At midlatitudes (40 & DEG; -65 & DEG;S), stratospheric ozone loss has been linked to a variety of physico-chemical effects. In this study, however, we examine how ozone responded to a fortuitous magnetic cloud (MC)-triggered storm on 20 November 2003 using a synthesis of mul-tiple platform approach. We employed this approach to predict how energetic particles interact with the atmosphere by determining real-istic NOx production rates without having to run a full ion chemistry model. Using simplified numerical approximations and synthetic satellite observations, we found that this method of ion pair production might be more economical in terms of numerical cost. Following three days of MC, the tendency for ozone reduction became apparent. According to this study, relativistic electron microbursts may pro-mote particle precipitation, which in turn induced the occurrence of NOx around 55 km midlatitude. This corroborates the hypothesis that existing electron precipitation proxies, which do not yet take relativistic microburst energies into account, are likely overlooking a significant source of precipitation that affects atmospheric ozone levels.& COPY; 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

In this study we use the CHAMP wind observations from 2002 to 2004 to investigate features of mid-latitude thermospheric winds at low and high geomagnetic activity levels. The winds are observed between the geomagnetic latitudes 40-50 degrees north or south within an altitude range of 370-450 Km and averaged longitudinally. The low geomagnetic activity level wind generally leads eastward in both hemispheres for all the seasons. Maximum westward speeds going above 100 m/s are observed at all levels of geomagnetic activity. Most investigations on F-region parameters assume the equinoxes are fundamentally the same. A first time separation of Equinox season using CHAMP winds reveals equinoctial asymmetric behavior of the wind speed at the two levels considered. Interhemispheric asymmetry in neutral wind behaviour is clearly evident in the solstice seasons with local winds considered in each of the hemispheres. This is consistent with asymmetry in the amount of solar illumination received in each of the hemispheres and differences in the magnetic field strength at magnetically conjugate regions. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

Monthly variations of averaged nighttime thermospheric winds have been investigated over Abuja, Nigeria (Geographic: 9.06°N, 7.5°E; Geomagnetic: 1.60°S). The reports are based on Fabry-Perot interferometer measurements of Doppler shifts and Doppler broadening of the 630.0 nm spectral emission. The results were obtained during a period of weak solar activity with the solar flux (F10.7) typically below 70 solar flux units. Inspection of the average monthly thermospheric winds from October 2017 to December 2017 found December meridional winds to be more equatorward than the October and November winds. Zonal winds are eastward with pre-midnight maximum speeds going above 100 m/s. Compared to Jicamarca zonal winds in the Peruvian sector for the same month of October, the magnitude of maximum Abuja zonal wind speed is weaker. We compare the observed diurnal variation with the recently updated Horizontal wind model (HWM 14). Most of the observational features of thermospheric wind diurnal variation are captured in the model variation. The HWM14 generally showed good agreement with the Abuja October and November zonal wind observations but overestimates the December meridional winds. Expected longer period analysis of the results from Abuja will stimulate a better understanding of wind climatology over the West African sector.

We present nighttime variation of thermospheric winds estimated from the Fabry-Perot interferometer (FPI) which was recently deployed at Abuja in Nigeria. These results apply to the thermosphere region around 250 km and were obtained during the period of weak solar activity with solar flux values generally below 70 s.f.u. The results presented cover three months, from October 2017 to December 2017. The high geomagnetic activity level zonal winds generally lag the low geomagnetic activity level winds. Low geomagnetic activity level wind speeds are maintained between 80 and 100 m/s. High activity zonal wind speeds revealed minimum  speed values between 0030 LT to about 0330 LT. We also present comparison between our observations and the latest version of the horizontal wind model (HWM14). The model predicts are generally in good agreement with our zonal wind observations. Our limited data used in the investigation is likely responsible for the significant discrepancies observed in the meridional winds. Key words: Equatorial thermosphere, Low solar activity, thermospheric dynamics, meridional and zonal winds.

