Scintillation Occurrence Research Articles

A Method for Automatic Detection of Plasma Depletions by Using GNSS Measurements

AbstractEnhanced scintillation activities observed at transionospheric radio signals are often correlated with slant total electron content (STEC) depletions in the equatorial ionosphere. In this study, the data derived from high‐rate Global Navigation Satellite System (GNSS) receivers were used to analyze the observed STEC depletions, commonly associated with plasma bubbles causing radio scintillations in the equatorial ionosphere. We found that the observed STEC depletion can be described by a wedge‐shaped structure. To quantitatively describe the structure of STEC depletions, we developed an effective method to routinely characterize the depth and the width of equatorial plasma depletion in automatized data screening. The developed method has been validated by analyzing data obtained from mostly African GNSS stations in 2014 and 2015. The results confirm current knowledge regarding the seasonal occurrence of radio scintillations and related bubbles. The detection results are compared with those from other published plasma bubble detection techniques.

Occurrence of pre-sunset L-band scintillation due to strong presence of sporadic-E over Arabian Peninsula

Rapid fluctuations in the amplitude and phase of L-band GNSS signals occur, when they pass through the ionosphere, because of electron density irregularities. This so-called scintillation is a consequence of electron density irregularities mainly in the E and F regions of the ionosphere. The daytime scintillation is proven to be linked with E-region irregularities by the existence of a cloud of intense sporadic-E (Es) in the path of radio waves, which produces scintillation even during the daytime at low latitudes. In this study, we have reported the presence of L-band (lower and upper) scintillation observed using data from a newly established GNSS station (SHJ1) situated under the northern crest of the equatorial ionization anomaly (EIA) near Arabian Peninsula at 25.3°N and 55.5°E. The presence of daytime L-band scintillation is found to be consistent with the appearance of Es. A significant presence of weak, moderate and strong scintillation has been observed throughout the day, in general, and in the late afternoon hours (LT 1500–1800), in particular. However, no significant post-sunset and post-midnight scintillation has been observed. The reported pattern of scintillation occurrences over Sharjah differs in many regards to the previous studies, specifically, the local time of peak occurrences. Using RO data, it has been concluded that Es does exist around Sharjah and the occurrence of it appears to be associated with the appearance of scintillation during pre-sunset hours.

Amplitude scintillation index derived from C/N0 measurements released by common geodetic GNSS receivers operating at 1 Hz

Two widely used scintillation indexes $$ S_{4} $$ and $$ \sigma_{\varphi } $$ are generated by dedicated ionospheric scintillation monitoring receivers (ISMRs), which typically have 50-Hz temporal resolution and thus require substantial memory capabilities. Taking into consideration the huge number of common Global Navigation Satellite System (GNSS) receivers, the derivation of a GNSS receiver-based scintillation index as a supplement to the ISMR indexes is expected to improve the study of ionospheric scintillation. We developed an amplitude scintillation index, $$ S_{{4{\text{c}}}} $$, which is derived from the carrier-to-noise density ratio ($$ C/N_{0} $$) data released by common geodetic GNSS receivers operating at 1-Hz. The reliability of the $$ S_{{4{\text{c}}}} $$ index is compared with the typical scintillation index $$ S_{4} $$ of three ISMRs located in Hong Kong and Brazil. The statistics indicate that during scintillation activity, the correlation coefficient between $$ S_{{4{\text{c}}}} $$ (derived from common receivers) and $$ S_{4} $$ (generated by ISMRs) is generally higher than 0.9. The reliability of $$ S_{{4{\text{c}}}} $$ was also verified based on 1-year observations from two adjacent stations in Hong Kong, which are equipped with Leica GR50 and Septentrio PolaRxS Pro receivers, respectively. Long-term scintillation occurrence ($$ S_{{4{\text{c}}}} $$ > 0.2 vs. $$ S_{4} $$ > 0.2) rates show good agreement between $$ S_{{4{\text{c}}}} $$ and $$ S_{4} $$. In addition, two-dimensional $$ S_{{4{\text{c}}}} $$ maps (1° × 1°) generated by GPS L1 and BDS B1 signals data collected at 117 continuous operation tracking stations in China clearly show post-sunset super plasma bubbles as the source of ionospheric scintillation during the main phase of the intense storm that occurred on September 8, 2017. These results demonstrate the feasibility of using the $$ S_{{4{\text{c}}}} $$ index derived from the large volume of GNSS observations recorded by non-scintillation GNSS receivers for the study and monitoring of ionospheric scintillation.

