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Keywords:

  • equatorial plasma bubble;
  • ionospheric irregularities;
  • ionospheric scintillation

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] The growth in altitude/latitude of equatorial plasma bubbles was monitored, using simultaneous recordings of VHF scintillations at five locations situated between 3° and 23°N magnetic latitudes along a common meridian (84°E) during February 1980. The onsets of postsunset scintillation were mostly abrupt in character, and their occurrence at higher latitudes was conditional on their prior appearance at lower latitudes, indicating a causal link to irregularities associated with rising equatorial plasma bubbles. The day-to-day occurrence and the latitudinal, and effectively altitudinal, growths are examined in relation to the prereversal enhancement in h′F during sunset hours and its rate of rise, the onset of a postsunset secondary maximum (PSSM) in ionospheric electron content (IEC), and equatorial electrojet strength (EEJ) variations. It is observed that the bubble and associated irregularities, after its onset over the magnetic equator, reached the highest altitudes/latitudes only on those days when a prior PSSM in IEC is observed there in addition to high values of h′F, dh′F/dt and bubble rise velocity; otherwise it will be confined to near equatorial latitudes only. Also, the equatorial h′F, dh′F/dt, magnitude of PSSM and intensity of 4 GHz scintillations at low latitude are all showing positive correlation with daytime EEJ strength variations. It is concluded that, after the initial development of a bubble, the ExB drift and the PSSM play an important role in the subsequent growth and evolution, and EEJ is a useful parameter for the prediction of the development.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] The equatorial ionosphere is highly dynamic, unpredictable and is characterized by the existence of intense equatorial plasma bubble associated irregularities. These irregularities affect almost all radio communication systems utilizing the Earth-space propagation path. Much of the current attention is directed toward understanding the cause and effect relationships of equatorial ionospheric irregularities in order to gain prediction capability in the equatorial and low region. Significant progress has been made during the past two decades in understanding the basic plasma processes governing the generation and growth of the equatorial plasma bubble and associated ionospheric irregularities. Multitechnique measurements in the equatorial regions have established that there exists a close association between saturated scintillations on VHF/UHF trans-ionospheric signals, range type spread F on ionograms, and plume like irregularity structures on backscatter radar [Woodman and La Hoz, 1976; Farley et al., 1970; Rastogi and Woodman, 1978; Basu and Kelley, 1979; Aarons et al., 1980]. Based on VHF nighttime scintillation data recorded simultaneously at a meridian chain of stations in the Indian zone, Somayajulu et al. [1984] and Dabas and Reddy [1986] have shown that the latitudinal extent of postsunset scintillation producing irregularities is essentially controlled by the generation and growth of F region irregularities over the magnetic equator. Dabas and Reddy [1990] utilized the systematic time delay observed in the occurrences of scintillations for estimating the equatorial plasma bubble rise velocity over the magnetic equator. Dabas et al. [1998] also reported that the day-to-day occurrence of scintillations at 4 GHz up to 21°N magnetic latitudes is strongly dependent on the evening hour h′F and dh′F/dt values over the magnetic equator. Also the intensity of GHz scintillations is positively correlated with the ionospheric electron content (IEC) values observed during postsunset hours (2000 LT) as well as with their diurnal maximum values. In equatorial and low latitudes, Garg et al. [1983] reported that onset and decay of postsunset equatorial ionization anomaly (PEA) is controlled by ExB drifts and meridional winds and the occurrence of postsunset secondary maximum (PSSM) in electron content at different latitudes in the low-latitude belt is the result of PEA. Fejer et al. [1999] studied the effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F, whereas Hysell [2000] gave an overview and synthesis of plasma irregularities in equatorial spread F. Whalen [2000], in a case study, used multistation ionosonde data from the American sector and examined the evolution of equatorial plasma bubble in relation to bottom side spread F and to the occurrence of postsunset Appleton ionization anomaly and reported that there is a strong link between the altitudinal/latitudinal growth of a plasma bubble and the development of postsunset hours Appleton anomaly. The present study has utilized simultaneous VHF scintillation and Faraday rotation recordings at a meridian chain of stations in the Indian zone to examine the day-to-day occurrence, latitudinal, and effectively altitudinal growths of equatorial plasma bubble and associated irregularities in relation to the prereversal enhancement in equatorial h′F and its rate of rise, plasma bubble rise velocity, and the onset of a postsunset secondary maximum (PSSM) in ionospheric electron content at low latitudes. The main objective here is to examine the critical parameters which actually controls the latitudinal growth of plasma bubbles up to 20°N magnetic latitude or beyond after its onset over the magnetic equator because the scintillations at these latitudes is much more intense than that at near equatorial locations and hence its effects on satellite communications. In the last, a possible correlation, if any, of these critical parameters are also examined with daytime equatorial electrojet strength variations for prediction purposes only.

