A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

The equatorial electrojet and associated currents as seen in Magsat data

Magsat data are re-examined with regard to the presence and character of fields due to the equatorial electrojet and meridional currents at dawn and dusk local times. Dip-latitude organized field variations at dawn are:

• 1.(1) extremely weak,
• 2.(2) extremely variable with longitude,
• 3.(3) inconsistent with the pattern expected from a line or narrow sheet current.

It is shown that the use of Magsat dusk data can ‘contaminate’ a main field model, introducing apparent equatorial electrojet effects into the dawn data.Fields due to the equatorial electrojet and (presumably) associated meridional currents are clearly present in the dusk data. They show a variation with longitude which is apparently associated with the longitudinal variation of the strength, or square of the strength, of the main field in the E-region. Also evident is a variation with time of the year, although data are available for only a six month period. The meridional currents are generally minimum during January and February and maximum either during November and December or March and April, depending upon longitude. The E-region horizontal currents are minimum in November and December and maximum in March and April, except for − 30° to −90° longitude when the maximum occurs in January and February.Assuming that field gradients in local time are considerably smaller than field gradients in dip-latitude, current densities are estimated to be 1–3.6μA/m² for the horizontal current at 110km and about 10–20 × 10⁻⁹ A/m² for the vertical currents at 400km altitude. These results confirm and extend earlier results of Takeda and Maeda.Most models of the electrojet system in the literature disagree severely with these measurements either because their scope is inadequate or because of the wind system they assume. Those models which best describe the data invoke an eastward wind and/or an eastward electric field at dusk local time.     

The effects of differential ion flows on EISCAT observations in the auroral ionosphere

During geomagnetic storms different partial pressure gradients in the auroral ionosphere may result in H⁺, He⁺, O⁺ and molecular ions drifting with different velocities along the Earth's magnetic field line. For relative drift velocities ⪡ 400 m s⁻¹ it is shown that differential ion flows may be identified by two signatures in the autocorrelation function (ACF) measured by EISCAT. For larger relative drifts numerical simulations show that these signatures still exist and may result in an asymmetry in the incoherent scatter spectrum for O⁺ and molecular ions. It is demonstrated that UHF data can be reliably analysed for k²λ_D² ≲ 1, but at high altitudes, where O⁺–H⁺ flows are expected, UHF observations will be restricted by large Debye lengths (k²λ_D² > 1). Examples of ACFs based on polar wind theory are presented and discussed for the VHF system and finally it is shown that large ion temperature ratios (T_i(H⁺) >T_i(O⁺)) can significantly affect the velocity determination.     

Solar flare effects on zonal and meridional currents at the equatorial electrojet station,Annamalainagar

During the normal electrojet period, a solar flare produces a positive change in the horizontal (H) field, negative changes in the eastward (Y) field and a negative change in the vertical (Z) field at a northern electrojet station. On average, the ΔY is about 40% of ΔH. During a counter electrojet period, ΔH, due to a solar flare, is negative and ΔY and ΔZ are positive. During a partial counter electrojet period, ΔH may be smaller at equatorial stations compared with other low latitude stations, and ΔY may be positive, or sometimes of very small magnitude. The observed change of ΔY at an electrojet station is suggested to be the combined effect of the flare on the associated Sq current system and on electrojet related meridional currents. These data confirm the seat of the equatorial meridional current to be in the ionospheric E layer.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

Simultaneous observations of E and F region electric field fluctuations at the magnetic equator

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.     

A numerical model of the ionospheric dynamo—III electric current at equatorial and low latitudes

Using the general dynamo model and its special cases derived in a previous paper, the distributions of three dimensional electric current density in a magnetic meridional plane in the equatorial and low latitude ionosphere are computed. The winds generating the ionospheric dynamo are tide-like and locally periodic, similar to those in an internal gravity wave. Very large (several μA m⁻²) field-aligned current density is obtained in the equatorial region at places of sharp vertical gradients of the wind velocity. The currents generated by locally periodic winds of latitudinal wavelength less than several hundred kilometers do not significantly affect the normal equatorial electrojet.     

