Penetration electric fields: Efficiency and characteristic time scale

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Abstract

Penetration of the interplanetary electric field (IEF) to the middle- and low-latitude ionosphere has been investigated for nearly four decades. Most previous studies focused on the correlation between the interplanetary and ionospheric electric field perturbations. Very little attention has been paid to a quantitative relationship except for a recent case analysis by Kelley et al. [2003. Penetration of the solar wind electric field into the magnetosphere/ionosphere system. Geophysical Research Letters 30(4), 1158. doi:10.1029/2002GL016321]. In this paper, we present a statistical result of the efficiency of IEF penetration to the dayside equatorial ionosphere; the efficiency is defined as the ratio of the change of the equatorial ionospheric electric field to the change of the IEF. The Jicamarca incoherent scatter radar has made continuous operation with a coherent scatter mode since 2001, and the radar data of equatorial ionospheric electric fields are used in our statistics. On the basis of data statistics, we derive an empirical value of 9.6% for the efficiency of penetration. We apply this empirical formula to the observations and numerical simulations of storm-time penetration electric fields over a prolonged interval of southward interplanetary magnetic field. The prediction of the formula is in good agreement with case studies and with results from first-principle simulations of the coupled magnetosphere–ionosphere–thermosphere system. We conclude that the IEF can continuously penetrate to the low-latitude ionosphere without significant attenuation for many hours during the main phase of magnetic storms.

Introduction

Penetration electric fields are the electric fields of solar wind/magnetospheric origin observed equatorward (earthward) of the shielding layer (Huang et al., 2006). The shielding layer acts to shield regions equatorward of it from the magnetospheric convection electric field and is identified with the inner edge of the plasma sheet/ring current. The low-latitude ionosphere may be shielded from the high-latitude electric field by the action of the region-2 field-aligned currents under geomagnetically quiet or steady conditions. As a result, the low-latitude ionospheric electric field during quiet times is mostly due to the dynamo action of the neutral atmosphere. During active times, the solar wind/magnetospheric electric fields can penetrate to the low- latitude ionosphere. Nishida (1968) showed for the first time that geomagnetic field perturbations at high and low latitudes were well correlated with interplanetary magnetic field (IMF) reorientations and suggested that the interplanetary electric field (IEF) penetrated deep to the equatorial ionosphere. Since then penetration of the IEF to the middle- and low-latitude ionosphere has been extensively studied with radar and magnetometer measurements (Kelley et al., 1979, Gonzales et al., 1979, Fejer et al., 1979, Fejer et al., 1990a, Kikuchi and Araki, 1979, Kikuchi et al., 1996, Kikuchi et al., 2003, Sibeck et al., 1998, Foster and Rich, 1998, Buonsanto et al., 1999, Huang and Foster, 2001, Huang et al., 2002, Huang et al., 2004, Huang et al., 2005a) and with numerical simulations (Spiro et al., 1988, Fejer et al., 1990b). These previous studies mostly focused on the correlation between IMF reorientations and ionospheric response.

An important issue of electric field penetration is the efficiency with which the penetration occurs. Here the efficiency is defined as the ratio of the change of the equatorial ionospheric electric field to the change of the IEF. It should be mentioned that this definition of penetration efficiency is different from the electric field penetration from high to low latitudes. In most of previous models and simulations (Kikuchi and Araki, 1979, Spiro et al., 1988, Fejer et al., 1990b), the potential across the polar cap was used as the driver of the penetration electric field. However, the potential across the magnetosphere, which is mapped along the magnetic field lines into the ionosphere, is never fully imposed across the polar cap. Instead, the IEF can be determined directly from the measured solar wind parameters, so we use the IEF, rather than the potential across the polar cap, in the definition of penetration efficiency. Kelley et al. (2003) found in one case that the ratio of the dawn-to-dusk component of the IEF to the dawn-to-dusk electric field in the equatorial ionosphere is 15:1. In other words, the penetration electric field at the equator is about 6.6% of the IEF in that case. However, it is uncertain whether the result derived from a single case has general applicability. A statistical study is necessary to derive the efficiency of electric field penetration.