Observations of TEC Depletion, Periodic Structure of TEC, ROTI and Scintillation Associated with ESF Irregularities over South China

The observations of Global Positioning System (GPS) scintillation, Total Electron Content (TEC) depletion, the periodic structure of TEC and Rate of TEC Index (ROTI) over south China were presented. Data were collected from GPS observations at stations of Shenzhen and Guangzhou from 2011 to 2012. This study reported that the ratio of simultaneous occurrences of TEC depletions with strong scintillations was higher than that of TEC depletions with weak scintillations in vernal and autumnal equinoxes of 2011 over South China. The number of the periodic structures of TEC with depletion contained was greater than that with no depletion contained corresponding to strong scintillations. The structure of the slab of plasma irregularities could be responsible for the simultaneous occurrences of TEC depletion with strong scintillations and ROTI. Before and during the occurrences of strong scintillation, there was Large-Scale Wave Structure (LSWS) which provided the seed ionization perturbation to trigger ESF irregularities and contributed to the periodic structure of TEC.

GNSS Ionosphere Sounding of Equatorial Plasma Bubbles

Ground- and space-based Global Navigation Satellite System (GNSS) receivers can provide three-dimensional (3D) information about the occurrence of equatorial plasma bubbles (EPBs). For this study, we selected March 2014 data (during solar maximum of cycle 24) for the analysis. The timing and the latitudinal dependence of the EPBs occurrence rate are derived by means of the rate of the total electron content (TEC) index (ROTI) data from GNSS receivers in China, whereas vertical profiles of the scintillation index S4 are provided by COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate). The GNSS receivers of the low Earth orbit satellites give information about the occurrence of amplitude scintillations in limb sounding geometry where the focus is on magnetic latitudes from 20° S to 20° N. The occurrence rates of the observed EPB-induced scintillations are generally smaller than those of the EPB-induced ROTI variations. The timing and the latitude dependence of the EPBs occurrence rate agree between the ground-based and spaceborne GNSS data. We find that EPBs occur at 19:00 LT and they are mainly situated above the F2 peak layer which descended from 450 km at 20:00 LT to 300 km at 24:00 LT in the equatorial ionosphere. At the same time, the spaceborne GNSS data also show, for the first time, a high occurrence rate of post-sunset scintillations at 100 km altitude, indicating the coexistence of equatorial sporadic E with EPBs.

Remote sensing ionospheric variations due to total solar eclipse, using GNSS observations

For years great interest has been taken in the effects of physical phenomena on ionosphere structure. A total solar eclipse was visible in North America on August 21st, 2017. This event offered a great opportunity for remote sensing the ionospheric behavior under the eclipse condition. In this study we investigated the effects of total solar eclipse on variations of Total Electron Content (TEC), and consequently deviations on regional models of Vertical TEC (VTEC), as well as variations in ionospheric scintillation occurrence. Although variations of TEC due to total solar eclipse are studied thoroughly by many authors, but the effect of solar eclipse on ionospheric scintillation has never been considered before. Our study is based on measurements from a high-rate GPS network over North America on the day of eclipse, a day before and after its occurrence, on the other hand, GPS measurements from ground-based stations on similar days were used to model TEC on the day of event, and also one day before and after it. The results of this study demonstrate that solar eclipse reduced scintillation occurrence at the totality region up to 28 percent and TEC values showed a decrease of maximum 7 TECU. Considering TEC models, our study showed apparent variations in the regional models, which confirms previous studies on ionospheric responses to eclipse as well as theoretical assumptions.

Statistical features of TEC and ionospheric scintillation over the low latitude of China

In this paper, a new index is established which is defined by the standard deviation of total electron content (TEC) fluctuation and denoted by the symbol σtec. It is demonstrated that the index σtec is equivalent to the phase scintillation index and can serve as an indicator of the strength of the phase scintillation. The receiver transiently loses lock on the signal, leading to cycle slips and jumps in the TEC time series. In order to obtain continuous TEC data in the situation of the disturbed ionosphere, a batch processing method is developed to detect and correct cycle slip. Based on the methods mentioned above, we investigate the statistical features of the phase scintillations and cycle slips by means of the data from a scintillation observation network in the south-central region of China during 2012–2015. The results show that the variations of phase scintillation occurrences with local time and season display very similar features to those of the cycle slip occurrences, implying that cycle slips are closely related to phase scintillations. Phase scintillations occur mainly during the night, most frequently before midnight and seldom in the daytime. It is found that phase scintillations occur mainly in equinox months but seldom in solstice months in the crest of the ionization equatorial anomaly and its adjacent regions, and an equinoctial asymmetry that phase scintillations occur more frequently in Spring than in Autumn is also found. Besides, a comparison of the TEC fluctuation index (σtec) and amplitude scintillation index (S4) indicates that there is close relation between the σtec and S4, indicating the co-existence of large and small scale irregularities in equatorial irregularity structures.

