Polar Cap Potential Drop Research Articles

State-switching Mechanism of Intermittent Pulsars

Abstract The emission feature of intermittent pulsars is significant for understanding the pulsar emission mechanism. Using the observational evidence of radio emissions turning on and off and the corresponding spin-down rates in these two states of an intermittent pulsar, we will examine the polar-cap potential drop, gap height, and curvature radii of a few intermittent pulsars within the regime of the pulsar polar-cap emission theory by applying the current loss and energy flux of particle flow to pulsar braking, which are generally associated with radio emission from the polar cap. It is seen that the polar-cap parameters of the intermittent pulsars are almost equal to their maximum values, which is the main prediction of the pulsar polar-cap theory with respect to the breaking of the radio emission. It is also noticed that the intermittent pulsars are distributed near the dipole death line in the diagram, which is consistent with their emission features and the calculated polar-cap parameters. To further confirm the state switching of the intermittent pulsar, the relationships among spin-down rate, gap height, potential drop, and activity duty cycles of PSR B1931+24 are discussed. It is found that the gap height has an anticorrelation with the activity duty cycle, which indicates that the intermittency of the radio emission has a close connection to the gap height, as indicated by the pulsar polar-cap emission theory.

How Do Auroral Substorms Depend on Earth's Dipole Magnetic Moment?

AbstractEarth's dipole magnetic moment M is known to decrease by ∼9% over the past 150 years. It has been argued that the decrease in M makes the near‐Earth space environment different. We investigated how the change in M affects the development of an auroral substorm by increasing and decreasing M by a factor of 1.5 in global magnetohydrodynamics simulation. The ionospheric conductivity decreases with increasing M, in accordance with the aid of empirical relations. When we imposed the southward interplanetary magnetic field, an auroral substorm took place regardless of M, but its development depends largely on M. When M is lower, (1) the expansion onset takes place later, (2) the auroral electrojet develops slowly, and (3) the maximum auroral electrojet increases. The first two consequences are probably associated with the slow magnetospheric convection as manifested by the polar cap potential drop. The third consequence is associated with the nonlinear dependence of substorm‐associated field‐aligned currents (FACs) on the ionospheric conductivity. The maximum values of the westward auroral electrojet and the net FACs increase with decreasing M, whereas the incident magnetic energy into the magnetosphere decreases with decreasing M. This implies that the efficiency of the generation of the substorm‐associated FACs increases with decreasing M. It is also found that, for the lower M‐value, the auroral oval shifts equatorward during the growth phase and expands more equatorward and poleward during the expansion phase. Evolution of substorms depends largely on the value of Earth's dipole moment and the ionospheric conductivity.

Strong induction effects during the substorm on 27 August 2001

We report on strong induction effects notably contributing to the cross polar cap potential drop and the energy balance during the growth and active phases of the substorm on 27 August 2001. The inductance of the magnetosphere is found to be crucial for the energy balance and electrical features of the magnetosphere in the course of the substorm. The inductive response to the switching on and off of the solar wind-magnetosphere generator exceeds the effect of the interplanetary magnetic field (IMF) variation. The induction effects are most apparent during the substorm expansion onset when the rapid growth of the ionospheric conductivity is accompanied by the fast release of the magnetic energy stored in the magnetotail during the growth phase. Using the magnetogram inversion technique, we estimated the magnetospheric inductance and effective ionospheric conductivity during the loading and unloading phases.

Polar cap response to the solar wind density jump under constant southward IMF

Sharp changes of the solar wind parameters determining the dynamic pressure jump lead to strong magnetosphere-ionosphere disturbances. Here the effect on the Earth’s ionospheric high latitudes of the solar wind dynamic pressure pulse caused only by the increase of the interplanetary plasma density under southward constant IMF is considered. We investigate reaction of the cross-polar cap potential on the increase of AL index and/or jump of the solar wind density. It is found that for the case of 10 January 1997 the main contribution to the polar cap potential drop increase gave the growth of AL index relative to the input of the solar wind density jump. We also study the influence of the solar wind density increase on the crosspolar cap potential for the quiet magnetospheric conditions. It occurred that the polar cap potential difference decreases with the great increase of the interplanetary plasma density. For the disturbed magnetosphere the main role in the polar cap potential drop increase plays increase of AL. Thus, we found the change of the cross-polar cap potential due to the AL index variations and/or the solar wind density drop even in a case when the interplanetary electric field is constant.

