The transition to overshielding after sharp and gradual interplanetary

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A01211, doi:10.1029/2010JA015985, 2011

The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning Yong Wei,1,2 Weixing Wan,1 Zuyin Pu,2 Minghua Hong,1 Qiugang Zong,2 Jianpeng Guo,3 Biqiang Zhao,1 and Zhipeng Ren1 Received 2 August 2010; revised 28 September 2010; accepted 2 November 2010; published 22 January 2011.

[1] Overshielding is referred to a shielding status, during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere, typically appearing when the interplanetary magnetic field (IMF) suddenly turns northward after a prolonged southward orientation. It is expected that the transition to overshielding after IMF northward turning can be affected by the shape of northward turning (sharp or gradual). Moreover, the initial shielding status (undershielding or goodshielding) prior to the transition may also have influence on the transition. Here we analyze two groups of cases, in which the transitions appear after sharp (duration less than 5 min) and gradual (duration more than 30 min) northward turning. Each group includes two cases, in which the transition initiated from undershielding and goodshielding. These cases show that (1) the beginning of the transition to overshielding coincides with sharp IMF northward turning but appears in the midst of gradual IMF northward turning; (2) the transition from goodshielding to overshielding is always associated with convection electric field drop and/or polar cap shrinkage, regardless of the shape of IMF northward turning; and (3) the typical solar wind condition in which the IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding. Furthermore, we will discuss the effect of substorm processes on overshielding. Citation: Wei, Y., W. Wan, Z. Pu, M. Hong, Q. Zong, J. Guo, B. Zhao, and Z. Ren (2011), The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning, J. Geophys. Res., 116, A01211, doi:10.1029/2010JA015985.

1. Introduction [2] It has long been known that the southward interplanetary magnetic field (IMF) often causes penetration of convection electric field into the inner magnetosphere at the first several hours during magnetic storm [Kelley, 1989; Fejer, 2002; Wolf et al., 2007]. In recent years, the penetration electric field has been used extensively to interpret various disturbances in the midlatitude and low‐latitude ionosphere [e.g., Kelley et al., 2003; Huang et al., 2005; Zhao et al., 2005, 2008; Wei et al., 2008a; Li et al., 2009, 2010; Guo et al., 2010; Huang, 2009; Zong et al., 2010]. The duskward penetration electric field sometimes can be completely canceled by the dawnward shielding electric field in the ring current, which is called “perfect shielding”

1 Beijing National Observatory of Space Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 2 School of Earth and Space Sciences, Peking University, Beijing, China. 3 SIGMA Weather Group, State Key Laboratory of Space Weather, CSSAR, Chinese Academy of Sciences, Beijing, China.

Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JA015985

[Goldstein, 2006] or “goodshielding” [Wolf et al., 2007]. In analogy with it, the penetration of duskward convection electric field is alternatively named as “undershielding,” while the case that shielding electric field dominates over penetration electric field is called “overshielding.” [3] To explain the polarity reversal of the equatorial ionospheric electric field associated with northward turning of the IMF, Kelley et al. [1979] recognized it as an overshielding effect and attributed it to a drop of the convection electric field. For this scenario, the typical historical pattern of overshielding is that the undershielding caused by the southward IMF occurs first, and then overshielding happens when the IMF turns northward. In many papers [e.g., Ebihara et al., 2008, paragraph 3], the IMF condition for overshielding is generally stated as “IMF suddenly turns northward after a prolonged southward orientation.” Under prolonged southward IMF, positive (negative) charges gradually accumulate in the duskside (dawnside) Alfven layer and then produce a dawnward shielding electric field, which takes time to reach the same level as the convection electric field. Therefore, undershielding usually lasts for several tens of minutes or longer after IMF southward turning [Huang et al., 2005]. If the duration of southward IMF is long enough, the shielding electric field may completely cancel convection electric field, and there will be a

