Transpolar Potential Research Articles

Automated Technique for the Detection of Step‐Like Solar Wind Dynamic Pressure Changes: Application to the Response of the Transpolar Potential to Solar Wind Dynamic Pressure Fronts

AbstractThe input from the solar wind (SW) and the accompanying Interplanetary Magnetic Field (IMF) is the defining feature of the solar wind‐magnetosphere interactions. One of the most important SW drivers is the solar wind dynamic pressure, and most significantly its sharp changes known as pressure fronts. Sudden enhancements in solar wind dynamic pressure, , have been shown to lead to intensification of the auroral electrojets (and Region 1 field‐aligned currents), increase in auroral emissions, and significant enhancement in ionospheric convection and the transpolar potential. Case studies of the interaction between pressure fronts and the terrestrial space environment shed great light on their importance to the structure and dynamics of the system. However, thorough understanding of the interaction requires fully statistical investigations. These statistical studies require a large number of suitable pressure front events. In the past these have been obtained through laborious manual inspection of solar wind data. We present an automated procedure that identifies sudden step enhancements in that satisfy a desired set of limiting step‐like characteristics, such as magnitude of the change, sign of the change (positive/negative), and rate of change. The procedure yields a list of suitable events, along with their step features within minutes. The technique is tested for missed events and false positives by direct comparison to manual inspection of the data. Its utility is illustrated by application to the statistical effect of pressure fronts to the transpolar potential as measured by the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique.

Changes in the Magnetic Field Topology and the Dayside/Nightside Reconnection Rates in Response to a Solar Wind Dynamic Pressure Front: A Case Study

AbstractOne of the most significant observations associated with a sharp enhancement in solar wind dynamic pressure, , is the poleward expansion of the auroral oval and the closing of the polar cap. The polar cap shrinking over a wide range of magnetic local times (MLTs), in connection with an observed increase in ionospheric convection and the transpolar potential, led to the conclusion that the nightside reconnection rate is significantly enhanced after a pressure front impact. However, this enhanced tail reconnection has never been directly measured. We demonstrate the effect of a solar wind dynamic pressure front on the polar cap closure, and for the first time, measure the enhanced reconnection rate in the magnetotail, for a case occurring during southward background Interplanetary Magnetic Field (IMF) conditions. We use Polar Ultra‐Violet Imager (UVI) measurements to detect the location of the open‐closed field line boundary, and combine them with Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials to calculate the ionospheric electric field along the polar cap boundary, and thus evaluate the variation of the dayside/nightside reconnection rates. We find a strong response of the polar cap boundary at all available MLTs, exhibiting a significant reduction of the open flux content. We also observe an immediate response of the dayside reconnection rate, plus a phased response, delayed by ∼15–20 min, of the nightside reconnection rate. Finally, we provide comparison of the observations with the results of the Open Geospace General Circulation Model (OpenGGCM), elucidating significant agreements and disagreements.

Field-aligned currents and magnetospheric generator in experiments on a laser-produced plasma flowing around a magnetic dipole

A laboratory experiment on modeling the magnetospheric generator of the field-aligned currents and the Earth’s transpolar potential in the absence of IMF is illustrated. The measurements of the total field-aligned current in the generator shorted mode and the transpolar potential in the circuit disconnection mode made it possible to determine the generator internal resistance. A model that explains the saturation current and internal resistance by the feedback between the field-aligned current and plasma flank motions has been proposed. This feedback is described through the effective resistance, which is proportional to the flow rate and the ratio of the boundary layer to the dimension of the magnetosphere. For the experimental conditions, the calculated generator resistance was in good agreement with the measured value. The estimates for the Earth’s magnetosphere indicate that the MHD generator internal resistance in the boundary layer is usually much lower than the reverse integral conductivity of the ionosphere.

