Region Vertical Plasma Drifts Research Articles

Modeling of Planetary Wave Influences on the Pre‐reversal Enhancement of the Equatorial F Region Vertical Plasma Drift

AbstractTemporal and longitudinal variations of the pre‐reversal enhancement (PRE) in the equatorial F region vertical plasma drift are examined based on idealized simulations by the thermosphere‐ionosphere‐electrodynamics general circulation model performed under geomagnetically quiet (Kp = 1) and high solar‐flux (F10.7 = 200) conditions. The model takes into account forcing by large‐scale waves from the middle and lower atmosphere, which leads to day‐to‐day variations of PRE. Simulations are performed under different wave forcing in order to separate contributions of various types of waves. It is shown that the simulated day‐to‐day variability of the PRE intensity is predominantly due to forcing by waves with periods less than 2 days, that is, tides and their modulation. Planetary‐wave forcing (periods of 2–20 days) makes contributions to periodic oscillations in the PRE intensity. Especially, the westward‐propagating quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 is found to be an important source of ∼6‐day oscillations of PRE. Not only the Q6DW from below but also the Q6DW generated within the thermosphere, as well as the secondary waves due to the nonlinear interaction between the Q6DW and migrating tides, is at play. The zonal wavenumber 1 nature of the ∼6‐day oscillations could contribute to longitudinal differences in the appearance of equatorial spread F and plasma bubbles, which are strongly controlled by PRE.

Significant Electric Field Perturbations in Low Latitude Ionosphere due to the Passage of Two Consecutive ICMEs During 6–8 September 2017

AbstractThis investigation shows that the significant electric field disturbances in the dip‐equatorial ionosphere during the geomagnetic storm of 6–8 September 2017 are due to the passage of two consecutive interplanetary coronal mass ejections (ICMEs). During the passage of the first ICME sheath, a long duration (∼10 hr) prompt penetration (PP) event is operational in which 60‐min periodic component is found to be present in vertical drift as well as in equatorial electrojet, but the 45‐min periodicity, though present, is not significant in equatorial electrojet. On 8 September, the shock associated with the second ICME enhances theFregion vertical plasma drift to ∼150 m/s in the evening hours which is one of the highest vertical drift ever measured over Jicamarca. The same PP electric field causes unusually large enhancement of the equatorial electrojet strength to ∼135 nT in the early morning hours over the Philippine sector. The disturbance dynamo (DD) that follows the storm causes an upward vertical drift of ∼55 m/s during postmidnight hours over Jicamarca which is one of the highest observed. These unusually large electric field perturbations cause significant changes in theFregion plasma fountain. It is shown that these electric field perturbations cannot be accounted by PP/DD electric field associated with the geomagnetic storm only and significant contribution from substorm is conspicuous. Therefore, the present investigation highlights the need to evaluate the role of substorm in unusually large electric field perturbations over equatorial ionosphere.

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 Occurrence Rate and Amplitudes of Local Time Distinguished Equatorial Plasma Depletions Observed by Swarm Satellite

AbstractIn this study, we developed an autodetection technique for the equatorial plasma depletions (EPDs) and their occurrence and depletion amplitudes based on in situ electron density measurements gathered by Swarm A satellite. For the first time, comparisons are made among the detected EPDs and their amplitudes with the loss of Global Positioning System (GPS) signal of receivers onboard Swarm A, and the Swarm Level‐2 product, Ionospheric Bubble Index (IBI). It has been found that the highest rate of EPD occurrence takes place generally between 2200 and 0000 magnetic local time (MLT), in agreement with the IBI. However, the largest amplitudes of EPD are detected earlier at about 1900–2100 MLT. This coincides with the moment of higher background electron density and the largest occurrence of GPS signal loss. From a longitudinal perspective, the higher depletion amplitude is always witnessed in spatial bins with higher background electron density. At most longitudes, the occurrence rate of postmidnight EPDs is reduced compared to premidnight ones; while more postmidnight EPDs are observed at African longitudes. CHAMP observations confirm this point regardless of high or low solar activity condition. Further by comparing with previous studies and the plasma vertical drift velocity from ROCSAT‐1, we suggest that while the F region vertical plasma drift plays a key role in dominating the occurrence of EPDs during premidnight hours, the postmidnight EPDs are the combined results from the continuing of former EPDs and newborn EPDs, especially during June solstice. And these newborn EPDs during postmidnight hours seem to be less related to the plasma vertical drift.