2. Data Analysis and Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[3] Simultaneous recording of VHF scintillations and Faraday rotation from geostationary satellite ETS-II (130°E) at five locations situated between 3° and 23°N magnetic latitudes along a common geographic meridian (±1° of 84°E) in the Indian zone during February 1980 are used to monitor the growth in latitude, and consequently in altitude, of equatorial plasma bubbles and associated ionospheric irregularities. The locations and subionospheric points of the network of five stations, i.e., Bangalore, Hyderabad, Nagpur, Delhi, and Kurukshetra, are given in Table 1 and shown in Figure 1. Magnetic field lines along 84°E meridian and ray paths to ETS-II geostationary satellites (130°E) from all the observing stations are shown in Figure 2. The field lines (see Figure 2) corresponding to F region heights (mean h′F value, as measure by ionosonde, is roughly equal to 420 km for near equatorial stations Bangalore, Hyderabad and Nagpur; and 250 km in case of low-latitude stations Delhi and Kurukshetra) over Bangalore, Hyderabad, Nagpur, Delhi and Kurukshetra map over the magnetic equator at the altitudes of about 450, 550, 710, 1140 and 1270 km, respectively. Information about the evening hour ionospheric F layer heights (h′F) variations over the magnetic equator has been obtained from published hourly Ionosonde data from Kodaikanal (10.2°N, 77.5°E; dip 3.5°N) and from that dh′F/dt (ExB drift) are estimated using h′F values around sunset hours. It is to be pointed out here that the estimation of ExB values in this way gives only the rough idea about the vertical drift because of using hourly h′F available values which again are not always accurate especially when range spread is there. In addition, 15 min ionospheric electron content (IEC) data from these stations are used to study the development and decay of postsunset equatorial ionization anomaly (PEA) and the occurrences of postsunset maximum in electron content (PSSM) at low latitudes. To gain prediction capability, an attempt is also made just to examine the relationship, if any, between the daytime Electrojet strength and the equatorial plasma bubble controlling parameters like magnitudes of h′F, dh′F/dt or ExB drift, bubble rise velocity and the postsunset secondary maximum (PSSM) in IEC. At VHF frequencies, observed scintillation at all the locations are mostly of saturated type, therefore, it is difficult to examine their intensity variations with other controlling parameters such as daytime EEJ strength. For that purpose, scintillation observations at 4 GHz from INSAT-1B (74°E) and INSAT-1C (94°E) satellites at two stations, namely, Chenglepet (10.4°N, 79.5°E, magnetic latitude 3.5°N subionospheric at 420 km) and Sikandarabad (26.8°N; 77.8°E; magnetic latitude 20.8°N) (for details, see Dabas et al. [1991] and Dabas et al. [1998]) for September–October 1989 periods have been utilized. The electrojet (EEJ) strength is calculated using magnetic field data from Trivandrum (8.5°N; 76.8°E; magnetic latitude 0.3°N) and Alibag (18.6°N; 72.8°E; magnetic latitude 13.4°N) as H(TRV) - H(ALB) in nT units [Chandra and Rastogi, 1974].

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Figure 1. Locations of scintillation observing stations and their subionospheric points corresponding to 420 km height.

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Figure 2. Magnetic field lines along 84°E meridian corresponding to F region heights of the respective locations and the ray paths to the Engineering Test Satellite, version 2 (ETS II) geo-stationary satellite (130°E).

