High Solar Flux Conditions Research Articles

A comparison of ground and satellite observations of F region vertical velocity near the dip equator

Nighttime F region vertical electrodynamic drifts were made at the magnetic equatorial stations in Africa, Ibadan (7.4°N, 3.9°E; 6°S dip), and Ouagadougou (12°N, 1.5°W; 5.9°N dip) using ionosondes hmF2 data during 1957/1958 International Geophysical Year (IGY) and 1990 periods, respectively; for high solar flux and geomagnetic quiet time conditions. We compare the seasonal averages of vertical drifts with observations made by Incoherent Scatter Radar (ISR) at Jicamarca (11.95°S, 76.87°W; 2°N dip) and Atmospheric Explorer E (AE‐E) satellite for equatorial F layer vertical drifts reported by Fejer et al. (1995). The results indicate good accord between the three techniques at periods when convection dominates other factors (e.g., around prereversal enhancement) except for June solstice drifts. However, when the drifts are completely downward (negative) between 2000 and 0500 LT sector, the mean discrepancies between ionosondes and ISR drifts are of 8–11 m/s (December solstice), 14–17 m/s (equinoxes) but comparable during June solstice with a value of about 14 m/s. Conversely, the typical values of the differences among ionosondes and AE‐E downward velocities are 7–10 m/s (December solstice), 9–12 m/s (equinoxes), yet again similar in winter with a significantly smaller value of about 3 m/s. The evening reversal times are in excellent agreement, apart from June solstice drifts, which exhibits large fluctuations. The morning reversal times show small variations. Equinoctial prereversal enhancement velocities have amplitudes of approximately 17–35 m/s between the methods. Our data are useful for global ionospheric modeling and for the predictions of development of nighttime equatorial F region irregularities at the African region where there is paucity of data.

Longitudinal variability of low‐latitude total electron content: Tidal influences

Recently, nighttime ultraviolet (UV) observations obtained by IMAGE FUV and TIMED GUVI instruments have revealed a longitudinal wave number four pattern in the nighttime airglow intensity and in the position of the equatorial anomalies during equinox and high solar flux conditions. In the present study, we have extended this work and determined the longitudinal variability of the low‐latitude total electron content (TEC) climatology during different geophysical conditions with a special emphasis on the longitudinal wave number four structure in the low‐latitude ionosphere. We have used more than 5 million low‐latitude TOPEX TEC observations covering the entire 13 years of TOPEX TEC data from August 1992 until October 2005. This data set was used to determine the local time, seasonal, solar cycle, and geomagnetic activity dependence of the longitudinal variability of TEC at equatorial and low latitudes, and in particular, to address the existence and evolution of the wave number four longitudinal pattern under these conditions. Our study shows that the wave number four pattern is created during the daytime hours at equinox and June solstice but is absent, or washed out by other processes, during December solstice. During equinox the wave number four pattern is created around noon with well‐defined longitudinal enhancements in the low‐latitude TEC. These enhancements, which are symmetric about the geomagnetic equator during this season, last for many hours and can be clearly seen past midnight. The longitudinal patterns are found to be nearly identical between the vernal (March/April) and autumnal (September/October) equinoxes and largely independent of the solar cycle conditions. The wave number four pattern is also observed during geomagnetically active conditions, indicating that the processes that create this pattern are also present during active times. The variations between the well‐defined longitudinal maxima and minima are of the order of 20%. During June solstice, the wave number four pattern is also observed in the afternoon hours but, in contrast to the equinox cases, it exhibits a strong hemispheric asymmetry and is not observed during the night. The low‐latitude TEC exhibits clear longitudinal variations during December solstice, with large daytime enhancements over the east Asian and Pacific regions and a third enhancement emerging in the afternoon over the Atlantic Ocean, but a clear wave number four pattern is not observed during this season. Although the equatorial and low‐latitude TEC values exhibit clear longitudinal patterns during all seasons, a significant amount of scatter remains in the TEC data that is not accounted for by changes in the solar cycle, the season, or the local time or by the longitudinal variability. This remaining scatter is largest near the poleward edges of the anomalies and is of the order of 40%.

DaytimeFregion ion energy balance at Arecibo for moderate to high solar flux conditions

[1] Ion energy balance studies using incoherent scatter radar (ISR) data involve the estimation of the neutral density and exospheric temperature from the ISR-measured parameters using theoretical collision cross sections. The ratio [O]radar/[O]MSIS in long-term averages is an estimate of the so-called Burnside factor and can be derived from these studies. This parameter is thought to be associated with errors in the O+-O collision cross section. The most recent comparison between the Mass Spectrometer Incoherent Scatter (MSIS) neutral atomic oxygen densities and values derived from Arecibo ISR measurements using ion energy balance shows large discrepancies for high solar flux conditions. In contrast to ion momentum studies, which typically lead to a Burnside factor greater than 1 (usually near 1.2–1.3), the discrepancies between MSIS densities and radar-derived values tend to result in low values (near 0.8) for the Burnside factor. Various interpretations of this discrepancy have been put forward. We have reanalyzed the Arecibo ISR World Day data from 1988 to 1994, corresponding to the moderate to high solar flux period of solar cycle 22. By extending the analysis to the upper F region/lower topside (and explicitly including the role of H+, which is often neglected) we obtain consistent results for the neutral density and exospheric temperature that show no significant long-term discrepancy from the MSIS predictions. For this period, we obtain a median ratio [O]radar/[O]MSIS of 1.26 ± 0.02 using the O+-O collision cross section from Banks (1966). The standard deviation of the data is about 0.35. This ratio is close to the most recently published theoretical simulations of the collision cross section within the uncertainties. Knowing the correct O+-O collision cross section allows one to extract the neutral parameters from the radar data and study short-term (day-to-day) variations in those parameters.

