Polar Cap Potential Saturation Research Articles

The Role of Solar Wind Density in Cross Polar Cap Potential Saturation Under Northward Interplanetary Magnetic Field

AbstractThe role of solar wind density in the cross polar cap potential (CPCP) response under northward interplanetary magnetic field is investigated with observation‐based global simulations. A rare event was reported by Clauer et al. (2016) during which the ionospheric electric field EISP does not saturate under extreme interplanetary electric field (IEF) of ∼15 mV/m. While commonly utilized coupling functions based on IEF fail to provide an unambiguous explanation for the linear response, the Lyon‐Fedder‐Mobarry‐Magnetosphere‐Ionosphere Coupler/Solver model is used to explore the mechanisms in this study. The model first reproduces the observed linear features of the EISP. The simulated CPCP also responds linearly to IEF variations. A controlled simulation is designed with solar wind density artificially reduced to 10% of the observed value while all other parameters such as the IEF are kept the same. The controlled simulation shows saturation of the EISP as well as the CPCP. Further analysis shows the difference in the magnetosheath plasma β, implying the distinct dominant forces between the two simulations. The Lopez magnetosheath force balance theory is used to explain the CPCP responses under different solar wind densities. This comparison study highlights the role of solar wind density in determining the magnetosphere‐ionosphere response to extreme interplanetary drivings.

Polar cap potential saturation during the Bastille Day storm event using global MHD simulation

AbstractWe investigated the temporal variations and saturation of the cross polar cap potential (CPCP) in the Bastille Day storm event (15 July 2000) by global magnetohydrodynamics (MHD) simulation. The CPCP is considered to depend on the electric field and dynamic pressure of the solar wind as well as on the ionospheric conductivity. Previous studies considered only the ionospheric conductivity due to solar extreme ultraviolet (EUV) variations. In this paper, we dealt with the changes in the CPCP attributable to auroral conductivity variations caused by pressure enhancement in the inner magnetosphere owing to energy injection from the magnetosphere because the energy injection is considerably enhanced in a severe magnetic storm event. Our simulation reveals that the auroral conductivity enhancement is significant for the CPCP variation in a severe magnetic storm event. The numerical results concerning the Bastille Day event show that the ionospheric conductivity averaged over the auroral oval is enhanced up to 18 mho in the case of Bz of less than −59 nT. On the other hand, the average conductivity without the auroral effect is almost 6 mho throughout the entire period. Resultantly, the saturated CPCP is about 240 kV in the former and 704 kV in the latter when Bz is −59 nT. This result indicates that the CPCP variations could be correctly reproduced when the time variation of auroral conductivity caused by pressure enhancement due to the energy injection from the magnetosphere is correctly considered in a severe magnetic storm event.

Magnetosheath control of solar wind‐magnetosphere coupling efficiency

AbstractWe examine the role of the magnetosheath in solar wind‐magnetosphere‐ionosphere coupling using the Time History of Events and Macroscale Interactions during Substorms plasma and magnetic field observations in the magnetosheath together with OMNI solar wind data and auroral electrojet recordings from the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain. We demonstrate that the electric field and Poynting flux reaching the magnetopause are not linear functions of the electric field and Poynting flux observed in the solar wind: the electric field and Poynting flux at the magnetopause during higher driving conditions are lower than those predicted from a linear function. We also show that the Poynting flux normal to the magnetopause is linearly correlated with the directly driven part of the auroral electrojets in the ionosphere. This indicates that the energy entering the magnetosphere in the form of the Poynting flux is directly responsible for driving the electrojets. Furthermore, we argue that the polar cap potential saturation discussed in the literature is associated with the way solar wind plasma gets processed during the bow shock crossing and motion within the magnetosheath.

Predicted dependence of the cross polar cap potential saturation on the type of solar wind stream

We study estimations of the cross polar cap potential (CPCP) saturation during magnetic storms induced by various types of solar wind drivers. Using the model of Siscoe–Hill (Hill et al., 1976; Siscoe et al., 2002a,b, 2004; Siscoe, 2011), we evaluate the saturation parameter Q and compare it with the criterion of the CPCP saturation in the main phases of 257 magnetic storms (Dstmin⩽-50nT) induced by the following types of solar wind streams: magnetic clouds (MC), Ejecta, the compressed region, Sheath, before MC (ShMC) and before Ejecta (ShE), corotating interaction regions (CIR), and indeterminate type (IND). Our analysis shows that expected number of events with CPCP saturation is higher for storms induced by ShMC (80%) than for those driven by MC. It is also higher for storms induced by interplanetary coronal mass ejection (ICME) (13.2%) than for storms driven by CIR (3.5%) or by IND (3.5%). The estimated CPCP saturation criterion is satisfied more often for storms initiated by MC (25%) than by Ejecta (2.9%); it is satisfied for 8.6% of magnetic storms induced by the sum of MC and Ejecta, and for 21.5% magnetic storms induced by Sheath before them (sum of ShMC and ShE). These results allow us to conclude that the rate of occurrence of CPCP saturation estimated in the main phase of magnetic storms depends on the type of solar wind driver.