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goodshielding status [Wei et al., 2010]. When the IMF suddenly turns northward, the convection electric field abruptly decreases, and then the shielding electric field becomes dominant in the inner magnetosphere, and thus, overshielding is created. [4] The overshielding scenario can successfully explain some observations in space [e.g., Goldstein et al., 2002, 2003] and on the ground [e.g., Kikuchi et al., 2000, 2008]. However, the physics behind overshielding still require more extensive study. The numerical simulations suggested that the duration of overshielding due to drop of the convection electric field is about 10–20 min, which is insufficient to explain the observed duration (typically 1–2 h) [Spiro et al., 1988]. With a time‐independent magnetic field model, Peymirat et al. [2000] also found that the simulated duration of overshielding is about half of the observed value. Fejer et al. [1990] proposed that the magnetic field reconfiguration of the magnetosphere due to a northward turning and the associated shrinkage of the polar cap may effectively prolong the overshielding time period. This effect has been confirmed by the Rice Convection Model (RCM) with a time‐dependent magnetic field model [Sazykin, 2000; De Zeeuw et al., 2004; Maruyama et al., 2007; Zhang et al., 2009] and has been discussed with observations [Wei et al., 2008b, 2009]. On the other hand, the IMF northward turning is preferable to trigger substorms [Lyons et al., 1997], but the effect of substorm on overshielding, reduction [e.g., Huang et al., 2004; Huang, 2009] or enhancement [Wei et al., 2009; Zhang et al., 2009] is still a controversial issue. [5] These considerations mentioned above imply that the relationship between overshielding and IMF northward turning may be very complicated. It is shown that overshielding can occur in association with a decrease of the magnitude of the southward IMF (in this paper we also refer it to northward turning though the IMF does not become northward) [Kikuchi et al., 2008]. Moreover, the overshielding signatures are observed even under stable southward IMF [Fejer et al., 2007; Ebihara et al., 2008], implying that IMF northward turning is not a necessary condition for overshielding. What has become clear is that IMF northward turning is the most effective factor to cause overshielding, but some other factors, either outside or inside the magnetosphere‐ionosphere system, can also contribute to the overshielding effect, such as a large drop of solar wind dynamic pressure [e.g., Wei et al., 2008b] and substorm processes [e.g., Ebihara et al., 2008; Wei et al., 2009; Zhang et al., 2009]. [6] In this paper we examine the transitions to overshielding after sharp and gradual IMF northward turning. The durations of northward turning are less than 6 min in the two “sharp” cases and are more than 30 min in the two “gradual” cases. We also consider the influences of the initial shielding status prior to overshielding, i.e., undershielding and goodshielding. The overshielding is identified as westward equatorial electric field disturbances deviating from the quiet time pattern. All cases were chosen on the daytime because (1) the penetration efficiency (ratio of solar wind electric field to penetration electric field) is almost constant [e.g., Nopper and Carovillano, 1978; Huang et al., 2007], and thus the local time effect can be excluded; (2) the equatorial ionospheric electric field can be inferred from the

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difference between geomagnetic H components (DH) of a pair of stations, of which one is located near the geomagnetic equator and the other is located off the equator [Anderson et al., 2002].

2. Data and Method [7] The solar wind parameters are measured by the Geotail satellite in the region near the dayside magnetopause and are shifted by matching the closest sudden storm commencement (SSC) and solar wind dynamic pressure front. The magnetic field and plasma parameters are observed by the Magnetic Field Experiment (MGF) and Comprehensive Plasma Instrument (CPI), respectively. For comparison, we also plot the IMF of the OMNI database derived form ACE observations at the L1 point. The IMF and solar wind velocity are in GSM. [8] The two geomagnetic stations in Peru (LT = UT minus 5 h), Jicamarca (JIC; 11.9°S, 283.1°E; dip 0.8°N), and Piura (PIU; 5.2°S, 279.4°E; dip 6.8°N) fairly meet the requirement to estimate the ionospheric electric field and are used for three cases in 2002. Because of lack of data in JIC for the case in 2000, another pair of stations adjacent to JIC are chosen: Huancayo (HUA; 12.0°S, 284.7°E; dip 1.8°S) and Kourou (KOU; 5.21°N, 307.3°E; dip 14.9°S). The positive (negative) value of DH denotes the eastward (westward) equatorial electric field. [9] The cross‐polar cap potential (CPCP) and polar cap area (PCA) calculated by the assimilated mapping of ionospheric electrodynamics (AMIE) procedure, which is a technique used to reconstruct the high‐latitude ionospheric electrodynamic parameters by combining the various data sets [Richmond and Kamide, 1988; Ridley et al., 1998]. The CPCP can be regarded as a proxy of plasma convection and the associated convection electric field, while the PCA is the area of open magnetic field line calculated from the auroral precipitation results [Ridley and Kihn, 2004]. The overshielding is closely related to polar cap shrinkage because of the geometrical attenuation of the penetrated electric field [Kikuchi and Araki, 1979]. [10] The symmetric ring current index (SYMH) and asymmetric ring current indices, ASYD (D component), are derived from observations primarily from six midlatitude stations, which are randomly selected from a station group consisting of 10 low‐ and middle‐latitude stations in which only two are from low latitudes. Thus, the SYMH and ASYD generally represent the symmetric or asymmetric variation of H and D components at middle latitude, respectively [Iyemori and Rao, 1996]. The ASYD can be used as an indicator of the intensity of the total field aligned currents (FACs) because the ring current and the Chapman‐Ferraro current do not contribute significantly to the D perturbation; only FACs do [Shi et al., 2005]. [11] The shielding statuses can be discerned according to the deviation of DH from the quiet time pattern. The quiet time pattern was chosen as the pattern of DH during the closest quiet day. Please note that the magnitude of quiet time field may be not equal to the background field during the event day because of day‐to‐day variability. Here we only assume the background field (essentially the Sq field) during the event day has the same shape as the quiet time pattern. Considering all of our cases observed in the day-

