Polar Cap Potential Drop Research Articles

Saturation of the ionospheric polar cap potential during the October–November 2003 superstorms

The question of whether the cross polar cap potential drop in the Earth's ionosphere saturates under conditions of extreme electric field in the solar wind has been tested observationally during the past several years. The challenge to proving the existence of this phenomenon is that periods of such extreme electric fields in the solar wind are relatively rare. The three superstorms of October and November 2003 provided ideal cases for testing this idea. We first review the earlier evidence of the saturation seen by the DMSP‐F13 spacecraft during the 31 March 2001 superstorm and other storm events during the 1998–2002 time period. Then we present observations from the DMSP‐F13 spacecraft during the October and November 2003 superstorms that show definite evidence of this saturation. In addition, some of the electric fields during these superstorms were almost twice as large as the largest fields previously studied, thus increasing the range of our sample set and further increasing our confidence in the existence of the saturation phenomenon. The data are compared with the saturated potentials predicted by the Hill‐Siscoe model to test its validity. The DMSP measurements indicate that the saturation limit of the cross polar cap is about 260 kV.

Storm‐time convection electric field in the near‐Earth plasma sheet

We have examined the electric field observations made by the Geotail spacecraft in the near‐Earth magnetotail during magnetic storms to study enhanced convection and the associated electric field that is thought to be key to causing the injection of particles into the ring current. Several recent modeling studies of the storm‐time ring current suggest that an enhanced convection electric field in the magnetosphere, which is induced by a continuous southward interplanetary magnetic field (IMF), is responsible for steady particle transport into the inner magnetosphere during the storm main phase. The enhanced convection is envisioned to weaken, or cease after the main phase in response to the decrease of southward IMF, leading to the formation of a relatively symmetric ring current around the Earth during the recovery phase. However, surprisingly, our present study has not found clear evidence for the existence of such a large, steady earthward convection during either the storm main phase or the recovery phase. The observed electric field properties in the near‐Earth plasma sheet are basically classified into two categories: One is characterized by intermittent bursts of fluctuating duskward electric fields associated with substorm expansions, and the other is dominated by a relatively steady, weak duskward electric field. The weak strength of the convection electric field in the latter category is seen even during storm main phase. The statistical study on this relatively steady, weak field shows that it has a weak duskward component of ∼0.3 mV/m on average during both the main and recovery phases, which is almost comparable to that observed during quiet times. Their comparison with the solar wind parameters and the polar cap potential drop calculated using the Boyle model and Weimer model reveals that the weak duskward electric field tends to show poor correlation with these parameters, suggesting that storm‐time convection electric field in the plasma sheet is not directly driven by either of them. These results imply that in the near‐Earth plasma sheet beyond geosynchronous distance, particle injection for the storm‐time ring current is not governed by enhanced convection induced by the solar wind, contrary to conclusions based on simulation studies of the storm‐time ring current. The present study suggests the importance of re‐examining the contribution to the ring current from the near‐Earth plasma sheet for both substorm and nonsubstorm time intervals on the basis of observations made in the magnetosphere.

Nonlinear impact of plasma sheet density on the storm‐time ring current

We investigated the nonlinear impact of the plasma sheet density on the total energy of the storm‐time ring current by means of a numerical simulation that self‐consistently solves the kinetic equation of ring current protons and the closure of the electric current between the magnetosphere and ionosphere. Results of the simulation indicate that when the convection electric field is self‐consistently coupled with the ring current, the total energy of the ring current ions trapped by the Earth's magnetic field is roughly proportional to ∼Nps1/2, where Nps is the plasma sheet density. This nonlinear response results from the strengthened shielding electric field with increasing Nps. The total energy is almost proportional to Nps when using an empirical convection electric field, which is independent of the condition of the simulated ring current. An empirical relationship between Nps and the solar wind density was used to estimate time‐dependent Nps. The result shows that the calculated Dst* tends to overshoot the observed one when the non‐self‐consistent electric field is employed. A better agreement was obtained with the self‐consistent electric field. We suggest that the nonlinear response of the ring current to Nps is one of the mechanisms that impedes the growth of the storm‐time ring current. Another mechanism is probably the saturation of the polar cap potential drop for high solar wind electric field.

