Anomaly Crest Region Research Articles

Comparative study of scintillations at the magnetic equator and at the crest region of the equatorial anomaly in Indian zone

Using simultaneous long-term observations of ionospheric scintillation at equator and anomaly crest region in the same longitude (Indian) zone comparative features of scintillation occurrence are brought out. The salient features are: (a) predominantly pre-midnight occurrence of scintillation at equator during winter and equinox seasons, (b) increase of pre-midnight scintillation occurrence with solar activity (c) shifting of occurrence peak during summer from post-midnight in low to pre-midnight in high solar activity periods (d) similarity of scintillation behaviour at these locations during winter and equinoxes but dissimilarity during summer. The solar activity response and magnetic effects indicate that the scintillations at the anomaly crest region in winter and equinox, particularly during high solar activity periods, are of equatorial origin while the summer events may be of local or mid-latitude origin.

Ionospheric disturbances at the equatorial anomaly crest region during the March 1989 magnetic storms

On March 6,1989, the largest sunspot group since 1982 came into view as it moved out of the eastern limb of the Sun. It was highly active during March 8–18, and a great many transient ionospheric and geomagnetic variations were triggered by this sunspot group. The intensive ionospheric observations at Lunping Observatory and Chungli Ionosphere Station during this period recorded 30 solar flares manifested as shortwave fade‐outs, sudden frequency deviations, and solar flare effects and three storm sudden commencement (SSC)‐type geomagnetic storms, among which the March 13 SSC‐type geomagnetic storm triggered an unusually severe ionospheric disturbance. The ionospheric total electron content, the critical frequency of the F2 layer, f0F2, and the virtual heights at given frequencies all show wavelike up‐and‐down oscillations of the ionosphere. This oscillatory ionospheric motion is explained as due to the compression and expansion of the plasmasphere.

Medium-scale traveling ionospheric disturbances around the equatorial anomaly crest as detected by the differential Doppler shift method

From January 1986 to December 1987, 968 medium-scale traveling ionospheric disturbances (TIDs) incident near the northern equatorial anomaly crest region were detected at Lunping Observatory (25.00°N, 121.17°E geographic; 14.27°N, 191.30°E geomagnetic) by observing the differential Doppler shift of 150 and 400 MHz coherent radio signals transmitted from six polar orbited satellites of the U.S. Navy Navigation Satellite System. By use of these TID data, it has been found that: 1. (1) most of the TIDs detected by the differential Doppler shift method have a peak-to-peak amplitude in the range of 0.2–0.4 Hz, and a wavelength in the range of 100–300km; 2. (2) most of the TIDs are observed during the daytime and early evening period; 3. (3) TIDs appear quite often in geomagnetically calm days or slightly disturbed days; 4. (4) TIDs have occurrence peaks around 29–32°N in summer and equinoxes, and around 23°N in winter.

Earthquake Sounds From Aftershocks Following the 1986 May 20 Hualien Eqrthquake

Solar activity dependence on the lunar daily variations of ionospheric totalelectron content at the equatorial anomaly crest region was studied using theChapman-Miller method by use of the total electron content data observed at Lunpingfrom March 4,1977 to December 31,1989.The result shows that the amplitudesof lunar daily variations increase with increasing sunspot numbers but becomesaturated when the sunspot number becomes larger than about 100.

On the equatorial anomaly of the ionospheric total electron content near the northern anomaly crest region

Daily contour charts of the ionospheric total electron content in latitude versus local time coordinates have been used to study the behavior of the development of the equatorial anomaly around the northern anomaly crest region. The daily development of the equatorial anomaly shows quite large day‐to‐day variabilities not only on magnetically disturbed days but also on quiet days. The daily maximum anomaly crest moves poleward as the magnitude of the total electron content of the daily maximum anomaly crest increases. The increase of the equatorial electrojet strength also results in a poleward movement of the anomaly crest. No significant correlation exists between the anomaly crest and the magnetic Ap index. The monthly mean diurnal development of the equatorial anomaly shows a remarkable seasonal difference, with the anomaly largest in equinoxes and slightly larger in winter than in summer.