Ground level enhancements (GLEs) of the cosmic-ray intensity have been observed seventy-one times over the past seven decades. GLEs are due to sudden increases in the intensity of solar energetic particles associated with large eruptive episodes. GLEs have been, controversially, divided into two distinct categories, gradual (classical) and impulsive events. Recent findings also argue that some GLEs are too impulsive to be accelerated in the eruptive episodes. Here we investigate this hypothesis by studying the time profiles of nine GLEs, which were observed with excellent data coverage of the associated solar eruptions. Results show that, when characterized solely on their time profile (i.e. pulse shape), GLEs do not separate into two distinct classes, but rather form a continuous distribution between these two extremes. Preliminary modelling results indicate that the interplanetary transport conditions may alter the GLE pulse shape in such a way as to obscure any source information by the time it reaches Earth. This implies that the shape of the GLE profile is, perhaps, a powerful indicator of propagation conditions between Sun and Earth.

It is known that the presence of nanometre-scale ice particles and neutral air turbulence in the Polar summer mesosphere modify the D-region plasma, resulting in strong backscatter. These strong backscatters are referred to as Polar Mesosphere Summer Echoes (PMSE). Although studies on PMSE have been ongoing for over three decades, aspects revealed by various instruments are still the subject of discussion. As a sequel to the paper by Ogunjobi et al. (2015), we report on the long term trends and variations in PMSE occurrence probability from Super Dual Auroral Radar Network (SuperDARN) high frequency (HF) radar measurements over the South African National Antarctic Expedition IV (SANAE IV). In this current paper, a simple multiple-filter technique is employed to obtain the occurrence probability rate for SuperDARN-PMSE during the summer periods for the years 1998 - 2007. The SuperDARN-PMSE occurrence probability rate in relation to geomagnetic activity is examined. The mesospheric neutral winds and temperature trends during these periods, are further studied and presented in this paper. Both the monthly and diurnal variations in occurrence are consistent with previous reports, confirming the presence of PMSE from SuperDARN SANAE IV radar measurements and the influence of pole to pole mesospheric transport circulation. The special mesospheric mean flow observed prior to the year 2002 is ascribed to the influence of solar activity. The SuperDARN-PMSE occurrence probability peaks with lowered geomagnetic activity. These present results support the hypothesis that the particle precipitation also plays an important role in SuperDARN-PMSE occurrence.

We study the temporal intensity profile, or pulse shape, of cosmic ray ground-level enhancements (GLEs) by calculating the rise ( τ_r) and decay (τ_d) times for a small subset of all available events. Although these quantities show very large inter-event variability, a linear dependence of τ_d≈ 3.5 τ_r is found. We interpret these observational findings in terms of an interplanetary transport model, thereby including the effects of scattering (in pitch-angle) as these particles propagate from (near) the Sun to Earth. It is shown that such a model can account for the observed trends in the pulse shape, illustrating that interplanetary transport must be taken into account when studying GLE events, especially their temporal profiles. Furthermore, depending on the model parameters, the pulse shape of GLEs may be determined entirely by interplanetary scattering, obscuring all information regarding the initial acceleration process, and hence making a classification between impulsive and gradual events, as is traditionally done, superfluous.

Energetic electrons are trapped in the Earth’s radiation belts which occupy a toroidal region between 3 and 7 \(\hbox {R}_{\mathrm{E}}\) above the Earth’s surface. Rapid loss of electrons from the radiation belts is known as dropouts. The source and loss mechanisms regulating the radiation belts population are not yet understood entirely, particularly during geomagnetic storm times. Nevertheless, the dominant loss mechanism may require an event based study to be better observed. Utilizing multiple data sources from the year 1997–2007, this study identifies radiation belt electron dropouts which are ultimately triggered when solar wind stream interfaces (SI) arrived at Earth, or when magnetic clouds (MC) arrived. Using superposed epoch analysis (SEA) technique, a synthesis of multiple observations is performed to reveal loss mechanism which might, perhaps, be a major contributor to radiation belt losses under SI and MC driven storms. Results show an abrupt slower decaying precipitation of electron peak (about 3000 counts/sec) on SI arrival within 5.05 \(< L\) < 6.05, which persist till 0.5 day before gradual recovery. This pattern is interpreted as an indication of depleted electrons from bounce lost cone via precipitating mechanism known as relativistic electron microburst. On the other hand, MC shows a pancake precipitating peak extending to lower L (Plasmapause); indicating a combination of electron cyclotron harmonic (ECH) and whistler mode waves as the contributing mechanisms.