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located at (5.4, 19.8, 1.3) RE in GSM at 1200 UT, while the thin lines are OMNI data derived from ACE observations at L1 point. The discrepancy between the thick and thin lines suggests that the solar wind had significantly changed during the propagation from ACE to the Earth or that there were some small‐scale structures in the solar wind. This event occurred during the beginning of the recovery phase of the moderate storm (SYMH > −100 nT). Comparing the solar wind dynamic pressure front in Geotail observations and the SSC in the SYMH index, the time delay is estimated at 2.4 min and is applied for Figures 1 and 2. The same method will be applied for the other three cases. The dashed line marks the IMF northward turning at 1244 UT, of which the duration was about 2 min. [13] The first panel of Figure 2 replots Geotail BZ, and the rest of panels show CPCP, PCA, AE, ASYD, and JIC‐PIU DH. The thin line in the bottom panel represents DH of quiet time pattern on 24 May as a reference. The vertical bold line indicates the beginning of overshielding at 1256 UT, identified according to the appearance of the westward (negative) electric field. The IMF BZ became southward

Figure 1. Solar wind conditions and symmetric ring current H index (SYMH) during 1000–1530 UT on 23 May 2002. Shown are the observations of solar wind: solar wind dynamic pressure (PSW), solar wind speed VX, IMF X component (BX), Y component (BY), and Z component (BZ). The thick lines are time‐shifted Geotail data, while the thin lines are OMNI data derived from ACE observations at L1 point. The bottom panel is SYMH, which is essentially the same as the Dst index. time, we may discern the shielding status as follows. (1) In undershielding, the DH increases and the increase is consistent with enhancement of southward IMF because the eastward penetration electric field is generally driven by the southward IMF. (2) In goodshielding, the DH is approximately equal to the background field, which can be given as the DH before undershielding in our cases. (3) Since the overshielding electric field is westward, the DH during the overshielding time period starts from the background field level and almost monotonically decreases to the negative level.

3. Observations 3.1. Sharp IMF Northward Turning 3.1.1. Case 1: Overshielding Begins From Undershielding [12] Figure 1 shows from top to bottom the solar wind dynamic pressure (PSW) and X component of solar wind velocity (VX), IMF X component (BX), Y component (BY), Z component (BZ), and SYMH during 1000–1530 UT on 23 May 2002. The thick lines are Geotail data which are

Figure 2. Replot of IMF BZ measured by Geotail; cross‐ polar cap potential (CPCP) and polar cap area (PCA) calculated by the assimilated mapping of ionospheric electrodynamics (AMIE) procedure; the AE and ASYD indices; and JIC‐PIU DH derived from the geomagnetic H component in Jicamarca and Piura stations, which is linearly correlated to the equatorial electric field. The positive (negative) corresponds to eastward (westward) electric field. The thick line represents disturbed day, while the thin line is quiet day as a reference (see text).