High‐latitude thermospheric wind observations and simulations with SuperDARN data driven NCAR TIEGCM during the December 2006 magnetic storm

AbstractIon convection pattern derived from the Super Dual Auroral Radar Network potential pattern (SDP) data is developed for the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamic General Circulation Model. The December 2006 geomagnetic storm event was simulated with the SDP ion convection pattern and two other existing input options (Heelis and Weimer convection models). The high‐latitude thermospheric wind simulated with SDP showed very good agreement with the Fabry‐Perot interferometer thermospheric wind data inside the polar cap at Resolute, Canada (74.7°N, 94.8°W, magnetic latitude 84). The Heelis model overestimated the winds during the storm event, because the model does not consider the cross polar cap potential saturation at high global geomagnetic index (Kp) values. The Weimer model provides a better performance than the Heelis model in this case. However, it has a larger discrepancy compared to the SDP results. The SDP provides an alternative data‐based tool for study of the ionosphere/thermosphere interactions in the polar cap.

Two models of cross polar cap potential saturation compared: Siscoe‐Hill model versus Kivelson‐Ridley model

AbstractThe cross polar cap potential is considered an instantaneous monitor of the rate at which magnetic flux couples the solar wind to the Earth's magnetosphere‐ionosphere system. Studies have shown that the cross polar cap potential responds linearly to the solar wind electric field under nominal solar wind conditions but asymptotes to the order of 200 kV for large electric field. Saturation of the cross polar cap potential is also found to occur in MHD simulations. Several mechanisms have been proposed to explain this phenomenon. Two well‐developed models are those of Siscoe et al. (2002), herein referred to as the Siscoe‐Hill model, and of Kivelson and Ridley (2008), herein referred to as the Kivelson‐Ridley model. In this study, we compare the mathematical formulas as well as the predictions of the two models with data. We find that the two models predict similar saturation limits. Their difference can be expressed in terms of a factor, which is close to unity during a saturation interval. A survey of the differences in the model predictions show that, on average, the potential of the Kivelson‐Ridley model is smaller than that of the Siscoe‐Hill model by 10 kV. Measurements of AMIE, DMSP, PC index, and SuperDARN are used to differentiate between the two models. However, given the uncertainties of the measurements, it is impossible to conclude that one model does a better job than the other of predicting the observed cross polar cap potentials.

The linear dependence of polar cap index on its controlling factors in solar wind and magnetotail

The solar wind is coupled to the magnetosphere‐ionosphere system through various interactions, e.g., magnetic reconnection at the dayside magnetopause, and viscous interactions at the low latitude boundary layer. The polar cap, a region of open magnetic flux connecting the magnetic field of the Earth to that of the solar wind, is an ideal region in which to investigate how solar wind drives the magnetosphere‐ionosphere dynamo. For such studies, the polar cap (PC) index provides a useful characterization of the state of the polar ionosphere. A previous study by Gao et al. (2012a) found that polar cap dynamics, characterized by the PC index, responds to both solar wind driving quantified by the electric field (EK‐R) proposed by Kivelson and Ridley (2008) which is a representative of the electric field imposed on the ionosphere by magnetopause reconnection that takes cross polar cap potential saturation into account, and the energy release in the magnetotail, described by a modified AL index (ALU). In that study, the dependence of the PC index on EK‐R and ALU was investigated assuming a linear relationship. In this study, we test the assumption that the relationship is linear by performing a similar analysis applying a more general, nonlinear model to the events of the Gao et al. (2012a) study. A nonlinear relationship can be established by use of a statistical approach referred to as the additive model. We find that the more flexible additive model outperforms the linear model. However, the improvement is small. Provided that EK‐R is used to characterize the solar wind input, results obtained from the additive model are very similar to those from the linear model. This result indicates that the linear relation between the PC index and EK‐R, ALU obtained by Gao et al. (2012a) represents the data within fluctuations.

The nonlinear response of the polar cap potential under southward IMF: A statistical view

[1] We report the results of an investigation into the effect of solar wind properties on the saturation of the polar cap potential (CPCP) during periods of strongly southward IMF. We use propagated solar wind data to search for periods between 1998 and 2007 when the interplanetary electric field is stable for more than 50 min and placed further conditions on the availability of SuperDARN and DMSP velocity data. CPCP values are calculated from these data sets and various fits of the polar cap potential to the interplanetary electric field (IEF) are compared. It is found that the trend is nonlinear, with a square root function fitting better than a straight line, and that the CPCP does not appear to exhibit asymptotic behavior. The nonlinearity of the CPCP is then correlated with various interplanetary parameters to test the various models of polar cap potential saturation. It is also found that the deviation of the CPCP from a linear fit has statistically significant correlation with solar wind Alfvènic Mach number and no significant correlation with solar wind dynamic pressure.

