Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Ionospheric electrodynamic model with an eccentric dipole geomagnetic field

A computer model of ionospheric electrodynamic processes using an eccentric dipole (ED) for the geomagnetic field has been developed. This is a development from existing models which are based on the centred dipole (CD) coaxial with the geographic axis. The ED dynamo model introduces or modifies the effects of hemispherical asymmetry and longitudinal variation in the dynamo processes through two explicit parameters—the geomagnetic field intensity and the length of the field lines. These parameters of the ED field have been quantified and displayed. An additional contribution to the above effects comes implicitly from the ionospheric parameters—plasma density and atmospheric tidal winds—which become asymmetric relative to the ED dip equator. The integrated effect of the geomagnetic and ionospheric parameters produces significant variation in the field line integrated ionospheric conductivity. The ED dynamo model shows that the peak height of the equatorial electrojet (EEJ) moves by over 2 km and height profiles of the EEJ display strong hemispherical asymmetry.     

Solar cycle variation of the total electron content around equatorial anomaly crest region in east asia

The monthly mean hourly values of total electron content data obtained at Lunping Observatory (geographic coordinates 25.00°N, 121.17°E; geomagnetic coordinates 14.3°N, 191.3°E) by using the ETS2 satellite beacon signal during the period from March 1977 to December 1990 have been used to analyze the solar cycle variations of total electron content (TEC) around equatorial anomaly crest region in East Asia. Positive, correlations were found between the 12 month running average of monthly mean TECs and sunspot numbers. By using the linear regression analysis method, the contour charts for real diurnal and seasonal variations of TEC at certain sunspot numbers were constructed and described. The diurnal variation of TEC was represented by the sum of its diurnal mean and first three harmonic components. The solar cycle variations of these components have also been discussed.     

Indirect phase height measurements of the lower ionosphere compared with rocket measurements of D-region electron density

A comparison between rocket-measured electron density profiles of the lower ionosphere and the results of ground-based indirect phase height measurements in the LF range, carried out near the Soviet rocket sounding station Volgograd over several years, confirms—to a first-order approximation—the height of the level of electron density which according to magnetoionic ray theory is necessary for reflection of the waves. In a second-order approximation, however, an additional phase path change has to be taken into account, which is caused by the ionization below the reflection level. This makes the observed phase height always slightly smaller than the real geometric height, on average by −1 km, but in extreme cases by up to −4 km, depending on the actual height gradient of electron density below the reflection level. Due to systematic diurnal and seasonal variations of this gradient, the amplitude of the diurnal variation of the observed phase height is found to be slightly larger than that of the real geometric height, whereas the reverse is true for the seasonal variation at constant solar zenith angle.     

Monitoring schumann resonances-11. Daily and seasonal frequency variations

The time variations of the Schumann resonance peak frequencies for the first three modes are presented in the vertical electric component measured in the Nagycenk Observatory (47.6°N, 16.7°E) from May 1993 to August 1994. The average daily frequency patterns are different for the three modes, and each mode shows a distinct seasonal variation. The recurrence of this seasonal variation is also shown. The daily frequency range, in which the frequencies shift, is wider in winter than in summer in all three modes. The mean frequency level also shows a seasonal variation in the third mode. A spring-autumn asymmetry has been found in case of the first mode.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Can the high latitude ionosphere support large field-aligned ion drifts?

Ion velocities perpendicular and parallel to the geomagnetic field have recently been deduced by Smith et al. from bistatic measurements at 71° geomagnetic latitude in the afternoon sector. The results of this experiment include large (>400 m s⁻¹) downward ion velocities parallel to the magnetic field that persist for hours, small (100 m s⁻¹) ion velocities perpendicular to the magnetic field and electron density profiles with extremely narrow full-width at half-maximum. The explanation of these results was that the ionospheric flux tubes observed were near the terminator, and thus, sunlit at the top and in darkness at the bottom. The difference in production between the top and bottom of the flux tube creates an excess of ions at the top, which rapidly diffuse downwards. A three-dimensional, time-dependent model of the ionosphere has been used to test this explanation. Numerical experiments were performed to determine upper limits for the downward ion velocity. Assuming reasonable vertically-induced ion drifts due to either neutral winds or plasma convection, these upper limits were substantially smaller than the measurements. The location of the terminator was found to contribute a maximum of about 60 m s⁻¹ to the vertical ion velocity due to diffusion in a partially illuminated flux tube. In an attempt to explain the narrow density profiles without invoking an additional ionization source, the downward force in the model was arbitrarily increased, as would occur due to parallel electric fields in the ionosphere. Since the interpretation of these measurements as large field-aligned flows seems untenable by a model thought to be consistent with the currently accepted physics of the atmosphere, an alternate hypothesis is presented. If the common volume measurement is made in a region of O⁺ precipitation, then the line profile would not be Doppler shifted when viewed off-zenith. Therefore, the field-aligned velocities would be small, and the narrow width of the profiles would be due to enhanced electron densities in an O⁺ arc.     