Local time resolved dynamics of field‐aligned currents and their response to solar wind variability

AbstractUsing 10 years of CHAMP measurements condensed into the empirical model of field‐aligned currents through empirical orthogonal function analysis, the dynamics of field‐aligned currents (FACs) is modeled and studied in separate magnetic local time (MLT) sectors. We investigate the distributions of FAC intensity and latitude and evaluate their predictability in terms of geospace parameters which are ranked according to their relative importance measured by a multivariate regression procedure. The response time to changes in solar wind variables is studied in detail and found to be much shorter for dayside FACs than on the nightside (15–25 min versus 35–95 min). Furthermore, dayside FACs can be parameterized more accurately: R2 values maximize greater than 0.7 for FAC latitude and greater than 0.3 for FAC intensity, whereas the corresponding values on the nightside are smaller than 0.3 and 0.15, respectively. The results support the separation between directly driven coupling processes acting on the dayside and unloading processes controlling the nightside. In addition, the MLT‐resolved standardized regression coefficients suggest that (1) FAC latitude is affected most significantly by the transpolar potential, substorm evolution, solar activity as represented by the F10.7 index and its square, and the dipole tilt; (2) Region‐1/2 current intensity is controlled most efficiently by substorm evolution, IMF Bz and IMF By; and (3) cusp current intensity is influenced by conductivity, IMF By and their cross item.

The relation between transpolar potential and reconnection rates during sudden enhancement of solar wind dynamic pressure: OpenGGCM‐CTIM results

AbstractThis study investigates how solar wind energy is deposited into the magnetosphere‐ionosphere system during sudden enhancements of solar wind dynamic pressure (Psw), using the coupled Open Geospace General Circulation Model–Coupled Ionosphere Thermosphere Model (OpenGGCM‐CTIM) 3‐D global magnetosphere‐ionosphere‐thermosphere model. We simulate three unique events of solar wind pressure enhancements that occurred during negative, near‐zero, and positive interplanetary magnetic field (IMF) Bz. Then, we examine the behavior of the dayside and nightside reconnection rates and quantify their respective contributions to cross polar cap potential (CPCP), a proxy of ionospheric plasma convection strength. The modeled CPCP increases after a Psw enhancement in all three cases, which agrees well with observations from the Defense Meteorological Satellite Program spacecraft and predictions from the assimilative mapping of ionospheric electrodynamics technique. In the OpenGGCM‐CTIM model, dayside reconnection increases within 9–13 min of the pressure impact, while nightside reconnection intensifies about 13–25 min after the pressure increase. As the strong Psw compresses the dayside magnetosheath and, subsequently, the magnetotail, their magnetic fields intensify and activate stronger antiparallel reconnection on the dayside magnetopause first and near the central plasma sheet second. For southward IMF, dayside reconnection contributes to the CPCP enhancement 2–4 times more than nightside reconnection. For northward IMF, the dayside contribution weakens, and nightside reconnection contributes more to the CPCP enhancement. We find that high‐latitude magnetopause reconnection during northward IMF produces sunward ionospheric plasma convection, which decreases the typical dawn‐to‐dusk ionosphere electric field. This results in a weaker dayside reconnection contribution to the CPCP during northward IMF.

Simulated ring current response during periods of dawn‐dusk oriented interplanetary magnetic field (B y )

[1] Ring current formation is mainly attributed to enhanced global magnetospheric convection and particle injection. One of the indicators of enhanced global magnetospheric convection is the transpolar potential. The transpolar potential has been shown to respond to dawn-dusk oriented interplanetary magnetic field (IMF), enhancing as the IMF magnitude grows. This suggests that the ring current should respond to dawn-dusk oriented IMF. This work examines the ring current response during periods of dawn-dusk oriented IMF using the Lyon-Fedder-Mobarry and Comprehensive Ring Current Model simulations. Exploring three hypotheses, this work shows through simulation results that the ring current does respond during periods of dawn-dusk oriented IMF, that the inner magnetospheric response is different for periods of dawn-dusk oriented IMF than for periods of southward IMF, and that these differences are attributed to both lower magnetospheric convection on closed field lines and the location of the reconnection region on the nightside. Specifically, the simulation results show that as the magnitude of the dawn-dusk oriented IMF increases, producing a corresponding increase in the transpolar potential, the ring current response increases. The response is always much less than a comparable southward IMF would produce. This lower response is due to both magnetospheric convection on closed field lines, which builds the ring current but at a slower rate, and flank reconnection, which allows energy to flow through the inner magnetosphere without building the ring current plasma population (as a conduit).

Measurement of the transpolar potential in laboratory magnetosphere

Results of measurements of the transpolar potential in a laboratory magnetosphere are presented. Its approximately linear dependence on the kinetic energy of ions of the incoming flow is found. The measurements of the electric potential in plasma have shown the presence of an asymmetry along the dawn-dusk line. Near the boundary layer at the dawn side, the potential is systematically higher than the average values, while at the dusk side it is systematically lower. The observed difference in the plasma potential in the lowlatitude equatorial part of the magnetosphere by its sign and magnitude approximately corresponds to the transpolar potential at the poles of a dipole. The obtained laboratory data give a direct confirmation of the magnetospheric generator model.