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Table 1. Coordinates of the Observing Stations and Their Subionospheric Points at 420 km for Bangalore, Hyderabad, and Nagpur and at 250 km for Delhi and Kurukshetra
StationCoordinates of Observing StationsCoordinates of Subionospheric Point
GeographicDip Lat.SubionosphericDip Lat.
Bangalore13.0°N, 77.5°E5.0°N11.8°N, 83.5°E3.0°N
Hyderabad17.3°N, 78.5°E10.5°N16.0°N, 84.5°E7.6°N
Nagpur21.0°N, 79.1°E14.6°N19.3°N, 85.1°E12.0°N
Delhi28.6°N, 77.2°E24.8°N26.1°N, 84.5°E20.9°N
Kurukshetra29.9°N, 78.6°E27.0°N27.2°N, 85.7°E22.8°N

[4] Observed scintillations at most of the locations were essentially of saturated type having Faraday Polarization fluctuations observed in IEC and their onsets were often abrupt. The occurrence of scintillations at a higher latitude station was conditional to their prior occurrence at lower latitudes. The onset of scintillations up to 12°N magnetic latitudes was nearly simultaneous (to within 15 min), while some time delay (varying from about 30 min to several hours) was observed in their onset at Delhi and Kurukshetra as compared to lowest latitude station Bangalore. The observed characteristics and systematic occurrence pattern of scintillations at a chain of stations suggest that these are caused by the ionospheric irregularities associated with rising equatorial plasma bubbles in the postsunset hours [Somayajulu et al., 1984; Dabas and Reddy, 1986]. Hence forth, in the present study the times of occurrences of scintillations at different locations will be assumed to represent the latitudinal, and effectively altitudinal, growths of equatorial plasma bubbles and associated ionospheric irregularities.