A comparison of foF2 obtained from a time-dependent ionospheric model with Argentine Islands' data for quiet conditions

In this study a comparison is made of the Utah State University Time-Dependent Ionospheric Model (TDIM) and an ionosonde data set from Argentine Islands. This study is unique in that the Argentine Islands data set of foF2 spans complete diurnal, seasonal and solar cycle conditions for low geomagnetic activity. The TDIM reproduces these foF2 variations extremely well. Although the observed winter and summer solstice foF2 diurnal curves have opposite phases, they are readily modelled. At equinox where a sharp transition occurs from winter to summer, or vice versa, the monthly average is complicated by this feature and hence the TDIM does not reproduce the diurnal fine structure.The neutral wind induced vertical plasma drift is the only free parameter in this study. All the other inputs are fixed for the specific solar, seasonal and diurnal conditions. A vertical plasma drift variation is presented; although simplistic, it couples the geographic and geomagnetic frames. With additional information such as hmF2, it would be possible to deduce a unique vertically induced drift pattern.     

foF2 hysteresis variations and the semi-annual geomagnetic wave

Seasonal and solar cycle variations of the foF2 hysteresis magnitude are investigated. Data for the noon foF2 monthly medians for Slough (51.48°N, 0.57°W), the monthly means for the sunspot numbers, and for the geomagnetic activity index aa(N) for the northern hemisphere for the period 1933–1986, covering solar cycle from 17 to 21, are used. It is found that: (1) the greatest negative amplitudes of the foF2 hysteresis variation are near the equinoxes, and (2) the solar cycle average noon foF2 hysteresis magnitude is linearly correlated with the solar cycle average semi-annual geomagnetic amplitude of the aa-index. These results support the hypothesis that the foF2 hysteresis is due to the geomagnetic activity variation during the sunspot cycle.     

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and Z) were included in the study.The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude.The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq(H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude.The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.     

Dynamical coupling of the auroral F-region ionosphere and thermosphere: case studies

During two 24 h periods of EISCAT observations in the summer of 1982, the F-region ion temperature and density responded differently before and after midnight to large ion convective flows. Such observations were recently reported at Chatanika (Alaska), however, the mechanism invoked to interpret these measurements (large day-to-night variation in electron density affecting the coupling between ions and neutrals) appears insufficient, for summer conditions, to account for the EISCAT observations. Hence, it is proposed, with the support of Fabry-Perot observations and numerical models, that in addition to the electron density asymmetry, the presence of a large southward neutral wind around midnight induces, through Coriolis coupling, a zonal neutral wind of an opposite direction to the convective flow. This enhances considerably the frictional energy and momentum transfer between ions and neutrals in the post-midnight sector.     

Major ionospheric storms during July–September 1982 at lower latitudes in East Asia

The period July–September 1982 was the peak of geomagnetic activity during the 21st solar cycle. There were four very severe storms, which covered the strongest period of geomagnetic disturbances seen in recent years, namely the storms on 13/14 July, 7 August, 6/7 September and 21/22 September 1982. The present paper analyzes the corresponding ionospheric behaviour during these major magnetic storms, along with the storms that occurred around the local summer months during the preceding years within both hemispheres of the East Asia-Oceanian sector. It is shown that during the summer months in the low-latitudes and lower mid-latitudes of this sector the strong geomagnetic storms generally result in depletion of the ionospheric electron content and electron concentration at the F-region peak during both day-time and night-time. The situation in summer is quite different from other seasons and is basically unrelated to SSC time or/and maximum disturbance time and duration.     