Simulations of observed auroral brightening caused by solar wind dynamic pressure enhancements under different interplanetary magnetic field conditions

Solar wind dynamic pressure (P(dyn)) enhancements have been observed to cause large-scale auroral brightening. The mechanism for this kind of auroral brightening is still a topic of current space research. Using the global piecewise parabolic method Lagrangian remap (PPMLR)-MHD simulation model, we investigate three auroral brightening events caused by dynamic pressure enhancement under different interplanetary magnetic field (IMF) conditions: (1) B(z) 0 on 11 August 2000, (2) B(z) = 0 on 21 January 2005. We show that the auroral location depends on the IMF conditions. Under southward IMF conditions, when B(y) is negative, the duskside aurora is located more equatorward at around 70 degrees magnetic latitude (MLAT) for all magnetic local times; when B(y) is positive, the duskside aurora can even reach beyond 80 degrees MLAT. A smaller and more localized response is seen when the IMF B(z) is nearly zero or northward, as shown in previous studies. Our simulation results are consistent with these observations, indicating that the observed aurora activities could be caused by solar wind dynamic pressure enhancements. The simulation results suggest that the enhancement of P(dyn) can increase the ionospheric transpolar potential and the corresponding field-aligned currents, leading to the observed auroral brightening.

Comparison between the two basic modes of magnetospheric convection

[1] Magnetic flux in the magnetosphere can circulate in a forward sense (southward IMF) and a reverse sense (northward IMF). The point of this paper is to compare and contrast these two modes of circulation. Both have transient and persistent phases in response to a suddenly applied southward or northward IMF that then is held steady. In the initial, transient phases, reconnection voltage exceeds transpolar potential in the ionosphere because for forward circulation there is an inductive-like time lag while the region 1 current builds up in association with a net flow of magnetic flux from the dayside to the tail (magnetospheric erosion). For the transient phase of reverse convection, reconnection voltage exceeds transpolar potential because instead of over-the-pole, night-to-day transport to undo preexisting magnetospheric erosion, magnetic flux accretes directly onto the dayside magnetosphere. The persistent phase of forward circulation almost always manifests substorms, which modulate the amounts of open and closed flux on the nightside by about 25% in a zero-sum game that leaves the dayside flux mostly unaltered. Energy to power the transient and persistent phases of forward circulation comes from the solar wind flow through magnetic stresses created as the field advects tailward. Energy to power the transient phase of reverse convection, however, comes from tapping magnetic energy stored inductively in association with the region 1 and tail current systems. The persistent phase of reverse circulation is driven possibly by the Song-Russell mechanism, which operates on a day-to-night pressure gradient of plasma entrained by dayside flux accretion. The resulting circulation is slow.

Role of IMF Bx in the solar wind‐magnetosphere‐ionosphere coupling

As far as the role of the interplanetary magnetic field (IMF) in the solar wind‐magnetosphere‐ionosphere (SMI) coupling is concerned, the role of the IMF Bx has more or less been ignored. Recent studies have shown that the IMF Bx plays an important role in the geometry of the bow shock under low Alfvén Mach numbers. Using global MHD simulations, this paper presents a further examination of the effects of the IMF Bx on the geometry of the magnetopause, the ionospheric transpolar potential, and the magnetopause reconnection rate, which quantify the SMI coupling process, under low Alfvén Mach numbers. The role of the IMF Bx manifests itself in three aspects: (1) the magnetopause shifts toward either north or south, depending on whether the Bx · Bz is negative or positive, whereas the bow shock expands in the opposite direction; (2) during southward IMF, the magnetic merging line shifts northward (southward) on the day side and southward (northward) on the night side for Bx > 0 (Bx < 0); (3) both the ionospheric transpolar potential and the magnetopause reconnection rate decrease with increasing Bx, and the relative reduction may reach as high as 20% under extreme cases. The physical mechanism for this reduction is attributed to the change in the width of the magnetosheath, which is sensitive to the variation of Bx under low Alfvén Mach numbers.

