Post-sunset Enhancement Research Articles

The role of the equatorial electrojet in the evening ionosphere

This paper focuses on the role of the equatorial E region in the electrodynamics of the evening ionosphere. The influence and reaction of the electrojet current on the equatorial ionosphere at sunset is investigated using a field line integrated, one‐dimensional, electrodynamic model. The one‐dimensional, time‐varying model predicts the divergence of the horizontal current of the equatorial electrojet for a given time variation of the horizontal electric field. The negative divergence of the horizontal current during the evening hours provides a net upward current out of the equatorial E region into the integrated ionosphere of higher equatorial altitudes (and equivalent latitudes). This upward current affects the vertical electric field magnitudes and subsequent horizontal plasma drifts of the overlying ionosphere. The model allows for chemical recombination and dynamic redistribution of ionization within the electrojet region under the assumption that the profile of the ionization density along a field line is proportional to the chemical equilibrium profile. The eastward horizontal electric field and the net upward current during the 2 hours after sunset combine to lift the ionization out of the E region resulting in ionization densities less than the equilibrium values. As the ionization densities (conductivities) are reduced, the electrodynamics of the equatorial ionosphere is altered. This model of the equatorial electrojet current divergence can be used as a lower boundary to global, two‐dimensional models of the equatorial electric fields.Finally, it is proposed that the equatorial electrojet current near sunset has a significant role in the determination of the postsunset enhancement of the horizontal electric field. The electrojet region provides the best avenue for current to be channeled from the dayside to meet the vertical current demands of the F region neutral wind dynamo after sunset. The conductivity reduction in the E region due to the recombination of ionization and the plasma uplift enhances the horizontal (eastward) electric field and thereby increases the speed of the uplift. Thus the dynamic adjustment of ionization has an unstable, feedback relationship with the electric fields which may explain the night to night variability of the horizontal electric field enhancement.

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index ( S 10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread- F ( ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.

Frequency and duration of disturbances in the mid‐latitude F region of the ionosphere

Disturbances of the ionospheric F region at two well‐separated mid‐latitude stations were identified on rapid run ionograms by spread echoes, blackouts, and anomalies in structure or critical frequency. The diurnal variation in the probability of disturbance consistently shows minima around sunrise and sunset. Details such as a brief postsunset enhancement in winter are revealed by the fine time resolution of the data. Disturbance probabilities were lowest in autumn and spring (8% and 9%), while that for winter (33%) exceeded the summer level (20%). Spread F constituted 32% of the events, and disturbances producing sharply defined ionogram signatures 77%, with some overlapping of types. The majority of disturbances were not correlated with geomagnetic phenomena. Disturbances at the two stations were essentially independent (spatial correlation of 0.15). Spread F events were of longer duration (25 min) than sharp disturbances (20 min). TIDs were the exception, with a most probable duration of 80 min, but accounted for only 8.3% of the sharp disturbances. The absence of disturbances in the F region at dusk and especially at dawn coincides with periods of instability in the E region.

Modeling the total electron content observations above Ascension Island

Recent Faraday rotation (136 MHz) measurements between Ascension Island (8° S geographic latitude; 345° E geographic longitude; 15° S. dip latitude) and the Sirio satellite (azimuth 0°, elevation 80°) reveal two characteristic features associated with the undisturbed, ambient ionosphere: a late afternoon decrease in TEC followed by a postsunset enhancement that lasts for two to three hours. We theoretically investigate the ambient ionosphere above Ascension Island by numerically solving the time‐dependent plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, drift and neutral wind (both zonal and meridional components) appropriate for an equinoctial, solar cycle maximum period. It is found that the postsunset enhancement in upward drift, which is a characteristic feature observed by the Jicamarca incoherent scatter radar facility during solar cycle maximum periods, is primarily responsible for the postsunset increase in TEC observed at Ascension Island between 1900 and 2300 LT. The late afternoon decrease in TEC is caused by an increase in the poleward neutral wind velocity, which lowers the F layer into a higher loss rate region. Inclusion of the meridional wind not only reproduces the observed decrease but also modulates the postsunset peak in TEC so that it is in much better agreement with the observed values. Calculated north‐south asymmetries in TEC caused by a zonal wind component are also discussed.