F-region drift velocities in the dusk sector mid-latitude trough

Analysis of 6 months of ground-based ionosonde data from mid/high-latitude Digisonde stations at Millstone Hill, Argentina and Goose Bay, shows the relation between the formation of the mid-latitude trough in the dusk sector and the measured F-region drift velocities. The observed westward drift velocities in the trough are comparable in magnitude with the velocity of the Earth's rotation as required by the stagnation theory of trough formation. Using the Digisonde database of 15 min samples of electron density profiles and F-region drifts, a new trough detection algorithm automatically identifies the occurrence of the trough at any of the three stations. Correlating trough occurrence with the measured drift velocities indicates that troughs develop due to an increase in the horizontal westward velocity component. The extent of the trough formation relates to the magnitude of the horizontal velocity.     

Response of the outer ionosphere to the magnetic storm of 20–22 March 1990

Measurements of the thermal plasma parameters in the interval 17–23 March 1990, made within the scope of the ACTIVE Project, were analysed to study the response of the outer ionosphere to the storm with SSC at 22.45 UT on 20 March. These measurements in the morning sector at altitudes around 2000 km clearly reflect the enhanced geomagnetic activity. They allow us to estimate the radial depth to which the plasmasphere has been affected by increased magnetospheric convection and at which a new equatorial plasmapause has formed. They also provide the possibility of monitoring the initial phase of recovery.The measurements in the dusk altitudes of 500–1000 km indicate a distincttrough in electron concentration and also a trough of light ions under relatively quiet, as well as under disturbed, conditions. The position of the equatorial edge of these troughs and the position of electron temperature peaks enable us to judge whether a plasmaspheric bulge has formed, and whether an inner plasmapause exists.     

The seasonal variations in the nighttime 6300 Å emission at midlatitudes from ISIS-II spacecraft

Optical limb observations at F-region heights from the ISIS-II satellite have been used to study the seasonal variations in the 6300 Å limb emission for nighttime conditions and the aeronomic implications. The observations were carried out over the American zone at northern midlatitudes, and refer mainly to the period 1973–1975 of low solar activity.The observed seasonal variations in the emission seem to be mainly controlled by the electron density at F-region heights for nighttime and quiet geomagnetic conditions. The winter minimum is found to be deeper than the summer minimum. The obervations give clear evidence of semiannual variation in the emission. The phase variations agree closely with that of the semiannual variations in electron density and neutral atmospheric density at F-region heights. However, the amplitude variations of the semiannual variations are found to be larger than suggested by the observed F-region electron density. The observations during highly disturbed conditions possibly show the presence of gravity waves with wavelengths around 500 km, which could transport auroral energy to lower latitudes. The midlatitude enhancements observed during disturbed conditions seem to be related to the inward movement of the plasmapause.     

Neutral density cells in the high latitude thermosphere—2. Mechanisms

NCAR-TIGCM simulations predict mesoscale cellular structures in the high latitude neutral density at altitudes from 120–350 km. During magnetically active conditions, the density structure at 200 km consists of low-density cells near dawn and dusk and high-density cells near noon and midnight. Mechanisms causing the structured density cells are a result of thermosphere-ionosphere coupling and can be explained in terms of dynamic meteorology. For example, at high latitudes ion drag causes the neutral circulation to flow cyclonically in the dawn sector and anticyclonically in the dusk sector. Low densities are contained within the cyclonic circulation at all altitudes. Below about 170 km, the densities inside the anticyclonic flow are high, while above that altitude densities within the anticyclonic flow are low. While typical dynamic meteorology explains low densities in the centre of cyclonic circulation and high densities inside anticyclonic circulation, the dusk low-density cell in the centre of anticyclonic flow is unexpected. The anticyclonic dusk low-density cell is explained by anomalous antibaric flow due to high-speed winds. 120 km and 200 km altitudes are used to demonstrate the relationship between the high latitude densities and winds as well as the effect of joule heating and auroral particle precipitation on the density structures.     

