Plasma Bubble Occurrence Research Articles

An analysis of the quiet time day‐to‐day variability in the formation of postsunset equatorial plasma bubbles in the Southeast Asian region

AbstractPresented is an analysis of the occurrence of postsunset Equatorial Plasma Bubbles (EPBs) detected using a Global Positioning System (GPS) receiver at Vanimo. The three year data set shows that the EPB occurrence maximizes (minimizes) during the equinoxes (solstices), in good agreement with previous findings. The Vanimo ionosonde station is used with the GPS receiver in an analysis of the day‐to‐day EPB occurrence variability during the 2000 equinox period. A superposed epoch analysis (SEA) reveals that the altitude, and the change in altitude, of the F layer height is ∼1 standard deviation (1σ) larger on the days for which EPBs were detected, compared to non‐EPB days. These results are then compared to results from the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), which show strong similarities with the observations. The TIEGCM is used to calculate the flux‐tube integrated Rayleigh‐Taylor (R‐T) instability linear growth rate. A SEA reveals that the modeled R‐T growth rate is 1σ higher on average for EPB days compared to non‐EPB days, and that the upward plasma drift is the most dominant contributor. It is further demonstrated that the TIEGCM's success in describing the observed daily EPB variability during the scintillation season resides in the variations caused by geomagnetic activity (as parameterized by Kp) rather than solar EUV flux (as parameterized by F10.7). Geomagnetic activity varies the modeled high‐latitude plasma convection and the associated Joule heating that affects the low‐latitude F region dynamo, and consequently the equatorial upward plasma drift.

Zonal velocity of the equatorial plasma bubbles over Kolhapur, India

Abstract. This paper presents the observations of zonal drift velocities of equatorial ionospheric plasma bubbles and their comparison with model values. These velocities are determined by nightglow OI 630.0 nm images. The nightglow observations have been carried out from the low latitude station Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) during clear moonless nights. Herein we have presented the drift velocities of equatorial plasma bubbles for the period of February–April 2011. Out of 80 nights, 39 showed the occurrence of equatorial plasma bubbles (49%). These 39 nights correspond to magnetically quiet days (ΣKp < 26). The average eastward zonal velocities (112 ± 10 m s−1) of equatorial plasma bubbles increased from evening sector to 21:00 IST (Indian Standard Time = Universal Time + 05:30:00 h), reach maximum about 165 ± 30 m s−1 and then decreases with time. The calculated velocities are in good agreement with that of recently reported values obtained with models with occasional differences; possible mechanisms of which are discussed.

Occurrence of equatorial plasma bubbles over Kolhapur

This paper reports the nightglow observations of OI 630.0nm emissions, made by using all sky imager operating at low latitude station Kolhapur (16.8°N, 74.2°E and dip lat. 10.6°N) during high sunspot number years of 24th solar cycle. The images are analyzed to study the nocturnal, seasonal and solar activity dependence occurrence of plasma bubbles. We observed EPBs in images regularly during a limited period 19:30 to 02:30LT and reach maximum probability of occurrence at 22:30LT. The observation pattern of EPBs shows nearly no occurrence during the month of May and it maximizes during the period October–April. The equinox and solstice seasonal variations in the occurrence of plasma bubbles show nearly equal and large differences, respectively, between years of 2010–11 and 2011–12.

Electron temperature enhancements in nighttime equatorial ionosphere under the occurrence of plasma bubbles

Simultaneous in-situ measurements of electron density and temperature in the nighttime equatorial region were performed by a rocket experiment launched under solar minimum and geomagnetic quiet conditions from the station of Alcântara (2.24°S; 44.24°W), Brazil, on Dec. 18, 1995, 21:17 LT. These measurements detected during the upleg flight a large overheated area around the base of density profile. The presence of plasma bubbles was revealed during the downleg phase, as well as temperature enhancements detected preferentially at altitudes where plasma depletions are found. It was assumed that the region traversed by the rocket during the downleg was preceded before the bubble's onset by a large overheating as observed during the upleg flight. Analyzing this framework under the light of the Global Self-Consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP), as well as a 2D numerical code that simulate the growth of an instability and the evolution of thermal energy inside a bubble, we found that despite an overheated area in the F-region bottomside can disturb the electron density profile around the altitude interval where such heat is deposited, it seems not have a direct influence over parameters responsible for the bubble onset. Additionally, the phenomenon of the intra-bubble thermal enhancement could be formed due to the convection of hot-electron fluid transported from the overheated region surrounding the base of the F-region to upper altitudes by the underlying mechanism of bubble generation.