2.1. Plasma Bubble Onsets and Latitudinal Growth With Local Time

[5] Out of the 21 days of simultaneous scintillation observations at a chain of stations extending from near equator to about 23°N magnetic latitudes, only on 7 days scintillations could be observed up to about 23° magnetic latitudes (i.e., of Kurukshetra) and on 13 days up to about 21°N (i.e., up to Delhi). However, near the equatorial location Bangalore (magnetic latitude 3°N) scintillations were observed daily except on one day (i.e., on February 16, 1980), when there was a partial solar eclipse during evening hours and it was magnetically disturbed day. On that day there were no scintillations at any of the stations. As mentioned above in section 2, the occurrence and growths of plasma bubbles are studied in terms of scintillation onset times at different latitudes starting from the lowest station Bangalore moving northwards to higher latitudes. Figures 3a, 3b, 3c, and 3d are four examples showing the onsets of four individual plasma bubbles observed on 21- 2-1980, 11-2-1980, 15-2-1980, and 17-2-1980, respectively, and also their subsequent latitudinal/altitudinal growth with local time. In the first two cases (see Figures 3a and 3b), scintillation occurred at all the stations from Bangalore (magnetic latitude 3.0°N) up to Kurukshetra (magnetic latitude 22.8°N) indicating that the plasma bubble and associated irregularities mapped up to 1270 km in altitudes over the magnetic equator whereas, in the next two cases (see Figures 3c and 3d) the bubble could reach only up to the altitude of 710 km as scintillations were observed up to Nagpur (magnetic latitude 12.0°N). Figure 3a shows a case of very rapidly rising bubble which takes less than an hour to rise between 450 km to 1270 km, whereas Figure 3b shows a case of comparatively slow rising bubble which takes almost double the time to rise over the same altitudes range over the magnetic equator. In these figures, bubble rise velocities over the magnetic equator are also given which are derived using successive time delay in the occurrence of scintillations at different locations and the altitudinal separation between their respective field lines over the magnetic equator [Dabas and Reddy, 1990] for different altitudinal slabs. The detailed results about plasma bubble rise velocity, i.e., their estimation; variations with altitudes and correlation with ExB etc. are given in a separate paper by Dabas and Reddy [1990]. It is to be pointed out here that a rising plasma bubble also drift (usually) eastward in such a way that their eastward velocity increases as they grow vertically upward. In the Indian sector, using 4 GHz scintillation observations (as mentioned in section 2 above) from two INSAT satellites separated by 20 degrees in longitude and at two stations, one near equator and another at low latitude near Delhi, the average values of eastward drift velocity reported by Dabas et al. [1992] were of the order of 100 to 175 m/s near F region, i.e., around 400 km altitude and of the order of 60 to 90 m/s in the topside ionosphere, i.e., around 1200 km altitude. While calculating the plasma bubble rise velocity (as shown in Figures 3a to 3d) the effect of bubble eastward drift velocity has not been incorporated and hence there can be a systematic error in the calculated rise velocities depending upon the observed time delays between the onsets of scintillations at successive stations. The larger the time delay, the more will be the error. The observed time delay between the onsets of scintillation at Bangalore and Hyderabad was minimum (varies from a few minutes to a maximum of about 10 min), and hence the error introduced by horizontal drift in calculating the initial plasma bubble rise velocity should also be minimum as compared to the other altitudinal slabs shown in Figures 3a to 3d. Here our main objective is to examine the role of initial plasma bubble rise velocity on the altitudinal/latitudinal growth of plasma bubble. In general, results of Figures 3a and 3b suggest that the bubble rise velocity over the magnetic equator decreases with altitudes and seems to be at a maximum near the peak of F region. The two examples shown in Figures 3a and 3c are cases of very rapidly rising bubble where initial plasma bubble rise velocity was more than 400 m/s, and on both these days evening hours h′F was more than 500 km with dh′F/dt greater than 30 m/s. However, still in the first case the bubble rises up to 1270 km in altitude over the magnetic equator reaching up to Kurukshetra latitude, whereas in the second case it was confined up to about 700 km, i.e., up to Nagpur latitude only. In Figure 3b the initial rise velocity (208 m/s) is lower than that of the case shown in Figure 3d (333 m/s), but still in the former scintillations are observed at all the stations, and in the later case these are again confined up to Nagpur only. In these cases also evening hours h′F was more than 450 km with dh′F/dt greater than 25 m/s. The results of the above examples show that the maximum vertical growth, i.e., up to 1270 km (or the latitudinal extent up to Kurukshetra magnetic latitude of 22.8°N) of an individual plasma bubble does not depend only on the initial plasma bubble rise velocity, h′F and ExB as is evident from Figures 3a to 3d, but there seems to be some other contributing factor as well, e.g., the development of postsunset equatorial ionization anomaly (PEA) as indicated by the occurrence of PSSM in IEC suggested by Whalen [2000], which will be examined in detail in the following sections.

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Figure 3. (a) Onset of equatorial plasma bubble and its altitudinal growth on 21-2-1980. Bubble rise velocity is also given for different altitude ranges. (b) Onset of equatorial plasma bubble and its altitudinal growth on 11-2-1980. (c) Onset of equatorial plasma bubble and its altitudinal growth on 15-2-1980. (d) Onset of equatorial plasma bubble and its altitudinal growth on 17-2-180.

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2.2. Altitudinal/Latitudinal Growth of Plasma Bubbles and Their Association With Other Controlling Parameters

[6] This section examines the onset as well as the latitudinal/altitudinal growth of equatorial plasma bubble in relation to the occurrence of postsunset secondary maximum in IEC (PSSM) at low latitudes, variations in the evening hour h′F and dh′F/dt values and also with daytime equatorial electrojet strength (EEJ). In the recent past, association between the strength of daytime equatorial ionization anomaly and the occurrence of nighttime equatorial and low-latitude scintillation has been studied by several authors [e.g., Balan and Rao, 1984; Dabas et al., 1998]. Though it is difficult to understand the exact relationship between the strength of daytime ionization anomaly and the occurrences of scintillation at equatorial and low-latitude regions, the main objective remains to be to gain prediction capability of the day-to-day occurrences of scintillations over these regions. Since the strength of daytime equatorial ionization anomaly depends upon the equatorial electrojet strength (EEJ), a parameter that is easily available on regular basis from magnetic data, it is therefore more appropriate to use electrojet strength for this purpose. Hence, to examine the relationship, if any, between the day-to-day occurrence of nighttime scintillations at different stations (i.e., latitudes) and the corresponding daytime electrojet strength values, daily EEJ strength values at 1100 hours (75°EMT) has been used.