Dependence of geomagnetic activity during magnetic storms on the solar wind parameters for different types of streams

The dependence of the maximal values of the |Dst| and AE geomagnetic indices observed during magnetic storms on the value of the interplanetary electric field (Ey) was studied based on the catalog of the large-scale solar wind types created using the OMNI database for 1976–2000 [Yermolaev et al., 2009]. An analysis was performed for eight categories of magnetic storms caused by different types of solar wind streams: corotating interaction regions (CIR, 86 storms); magnetic clouds (MC, 43); Sheath before MCs (ShMC, 8); Ejecta (95); Sheath (ShE, 56); all ICME events (MC + Ejecta, 138); all compression regions Sheaths before MCs and Ejecta (ShMC + ShE, 64); and an indeterminate type of storm (IND, 75). It was shown that the |Dst| index value increases with increasing electric field Ey for all eight types of streams. When electric fields are strong (Ey > 11 mV m−1), the |Dst| index value becomes saturated within magnetic clouds MCs and possibly within all ICMEs (MC + Ejecta). The AE index value during magnetic storms is independent of the electric field value Ey for almost all streams except magnetic clouds MCs and possibly the compressed (Sheath) region before them (ShMC). The AE index linearly increases within MC at small values of the electric field (Ey 11 mV m−1). Since the dynamic pressure (Pd) and IMF fluctuations (σB) correlate with the Ey value in all solar wind types, both geomagnetic indices (|Dst| and AE) do not show an additional dependence on Pd and IMF δB. The nonlinear relationship between the intensities of the |Dst| and AE indices and the electric field Ey component, observed within MCs and possibly all ICMEs during strong electric fields Ey, agrees with modeling the magnetospheric-ionospheric current system of zone 1 under the conditions of the polar cap potential saturation.

Saturation of polar cap potential: Nonlinearity in quasi‐steady solar wind‐magnetosphere‐ionosphere coupling

We propose a quasi‐steady nonlinear circuit model for the solar wind‐magnetosphere‐ionosphere (SW‐M‐I) coupling to study the observed saturation of polar cap potential. The oval conductance is shown to be a nonlinear circuit element since it increases with increasing dayside reconnection E field driving the proposed circuit. Oval conductance is produced by precipitating particles energized by enhanced sunward convection in the plasma sheet driven by reconnection at the dayside magnetopause and in the plasma sheet. The asymptotic saturation potential is shown to increase with (1) decreasing internal resistance of the dynamo region, (2) increasing length of dayside reconnection line, (3) increasing ratio of nightside to dayside reconnection potentials, and (4) increasing ratio of nightside to dayside internal resistances.

On the SuperDARN cross polar cap potential saturation effect

Abstract. Variation of the cross polar cap potential (CPCP) with the interplanetary electric field (IEF), the merging electric field EKL, the Polar Cap North (PCN) magnetic index, and the solar wind-magnetosphere coupling function EC of Newell et al. (2007) is investigated by considering convection data collected by the Super Dual Auroral Radar Network (SuperDARN) in the Northern Hemisphere. Winter and summer observations are considered separately. All variations considered show close to linear trend at small values of the parameters and tendency for the saturation at large values. The threshold values starting from which the non-linearity was evident were estimated to be IEF*~EKL*~3 mV/m, PCN*~3–4, and EC*~1.5×104. The data indicate that saturation starts at larger values of the above parameters and reaches larger (up to 10 kV) saturation levels during summer. Conclusions are supported by a limited data set of simultaneous SuperDARN observations in the Northern (summer) and Southern (winter) Hemispheres. It is argued that the SuperDARN CPCP saturation levels and the thresholds for the non-linearity to be seen are affected by the method of the CPCP estimates.