Total electron content of the ionosphere at two stations in East Africa during the 24–25 October 2011 geomagnetic storm

The equatorial ionosphere has been known to become highly disturbed and thus rendering space-based navigation unreliable during space weather events, such as geomagnetic storms. Modern navigation systems, such as the Global Positioning System (GPS) use radio-wave signals that reflect from or propagate through the ionosphere as a means of determining range or distance. Such systems are vulnerable to effects caused by geomagnetic storms, and their performance can be severely degraded. This paper analyses total electron content (TEC) and the corresponding GPS scintillations using two GPS SCINDA receivers located at Makerere University, Uganda (Lat: 0.3o N; Lon: 32.5o E) and at the University of Nairobi, Kenya (Lat: 1.3o S; Lon: 36.8o E), both in East Africa. The analysis shows that the scintillations actually correspond to plasma bubbles. The occurrence of plasma bubbles at one station was correlated with those at the other station by using observations from the same satellite. It was noted that some bubbles develop at one station and presumably “die off” before reaching the other station. The paper also discusses the effects of the geomagnetic storm of the 24–25 October 2011 on the ionospheric TEC at the two East African stations. Reductions in the diurnal TEC at the two stations during the period of the storm were observed and the TEC depletions observed during that period showed much deeper depletions than on the non-storm days. The effects during the storm have been attributed to the uplift of the ionospheric plasma, which was then transported away from this region by diffusion along magnetic field lines.

Equatorial plasma bubbles and L-band scintillations in Africa during solar minimum

Abstract. We report on the longitudinal, local time and seasonal occurrence of equatorial plasma bubbles (EPBs) and L band (GPS) scintillations over equatorial Africa. The measurements were made in 2010, as a first step toward establishing the climatology of ionospheric irregularities over Africa. The scintillation intensity is obtained by measuring the standard deviation of normalized GPS signal power. The EPBs are detected using an automated technique, where spectral analysis is used to extract and identify EPB events from the GPS TEC measurements. Overall, the observed seasonal climatology of the EPBs as well as GPS scintillations in equatorial Africa is adequately explained by geometric arguments, i.e., by the alignment of the solar terminator and local geomagnetic field, or STBA hypothesis (Tsunoda, 1985, 2010a). While plasma bubbles and scintillations are primarily observed during equinoctial periods, there are longitudinal differences in their seasonal occurrence statistics. The Atlantic sector has the most intense, longest lasting, and highest scintillation occurrence rate in-season. There is also a pronounced increase in the EPB occurrence rate during the June solstice moving west to east. In Africa, the seasonal occurrence shifts towards boreal summer solstice, with fewer occurrences and shorter durations in equinox seasons. Our results also suggest that the occurrence of plasma bubbles and GPS scintillations over Africa are well correlated, with scintillation intensity depending on depletion depth. A question remains about the possible physical mechanisms responsible for the difference in the occurrence phenomenology of EPBs and GPS scintillations between different regions in equatorial Africa.

