Vertical Drift Velocity Research Articles

On monthly/seasonal/longitudinal variations of equatorial irregularity occurrences and their relationship with the postsunset vertical drift velocities

Monthly variation of global equatorial density irregularity distribution has been obtained with the data taken by ROCSAT‐1 at the 600 km topside ionosphere from March 1999 to June 2004 during high to moderate solar activity years. This global distribution of monthly irregularity occurrence rate not only provides the best spatial/temporal distribution in existence so far but also serves to fill the gap of irregularity distribution missing over some eastern Pacific region where no ground observation is available. The 5 1/2‐year result of the monthly occurrence pattern indicates a smooth variation across the longitudes contrary to some beliefs that a drastic change in irregularity occurrence pattern has occurred across the eastern Pacific longitudes. Excellent agreement is noted for the current results with Aarons' (1993) conjectured sketch of the global scintillation distribution. Furthermore, the seasonal/longitudinal (s/l) variations of quiet time postsunset vertical drift velocities are found to track closely with the s/l variations of irregularity occurrences. Linear regression analysis between the vertical drift velocity and the irregularity occurrence rate indicates that the vertical drift velocities at three different longitude zones of magnetic declination have good correlations with the irregularity occurrences for all seasons. This implies that the averaged postsunset vertical drift velocity is a good indicator for the occurrences of equatorial density irregularities in a longitude region of similar magnetic declination. The smooth variation of monthly/seasonal/longitudinal distributions of the vertical drift velocities that results in the smooth variation of topside density irregularity occurrences implies that the global occurrence pattern of either distribution is controlled by the smooth global variation of the postsunset ionospheric condition, related to the magnetic declination effect and the seasonal variation of the ionospheric density level at the dip equator located with respect to the geographic equator. There seems little need to include the global distribution of instability perturbation seeds from atmospheric disturbances to complete the global distribution of topside irregularity occurrences.

Modeling the relationship between E × B vertical drift and the time rate of change of hmF2 (ΔhmF2/Δt) over the magnetic equator

A middle and low latitude ionospheric model is used to model the relationship between E × B vertical drift velocity and time rate of change of hmF2 (ΔhmF2/Δt) over the magnetic equator. F107 (10.7 cm solar radiation flux) is chosen to be equal to 100, 150, and 200 to represent low, middle, and high solar activities, respectively, and Ap is equal to 1 to represent quiet geomagnetic activity. Our simulations show that hmF2 derived vertical drifts over the magnetic equator are in good agreement with the E × B vertical drift imposed in the model during 0600–0730 and 1700–2100 LT. This is consistent with previous comparisons between vertical drift velocities derived from ionosonde, Jicamarca incoherent scatter radar, and AE‐E observations well. However, the amplitudes of hmF2 derived drift are smaller than the given drift velocities during other local times. During disturbed conditions, the variations of hmF2 can be used to determine the occurrence of intense electric field disturbance.

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations of the Eastern Hemisphere. It is found that after about 1980, three types of behaviors of this ratio with time are observed: an increase with time (a positive trend), a decrease with time (a negative trend) and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above-indicated trend is caused by long-term variations in the zonal component V ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are not yet known.

Effects of geomagnetic storm on GPS ionospheric scintillations at Sanya

The effects of geomagnetic storm on GPS ionospheric scintillations are studied here using GPS scintillation data recorded at Sanya (18.31N, 109.51E; geomagnetic: 7.61N, 180.81E), the southmost station in the Chinese longitude region. GPS scintillation/TEC and DMSP data are utilized to show the development of irregularities during the period year 2005 (solar minimum). Statistical analysis of K planetary index (Kp) and amplitude scintillation index (S4) indicates that most storms of the year did not trigger the scintillation occurrence at Sanya. However, cases of scintillation occurring during moderate and strong storm (Dsto� 100) periods show clearly that the development of irregularities producing scintillations can be triggered by geomagnetic storms during the low scintillation occurrence season. The effects (trigger or not trigger/inhibit) depend on the maximum dDst/dt determined local time sector, and can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance electric fields. For station Sanya, the maximum dDst/dt determined local time is near the noon (or post-midnight) sector for most storms of the year 2005, which inhibited (or did not trigger) the post-sunset (or post-midnight) scintillation occurrence and then led to the phenomena that the statistical results presented. r 2008 Elsevier Ltd. All rights reserved.

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F 2 layer

The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations in the Eastern Hemisphere. It is found that three types of time variations in this ratio are observed after 1980: an increase with time (a positive trend), a decrease with time (a negative trend), and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above trend is caused by long-term variations in the zonal component V ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are still unknown; however, it is quite probable that they are related to anthropogenic changes in the atmosphere.