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

On new detailed ionogram signatures of Equatorial Plasma Bubbles from non-equatorial station during low solar activity

Altitudinal dependence of evening equatorial F region vertical plasma drifts

AbstractWe use Jicamarca incoherent scatter radar measurements to study for the first time the altitudinal variations of late afternoon and early night equatorial F region vertical plasma drifts. We also present the initial vertical drift measurements over the altitudinal range from about 200 to 2000 km. These data show that the afternoon drifts decrease weakly with altitude. Near their evening prereversal enhancements, the vertical drifts generally increase with altitude below about the F layer peak, decrease with height near the F layer peak and above, and are nearly height independent in the (solar flux dependent) topside ionosphere. The transition altitudes from height‐decreasing to height‐independent evening upward drifts decrease with altitude from solar maximum to solar minimum. After their reversal to downward, the vertical drifts do not change much with height. The altitudinal dependence of the evening vertical drifts has large day‐to‐day variability and is closely related to the time dependence of the zonal drifts, as expected from the curl‐free electric field condition.

A theory of ionospheric response to upward‐propagating tides: Electrodynamic effects and tidal mixing effects

The atmospheric tide at ionospheric heights is composed of those locally generated and those propagated from below. The role of the latter in producing the variability of the daytime ionosphere is examined using the National Center for Atmospheric Research Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model. The impact of upward‐propagating tides is evaluated by running simulations with and without tidal forcing at the lower boundary (approximately 96 km), which imitates the effect of tides from below. When migrating diurnal and semidiurnal tides at the lower boundary is switched on, the intensity of E region currents and the upward velocity of the equatorial F region vertical plasma drift rapidly increase. The low‐latitude ionospheric total electron content (TEC) first increases, then gradually decreases to below the initial level. The initial increase in the low‐latitude TEC is caused by an enhanced equatorial plasma fountain while the subsequent decrease is due to changes in the neutral composition, which are characterized by a global‐scale reduction in the mass mixing ratio of atomic oxygen O1. The results of further numerical experiments indicate that the mean meridional circulation induced by dissipating tides in the lower thermosphere is mainly responsible for the O1 reduction; it acts like an additional turbulent eddy and produces a “mixing effect” that enhances net downward transport and loss of O1. It is stressed that both electrodynamic effects and mixing effects of upward‐propagating tides can be important in producing the variability of ionospheric plasma density. Since the two mechanisms act in different ways on different time scales, the response of the actual ionosphere to highly variable upward‐propagating tides is expected to be complex.

Abnormal evening vertical plasma drift and effects on ESF and EIA over Brazil‐South Atlantic sector during the 30 October 2003 superstorm

Equatorial F region vertical plasma drifts, spread F and anomaly responses, in the south American longitude sector during the superstorm of 30 October 2003, are analyzed using data from an array of instruments consisting of Digisondes, a VHF radar, GPS TEC and scintillation receivers in Brazil, and a Digisonde and a magnetometer in Jicamarca, Peru. Prompt penetrating eastward electric field of abnormally large intensity drove the F layer plasma up at a velocity ∼1200 ms−1 during post dusk hours in the eastern sector over Brazil. The equatorial anomaly was intensified and expanded poleward while the development of spread F/plasma bubble irregularities and GPS signal scintillations were weaker than their quiet time intensity. Significantly weaker F region response over Jicamarca presented a striking difference in the intensity of prompt penetration electric field between Peru and eastern longitudes of Brazil. The enhanced post dusk sector vertical drift over Brazil is attributed to electro‐dynamics effects arising energetic particle precipitation in the South Atlantic Magnetic Anomaly (SAMA). These extraordinary results and their longitudinal differences are presented and discussed in this paper.

Quiet time equatorial F region vertical plasma drift model derived from ROCSAT‐1 observations

We have used five years of measurements on board the ROCSAT‐1 satellite to develop a detailed quiet time global empirical model for equatorial F region vertical plasma drifts. This model describes the local time, seasonal and longitudinal dependence of the vertical drifts for an altitude of 600 km under moderate and high solar flux conditions. The model results are in excellent agreement with measurements from the Jicamarca radar and also from other ground‐based and in situ probes. We show that the longitudinal dependence of the daytime and nighttime vertical drifts is much stronger than reported earlier, especially during December and June solstice. The late night downward drift velocities are larger in the eastern than in the western hemisphere at all seasons, the morning and afternoon December solstice drifts have significantly different longitudinal dependence, and the daytime upward drifts have strong wave number‐four signatures during equinox and June solstice. The largest evening upward drifts occur during equinox and December solstice near the American sector. The longitudinal variations of the evening prereversal velocity peaks during December and June solstice are anti‐correlated, which further indicates the importance of conductivity effects on the electrodynamics of the equatorial ionosphere.