Ionospheric Scintillations: Indices and Modeling

AbstractThis paper presents a review of the ionosphere scintillation characteristics recorded at low and high latitudes. The European Space Agency (ESA) MONITOR database has been used to support the analysis. We review in particular the different indices and the values recorded by the different kinds of receivers deployed during the measurement campaign. The climatology of the scintillation occurrences and their dependencies are presented on global maps. At low latitudes, together with the most used scintillation indices, additional parameters have been investigated, such as the time and frequency coherences and the fades and interfade time duration. These additional parameters have been obtained through an analysis of the raw data files recorded on extreme scintillation events. At high latitudes, the different nature of the scintillations recorded is analyzed and compared with the low‐latitude case. The last section of the paper shows a comparison between modeling and measured results for a 1‐month period at low latitudes.

VHF Scintillation and Drift Studied Using Spaced Receivers in Southern Taiwan

AbstractA statistical study of the occurrence and drift characteristics of Very High Frequency (VHF) amplitude scintillations during 2015–2017 based on the observations of recently installed spaced receiver system at Pingtung in Southern Taiwan is presented in this paper. Drift characteristics during quiet periods have been examined in details and compared with the drift recorded during 08 September 2017 geomagnetic storm. Analysis of the recorded observations indicated a strong dependence of VHF scintillation on the solar activity level. Equinoctial asymmetry in the occurrence of VHF scintillation was seen in 2015 and 2016. However, such asymmetry was not apparent in 2017 (low solar flux). Quiet period spaced receiver drift indicated a gradual reduction in the zonal drift of irregularities between postsunset and postmidnight period. Intriguingly, the average quiet period drift of irregularities was found to be independent of solar activity level. A few investigations in recent past have already reported similar observations and can be generally attributed to the polarization process inside equatorial plasma bubbles; however, a definite explanation for this behavior is not available. During the active phase of the 08 September 2017 geomagnetic storm, a complex interplay of disturbance dynamo and prompt penetration electric field was observed in the spaced receiver derived drift velocity of ionospheric irregularities. Approximately −200% deviation from quiet period drift was observed during this moderate storm. A significant enhancement to the spaced receiver derived characteristic random velocity, Vc, was also seen during the active phase of storm, which indicates that the drift of irregularities was highly turbulent.

Ionospheric scintillation intensity fading characteristics and GPS receiver tracking performance at low latitudes

Ionospheric scintillation refers to rapid fluctuations in signal amplitude/phase when radio signals propagate through irregularities in the ionosphere. The occurrence of ionospheric scintillation can severely degrade the Global Navigation Satellite System (GNSS) receiver tracking loop performance, with consequential effects on positioning. Under strong scintillation conditions, receivers can even lose lock on satellites, which poses serious threats to safety–critical GNSS applications and precise positioning. The characteristics of intensity fading on Global Positioning System (GPS) L1 C/A signals during the peak of the last solar cycle at the low latitude station of Presidente Prudente (Lat. 22.12°S, Long. 51.41°W, Magnetic Lat. 12.74°S) are investigated. The results show that the occurrence of scintillation at this station is extremely frequent. An analysis of the fading events revealed an inverse relationship between fading depth and duration. Mathematical models are built to investigate and explain the statistical relationship between intensity fading and the commonly used amplitude scintillation index S4. Then the GPS receiver tracking loop performance is studied in relation to fading. A conclusion can be drawn that both fading depth and duration can affect the tracking loop performance, but the tracking error variance is more strongly related to fading speed, defined as the ratio of fading depth to fading duration. The proposed study is of great significance for better understanding the ionospheric scintillation intensity fading characteristics at low latitudes. It can also contribute to the research on the effects of scintillation on GNSS as well as support the design and development of scintillation robust GNSS receivers.