Effect of vertical plasma transport on ionospheric F2-region parameters at equatorial latitude

The variability of the F2-layer even during magnetically quiet times are fairly complex owing to the effects of plasma transport. The vertical E × B drift velocities (estimated from simplified electron density continuity equation) were used to investigate the seasonal effects of the vertical ion drifts on the bottomside daytime ionospheric parameters over an equatorial latitude in West Africa, Ibadan, Nigeria (Geographic: 7.4°N, 3.9°E, dip angle: 6°S) using 1 year of ionsonde data during International Geophysical Year (IGY) of 1958, that correspond to a period of high solar activity for quiet conditions. The variation patterns between the changes of the vertical ion drifts and the ionospheric F2-layer parameters, especially; f oF 2 and h mF 2 are seen remarkable. On the other hand, we observed strong anti-correlation between vertical drift velocities and h′F in all the seasons. We found no clear trend between N mF 2 and h mF 2 variations. The yearly average value of upward daytime drift at 300 km altitude was a little less than the generally reported magnitude of 20 ms −1 for equatorial F-region in published literature, and the largest upward velocity was roughly 32 ms −1. Our results indicate that vertical plasma drifts; ionospheric F2-layer peak height, and the critical frequency of F2-layer appear to be somewhat interconnected.

Nighttime thermospheric meridional neutral winds inferred from ionospheric h′ F and hpF2 data

Nighttime thermospheric meridional winds aligned to the magnetic meridian have been inferred using h′ F and hpF2 ionosonde data taken from two equatorial stations, Manaus (2.9°S, 60.0°W, dip latitude 6.0°N) and Palmas (10.17°S, 48.2°W, dip latitude 6.2°S), and one low-latitude station, Sao Jose dos Campos (23.21°S, 45.86°W, dip latitude 17.26°S), during geomagnetic quiet days of August and September, 2002. Using an extension of the ionospheric servo model and a simple formulation of the diffusive vertical drift velocity, the magnetic meridional component of the thermospheric neutral winds is inferred, respectively, at the peak ( hpF2) and at the base ( h′ F) heights of the F region over Sao Jose dos Campos. An approach has been included in the models to derive the effects of the electrodynamic drift over Sao Jose dos Campos from the time derivative of hpF2 and h′ F observed at the equatorial stations. The magnetic meridional winds inferred from the two methods, for the months of August and September, are compared with winds calculated using the HWM-90 model and with measurements from Fabry–Perot technique. The results show varying agreements and disagreements. Meridional winds calculated from hpF2 ionospheric data (servo model) may produce errors of about 59 m/s, whereas the method calculated from the F-region base height ( h′ F) ionospheric data gives errors of about 69 m/s during the occurrence of equatorial spread-F.

Modeling storm-time electrodynamics of the low-latitude ionosphere–thermosphere system: Can long lasting disturbance electric fields be accounted for?

Storm-time ionospheric disturbance electric fields are studied for two large geomagnetic storms, March 31, 2001 and April 17–18, 2002, by comparing low-latitude observations of ionospheric plasma drifts with results from numerical simulations based on a combination of first-principles models. The simulation machinery combines the Rice convection model (RCM), used to calculate inner magnetospheric electric fields, and the coupled thermosphere ionosphere plasmasphere electrodynamics (CTIPe) model, driven, in part, by RCM-computed electric fields. Comparison of model results with measured or estimated low-latitude vertical drift velocities (zonal electric fields) shows that the coupled model is capable of reproducing measurements under a variety of conditions. In particular, our model results suggest, from theoretical grounds, a possibility of long-lasting penetration of magnetospheric electric fields to low latitudes during prolonged periods of enhanced convection associated with southward-directed interplanetary magnetic field, although the model probably overestimates the magnitude and duration of such penetration during extremely disturbed conditions. During periods of moderate disturbance, we found surprisingly good overall agreement between model predictions and data, with penetration electric fields accounting for early main phase changes and oscillations in low-latitude vertical drift, while the disturbance dynamo mechanism becomes increasingly important later in the modeled events. Discrepancies between the model results and the observations indicate some of the difficulties in validating these combined numerical models, and the limitations of the available experimental data.