Comparison between nighttime ionosonde, incoherent scatter radar, AE‐E satellite, and HF Doppler observations ofFregion vertical electrodynamic plasma drifts in the vicinity of the magnetic equator

NighttimeFregion vertical drifts were made using an ionosonde for the equatorial station Ibadan (7.4°N, 3.9°E, 6°S dip) from 1 year of data during 1957–1958 International Geophysical Year (IGY) that corresponds to a period of solar maximum for undisturbed condition. We compare the seasonal vertical drifts with measurements made by incoherent scatter radar, AE‐E satellite, and HF Doppler for equatorialFregion vertical drifts. We find a comparable variability pattern during periods of highFlayer heights during equinox and the December solstice, and the opposite behavior occurs during June solstice. The drifts are predominantly downward between 2000 and 0500 LT intervals. Ionosonde drifts are smaller in values by either a factor of two or three than other methods, except for consistent June solstice ionosonde and satellite magnitudes. The equinoctial average prereversal enhancements measured by the four techniques are roughly comparable (about 36 m/s) and occur at the same local time (1900 LT) for all the seasons. The evening reversal times are similar, apart from June solstice that exhibits large variations. The morning reversal times are also in accord except for the equinoctial Jicamarca drift. Our observations indicate that ionosonde drifts measurements are in better agreement with vertical drifts results at other equatorial stations.

Planetary wave oscillations in mesospheric winds, equatorial evening prereversal electric field and spread F

Analysis of the MLT region winds measured by a meteor radar and the evening F region vertical plasma drift (prereversal zonal electric field ‐PRE) measured by digisondes over low latitude sites in Brazil, provide evidence of planetary wave (PW) scale oscillations of episodic nature simultaneously at mesospheric and F region heights. ∼4‐day and 7‐day periods are found to dominate the event analyzed. The PW scale oscillations in the PRE produces strong modulation in the equatorial spread F (ESF) irregularity processes as diagnosed by the digisondes. Considerations on the PRE development mechanism involving the E layer integrated conductivity including the effect of metallic ions and tidal winds point to the source of the PRE oscillations to be PW modulation of E region tidal winds. The PW oscillations in PRE appear to be an important source of the day‐to‐day variability in the ESF.

Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

Abstract. Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp < 4-). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H/X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.Key words: Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)

Electric field fluctuations (25–35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2∞N, 77.5∞E, geomagnetic latitude 0.8∞N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp <4 ) ). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplan- etary magnetic field (IMF), in geomagnetic H/X com- ponents at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.

Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F

We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable. The evolution of the unstable layer is controlled by the history of the vertical drift velocity. When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present. The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux. The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent. These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset. The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour. These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux.

Storm time dependence of equatorial disturbance dynamo zonal electric fields

We use Jicamarca radar observations of F region vertical plasma drifts and auroral electrojet indices during 1968–1988 to study the characteristics and temporal evolution of equatorial disturbance dynamo zonal electric fields. These electric fields result from the dynamo action of storm time winds and/or thermospheric composition changes driven by enhanced energy deposition into the high‐latitude ionosphere during geomagnetically active conditions. The equatorial vertical drift perturbations last for periods of up to 30 hours after large increases in the high‐latitude currents. On the average, this process can be described by two basic components with time delays of about 1–12 hours and 22–28 hours between the high‐latitude current enhancements and the equatorial velocity perturbations. Our data indicate strong coupling between dynamo processes with different timescales. The short‐term disturbance dynamo drives upward equatorial drifts (eastward electric fields) at night with largest amplitudes near sunrise and small downward drifts during the day. These perturbation drifts are in good agreement with results from the Blanc‐Richmond disturbance dynamo theory. The dynamo process with time delays of about a day drives upward drift velocities at night with largest values near midnight and downward drifts in the sunrise‐noon sector. In this case, the amplitudes of the disturbance drifts maximize during geomagnetically quiet times preceded by strongly disturbed conditions. We also present results of a new equatorial storm time dependent empirical model which illustrate the characteristics of the vertical disturbance dynamo drifts.