Characterization of GPS and EGNOS amplitude scintillations over the African equatorial/low-latitude region

This study characterizes GPS and EGNOS amplitude scintillations over the African equatorial/low-latitude region. The data set covers January–December 2013 at three stations; namely, Cape-Verde Island (Lat: 16°N, Lon: 24°W, Mag. Lat: 11°N), Dakar (Lat: 14.75°N, Lon: 17.45°W, Mag. Lat: 5.88°N) [Senegal] and Addis Ababa (Lat: 9.03°N, Lon: 38.77°W, Mag. Lat: 0.18°N) [Ethiopia]. Firstly, we investigated the seasonal variations of scintillations over the study sites. In doing this, we suppressed multipath effects on the data by imposing a 30° elevation masking on the data. Seasonally, scintillations recorded highest occurrences during equinoxes, and the least during June solstice. Secondly, we investigated scintillation occurrences at the three sites on a satellite-by-satellite basis at varying elevation and azimuthal angles. Generally, scintillations (at both low- and high-elevation angles) recorded the highest occurrences at Cape-Verde, followed by Dakar, and the least at Addis Ababa. Scintillation activities were generally localized within the northern skies of the study locations. EGNOS satellites signals scintillated at Dakar and Addis Ababa during the time intervals when GPS satellites signals experienced scintillations. Data showed that Cape-Verde is off the EGNOS geostationary satellite footprint. These results have the potentials of supporting the development of scintillation models for equatorial Africa. Furthermore, with a view to providing robust services for the African user community, the current research effort is also expected to provide value addition to GPS and EGNOS service providers and system designers.

Characteristics of ionospheric scintillation climatology over Indian low-latitude region during the 24th solar maximum period

The amplitude and phase of L-band satellite signals are fluctuated randomly due to small scale electron density irregularity structures in the ionosphere which result in fleeting variations, known as ‘ionospheric scintillations’. The Global Navigation Satellite System (GNSS) is a profound remote sensing tool to monitor, model and forecast the ionospheric weather conditions. In this paper, the GNSS amplitude scintillation data has been analyzed during the year 2013 at Bengaluru (12.9°N, 77.59°E) and Lucknow (26.8467°N, 80.9462°E) stations to reinforce climatology of ionospheric scintillation over Indian low-latitude region. The probability of scintillation occurrence and their variations over equatorial and Equatorial Ionization Anomaly (EIA) regions in India are analyzed during various geomagnetic quiet and disturbed days, months and seasons. The annual occurrence of amplitude scintillations are mapped with the function of local time. It is observed from the experimental results that the probability of scintillations occurrences is higher over EIA region than over the equatorial region. The probability of scintillations is higher during March equinox and December solstice, and lowest during June solstice. Distribution of scintillations is intense during post–sunset period. The maximum percentage of scintillation occurrences at the two stations are recorded in November. Moreover, the highest percentage of scintillation occurrences took place on storm day (March 17, 2013) at the two stations. This work would be helpful for understanding the features of GNSS amplitude scintillations over Southern and Northern Indian regions. Moreover, these kinds of investigations are helpful for developing new algorithms to nowcast and forecast ionospheric scintillations over Indian Sub-continent.

Role of the external drivers in the occurrence of low-latitude ionospheric scintillation revealed by multi-scale analysis

We analyze the amplitude scintillation on L-band signals over San Miguel de Tucumán (Argentina), focusing on the multi-scale variability and speculating on the possible relationship between forcing factors from the geospace and the ionospheric response. The site is nominally located below the expected position of the southern crest of the Equatorial Ionospheric Anomaly (EIA). For this scope, we concentrate on the period 1–31 March 2011, during which one minor and one moderate storm characterize the first half of the month, while generally quiet conditions of the geospace stand for the second half. By leveraging on the Adaptive Local Iterative Filtering (ALIF) signal decomposition technique, we investigate the multi-scale properties of Global Navigation Satellite Systems (GNSS) amplitude scintillation and helio-geophysical parameters, looking for possible cause-effect mechanisms relating the former to the latter. Namely, we identify resonant modes in the Akasofu (ε) parameter as likely related to the frequency components in the time evolution found for the amplitude scintillation index, hence modulating the scintillation itself.