On the post‐sunset rise of the equatorial F layer and superposed upwellings and bubbles

Usual practice has been to describe the post‐sunset rise (PSSR) of the equatorial F layer with the vertical drift velocity measured overhead, as a function of local time, by a ground‐based sensor. However, with the finding that large‐scale wave structure (LSWS) develops with its own eastward polarization electric field, during a PSSR, the measured upward velocity must now be thought of as containing two components, one associated with PSSR, and the other with LSWS. This distinction may hold the key to understanding the lack of correlation, on a day‐to‐day basis, between measured vertical drift and ensuing plasma structure in the form of plasma bubbles. Measurements of backscatter from the bottomside of the F layer, using a three‐beam radar at Pohnpei, are presented in this paper to further validate this interpretation.

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.

Plasma bubble phenomenon in the topside ionosphere

There are the indications that plasma bubbles/flux tube aligned plasma density depletions, produced by Rayleigh–Taylor instability at the bottomside of ionosphere, could rise up to the topside ionosphere and plasmasphere. Maruyama and Matuura [Maruyama, T., Matuura, N. Longitudinal variability of annual changes in activity of equatorial spread-F and plasma bubbles. J. Geophys. Res. 89(A12), 10903–10912, 1984.], using ISS-b satellite data for the high solar activity period, 1978–1979, have seen the plasma bubbles over equator at 1100 km altitudes in 46 cases in 1700 passes. That is ∼3% only. However, there is distinctly another picture in He + density depletions (subtroughs) according to the ISS-b data for the same period. He + density subtroughs were observed in the topside ionosphere over equatorial and low-latitudinal regions ( L ∼ 1.3–3) in 11% of the cases [Karpachev, A.T., Sidorova, L.N. Occurrence probability of the light ion trough and subtrough in He + density on season and local time. Adv. Space Res. 29, 999–108, 2002; Sidorova, L.N., He + density topside modeling based on ISS-b satellite data. Adv. Space Res. 33, 850–854, 2004.]. We have carried out a statistical study of the He + density subtrough characteristics. The subtrough depth (depletion value) as function of local time (evening–night hours) was compared with the vertical plasma drift velocity variations, obtained for the same periods from the AE-E satellite and IS radar (Jicamarca) data. Striking similarity in development dynamics is revealed for the different seasons. It is noted also that the He + density subtroughs are mostly observed in the evening–night sector (18–05 LT) from October till May, which is very similar to the peculiarities of the equatorial spread-F (ESF), usually associated with plasma bubbles. The monthly mean He + density subtrough occurrence probability, plotted in local time versus month, was compared with the similar plots for ESF occurrence probability derived by Abdu et al. [Abdu, M.A., Sobral, J.H.A., Batista, I.S. Equatorial spread-F statistics in the american longitudes: some problems relevant to ESF description in the IRI scheme. Adv. Space Res. 25, 113–124, 2000.] from ground-based ionograms obtained over Brazilian region for the same years. The comparison shows good enough correlation ( R ∼ 0.67). It is concluded that: (a) He + density subtroughs like ESF are controlled by pre-reversal enhancement electric field (vertical drift); (b) He + density subtroughs and ESF/bubble irregularities may be considered as phenomena of the same plasma bubble origin; (c) it seems, plasma bubbles, reaching the topside ionosphere altitudes, are most easily observable in He + density as depletions.

F-region vertical drift measurements at Ebro, Spain

Eighteen months values of vertical drift velocity measured with a Digisonde 256 at the mid-latitude station of Ebro Observatory (40.5°N, 0.5°E) have been analyzed in order to establish the typical daily variations. The daily pattern of the vertical drift velocity can easily be represented by the main daily harmonics. The summer and winter patterns are quite different. The diurnal component is most prominent for each month, with a practically constant phase and larger equinoctial amplitudes. The semidiurnal component displays the same seasonal amplitude variation as the diurnal one, but its phase is sun-synchronous. The terdiurnal component displays a constant phase and it plays a role during the summer half year, with amplitudes larger than those of the semidiurnal component. The variability is larger at nighttime than at daytime, and in general winter shows a larger variability. The usefulness of monitoring the vertical F-region drift motions with Digisonde drift measurements is discussed.

A screw-like meteoric train photographed by double-station observations

The double-station observation of a meteor is a kind of method for measuring the physical parameters of the meteor effectively. Four special groups of photographs about meteoric trains taken at two stations during Leonids 2001 have been collected. One representative group of them has been measured and analyzed in detail. The results reveal that this train has a screw-like structure and looks to have many spoke beams. The train moves forward in the form of left-hand screw. The horizontal drift velocity northward is 29.6 m s −1 and the vertical drift velocity upward is 12.3 m s −1. In addition, the material of the meteoric train has an outward expanding velocity up to about 79 m s −1. The possible explanation is that the persistent meteoric train is disturbed by the high altitude wind.