2.2.1. Plasma Bubble Growth and Its Relationship With PEA/PSSM

[7] From the examination of daily multistation scintillation and simultaneous TEC data it is noted that the occurrences of PSSM at Delhi were observed mostly between 1930 to 2100 hours local time (LT) whereas scintillations were observed between 1930 to 0030 LT. It is noted that the scintillation at low latitudes, i.e., at Delhi and Kurukshetra is observed only on those days when a prior occurrence of PSSM at Delhi is also observed whereas the reverse may not be true, i.e., there may be PSSM without the occurrence of scintillation. In other words, plasma bubble and associated irregularities reached up to Delhi latitudes (21°N magnetic latitude) or beyond only on those days when it is also accompanied by high background electron density as indicated by the occurrence of PSSM there. In addition, it was also noticed that the magnitude of PSSM at Delhi was higher on those days when scintillations were also observed at Kurukshetra latitudes (23°N magnetic latitude) as compared to those days when scintillation activity was confined up to Delhi latitudes only. As an example, TEC variation during nighttime at different stations on 21st and 15th February 1980 corresponding to the scintillation observations shown in Figures 3a and 3c are plotted in Figures 4a and 4b, respectively. In Figure 4a, TEC data from three stations, namely, Hyderabad, Nagpur, and Delhi are plotted because Bangalore TEC data were not available for that day, whereas in Figure 4b TEC data from all four stations are shown. As seen from these figures, on 21st February (see Figures 3a and 4a) when scintillations were observed at all the locations, well developed PSSM (TEC was more than 120 × 1016 el/m**2) was observed at Delhi before the onset of scintillations at Delhi and Kurukshetra, whereas on 15th February (see Figures 3c and 4b) when scintillations were confined up to Nagpur only, PSSM was also developed up to Nagpur only and there was no PSSM observed at Delhi where TEC was less than 60 × 1016 el/m**2 around 1800 LT and there after it start decreasing. Similarly on 11th February (see Figure 3b), when scintillations were observed at all locations, a well developed PSSM (figure not shown) was observed around 1940 LT at Delhi but the same was not there on 17th February (see Figure 3d) when scintillations activity was confined up to Nagpur only. The above result does indicate that the prior occurrence of PSSM provides a suitable environment for the development of equatorial plasma bubble up to low latitudes after its onset in the bottom of equatorial F region; otherwise it will remain confined to near equatorial latitudes only. Therefore, background ionization also seems to plays an important role for the altitudinal/latitudinal growth of plasma bubble and associated irregularities in addition to other controlling parameters like high values of h′F, ExB etc. that will be discussed in more detail in the subsequent sections.

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Figure 4. (a) Variations of nighttime TEC at different stations on the night of 21st February 1980 when scintillations were observed at all the stations shown in Figure 3a. (b) Variations of nighttime TEC at different stations on the night of 15th February 1980 when scintillations were observed up to Nagpur only shown in Figure 3c.

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2.2.2. Occurrence of Plasma Bubble and Variations of h′F and dh′F/dt

[8] From the analysis of 21 simultaneous scintillation data from different stations and the Kodaikanal (10.2°N, 77.5°E; dip 3.5°) ionosonde data, it is noted that plasma bubbles and associated irregularities are observed up to 21°N magnetic latitude or even higher mainly on those days when evening hour h′F values over the magnetic equator reaches up to 500 km or even more with corresponding dh′F/dt (interpreted as ExB drift velocities) values more than 20 m/s. Whereas at near equatorial locations, like Bangalore, Hyderabad and Nagpur, scintillations are observed almost daily even when the evening hours h′F was about 400 km. Only on one day, i.e., on February 16, 1980, when there was a solar eclipse and a moderate magnetic storm also occurred, no scintillations were observed at any of the locations, and the evening hours h′F and dh′F/dt values at Kodaikanal were lower than 400 km and 20m/s, respectively. This clearly indicates that after the onset of a plasma bubble over the magnetic equator its altitudinal/latitudinal growth is strongly dependent on the h′F and ExB drift velocities in addition to the occurrence of PSSM discussed above and their growth to higher latitude is more likely when h′F and ExB values are more than a threshold value.