On the importance of wave‐like structures in the occurrence of equatorial plasma bubbles: A case study

Coordinated observations of equatorial plasma bubbles (EPBs) have been made with an all‐sky airglow imager, narrow bandwidth photometer, VHF radar, and ionosonde over the Indian sector on the night of 23 March 2009. The prereversal enhancement (PRE) in the vertical plasma drift during the postsunset hours on this day was moderate. Range type spread F was found to occur immediately after the satellite traces were noted in the ionograms. This was well recorded in measurements made by all‐sky imager, narrow band photometer, and VHF radar. The airglow emission intensities also revealed the presence of a large‐scale wave‐like structure (LSWS) together with the plasma bubbles that coincided with plume structures observed in the VHF radar echoes. The periodicity of the occurrence of bubbles (and plumes) and their interdepletion distances suggest the presence of small‐scale wave‐like structures (SSWS) on this night. The results are compared with the ionosonde observations made on the night of 21 February 2008. The PRE and the maximum height attained by the F layer were very similar to that of 23 March 2009. In addition, the ionograms showed the presence of satellite traces. However, no subsequent evolution of spread F was noticed. Considering the satellite traces to have their origin in LSWS, these observations imply that though the presence of LSWS is important for the triggering of EPBs, they alone are not sufficient. However, the coexistence of both LSWS and SSWS may have the potential to trigger EPBs.

Occurrence of Equatorial Plasma Bubbles during Intense Magnetic Storms

An important issue in low-latitude ionospheric space weather is how magnetic storms affect the generation of equatorial plasma bubbles. In this study, we present the measurements of the ion density and velocity in the evening equatorial ionosphere by the Defense Meteorological Satellite Program (DMSP) satellites during 22 intense magnetic storms. The DMSP measurements show that deep ion density depletions (plasma bubbles) are generated after the interplanetary magnetic field (IMF) turns southward. The time delay between the IMF southward turning and the first DMSP detection of plasma depletions decreases with the minimum value of the IMFBz, the maximum value of the interplanetary electric field (IEF)Ey, and the magnitude of the Dst index. The results of this study provide strong evidence that penetration electric field associated with southward IMF during the main phase of magnetic storms increases the generation of equatorial plasma bubbles in the evening sector.

Handling cycle slips in GPS data during ionospheric plasma bubble events

[1] During disturbed ionospheric conditions such as the occurrence of plasma bubbles, the phase and amplitude of the electromagnetic waves transmitted by GPS satellites undergo rapid fluctuations called scintillation. When this phenomenon is observed, GPS receivers are more prone to signal tracking interruptions, which prevent continuous measurement of the total electron content (TEC) between a satellite and the receiver. In order to improve TEC monitoring, a study was conducted with the goal of reducing the effects of signal tracking interruptions by correcting for “cycle slips,” an integer number of carrier wavelengths not measured by the receiver during a loss of signal lock. In this paper, we review existing cycle-slip correction methods, showing that the characteristics associated with ionospheric plasma bubbles (rapid ionospheric delay fluctuations, data gaps, increased noise, etc.) prevent reliable correction of cycle slips. Then, a reformulation of the “geometry-free” model conventionally used for ionospheric studies with GPS is presented. Geometric information is used to obtain single-frequency estimates of TEC variations during momentary L2 signal interruptions, which also provides instantaneous cycle-slip correction capabilities. The performance of this approach is assessed using data collected on Okinawa Island in Japan during a plasma bubble event that occurred on 23 March 2004. While an improvement in the continuity of TEC time series is obtained, we question the reliability of any cycle-slip correction technique when discontinuities on both GPS legacy frequencies occur simultaneously for more than a few seconds.

Comparing plasma bubble occurrence rates at CHAMP and GRACE altitudes during high and low solar activity

Abstract. Based on the multi-year data base (2001–2009) of CHAMP Planar Langmuir Probe (PLP) data and GRACE K-Band Ranging (KBR1B) data, typical features of ionospheric plasma irregularities are studied at the altitudes of CHAMP (300–400 km) and GRACE (~500 km). The phenomena we are focusing on are the equatorial plasma bubbles (EPBs). Similar seasonal/longitudinal (S/L) distributions of EPB have been found at both CHAMP and GRACE altitudes during solar active and quiet years. Peak EPB occurrence rates, defined as number of events within an S/L bin divided by the number of passes over that bin, decrease from the high and moderate solar flux period (2001–2005) to the low solar flux period (2005–2009) from 80% to 60% and 60% to 40% at CHAMP and GRACE altitudes, respectively. On average the occurrence rate increases linearly with solar flux at about the same rate at CHAMP and GRACE. For high flux levels (P10.7>200) non-linear increases are observed at GRACE. The occurrence rate increases rapidly after 19:00 local time (LT) during high solar flux periods. Around solar minimum rates increase more gently and peak around 22:00 LT. The highest occurrence rates are encountered at latitudes around 10° north and south of the dip equator. Results from the two altitudes support the notion that EPBs form regions of depleted plasma along geomagnetic fluxtubes. It is shown for the first time that in regions of high occurrence rates EPBs are associated with fluxtubes reaching greater apex heights than those in regions of low rates.