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Figure 3. The same parameters as Figure 1 but for 1500– 1900 UT on 23 May 2002. after 1100 UT, and the eastward (positive) penetration electric field (undershielding) appeared 55 min later when JIC and PIU crossed the terminator at 1155 UT, entering the daytime equatorial electrojet region. [14] The IMF turned northward at 1244 UT after remaining southward for about 100 min, and the CPCP simultaneously decreased. The penetration electric field started to decrease at 1244 UT, but the expected overshielding signature (negative DH) did not appear. During 1244–1256 UT, the IMF remained northward except for the southward spike at ∼1251 UT, and the DH almost monotonously decreased. However, the equatorial electric field seemingly did not response to the spike. The westward overshielding electric field, as seen from the negative DH, emerged at 1256 UT. The PCA data during 1215–1300 UT is not reliable, but the PCA obviously decreased after 1300 UT. During 1244–1256 UT, the increase of AE suggests that a substorm happened, and GOES 10 in the postmidnight sector observed a magnetospheric dipolarization around 1244 UT (not shown). [15] This case shows a gradual transition from undershielding to overshielding following the sharp IMF northward turning associated with CPCP drop. The overshielding electric field (negative DH) lasted for 2.5 h and gradually became weak, implying the weakening of R2 FAC consistent with the decrease of ASYD. The long‐duration overshielding effect should involve magnetic reconfiguration according to the simulation results by Sazykin [2000], and this was consistent with the gradual polar cap shrinkage.

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3.1.2. Case 2: Overshielding Begins From Goodshielding [16] Figures 3 and 4 show the same parameters as Figures 1 and 2 during 1500–1900 UT on 23 May 2002, respectively. This is another magnetic storm following the recovery phase of the storm shown in case 1. The thick lines are Geotail data located at (3.2, 21.8, 4.7) RE in GSM at 1540 UT. We mark two northward turnings at 1648 UT and 1734 UT. The first one was consistent with the typical condition for overshielding; that is, IMF suddenly turns northward after a prolonged southward orientation. The second one, with a duration of about 4 min, had a polarity reversal in IMF BZ. [17] Compared to the quiet time pattern during 24 May, the JIC‐PIU DH showed three shielding stages: undershielding (1544–1711 UT), goodshielding (1711–1734 UT), and overshielding (1734–1803 UT). During 1711–1734 UT, the DH was seemingly in the same level as that before 1544 UT when the IMF was northward. Therefore, the shielding electric field almost completely canceled the penetration electric field, and this kind of shielding status is called “goodshielding” [Wei et al., 2010]. Unexpectedly, the overshielding signature did not occur after the first northward turning at 1648 UT; instead, it was triggered by the second sharp northward turning at 1734 UT from the goodshielding status. The polar cap shrinkage, starting from 1648 UT, implied that the R1 FAC shifted to higher latitude and thus contributed to the reduction of the penetration electric field [Kikuchi and Araki, 1979]. The ASYD curve showed anticorrelation to the overshielding electric field

Figure 4. The same parameters as Figure 2 but for 1500– 1900 UT on 23 May 2002.

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negative DH was significant. Those DH fluctuations without IMF counterparts may be attributed to some dynamic processes inside the magnetosphere‐ionosphere system. The overshielding signature is also confirmed by IMAGE observations and corresponding computer simulations through identifying the “shoulder” structure at the plasmapause [Goldstein et al., 2003]. We divided the time period of overshielding into two parts: (1) During 1615–1723 UT, the IMF was turning northward but had not become northward yet, while the ASYD did not significantly change. Because of the pulse‐like shape of DH around 1611 UT, it is difficult to determine the beginning time of overshielding. Assuming that at the beginning moment the DH was approximately equal to the value (∼40 nT) during 1310–1410 UT, we may choose 1615 UT as the beginning moment. The overshielding appeared after northward turning with time delay and was associated with the CPCP decrease. (2) During 1723–2010 UT, the IMF was turning northward and remained northward. The ASYD first increased and then decreased, showing a rough anticorrelation with the DH. This suggests that the enhancement of R2 FAC was at least partly responsible for the overshielding. Furthermore, the overshielding also coincided with the polar cap shrinkage indentified from PCA data, which had been suggested as a possible cause for overshielding [Kikuchi and Araki, 1979]. This case suggests that the overshielding can occur in the midst of gradual IMF northward turning. Moreover, the