Effects of energetic electrons on the electrodynamics in the ionosphere

Abstract. From the observations by the PIXIE and UVI cameras on board the Polar satellite, we derive global maps of the precipitating electron energy spectra from less than 1keV to 100keV. Based on the electron spectra, we generate instantaneous global maps of Hall and Pedersen conductances. The UVI camera provides good coverage of the lower electron energies contributing most to the Pedersen conductance, while PIXIE captures the high energy component of the precipitating electrons affecting the Hall conductance. By characterizing the energetic electrons from some tens of keV and up to about 100keV using PIXIE X-ray measurements, we will, in most cases, calculate a larger electron flux at higher energies than estimated from a simple extrapolation of derived electron spectra from UVI alone. Instantaneous global conductance maps derived with and without inclusion of PIXIE data have been implemented in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure, to study the effects of energetic electrons on electrodynamical parameters in the ionosphere. We find that the improved electron spectral characterization using PIXIE data most often results in a larger Hall conductance and a smaller inferred electric field. In some localized regions the increase in the Hall conductance can exceed 100%. On the contrary, the Pedersen conductance remains more or less unaffected by the inclusion of the PIXIE data. The calculated polar cap potential drop may decrease more than 10%, resulting in a reduction of the estimated Joule heating integrated over the Northern Hemisphere by up to 20%. Locally, Joule heating may decrease more than 50% in some regions. We also find that the calculated energy flux by precipitating electrons increases around 5% when including the PIXIE data. Combined with the reduction of Joule heating, this results in a decrease in the ratio between Joule heating and energy flux, sometimes exceeding 25%. An investigation of the relationship between Joule heating and the AE index shows a nearly linear correspondence between the two quantities, in accordance with previous studies. However, we find lower proportionality factors than reported by others when taking geomagnetic conditions into account, ranging between 0.13 and 0.23GW/nT. We also find that the contribution from auroral particles to the energy budget is more important than most previous studies have reported. Key words. Ionosphere (auroral ionosphere; particle precipitation) – Magnetospheric physics (storms and substorms)

Ring current intensification and convection‐driven negative bays: Multisatellite studies

The original view on the cause of ring current intensifications was a frequent occurrence of intense substorm expansion phases. Results from many studies have supported this view. However, whether this is the only mechanism of ring current buildup has been a controversy. Kamide [1992] asserted that ring current intensification is due to “sustained, southward IMF, not because of frequent occurrence of intense substorms.” Lui et al. [2001] have shown that the ring current can be intensified during enhanced convection without substorm occurrence. Tsurutani et al. [2003] have found that there was a lack of substorm expansion phases for long periods of time (up to 7 hours) in 5 out of 11 storm main phases (in 1997) that were induced by the smoothly varying Bz component of the interplanetary magnetic field (IMF) within interplanetary magnetic clouds. In this paper, a relatively weak magnetic storm event (with minimum SYM‐H at −47 nT) that occurred on 15 July 1997 is studied using ground‐based magnetograms, polar cap potentials from Super Dual Auroral Radar Network, and Los Alamos National Laboratory (LANL) geosynchronous energetic particle data as well as the Polar UV imaging (for aurorae) and Wind (for the solar wind) data. It is shown that during the storm main phase, there was a lack of substorm expansion phase activity (from imaging and the ground‐based data) and a lack of energetic particle injections at the geostationary orbit. The most prominent auroral forms were north‐south aligned auroral patches and torches. Dawn and dusk aurorae were more intense than the aurorae near midnight, where auroral gaps occurred. In addition, this paper shows that there was a significant directly driven activity during the storm main phase when the IMF was continually southward. We argue that during this event the ring current intensification was more strongly associated with enhanced magnetospheric convection than with impulsive energy unloading. Three scenarios are suggested to explain the relatively low intensity of the magnetic storm induced by a magnetic cloud. They are (1) weak nightside auroral zone ionospheric ion outflows (due to lack of substorms), (2) choked penetration of the tail plasma flow (due to lack of substorms), and (3) retarded magnetospheric convection (due to reduced solar wind‐magnetosphere reconnection). The observed saturation of the polar cap potential drop is in support of this latter mechanism.

