Showing papers on "Earth's magnetic field published in 2000"

Mountains versus valleys: Semiannual variation of geomagnetic activity

Abstract: The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field Bs, near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the Ψ angle between the solar wind flow direction and Earth's dipole axis) that makes Bs, coupling less effective (by ∼25% on average) at the solstices. Thus the semiannual variation is not simply due to “mountain building” (creation of Bs) at the equinoxes but results primarily from “valley digging” (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for ∼65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.

Penetration of auroral electric fields to the equator during a substorm

Abstract: We have studied the negative magnetic bay associated with the substorm that occurred on April 20, 1993, and have found that it is markedly enhanced at the daytime dip equator, coherent with that at afternoon subauroral latitudes. The amplitude of the negative bay decreases monotonously with the latitude, but it is amplified at the dip equator by a factor of 2.5 compared to the low-latitude negative bay. This latitudinal profile implies that in addition to the three-dimensional current system in the magnetosphere, DP ionospheric currents originating in the polar ionosphere contribute greatly to negative bays. Penetration of the convection electric field and the effect of a shielding electric field due to Region 2 (R2) field-aligned currents (FACs) are examined on the basis of European Incoherent Scatter (EISCAT) and International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer observations made in the afternoon sector. The northward electric field at EISCAT (66° corrected geomagnetic latitude (CGMLAT)) is well correlated with the magnetic field X component at Nurmijarvi (56° CGMLAT) during the presubstorm period, but the coherency breaks down during the substorm cycle. By assuming that the R2 FACs intensify the northward electric field at EISCAT but reduce it at Nurmijarvi, we demonstrate that the R2 FACs grow concurrently, although delay by some 17 min, with the convection electric field. Our analytical results indicate that the convection electric field decreases abruptly during the substorm and that the shielding electric field overcomes the convection electric field at around the peak of the negative bay, owing to its delayed reaction. The equatorial negative bay is thus due to an overshielding effect caused by the electric field associated with the R2 FACs.

Coronal mass ejections, interplanetary ejecta and geomagnetic storms

Abstract: StudiesusingSOHOspacecraftdatahavedemon- strated that frontside halo coronal mass ejections (CMEs) detected by the LASCO coronagraphs generally precede ge- omagnetic storms. Nonetheless, about three quarters of such CMEs do not result in even moderate geomagnetic activity. We study the relationship of all the ejecta (in- terplanetary CMEs) which passed Earth during 1996-1999 to coronagraph CMEs and geomagnetic activity. We reach the following conclusions: (1) Only about half of frontside halo CMEs encounter the Earth; (2) The geoeectiveness of ejecta depends strongly on the southward magneticeld strengthand, for thesame southwardeld, is irrespective of whetherornottheejectahasamagneticcloudstructure;(3) Transit speeds of ejecta to Earth are only loosely correlated with CME speeds, one influence being the prevailing solar windconditions betweentheSunandEarth; (4)Ejecta may be detected at Earth even when there is no preceding halo CME observed by LASCO. Such ejecta are not particularly geoeective.

Convection and auroral response to a southward turning of the IMF : Polar UVI, CUTLASS, and IMAGE signatures of transient magnetic flux transfer at the magnetopause

Abstract: We present the first spacecraft-borne imager observations of the auroral manifestation of transient magnetic flux transfer at the magnetopause. During an interval of interplanetary magnetic field Bz ≈ −10 nT, By ≈ 10 nT, and solar wind dynamic pressure and velocity Psw ≈ 5 nPa and vsw ≈ 650 km s−1, Polar Ultraviolet Imager (UVI) images show a sequence of events, each of which begins as a bifurcation of the main auroral oval in the 14 to 16 magnetic local time (MLT) sector which subsequently progresses antisunward (eastward) at 2 km s−1 toward the 19 MLT sector. The poleward portion of the bifurcation is interpreted as a poleward-moving auroral form (PMAF) as has previously been observed by ground-based optical instrumentation and identified as the auroral signature of flux transfer events. Ground-based measurements of the associated plasma drift, made with the Cooperative U.K. Twin Located Auroral Sounding System (CUTLASS) Finland HF radar, show poleward (1 km s−1) and westward (1 km s−1) convection flow, consistent with the By tension force, as well as poleward-moving regions of backscatter. International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers within the radar field of view observe poleward-progressing, 10 min period, X component magnetic deflections, which are consistent with the effect of Hall currents associated with the plasma flow. The combined radar and optical observations suggest that the PMAFs can be 3500 km or 7 hours of MLT in length. The antisunward motion of the bifurcation of the auroral oval is interpreted as an expansion of the reconnection X line along the flank of the magnetopause.

