Vertical Polarization Electric Field Research Articles

On the unique divergent response of the equatorial electrojet vertical polarization electric field to different solar flare events

AbstractThe response of ionospheric E region to different flare events is investigated using coherent HF backscatter radar to bring out divergent observations. The study reveals the following aspects: (i) increase of absolute mean Doppler frequency by ~25–52% during the initial phase of the morning time flares of 9 September 2005 with concurrent fall in backscattered power and (ii) decrease of absolute mean Doppler frequency by ~11.6–16.2% during the initial phase of the flare of 20 February 2002 with concurrent fall in backscattered power. The Doppler frequency is directly related to vertical polarization electric field. Therefore, these observations are unique and in contrast to earlier works which have, in general, reported only a decrease in equatorial electrojet (EEJ) vertical polarization electric field during the initial phase of flare events. The present study also brings out the possible important role played by the height‐integrated conductivities in producing the divergent response of the EEJ. The large reduction in backscattered power for weak M class flare events is also observed in this study. In view of the importance of the ionosphere as a major source of error in GPS‐based navigation, the present result assumes significance.

Daytime gigahertz scintillations near magnetic equator: relationship to blanketing sporadic E and gradient-drift instability

Observations made in non-equatorial regions appear to support the hypothesis that the daytime scintillation of radio signals at gigahertz (GHz) frequencies is produced by the gradient-drift instability (GDI) in the presence of a blanketing sporadic E (Esb) layer. However, the only evidence offered, thus far, to validate this notion, has been some observations of Esb in the vicinity of GHz scintillations. A more comprehensive evaluation requires information about electric field, together with the presence of a steep gradient, which is presumed to be that of Esb. In this regard, the region in the vicinity of the equatorial electrojet (EEJ) appears to be an ideal “laboratory” to conduct such experiments. The dominant driver of electron drift there is the same as that of the EEJ, the vertical polarization electric field, and indications are that the presence of Esb in that vicinity is controlled by a balance in horizontal transport of Esb, between the EEJ electric field and the neutral wind, as described in a model by Tsunoda (On blanketing sporadic E and polarization effects near the equatorial electrojet, 2008). In this paper, we present, for the first time, results from a comprehensive study of daytime GHz scintillations near the magnetic equator. The properties, derived from measurements, are shown, for the first time, to be consistent with a scenario in which Esb presence is dictated by the Tsunoda model, and the plasma-density irregularities responsible for GHz scintillations appear to be produced by the GDI.

Solar cycle dependent characteristics of the equatorial blanketing E<sub><i>s</i></sub> layers and associated irregularities

Abstract. The occurrence of blanketing type Es (Esb) layers and associated E-region irregularities over the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip ~0.5°) during the summer solstitial months of May, June, July and August has been investigated in detail for the period 1986–2000 to bring out the variabilities in their characteristics with the solar cycle changes. The study has been made using the ionosonde and magnetometer data of Trivandrum from 1986–2000 along with the available data from the 54.95 MHz VHF backscatter radar at Trivandrum for the period 1995–2000. The appearance of blanketing Es layers during these months is observed to be mostly in association with the occurrence of afternoon Counter Electrojet (CEJ) events. The physical process leading to the occurrence of a CEJ event is mainly controlled by the nature of the prevailing electro dynamical/neutral dynamical conditions before the event. Hence it is natural that the Esb layer characteristics like the frequency of occurrence, onset time, intensity, nature of gradients in its top and bottom sides etc are also affected by the nature of the background electro dynamical /neutral dynamical processes which in turn are strongly controlled by the solar activity changes. The occurrence of Esb layers during the solstitial months is found to show very strong solar activity dependence with the occurrence frequency being very large during the solar minimum years and very low during solar maximum years. The intensity of the VHF radar backscattered signals from the Esb irregularities is observed to be controlled by the relative roles of the direction and magnitude of the prevailing vertical polarization electric field and the vertical electron density gradient of the prevailing Esb layer depending on the phase of the solar cycle. The gradient of the Esb layer shows a more dominant role in the generation of gradient instabilities during solar minimum periods while it is the electric field that has a more dominant role during solar maximum periods.

Effects of large horizontal winds on the equatorial electrojet

The effects of large winds on the low‐latitudeEregion ionosphere and the equatorial electrojet in particular are analyzed theoretically, computationally, and experimentally. The principles that govern the relationship between electric fields, currents, and winds in steady flows in the ionosphere are reviewed formally. A three‐dimensional numerical model of low‐latitude ionospheric electrostatic potential is then described. Scaled wind profiles generated by the National Center for Atmospheric Research (NCAR) thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME‐GCM) are used as inputs for the potential model. The model shows that the horizontal wind component drastically modifies the vertical polarization electric field in the electrojet and drives strong zonal and meridional currents at higher dip latitudes outside the electrojet region. Comparison between the model output and coherent scatter radar observations of plasma irregularities in the electrojet indicate that strong winds and wind shears are present in theEregion over Jicamarca that are roughly consistent with NCAR model wind predictions if the amplitudes of the latter are increased by about 50%.

