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Keywords:

  • interplanetary electric field;
  • ionospheric electric field;
  • electric field penetration;
  • magnetic storms;
  • positive ionospheric storms

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[1] It is well known that the interplanetary electric field can penetrate to the low-latitude ionosphere. It is generally believed that the penetration of electric fields can last only for ∼30 min because of the shielding effect in the ring current. In this paper we present the observations of the dayside ionospheric electric field enhancements at middle and low latitudes in association with reorientations of the interplanetary magnetic field (IMF). In six cases, the eastward electric field in the dayside equatorial ionosphere, measured by the Jicamarca incoherent scatter radar, was enhanced for 2–3 hours after the IMF turned southward and remained continuously southward. In one case the eastward electric field in the dayside midlatitude ionosphere, measured by the Millstone Hill incoherent scatter radar, was continuously enhanced for ∼10 hours during southward IMF. Since Millstone Hill is close to the equatorward boundary of the auroral zone during magnetic storms, the penetration electric field there may be different from that at the equatorial ionosphere. The most striking feature of the measurements is that the enhancements of the ionospheric electric field can last for many hours without significant decay. The electric field enhancements in the middle- and low-latitude ionosphere are closely related to magnetic activity and occur during the main phase of magnetic storms. The observations show that the interplanetary electric field can continuously penetrate to the low-latitude ionosphere without shielding for many hours as long as the strengthening of the magnetic activity is going on under storm conditions.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[2] It has been known that the interplanetary electric field (IEF) can penetrate to the low-latitude ionosphere. Nishida [1968] found that geomagnetic field perturbations at high and low latitudes were well correlated with interplanetary magnetic field (IMF) reorientations and suggested that the IEF penetrated deep into the magnetosphere down to the equatorial ionosphere. Penetration of the IEF to the middle- and low-latitude ionosphere has been extensively studied since then through geomagnetic field perturbations and in particular by the incoherent scatter radar chain including the Sondrestrom, Millstone Hill, Arecibo, and Jicamarca radars at ∼75°W geographic longitude [Kelley et al., 1979, 2003; Gonzales et al., 1978, 1979, 1983; Fejer et al., 1979, 1990a; Blanc, 1983; Earle and Kelley, 1987; Bastista et al., 1991; Sastri et al., 1992; Pi et al., 1993; Kikuchi et al., 1996; Foster and Rich, 1998; Reddy and Mayr, 1998; Buonsanto et al., 1999]. Spiro et al. [1988] and Fejer et al. [1990b], using the Rice Convection Model, simulated the temporal, longitudinal, and latitudinal variations of penetration electric fields and showed reasonable agreement with observations. Besides the IEF, substorms are another source of magnetospheric electric field perturbations. Penetration of substorm-associated magnetospheric electric fields to the low-latitude ionosphere has been studied by Kikuchi et al. [2000, 2003], Sastri et al. [2001, 2003], and Huang et al. [2004].

[3] A shielding/overshielding mechanism has been proposed for a long time to explain how the interplanetary/magnetospheric electric field can or cannot penetrate to the low-latitude ionosphere [Vasyliunas, 1972; Jaggi and Wolf, 1973; Wolf, 1974; Southwood, 1977]. Electric field penetration is influenced by the magnetospheric hot plasma that is the source of the region-2 field-aligned currents. When the magnetospheric plasma convection from the tail toward the Earth is enhanced during southward IMF, the plasma is deflected due to gradient curvature drift in the ring current. Strong pressure gradient there results in enhanced region-2 currents. In a steady state, the region-2 currents tend to minimize the electric field at low latitudes, producing the shielding effect. An alternative interpretation of the shielding effect is that polarization charges accumulate at the inner boundary of the ring current during enhanced magnetospheric convection, creating a secondary potential drop which more or less cancels out the effect of the primary magnetospheric electric field [Senior and Blanc, 1984]. In the opposite case, if the magnetospheric convection is suddenly weakened when the IMF turns from southward to northward, the shielding charges will be the source of the dusk-to-dawn electric field in the inner magnetosphere, resulting in an eastward electric field in the nightside ionosphere and a westward electric field on the dayside [Kelley et al., 1979]. This is termed the overshielding effect.