Perturbation electric fields and disturbance currents investigated during the 25 September 1998 great storm

AbstractThis study investigates the ionosphere's response to the 25 September 1998 great storm and utilizes multi‐instrument observations covering the Australian (140°E; geographic) and Indian (75°E) longitude sectors. Results show the domination of eastward (westward) electrojet at 140°E (75°E). Its causative net eastward (westward) perturbation electric (E) field drove the forward (reverse) plasma fountain, caused the presence (absence) of the equatorial ionization anomaly (EIA). These strong longitudinal differences were due to a combination of various LT‐dependent, E field‐driven, and competing mechanisms. Perturbation E fields are identified as prompt penetration E field (PPEF) and disturbance dynamo E field (DDEF). Due to the later (earlier) local time at 140°E (75°E), the undershielding PPEF was eastward (westward) directed early in the main phase. A series of periodic substorms occurred during the recovery phase. The substorm‐related eastward PPEFs became overpowered by westward DDEFs over India but remained dominant in the Australian sector. Thus, eastward PPEFs (westward DDEFs) dominated at 140°E (75°E). At 140°E these eastward PPEFs exhibited a strong positive correlation with the variations of both the cross polar cap potential drop and the asymmetric ring current, significantly increased the net equatorial upward E × B drift and thus caused EIA development with plasma bubbles scintillating GPS signals. Based on the strong and independent correlation of these asymmetric ring current events with both the EIA development and the scintillation activity, we propose that the ASY‐H index could provide a natural tool for modeling EIA development and scintillation episodes during severe storms characterized by periodic substorm‐related eastward PPEFs.

Large plasma density enhancements occurring in the northern polar region during the 6 April 2000 superstorm

AbstractWe focus on the ionospheric response of northern high‐latitude region to the 6 April 2000 superstorm and aim to investigate how the storm‐enhanced density (SED) plume plasma became distributed in the regions of auroral zone and polar cap plus to study the resultant ionospheric features and their development. Multi‐instrument observational results combined with model‐generated, two‐cell convection maps permitted identifying the high‐density plasma's origin and the underlying plasma transportation processes. Results show the plasma density feature of polar cap enhancement (PCE; ~600 × 103 i+/cm3) appearing for 7 h during the main phase and characterized by increases reaching up to 6 times of the quiet time values. Meanwhile, strong westward convections (~17,500 m/s) created low plasma densities in a wider region of the dusk cell. Oppositely, small (~750 m/s) but rigorous westward drifts drove the SED plume plasma through the auroral zone, wherein plasma densities doubled. As the SED plume plasma traveled along the convection streamlines and entered the polar cap, a continuous enhancement of the tongue of ionization (TOI) developed under steady convection conditions. However, convection changes caused slow convections and flow stagnations and thus segmented the TOI feature by locally depleting the plasma in the affected regions of the auroral zone and polar cap. From the strong correspondence of polar cap potential drop and subauroral polarization stream (SAPS), we conclude that the SAPS E‐field strength remained strong, and under its prolonged influence, the SED plume provided a continuous supply of downward flowing high‐density plasma for the development and maintenance of PCEs.

Ionospheric Response to Solar Wind Pressure Pulses Under Northward IMF Conditions

Enhancements of aurora and auroral electrojets in response to sudden compression of the magnetosphere by shocks/ pressure pulses are well known and have been attributed by some to compression-enhanced magnetic field reconnection. To examine such a view, we analyze a fortuitous event that is comprised of a series of pressure pulses (< 20 min) on November 8, 2000. These pressure pulses were preceded by a large, northward interplanetary magnetic field (IMF) that lasted more than 15 hours such that effects from reconnection can be minimized. Auroral images acquired by ultraviolet imager on board the Polar satellite clearly show intensifications of the aurora that occurred first near local noon and progressively extended from dayside to nightside. The area-integrated global auroral power reached ~30 gigawatts (GW). It is also found that the global auroral power is well correlated with the solar wind dynamic pressure (correlation coefficient r ~0.90), rather than the change in the solar wind dynamic pressure. In-situ measurements of particle data from the Defense Meteorological Satellite Program satellite indicate that the magnetospheric source for the pressure-enhanced auroras is most likely the central plasma sheet. Other ionospheric parameters such as the auroral electrojet (AE index), magnetic storm index (Sym-H), and the cross polarcap potential drop also show a one-to-one correspondence to the pressure pulses. In one instance the auroral electrojets AE index reached more than 200 nT, the cross polar-cap potential drop (φpc) inferred from the SuperDARN radar network ionospheric plasma convection increased to ~60 kV. The observed increases in the auroral emissions, AE, and polar cap potential were not associated with substorms. Our result strongly suggests that solar wind pressure pulses are an important source of geomagnetic activity during northward IMF periods.