Radar measurements of electric fields in the topside of the equatorial electrojet: First results

We show that the vertical polarization electric field (Ep) in the topside of the equatorial electrojet can be determined from the Doppler spectra of type‐2 echoes obtained with a radar that is appropriately displaced in latitude from the magnetic dip equator. Using a 49.92 MHz radar on Christmas Island (2.9° magnetic dip latitude), we found unexpectedly large Ep at altitudes at least as high as 120 km over the dip equator. Another new and related finding is the transient appearance of type‐1 echoes at 99 km over Christmas Island, which likely was produced by an Ep of at least 11 mV/m that must have appeared at 114 km altitude over the dip equator. Elevated electron temperatures inferred from the type‐1 Doppler spectra are consistent with the presence of anomalous energetics.

Modeling investigation of the evening prereversal enhancement of the zonal electric field in the equatorial ionosphere

The neutral winds in the E and F regions are the drivers of the low‐latitude ionospheric current system and observed electric field structure. A flux‐tube‐integrated model of ionospheric electrodynamics is used to examine three proposed causes of the prereversal enhancement of the zonal electric field. All three mechanisms try to define the obscure relationship between the F region dynamo and the evening prereversal enhancement. The results show that the original mechanism proposed by Rishbeth [1971] is the fundamental mechanism. The rapid change in the vertical polarization electric field near local sunset, which is due to the accelerating neutral wind dynamo, produces a corresponding change in the zonal electric field through curl‐free requirements. The other two mechanisms do not produce the evening enhancement but alter the enhancement shape and magnitude.

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field E W(z), or from the radar-measured phase velocity V p II(z) of the type II plasma waves. The method is found to be satisfactory when E w ⪢ E p, where E p is the vertical polarization electric field generated by the global scale east-west electric field, EY, and E y < 0.2 mV m −1. Measurements of V p II by a VHF backscatter radar can be inverted to obtain the causative wind profile by this method. The method is tested using a simulation study in which E w(z) and V p II(z) as generated by two different wind models are used. The retrieved winds are compared with the original wind profiles and it is found that the error in the retrieved winds is mostly under 5%, for the case of no errors in the model Pedersen conductivity ( σ 1) profile and the E w(z) or V p II(z)(z) profiles used in the inversion. Even with a ±20% error in the above profiles, the errors in the retrieved winds are found to be less than 20% over 75% of the altitude range and 20–30% for the remaining 25% of the altitude range, on the average.

Modeling equatorial ionospheric electric fields

This paper gives a brief overview of the processes responsible for the equatorial electric field, and reviews relevant modeling work of these processes, with emphases on basic aspects and recent progress. Modeling studies have been able to explain most of the observed features of equatorial electric fields, although some uncertainties remain. The strong anisotropy of the conductivity and the presence of an east-west electric field lead to a strong vertical polarization electric field in the lower ionosphere at the magnetic equator, whose magnitude can be limited by plasma irregularities. Local winds influence the structure of the equatorial polarization field in both the E and F regions. The evening pre-reversal enhancement of the eastward electric field has been modeled by considering a combination of effects due to the presence of a strong eastward wind in the F region and to east-west gradients of the conductivity, current, and wind. Models of coupled thermosphere-ionosphere dynamics and electrodynamics have demonstrated the importance of mutual-coupling effects. The low-latitude east-west electric field arises mainly from the global ionospheric wind dynamo and from the magnetospheric dynamo, but models of these dynamos and of their coupling have not yet attained accurate predictive capability.

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread- F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.

A Van de Graaf source mechanism for middle atmospheric vertical electric fields

It is proposed that meteoric and other debris descending through the mesosphere constitute a natural Van de Graaf generator for vertical electric fields within the mesosphere. Dust and aerosol particles falling from above 85 km are charged negatively in the upper D-region. Charge is lost in the region below 70 km. This net charge transport creates a vertical polarization electric field. Calculated fields are in the range of 10 mV/m for the average input of meteoric debris. Observed vertical electric fields are confined to a few occasions when large fields of the order of 4 V/m are observed to maximize at 65 km. Calculated fields from this model also maximize at this altitude, but a special event with increased dust density or another mechanism to increase relative vertical velocity is required to explain the large fields. Such large values are the exception rather than the rule for D-region vertical electric fields.

The morphology of equatorial Mg+ ion distribution deduced from 2800‐Å airglow observations

The Visible Airglow Experiment on the Atmosphere Explorer E satellite has observed the resonantly scattered emission from Mg II at 2800 Å in the equatorial ionosphere. Altitude profiles of the Mg+ ion distribution have been obtained from the inversion of the surface brightness measurements made on spinning orbits. These data show a daytime metallic ion layer between 150 and 200 km developing in the early morning and reaching about 100 ions/cm³ in the afternoon. Mg+ ions are also seen in the F2 region mostly in the late afternoon hours within a few degrees of the dip equator. The study of the vertical column density measured in the despun mode indicates that the amount of Mg+ in the F region is most variable in the afternoon hours at low dip latitudes. These results can be explained in part by the diurnal variation of the E × B drift velocity which lifts the metallic ions up into the F region. The observations suggest that the vertical polarization electric field is not the primary transport mechanism extracting the Mg+ ions from the low‐altitude source layer.