[4] A time constant of the shielding/overshielding process provides an estimate as to how long the penetration electric field will last. Senior and Blanc [1984] derived a shielding time constant of ∼30 min. The simulations of Spiro et al. [1988] and Fejer et al. [1990b] showed that the penetration electric field decayed significantly after 10–20 min. Somayajulu et al. [1987] compared the equatorial ionospheric electric field increase with the corresponding increase in the interplanetary electric field and suggested that the region 2 shielding current might take ∼20 min to develop. Kikuchi et al. [2000] identified a time delay of 17 min between the sudden changes in the signatures of the region 1 and region 2 currents and interpreted the time delay as the shielding time. Kobea et al. [2000] observed a strong westward perturbation of the equatorial electrojet of ∼3 hours in the local morning sector and interpreted the enhancement of the equatorial electrojet as the signature of the overshielding effect. The numerical simulations of Peymirat et al. [2000] for this case showed that the overshielding electric field decayed significantly within the first ∼30 min and nearly vanished within 1.5 hours. In the empirical model of Fejer and Scherliess [1995] and Scherliess and Fejer [1998], the prompt penetration of electric field is represented by an enhancement of the equatorial ionospheric electric field during the first 10–30 min.

[5] Enhanced ionospheric electric fields associated with IMF reorientations have important effects on the ionospheric electron density. Huang et al. [2005] reported the observations of a strong positive ionospheric storm measured by the Millstone Hill radar. During the positive storm, the dayside midlatitude F region electron density was increased by more than 100% over the previous quiet time. The positive storm phase lasted for ∼10 hours, from local morning (∼0912 LT) to local evening (∼1900 LT). The Millstone Hill radar detected a continuous enhancement of the eastward ionospheric electric field during this interval. They suggest that the long-duration enhancement of the electric field moved the F region plasma to higher altitudes, resulting in the large increases of the plasma density. Huang and Foster [2001] and Huang et al. [2002] found that oscillations of the IMF may cause variations of ∼30% in the midlatitude ionospheric F region electron density.

[6] Observations of long-duration (>1 hour) penetration of the IEF to the low-latitude ionosphere were very few in previous studies except for the one reported recently by Huang et al. [2005]. It is not understood how and when electric field penetration can last for hours without shielding. In this paper we present the measurements of the Jicamarca and Millstone Hill radars and show that the ionospheric electric field at low latitudes can be continuously enhanced over many hours during the main phase of magnetic storms. We will discuss the possible mechanisms responsible for the long-duration electric field penetration.

2. Observations

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[7] Large-scale equatorial spread F plasma bubbles often occur during the night times. The electric fields within the plasma bubbles are very strong. In order to avoid any enhanced electric fields associated with spread F, we choose only the cases in which electric field enhancements occur during the daytime and are unambiguously not related to spread F processes.

[8] We first present a case in which the IMF is oscillating with a period of ∼1.5 hour. The purpose of showing this case is to reproduce the characteristic features of oscillating electric field penetration in previous studies. Figure 1 displays the solar wind pressure, IMF Bz component, IEF Ey component, and ionospheric electric field eastward (Ey) component on 17 April 2002. The solar wind parameters were measured by the Wind satellite. During the period of interest, Wind was located at XGSM = 12 RE, YGSM = 195 RE, and ZGSM = 45 RE. Since Wind was far from the Sun-Earth line, we compare a large solar wind pressure impulse around 1100 UT with the corresponding compression of the magnetosphere to best determine the time of the solar wind arrival at the magnetosphere. Accordingly, the solar wind data have been shifted by 6 min. In all cases studied in this paper, the solar wind parameters were measured by the ACE and Wind satellites. We have considered the locations of the satellites and made necessary time shift to allow for the solar wind to travel from the satellite position to the magnetosphere. In the following, we will simply present the shifted solar wind data and no longer give the details of the satellite location and time shift. The IMF Bz is given in GSM coordinate in all figures.