Observations and modeling of post-midnight uplifts near the magnetic equator

Abstract. We report here on post-midnight uplifts near the magnetic equator. We present observational evidence from digital ionosondes in Brazil, a digisonde in Peru, and other measurements at the Jicamarca Radio Observatory that show that these uplifts occur fairly regularly in the post-midnight period, raising the ionosphere by tens of kilometers in the most mild events and by over a hundred kilometers in the most severe events. We show that in general the uplifts are not the result of a zonal electric field reversal, and demonstrate instead that the uplifts occur as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination and plasma flux. The decreasing westward electric field may be caused by a change in the wind system related to the midnight pressure bulge, which is associated with the midnight temperature maximum. In order to agree with observations from Jicamarca and Palmas, Brazil, it is shown that there must exist sufficient horizontal plasma flux associated with the pressure bulge. In addition, we show that the uplifts may be correlated with a secondary maximum in the spread-F occurrence rate in the post-midnight period. The uplifts are strongly seasonally dependent, presumably according to the seasonal dependence of the midnight pressure bulge, which leads to the necessary small westward field in the post-midnight period during certain seasons. We also discuss the enhancement of the uplifts associated with increased geomagnetic activity, which may be related to disturbance dynamo winds. Finally, we show that it is possible using simple numerical techniques to estimate the horizontal plasma flux and the vertical drift velocity from electron density measurements in the post-midnight period.

The role of post-sunset vertical drifts at the equator in predicting the onset of VHF scintillations during high and low sunspot activity years

Abstract. The day-to-day variability in the occurrence of ionospheric scintillations, which are of serious concern in the trans-ionospheric communications, makes their prediction still a challenging problem. This paper reports on a systematic study in quantitatively identifying the precursors responsible, such as pre-reversal E×B drift velocity, geo-magnetic activity index (Kp) and the Equatorial Ionization Anomaly (EIA) gradient, for the onset of VHF scintillations over a low-latitude station, Waltair (20° N dip), during high (2001) and low (2004) sunspot activity years. The percentage of occurrences of VHF scintillations over Waltair show a good correlation with the monthly mean post-sunset vertical drift velocities at the equator, during both the high and low sunspot activity years. During the days on which intense (>10 dB) scintillations occur, the ionization anomaly gradient (dN/dL), measured from ionosonde data of an equatorial (Trivandrum, 0.9° N dip) and an off-equatorial station (Waltair, 20° N dip) shows an enhancement in the gradient prior to the onset of the scintillations. However, this enhancement is not seen on days when the scintillations are weak (<10 dB) or absent. The day-to-day post sunset enhancement in the E×B drift is found to decrease with increasing Kp-index and this decrease is more prominent in the equinoxes, less in winter and insignificant in the summer months. On a day-to-day basis, it is found that the value of the upward drift velocity at the equator should be ≥30 m/s for the onset of strong scintillations over Waltair for magnetically quiet days with average Kp≤2 (6 h prior to the local sunset) during the high sunspot year, 2001. This threshold value of the upward drift reduces to 20 m/s with the decrease in the sunspot activity during 2004. Further, these conditions for the onset of intense scintillations is well defined in equinoxes, less in winter and least in the summer solstices.

A comparison of ionospheric vertical drift velocities measured by Digisonde and Incoherent Scatter Radar at the magnetic equator

Using data acquired with a Digisonde DPS-4 and an Incoherent Scatter Radar both from Jicamarca Radio Observatory, Peru, comparisons of ionospheric vertical drift velocities derived from these two instruments are carried out in this work. They show good agreement during sunset and evening hours (around sunset, up to approximately 22 LT), and also, during the post-midnight (between 02 and 03 LT) and sunrise hours (until 08 LT), when the F-layer bottom side height is around and above 300 km, as well as during events of magnetically disturbed periods when the ionospheric plasma drift may be influenced by the perturbation electric fields arising from (1) direct penetration from high to low magnetic latitudes, and (2) disturbance winds dynamo action driven by Joule heating due to increased energy and momentum deposition into the high-latitude thermosphere–ionosphere system. The results of this study lend confidence to using Digisonde drift data to study phenomena related to night-time magnetically disturbed periods.

