Prereversal Vertical Drift Research Articles

Role of Bottom‐Side Density Gradient in the Development of Equatorial Plasma Bubble/Spread F Irregularities: Solar Minimum and Maximum Conditions

AbstractFrom the analysis of Digisonde data over Brazilian equatorial and low‐latitude sites, we investigate the relative importance of the different parameters driving the generation of rising bubble‐type and bottom‐type spread F (SF) irregularities. Data for the complete month of October 2001, a solar maximum epoch (F10.7 = 210), and that of October 2008, an extended solar minimum period (F10.7 = 70), are analyzed to examine the SF intensity and occurrence rate as a function of the evening prereversal vertical drift velocity and the corresponding F layer heights and the bottom‐side density gradient. While the SF at the equatorial site is indicative of both the bottom‐side irregularities and rising bubbles, the SF at the low latitude represents exclusively the latter. Comparison of the results, from the two epochs, reveals a large decrease in the intensity and occurrence rate of plasma bubbles, with a decrease in solar flux. But a notable increase in these characteristics is observed in the case of bottom‐side SF. It is found that a larger (steeper) density gradient of the F layer bottom side that exists in the low solar flux condition is responsible for an enhanced Raleigh‐Taylor instability growth, counterbalancing a reduction in this rate that may arise from a smaller prereversal vertical drift and lower layer height that also characterize the low solar flux condition. Thus, the role of the bottom‐side density gradient in the ESF instability growth has been identified for the first time in terms of its ability to explain the contrasting irregularity features as observed during solar flux maximum and minimum years.

Further complexities on the pre-reversal vertical drift modeling over the Brazilian region: A comparison between long-term observations and model results

The vertical component of the plasma drift, especially the evening-time pre-reversal drift, constitutes an important aspect of the nighttime electrodynamics of the equatorial ionosphere. Over the years, several studies using measurements and models have been performed to understand the characteristics of this process and its implications for the development of the instabilities leading to the plasma bubble formation and ionospheric scintillation. However, the Brazilian region presents some unique features that bring some difficulties for the vertical drift prognosis, which is required for the scintillation forecasting. These features are mainly related to the geomagnetic field lines topology that presents strong differences when compared to those of other equatorial longitudes. In this work, some of the difficulties for the pre-reversal vertical drift modeling and estimation are discussed; also, a dataset containing long-term observation (2001–2009) is compared with a widely used empirical model. The results show an intrinsic trend of underestimation by the model, which seems to be independent of latitude and seasonality thus suggesting an additional contribution arising from sources other than solely the geomagnetic topology. Also, the results indicate that the deviation can vary in the range of 0–40 m/s and the percentage error enhances with increasing values of pre-reversal vertical drift peak and reduces with increasing F10.7 values, thereby, indicating a clear possibility of meridional winds contribution which is not included in the empirical model used and may account for these differences.

A Numerical Study on the 3‐D Approach of the Equatorial Plasma Bubble Seeded by the Prereversal Vertical Drift

AbstractNumerical studies concerning equatorial plasma bubble (EPB) seeding source mechanisms have been performed for the last decades. Density perturbations, gravity waves, shear instabilities among others were used in the literature. In all the cases the prereversal vertical drift has a central role, being a necessary condition in addition to all these seeding mechanisms. Notwithstanding, since the prereversal vertical drift presents an asymmetric distribution, this longitudinal variation was hypothesized to act as a seeding source mechanism. This mechanism is capable of providing both contributions: elevate the ionosphere and seed the collisional interchange instability. Two‐dimensional investigation was already done, showing the validity of this hypothesis in a bidimensional scenario. Nonetheless, the inclusion of the parallel dynamics could alter the outcome of such analysis, once the component of the conductivity along the geomagnetic field direction causes a load effect reducing the growth rate and shifting its altitude of maximization, thus a 3‐D model was developed, named MATPLAB_3D (Mathematical Plasma Bubble Model 3D), and the hypothesis was tested. A progressive approach was performed starting with an oversimplified configuration and concluding with a more realistic profile of prereversal vertical drift obtained using the SAMI2 model. The numerical simulations revealed the existence of a quasi‐gaussian seeding source. Also, in case of a prereversal vertical drift varying within 20–60 m/s, the seeded instability evolves into an EPB structure with a longitudinal extension of ~2° within ~22 min. This EPB maps to low latitude regions and presents secondary structures in its west side. This result further suggests that even though a decrease in the EPB growth due to the inclusion of the parallel conductivity occurred, the hypothesis remains valid.