2.2.3. Magnitude of PSSM at Low Latitudes and Its Relationship With ExB Drift and Daytime EEJ Strength

[9] In the previous section we have seen that latitudinal/altitudinal growth of equatorial plasma bubble and associated irregularities is a very complex phenomena which requires various suitable controlling parameters as an essential environment for its growth and strongly dependent on the postsunset hours h′F, dh′F/dt, plasma bubble rise velocity and the development of PSSM at low latitudes. It is difficult to pin point, which one is the main controlling parameter suitable for the predictions of plasma bubble especially up to low latitudes where the observed intensity of scintillations is very high as compared to that at near equatorial locations. Therefore, it will be interesting to examine the relationship between the magnitude of PSSM enhancement at Delhi with ExB estimated as dh′F/dt as well as with daytime EEJ strength variations. The magnitude of PSSM is determined roughly as TEC enhancement from the average behavior over few hours centered on a time interval around the PSSM occurrence on a given day. In Figures 5a and 5b, magnitude of PSSM is plotted versus ExB drift speed and EEJ strength, respectively. Results of these figures indicate that there exist a positive correlation between the magnitude of PSSM and that of ExB and EEJ strength variations. The above relationship between PSSM and ExB is seems to be obvious because both are driven by prereversal F region electric field enhancement over the magnetic equator whereas the positive relationship with daytime EEJ strength is difficult to understand or explain at this stage because EEJ is driven by daytime E region dynamo electric field.

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Figure 5. (a) Variations of the magnitude of PSSM with dh′F/dt (or ExB drift). (b) Variations of the magnitude of PSSM with EEJ strength expressed in magnetic field H component intensity denoted by H(TRV) - H(ALB).

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2.2.4. EEJ Relationship With Equatorial h′F and dh′F/dt (or the ExB)

[10] It will also be interesting to examine the relationship, if any, between the daily values of evening hours h′F and dh′F/dt with that of daytime EEJ strength. To do that, daily evening hours (around 1800 and 1900 LT) peak values of h′F and the calculated dh′F/dt values (between the time interval 1600 to 1800 or 1900 LT depending upon the occurrence of peak h′F value) are plotted against daytime (1100 LT) EEJ values in Figures 6a and 6b, respectively. It is interesting to note that both h′F and dh′F/dt (ExB drift) showing good positive correlation with correlation coefficient of about 0.85 with EEJ although there is a large scatter of points. The above study is based on only 20 days data, therefore, to further verify the above relationship, September–October 1989 ionosonde h′F data from Kodaikanal, corresponding to the period of 4 GHz scintillation observations (details of which are mentioned in section 2 above) are also examined with corresponding daytime EEJ strength. Figures 6c and 6d show the plots of h′F and dh′F/dt, respectively, with daytime EEJ values. It is interesting to note that similar to the results of Figures 6a and 6b, here again both h′F and dh′F/dt are showing positive correlation with daytime EEJ strength variations although in the later case correlation coefficient reduced to about 0.7 as compared to 0.85 in the former. The large scatter seen in all these plots are may be because of the fact that here only published hourly h′F values are used and some times due to presence of spread F these values are approximate one. For further evaluation of a possible control by electrojet process, it is also examined how the amplitude of EEJ strength varies during the months of occurrence and non-occurrence of scintillations. For this purpose daily 1100 LT values of EEJ strength during four months, i.e., September–October 1989, when scintillation occurrence in this longitude zone is maximum and during November–December 1989 when it is less frequents, are examined. It is noted that the average values of EEJ strength during September–October periods was comparatively higher (about 85 nT) than that during November–December periods (about 50 nT). In general, like scintillation occurrences, EEJ strength is also noted to be maximum during equinoxial months. Though, it is difficult to understand the physical relationship between the daytime EEJ strength and the evening hour h′F and dh′F/dt (ExB) variations but these statistical results do show their importance for prediction of occurrences and latitudinal development of postsunset hour plasma bubble and associated irregularities.