We report on the polar mesosphere summer echoes (PMSE) occurrence probability over SANAE (South African National Antarctic Expedition) IV, for the first time. A matching coincidence method is described and implemented for PMSE extraction from SuperDARN (Super Dual Auroral Radar Network) HF radar. Several SuperDARN-PMSE characteristics are studied during the summer period from years 2005 - 2007. The seasonal and interannual SuperDARN-PMSE variations in relation to the mesospheric neutral winds are studied and presented in this paper. The occurrence probability of SuperDARN-PMSE on the day-to-day scale show, predominantly, diurnal variation, with a broader peak between 12 - 14 LT and distinct minimum of 22 LT. The SuperDARN-PMSE occurrence probability rate is high in the summer solstice. Seasonal variations show a connection between the SuperDARN-PMSE occurrence probability rate and mesospheric temperature from SABER (Sounding of the Atmosphere using Broadband Emission Radiometry). The seasonal trend for both meridional and zonal winds is very stable year-to-year. Analysis of the neutral wind variations indicates the importance of pole-to-pole circulations in SuperDARN-PMSE generation.

The response of mesosphere and lower thermosphere (MLT) temperature to energetic particle precipitation over the Earth’s polar regions is not uniform due to complex phenomena within the MLT environment. Nevertheless, the modification of MLT temperatures may require an event-based study to be better observed. This work examines the influence of precipitation, triggered by solar wind stream interfaces (SI) event from 2002 to 2007, on polar MLT temperature. We first test the relationship between the ionospheric absorption measured by the SANAE IV (South African National Antarctic Expedition IV) riometer and the layer of energetic particle precipitation from POES (Polar Orbiting Environmental Satellites). The combined particle measurements from POES 15, 16, 17 and 18 were obtained close in time to the pass of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature retrieval. Here, a superposed epoch technique is described and implemented to obtain average temperature profiles during SI-triggered particle precipitation. The superposed epoch average shows no significant temperature decrease below 100km prior to the onset of SI-triggered precipitation, whereas a clear superposed average temperature decrease is observed at 95km after the SI impact. A case study of SI event also yields similar observations. Results indicate that cooling effects due to the production of mesospheric odd hydrogen might be major contributors to temperature decrease under compressed solar wind stream.

Several possible characteristics of magnetic clouds (MCs) have been discussed in the literature, but none appears to explain all the effects from accumulated observations. MC characteristics range from low proton temperature and plasma beta, to high magnetic field magnitude, to smooth rotation in the direction of the magnetic field thus resulting in strong geomagnetic disturbances. Varied instrumentation which is located not only in SANAE IV, Antarctica, but also at Halley, a same radial distance (L ~ 4) in the southern hemisphere and in the vicinity of a conjugate location in northern hemisphere provide an opportunity to test theories applied to high latitude heating rates on the arrival of MC. The Halley riometer is used to monitor coincidences of absorption with the arrival of MC which was observed on 8 November 2004. Using the Monte Carlo Energy Transport Model (MCETM), the corresponding altitude of electron and proton energy distribution indicates the importance of MC triggered geomagnetic storms on mesosphere dynamics.

The high resolution OMNI-2 data set enables a correlative study of Stream Interfaces events (SIs). Statistical analysis of energetic electron flux indicates a gradual dropout as measured by LANL/SOPA, reaching deepest minimum at the arrival of SI. Additionally, precipitation from POES 15-18 measurement of >;30 keV electrons at LEO shows slower-decaying peaks at the time of SI arrival. The precipitating electron flux is attributed to passage of SIs thus can be used to explain some loss mechanisms under high-speed-stream interference.