Polar cap potential saturation, dayside reconnection, and changes to the magnetosphere

Global MHD simulations of the Earth's magnetosphere using the Block‐Adaptive‐Tree Solarwind Roe Upwind Scheme code at the Community Coordinated Modeling Center are run with a high‐resolution dayside magnetopause and variable‐resolution near‐Earth regions to study the phenomena of polar cap saturation. In the simulations a resistive spot is maintained on the moving magnetopause to ensure that the correct dayside reconnection rate is obtained. The solar wind parameters are held fixed, and the ionospheric Pedersen conductivity is varied. Strong reduction of the cross–polar cap potential is observed, with modest increases in the cross–polar cap current, and no changes in the local and global reconnection rates. Changes in the magnetosphere associated with the saturation of the polar cap are explored: these include a weakening of the dayside magnetic field strength, an equatorward shift of the cusps, a flattening in Z of the closed field line region of the dayside magnetosphere, and a taillike stretching of the dipole field lines at the terminator. Measurements of the currents and voltages of the polar cap indicate that the solar wind acts like a current‐limited voltage generator. A simple circuit model is analyzed using measurements from the MHD simulations, and physically reasonable values for the circuit elements are obtained. Nine models for polar cap saturation are assessed against the findings of the present study.

Altered solar wind‐magnetosphere interaction at low Mach numbers: Coronal mass ejections

We illustrate some fundamental alterations of the solar wind‐magnetosphere interaction that occur during low Mach number solar wind. We first show that low Mach number solar wind conditions are often characteristic of coronal mass ejections (CMEs) and magnetic clouds in particular. We then illustrate the pivotal role of the magnetosheath. This comes from the fact that low Mach number solar wind leads to the formation of a low thermal β magnetosheath downstream of the bow shock. This property influences magnetic forces and currents, in particular, and in turn alters magnetosheath‐magnetosphere coupling. The implications of this unusual regime of interaction have generally been overlooked. Potentially affected phenomena include the following: (1) asymmetric magnetosheath flows (with substantial enhancements), (2) asymmetric magnetopause and magnetotail shapes, (3) changes in the development of the Kelvin‐Helmholtz instability and giant spiral auroral features, (4) variations in the controlling factors of dayside magnetic reconnection, (5) cross polar cap potential saturation and Alfvén wings, and (6) global sawtooth oscillations. Here we examine these phenomena, primarily by use of global magnetohydrodynamic simulations, and discuss the mechanisms that rule such an altered interaction. We emphasize the fact that all these effects tend to occur simultaneously so as to render the solar wind‐magnetosphere interaction drastically different from the more typical high Mach number case. In addition to the more extensively studied inner magnetosphere and magnetotail processes, these effects may have important implications during CME‐driven storms at Earth, as well as at other astronomical bodies such as Mercury.

Polar cap potential saturation: An energy conservation perspective

In the long run, energy entering the magnetosphere from the solar wind must be balanced by energy dissipation in or escape from the system. It then follows that the Joule heating rate in the ionosphere statistically should be bounded from the upside by the solar wind energy input function (e.g., the Perrault‐Akasofu parameter). We show that this energy constraint, coupled with some observationally motivated assumptions about the behavior of the auroral oval under escalating solar wind conditions, leads to the prediction of polar cap potential saturation.

Polar cap potential saturation: Observations, theory, and modeling

The polar cap potential ( Φ PC ) has long been considered an indicator for the amount of energy flowing into and through the magnetosphere–ionosphere (M–I) system. Studies have shown that Φ PC reaches an upper limit, or saturates, during geomagnetic storms where the solar wind electric field becomes increasingly large. Numerous observational, theoretical, and modeling studies of Φ PC have seemingly confirmed that saturation is real, however, many mechanisms for saturation have been suggested with no direct observational evidence confirming any single cause. This paper presents a review of Φ PC saturation from various perspectives and attempts to identify some of the outstanding issues in determining the causal mechanism of this important piece necessary to understanding the coupling between the solar wind and M–I system.

Polar cap potential saturation during large geomagnetic storms

We present evidence that the cross polar cap potential saturates during the Bastille Day geomagnetic storm which is characterized by large values of the driving interplanetary electric and magnetic fields. Empirical models that were derived and constrained for more benign solar wind conditions severely over-predict the cross polar cap potential for this event. Global simulation results show that the magnetopause moves significantly inward as a result of compression and erosion of the magnetosphere. We propose a model that explains the potential saturation primarily as an effect of the shortened x-line. An appropriately modified empirical model gives much better potential prediction, supporting our hypothesis.

Saturation of polar cap potential by intense solar wind electric fields

We have examined the statistical relationship between solar wind parameters and the polar cap potential (ΦPC) using data from the Wind and ACE satellites and the northern polar cap index (PCN) as a proxy for ΦPC. We found that PCN tended to be saturated when the value of the merging electric field (Em) was high (greater than 5 mV/m). Furthermore, it was found that the degree of saturation does not depend on the intensity of the interplanetary magnetic field but depends on the value of Em. These results are in quite reasonable agreement with the predictions of the model of Hill et al. [1976]. This fact suggests that the development of magnetospheric convection is not controlled by the efficiency of the dayside merging process itself but is controlled by the coupling among the solar wind, the magnetosphere, and the ionosphere.