetic and solar disturbances agree with the variations expected on the basis of current models. The semi-annual variation is persisting to higher levels of solar activity, and although its amplitude is diminishing the factor of variation was still 1.6 in 1966.