Periodic spacing between consecutive equatorial plasma bubbles

We analyze three‐years of data collected by a field‐aligned airglow imaging system located at the Cerro Tololo Inter‐American Observatory near La Serena, Chile to determine the occurrence of equatorial plasma bubbles (EPBs). On 317 of the 552 predominately clear nights of observations, structure indicative of EPBs is present. On 123 of these nights, multiple EPBs with periodic spacings were recorded with 88 nights showing 3 or more consecutive bubbles. We suggest that the periodic spacing of EPBs could be related to the properties of an underlying seed mechanism, namely gravity waves (GWs). The distribution of spacings compares favorably to the spectrum of GW induced traveling ionospheric disturbances (TIDs) measured by Vadas and Crowley (2010) from a similar geographic latitude in the northern hemisphere. Furthermore, the distribution of spacings decreases from 2006 through 2009, tracking the corresponding decrease in the thermospheric neutral temperature, Tn. As Tn decreases, GWs with larger horizontal wavelengths have smaller initial amplitudes and cannot propagate as easily to EPB seeding altitudes. Thus, our observations are consistent with GW theory.

Global Ultraviolet Imager equatorial plasma bubble imaging and climatology, 2002–2007

A comprehensive database of plasma bubble reconstructions is under development, with results reported here from more than 5 years of Global Ultraviolet Imager (GUVI) data. Climatological statistics of plasma bubble occurrence from this database are presented, including the effects of longitudinal, seasonal, geomagnetic, and solar cycle variations on plasma bubble occurrence. The relationship between the latitudinal separation and peak electron density values of the equatorial arcs and plasma bubble occurrence is also discussed. Since its launch on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite in December 2001, GUVI has more than 7 years of observations of the nightside equatorial ionosphere. GUVI is capable of detecting and imaging plasma bubbles within the northern and southern equatorial arcs. An automated algorithm was developed to locate the peaks of the equatorial arcs and detect the presence of equatorial plasma bubbles. This algorithm was integrated with a tomographic imaging model and a statistical inversion technique to reconstruct electron density and produce multidimensional images of plasma depletion structures.

Modeling the low-latitude ionospheric electron density and plasma turbulence in the November 2004 storm period

The storm period of 8–12 November 2004 offers an opportunity for insight into the phenomena of low-latitude ionospheric structure during geomagnetically disturbed times because of the strength of the disturbances, the timing of the storms, and the instrumentation that was operating during the interval. We will take advantage of these factors to model the ambient ionosphere and the plasma turbulence responsible for radio scintillation within it, using the AFRL low-latitude ambient/turbulent ionospheric model and the storm-time model features described in the companion paper [Retterer, J.M., Kelley, M.C., 2009. Solar-wind drivers for low-latitude ionospheric models during geomagnetic storms. J. Atmos. Solar-Terr. Phys., this issue]. The model plasma densities show very good agreement with the densities measured by the Jicamarca ISR as well as with the total electron content (TEC) measured by the Boston College South American chain of GPS receivers. The detection by the radar of coherent returns from plasma turbulence match well the times of predicted ionospheric instability. The predicted geographic extent of the occurrence of equatorial plasma bubbles was matched by DMSP satellite observations and our forecasts of scintillation strength were validated with measurements of S4 at Ancon and Antofagasta by stations of the AFRL SCINDA network.