Observed saturation of the ionospheric polar cap potential during the 31 March 2001 storm

Theoretical arguments and MHD simulations have suggested that the potential drop across the polar‐cap ionosphere approaches a constant value when the IMF is very large and southward. This idea has been difficult to test because the conditions producing this effect are extremely rare. During the 31 March 2001 storm the IMF was directed southward for over 6 hours with a magnitude of ∼36 nT to ∼20 nT while the DMSP‐F13 satellite crossed the polar region, obtaining a good measure of the true polar cap potential drop. The observed potentials are compared with predictions from a theoretical model of the saturation process [Hill et al., 1976; Siscoe et al., 2002]. Clear evidence of a non‐linear response consistent with the model predictions for height‐integrated Pedersen conductivities in the range of 5–10 S are shown.

Modeling the inner plasma sheet protons and magnetic field under enhanced convection

In order to understand the evolution of the protons and magnetic field in the inner plasma sheet from quiet to disturbed conditions, we incorporate a modified version of the Magnetospheric Specification Model (MSM) with a modified version of the Tsyganenko 96 (T96) magnetic field model to simulate the protons and magnetic field under an increasing convection electric field with two‐dimensional (2‐D) force balance maintained along the midnight meridian. The local time dependent proton differential fluxes assigned to the model boundary are a mixture of hot plasma from the mantle and cooler plasma from the low latitude boundary layer (LLBL). We previously used this model to simulate the inner plasma sheet under weak convection corresponding to a cross polar cap potential drop (ΔΦPC) equal to 26 kV and obtained 2‐D quiet time equilibrium for proton and magnetic field that agrees well with observations. We start our simulation for enhanced convection with this quiet time equilibrium and time‐independent boundary particle sources and increase ΔΦPC steadily from 26 to 146 kV in 5 hours. Simulations are also run separately to steady states by keeping ΔΦPC constant after it is increased to 98 and 146 kV. The magnitudes of proton pressure, number density, and temperature and their increase from quiet to moderate activity (ΔΦPC = 98 kV) are consistent with most observations. Our simulation at high activity (ΔΦPC = 146 kV) underestimates the observed pressure and temperature. This disagreement indicates possible dependence of the boundary particle sources on activity and possible effects of solar wind dynamic pressure enhancements that have not yet been included in our simulation. The simulated equatorial pressures and temperatures show stronger enhancement on the dusk side than on the dawn side as convection is increased, while density profiles show an increase on the dawn side and a decrease on the dusk side. The simulated proton flow speed at the equatorial plane increases with enhancing convection while the overall flow direction does not change significantly, a result of enhancement in both the earthward electric drift and the azimuthal diamagnetic drift. The equatorial magnetic field strength decreases more in the near‐Earth plasma sheet than at larger radial distances as ΔΦPC increases, resulting in an increasing flat radial profile with enhancing convection. The feedbacks from diamagnetic drift and magnetic fields to increasing convection are found to restrain the pressure increase. Based on the good agreement between our results and observations at moderate activity, our magnetic field indicates that the plasma and magnetic field in the plasma sheet can be in a state far from possible force balance inconsistency during periods of moderately enhanced convection. A scale analysis of our results shows that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet for moderate activity.

Ring Current Energy Input and Decay

A new view of the ring current as an active element in the geospace system has emerged in which the ring current responds not only to changing convection electric fields imposed by solar wind interactions but to internal dynamics of the magnetosphere-ionosphere-atmosphere (geospace) system. Variations in the plasma sheet density, temperature and composition, saturation of the polar cap potential drop (and presumably the cross-tail potential drop), modifications to the imposed convection potential in the inner magnetosphere due to ring current shielding effects, the presence of a pre-existing ring current population, storm-substorm coupling, and strong convection with and without accompanying substorm activity all have an impact on the ring current strength, formation and loss. All of these internal processes imply that the geoeffectiveness of a solar wind driver cannot be predicted on the basis of the characteristics of the driver alone but must reflect key aspects of the dynamically changing geospace environment, itself. This review gives a summary of new information on ring current input and decay processes focusing on implications for the global geospace response to solar wind drivers during magnetic storms and on open questions that can be addressed with new ENA imaging techniques.