The electron content and its variations at Natal, Brazil

The Faraday rotation and amplitude data for the period September 1978 through August 1979, recorded from the geostationary satellite SMS 1 (90°W) at Natal (5.9°S, 35.2°W), have been utilized to study the behavior of total electron content (TEC). Apart from diurnal and seasonal behavior of TEC, two characteristic features of the nighttime ionosphere are discussed: (1) a postsunset enhancement in TEC which lasts for several hours following the rapid sunset decrease and (2) the sharp, isolated changes in TEC. In particular, relating to phenomenon 2, the depletions in TEC are usually accompanied by a simultaneous increase in fading rate, scintillation index, and amplitude level, while the enhancements are accompanied by a simultaneous decrease in fading rate, scintillation index, and amplitude level. The statistics of their occurrence and nature are described. The average behavior of the signal amplitude after propagating through either a depleted or an enhanced ionosphere is modeled theoretically and shown to be in agreement with the observed experimental behavior.

Modeling the ambient, low latitude F-region ionosphere—a review

This paper describes some recent theoretical modeling efforts which primarily study the effects of E×B drift, both vertical and horizontal, on the equatorial ionosphere. Solving the time-dependent ion continuity equation for plasma densities as a function of altitude, latitude and local time it is shown that: (1) by incorporating F-region, vertical E×B drifts which are typical of solar cycle maximum and minimum equinoctial conditions, good agreement is achieved between observed and calculated ƒoF2 values at the magnetic equator (American sector) for these two periods. (2) Modeling the seasonal anomaly in ƒof2 at Kodaikanal for October 1975 and July 1976 requires different vertical E×B drift patterns for these two months and both of these patterns are significantly different from the drifts measured at Jicamarca. (3) By incorporating a realistic E×B drift model which includes the post-sunset enhancement in upward drift, bottomside F-region electron density scale heights are found to be in good agreement with measured values in the 1900–2400 LT period. (4) When a realistic east-west E×B drift pattern is included, calculated electron density profiles at the magnetic equator between 1800 and 2400 LT do not differ significantly from calculated profiles obtained when this component of E×B drift is omitted.

Seasonal variations in the low latitude dynamo current system near sunspot maximum

Strong seasonal variations in the equatorial dynamo-induced electric fields have been deduced from incoherent-scatter sounder data. These seasonal variations are studied in this paper using a dynamo model in conjunction with the thermospheric neutral wind system. The E-region dynamo system is studied using an equivalent mesh circuit model. The model contains a detailed magnetic field system, includes the influence of conjugate effects and employs a quadrature synthesis technique to fit experimental and theoretical results. The thermospheric wind system is restricted to two tidal modes; the first evanescent diurnal mode and the semidiurnal mode. The wind systems are converted to geomagnetic coordinates and global dynamo currents and electric fields are derived for equinox and solstice periods. It is shown that the post sunset enhancement in electric field can be explained by this simple E-region dynamo model.

Conjugate photoelectrons at L = 5·6 and the 6300 Å postsunset enhancement

A series of simultaneous incoherent-backscatter and 6300 Å tilting-filter photometer observations were made in the winter and early spring of 1972 at Chatanika, Alaska. These observations have been combined for magnetically quiet nights to deduce the presence of excitation by two sources in addition to dissociative recombination. The first appears to be a source that is steady for at least three hours, centered about local midnight, but that varies from night to night, taking on values between 10 and 40 Rayleighs for the nights in question. It is suggested that this source may be a flux of low-energy electrons or protons. The second source is impact excitation by electrons from the magnetic conjugate point. The emission due to this source appears to be small immediately after local sunset, rises to a maximum at about 92° conjugate solar zenith angle (CSZA), and then decreases to zero in the vicinity of 105° CSZA. The deduced intensities for this source at 92° CSZA are in the region of 15–30 Rayleighs for local F-layer critical frequencies of 3−2 MHz, respectively.

Post-sunset cooling behavior of the lunar surface

Differential infrared flux scans at 22μm have been made across the nighttime lunar surface over a range of phases before and after new Moon. The differential chopping technique effectively cancels atmospheric emission in the beam path but records the flux difference between adjacent resolution elements on the lunar disk. The scans show that the brightness temperature gradient across the highlands after sunset is much greater than that across the western maria. The large gradients consistently disappear approximately 3.5 days after sunset. The post-sunset enhancement could be due to surface roughness in the highlands or to a significant surface rock population with a mean size of approximately 0.5 m. The effect can be seen in the 10μm measurements of other investigators, but its global nature was not detected in their limited data sets.