Figure 5. The same parameters as Figure 1 but for 1200– 2200 UT on 28 July 2000. (negative DH), implying that the R2 FAC controlled the variations of shielding electric field. [18] This case shows that overshielding coincides with an abrupt northward turning and the associated large drop of CPCP. However, the other northward turnings prior to this northward turning did not cause overshielding. 3.2. Gradual IMF Northward Turning 3.2.1. Case 3: Overshielding Begins From Undershielding [19] Figures 5 and 6 show the same parameters as Figures 1 and 2 during 1200–2200 UT on 28 July 2000, respectively. The thick lines are Geotail data located at (5.1, 22.6, −3.2) RE in GSM at 1600 UT. The excellent overlap between OMNI data (thin lines) and Geotail data suggests that the solar wind condition was stable during its propagation to the Earth. The time delay is determined by the SSC at 0630 UT (not included in Figure 5). The quite gradual northward turning of IMF BZ persisted for about 4 h (1455–1900 UT), and the polarity reversal appeared at 1730 UT. The HUA‐KOU DH on 27–28 July showed significant day‐to‐day variability. Assuming that the DH during 1310–1410 UT on 28 July (∼40 nT) was the solar quiet (Sq) field because the IMF was northward, it is seen that the magnitude of the Sq field on 28 July was higher than that on 27 July by about 32 nT. [20] The HUA‐KOU DH exhibited some fluctuations, and this brought some difficulties in accurately determining the beginning time of the overshielding period, though the

Figure 6. The same parameters as Figure 2 but for 1200– 2200 UT on 28 July 2000. Note that DH is derived from the geomagnetic H component in HUA and KOU stations.

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northward turning and is associated with a significant drop of CPCP.

4. Discussion [23] The four overshielding cases happened during storms, and the westward equatorial electric fields appeared during the recovery phases, less than 3 h after the IMF southward turning. Moreover, the durations of overshielding in our cases were less than 5 h. The neutral wind disturbance dynamo [Blanc and Richmond, 1980] thus might be ignored in our cases because it usually takes several hours to become effective and the dynamo continues to work for a longer time (up to tens of hours) [Fejer and Scherliess, 1997].

Figure 7. The same parameters as Figure 1 but for 1600– 2100 UT on 7 September 2002. appearances of overshielding electric field are indeed associated with the CPCP decrease or polar cap shrinkage. 3.2.2. Case 4: Overshielding Begins From Goodshielding [21] Figures 7 and 8 show the same parameters as Figures 1 and 2 during 1600–2100 UT on 7 September 2002, respectively. The thick lines are Geotail data located at (12.2, 21.6, −4.9) RE in GSM at 1640 UT. The southward IMF of −20 nT magnitude, stably persisting from 1640 to 1827 UT, caused a storm with the minimum of SYMH about −150 nT. The IMF BZ gradually turned northward, and the duration was about 30 min (1827–1957 UT). The CPCP enhancement around 1843 UT was inconsistent with the pattern of IMF BZ, but the enhanced antisunward plasma flow in the throat region observed by Super Dual Auroral Radar Network (SuperDARN) confirmed the AMIE results [Wei et al., 2010]. [22] Compared to the quiet time pattern during 8 September, the JIC‐PIU DH shows again three shielding stages: undershielding (1635–1817 UT), goodshielding (1817– 1843 UT), and overshielding (1843–2009 UT). The IMF started to turn northward at 1827 UT during the goodshielding time period; however, the CPCP did not decrease, and the shielding status did not change. When the IMF reached zero at 1843 UT, the CPCP began to decrease, and meanwhile, the overshielding took place. The ASYD showed a rough anticorrelation to the DH, implying that the enhancement of R2 FAC played an important role in production of the overshielding signature. This case shows that the overshielding happens in the midst of gradual IMF

4.1. The Shape of IMF Northward Turning [24] When the IMF turns northward from southward orientation, the magnetic reconnection on the dayside magnetopause is weakened, and thus, the magnetospheric convection is reduced. One consequence is the drop of convection electric field and penetration electric field. However, this effect alone is insufficient to explain the observed overshielding duration [Spiro et al., 1988]. The other consequence is the magnetotail expansion resulting from reduction of cross‐tail current, which may be able to help to explain the observations [Sazykin, 2000]. [25] The shape of IMF northward turning, sharp or gradual, determines the decreasing rate of the magnetospheric convection electric field. A sharp northward turning after prolonged southward IMF orientation causes an abrupt drop

Figure 8. The same parameters as Figure 2 but for 1600– 2100 UT on 7 September 2002.