Nonlinear response of the polar ionosphere to large values of the interplanetary electric field

During large geomagnetic storms the polar cap potential drop fluctuates, but its average value seldom reaches 150 kV and its peak value seldom reaches 200 kV. Since the quiet time potential drop is about 40 kV and since the interplanetary electric field (IEF) is greatly enhanced during storms, this relative constancy appears to be due to a saturation effect. This apparent saturation phenomenon has been previously noted but has not received much attention in the recent literature, in spite of its importance during magnetic storms and in spite of the fact that it seems not to be understood. The IEF and the polar cap potential drop appear to have a very linear relationship up to about 80 kV. At greater IEF the potential drop increases only slightly. We use the assimilative mapping of ionospheric electrodynamics procedure to illustrate this saturation on a case‐by‐case basis during a number of recent storms. We confirm using DE 2 data that the saturation of the potential drop is a real effect and is not an artifact of the measurement process. Observations of the plasmapause and the buildup of the ring current indicate a linear response to the IEF over a wide range of solar wind conditions. Thus the convection of the magnetospheric plasma and the injection of energy into the ring current seem not to be affected by this saturation.

An investigation of the influence of data and model inputs on assimilative mapping of ionospheric electrodynamics

The Geospace Environment Modeling (GEM) substorm challenge event of November 24, 1996, has been used as a test case to investigate the influence of different data and model inputs on the assimilative mapping of ionospheric electrodynamics (AMIE) outputs. During that period the interplanetary magnetic field (IMF) went from northward to southward and then returned to northward. In the later part of the day a moderate substorm with AL ∼ −600 nT took place. The AMIE convection patterns derived from the Super Dual Auroral Radar Network (SuperDARN) data alone are generally similar to those derived using ground magnetometer alone, especially during the relatively stable southward IMF period. However, some differences are found during the northward IMF period and during the substorm; namely, the reversed convection configuration near local noon imaged by SuperDARN is absent in the magnetometer observations while the strong convection on the nightside recorded by the magnetometers during the substorm expansion phase is not seen by the radars. Different conductance models do not seem to have a big effect on the large‐scale distributions of ionospheric convection and Joule heating, but they do alter the cross polar cap potential drop and the hemispheric integrated Joule heating rate by nearly a factor of 2. When the AMIE‐derived electric potential drop was compared with the in situ measurements along the satellite track, it is found that AMIE underestimated the potential drop by 20 kV, amounting to a 22% underestimation. The analysis of the AMIE results based on the SuperDARN and ground magnetometer data reiterates the view that the response of ionospheric convection to a sudden IMF southward turning is global and nearly simultaneous.

Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere‐Thermosphere‐lonosphere‐Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well‐known overshielding phenomenon corresponding to a sudden reversal of the low‐latitude electric field. For quasi‐periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential‐drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region‐2 field‐aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.

A magnetospheric specification model validation study: Geosynchronous electrons

The Rice University Magnetospheric Specification Model (MSM) is an operational space environment model of the inner and middle magnetosphere designed to specify charged particle fluxes up to 100 keV. Validation test data taken between January 1996 and June 1998 consist of electron fluxes measured by a charge control system (CCS) on a defense satellite communications system (DSCS) spacecraft. The CCS includes both electrostatic analyzers to measure the particle environment and surface potential monitors to track differential charging between various materials and vehicle ground. While typical RMS error analysis methods provide a sense of the models overall abilities, they do not specifically address physical situations critical to operations, i.e., how well does the model specify when a high differential charging state is probable. In this validation study, differential charging states observed by DSCS are used to determine several threshold fluxes for the associated 20–50 keV electrons and joint probability distributions are constructed to determine Hit, Miss, and False Alarm rates for the models. An MSM run covering the two and one-half year interval is performed using the minimum required input parameter set, consisting of only the magnetic activity index Kp, in order to statistically examine the model's seasonal and yearly performance. In addition, the relative merits of the input parameter, i.e., Kp, Dst, the equatorward boundary of diffuse aurora at midnight, cross-polar cap potential, solar wind density and velocity, and interplanetary magnetic field values, are evaluated as drivers of shorter model runs of 100 d each. In an effort to develop operational tools that can address spacecraft charging issues, we also identify temporal features in the model output that can be directly linked to input parameter variations and model boundary conditions. All model output is interpreted using the full three-dimensional, dipole tilt-dependent algorithms currently in operational use at the Air Force 55th Space Weather Squadron (55 SWXS). Results indicate that both diurnal and seasonal activity related variations in geosynchronous electrons are reproduced in a regular and consistent manner regardless of the input parameter used as drivers. The ability of the MSM to specify DSCS electrons in relation to thresholds indicative of spacecraft charging varies with the combination of input parameters used. The input parameter of greatest benefit to the MSM, after the required Kp index, is the polar cap potential drop as determined by DMSP spacecraft. Regarding the highest electron flux threshold, the model typically achieves high HIT rates paired with both high False Alarm rates and higher RMS error. Suggestions are made regarding the utilization of proxy values for the polar cap potential parameter and Kp-dependent model boundary conditions. The importance of generating accurate real-time proxy input data for operational use is stressed.