Initial results from a dynamic coupled magnetosphere‐ionosphere‐ring current model

In this paper we describe a coupled model of Earth's magnetosphere that consists of the Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamics (MHD) simulation, the MIX ionosphere solver and the Rice Convection Model (RCM) and report some results using idealized inputs and model parameters. The algorithmic and physical components of the model are described, including the transfer of magnetic field information and plasma boundary conditions to the RCM and the return of ring current plasma properties to the LFM. Crucial aspects of the coupling include the restriction of RCM to regions where field‐line averaged plasma‐β≤ 1, the use of a plasmasphere model, and the MIX ionosphere model. Compared to stand‐alone MHD, the coupled model produces a substantial increase in ring current pressure and reduction of the magnetic field near the Earth. In the ionosphere, stronger region‐1 and region‐2 Birkeland currents are seen in the coupled model but with no significant change in the cross polar cap potential drop, while the region‐2 currents shielded the low‐latitude convection potential. In addition, oscillations in the magnetic field are produced at geosynchronous orbit with the coupled code. The diagnostics of entropy and mass content indicate that these oscillations are associated with low‐entropy flow channels moving in from the tail and may be related to bursty bulk flows and bubbles seen in observations. As with most complex numerical models, there is the ongoing challenge of untangling numerical artifacts and physics, and we find that while there is still much room for improvement, the results presented here are encouraging.

Response of the topside ionosphere to high-speed solar wind streams

[1] During an 80 d period in 2008 the ionosphere was driven very coherently by periodic high-speed solar wind streams. An examination of the high-latitude ionospheric dynamics during this period indicates that changes in the polar cap potential drop from 20 to 60 kV induce global changes in the dynamics, composition, and temperature of the topside ionosphere. These changes in the potential are small compared to those usually seen during times of higher solar activity. Changes in the polar cap potential produce significant increases in the frictional heating of the ions and neutral gas associated with antisunward flows in the polar cap. These increases are accompanied by expansions in the latitude extent over which the return sunward flows exist. Thus, no significant increase in the magnitude of the flows is seen in this region, and the plasma in the auroral zone remains relatively cool compared to that seen in the polar cap. Associated with changes in the high-latitude convection are changes in the topside ion temperature of 100 to 150 K that extend from the auroral zone itself to latitudes as low as 20°.

Statistical study of the effect of ULF fluctuations in the IMF on the cross polar cap potential drop for northward IMF

[1] Recent studies showed that, regardless of the orientation of the Interplanetary Magnetic Field (IMF), ULF wave activity in the solar wind can substantially enhance the convection in the high latitude ionosphere, suggesting that ULF fluctuations may also be an important contributor to the coupling of the solar wind to the magnetosphere‐ionosphere system. We conduct a statistical study to understand the effect of ULF power in the IMF on the cross polar cap potential, primarily focusing on northward IMF. We have analyzed the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) calculations of the polar cap potential, a IMF ULF index that is defined as the logarithm of Pc5 ULF power in IMF, and solar wind velocity and dynamic pressure for 249 days in 2003. We find that, separated from the effects of solar wind speed and dynamic pressure, the average cross polar cap potentials show a roughly linear dependence on the ULF index, with a partial correlation coefficient of 0.19. Highly structured convection flow patterns with a number of localized vortices are often observed under fluctuating northward IMF. For such a convection configuration, it is hard to estimate properly the cross polar cap potential drop, as the enhanced flows around the vortices that may be associated with IMF fluctuations do not necessarily yield a large potential drop. Thus, despite the relatively small correlation coefficient, the linear trend we found gives support to the significant role of IMF ULF fluctuations on the coupling of the solar wind to the magnetosphere‐ionosphere system.