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of convection electric field, and thus, it is preferable to create overshielding because the shielding electric field usually takes time to increase or decay depending on the charge accumulation [Kelley et al., 1979]. On the other hand, a very gradual northward turning after prolonged southward IMF orientation is not preferable for overshielding because there may be enough time allowing the shielding electric field to adjust to cancel the convection electric field. As a result, it could be difficult for the shielding electric field to become dominant in the inner magnetosphere. However, it is not known how gradual northward turning can preclude overshielding. In case 3, the northward turning persisted more than 4 h (1455–1900 UT), but the overshielding signatures were still discernable. [26] The IMF northward turning is closely related to the onset of the substorm expansion phase [Lyons et al., 1997], involving several processes that have opposite influences on the shielding status. On one hand, substorms can enhance plasma convection [e.g., Miyashita et al., 2008] and thus enhance the duskward penetration electric field [Huang et al., 2004; Huang, 2009]. Some simulations show that the enhancement of auroral conductance enables further penetration of convection electric field to lower latitude and reduces the shielding electric field that is fed by the region 2 FACs, though the enhanced auroral conductance results in a relatively stronger ring current and stronger region 2 FACs [Zheng et al., 2008]. On the other hand, substorms can contribute to the overshielding effect through dipolarization [Fejer et al., 1990; Sazykin, 2000] and polar cap shrinkage [Ebihara et al., 2008]. Moreover, since the shielding electric field is proportional to the strength of the R2 FAC, which is mainly driven by the component of the plasma pressure gradient perpendicular to the gradient of the magnetic flux tube volume at the Alfven layer [Vasyliunas, 1970], the redistribution of plasma pressure must change the R2 FAC. Wing et al. [2007] found that the pressure profile varies with the phases of substorm: the pressure peaks at the inner edge of the plasma sheet (the Alfven layer) during the growth phase; after the substorm onset, the pressure at the inner edge diminishes, and instead, the pressure peaks at premidnight from X = −10 to −40 RE. These findings suggest that the plasma pressure distribution can enhance the R2 FAC during the growth phase, but this effect may be weakened during the expansion phase. Therefore, there are some uncertainties in the total equatorial electric field associated with substorms triggered by IMF northward turning. 4.2. The Initial Shielding Status Prior to Overshielding [27] The shielding status corresponds to the ratio of the penetration electric field to the shielding electric field. For the goodshielding case, the penetration electric field is approximately equal to the shielding electric field. Therefore, the shielding electric field should immediately dominate the inner magnetosphere when the convection electric field suddenly drops as shown in cases 2 and 4, but the transition to overshielding from undershielding needs more time as shown in cases 1 and 3. The long time transition from undershielding is expected to be more significant after the “long‐duration penetration.” Huang et al. [2005] found that the convection electric field can continuously penetrate to the low‐latitude ionosphere without shielding for many