A numerical study on the three-dimensional approach of the equatorial plasma bubble seeded by the pre-reversal vertical drift

A numerical study on the three-dimensional approach of the equatorial plasma bubble seeded by the pre-reversal vertical drift

Impact of disturbance electric fields in the evening on prereversal vertical drift and spread F developments in the equatorial ionosphere

Abstract. Equatorial plasma bubble/spread F irregularity occurrence can present large variability depending upon the intensity of the evening prereversal enhancement in the zonal electric field (PRE), that is, the F region vertical plasma drift, which basically drives the post-sunset irregularity development. Forcing from magnetospheric disturbances is an important source of modification and variability in the PRE vertical drift and of the associated bubble development. Although the roles of magnetospheric disturbance time penetration electric fields in the bubble irregularity development have been studied in the literature, many details regarding the nature of the interaction between the penetration electric fields and the PRE vertical drift still lack our understanding. In this paper we have analyzed data on F layer heights and vertical drifts obtained from digisondes operated in Brazil to investigate the connection between magnetic disturbances occurring during and preceding sunset and the consequent variabilities in the PRE vertical drift and associated equatorial spread F (ESF) development. The impact of the prompt penetration under-shielding eastward electric field and that of the over-shielding, and disturbance dynamo, westward electric field on the evolution of the evening PRE vertical drift and thereby on the ESF development are briefly examined. Keywords. Ionosphere (ionospheric irregularities)

Climatology of the scintillation onset over southern Brazil

Abstract. This work presents an analysis of the climatology of the onset time of ionospheric scintillations at low latitude over the southern Brazilian territory near the peak of the equatorial ionization anomaly (EIA). Data from L1 frequency GPS receiver located in Cachoeira Paulista (22.4∘ S, 45.0∘ W; dip latitude 16.9∘ S), from September 1998 to November 2014, covering a period between solar cycles 23 and 24, were used in the present analysis of the scintillation onset time. The results show that the start time of the ionospheric scintillation follows a pattern, starting about 40 min earlier, in the months of November and December, when compared to January and February. The analyses presented here show that such temporal behavior seems to be associated with the ionospheric prereversal vertical drift (PRVD) magnitude and time. The influence of solar activity in the percentage of GPS links affected is also addressed together with the respective ionospheric prereversal vertical drift behavior. Based on this climatological study a set of empirical equations is proposed to be used for a GNSS alert about the scintillation prediction. The identification of this kind of pattern may support GNSS applications for aviation and oil extraction maritime stations positioning. Keywords. Ionosphere (ionospheric irregularities; modeling and forecasting) – radio science (space and satellite communication)

Spread F modeling over Brazil

Based on a numerical model developed by Carrasco et al. (2014), the dynamics and morphology of ionospheric plasma bubbles for both magnetic hemispheres over Brazil are examined. Observational results show that the post sunset maximum velocity of the pre-reversal vertical drift velocity, VP, has different values at the conjugate points. This information on the asymmetry of the vertical drift or zonal electric field is used in the simulation of bubbles over Brazilian sector. The simulations were conducted considering two different cases: (a) simulation of trans-equatorial bubble neglecting the meridional winds, and (b) considering the effect of meridional winds. In case (a) the bubble development is symmetric in relation to the magnetic equator, while in case (b) we raise the conjecture of a possible asymmetry. The simulation shows that the trans-equatorial bubbles may develop between ±30° of latitude and present internal structures of different sizes and shapes. In addition, three spatial perspectives of the bubbles are displayed, where each perspective reveals interesting aspects of the bubble development.

Wave structure and polarization electric field development in the bottomside F layer leading to postsunset equatorial spread F

AbstractIn this paper we present the results of a study on the characteristics of large‐scale wave structure in the equatorial ionospheric F region that serve as precursor to postsunset development of the spread F/plasma bubble irregularities. The study is based on analysis of Digisonde data from three equatorial sites in Brazil (Fortaleza, Sao Luis, and Cachimbo) for a period of about 2 months at a medium solar activity phase. Small‐amplitude oscillations in the F layer heights, extracted at a number of plasma frequencies, present characteristics as them being generated from upward propagating gravity waves. They represent wave structures in polarization electric field having zonal scale of a few hundred kilometers. Their amplitudes in the afternoon hours undergo amplification toward evening, leading to postsunset development of equatorial spread F/plasma bubble irregularities, on a statistical basis. On the days of their larger amplitudes they appear to occur in phase coherence on all days, and correspondingly, the evening prereversal vertical drift velocities are larger than on days of the smaller amplitudes of the wave structure that appear at random phase on the different days. The sustenance of these precursor wave structures is supported by the relatively large ratio (approaching unity) of the F region‐to‐total field line‐integrated Pedersen conductivities as calculated using the Sheffield University Plasmasphere‐Ionosphere Model simulation of the low‐latitude ionosphere. The significant amplification in the wave structure toward sunset and the “phase coherent” nature of their occurrences on different days are explained tentatively on the basis of the spatial resonance mechanism.