Another important issue is the characteristic time scale within which the IEF penetrates to the low-latitude ionosphere without shielding. Previous theoretical and modeling studies provided widely varying estimates ranging from a few (Southwood, 1977, Siscoe, 1982) up to 5 h (Jaggi and Wolf, 1973) as contrasted with Senior and Blanc (1984) estimate of shielding time scale of 30min. Fejer and Scherliess (1995) and Scherliess and Fejer (1998) termed the short-lived (10–30 min) enhancement of the equatorial ionospheric electric field in response to IMF southward turnings as prompt penetration and attributed the longer-lasting enhancement of the equatorial ionospheric electric field to neutral wind disturbance dynamo. From the physics of the solar wind–magnetosphere–ionosphere coupling point of view, it is the sign and magnitude of the IMF Bz that is the most important parameter controlling the penetration effect. More negative (southward) IMF Bz values imply a faster rate of reconnection and flux transfer into the magnetosphere, which is usually expressed in terms of the polar cap potential (PCP) drop across the open-closed field line boundary (identified here with the polar cap boundary). Positive (northward) IMF Bz values imply a reduction in the rate of reconnection, resulting in low values of the PCP drop. Thus, for discussion purposes here, we adopt the IMF Bz, or the equivalent motional IEF Ey, value as the controlling factor, or the “driver”, of the disturbance electric fields. Under certain assumptions, this can be related to the polar cap potential drop or its proxies such as the geomagnetic AE index as was done by Fejer and Scherliess (1995).

It is important to mention that most of the previous results were for short-lived ionospheric response and for moderate geomagnetic disturbance levels. However, Huang et al. (2005b) show that penetration electric fields over the equator can last for several hours without significant decay during the main phase of magnetic storms. If the IEF can indeed continuously penetrate to the low latitudes for many hours, it will greatly affect the global ionospheric electrodynamics and cause significant redistribution of the storm-time ionospheric plasma.

In this paper, we present the statistical result of the efficiency of IEF penetration to the dayside equatorial ionosphere. The ionospheric electric field data are obtained with the Jicamarca radar in coherent scatter mode. The purpose of this study is to find how much of the IEF can penetrate to the equatorial ionosphere and whether the efficiency of electric field penetration changes with the duration of southward IMF. We derive an empirical formula of the penetration efficiency. We also show the simulation results of penetration electric field from a combined numerical model of coupled thermosphere–ionosphere–plasmasphere–electrodynamics (CTIPe) model (Millward et al., 2001, Fuller-Rowell et al., 2002) and Rice Convection Model (RCM) (Wolf, 1983, Toffoletto et al., 2003) in a storm case, as reported in Maruyama et al., 2005, Maruyama et al., 2007, and compare the empirical formula with the measured and simulated penetration electric fields.

Section snippets

Observations and data analysis

We use the solar wind data measured by the ACE satellite to derive the interplanetary electric field by assuming that the plasma motion is determined by E×B drift. The time shift of the solar wind data is determined as follows. We first use the satellite position and solar wind velocity to calculate the possible propagation delay, and then compare the changes in the interplanetary and ionospheric electric fields. There are two different methods to calculate the solar wind propagation time. One

Discussion

In our statistics, we choose the cases with rapid southward turnings of the IMF, as shown in Fig. 1. In these cases, IMF turnings from northward to southward occur within 0.5–1 h. We may assume that the shielding effect is negligible within this time interval. We characterized observations of apparent penetration of IEF to low-latitude ionosphere in terms of (A) efficiency (ratio of horizontal east–west electric field component measured in the F-region ionosphere at the magnetic equator to IEF y

Conclusions

In this paper, we have performed a statistical study of observations of penetration of the interplanetary electric field to the equatorial ionosphere, using the dayside ionospheric electric field data obtained by the Jicamarca radar with coherent scatter mode between 1000 and 1500 LT over five years (2001–2005). We have derived an empirical formula of the efficiency of IEF penetration to the equatorial ionosphere. The efficiency of IEF penetration to the equatorial ionosphere is found to be

Acknowledgments

Work at MIT Haystack Observatory was supported by an NSF cooperative agreement with Massachusetts Institute of Technology. The Jicamarca Radio Observatory is a facility of the Instituto Geofísico del Perú and is operated with support from the NSF Cooperative Agreement ATM-0432565 through Cornell University. Work at Rice University was supported in part by the NASA Sun-Earth-Connection Theory Program Grant NNG05GH93G and in part by the Center for Integrated Space Weather Modeling (CISM), which

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