Response of the midtail electric field to enhanced solar wind energy input

We study the response of midtail plasma and field parameters to enhanced solar wind electric field input for two substorm intervals on November 22, 1995. The solar wind input signatures were quite different for these two substorms, which had major Pi2 onsets at 1108 and 1502 UT. The solar wind input for the 1108 UT substorm had a short timescale (∼0.5 hour) electric field enhancement up to 1.5 mV/m, whereas the solar wind electric field for the 1502 UT substorm continuously exceeded 1.5 mV/m for ∼2 hours. In association with the expansion phase onsets of both substorms, the electric field fluctuation in the midtail plasma sheet commenced. The electric field disturbances lasted for a time interval close to the length of time, in which the solar wind electric field was enhanced. The midtail plasma sheet electric field responded well to the enhanced solar wind input with a time delay of 45–80 min. The pressure decrease, which started at the onset of both substorms, ended quite differently for these two substorms: the pressure decreased until it approached the quiet time level in the course of the 1108 UT substorm, whereas the pressure dropped below the quiet time level during the expansion phase of the 1502 UT substorm and stayed at this low level until late in the recovery phase. Using polar cap potential drop and the pressure profile in the midtail, we estimate the relationship between the dayside and nightside potential drop. We suggest that the difference in the nightside flux transport rate could control the configuration of the midtail and could explain why the tail pressure responded differently during the expansion and recovery phase of the two substorms.

Analysis of the ionospheric cross polar cap potentialdrop and electrostatic potential distribution patternsduring the January 1997 cme event using DMSP data

On 6 January 1997 a coronal mass ejection (CME) occurred on the sun producing amagnetic cloud in the solar wind that passed the Earth on 10–11 January 1997 and caused ageomagnetic storm. During this period there were two polar-orbiting DMSP spacecraft (F12 andF13) at 840 km carrying the SSIES plasma instruments on board, which measure the bulk flowvelocity of the thermal ions of the ionospheric plasma. These data are used to calculate theelectric field and electrostatic potential along the flight track of the spacecraft in the polar regions.The time history of the observed total cross polar cap potential drop for this event is presentedalong with comparisons to the estimates from the Weimer model and observations of the aurorafrom the UVI instrument on the POLAR spacecraft. A surprisingly large potential drop of 235 kVis observed at one point and appears to be the result of a pressure pulse in the solar windtriggering an enhancement of the geomagnetic activity. An examination of the electrostaticpotential distribution patterns in the polar ionosphere during the event is also presented.

Foreword [to Special Section on The November 1993 Geomagnetic Storm

Foreword [to Special Section on The November 1993 Geomagnetic Storm

Analysis of the ionospheric cross polar cap potential drop using DMSP data during the National Space Weather Program study period

During the National Space Weather Program (NSWP) event study period of November 2–11, 1993 there were three operational DMSP meteorological satellites (F8, F10, and F11) in orbit, each carrying the Special Sensor for Ions, Electrons, and Scintillation (SSIES) plasma instrument package. Ion flow data from these instruments are used to determine the electrostatic potential drop across both the northern and southern polar caps. The magnitude and distribution of the potential are used to characterize the convection patterns present in the polar ionospheres. The results from all three satellites ate presented to show an overall observational history of the potential drop and the convection pattern during the study period. These observational parameters provide crucial inputs and checks to several ionospheric and magnetospheric models being used in this study. Evidence is presented of an unambiguous difference in the cross polar cap potential drop between the two hemispheres for an extended period of time.