Comments on the percentage of occurrence methodology used in “a study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station” by H J Tanna, S P Karia and K N Pathak

Following Tanna et al. (2013), we computed the percentage of occurrence of S4 index for the period of 2012–2015 using the data of the dual frequency GPS receiver at the Tripura University, Agartala station (23.76°N, 91.26°E) situated at the northern crest of the equatorial ionization anomaly (EIA) region of the Indian Subcontinent. We have observed discrepancy in the results contradicting the actual scintillation occurrence. The distinctly noticeable discrepancy is that the maximum occurrence month is shifted to April 2013 instead of March 2014. The problem arises due to the denominator term used in the percentage of occurrence ratio i.e. the total number of days of observed scintillation activity during the complete period under consideration. But the conventional percentage of occurrence methodology uses the number of days of observation (the total number of days for which data is available) during each month in the denominator. It correctly assigns the maximum occurrence to March 2014 instead of April 2013 and the obtained monthly statistics follow the solar activity during this period.

A comparative study of ionospheric spread-F and scintillation at low- and mid-latitudes in China during the 24th solar cycle

The spread-F echo of ionograms and scintillation of satellite signal propagation along the Earth-space path are two typical phenomena induced by ionospheric irregularities. In this study, we obtained spread-F data from HF (high frequency) digital ionosonde and scintillation index (S4) data from L-band and UHF receivers at low- and mid-latitudes in China during the 24th solar cycle. These four sites were located at Haikou (HK) (20°N, 110.34°E), Kunming (KM) (25.64°N, 103.72°E), Qingdao (QD) (36.24°N, 120.42°E), and Manzhouli (MZL) (49.56°N, 117.52°E). We used these data to investigate spread-F and scintillation occurrence percentages and variations with local time, season, latitude and solar activity. A comparative study of spread-F and scintillation occurrence rates has been made. The main conclusions are as follows: (a) FSF occurred mostly during post-midnight, while RSF and scintillation appeared mainly during pre-midnight at HK and KM; (b) FSF occurrence rates were larger at QD and MZL than expected; (c) the FSF occurrence percentages were anti-correlated with solar activity at HK and KM; meanwhile RSF and scintillation occurrence rates increased with the increase of solar activity at this two sites; (d) the highest FSF occurrence rates mostly appeared during the summer months, while RSF and scintillation occurred mostly in the equinoctial months at HK and KM; (e) the scintillation occurrence was usually associated with the appearance of RSF, probably due to a different physical mechanism comparing with FSF. Some of these results verified the conclusions of previous papers, whereas some show slight difference. These results are important in understanding ionospheric irregularities variations characteristic at low- and mid-latitudes in China.

Spatial Features of L‐Band Equinoctial Scintillations From Equator to Low Midlatitude at Around 95°E During 2015–2016

AbstractThe nature and extent of the irregularities causing L‐band nighttime scintillations at a group of five stations Dibrugarh (27.5°N, 95°E, 43° dip), Kohima (25.6°N, 94.1°E, 39° dip), Aizawl (23.7°N, 92.8°E, 36° dip), Port Blair (11.6°N, 92.7°E, 9° dip), and Cocos Islands (12.2°S, 96.8°E, 43° dip) from the northern low midlatitudes to southern midlatitudes along 95°E meridian is investigated. Global Navigation Satellite System/Global Positioning System/ionosonde measurements during the equinoctial months of 2015–2016 have been utilized. The northernmost station Dibrugarh and the southernmost station Cocos Islands are magnetically conjugate. Scintillations occur more frequently around the equatorial ionization anomaly crest where the background charge density (total electron content crest‐to‐trough ratio) plays an important role. It is also observed that L‐band scintillations around this sector decrease considerably from equinoctial months of 2015 to equinoctial months of 2016. The anomalous El Niño‐Southern Oscillation and quasi‐biennial oscillation recorded during the 2015–2016 winter might have contributed partly to the suppression of irregularities (and hence scintillations) in the succeeding equinox. Strong pre‐midnight scintillations when triggered by equatorial spread F occur simultaneously at all stations. The zonal/vertical drift velocities of irregularities estimated from the time delay of occurrence of scintillations decrease from postsunset to midnight hours. On the other hand, simultaneous sporadic E and post‐midnight scintillations occur over Dibrugarh and Cocos Islands in the absence of scintillations at the equator. Azimuthal position of post‐midnight scintillations at these two locations suggests that they are the manifestation of a frontal structure of sporadic E.