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Figure 1. Solar wind pressure, interplanetary magnetic field (IMF) Bz component, and interplanetary electric field (IEF) Ey component measured by the Wind satellite, and ionospheric electric field eastward (Ey) component measured by the Jicamarca radar on 17 April 2002. The vertical dotted lines indicate negative peaks in the IMF Bz and positive peaks in the IEF Ey.

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[9] The IMF was oscillating between southward and northward with a period of ∼1.5 hours in this case. Each southward turning of the IMF corresponds to an enhancement of the IEF Ey in the duskward direction. The vertical dotted lines at 1230, 1415, 1540, 1645, and 1833 UT in Figure 1 indicate the negative peaks in the IMF Bz and positive peaks in the IEF Ey. The bottom panel shows the ionospheric electric field measured by the Jicamarca radar with incoherent scatter mode. The electric field data of the radar are averaged over the altitude range of 330–490 km. The time resolution of the Jicamarca radar data in all cases presented in this paper was 5 min. Local time at Jicamarca is UT - 5 hours. It is clear that each southward turning of the IMF (or each positive enhancement of the IEF Ey) resulted in an enhancement of the eastward electric field in the dayside equatorial ionosphere. The ionospheric electric field oscillated with the same period as the IMF. This is a typical event of IEF penetration to the low-latitude ionosphere. Because the IMF did not maintain a stable value, it is not possible to determine whether the ionospheric electric field can be continuously enhanced for >1 hour from this case.

[10] We now study electric field penetration when the IMF remains stable for >1 hour. Figure 2 depicts the solar wind pressure, IMF Bz, IEF Ey, and ionospheric electric field Ey on 12 and 13 January 1999. The ionospheric electric field data are averaged over the altitude range of 248–368 km. The day of 12 January was a quiet day, and the data on this day are plotted as a quiet-time reference. Our purpose is to compare the change of the low-latitude ionospheric electric field in response to IMF reorientations with the values over a relatively quiet day. The quiet-time reference used in this paper is only for comparison but does not represent the statistical average variations. A detailed study of the average plasma drift over Jicamarca has been given by Fejer et al. [1991]. In the case of Figure 2, the IMF turned strongly southward at 1615 UT, decreased gradually in magnitude, and became close to the quiet-day value at ∼2000 UT. The IEF Ey was enhanced between 1615 and 2000 UT. The equatorial ionospheric electric field measured by the Jicamarca radar (incoherent scatter mode) was also enhanced between 1615 and 2000 UT. An important feature is that the magnitude of the ionospheric electric field did not show any decay between 1615 and 1820 UT. This may indicate that the IEF penetrated to the equatorial ionosphere without shielding for ∼2 hours. However, the ionospheric electric field became much smaller after 1900 UT, as indicated by the vertical dashed line. It appears that the IEF was largely shielded after 1900 UT. We will discuss why the penetration electric field behaved very differently later.

image

Figure 2. Solar wind pressure, IMF Bz, and IEF Ey measured by the ACE satellite, and ionospheric electric field Ey measured by the Jicamarca radar on 12 and 13 January 1999. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1615 and 2000 UT on 13 January.

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[11] Figure 3 shows a case in which the IMF turned southward for 2 hours and then turned northward for ∼5 hours on 9–10 November 2004. The data on 11–12 November are plotted as a quiet-time reference. The vertical dotted lines indicate the IMF reorientations on 9–10 November. The IMF was small before 1848 UT on 9 November, became southward between 1848 and 2048 UT, turned northward at 2048 UT, and remained strongly northward until 0130 UT on 10 November. The ionospheric electric field Ey measured by the Jicamarca radar (incoherent scatter mode) are presented in the bottom panel; the ionospheric electric field data are averaged over the altitude range of 248–368 km. When the IMF turned southward between 1848 and 2048 UT, the ionospheric electric field became strongly eastward. The shape of the enhanced eastward ionospheric electric field is very similar to the positive enhancement of the IEF Ey, indicating that the electric field penetration occurred during the entire interval of southward IMF. When the IMF turned northward between 2048 and 0130 UT, the ionospheric electric field became westward. In particular, the magnitude of the westward electric field increased gradually without decay between 2048 and 0130 UT. After the IMF turned southward again at 0130 UT, the westward ionospheric electric field remained for an additional ∼20 min. The temporal difference between the IMF southward turning and the end of the westward ionospheric electric field may be caused by the time shift. In Figure 3, a constant time shift of the solar wind data is used to match the solar wind pressure impulse at 1848 UT on 9 November with the sudden compression of the magnetosphere. However, because the Wind satellite was far from the Earth, the large changes in the IMF direction and solar wind speed after 1848 UT may require a nonconstant time shift for the solar wind to travel from Wind to the magnetosphere. Another striking phenomenon in this case is that the ionospheric electric field remained strongly westward for ∼5 hours during the entire interval of northward IMF.

