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Found 5 entries in the Bibliography.
Showing entries from 1 through 5
| 2015 |
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This paper presents a statistical analysis of ionospheric response over ionosonde stations Grahamstown (33.3\textdegreeS, 26.5\textdegreeE, geographic) and Madimbo (22.4\textdegreeS, 30.9\textdegreeE, geographic), South Africa, during geomagnetic storm conditions which occurred during the period 1996\textendash2011. Such a climatological study is important in establishing local ionospheric behavior trend which later forms a basis for accurate modeling and forecasting electron density and critical frequency of the\ F2\ layer (foF2) useful for high-frequency communication. The analysis was done using\ foF2\ and total electron content (TEC), and to identify the geomagnetically disturbed conditions, the\ Dst\ index with a storm criterion of\ Dst\ <=\ nT was used. Results show a strong solar cycle dependence with negative ionospheric storm effects following the solar cycle and positive ionospheric storm effects occurring most frequently during solar minimum. Seasonally, negative and positive ionospheric storm effects occurred most in summer (63.24\%) and in winter (53.62\%), respectively. An important finding is that only negative ionospheric storms were observed during great geomagnetic storm activity (Dst\ <=\ nT). For periods when both\ foF2\ and TEC data (from colocated ionosonde and GPS receiver stations) were available, a similar response in terms of variational trend was observed. Hence, GPS data can be used to effectively identify the ionospheric response in the absence of ionosonde data. Matamba, Tshimangadzo; Habarulema, John; McKinnell, Lee-Anne; Published by: Space Weather Published on: 09/2015 YEAR: 2015 DOI: 10.1002/swe.v13.910.1002/2015SW001218 Geomagnetic storms; ionospheric storm effects; midlatitude ionosphere |
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The annual asymmetry in the F 2 layer during deep solar minimum (2008-2009): December anomaly Annual January/July midlatitude daytime asymmetry in monthly median NmF2 and model thermospheric parameters has been considered during deep solar minimum, (2008\textendash2009), when solar and geomagnetic activities were at the lowest level, to analyze the background effect due to the Sun-Earth minimum distance, perihelion, in the vicinity of the December solstice. Averaged over 10 midlatitude station pairs, the NmF2 asymmetry was found to be ≈1.23, while the average asymmetry for the annual component in NmF2 variations is ≈1.17. Annual asymmetry in monthly median neutral composition and temperature predicted by Mass Spectrometer Incoherent Scatter 86 (MSIS86) and MSISE00 thermospheric models along with the 7\% increase in solar EUV flux in the vicinity of the December solstice is sufficient to explain the observed annual asymmetry in NmF2. A hierarchy of aeronomic parameters responsible for the observed asymmetry in NmF2 has been established: the main contributor is atomic oxygen\textemdashabout 50\% of the total effect, [N2] contributes around 35\% strongly compensating the [O] contribution, and solar EUV and Tn provide \<10\% each. The zonal mean annual asymmetry in MSIS86 atomic oxygen column density was shown to be 1.18 at low and middle latitudes, and this is close to the estimated asymmetry for the annual component in NmF2 variations. The earlier proposed mechanism of the December anomaly is considered as a plausible one to explain the 1.18 January/July asymmetry in the atomic oxygen variations and consequently the NmF2 annual daytime asymmetry at middle latitudes under the deep solar minimum. Published by: Journal of Geophysical Research: Space Physics Published on: 02/2015 YEAR: 2015 DOI: 10.1002/2014JA020929 midlatitude ionosphere; thermosphere composition and structure |
| 2014 |
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Matamba, Tshimangadzo; Habarulema, John; McKinnell, Lee-Anne; Published by: Space Weather Published on: YEAR: 2014 DOI: https://doi.org/10.1002/2015SW001218 ionospheric storm effects; Geomagnetic storms; midlatitude ionosphere |
| 2013 |
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Ionospheric symmetry caused by geomagnetic declination over North America We describe variations in total electron content (TEC) in the North American sector exhibiting pronounced longitudinal progression and symmetry with respect to zero magnetic declination. Patterns were uncovered by applying an empirical orthogonal function (EOF) decomposition procedure to a 12 year ground-based American longitude sector GPS TEC data set. The first EOF mode describes overall average TEC, while the strong influence of geomagnetic declination on the midlatitude ionosphere is found in the second EOF mode (or the second most significant component). We find a high degree of correlation between spatial variations in the second EOF mode and vertical drifts driven by thermospheric zonal winds, along with well-organized temporal variation. Results strongly suggest a causative mechanism involving varying declination with longitude along with varying zonal wind climatology with local time, season, and solar cycle. This study highlights the efficiency and key role played by the geomagnetic field effect in influencing mesoscale ionospheric structures over a broad midlatitude range. Zhang, Shun-Rong; Chen, Ziwei; Coster, Anthea; Erickson, Philip; Foster, John; Published by: Geophysical Research Letters Published on: 10/2014 YEAR: 2013 DOI: 10.1002/grl.v40.2010.1002/2013GL057933 geomagnetic field; midlatitude ionosphere; thermospheric winds; total electron content |
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The global configuration of the geomagnetic field shows that the maximum east-west difference in geomagnetic declination of northern middle latitude lies in the US region (~32\textdegree), which produces the significant ionospheric east-west coast difference in terms of total electron content first revealed by Zhang et al. (2011). For verification, it is valuable to investigate this feature over the Far East area, which also shows significant geomagnetic declination east-west gradient but smaller (~15\textdegree) than that of the US. The current study provides evidence of the longitudinal change supporting the thermospheric zonal wind mechanism by examining the climatology of peak electron density (NmF2), electron density (Ne) of different altitudes in the Far East regions with a longitude separation of up to 40\textendash60\textdegree based on ground ionosonde and space-based measurements. Although the east-west difference (Rew) over the Far East area displays a clear diurnal variation similar to the US feature, that is negative Rew (West Ne \> East Ne) in the noon and positive at evening-night, the observational results reveal more differences including: (1) The noontime negative Rew is most pronounced in April\textendashJune while in the US during February\textendashMarch. Thus, for the late spring and summer period negative Rew over the Far East region is more significant than that of the US. (2) The positive Rew at night is much less evident than in the US, especially without winter enhancement. (3) The magnitude of negative Rew tends to enhance toward solar maximum while in the US showing anticorrelation with the solar activity. The altitude distribution of pronounced negative difference (300\textendash400 km) moves upward as the solar flux increases and hence produces the different solar activity dependence at different altitude. The result in the paper is not simply a comparison corresponding to the US results but raises some new features that are worth further studying and improve our current understanding of ionospheric longitude difference at midlatitude. Zhao, Biqiang; Wang, Min; Wang, Yungang; Ren, Zhipeng; Yue, Xinan; Zhu, Jie; Wan, Weixing; Ning, Baiqi; Liu, Jing; Xiong, Bo; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2013 YEAR: 2013 DOI: 10.1029/2012JA018235 geomagnetic declination; longitudinal variation; midlatitude ionosphere |
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