Saturation of transpolar potential for large Y component interplanetary magnetic field

This study examines the response of the transpolar potential to a large Y component interplanetary magnetic field (IMF By). The transpolar potential responds nonlinearly and saturates for large IMF By in the Lyon‐Fedder‐Mobarry (LFM) global MHD simulation, just as it does for large southward IMF (−Bz). Data from Defense Meteorological Satellite Program (DMSP) satellites and Assimilative Mapping of Ionospheric Electrodynamics (AMIE) results confirm the saturation of the transpolar potential during large IMF By. The magnitude of the saturated transpolar potential is significantly smaller for large IMF By than for large negative IMF Bz. This indicates that transpolar potential saturation does not depend on the strength of the Region 1 current. However, the magnitude of the IMF at which the transpolar potential becomes nonlinear and begins to exhibit saturation behavior is the same for large By as it is for large Bz. Furthermore, when the IMF (By or Bz) reaches the value that produces saturation, the magnetosheath becomes magnetically dominated, with β < 1. This suggests that the saturation of the transpolar potential is related to a change in the force balance in the magnetosheath from a plasma‐pressure‐dominated magnetosheath to a magnetically‐dominated magnetosheath.

Electrodynamic coupling in the solar wind-magnetosphere-ionosphere system

This paper presents a brief summary of our recent work based on global MHD simulations of the Solar wind-Magnetosphere-Ionosphere (SMI) system with emphasis on the electrodynamic coupling in the system. The main conclusions obtained are summarized as follows. (1) As a main dynamo of the SMI system, the bow shock contributes to both region 1 Field-Aligned Current (FAC) and cross-tail current. Under strong interplanetary driving conditions and moderate Alfven Mach numbers, the bow shock's contribution may exceed more than fifty percent of the total of either region 1 or cross-tail currents. (2) In terms of more than 100 simulation runs with due southward Interplanetary Magnetic Field (IMF), we have found a combined parameter<EM> f</EM> = <EM>E</EM><SUB>sw</SUB><EM>P</EM><SUB>sw</SUB><EM>M</EM><SUB>A</SUB>^-1/2 (<EM>E</EM><SUB>sw</SUB>, <EM>P</EM><SUB>sw</SUB>, and <EM>M</EM><SUB>A</SUB> are the solar wind electric field, ram pressure, and Alfven Mach number, respectively): both the ionospheric transpolar potential and the magnetopause reconnection voltage vary linearly with f for small f, but saturate for large <EM>f.</EM> (3) The reconnection voltage is approximately fitted by sin^3/2<EM>θ</EM><SUB>IMF</SUB>/2, where <EM>θ</EM><SUB>IMF</SUB> is the IMF clock angle. The ionospheric transpolar potential, the voltage along the polar cap boundary, and the electric fields along the merging line however defined they may be, respond differently to <EM>θ</EM><SUB>IMF</SUB>, so it is not justified to take them as substitutes for the reconnection voltage.

Laboratory simulation of field aligned currents in an experiment on laser-produced plasma interacting with a magnetic dipole

In an experiment on a magnetic dipole interacting with a laser-produced plasma the generation of an intense field aligned current (FAC) system was observed for the first time in a laboratory. The detailed measurements of the total value and local current density, of the magnetic field at the poles and in the equatorial magnetopause, and particular features of electron motion in the current channels revealed its similarity to the Region-1 current system in the Earth magnetosphere. Such currents were found to exist only if they can close via conductive cover of the dipole. Comparison of conductive and dielectric cases revealed specific magnetic features produced by FAC and their connection with electric potential generated in the equatorial part of the magnetopause. To interpret the data we consider a model of electric potential generation in the boundary layer which agrees with experiment and with measurements of the Earth's transpolar potential in the absence of an interplanetary magnetic field as well. The results could be of importance for the investigation of Mercury as a magnetic disturbance due to FAC could be especially large because of the small size of the Hermean magnetosphere.

Magnetic merging line and reconnection voltage versus IMF clock angle: Results from global MHD simulations