image

Figure 3. Solar wind pressure, IMF Bz, and IEF Ey measured by the Wind satellite, and ionospheric electric field Ey measured by the Jicamarca radar on 9–10 and 11–12 November 2004. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1848 and 2048 UT and the interval of northward IMF and enhanced westward ionospheric electric field between 2048 and 0130 UT on 9–10 November.

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[12] The continuous enhancement of the westward ionospheric electric field for ∼5 hours with stable northward IMF occurred during the initial recovery phase of a magnetic storm. Different mechanisms may cause such an enhancement. One is the overshielding effect proposed by Kelley et al. [1979]. If the IMF turns northward after several hours of being southward, the polarization charges generated during the interval of southward IMF may exist until the weakening of the magnetospheric convection ends. This explanation arises from the coincidence between the northward IMF and enhanced westward ionospheric electric field. Another possible mechanism is the disturbance dynamo electric field. As shown by Fejer and Scherliess [1995] and Fejer and Emmert [2003], the disturbance dynamo at low latitudes will develop after southward IMF of several hours and cause enhanced westward electric field. Since we do not have simultaneous measurements of electric fields at other latitudes in this case, it is difficult to exclusively determine the source of the enhanced westward ionospheric electric field. Further investigations are required to solve this problem. In this paper we focus on the penetration electric field during southward IMF.

[13] Figure 4 shows another case which occurred on 8 April 1993, and the data on 7 April are plotted as a quiet-time reference. In this case the solar wind data from the Geotail and IMP 8 satellites are used. During the period of interest, IMP 8 was located at XGSM = 1 RE, YGSM = −23 RE, and ZGSM = −30 RE. Because of the Y-Z position, IMP 8 was in the solar wind. However, the IMP 8 data are available only between 1611 and 1813 UT on 8 April. Geotail was located at XGSM = −46 RE, YGSM = −22 RE, and ZGSM = −12 RE and might be close to the boundary of the magnetospheric tail. The shape of the magnetic field measured by Geotail was very similar to that measured by IMP 8 but had a slightly larger amplitude, indicating that Geotail was in the solar wind. The larger amplitude of the Geotail measured IMF Bz is the effect of the bow shock. The solar wind pressure measured by IMP 8 was almost exactly the same as that measured by Geotail, further indicating the presence of Geotail in the solar wind. The ionospheric electric field measured by the Jicamarca radar (incoherent scatter mode) is plotted in the bottom panel of Figure 4, and the data are averaged over an altitude range of 260–460 km. The ionospheric electric field was enhanced starting from 1412 UT on 8 April, corresponding to a southward turning of the IMF, and showed a decrease between 1500 and 1518 UT, corresponding to a short-lived northward turning of the IMF. The period we are interested in is between 1518 and 1745 UT, during which the IMF was continuously southward, as indicated by the vertical dotted lines. The difference in the IMF Bz during this period between the event and quiet days was nearly constant. The difference in the ionospheric electric field was also approximately constant between the 2 days, although the magnitude of the electric field decreased with time. The observations suggest that the penetration electric field did not decay over the interval of 2.4 hours.

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Figure 4. Solar wind pressure and IMF Bz measured by the Geotail and IMP 8 satellites, and ionospheric electric field Ey measured by the Jicamarca radar on 7 and 8 April 1993. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1518 and 1745 UT on 8 April.