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hours as long as the strengthening of the magnetic activity is going on under storm conditions. Maruyama et al. [2007] suggested that enhanced reconnection at the dayside magnetopause results in constant transfer of magnetic flux to the lobes of the magnetotail, causing stretching of nightside plasma sheet magnetic field lines and further prevent the shielding electric field from building. Under this condition, there is no effective shielding electric field when the IMF begins to turn northward; thus, the shielding electric field must develop during the transition to overshielding. [28] For instance, now we discuss case 4, which had been studied as a long‐duration penetration case by Huang et al. [2005]. The ASYD index remained ∼45 nT before and after 1644 UT, when the magnitude of southward IMF BZ increased from about 5 to 20 nT. This suggests that R2 FAC did not significantly enhance during the undershielding period time (1644–1810 UT), confirming that there was no effective shielding electric field. At 1810 UT, the ASYD began to increase and the DH simultaneously started to decrease, implying that the shielding electric field was developing. These changes probably resulted from the substorm injection, which also caused the O+ flux enhancement as discerned from the High Energy Neutral Atom (HENA) imager onboard the IMAGE satellite (P. C. Brandt, private communication, 2009). An energetic particle injection at 1843 UT was recorded by LANL 01A and 02A in the premidnight sector; meanwhile, the AE index began to increase from 730 nT and finally reached 1950 nT, suggesting an onset of the substorm expansion phase. After 1843 UT, the ASYD further increased and the CPCP gradually decreased. These features imply that the shielding electric field was enhanced but the convection electric field and duskward penetration electric field became weak, which was consistent with the observed overshielding effect. Specially, the enhancement of the R2 FAC and the associated shielding electric field could not be explained by the drop of convection electric field, and they might be attributed to the substorm effects as discussed in section 4.1. 4.3. Some Further Comments [29] Fejer et al. [2007] showed 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 7 November 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. Ebihara et al. [2008] found an antisunward flow in the predusk sector subauroral region during a period of southward oriented IMF. They explained the flow as an overshielding effect and speculated that it was due to a sudden contraction of the polar cap associated with the substorm or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions and further suggested that the shielding/overshielding condition is not simply caused by the northward turning of IMF. These two studies implied that the overshielding effect can take place without IMF northward turning; in other words, IMF northward turning is not a necessary condition for overshielding.

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[30] IMF northward turning after a prolonged southward orientation is not a sufficient condition for overshielding. In our case 2, the first northward turning (1648 UT) causes gradual reduction of the penetration electric field and the subsequent goodshielding effect rather than overshielding. Instead, the other sharp northward turning (1734 UT) with polarity reversal causes the transition from goodshielding to overshielding. This case suggests that there is no one‐to‐one correspondence between IMF northward turning and the overshielding signature. [31] The changes in solar wind condition are not directly responsible for overshielding. The shielding status is determined by the balance status between the convection electric field and shielding electric field, which is controlled by various magnetospheric and ionospheric dynamic processes. It is clearly seen from the two goodshielding‐ overshielding cases that the overshielding effects are closely related to a drop of the convection electric field, regardless of the shape of IMF northward turning. Particularly, in case 4, the CPCP was enhanced during the gradual northward turning (1827–1843 UT), probably because of magnetic reconnection caused by strong positive BY [Ridley et al., 1998]. Therefore, the overshielding decouples from IMF northward turning, but it is still consistent with the drop of the convection electric field as expected by the conventional overshielding concept [Kelley et al., 1979].

5. Summary and Conclusions [32] To study the relation between overshielding and IMF northward turning, we have analyzed four overshielding cases: (1) starting from undershielding and associated with sharp northward turning, (2) starting from goodshielding and associated with sharp northward turning, (3) starting from undershielding and associated with gradual northward turning, and (4) starting from goodshielding and associated with gradual northward turning. [33] The typical solar wind condition in which IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition [Fejer et al., 2007; Ebihara et al., 2008] nor a sufficient condition for overshielding. For the transition from goodshielding to overshielding, the overshielding effects immediately take place in association with a drop of the convection electric field, regardless of the shape of IMF northward turning. For the transition from undershielding to overshielding without goodshielding, it takes time to enhance the shielding electric field to finally create the overshielding. The substorm, if it is triggered by northward turning during the transition to overshielding, can significantly influence shielding electric field and the subsequent overshielding effects. [34] Acknowledgments. We sincerely thank A. J. Ridley for providing AMIE data. We appreciate the valuable discussion with P. C. Brandt. This work is supported by the KIP Pilot Project (kzcx2‐yw‐123) of CAS, National Science Foundation of China (41004072, 40874088, 40974090, 40890163, and 40636032), National Important Basic Research Project (2006CB806306 and 2011CB811405), and Postdoctoral Science Foundation funded project and Open Research Program of SOA Key Laboratory for Polar Science (KP2008008). We acknowledge the CDAWeb for access to the OMNI and Geotail data. The SYMH, ASYD, and AE data are provided by the World Data Center for Geomagnetism at Kyoto University. The Jicamarca Radio Observatory is a facility of the

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Instituto Geofísico del Perú operated with support from the NSF Cooperative Agreement ATM‐0432565 through Cornell University. [35] Robert Lysak thanks the reviewers for their assistance in evaluating this paper.

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