Solar cycle variability of nighttime topside helium ion concentrations over Arecibo

The incoherent scatter radar mode and data extraction method used in topside experiments at the Arecibo Observatory are discussed. Helium ion concentrations in the lower topside ionosphere over Arecibo are presented for low and high solar flux periods of the same season (October). Data from 8–9 October 1988, 21–22 October 1995, 26–27 October 1997, and 12–13 October 2001 are compared. The data from October 1988 and 2001 are representative of high solar flux conditions, while the other two periods (1995 and 1997) are from low solar flux conditions. All data sets represent geomagnetic quiet periods over Arecibo and serve as examples of typical equinox topside conditions. For solar minimum the altitude distribution of the helium ions usually has a maximum near the O+ to H+ transition altitude () during the night, usually around 550–650 km. The He+ number densities tend to be quite low, 2 or 3 × 103 cm−3 or <15% of the topside plasma density at the peak of the layer. For the higher solar flux case, however, He+ forms a more distinct layer at an altitude that is slightly above at night. The O+‐to‐He+ transition altitude () is reported to be below after the postsunset collapse in these solar‐maximum conditions, meaning that He+ is the dominant species. We find that the relative abundance of He+ can reach 60–65% of the topside plasma at the peak of this layer during these equinox conditions or up to 4 × 104 cm−3, with as low as 700 km and the He+ to H+ transition () extending over 1000 km at night.

Climatology and storm time dependence of nighttime thermospheric neutral winds over Millstone Hill

We use 630.0 nm nightglow Fabry‐Perot measurements over Millstone Hill from 1989–1999 to study the climatology and storm time dependence of the midlatitude thermospheric winds. Our quiet time wind patterns are consistent with results from earlier studies. We determine the perturbation winds by subtracting from each measurement the corresponding quiet time averages. The climatological zonal disturbance winds are largely independent of season and solar flux and show large early night westward and small late‐night eastward winds similar to disturbance ion drifts. The meridional perturbation winds vary strongly with season and solar flux. When the solar flux is low, the winter and equinox average meridional winds change from equatorward to poleward at ∼2200 LT, and the summer winds are equatorward throughout the night. The high solar flux meridional winds are poleward, with magnitudes increasing from dawn to dusk at all seasons. These disturbance winds patterns are in poor agreement with results from the empirical horizontal wind model, HWM‐93. The zonal and meridional disturbance winds show very large variations relative to their average values. We have also studied the time‐dependent response of the midlatitude thermospheric winds to enhanced magnetic activity. The early night westward winds build up to large amplitudes (about twice their climatological values) in ∼6 hours; the late‐night eastward winds are smaller and reach their peak values ∼3 hours after the increase in magnetic activity. The storm time dependence of the meridional winds is considerably more complex than that of the zonal winds, and it varies with season and solar flux. Following enhanced magnetic activity, equatorward winds are observed at all local times and seasons, but the increase of their amplitudes with storm time is fastest in the late local time sector. Near midnight, and when the solar flux is low, the meridional winds reverse from equatorward to poleward ∼6–10 hours after the increase in magnetic activity. This reversal is fastest (slowest) during December (June) solstice. At later local times, and for high solar flux conditions, the variation of the meridional disturbance winds is season independent. The observed storm time dependence partly explains the large variability of the disturbance winds.

Longitudinal dependence of equatorial F region vertical plasma drifts in the dusk sector

The effect of solar activity on the postsunset peak of F region vertical plasma drift at Kodaikanal (77°28′E, dip 3°N) is determined as a function of season and for two levels of magnetic activity using extensive HF phase path observations and compared with that at Jicamarca (75° W, dip 2°N) reported in the literature. The postsunset peak vertical velocity Vzp at Kodaikanal increases linearly with 10.7‐cm solar flux (F10.7) irrespective of season and level of magnetic activity. The sensitivity of Vzp to solar flux changes is more at Jicamarca than at Kodaikanal during equinoxes and December solstice. This feature is virtually absent in June solstice. The seasonally averaged values of dusktime vertical velocity at Kodaikanal for moderate to high solar flux conditions and quiet magnetic conditions(Kp ≤ 3) are in reasonable agreement with those reported very recently for the Indian sector (80°E) from AE‐E satellite data [Fejer et al., 1995]. In June solstice, the evening upward vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160°–200°E), while the postreversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230°–310°E) where there they are higher. The present study supports the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June solstice.

Nitrogen escape from Mars

The escape rate of nitrogen from Mars is calculated to be 2.3×105 s−1 for low solar flux conditions and 8.9×105 s−1 for high solar flux conditions. The major source of energetic atoms is dissociative recombination of ground state and vibrationally excited N2+ ions. The measured 15N/14N isotope ratio can be reproduced by postulating an early dense atmosphere during which little differentiation occurred.