Plasma density irregularities in low-latitude and midlatitude F regions are normally understood in terms of equatorial plasma bubbles (EPBs) and traveling ionospheric disturbances (TIDs), respectively. Referring to the ground-based observations, such as total electron content (TEC) maps and all-sky airglow images, EPBs and TIDs can be distinguished by their propagation directions or alignments (
Deep fades observed in a GPS L1/L5 strong scintillation dataset have been characterized in terms of availability for GNSS navigation. A stochastic model based on a Markov chain accurately generates realistic correlated fading processes of GNSS signals under scintillation. New Markov chain model confirms that use of dual-frequency GNSS signals significantly enhances aviation availability during scintillation.
Large amplitude plasma density irregularities have occasionally been detected at night in the midlatitude F region during geomagnetic storms. They are often interpreted in terms of equatorial plasma bubbles (EPBs) because midlatitude irregularities have the morphology of EPBs. This study assesses whether morphology can be a determining factor in ascribing the origin of such midlatitude ionospheric irregularities. We address this question by analyzing the observations of the First Republic of China satellite (ROCSAT‐1) and Defense Meteorological Satellite Program (DMSP)‐F14 and ‐F15 satellites during the geomagnetic storms on 12 February 2000 and 29 October 2003. On both days, ROCSAT‐1 detects plasma depletions at midlatitudes in broad longitude regions and DMSP satellites detect isolated severe plasma depletions whose widths and depths are much wider and deeper than those of typical EPBs. The distinguishing characteristics during the storms are the detection of midlatitude depletions only in the Southern Hemisphere and the occurrence of some of these depletions before 19 hr local time and at the longitudes where EPBs are absent in the equatorial region. These characteristics are not explained satisfactorily by the characteristics of EPBs. Considering the detection of some of the midlatitude depletions at the equatorward edge of ionospheric perturbations in midlatitudes, midlatitude depletions are likely ionospheric perturbations that originated from higher latitudes. Because midlatitude depletions can originate from different sources, the morphology alone is not a determining factor of their origin.
The intensity scintillation index (S4) exhibits a linear frequency dependence derived from a power law phase screen model, with the exponent determined by the phase spectral index (p). However, it is well known that a departure from linearity occurs as S4 increases and saturates under strong scatter conditions. Additionally, statistical errors over a finite time window can also cause the scintillation statistic to deviate from the power law dependence on frequency. This paper presents a parametric analysis on the deviation in the S4 frequency dependence from the power law form. This investigation focuses on the contribution of statistical uncertainty in S4 calculations to the deviation in the frequency dependence according to the selected parameters: S4 and Fresnel frequency (fF). The deviation is determined by comparing p estimates calculated from the power law relationship of S4 at different L‐band frequencies with those inferred from a model‐fitting method using a multi‐frequency global navigation satellite system scintillation data set. The results show the well‐known departure from linearity as S4 increases, and reveal increasing deviations with decreasing fF. The spectral analysis indicates that the variability at the lowest frequencies of intensity spectra contributes to the statistical errors in S4 calculations due to the finite time window, particularly when scintillations are dominated by low‐frequency contributions at low fF. Simulation results of scintillation realizations show much lower deviations with a 30‐min compared to 1‐min window. This suggests a 1‐min window can be too short for reliable S4 calculations due to statistical uncertainty, especially at low fF.
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