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Figure 6. (a) Variations of h′F with EEJ strength, denoted by H(TRV) - H(ALB), during February 1980. (b) Variations dh′F/dt (or ExB drift) with EEJ strength, denoted by H(TRV) - H(ALB), during February 1980. (c) Variations of h′F with EEJ strength, denoted by H(TRV) - H(ALB), during September–October 1989. (d) Variations dh′F/dt (or ExB drift) with EEJ strength, denoted by H(TRV) - H(ALB), during September–October 1989.

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2.2.5. EEJ Strength and the Intensity of 4 GHz Scintillations at Low Latitude

[11] As mentioned in section 2 above, observed VHF scintillations, at all the locations and especially at low latitudes, were of saturated type, and it is difficult to examine their intensity variations with other controlling parameters such as magnitude of PSSM or ExB. For this purpose, here we have used 4 GHz scintillation data pertaining to the premidnight period only, from a low-latitude station, Sikandarabad (26.8°N; 77.8°E; magnetic latitude 20.8°N), near Delhi for September–December, 1989 period. The scintillation occurrence was maximum during September–October period whereas only few cases were observed during November–December Period. Using the same data, Dabas et al. [1998] showed that there is a good correlation between the intensity of 4 GHz scintillation and the strength of PSSM in IEC over Delhi as well as with the diurnal maximum IEC values. Since daytime IEC in low latitude is again associated with EEJ strength variations, therefore it will be interesting to examine further if there is any relationship between the scintillation intensity with daytime EEJ strength as well. In doing that, intensity (peak-to-peak fluctuations in dB) of 4 GHz nighttime (premidnight hours) scintillation, observed during September–December 1989 period, is plotted against EEJ strength in Figure 7. From the figure it is interesting to note that there is a linear relationship between the two with correlation coefficient of 0.76. The above results, though preliminary and statistical, but again shows that daytime EEJ strength is an important parameter and can be useful for prediction of scintillation strength at low latitudes even at GHz frequencies.

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Figure 7. Correlation between the intensity of 4 GHz scintillation observed during September–December 1989 at low-latitude station Sikandanabad (26.8°N; 77.8°E; magnetic latitude 20.8°N) and the daytime EEJ strength, denoted by H(TRV) - H(ALB).

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3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[12] The above results show that the altitudinal/latitudinal growth of equatorial plasma bubble and associated irregularities is seems to be strongly dependent on the development of PEA in the form of occurrence of PSSM at low latitudes in addition to the magnitudes of evening hours equatorial h′F, dh′F/dt and plasma bubble rise velocity values. There are several factors that control the generation of plasma bubble and its subsequent vertical growth velocity. The linear growth rate of the plasma bubble and associated irregularities is inversely proportional to the ion-neutral collision frequency, and consequently when the F layer is high, the growth rate will be high [Ossakow and Chaturvedi, 1978] because of reduced ion drag at higher altitudes. In addition, the vertical growth rate and hence its latitudinal spread depend on the rate of rise of the F layer (interpreted as ExB drift velocity) during presunset hours as noted above. Anderson and Haerendel [1979] have shown that, in general, the bubble growth rates are small (large) when the ExB vertical drift velocity is small (large) which is also evident from the present results as well. This implies its dependence on the prereversal electric field enhancement in the equatorial region and rather ExB is central to any study related to postsunset equatorial ionosphere. Present results also further reconfirm the above observations about h′F and its rate of rise during evening hours.