Comparison of empirical and theoretical polar cap convection patterns for the January 1992 GEM interval

We compare empirical and theoretical models of polar cap boundary locations and ionospheric convection patterns for the Geospace Environment Modeling (GEM) Boundary Layer Campaign Interval during January 1992. The empirical model is described by Lyons [this issue], and the theoretical model is described by Toffoletto and Hill [1993]. The empirical and theoretical models agree on the large‐scale pattern of polar cap flow, including its dawn‐dusk deflection in response to the dawn‐dusk component of the interplanetary magnetic field (IMF). There is, however, a discrepancy with respect to the magnitude of the flow, as measured by the polar cap potential drop. This parameter is an input to the theoretical model, for given solar wind/IMF parameters, based on previous statistical studies, and the standard (empirical) input exceeds the empirical value obtained for these cases by a factor of order 2. There is also a discrepancy with respect to the size of the polar cap, which is smaller in the theoretical model than in the empirical one. The size discrepancy is partly, but not entirely, attributable to the potential‐drop discrepancy. The comparison provides clear directions for improvement of the theoretical model and perhaps also for refinement of the empirical modeling technique.

Empirical models of storm time equatorial zonal electric fields

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

HF‐radar observations of the dayside magnetic merging rate: A Geospace Environment Modeling boundary layer campaign study

Goose Bay HF‐radar data have been used to determine the dayside reconnection electric field which transports energy from the solar wind into the Earth's magnetosphere and ionosphere. The speed of the ionospheric plasma flow perpendicular to the open/closed boundary is determined in the rest frame of the boundary along each of the 16 beam directions of the HF radar. The observations were made during one of the Geospace Environment Modeling program's boundary layer campaigns. The period from 1200 to 1600 UT on March 29, 1992, was one of generally southward interplanetary magnetic field (IMF). The y component of the IMF was negative for most of the time. Despite the generally steady IMF conditions, the merging rate observed by the radar shows a great deal of temporal structure. The radar observations have been compared with the results from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. Initially, the merging inferred from the radar observations accounts for a significant portion of the total polar cap potential drop, suggesting that a majority of the potential drop was generated within the radar field of view and must therefore be due to magnetic merging at the magnetopause. At the end of the period, however, the potential drop derived from the radar measurements is distinctly less than that derived from the AMIE procedure. At that time, however, satellite and ground magnetometer data show that a substorm was in progress, and there is substantial evidence for a strong nightside contribution to the polar cap potential drop. An additional feature that appears in this data set is that the orientation of the open/closed magnetic field separatrix with respect to magnetic latitude is well correlated to the y component of the IMF.

Empirical polar cap potentials

DMSP satellite plasma flow data from 1987–1990 are used to derive empirical models of the polar cap potential for quasi‐steady interplanetary magnetic field (IMF) conditions. The large data set, due to the high duty cycle and nearly Sun synchronous DMSP orbits, allowed very stringent data selection criteria. The analysis indicates that a good description of the unskewed (Heppner Maynard pattern A) steady state polar cap potential is ΦA = 10−4v2+11.7B sin3 (θ/2) kV, where v is the solar wind velocity in kilometers per second, B is the magnitude of the interplanetary magnetic field in nanoteslas, and θ = arccos (Bz/|B|)GSM. The IMF‐dependent contribution to the cross polar cap potential does not depend significantly on solar wind pressure. Functional forms for the potential do benefit from inclusion of an IMF independent term proportional to the solar wind flow energy. Best fits to IMF‐independent contributions to the steady state polar cap potential yield ∼16 kV for vsw = 400 kilometers per second. During steady IMF the total unskewed polar cap potential drop is shown to be approximately ΦA = 16.5 + 15.5 Kp kV. The distribution of potential around the polar cap is examined as a function of magnetic local time. A sinusoidal distribution is an excellent description of the distribution, and more complex forms are not justified by this data set. Analysis of this data set shows no evidence of saturation of the polar cap potential for large |MF|. A simple unified description of the polar cap potential at all magnetic local times (MLT) and IMF, Φ(IMF, MLT) = −4.1 + 0.5 sin ((2π/24) MLT + 0.056 + 0.015 Byeff) (1.1 × 10–4 v2 + 11.1 B sin3 (θ/2)) kV, is generated, where Byeff is By (−BY) in the northern (southern) hemisphere. If IMF data is unavailable, the polar cap potential is well described by ΦA(Kp, MLT) = − 4.1 + 1/2 sin ((2π/24) MLT + ϕHM)(16.4 + 15.2 Kp) kV, where ÖϕHM is a small phase correction of (−0.054, −0.031, 0.040) for Heppner‐Maynard convection patterns (BC, A, DE), respectively.