Long-term indirect indices of solar variability

Abstract: Continuous direct records of solar variability are limited to the telescopic era covering approximately the past four centuries. For longer records one has to rely on indirect indices such as cosmogenic radionuclides. Their production rate is modulated by magnetic properties of the solar wind. Using a parameterisation of the solar activity and a Monte Carlo simulation model describing the interaction of the cosmic rays with the atmosphere, the production rate for each cosmogenic nuclide of interest can be calculated as a function of solar activity. Analysis of appropriate well-dated natural archives such as ice cores or tree rings offers the possibility to reconstruct the solar activity over many millennia. However, the interpretation of the cosmogenic nuclide records from these archives is difficult. The measured concentrations contain not only information on solar activity but also on changes in the geomagnetic field intensity and the transport from the atmosphere into the archive where, under ideal conditions, no further processes take place. Comparison of different nuclides (e.g. 10Be and 14C) that are produced in a very similar way but exhibit a completely different geochemical behaviour, allows us to separate production effects from system effects. The presently available data show cyclic variability ranging from 11-year to millennial time scale periodicities with changing amplitudes, as well as irregularly distributed intervals of very low solar activity (so called minima, e.g. Maunder minimum) lasting typically 100 years.

ULF electromagnetic precursors for an earthquake at Biak, Indonesia on February 17, 1996

Abstract: ULF electromagnetic emissions associated with a large earthquake occurred at Biak Island, Indonesia at 5h 59m UT on February 17, 1996 (magnitude (Mw) = 8.2 and depth = 20 km from USGS catalog), have been investigated on the basis of ULF magnetic observations at two stations, Biak and Darwin in Australia (about 1,200 km apart). Though a simple analysis of magnetic field intensity (horizontal and vertical magnetic field components) at both stations was found to be closely correlated with geomagnetic ΣKp activity, a detailed analysis of the difference of H and Z components at the two stations, the polarization analysis (Z/H) and fractal analysis (frequency spectrum slope) at these two stations has yielded that the ULF emissions (in the frequency range from 5 mHz to 30 mHz) about 1.5–1.0 months before the quake are likely to be a precursory signature of the quake with its intensity on the order of 0.2–0.3 nT.

Numerical models of the geodynamo and observational constraints

Abstract: [1] The past few years have seen the emergence of a large number of numerical simulations of the geodynamo. In parallel, both new and old geomagnetic, archeomagnetic, and paleomagnetic observations have been interpreted as actual geomagnetic features and used as constraints for dynamo models. Naturally, model predictions should be tested against actual characteristics of the geomagnetic field. Despite huge differences (sometimes in excess of a billion) between the values of parameters used in the simulations and those estimated for the Earth, it is intriguing that many available simulations succeed in producing largely axial dipolar magnetic fields with weaker nondipolar structures, in agreement with the first-order characteristics of the geomagnetic field. Yet, when considering finer characteristics, there are significant differences, and failures to actually produce a number of fundamental characteristic features. In this presentation, we first review numerical results obtained to date, then we attempt to summarize which field characteristics derived from observational data sets can be considered robust. On the basis of simple criteria used to evaluate the degree of confidence that can be placed in each datum, we sort presumably characteristic geomagnetic features into three categories (robust, controversial, and unlikely). We conclude that numerical models should be illustrated with a number of key “predictions,” averaged over at least 10 dipole diffusion times. These predictions should be tested against the subset of robust observations only. Controversial observations should await additional confirmation.