This paper proposes diagnosis methods to trace the magnetic merging line and to calculate the electric potential along it for Earth's magnetospheric magnetic fields obtained by global magnetohydrodynamic (MHD) simulations of the solar wind‐magnetosphere‐ionosphere (SMI) system. The points with minimum magnetic field strength along last closed magnetic field lines and properly selected closed field lines are combined to trace the whole merging line, and the radial ray integration of convectional electric field is used to calculate the electric potential on the merging line. The diagnosis methods are then applied to magnetospheric magnetic fields associated with different interplanetary magnetic field (IMF) clock angles, and a preliminary analysis is presented on the clock angle response of the geometry of the merging line and the associated reconnection voltage. The merging line is found to be similar in geometry to that of the compound field superposed by the Earth's dipole field and the IMF, whereas the reconnection voltage is approximately fitted by sin3/2(θIMF/2) for its response to the IMF clock angle θIMF. The ionospheric transpolar potential and the voltage along the polar cap boundary show different dependence from that of the reconnection voltage, so it is not justified to take them as substitutes for the reconnection voltage. The length of the sunward merging line between the two peaks of reconnection potential shows a nonmonotonic variation in response to θIMF, peaked at θIMF = 90°, so it is also not justified to take electric fields along the merging line, however defined they may be, to characterize the total reconnection rate and the coupling strength between the solar wind and the magnetosphere. The reconnection nearby magnetic nulls closest to the subsolar point is found to be negligible, which gives support to the component reconnection hypothesis for dayside reconnection of quasi‐steady states of the SMI system.

Transpolar potential and reconnection voltage of the Earth from global MHD simulations

[1] The ionospheric transpolar potential (Vpc) and the magnetospheric reconnection voltage (Vr) of the Earth are investigated in terms of global MHD simulations of the solar wind-magnetosphere-ionosphere (SMI) system. A spherical shell approximation is used for the ionosphere with a uniform Pedersen conductance and a zero Hall conductance, the interplanetary magnetic field is due southward, and quasi-steady solutions are obtained for the system. Each solution is characterized by the following four parameters: the ionospheric Pedersen conductance ΣP, the solar wind electric field Esw, ram pressure Psw, and Alfven Mach number MA. More than 100 cases are treated separately and Vpc and Vr are calculated in each case. It is found that both Vpc and Vr increase monotonically with increasing Psw and decreasing MA regardless of the magnitude of Esw. The simulation results are well organized by a combined parameter f = EswPswMA−1/2 and approximately fitted by functions of f and ΣP. When the units are S for ΣP, kV for Vpc and Vr, mV/m for Esw, and nPa for Psw, the functions are found to be Vpc = 2.3 × 103 (f + 0.8)(ΣP + 2)−1/(f + 8.2) and Vr = 1.8 × 103[f + 0.45(ΣP1/2 + 1.2)] (ΣP1/2 + 0.45)−1/(f + 15.5). Three conclusions are made as follows: (1) Both Vpc and Vr saturate with respect to the increase of f; any variation of the interplanetary conditions in favor of the increase of f may cause the saturation. (2) The saturation point is found to be fc = 6.6 for Vpc and fc = 14.4 − 0.9ΣP1/2 for Vr, whereas the value of ΣP controls the saturation levels. (3) The two potentials, Vpc and Vr, stem from the SMI coupling and exhibit similar saturation behaviors. They are positively correlated because of sharing the same driving source and the close coupling between the ionosphere and the magnetosphere.

Contribution from the Earth's Bow Shock to Region 1 Current under Low Alfvén Mach Numbers

Using global MHD simulations of the solar wind–magnetosphere–ionosphere system, we investigate the dependence of the contribution from the Earth's bow shock (I1bs) to ionospheric region 1 field aligned current (FAC) (I1). It is found that I1bs increases with increasing southward interplanetary magnetic field (IMF) strength Bs, if the Alfvén Mach number MA of the solar wind exceeds 2, a similar result as obtained by previous authors. However, if MA becomes close to or falls below 2, I1bs will decrease with Bs in both magnitude and percentage (i.e., I1bs/I1) because of the resultant reduction of the bow shock strength. Both the surface current density Jbs at the nose of the bow shock and the total bow shock current Ibs share nearly the same relationship with MA, and vary non-monotonically with MA or Bs. The maximum point is found to be located at MA = 2.7. Three conclusions are then made as follows: (1) The surface current density at the nose, which is much easier to be evaluated, may be used to largely describe the behaviour of the bow shock instead of the total bow shock current. (2) The peak of the total bow shock current is reached at about MA = 2.7 when only Bs is adjusted. (3) The non-monotonic variation of the bow shock current with MA causes a similar variation of its contribution to region 1 FAC. The turning point for such contribution is found to be nearly MA = 2. The implication of these conclusions to the saturation of the ionospheric transpolar potential is briefly discussed.