Effect of magnetic activity on plasma bubbles over equatorial and low-latitude regions in East Asia

Abstract. The dependence of plasma bubble occurrence in the eveningside ionosphere, with magnetic activity during the period years 2001–2004, is studied here based on the TEC observations gathered by ground-based GPS receivers which are located in the equatorial and low-latitude regions in East Asia. The observed plasma bubbles consist of the plasma-bubble events in the equatorial (stations GUAM, PIMO and KAYT), and low-latitude regions (stations WUHN, DAEJ and SHAO). It is shown that most equatorial plasma-bubble events commence at 20:00 LT, and may last for >60 min. The magnetic activity appears to suppress the generation of equatorial plasma bubbles with a time delay of more than 3 h (4–9 h). While in the low-latitude regions, most plasma-bubble events commence at about 23:00 LT and last for <45 min. The best correlation between Kp and low-latitude plasma-bubble occurrence is found with an 8–9 h delay, a weak correlation exists for time delays of 6–7 h. This probably indicates that over 3 h delayed disturbance dynamo electric fields obviously inhibit the development of plasma bubbles in the pre-midnight sector.

Correlations between Strong Range Spread-F and GPS L-Band Scintillations Observed in Hainan in 2004

Data from the DPS-4 digisonde and the GPS L-band ionospheric scintillation monitor are employed to study the correlations between strong range spread-F (SSF) and GPS L-band scintillations observed in the ionosphere over Hainan Island, China (19.5°N, 109.1°E geogr., dip lat. 9°N) in 2004. The SSF in the ionogram is different from the general range spread-F because it extends in frequency well beyond FoF2 and makes FoF2 difficult to be determined. The observations show that the SSF phenomenon is frequently accompanied by the occurrence of GPS L-band scintillations. The SSF and GPS L-band scintillations occur frequently in the equinoctial months (March, April, September, and October), but rarely in the winter (January, February, November, and December) and summer (May–August); especially, occurrence variations of the SSF and GPS L-band scintillations nearly have a same trend. The SSF and scintillations may be associated with the occurrence of topside plasma bubbles and could be explained by the generalized Rayleigh–Taylor instability.

Observations of small- to large-scale ionospheric irregularities associated with plasma bubbles with a transequatorial HF propagation experiment and spaced GPS receivers

The results from simultaneous observations of the nighttime transequatorial propagation (TEP) of HF radio waves between Australia and Japan and the GPS scintillation measurements in south China and Vietnam are presented in this paper. The results showed that there was good correspondence between the nighttime eastward traveling off-great circle propagation (OGCP) of broadcasting waves of Radio Australia from Shepparton, Australia, measured at Oarai, Japan, and the scintillations in GPS radio waves at Hainan, China. This shows that the nighttime eastward traveling OGCP in HF TEP is caused by a large- scale ionospheric structure associated with a plasma bubble. The zonal drift velocities of the large- scale ionospheric structure estimated by the change in the direction of arrival of the OGCP were similar to those of the small- scale irregularities associated with plasma bubbles measured by the GPS scintillation spaced- receiver technique. Our results show that the HF TEP measurement is quite useful for monitoring the plasma bubble occurrence over a wide area and for forecasting the arrival of the plasma bubble at places located to the east of it.

Correlative study of plasma bubbles, evening equatorial ionization anomaly, and equatorial prereversal E × B drifts at solar maximum

Previous ground observations have revealed a correlation that exists between equatorial plasma bubbles, evening equatorial ionization anomaly (EIA), and prereversal E × B drift velocity using latitudinal arrays of ionospheric sounders, such as in the Indian and American regions. Besides the ground measurements, the space‐based observations also provide a convenient way to study the global‐scale variations. On the basis of in situ data collected from DMSP, ROCSAT‐1, and CHAMP satellites, we investigated the correlation of seasonal/longitudinal variations of plasma bubble (PB) occurrence, evening EIA, and prereversal E × B drifts on magnetically quiet days during the solar maximum years (2000–2002). In general, the observational results provide consistent evidences that the large‐scale variations in seasonal/longitudinal distribution of evening EIA and PB occurrence rates are well‐correlated with the observed evening prereversal E × B drifts and that some of the small‐scale longitudinal variations (such as wave number‐4 structure during equinox) of evening EIA exist before the occurrence of the prereversal enhancement and coincide with the daytime equatorial electrojet. In such cases, the small‐scale longitudinal variation of PB occurrence rates may result from the evening EIA small‐scale longitudinal structures. The evening prereversal E × B drifts, together with the longitudinal variations of evening EIA, are assumed to play a role in determining the longitudinal variations of equatorial and low‐latitude PB occurrence rates.