Annual and semiannual variations in the ionospheric F2-layer : I. Modelling

Abstract: Annual, seasonal and semiannual variations of F2-layer electron density (NmF2) and height (hmF2) have been compared with the coupled thermosphere-ionosphere-plasmasphere computational model (CTIP), for geomagnetically quiet conditions. Compared with results from ionosonde data from midlatitudes, CTIP reproduces quite well many observed features of NmF2, such as the dominant winter maxima at high midlatitudes in longitude sectors near the magnetic poles, the equinox maxima in sectors remote from the magnetic poles and at lower latitudes generally, and the form of the month-to-month variations at latitudes between about 60°N and 50°S. CTIP also reproduces the seasonal behaviour of NmF2 at midnight and the summer-winter changes of hmF2. Some features of the F2-layer, not reproduced by the present version of CTIP, are attributed to processes not included in the modelling. Examples are the increased prevalence of the winter maxima of noon NmF2 at higher solar activity, which may be a consequence of the increase of F2-layer loss rate in summer by vibrationally excited molecular nitrogen, and the semiannual variation in hmF2, which may be due to tidal effects. An unexpected feature of the computed distributions of NmF2 is an east-west hemisphere difference, which seems to be linked to the geomagnetic field configuration. Physical discussion is reserved to the companion paper by Rishbeth et al.

Ørsted Initial Field Model

Abstract: Magnetic measurements taken by the Orsted satellite during geomagnetic quiet conditions around Jan-uary 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is < 2 nT for the scalar intensity and < 3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Orsted mission, this model supercedes IGRF 2000.

Reconstruction of the geomagnetic field between 20 and 60 kyr BP from cosmogenic radionuclides in the GRIP ice core

Abstract: A pure physical model for the simulation of cosmic ray particle interactions with the Earth’s atmosphere was used to investigate the effects of a changing geomagnetic field on the production rates of cosmogenic nuclides. Analytical dependencies of the production rates of 3H, 7Be, 10Be, 14C and 36Cl on geomagnetic field intensity were developed. Applying those relations to the 10Be and 36Cl fluxes measured in the GRIP ice core, the geomagnetic field intensity for the period between 20 and 60 kyr BP was reconstructed. Comparison with remnant magnetism records from marine sediment cores shows excellent agreement. This validates the use of cosmogenic nuclides in ice cores to reconstruct geomagnetic field variations.

Alfv?n Waves, Density Cavities and Electron Acceleration Observed from the FAST Spacecraft

Abstract: Inertial Alfven waves propagating in regions of auroral electron acceleration are observed from the FAST (Fast Auroral Snapshot) spacecraft over its entire altitude range (350–4180 km). These electron skin depth sized field structures are dispersive and carry an electric field component parallel to the geomagnetic field leading to a variety of non-linear effects including the formation of magnetic field-aligned density cavities and electron acceleration. High resolution measurements show that the electron distribution inside the impulsive wave field envelope or density cavity consists of a cold ionospheric component and an accelerated and heated field-aligned component comprised of downgoing and reflected ionospheric and magnetosheath (solar wind like) electrons. In large amplitude examples the plasma within the wave may be dominated by the accelerated component and the depletion of plasma within the cavity approaches 100%. Comparison of the observed density depletion with the predictions of ponderomotive density cavity formation in association with electron heating shows good agreement. Furthermore, simulations show that most of the observed features of the accelerated component can be explained through Landau resonance of the cold ionospheric and magnetosheath electrons with the inertial Alfven wave as it propagates through an altitude dependent density profile.

Application of ionospheric tomography to real‐time GPS carrier‐phase ambiguities Resolution, at scales of 400–1000 km and with high geomagnetic activity

Abstract: Theinfluenceoftheionospherecanbeoneofthe mainobstacles toGPScarrierphaseambiguityresolution in real-time,particularlyoverlongbaselines. Thisisimportant toallusersofGPSrequiringsub-decimeterpositioning, per- haps in real time, especially with high geomagnetic activity or close to the Solar Maximum. Therefore, it is desirable to have a precise estimation of the ionospheric delay in real- time, to correct the data. In this paper we asses a real-time tomographic model of the ionosphere created using dual- frequency phase data simultaneously collected with the re- ceiversofanetworkofstationsintheUSAandCanada,with separationsof400-1000km,duringaperiodofhighgeomag- netic activity (Kp=6). When the tomographic ionospheric correctionisincluded,theresolutionon-the-fly(OTF)ofthe widelanedouble-dierencedambiguitiesatthereferencesta- tions is nearly 100% successful for satellite elevations above 20 degrees, while theresolution of theL1,L2ambiguities at the rover is typically more than 80% successful.