Fast and ultrafast Kelvin wave modulations of the equatorial evening F region vertical drift and spread F development

In this paper, we investigate the role of eastward and upward propagating fast (FK) and ultrafast Kelvin (UFK) waves in the day-to-day variability of equatorial evening prereversal vertical drift and post sunset generation of spread F/plasma bubble irregularities. Meteor wind data from Cariri and Cachoeira Paulista (Brazil) and medium frequency (MF) radar wind data from Tirunelveli (India) are analyzed together with Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature in the 40- to 100-km region to characterize the zonal and vertical propagations of these waves. Also analyzed are the F region evening vertical drift and spread F (ESF) development features as diagnosed by Digisonde (Lowell Digisonde International, LLC, Lowell, MA, USA) operated at Fortaleza and Sao Luis in Brazil. The SABER temperature data permitted determination of the upward propagation characteristics of the FK (E1) waves with propagation speed in the range of 4 km/day. The radar mesosphere and lower thermosphere (MLT) winds in the widely separated longitude sectors have yielded the eastward phase velocity of both the FK and UFK waves. The vertical propagation of these waves cause strong oscillation in the F region evening prereversal vertical drift, observed for the first time at both FK and UFK periodicities. A delay of a few (approximately 10) days is observed in the F region vertical drift perturbation with respect to the corresponding FK/UFK zonal wind oscillations, or temperature oscillations in the MLT region, which has permitted a direct identification of the sunset electrodynamic coupling process as being responsible for the generation of the FK/UFK-induced vertical drift oscillation. The vertical drift oscillations are found to cause significant modulation in the spread F/plasma bubble irregularity development. The overall results highlight the role of FK/UFK waves in the day-to-day variability of the ESF in its occurrence season.

Equatorial evening prereversal vertical drift dependence on solar EUV flux and F10.7 index during quiet and disturbed periods over Brazil

A study of the dependence of the equatorial evening F region vertical drift velocity on solar EUV flux and F10.7 index is presented here, based on the vertical drift data obtained from Digisondes operated in São Luis (44.2° W, 2.33° S, dip angle: −2.7°) and Fortaleza (38.45° W, 3.9° S, dip angle: −11.5°) in Brazil. Previous studies on the vertical drift dependence on solar flux have addressed only the dependence on F10.7 index. The data analyzed here are from the months of October, November, and December of the years from 2001 to 2009, and the analysis was done for magnetically quiet and disturbed conditions. The results show that while the drift velocity peak is strongly dependent on both solar emission fluxes, the degrees of such dependence are higher for the EUV flux than for the F10.7 index in a consistent way as judged from its identical behavior at both São Luis and Fortaleza. The study also reveals different degrees of the vertical drift dependence on solar flux for magnetically quiet and disturbed conditions, the nature of which is investigated using an example of a storm time case study.

Equatorial ionosphere responses to two magnetic storms of moderate intensity from conjugate point observations in Brazil

Equatorial ionospheric responses during two magnetic storms of moderate intensity are investigated, for the first time, by conjugate point observations in Brazil. The study focuses on storm‐induced changes in the evening prereversal vertical drift, thermospheric trans‐equatorial winds, spread F/plasma bubble irregularity development, electron density/plasma frequency heights, the EIA strength, and zonal plasma drifts. It is based on data obtained from five Digisondes operated in Brazil, three of them being part of a conjugate point equatorial experiment (COPEX) involving a dip equatorial and two magnetic conjugate sites at ±12°. The other two were operated at the equatorial ionization anomaly (EIA) trough and crest locations at nearby magnetic meridians. The results bring out, and clarify, many outstanding aspects of the strong influence of storm time electric fields on the equatorial ionosphere at different phases of the two long lasting storm sequences. During both storms prompt penetration electric fields dominated the ionospheric response features as compared to the disturbance wind dynamo effects that were not very conspicuous. An under‐shielding (over‐shielding) electric field occurring in the evening hours causes enhancement (suppression) of the prereversal vertical drift and post sunset spread F/plasma bubble generation. The same electric fields cause post sunset EIA enhancement and suppression, respectively. Post sunset (post midnight) spread F can develop from under‐shielding (over‐shielding) electric fields, while it can be disrupted by over‐shielding (under‐shielding) electric field. Trans‐equatorial winds are found to be ineffective to stabilize the post sunset F region against the destabilizing effect of strong prereversal vertical drift. Storm time westward plasma drifts are found to be driven by prompt penetration eastward electric fields (through their effect of inducing vertical Hall electric fields), rather than by a disturbance westward thermospheric wind during these storms.