Occurrence characteristics of plasma bubble derived from global ground‐based GPS receiver networks

Occurrence characteristic of plasma bubble was studied using ground‐based GPS receiver networks. The occurrence rate of plasma bubble derived from the global GPS network has higher spatial and temporal resolution than that derived from the other observational techniques because of its wide coverage of the observation. The physical characteristics of plasma bubble occurrence were studied in detail with this novel data set. Twenty‐three GPS receivers around the dip equator were used to reveal the occurrence from 2000 to 2006. Characteristics of the monthly occurrence rates were different among the regions. Although it was found that sunset time lag effect plays an important role for the monthly variation, two asymmetries which could not be explained with the sunset time lag scenario were found: (1) asymmetry between two solstices and (2) asymmetry between two equinoxes. The seasonal variation of the F‐region conductivity integrated along the geomagnetic field line could partially explain the solstice asymmetry. Semiannual occurrence rates from 2000 to 2006 were used to study the year‐to‐year variation from the high solar activity period to the low solar activity period. The dependency of the occurrence on the solar activity was different among the regions. Occurrence rates against the latitude/altitude were investigated in the Asian region in 2004. It was found that the occurrence was high and constant for a station whose height on the dip equator (HODE) was less than 700 km. They began to decrease when HODE was higher than 700 km and was almost zero where it was higher than 900 km.

Optical observations of the growth and day‐to‐day variability of equatorial plasma bubbles

A new narrow‐field ionospheric imaging system, the Portable Ionospheric Camera and Small‐Scale Observatory, has been installed at the Cerro Tololo Inter‐American Observatory near La Serena, Chile (geographic 30.17°S, 289.19°E; geomagnetic 16.72°S, 0.42°E). We present observations of the naturally occurring nightglow emission at 630.0 nm on three consecutive nights demonstrating the day‐to‐day variability in the occurrence of equatorial plasma bubbles or depletions. On two nights, large‐scale undulations with a wavelength on the order of 300–600 km are observed in the emission regions magnetically connected to the bottomside of the equatorial F layer. We demonstrate that at each crest of these large‐scale waves, zero, one, or multiple depletions may grow. Thus, the presence of a large‐scale wave on the bottomside alone is not sufficient for irregularity growth. This variability is presumably due to the presence, or lack, of small‐scale seed waves or some other mechanism needed to increase the instability growth rate past the critical threshold.

The correlation of longitudinal/seasonal variations of evening equatorial pre-reversal drift and of plasma bubbles

Abstract. The evening pre-reversal vertical drift velocity enhancement (PRE) constitutes an important seeding mechanism for the generation of F region irregularities. Ion density and drift measurements from ROCSAT-1 and DMSP satellites are used to examine the correlation of longitudinal/seasonal (l/s) variations in the evening pre-reversal vertical drift velocity at the magnetic equator in the topside ionosphere and the plasma bubble (PB) occurrence probability. The analysis performed for three years 2000–2002 (solar maximum), provides consistent evidence as the ground observations that the equatorial PB occurrence is dependent on and increases approximately linearly with PRE, and the l/s variations of PRE play an important role in the global l/s distribution of PB occurrence. The solstitial evening PRE and equatorial PB occurrence show similar longitudinal variations: During June solstice, two peaks appear in the African and Pacific longitude sectors, and two minimums are observed in the Indian and American regions; During December solstice, the situation is approximately opposite. The equinoctial longitudinal effects are comparably small. It is concluded that the large-scale l/s variations of equatorial PB occurrence can be closely related to the l/s variations of PRE.