A statistical study of the inner edge of the electron plasma sheet and the net convection potential as a function of geomagnetic activity

[1] A widely accepted explanation of the location of the inner edge of the electron plasma sheet and its dependence on electron energy is based on drift motions of individual particles. The boundary is identified as the separatrix between drift trajectories linking the tail to the dayside magnetopause (open paths) and trajectories closed around the Earth. A statistical study of the inner edge of the electron plasma sheet using THEMIS Electrostatic Analyzer plasma data from November 2007 to April 2009 enabled us to examine this model. Using a dipole magnetic field and a Volland‐Stern electric field with shielding, we find that a steady state drift boundary model represents the average location of the electron plasma sheet boundary and reflects its variation with the solar wind electric field in the local time region between 21:00 and 06:00, except at high activity levels. However, the model does not reproduce the observed energy dispersion of the boundaries. We have also used the location of the inner edge of the electron plasma sheet to parameterize the potential drop of the tail convection electric field as a function of solar wind electric field (Esw) and geomagnetic activity. The range of Esw examined is small because the data were acquired near solar minimum. For the range of values tested (meaningful statistics only for Esw < 2 mV/m), reasonablygoodagreementisfoundbetweenthepotentialdropofthetailconvectionelectric field inferred from the location of the inner edge and the polar cap potential drop calculated from the model of Boyle et al. (1997).

Occurrence and properties of substorms associated with pseudobreakups

We investigate how substorms with and without growth‐phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth‐phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field Em is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating Em over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much Em reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth‐phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with Em, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by Em and AE, respectively. Comparing growth‐phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth‐phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth‐phase pseudobreakups or substorms.

Energetics of the magnetospheric superstorm on November 20, 2003

The magnetospheric storm on November 20, 2003 was one of two greatest events in 1957–2003. The Dst* index reached −472 nT, the polar cap potential drop exceeded 200 kV, the polar cap boundary expanded up to Φ = 60°, the plasma layer density in the synchronous orbit reached 5 cm−3, and the inner edge of the plasma sheet penetrated up to L ∼ 1.5RE. The sequence of disturbance modes including some previously unknown is described. The distribution of the total power input into the magnetosphere between the ionosphere (power Qi) and the ring current (QDR), as well as the relative roles of the spontaneous substorms and the driven disturbances in the creation of the DR current, is analyzed. The values of the parameter α = QDR/Qi are calculated with a step of 5 min. It is shown that intervals with α ≪ 1 and with maximums α ≫ 1 were observed in the events under consideration. These results contradict the dominant opinion that the energy input into the magnetosphere during disturbances is primarily dissipated in the ionosphere. The two types of α maximums are observed: one in the mode of a prevailing spontaneous substorm and the other in the mixed mode of the substorm and driven disturbance. It is concluded that both types of the maximums and corresponding enhancements of the DR current appeared due to the plasma turbulization processes in the substorm current wedge. The parameter α appears to slowly increase from α ≪ 1 to α > 1 with increasing activity level; this trend supports the driven model of creating the DR current due to an increase in the electric field of the solar wind.

Equatorial Ionospheric Electric Fields During the November 2004 Magnetic Storm

We use radar measurements from the Jicamarca Radio Observatory, magnetometer observations from the Pacific sector and ionosonde data from Brazil to study equatorial ionospheric electric fields during the November 2004 geomagnetic storm. Our data show very large eastward and westward daytime electrojet current perturbations with lifetimes of about an hour (indicative of undershielding and overshielding prompt penetration electric fields) in the Pacific equatorial region during the November 7 main phase of the storm, when the southward IMF, the solar wind and reconnection electric fields, and the polar cap potential drops had very large and nearly steady values. This result is inconsistent with the recent suggestion that solar wind electric fields penetrate without attenuation into the equatorial ionosphere for several hours during storm main phase. The largest daytime prompt penetration electric fields (about 3 mV/m) ever observed over Jicamarca occurred during the November 9 storm main phase, when large equatorial electrojet current and drift perturbations were also present in the Pacific and Brazilian equatorial regions. The rise and decay times of these equatorial electric fields were about 20 min longer than of the corresponding solar wind electric fields. The ratios of prompt penetration electric fields and corresponding solar wind electric field changes were highly variable even during the day, and had largest values near dawn. Also, the prompt penetration electric fields did not show polar cap potential drop saturation effects. Our results clearly highlight that the relationships of prompt penetration and solar wind electric fields, and polar cap potentials are far more complex than implied by simple proportionality factors.