Polar traveling ionospheric disturbances inferred with the B-spline method and associated scintillations in the Southern Hemisphere

A new method for analyzing travelling ionospheric disturbances (TIDs) is developed by using two B-spline basis functions of degree 4 on the Total Electron Content (TEC) data from the ground-based Global Positioning System (GPS) receivers. This method enhances the spatial resolution to about 0.1° (geographic latitude) × 0.1° (geographic longitude), which is useful in studying all scales (small, medium and large) TIDs. Using this method, we investigated TIDs and their associated scintillation on 18–19 July 2013 at Southern Hemisphere and found phase scintillation is more sensitive than amplitude scintillation to the TIDs at South Pole. To see the full impact of TIDs on scintillation, we have used a proxy phase scintillation index, calculated using geodetic GPS receivers over Antarctica. We have verified the presence of TIDs during these two days by using a Global Navigation Satellite System (GNSS)-TEC single station approach and SuperDARN slant range signals. Our results show the TEC fluctuations are associated with ionospheric scintillation. The shape of TIDs, their elongation and flattening along/across the geographic latitude/longitude, seems to be related to the magnitude and occurrence of ionospheric scintillations. Magnetospheric particle precipitation boost TEC gradients and generate stronger amplitude scintillation, however, large-scale plasma irregularities cause overall enhancement in magnitude of the phase scintillation index. Due to the high turbulence in the polar ionosphere, TIDs change their shapes quite quickly and/or may disappear in the background ionosphere. B-spline TIDs analysis method is very useful in identifying the visible as well as hidden TIDs parts in the polar ionosphere. For the first time, ionospheric scintillation has been investigated in the vicinity of TIDs at high- latitude in the southern hemisphere. Further, the presented B-spline TIDs analysis method is unique and simple in itself as it uses GPS receiver processed TEC data as the primary input. Our results show that at polar latitude it is not necessary that TIDs always appear near the high TEC regions. Usefulness of the B-spline TIDs detection method has been demonstrated in analyzing TIDs at all geographic locations and different solar activity conditions by comparing B-spline TIDs method produced results with the previous case studies.

Study of equatorial plasma bubbles using all sky imager and scintillation technique from Kolhapur station: a case study

The nightglow observations of OI 630.0 nm emission carried out from low latitude station Kolhapur using All Sky Imager (ASI) with $140^{\circ}$ field of view (FOV) for the month of April 2011 are used. The images were processed to study the field aligned irregularities often called as equatorial plasma bubbles (EPBs). The present study focuses on the occurrence of scintillation during the traversal of EPBs over ionospheric pierce point (IPP). Here we dealt with the depletion level (depth) of the EPB structures and its effect on VHF signals. We compared VHF scintillation data with airglow intensities at Ionospheric pierce point (IPP) from the same location and found that the largely depleted EPBs make stronger scintillation. From previous literature, it is believed that the small scale structures are present near the steeper walls of EPBs which often degrades the communication, the analysis presented in this paper confirms this belief.

Occurrence characteristics of ionospheric irregularities over Indian low latitude region Varanasi during ascending phase of solar cycle 24

Ionospheric irregularities degrade the performance of radio technological system by producing fluctuations in amplitude and phase of signal passing through them, a phenomenon which is known as scintillations. This study presents diurnal and seasonal variations of ionospheric irregularities during ascending phase of solar activity from 2009 to 2014 by using the amplitude scintillation index S 4 computed from a dual frequency GPS receiver installed at the low-latitude station of Varanasi (Lat. 25.3176° N, Long. 82.9739° E). Scintillation occurrences are found to be higher during nighttime hours (1930-0130 LT) and characterized by an equinoctial maximum throughout the years 2009-2014, except for the peculiar solar minimum year 2009. Gravity wave seed perturbation from lower atmosphere and pre-reversal enhancement (PRE) in zonal electric field have been considered to explain the observed seasonal occurrences, which have been also compared with the previous results obtained from observations and model. Influence of solar activity on scintillation occurrence has also been studied, and it was found that there is linear dependence between the solar activity and scintillation occurrence, which is seasonally variable.

Simulation and analysis of chemical release in the ionosphere

Ionospheric inhomogeneous plasma produced by single point chemical release has simple space-time structure, and cannot impact radio wave frequencies higher than Very High Frequency (VHF) band. In order to produce more complicated ionospheric plasma perturbation structure and trigger instabilities phenomena, multiple-point chemical release scheme is presented in this paper. The effects of chemical release on low latitude ionospheric plasma are estimated by linear instability growth rate theory that high growth rate represents high irregularities, ionospheric scintillation occurrence probability and high scintillation intension in scintillation duration. The amplitude scintillations and the phase scintillations of 150 MHz, 400 MHz, and 1000 MHz are calculated based on the theory of multiple phase screen (MPS), when they propagate through the disturbed area.