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[14] In recent years, the Jicamarca radar has run a coherent scatter mode which makes continuous measurements over many days and even months possible. In the coherent scatter radar mode, the equatorial F region E × B drifts are determined from the Doppler shifts of 150-km echoes [Chau and Woodman, 2004]. Huang et al. [2004] have used the coherent scatter measurements to study low-latitude ionospheric electric fields associated with substorms. The data of the coherent scatter radar mode are available only during the daytime, which is suitable for the purpose of studying daytime electric field penetration.

[15] Shown in Figure 5 are the solar wind pressure, IMF Bz, and IEF Ey measured by the Wind satellite, equatorial electric field measured by the Jicamarca radar with coherent scatter mode, and equatorial electric field derived from ground magnetometer measurements on 22 and 23 September 2002. The data of 23 September are plotted as a quiet-time reference. Near the equator, an eastward electric field causes an enhanced eastward current (the equatorial electrojet) through the Cowling effect during the daytime, and the electrojet produces horizontal magnetic field perturbations in the north-south direction. Anderson et al. [2002] showed that the difference in the H component (ΔH) between a magnetometer on the magnetic equator and one displaced 6–9 degrees away can be used to infer the vertical plasma drift velocity VE×B. Anderson et al. [2004] recently studied how the relationship between VE×B and ΔH varies with month and solar F10.7 cm flux. The magnetometers have continuous data coverage, and we can use the measured ΔH to calculate the vertical velocity VE×B and the eastward electric field through the relationship VE×B = E/B. In our paper we use the first formula in Table 1 of Anderson et al. [2004] to calculate the eastward electric field over the equator, and the result is given in the bottom panel of Figure 5. The IMF turned southward during 1442–1712 UT on 23 September, and the IEF Ey was enhanced. Note that the magnitude of the southward IMF was very small (∼4 nT). However, an enhancement of the ionospheric electric fields during this period were measured by both the Jicamarca radar and magnetometers. The duration of the electric field enhancement was 2.5 hours in this case.

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Figure 5. Solar wind pressure, IMF Bz, and IEF Ey measured by the Wind satellite, and ionospheric electric field Ey measured by the Jicamarca radar and derived from the Jicamarca and Piura magnetometer measurements on 22 and 23 September 2002. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1442 and 1712 UT on 22 September.

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[16] Figure 6 shows a case which occurred on 5 December 2003, and the data on 6 December are plotted as a quiet-time reference. The IMF turned southward during 1342–1618 UT on 5 December, although there were some oscillations in the magnitude of the IMF Bz. The IEF Ey was enhanced during the interval of southward IMF. The radar data on 5 December showed a large electric field during the period of southward IMF. The magnetometers had continuous data over the 2 days, and the electric field derived from the Anderson et al. [2004] formula is similar to the radar measurement. The ionospheric electric field showed a sudden increase in response to the IMF southward turning at 1342 UT, maintained a large and stable amplitude for ∼2 hours until 1545 UT, and then started to decrease. The ionospheric electric field had a nearly constant amplitude without any decay for ∼2 hours between 1342 and 1545 UT. The IMF had additional two southward turnings around 1700 and 1820 UT, and the corresponding enhancements in the ionospheric electric field were clearly monitored by the radar and magnetometers.

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Figure 6. Solar wind pressure, IMF Bz, and IEF Ey measured by the ACE satellite, and ionospheric electric field Ey measured by the Jicamarca radar and derived from the Jicamarca and Piura magnetometer measurements on 5 and 6 December 2003. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1342 and 1618 UT on 5 December.

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[17] Figure 7 shows an additional case that occurred on 7 September 2002. The IMF was continuously northward on 8 September, and the data of this day are plotted as a quiet-time reference. On 7 September, the IMF was continuously southward (between −4 and −7 nT) from 1100 UT, suddenly became strongly southward (about −22 nT) at 1636 UT, and then turned northward at 1815 UT. The Jicamarca radar data were not available during the interval of strongly southward IMF. Nevertheless, the magnetometers provided continuous data coverage. As demonstrated in Figures 5 and 6, the ionospheric electric field derived from the magnetometer data is in good agreement with the radar data and can be used as a reliable measurement. As shown in the bottom panel of Figure 7, the ionospheric electric field was greatly enhanced during the interval of strongly southward IMF for ∼1.6 hours between 1636 and 1815 UT. Another important feature in Figure 7 is that the ionospheric electric field was also enhanced during a long interval of relatively weak southward IMF between 1100 and 1636 UT, and the amplitude of the ionospheric electric field did not show any decay. It appears that the ionospheric electric field will maintain an enhanced amplitude as long as the IMF remains southward and stable.