Global geomagnetic field models for the past 3000 years: transient or permanent flux lobes?

Abstract: PSVMOD1.0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts, lava flows, and lake sediments at 24 sites evaluated at 100 year intervals from 1000 BC to AD 1800. We estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time–intervals of overlap, and use the 100–year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models. Global predictions of radial magnetic field at the coremantle boundary (CMB), as well as inclination and declination anomalies at the Earth9s surface, provide an unprecedented view of geomagnetic secular variations over the past 3000 years, and demonstrate a consistent evolution of the field with time. Resolution of the models is poorest in the Southern Hemisphere, where only six of the 24 sites are located, several with incomplete temporal coverage. Low–flux regions seen in the historical field near the North Pole are poorly resolved, but the Northern Hemisphere flux lobes are clearly visible in the models. These lobes are not fixed in position and intensity, but they only rarely venture into the Pacific hemisphere. The Pacific region is seen to have experienced significant secular variation: a strong negative inclination anomaly in the region, like that seen in 0–5 Ma models, persists from 1000 BC until AD 1000 and then gradually evolves into the smaller positive anomaly seen today. On average between 1000 BC and AD 1800, the non–axial–dipole contribution to the radial magnetic field at the core–mantle boundary is largest in the north–central Pacific, and beneath Central Asia, with clear non–zonal contributions. At the Earth9s surface, average inclination anomalies are large and negative in the central Pacific, and most positive slightly to the east of Central Africa. Inclination anomalies decrease with increasing latitude. Average declinations are smallest in equatorial regions, again with strong longitudinal variations, largest negative departures are centred over Australia and Eastern Asia. Secular variation at the Earth9s surface is quantified by standard deviation of inclination and declination about their average values, and at the CMB by standard deviation in radial magnetic field. All three show significant geographical variations, but appear incompatible with the idea that secular variation in the Pacific hemisphere is permanently attenuated by greatly enhanced conductivity in D 00 beneath the region.

Cosmic Rays in Relation to Space Weather

Abstract: A review of selected experimental results relevant for the use of cosmic ray records in Space Weather research is presented Interplanetary perturbations, initiated in the solar atmosphere, affect galactic cosmic rays In some cases their influence on the cosmic ray intensity results in data signatures that can possibly be used to predict geomagnetic storm onsets Case studies illustrating the complexity of the cosmic ray effects and related geomagnetic activity precursors are discussed It is shown that some indices for cosmic ray activity are good tools for testing the reliability of cosmic ray characteristics for Space Weather forecasts A brief summary of the influence of cosmic rays on the ozone layer is also given The use of cosmic ray data for Space Weather purposes is still in its infant stage, but suggestions for both case and statistical studies are made

SAMPEX observations of precipitation bursts in the outer radiation belt

Abstract: The occurrence frequency of precipitation bursts of > 1 MeV electrons in the outer radiation belt is examined using data from the SAMPEX satellite. Electron burst characteristics shown in this paper include the dependence of the precipitation on magnetic local time, radial distance and geomagnetic activity. Precipitation bursts with timescales < 1 s, i.e., microbursts, are studied in detail, including their dependence on the phases of geomagnetic storms. It is found that precipitation bursts occur typically in the region between L = 4 and L = 6. Microbursts tend to occur at L lower than the bursts with timescales of several tens of seconds. The number of observed microbursts significantly increases during storms, appearing mainly in the morning sector early in the recovery phase of storms. These findings suggest that the microbursts may be due to interactions with electron whistler waves, which take place near the dawnside plasmapause in the density irregularities that are perhaps created in the "recovering" plasmasphere. The prevalence of bursty precipitation indicates that this enhanced loss component of the relativistic electron flux should be taken into account in any quantitative model of relativistic electron acceleration processes.