Solar flux effects on the equatorial evening vertical drift and meridional winds over Brazil: A comparison between observational data and the IRI model and the HWM representations

We present the results of a study of the equatorial evening F region prereversal vertical drift enhancement over Brazil as a function of solar flux (F10.7). The vertical drift is measured from true heights obtained from digisondes operated at an equatorial site, Sao Luis, and at an off equatorial site, Fortaleza. The different magnetic inclinations of the two sites cause different degree of dependence of the vertical drift on magnetic meridional winds permitting an evaluation of the solar flux control on both the evening vertical drift (zonal electric field) and meridional winds. The analysis period covers the year 2001 and 2004 during which the monthly mean F10.7 varied from 245 to 80. The solar flux dependence of the vertical drift of the evening F layer as obtained from the data are compared with the representation of such dependence in the IRI model. While the IRI description of such dependence appears comparable to that is determined from observational data there is significant difference between the drift values in the two cases, the IRI representation being an under-estimation of the observed drift for all flux values. The results are discussed from the perspective of validating the IRI model representations of the vertical drift. The results of the study also shows that the HWM description of the evening thermospheric meridional wind in the Brazilian sector, shows agreement with observation for lower F10.7 values, while it significantly deviates from the observed/calculated meridional wind for higher solar flux values.

Equatorial evening prereversal vertical drift and spread F suppression by disturbance penetration electric fields

This paper presents new evidences on the responses of the evening sector equatorial ionosphere to prompt penetrating (PP) magnetospheric electric fields. In particular we show for the first time that the evening prereversal enhancement in the vertical drift/zonal electric field (PRE) and the consequent spread F (ESF) generation can be totally suppressed by penetration electric field of westward polarity arising from over‐shielding processes. In contrast the undershielding PP electric field of eastward polarity is found to enhance the PRE and ESF in agreement with recent findings. These results are evaluated using collisional interchange instability growth rate model calculations.

Gravity wave initiation of equatorial spread F/plasma bubble irregularities based on observational data from the SpreadFEx campaign

Abstract. The data from ground based experiments conducted during the 2005 SpreadFEx campaign in Brazil are used, with the help of theoretical model calculations, to investigate the precursor conditions, and especially, the role of gravity waves, in the instability initiation leading to equatorial spread F development. Data from a digisonde and a 30 MHz coherent back-scatter radar operated at an equatorial site, Sao Luis (dip angle: 2.7°) and from a digisonde operated at another equatorial site (dip angle: −11.5°) are analyzed during selected days representative of differing precursor conditions of the evening prereversal vertical drift, F layer bottom-side density gradients and density perturbations due to gravity waves. It is found that radar irregularity plumes indicative of topside bubbles, can be generated for precursor vertical drift velocities exceeding 30 m/s even when the precursor GW induced density oscillations are marginally detectable by the digisonde. For drift velocities ≤20 m/s the presence of precursor gravity waves of detectable intensity is found to be a necessary condition for spread F instability initiation. Theoretical model calculations show that the zonal polarization electric field in an instability development, even as judged from its linear growth phase, can be significantly enhanced under the action of perturbation winds from gravity waves. Comparison of the observational results with the theoretical model calculations provides evidence for gravity wave seeding of equatorial spread F.

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.

Dependence of equatorial bubbles and bottomside spread F on season, magnetic activity, and E × B drift velocity during solar maximum

Equatorial bubbles occur sporadically, so that the resulting scintillation that disrupts transionospheric RF propagation is unpredictable. Furthermore, this randomness prevents the systematic observation of the formation of bubbles necessary for making progress in their prediction. However, recent results indicate that a necessary condition for bubble formation is bottomside spread F (BSSF); that BSSF is a function of the F region dynamo electric field; and that this field is dependent on magnetic activity and on season. This work measures these conditions using an array of ionospheric sounders in conjunction with other spread F observations, all of which are ground‐based, in the western American sector, during solar maximum. Three spread F levels are observed: total BSSF and strong BSSF, both measured near the dip equator, and macroscopic bubbles measured at the equatorial anomaly. The three independently measured thresholds form a hierarchy in which macroscopic bubbles are a subset of strong BSSF, which is a subset of total BSSF. Each level decreases as a linear function of Kp averaged in the 6 hours preceding observation during March, April, September, and October, and during November–February; however, during May–August the levels are small and are independent of Kp. As a result, the probability of occurrence of each level, most importantly of macroscopic bubbles, is a quantitative function of Kp and season. The seasonal variation of Kp dependence indicates that the suppression of irregularities results from the decrease in the maximum prereversal vertical drift velocity of the equatorial F layer and therefore from the decrease in the eastward electric field.