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Figure 7. Solar wind pressure, IMF Bz, and IEF Ey measured by the ACE satellite, and ionospheric electric field Ey measured by the Jicamarca radar and derived from the Jicamarca and Piura magnetometer measurements on 7 and 8 September 2002. The vertical dotted lines indicate the interval of southward IMF and enhanced eastward ionospheric electric field between 1636 and 1815 UT on 7 September.

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[18] A long-duration (∼10 hour) enhancement of the ionospheric electric field during continuous southward IMF was indeed measured by the Millstone Hill radar during a magnetic storm on 3 April 2004. The details of this case have been analyzed by Huang et al. [2005]. Here we only present some measurements related to the electric field. Shown in Figure 8 are the solar wind pressure, IMF Bz, and IEF Ey measured by the Wind satellite, and eastward ionospheric electric field and total electron content (TEC) measured by the Millstone Hill radar. The data on 2 April are plotted as a quiet-time reference. A solar wind pressure impulse hit the magnetosphere at 1412 UT on 3 April and triggered the magnetic storm. The IMF showed a northward spike at the time of the solar wind pressure impulse and then turned southward. The southward IMF continued for ∼10 hours, and the IEF Ey was enhanced during this interval. The ionospheric electric field at middle latitudes was enhanced immediately following the IMF southward turning, and the difference in the ionospheric electric field between the event and quiet days was almost constant over the entire interval of southward IMF. Shown in the bottom panel of Figure 8 is the F region TEC which is obtained by integrating the electron density over the altitude range of 180–600 km measured by the Millstone Hill radar. As reported in detail by Huang et al. [2005], the midlatitude ionospheric F peak moved upward rapidly during the interval of southward IMF, and the electron density was increased by a factor of 2–4 over the F region compared to the previous quiet-time values. The increase of the ionospheric electron density lasted for ∼10 hours and is classified as a long-duration ionospheric positive storm. Huang et al. [2005] suggest that the enhanced ionospheric electric field is responsible for the generation of the positive storm by moving the F region plasma upward through E × B drift.

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Figure 8. Solar wind pressure, IMF Bz, and IEF Ey measured by the Wind satellite, and ionospheric electric field Ey and total electron content (TEC) measured by the Millstone Hill radar on 2 and 3 April 2004. The vertical dotted line indicates a storm sudden commencement (SSC) at 1412 UT and subsequent southward IMF through 2400 UT on 3 April. The ionospheric electric field and TEC were enhanced after the SSC.

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[19] However, we should notice the possible difference in the electric field between higher midlatitudes such as at Millstone Hill and equatorial latitudes. Millstone Hill is close to the equatorward boundary of the auroral zone during magnetic storms, and the auroral electric field may have much stronger effects on the ionosphere over Millstone Hill than over the equator. For the first 2–3 hours after the IMF southward turning, the ionospheric electric field enhancement should be the direct consequence of penetration of the interplanetary electric field. A new circulation pattern of the neutral atmosphere may develop 2–3 hours after a sudden storm commencement, and the dynamo electric field may also contribute to the total electric field. The measured electric field during the late part of the 10-hour interval might be a combination of the penetration and disturbance dynamo electric fields. Therefore the 10-hour enhancement of the ionospheric electric field over Millstone Hill in the case of Figure 8 may not be exactly the same as the prompt penetration electric field of 2–3 hours at low latitudes.