Response of the Earth's Magnetosphere and Ionosphere to Solar Wind Driver and Ionosphere Load: Results of Global MHD Simulations

Three-dimensional global magnetohydrodynamic simulations of the solar wind–magnetosphere–ionosphere system are carried out to explore the dependence of the magnetospheric reconnection voltage, the ionospheric transpolar potential, and the field aligned currents (FACs) on the solar wind driver and ionosphere load for the cases with pure southward interplanetary magnetic field (IMF). It is shown that the reconnection voltage and the transpolar potential increase monotonically with decreasing Pedersen conductance (σP), increasing southward IMF strength (Bs) and solar wind speed (vsw). Moreover, both regions 1 and 2 FACs increase when Bs and vsw increase, whereas the two currents behave differently in response to σP. As σP increases, the region 1 FAC increases monotonically, but region 2 FAC shows a non-monotonic response to the increase of σP: it first increases in the range of (0,5) Siemens and then decreases for σP > 5 Siemens.

Geotail and LFM comparisons of plasma sheet climatology: 1. Average values

We compare the statistics of central plasma sheet properties from 6 years of Geotail observations with 2 months of Lyon‐Fedder‐Mobarry (LFM) global MHD simulations. This statistical validation effort represents an inherently new method of systematically characterizing and quantifying global MHD model performance. For our comparison, we identify the central plasma sheet in the observations and simulation by identical criteria and select the simulation interval to ensure statistically similar distributions of solar wind conditions in both studies. After verifying our plasma sheet selection by inspecting the magnetic signatures of both studies, we compare the resultant number densities, thermal pressures, thermal energies, and bulk flows as functions of position across the equatorial plane. We find that the LFM model successfully reproduces the gross features of the global plasma sheet in a statistical sense. However, our comparison also reveals certain systematic discrepancies between the model and the observations. The LFM predicts a plasma sheet which is too dense, too cool, and exhibits faster globally averaged bulk flows than the observed plasma sheet. By quantifying the LFM overestimate of ionospheric transpolar potential and showing that ΦPC correlates with plasma sheet flow speed, we demonstrate that 15% of the plasma sheet velocity discrepancy is reflected in a ΦPC overestimate. This statistical validation effort represents an essential first step toward the rigorous, quantitative evaluation of a global MHD model in the plasma sheet.

Temporal evolution of the transpolar potential after a sharp enhancement in solar wind dynamic pressure

Recent studies of ionospheric convection have shown that sudden enhancements in solar wind dynamic pressure have significant effect on the transpolar potential and the coupling efficiency between the solar wind and the terrestrial magnetosphere. Super Dual Auroral Radar Network observations of the dayside convection have demonstrated that the strength of convection correlates well with solar wind dynamic pressure variations, implying an enhancement of dayside reconnection induced by changes in solar wind pressure. At the same time, dynamic pressure increases have been shown to lead to closing of the polar cap, particularly on the nightside, and thus directly drive enhanced tail reconnection. The enhanced dayside and nightside reconnection potentials can both lead to changes in the transpolar potential, but their individual contributions and the balance between the two is not known. We present a case study of the transpolar potential evolution after a long‐lasting solar wind pressure step increase. We show that the potential first rises in response to the increase in pressure, then gradually subsides a few hours later despite the solar wind pressure remaining high. We interpret this behavior in terms of pressure‐driven changes in dayside and nightside reconnection.

Magnetohydrodynamic simulations of transient transpolar potential responses to solar wind density changes

Magnetohydrodynamic (MHD) simulations are used to examine the response of the transpolar potential (ΦTP) to changes in the solar wind density during periods of constant solar wind electric field. For increases (decreases) in the solar wind density ΦTP responds immediately by increasing (decreasing) from the steady state values. In both cases the response of ΦTP is transient, decaying to near initial steady state values even when the density change persists. The magnitude of the ΦTP response is proportional to both the rate of erosion of the dayside magnetopause and the ionospheric Pedersen conductance. In our MHD simulations ΦTP is driven entirely by the dayside merging rate and is insensitive to changes in the nightside reconnection rate. The observed relationship between the modeled dayside merging rate and ΦTP is well characterized by an L‐R circuit equation derived from integrating Faraday's Law around the Region 1 current loop. The inductive time constant for variations in the transpolar potential was found to be 6.5 (13) minutes for simulations using ionospheric Pedersen conductances of 6 (12) mhos. This corresponds in both cases to a magnetosphere‐ionosphere inductance of 65 Henries. Observations of the transpolar potential derived using the assimilative mapping of ionospheric electrodynamics (AMIE) model are presented and shown to be consistent with the simulation results.