Day-to-day variability of ionospheric electron content at low latitudes

The day-to-day variability of ionospheric electron content (N F) at low latitudes as given by the per cent standard deviation, shows values ranging from 10% to 25% during the day-time and from 40% to 60% during the night-time. The variability of N F seems to be significantly lower at equatorial stations and largest at stations near the anomaly crest region.

Daily observations of the development of the ionospheric equatorial anomaly by means of differential Doppler shift method

The differential Doppler frequency shifts observed by receiving coherent radio signals at frequencies of 150 and 400 MHz transmitted from the polar orbiting satellites of U.S. Navy Navigation Satellite System have been used to deduce the latitudinal variations of the ionospheric total electron content (TEC) near the ionospheric equatorial anomaly crest region. All latitudinal variations of TEC thus obtained for each passage of an NNSS satellite are used to construct daily contour plots of TEC in a latitude versus local time coordinates. It has been shown that these contour plot of TEC can be used to investigate the behavior of TEC around equatorial anomaly crest region. Some results are presented and discussed.

250 MHz/GHz Scintillation Parameters in the Equatorial, Polar, and Auroral Environments

Ionospheric scintillation effects encountered in the equatorial anomaly crest, polar cap, and auroral regions have been contrasted to provide information for the design and evaluation of the performance of multifrequency satellite communication links in these regions. The equatorial anomaly region is identified as the most disturbed irregularity environment where the amplitude and phase structures of 250 MHz and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> -band scintillations are primarily dictated by the strength of scattering rather than ionospheric motion. In the anomaly region, the spectra of intense amplitude scintillations at these frequencies are characterized by uniform power spectral density from the lowest frequency (10 MHz) to 4 Hz at 257 MHz and to 1 Hz at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> -band (1541 MHz) and steep rolloff at higher fluctuation frequencies with power law indexes of -5 to -7. Such structures are compatible with intensity decorrelation times of 0.1 and 0.3 s at 257 and 1541 MHz, respectively. The phase spectra at 244 MHz are described by power law variation of psd with frequency with typical spectral indexes of -2.4. The strong scattering at VHF induces extreme phase rates of 200° in 0.1 s. The 90th percentile values of rms phase deviation at 244 MHz with 100 s detrend are found to be 16 rad in the early evening hours, whereas amplitude scintillation can cover the entire dynamic range of 30 dB not only in the 250 MHz band but at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> -band as well. In the polar cap, the 50th and 90th percentile values of rms phase deviation at 250 MHz for 82 s detrend are 3 and 12 rad, respectively, with comparable values being obtained in the auroral oval. The corresponding values for the S <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> index of scintillation are 0.5 and 0.8 in the polar cap, which are slightly higher than those recorded in the auroral oval. The power law index of phase scintillation at high latitudes is in the vicinity of -2.3, which is not a result of very strong turbulence as in the equatorial region but is considered to be a consequence of shallow irregularity spectral indexes. The phase rates at auroral locations are an order of magnitude smaller than in the equatorial region and attain values of 100° in 0.5 s. The extreme variability of ionospheric motion in the auroral oval sensitively controls the structure of scintillations. The long-term morphology (period 1979-1984) of intensity scintillations at 250 MHz in the polar cap shows that, in addition to the absence of diurnal variation of scintillations, and the presence of an annual variation with a pronounced minimum during local summer, there exists a marked solar control of scintillation activity such that it abruptly decreases when the solar activity falls below a threshold level.

Ionospheric electron content depletion associated with amplitude scintillation at the equatorial anomaly crest region

The diurnal and seasonal behaviors of the ionospheric total electron content (TEC) depletion associated with VHF amplitude scintillation observed at Lunping, Taiwan (25.00°N, 121.17°E geographic), during the period March 1977 to February 1980 were presented. Diurnally, the maximum occurrence of TEC depletion appears during the late evening hours for all seasons except winter. Seasonally, the maximum occurrence of TEC depletion is not fixed at a certain season but varies with sunspot number because of the different sunspot number dependence of the occurrence of TEC depletion in different seasons. The most probable maximum TEC depletion ranges from 0.8 to 3.6×1016 el/m², and the duration ranges from 10 to 33 min. No correlation was found between the maximum TEC depletion and the duration or between the maximum TEC depletion and the associated ionospheric scintillation index. The association between TEC depletion and range spread F is much better than that between TEC depletion and frequency spread F.