Understanding storm‐time ring current development through data‐model comparisons of a moderate storm

With three components, global magnetosphere (GM), inner magnetosphere (IM), and ionospheric electrodynamics (IE), in the Space Weather Modeling Framework (SWMF), the moderate storm on 19 May 2002 is globally simulated over a 24‐hour period that includes the sudden storm commencement (SSC), initial phase, and main phase of the storm. Simulation results are validated by comparison with in situ observations from Geotail, GOES 8, GOES 10, Polar, LANL MPA, and the Sym‐H and Dst indices. It is shown that the SWMF is reaching a sophistication level for allowing quantitative comparison with the observations. Major storm characteristics at the SSC, in the initial phase, and in the main phase are successfully reproduced. The simulated plasma parameters exhibit obvious dawn‐dusk asymmetries or symmetries in the ring current region: higher density near the dawn and higher temperature in the afternoon and premidnight sectors; the pressure is highest on the nightside and exhibits a near dawn‐dusk symmetry. In addition, it is found in this global modeling that the upstream solar wind/IMF conditions control the storm activity and an important plasma source of the ring current is in the solar wind. However, the ionospheric outflow can also affect the ring current development, especially in the main phase. Activity in the high‐latitude ionosphere is also produced reasonably well. However, the modeled cross polar cap potential drop (CPCP) in the Southern Hemisphere is almost always significantly larger than that in the Northern Hemisphere during the May storm.

High latitude convection based on long-term incoherent scatter radar observations in North America

High latitude ionospheric convection patterns and models are obtained based on a composite analysis of incoherent scatter radar (ISR) observations in the North America sector at Millstone Hill and Sondrestrom (MHS). These observations span a combined latitude range of 55 ∘ ⩽ Λ ⩽ 82 ∘ over time periods starting from 1978 for Millstone Hill and from 1995 for Sondrestrom. These models, regional to the America sector, represent average and steady patterns of the convection as driven by the interplanetary magnetic field (IMF) By and Bz or by Kp and the By direction, and update the existing MHS models. This paper describes the ISR and IMF data processing and modeling technique and discusses convection changes under a variety of conditions. In particular, we have examined By and Bz effects on the electrostatic potential at dawn and dusk, the location of the cells, and the shape of the cells; we also present the convection change in response to magnetic activity. This long-term data set has enabled us to study seasonal variations of the convection. The cross polar cap potential is on average higher in equinox than in solstice. With the advance of season from winter to summer, the convection cells appear to move antisunward, and the axis across the two cells rotates toward earlier local times; meanwhile, for By negative, the crescent-shaped dawn cell and circular dusk cell become well defined with a reduced cross polar cap potential, while for By positive, the cross polar cap potential tends to be higher. The dusk–dawn cell similarity in summer is found to be high for By negative and low for By positive, supportive of the theory of lobe cell generation in summer. Because of the asymmetry of the convection response to By negative and positive, and of seasonal differences in the convection, it is speculated that there exist perceptible hemispheric differences in the cross polar cap potential drop.

Magnetopause mapping to the ionosphere for northward IMF

Abstract. We study the topological structure of the magnetosphere for northward IMF. Using a magnetospheric magnetic field model we study the high-latitude response to prolonged periods of northward IMF. For forced solar wind conditions we investigate the location of the polar cap region, the polar cap potential drop, and the field-aligned acceleration potentials, depending on the solar wind pressure and IMF By and Bx changes. The open field line bundles, which connect the Earth's polar ionosphere with interplanetary space, are calculated. The locations of the magnetospheric plasma domains relative to the polar ionosphere are studied. The specific features of the open field line regions arising when IMF is northward are demonstrated. The coefficients of attenuation of the solar wind magnetic and electric fields which penetrate into the magnetosphere are determined.