Concurrent study of bottomside spread F and plasma bubble events in the equatorial ionosphere during solar maximum using digisonde and ROCSAT-1

Abstract. Data from the Jicamarca digisonde and the ROCSAT-1 satellite are employed to study the equatorial ionosphere on the west side of South America during April 1999-March 2000 for the concurrent bottomside spread F (BSSF) and plasma bubble events. This study, using digisonde and ROCSAT-1 concurrently, is the first attempt to investigate the equatorial spread F. Results show that BSSF and plasma bubble observations appear frequently respectively in the summer (January, February, November, and December) and in the equinoctial (March, April, September, and October) months, respectively, but are both rarely observed in the winter (May-August) months. The upward drift velocity during the concurrent BSSF and bubble observations has been determined to study the driving mechanism. This analysis shows that large vertical drift velocities favor BSSF and bubble formations in the equinoctial and summer months. Conversely, the smaller upward velocities during the winter months cause fewer BSSF and bubble occurrences. For the geomagnetic effect, the BSSF/bubble occurrence decreases with an increasing Kp value in the equinoctial months, but no such correlation is found for the summer and winter months. Moreover, the anti-correlations between Kp and dh'F/dt are apparent in the equinoctial months, but not in the summer and winter months. These results indicate that in the equinoctial months the BSSF/bubble generations and the pre-reversal drift velocity can be suppressed by geomagnetic activity, because the disturbance dynamo effects could have decreased the eastward electric field near sunset. However, BSSF and bubble occurrences may not be suppressed by the geomagnetic activity in the summer and winter months.

Assimilative modeling of the equatorial ionosphere for scintillation forecasting: Modeling with vertical drifts

Knowledge of the vertical plasma drift velocity observed by the Jicamarca incoherent radar in seven events is assimilated into a theoretical model for the ambient F region plasma density. Comparisons of the calculated plasma density model and the observed plasma density show that, apart from the signature effects of equatorial plasma bubbles, the ambient model captures much of the detail of the plasma density profiles. Rayleigh‐Taylor growth rates calculated with the ambient model show a good correlation with the occurrence of spread F.

East‐west asymmetries of the equatorial electrojet 8.3 m type‐2 echoes observed over Trivandrum and a possible explanation

The east‐west asymmetries in the spectral parameters of the type‐2 echoes from the equatorial electrojet plasma irregularities observed using a 18 MHz radar from Trivandrum are presented. Observations show that the difference in signal strength, velocity, and spectral width of the type‐2 echoes observed in the west and east beam are as high as 15 dB, 60 m s−1, and 70 m s−1, respectively. Further, the asymmetry in velocity increases with height, while the asymmetries in signal strength and spectral width decrease with height. While the velocity asymmetry is consistent with the past results, the asymmetries in signal strength and spectral width of the type‐2 echoes are significant new results, not reported earlier. Finite vertical drift velocities of the irregularities, associated with the primary wave structures, are found to be responsible for the observed east‐west velocity asymmetry. The asymmetries in signal strength and spectral width are attributed to the orientation of kilometer‐scale plasma waves present in the equatorial electrojet. Finally, a unified picture is presented to show that the properties of the kilometer‐scale waves are the ones that are responsible for all three asymmetries observed in the spectral parameters of the type‐2 echoes.

Sequential observations of the local neutral wind field structure associated with E region plasma layers

The first sequential rocket experiment to study intermediate layers in over 30 years was launched from Wallops Island, Virginia on the night of 29–30 June 2003. Using an onsite digisonde to determine the presence of sporadic E and conditions indicating the possible presence of an intermediate layer, four rockets were launched over a 4‐hour period. Three of the rockets launched with at least an hour separation; they each contained chemical release experiments and plasma impedance probes. All four payloads encountered two plasma layers on both the ascent and descent of the flights. The lower‐altitude layer, located at approximately 100 km, is clearly a sporadic E layer. The higher‐altitude layer, located between 120 and 130 km, displays many characteristics of an intermediate layer, but it exhibits little downward motion over time. The neutral wind profiles resulting from the chemical tracer experiment are presented here along with the vertical drift velocities derived from the wind measurements. These are compared with the electron density profiles. They show a good agreement between the convergent regions in the velocity profiles and the location of the sporadic E layers. However, agreement between the center of the convergent vertical drift regions and the location of the higher‐altitude layer is poor. The inclusion in the drift calculation of electric field data from the instrumented rocket significantly improves the overall agreement between the convergent vertical drift region center and the intermediate layer center. The convergent region is within 4 km of the intermediate layer. Further, the density depletions surrounding the layers coincide with the regions of divergent drift. The relatively large discrepancy observed between the shear in the vertical drift and the location of the intermediate layer implies that other factors such as horizontal motion structure variations may be important. Thus intermediate layer formation theory and subsequent evolution is still not fully understood.