[13] Aarons et al. [1981], Basu et al. [1988], and Dabas et al. [1998] reported that intensity of scintillations is dependent on the background electron density distribution in the local ionosphere of the respective location. Whalen [2000], from the analysis of multistation ionosonde data from an equatorial low-latitude region, reported a strong link between the altitudinal/latitudinal growth of a plasma bubble and development of postsunset hours Appleton ionization anomaly and suggested that the anomaly and bubble are each an essential part of the environment of the other. It is also reported that the rate of increase of NmF2, and its final maximum are closely related to maximum ExB drift velocity [Whalen, 1998]. Garg et al. [1983] reported that the meridional winds and ExB drifts primarily control the development of postsunset equatorial ionization anomaly. Dabas et al. [1998] also reported that intensity of nighttime scintillations at 4 GHz at Delhi showed good correlation with the nighttime IEC enhancement over there. In addition, it is also reported by them that the intensity of 4 GHz scintillations at Delhi are showing positive correlation even with daytime diurnal IEC maximum values. It is well known that EEJ strength and the daytime equatorial ionization anomaly are related because both are basically driven by the same dynamo electric field. Results of the present analysis suggest that the prior occurrence of PSSM up to 21°N magnetic latitude is seems to be one of the important requirements for the latitudinal growth of plasma bubble up to that latitude. In other words high background ionization in the form of occurrence of PSSM appears to accompany the plasma bubble development and the present results further reconfirm the Whalen [2000] observations stated above.

[14] Since ionospheric irregularities associated with the equatorial plasma bubble cause scintillation that can seriously disrupt the operation of nearly all space and ground based communication systems that rely on trans-ionospheric propagation of radio frequencies [Aarons, 1993], therefore, one of the most important problems at present is to know their cause and effect relationship and finally to gain their prediction capability. In the present study, daytime Equatorial Electrojet strength was used to study its correlation with some of the parameters like h′F, ExB drift, magnitude of PSSM which actually controls the day-to-day occurrence as well as the latitudinal/altitudinal growth of equatorial plasma bubble as well as with the intensity of 4 GHz scintillations at low latitudes. Though, apparently there seems to be no physical relation, except the statistical ones, between these phenomena occurring at different times but here the main objective was just to gain prediction capability. During daytime the eastward electric field mainly controls the electron density distribution in equatorial and low latitudes through the vertical ExB drift, and the electrojet strength variations through the Hall effect. Therefore, on a particular day if the eastward electric field/electrojet strength is more then the residue ionization at low latitude during evening hours should be more as compared to the day when it is less. This may further add to by the development of postsunset hour ionization anomaly by F region dynamo field. Other than this there seems to be no connection but still interestingly three most important parameters (h′F, ExB and PSSM), which are related with the onset and latitudinal/altitudinal growth of a plasma bubble, are showing good correlation with daytime EEJ strength variations. Daytime EEJ strength also showing good correlation with the intensity of 4 GHz scintillations at low latitudes. Therefore, EEJ strength can be a very useful parameter for prediction of equatorial plasma bubble and associated irregularities for radio system applications.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[15] Simultaneous observations of VHF scintillations at five locations situated between 3°and 23°N magnetic latitudes were used to detect and monitor the altitudinal/latitudinal growth of equatorial equatorial plasma bubble and associated ionospheric irregularities. After its onset at the base of F region bubble grow in altitude over the magnetic equator with a velocity which maximized near the F region peak and then subsequently decreases at higher altitude. The day-to-day occurrence and their latitudinal growths found to be strongly dependent on evening hours h′F, ExB drift speed and the prior occurrence of postsunset hours enhancements in IEC. The magnitude of postsunset enhancement is found to more on those days when ExB drift speed was more. It was also found that the equatorial h′F and dh′F/dt, magnitude of postsunset hours enhancement in IEC and the intensity of 4 GHz scintillation at low latitudes are showing good positive correlation with daytime (1100 LT) electrojet strength (EEJ) values. It is concluded that, after the initial development of a bubble, the ExB drift and the postsunset ionization anomaly play an important role in the subsequent growth and evolution, and that electrojet strength is a useful parameter for the prediction of the development.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[16] This work was carried out while R.S.D. was a Leverhulme Trust Visiting Fellow at the Department of Physics, University of Wales, Aberystwyth, supported by the Leverhulme Trust, UK. Special thanks to Prof. L. Kersley of the Department of Physics, University of Wales, Aberystwyth, UK, for his valuable suggestions and helpful discussions.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Data Analysis and Results
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information
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rds4809-sup-0001tab01.txtplain text document1KTab-delimited Table 1.

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