Electrodynamic structural features of the auroral zone as deduced using data from a meridional chain of ionospheric and magnetic stations

This study has used ionospheric and magnetic observational data obtained at a meridional chain of stations during the high latitude geophysical experiment ‘Taimir-82’ in the winter of 1982–1983. Mean statistical latitude-time distributions of the occurrence probability of various types of E_s, their blanketing frequency and of the amplitude of geomagnetic field H-variations have been constructed. Based on these distributions and taking the E_s properties into account, an analysis is made of the mutual correspondence of large-scale structures of the auroral ionosphere and ionospheric currents.Ionospheric currents flow mainly in the region of high E-layer ionization. With increasing magnetic activity, the zone of currents and the zone of ionization expand simultaneously toward lower latitudes. The evening eastward electrojet and the morning westward electrojet are localized inside the zone of diffuse auroral precipitation which is responsible for the formation of E_s type r. The equatorial part of the midnight westward electrojet is also located in the zone of diffuse precipitation which coincides also with the region of maximum ionization of the E-layer. The polar part of this electrojet, which extends far into the dusk sector, is located in the zone of discrete auroral precipitation (a type E_s). Whereas there exists in the meridional cross-section quite a definite relationship between the Harang discontinuity and ionospheric parameters, such a relationship is not manifested in the zonal cross-section of the Harang discontinuity.     

Effect of severe auroral disturbances on the equatorial ionosphere

It is now an established fact that during extremely strong magnetic storms a sudden anomalous decrease in the F-layer critical frequency foF2 is sometimes noticed at the equator around noon-time and the duration of this effect is known to be anywhere between some tens of minutes to several hours. As an extension of earlier work by Turunen and Rao, 1980, seven severe auroral storm events based on AE index have been selected during the period July 1958–June 1960 and their effects on the equatorial ionosphere have been investigated utilizing the published ionospheric data for the chain of Indian stations starting from equatorial latitudes and extending up to the mid-latitudes. From this study, it is noted that at the equator around noontime the foF2 values decrease and the noon bite-out phenomena are enhanced. However, as one goes towards mid-latitudes this trend is reversed. Because of this, the Appleton anomaly is also enhanced during disturbed days. Besides, the fF_s values at the magnetic equator show an increase during disturbed days indicating thereby that the eastward equatorial electrojet current is enhanced on disturbed days. This suggests that the auroral electrojet current is coupled to the equatorial electrojet current possibly via the magnetosphere.     

Evidence for non-Maxwellian ion velocity distributions in the F-region

A study has been made of data taken with EISCAT using the Common Program CP-3-C (F-region meridian scan) which shows that regions of enhanced ion temperature (in excess of 3000K at all three EISCAT stations) are found on most days when K_p exceeds 2 or 3, usually accompanied by ion drift velocities of more than 1 km s⁻¹. These periods are often accompanied by anisotropy of the ion temperature and abnormally low apparent electron temperature, consistent with the presence of a non-Maxwellian ion velocity distribution such as would result from large but not exceptional ion drifts. Data for a selected period have been fitted using theoretical ion velocity distributions based on the relaxation collision model and assuming that the ion composition is 100% O⁺. The results confirm the presence of non-Maxwellian distributions, but a detailed comparison with theory reveals some discrepancies, indicating that the analysis may need to be extended to include effects due to, for example, molecular ions and instabilities.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.     

Vertical neutral wind in the equatorial F-region deduced from electric field and ion density measurements

DC electric field and ion density measurements near density depletion regions (that is, equatorial plasma bubbles) are used to estimate the vertical neutral wind speed. The measured zonal electric field in a series of density depletions crossed by the San Marco D satellite at 01.47-01.52 UT on 25 October 1988, can be explained if a downward neutral wind of 15–30 m s⁻¹ exists. Simultaneously, the F-region plasma was moving downward at a speed of 30–50 m s⁻¹ These events appear in the local time sector of 23.002̄23.15 in which strong downward neutral winds may occur. Indeed, airglow measurements suggest that downward neutral velocities of 25–50 m s⁻¹ are possible at times near midnight in the equatorial F-region.     