Geomagnetic intensity variations over the past 780 kyr obtained from near-seafloor magnetic anomalies

Abstract: Knowledge of past variations in the intensity of the Earth's magnetic field provides an important constraint on models of the geodynamo. A record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials, and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences. But a long-term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust Here we show that near-seafloor magnetic anomalies recorded over the southern East Pacific Rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr. Moreover, the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well-documented dipole intensity pattern for the past 50 kyr. A comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region. The record of geomagnetic intensity preserved in the oceanic crust should provide a higher-time-resolution record of crustal accretion processes at mid-ocean ridges than has previously been obtainable.

Electric field variability associated with the Millstone Hill electric field model

Abstract: Joule heating that is generated at high latitudes in the thermosphere because of the magnetospherically imposed electric potential is proportional to the average of the square of the electric field (E field). Most theoretical Joule heating computations use only average electric fields, resulting in heating that is proportional to the square of the average E field. The computation of the average of the square of the E field requires knowledge about the statistical characteristics of E field variability associated with the average electric field model. In this paper we present the variability associated with the Millstone Hill bin-averaged empirical E field model [Foster et al. 1986] and discuss the implications of variability as an upper atmosphere energy source. We rebinned the radar plasma drift measurements from Millstone Hill, Massachusetts, in magnetic latitude and local time as a function of auroral activity and calculated the average electric fields and the variability associated with them as reflected in the bin standard deviations. We present the E field patterns and the associated variability for both quiet and disturbed geomagnetic conditions for the four seasons. We show that for an electric field model with a Gaussian distribution of small-scale variability around the mean, the average field and the variability have equal contributions to Joule heating generation.

Quantitative modeling of the ionospheric response to geomagnetic activity

Abstract: . A physical model of the coupled thermosphere and ionosphere has been used to determine the accuracy of model predictions of the ionospheric response to geomagnetic activity, and assess our understanding of the physical processes. The physical model is driven by empirical descriptions of the high-latitude electric field and auroral precipitation, as measures of the strength of the magnetospheric sources of energy and momentum to the upper atmosphere. Both sources are keyed to the time-dependent TIROS/NOAA auroral power index. The output of the model is the departure of the ionospheric F region from the normal climatological mean. A 50-day interval towards the end of 1997 has been simulated with the model for two cases. The first simulation uses only the electric fields and auroral forcing from the empirical models, and the second has an additional source of random electric field variability. In both cases, output from the physical model is compared with F-region data from ionosonde stations. Quantitative model/data comparisons have been performed to move beyond the conventional "visual" scientific assessment, in order to determine the value of the predictions for operational use. For this study, the ionosphere at two ionosonde stations has been studied in depth, one each from the northern and southern mid-latitudes. The model clearly captures the seasonal dependence in the ionospheric response to geomagnetic activity at mid-latitude, reproducing the tendency for decreased ion density in the summer hemisphere and increased densities in winter. In contrast to the "visual" success of the model, the detailed quantitative comparisons, which are necessary for space weather applications, are less impressive. The accuracy, or value, of the model has been quantified by evaluating the daily standard deviation, the root-mean-square error, and the correlation coefficient between the data and model predictions. The modeled quiet-time variability, or standard deviation, and the increases during geomagnetic activity, agree well with the data in winter, but is low in summer. The RMS error of the physical model is about the same as the IRI empirical model during quiet times. During the storm events the RMS error of the model improves on IRI, but there are occasionally false-alarms. Using unsmoothed data over the full interval, the correlation coefficients between the model and data are low, between 0.3 and 0.4. Isolating the storm intervals increases the correlation to between 0.43 and 0.56, and by smoothing the data the values increases up to 0.65. The study illustrates the substantial difference between scientific success and a demonstration of value for space weather applications. Key words: Ionosphere (ionospheric disturbances; mid-latitude ionosphere; modeling and forecasting)

effects of driving mechanisms in geodynamo models

Abstract: We compare the influence of different mechanisms for driving convection in the Earth's core on the structure of the magnetic and velocity fields using a 3D numerical dynamo model. We find dynamos with a dipolar magnetic field in cases of chemical convection or convection driven by an imposed temperature contrast. With purely internal heating we obtain only dynamos with a quadrupolar or more complex field. The relative strength of convection and magnetic field generation in the regions close to the poles depends on whether a condition of fixed composition or of fixed chemical flux is specified on the inner core boundary. If applicable to the geodynamo, our results favor the dominance of chemical convection during the past 3 Gyr.