Empirical models of storm time equatorial zonal electric fields

Ionospheric plasma drifts often show highly complex and variable signatures during geomagnetically active periods due to the effects of different disturbance processes. We describe initially a methodology for the study of storm time dependent ionospheric electric fields. We present empirical models of equatorial disturbance zonal electric fields obtained using extensive F region vertical plasma drift measurements from the Jicamarca Observatory and auroral electrojet indices. These models determine the plasma drift perturbations due to the combined effects of short‐lived prompt penetration and longer lasting disturbance dynamo electric fields. We show that the prompt penetration drifts obtained from a high time resolution empirical model are in excellent agreement with results from the Rice Convection Model for comparable changes in the polar cap potential drop. We also present several case studies comparing observations with results obtained by adding model disturbance drifts and season and solar cycle dependent average quiet time drift patterns. When the disturbance drifts are largely due to changes in magnetospheric convection and to disturbance dynamo effects, the measured and modeled drift velocities are generally in good agreement. However, our results indicate that the equatorial disturbance electric field pattern can be strongly affected by variations in the shielding efficiency, and in the high‐latitude potential and energy deposition patterns which are not accounted for in the model. These case studies and earlier results also suggest the possible importance of additional sources of plasmaspheric disturbance electric fields.

Equatorial ionospheric vertical plasma drift model over the Brazilian region

Comparison between equatorial inospheric F region vertical plasma drift from satellite measurements [Fejer et al., 1995], for the Brazilian longitude sector, and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods. Using ionosonde measurements around sunset and satellite measurements at other local times, we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the Brazilian longitudes, where the magnetic declination is around −20°. The so derived drift model, here called IDM (ionosonde drift model), is used as an input to the Sheffield University plasmasphere‐ionosphere model (SUPIM). It is shown that the F layer heights given by SUPIM with IDM are in good agreement with ionosonde measurements over the Brazilian longitudes and that IDM better simulates the F layer heights than the averaged drifts given by the satellite drift model [Fejer et al., 1995].

Longitudinal dependence of equatorial F region vertical plasma drifts in the dusk sector

The effect of solar activity on the postsunset peak of F region vertical plasma drift at Kodaikanal (77°28′E, dip 3°N) is determined as a function of season and for two levels of magnetic activity using extensive HF phase path observations and compared with that at Jicamarca (75° W, dip 2°N) reported in the literature. The postsunset peak vertical velocity Vzp at Kodaikanal increases linearly with 10.7‐cm solar flux (F10.7) irrespective of season and level of magnetic activity. The sensitivity of Vzp to solar flux changes is more at Jicamarca than at Kodaikanal during equinoxes and December solstice. This feature is virtually absent in June solstice. The seasonally averaged values of dusktime vertical velocity at Kodaikanal for moderate to high solar flux conditions and quiet magnetic conditions(Kp ≤ 3) are in reasonable agreement with those reported very recently for the Indian sector (80°E) from AE‐E satellite data [Fejer et al., 1995]. In June solstice, the evening upward vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160°–200°E), while the postreversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230°–310°E) where there they are higher. The present study supports the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June solstice.

Global equatorial ionospheric vertical plasma drifts measured by the AE‐E satellite

Ion drift meter observations from the Atmosphere Explorer E satellite during the period of January 1977 to December 1979 are used to study the dependence of equatorial (dip latitudes ≤ 7.5°) F region vertical plasma drifts (east‐west electric fields) on solar activity, season, and longitude. The satellite‐observed ion drifts show large day‐to‐day and seasonal variations. Solar cycle effects are most pronounced near the dusk sector with a large increase of the prereversal velocity enhancement from solar minimum to maximum. The diurnal, seasonal, and solar cycle dependence of the longitudinally averaged drifts are consistent with results from the Jicamarca radar except near the June solstice when the AE‐E nighttime downward velocities are significantly smaller than those observed by the radar. Pronounced presunrise downward drift enhancements are often observed over a large longitudinal range but not in the Peruvian equatorial region. The satellite data indicate that longitudinal variations are largest near the June solstice, particularly near dawn and dusk but are virtually absent during equinox. The longitudinal dependence of the AE‐E vertical drifts is consistent with results from ionosonde data. These measurements were also used to develop a description of equatorial F region vertical drifts in four longitudinal sectors.

Time dependent response of equatorial ionospheric electric fields to magnetospheric disturbances

We use extensive radar measurements of F region vertical plasma drifts and auroral electrojet indices to determine the storm time dependence of equatorial zonal electric fields. These disturbance drifts result from the prompt penetration of high latitude electric fields and from the dynamo action of storm time winds which produce largest perturbations a few hours after the onset of magnetic activity. The signatures of the equatorial disturbance electric fields change significantly depending on the relative contributions of these two components. The prompt electric field responses, with lifetimes of about one hour, are in excellent agreement with results from global convection models. The electric fields generated by storm time winds have longer lifetimes, amplitudes proportional to the energy input into the high latitude ionosphere, and a daily variation which follows closely the disturbance dynamo pattern of Blanc and Richmond [1980]. The storm wind driven electric fields are responsible for the larger amplitudes and longer lifetimes of the drift perturbations following sudden decreases in convection compared to those associated with sudden convection enhancements.