Relationship between Region 2 field‐aligned current and the ring current: Model results

Detailed investigations on the relationship between the ring current and Region 2 field‐aligned currents (FACs) have been tenuous. Using the Comprehensive Ring Current Model (CRCM), we first studied their relationship for two GEM storm events (22 April 2001 and 21 October 2001) through model and data comparison. The simulated Region 2 FACs for two different magnetic storm events capture major characteristics of the Iridium measurements, and the simulated ENA flux has a reasonable agreement with the IMAGE/HENA data. We then examined the effects of polar cap potential drop, ionospheric conductivity, plasma sheet density and different magnetic field models on the development of Region 2 FACs, and the relationship between Region 2 FACs and the ring current, either through one of the two GEM events or the 12 August 2000 storm. It is shown that an increase in polar cap potential, ionospheric conductivity, or plasma sheet density generally results in an increase in Region 2 FACs' intensity, but Region 2 FACs display different local time and latitudinal distributions for changes in each parameter due to the different mechanisms involved. The CRCM run in the Tsyganenko 2004 magnetic field model for storm times (T04S), which includes a partial ring current, produces the dawn‐dusk asymmetry in Region 2 FACs during geomagnetically active times, as reported by Anderson et al. (2005): more intense currents flow into the ionosphere around dusk, compared to those that flow out of the ionosphere around dawn, and the dusk currents shift further equatorward than do the dawn currents.

Time constants of Earth's magnetotail for responding to a southward excursion of IMF: The super‐high‐pressure magnetotail event of 10 December 1996

Geomagnetic and plasma conditions in the nightside magnetosphere and the northern polar region are examined in detail for the first 6 hours of 10 December 1996. In association with a southward turning of the interplanetary magnetic field, the polar cap potential drop began to increase and maximized within several minutes after the peak of the energy inflow from the solar wind into the magnetosphere. The total pressure observed by Geotail in the midmagnetotail, however, continued increasing for ∼50 min after the peak of the polar cap potential drop, resulting in an exceptionally high total pressure in the midmagnetotail. Auroral images from Polar and magnetograms from ground stations show that there was no major breakup of substorms during the period concerned. This unique event provides us with an opportunity to study the solar wind–magnetosphere coupling without any intense disturbances associated with major substorms. Using an equivalent circuit model, it is attempted to reproduce variations in the polar cap potential drop and the midtail total pressure. The time lag between them is particularly focused on in this study. It is found that the characteristic time constants of the equivalent circuit model for this event are significantly longer than those estimated in previous papers.

Dependencies of high‐latitude plasma convection: Consideration of interplanetary magnetic field, seasonal, and universal time factors in statistical patterns

The database of the nine radars of the Super Dual Auroral Radar Network (SuperDARN) in the northern hemisphere has been analyzed for information on factors that influence the convection of plasma in the high‐latitude ionosphere. The velocity measurements were collected over the period 1998–2002. The data were first used to derive a new statistical model of convection that improves upon the earlier one‐radar model of Ruohoniemi and Greenwald (1996) in its specification of the dependence of the convection pattern on the magnitude and direction of the IMF in the GSM Y‐Z plane. We then derived average patterns for secondary sortings by season, year, and radar. Such dependencies as emerged were most clearly seen by contrasting the results for By+ and By−. The seasonal effect in the convection pattern is found to have similarities to that of the sign of By. In particular, the combination of By+/summer (By−/winter) reinforces the tendency of the By sign factor to sculpt the dusk and dawn cells into more round/crescent (crescent/round) shapes and to shift the crescent cell across the midnight MLT meridian. However, these combinations are associated with lower estimates of the total cross polar cap potential drop, ΦPC, while the nonreinforcing combinations produce elevated ΦPC, especially By−/summer. There is an overall tendency for ΦPC to increase from winter to summer, although the pure seasonal effect on the potential drop is weaker than that of the By−sign/season factor. We did not find pronounced differences among the patterns derived for the 5 individual years, which spanned the most recent interval of solar cycle maximum. Sorting by radar, we found few differences among the patterns for By+, but for By−, variations emerged that are consistent with a possible dependence on universal time (UT). The impacts of season and UT on convection in the high‐latitude ionosphere thus depends on the IMF, especially the sign of By. We speculate that variability in the ionospheric conductivity has a greater effect on magnetosphere‐ionosphere coupling under By− conditions.