Ionospheric sources of Pi(c) pulsations

Data collected at Macquarie Island (invariant latitude 64.5°S) shows average delays of 0.6 s and 0.2 s between D and H component Pi(c) pulsations, respectively, and the N₂⁺1NG (0,1) optical band pulsations. The dominant phase of the cross-correlations between the H component pulsations and the optical pulsations is consistent with the H component fluctuations being generated by increases in a westward electrojet. The D component fluctuations are consistent with either an increase in an equatorward Pedersen current or an increase in a field aligned current. The polarization of the Pi(c) pulsations may be explained in terms of the altitude of the respective currents.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

The equatorial electrojet and the day-to-day variability of Sq

The day-to-day variability of Sq(H) at equatorial stations has been studied, using the correlation between daily ranges for the IQSY at a large number of pairs of stations. It is found that the daily ranges at stations under the equatorial electrojet are significantly less well correlated with those at other equatorial stations than is the case for pairs of equatorial stations not involving an electrojet station, for comparable distances of separation of the pairs of stations.     

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical dynamo action in the ionosphere

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical ionospheric dynamo are calculated self-consistently, using the assumption of infinite parallel conductivity. Tidal winds of (1, −2) mode, which are generally accepted as a main cause of Sq fields, are adopted as a wind model. Variation in universal time (UT) is examined by considering the discordance between conductivity and wind distribution, which are assumed to follow the geographic coordinate system, and geomagnetic dipole field. Observed UT variation of Sq current system is partly reproduced by our calculation. Calculation for solstice condition is performed by shifting conductivity distribution by 23.5° in latitude. Height integrated westward currents are much smaller in the winter hemisphere than in the summer hemisphere, though eastward currents are not so different in both hemispheres. This unbalance is compensated by the field aligned currents mainly from summer to winter hemisphere in the morning and vice versa in the afternoon. In both above asymmetric cases, structure of the equatorial electrojet is almost symmetric with respect to the equator. Total field aligned currents are rather large and comparable to currents in the ionosphere.     

EISCAT measurements of ion temperatures which indicate non-isotropic ion velocity distributions

Substantial increases of the ion temperature can be observed at high latitudes as a consequence of strong convection electric fields. We have measured, with EISCAT, three independent components of the ion velocity vector and temperature in the same scattering volume, at about 300 km. During periods of strong variations in ion velocity (consequently of the E-field), the ion temperatures derived at the 3 sites are different. This difference, which appears to be systematic for the two experiments studied, can be interpreted in terms of different ion temperature perpendicular and parallel to the magnetic field, i.e. T_i⊥ greater than T_i∥. Assuming that a bi-Maxwellian distribution is present for convection electric field strengths as large as 50 mV m⁻¹, one obtains an anisotropy factor of approximately 1.5. It also appears that resonant charge exchange is the dominant collision process. During the evening sector events studied, the electron density was decreasing, whereas the electron temperature was generally increasing. Such events are strongly related to variations in the magnetic H component detected on the ground.     

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 (σ₁) 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.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

Ion-neutral collisions and the equatorial electrojet

There are various theoretical models of ion-neutral collision and the resultant collision frequencies (v_in) differ significantly in their values. Also there is a range of uncertainty associated with each of the theoretical values of v_in. The effects of the differing theoretical values and the uncertainty in the v_in on the estimates of the equatorial electrojet (EEJ) current strength are examined numerically. It is found that the differences in various v_in-models affect the amplitude of the EEJ, but leave the peak altitude of the EEJ nearly unchanged. However, modification to v_in by the order of its uncertainty does change the peak altitude of the EEJ by at least 2km.