Sensitivity of the geomagnetic axial dipole to thermal core-mantle interactions

Abstract: Since the work of William Gilbert in 1600 (ref. 1), it has been widely believed that the Earth's magnetic field, when suitably time-averaged, is that of a magnetic dipole positioned at the Earth's centre and aligned with the rotational axis. This ‘geocentric axial dipole’ (GAD) hypothesis has been the central model for the study of the Earth's magnetic field—it underpins almost all interpretations of palaeomagnetic data, whether for studies of palaeomagnetic secular variation, for plate tectonic reconstructions, or for studies of palaeoclimate2. Although the GAD hypothesis appears to provide a good description of the Earth's magnetic field over at least the past 100 Myr (ref. 2), it is difficult to test the hypothesis for earlier periods, and there is some evidence that a more complicated model is required for the period before 250 Myr ago3. Kent and Smethurst3 suggested that this additional complexity might be because the inner core would have been smaller at that time. Here I use a numerical geodynamo model and find that reducing the size of the inner core does not significantly change the character of the magnetic field. I also consider an alternative process that could lead to the breakdown of the GAD hypothesis on this timescale, the evolution of heat-flux variations at the core–mantle boundary, induced by mantle convection. I find that a simple pattern of heat-flux variations at the core–mantle boundary, which is plausible for times before the Mesozoic era, results in a strong octupolar contribution to the field, consistent with previous findings3.

Global Positioning System measurements of the ionospheric zonal apparent velocity at Cachoeira Paulista in Brazil

Abstract: Ionospheric irregularities and their zonal apparent drift were studied using Global Positioning System (GPS) measurements at Cachoeira Paulista (22.41°S, 45.00°W, −26° dip angle) in Brazil during November 6–19, 1998. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using four GPS receivers spaced geomagnetically east–west and north–south. Total electron content (TEC) was measured through the ionospheric advance of the GPS L1 and L2 (1.227 GHz) phases. Strong amplitude scintillations coincided with TEC fluctuations associated with spread F bubbles elongated along the magnetic field. Movement of the Presnel-scale (400 m) ionospheric irregularity layers caused the scintillation to drift, and their zonal apparent drift velocities were measured using a cross-correlation technique. Our measurements show that the apparent eastward velocity varies from 200 m/s to 150 m/s at 2000 LT, and then it decreases to 100–50 m/s at midnight. On a magnetically disturbed day, reversal of the zonal apparent drift was observed just after midnight, and the apparent westward velocities observed at early in the morning showed large variations with location in the sky. From the receivers spaced in the geomagnetic north–south direction we measured near-zero time shifts, from which we conclude that the correlation length of several-hundred-meter-scale irregularities is much larger than 70-m separation between the north and south receivers.

Polar cap index (PC) as a proxy for ionospheric electric field in the near‐pole region

Abstract: The ion drift measurements made by a number of DMSP satellites during some intervals in 1991, 1997, and 1998 are utilized for estimation of the ionospheric electric fields over the near-pole region; these estimates are then compared with the Polar Cap (PC) magnetic activity index obtained from ground geomagnetic observations at Qaanaaq (former Thule, Greenland) and Vostok (Antarc- tica). The analysis shows that the polar cap electric field is primarily controlled by variations in the near-Earth's inter- planetary electric field. The relationship between the polar cap ionospheric electric field and the PC-index can be ap- proximated by a quadratic polynomial. The polar cap iono- spheric electric field tends to saturate at the asymptote of -45-50 mV/m when the PC index reaches large positive values (PC > 10); the residual electric field (for near-zero interplanetary electric field applied to the Earth's magneto- sphere) is-12 mV/m. It is concluded that the PC-index can serve as a proxy of the ionospheric electric fields in the near-pole region.