Control of diurnal and seasonal variation of particle precipitation by regular changes of the interplanetary magnetic field

Systematic changes of the position of the dipole axis of the Earth's magnetic field with respect to the solar axis induce distinct daily and seasonal variations of the vertical B_z-component in the solarmagnetospheric coordinate system (B_ZSM). Depending on the direction of the interplanetary magnetic field (IMF), negative B_ZSM- values are produced in spring by T polarity and in autumn by A polarity, whereas in the diurnal variation lowest B_ZSM-values have been calculated to occur near 23 UT for T, and near 11 UT for A polarity, respectively. In different ionospheric and geomagnetic parameters measured at high and midlatitudes increased precipitation of high energetic particles into the lower thermosphere and upper mesosphere has been detected during periods with negative B_ZSM-components. The seasonal variation of the parameters investigated, with maximum values near the equinoxes, as well as a part of their diurnal variations, can thus be explained by particle precipitation being markedly controlled by the IMF sector structure.     

Some further results on long term mesospheric and lower thermospheric wind observations by the Arecibo UHF radar

A second series of long term mesospheric and lower thermospheric wind observations was conducted at Arecibo (18.4°N, 66.8°W) between 6 and 20 March 1981 using the UHF Doppler radar, following the first observations in August 1980 (Hirota et al., 1983). Zonal and meridional wind velocities were measured during the morning (8–10 LT) and afternoon (13–15 LT) periods. The mean wind profile averaged over the entire observational period shows the predominance of the diurnal tide. The fluctuating wind vector rotates clockwise relative to height with a characteristic vertical scale of about 10 km. The phase difference inferred by a cross correlation analysis between morning and afternoon profiles indicates that the dominant period is about 20–30 h. This oscillation is discussed in relation to internal inertia-gravity waves observed by the same radar in the lower stratosphere. On the other hand, wind fluctuation with a vertical scale larger than 20 km shows a substantial day-to-day variation with a period of 5–8 days. This long period oscillation shows a good correlation with the global scale geopotential height anomalies at 1 mb (46–48 km) observed by the Tiros-N satellite at 20°N. Our evidence suggests that westward travelling planetary-scale waves with zonal wavenumber one may propagate up to the lower thermosphere.     

Seasonal trend of geomagnetic activity derived from solar-terrestrial geometry confirms an axial-equinoctial theory and reveals deficiency in planetary indices

At the magnetopause, solar wind plasma interacts with the terrestrial magnetic field, with the consequent entry of solar wind energy into the magnetosphere and the ionosphere. Geomagnetic activity is one of the results. Planetary geomagnetic indices, e.g. Kp, Ap, Am, etc, have been designed to measure solar particle radiation by its magnetic effects. Long-term averages of these indices have established that solar wind energy input into the ionosphere maximizes around equinoctial months with minima around the solstices. Although considerable progress has been made to explain qualitatively the semiannual variation o1' geomagnetic activity, its component parts, representing the axial and equinoctial hypotheses, have not so far been put together with a high degree of quantitative precision. This paper demonstrates that the semiannual trend of geomagnetic activity can be reproduced quantitatively with good precision by using accurate astronomical data relating to the Sun-Earth geometry. The key factor is the combination of the varying solar declination and the heliographic latitude of the Earth during different months. Analysis shows that the seasonal trend of solar wind-magnetopause coupling is, in fact, controlled by a combination of the two competing theories, the axial and equinoctial, which have been advanced over the years to explain the semiannual variation in geomagnetic activity. Planetary ion density of the F2 layer of the ionosphere (F2pd) is another index of relatively higher accuracy which also shows marked maxima around the equinoxes. The observed seasonal trend of F2pd can be reproduced by using the semiannual trend of geomagnetic activity as derived from astronomical data with a correlation coefficient of 0.98. This analysis also brings out another important fact that the planetary indices, Kp, Ap, Am and AA, are somewhat deficient as they respond to solar declination only and do not bring out the contribution of the heliographic latitude of the Earth.     

Geomagnetic field variation and the equivalent current system generated by an ionospheric dynamo at the solstice

The geomagnetic field variation and equivalent current system produced by an asymmetrical ionospheric dynamo action under a solstitial condition are simulated and compared with the observational results. Results of our simulation reproduce well most of the observational features of the solstitial Sq system. For example, the latitude of the current vortex center is higher in summer than in winter and the local time of the center in the summer hemisphere is located earlier than that in the winter hemisphere. In the morning and afternoon sector the current vortex in the summer hemisphere invades the winter hemisphere. The first feature is attributed to the ionospheric currents, but the second and third features are due to the field-aligned currents generated by the asymmetry of the ionospheric dynamo.