[20] Finally, we examine how the long-duration enhancements of the ionospheric electric field are related to magnetic storms. Figure 9 shows the IMF Bz and Dst index in the seven cases of Figures 28. The Dst index has a time resolution of 1 hour. We study the penetration electric field in a time interval of 2–3 hours, and an index with higher time resolution can better show its correlation with the electric field. Therefore we also plot in Figure 9 the SYM-H index that has a time resolution of 1 min. The SYM-H index is also derived from midlatitude magnetometers and is a measure of the symmetric component of the ring current [Iyemori, 1990; Iyemori and Rao, 1996]. In fact, SYM-H is equivalent to Dst but has a higher time resolution than Dst. In general, the Dst index is very close to the SYM-H index. However, in the case of 22 September 2002 shown in Figure 9d, the magnitude of SYM-H is much smaller than that of Dst; it is not clear what causes the difference. Magnetic storms are classified by the minimum Dst value. Following Gonzalez et al. [1994], a magnetic storm is classified to be weak with minimum Dst between −30 and −50 nT, moderate with minimum Dst between −50 and −100 nT, and intense with minimum Dst of less than −100 nT. The minimum Dst value in the seven cases was −107, −231, −61, −35, −62, −163, and −108 nT, respectively. Accordingly, in the seven cases we analyzed, one case (22 September 2002 in Figure 9d) was a weak storm, two cases (8 April 1993 in Figure 9c and 5 December 2003 in 9e) were moderate storms, and four cases (13 January 1999 in Figure 9a, 9–10 November 2004 in Figure 9b, 7 September 2002 in Figure 9f, and 3 April 2004 in Figure 9g) were intense storms.

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Figure 9. IMF Bz and Dst/SYM-H index in seven storm cases. The shaded intervals indicate the occurrence of southward IMF and enhanced ionospheric electric fields.

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[21] A very important feature in Figure 9 is that the enhancements of the ionospheric electric field occurred during the main phase of the magnetic storms when the magnetic activity was strengthening, as indicated by the shaded intervals. The observations indicate that the interplanetary electric field can easily penetrate to the low-latitude ionosphere without shielding during the main phase of magnetic storms. The case of Figure 9a is particularly interesting. As shown in Figure 2, the electric field was enhanced between 1615 and 1900 UT on 13 January 1999 without decay but became much smaller between 1900 and 2000 UT. It can be seen from Figure 9a that the main phase of the magnetic storm ended around 1900 UT, as indicated by the vertical dashed white line, although the IMF remained continuously southward from 1615 through 2000 UT. It is beyond the scope of this paper to find why the main phase of the storm ended when the IMF remained southward. What is important for our study is that penetration of the IEF to the low-latitude ionosphere is not shielded during the main phase of magnetic storms. The case in Figure 9d is a rather weak storm. However, the electric field penetration occurred continuously over the entire interval (2.5 hours) during which the Dst/SYM-H was decreasing, even if the southward IMF was small and the magnetic activity was weak. A common feature in these storm cases is that the continuous penetration of electric fields occurred as long as the magnetic activity was strengthening.

[22] In Figure 9b the main phase of the magnetic storm occurred during southward IMF, and the storm started to recover from 2100 UT after the IMF turned northward, as indicated by the vertical dashed white line. As shown in Figure 3, the 5-hour continuous enhancement of the westward ionospheric electric field occurred during the recovery phase of the storm with northward IMF. The possible mechanisms for the enhanced westward electric field include the overshielding effect and disturbance dynamo, as discussed in association with Figure 3.

3. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[23] In most previous studies [Kelley et al., 1979, 2003; Gonzales et al., 1978, 1979, 1983; Fejer et al., 1979, 1990a; Blanc, 1983; Earle and Kelley, 1987; Bastista et al., 1991; Sastri et al., 1992; Pi et al., 1993; Kikuchi et al., 1996; Foster and Rich, 1998; Buonsanto et al., 1999; Reddy and Mayr, 1998], oscillating electric fields with a characteristic period of ∼1 hour were used to demonstrate penetration of the IEF to the low-latitude ionosphere because the correspondence between the oscillating IEF and ionospheric electric field can be easily identified. Figure 1 of our paper shows a typical case of such phenomena. Almost all theories, simulations, and empirical models predict that the prompt penetration of electric fields can last only for ∼30 min because of the shielding-overshielding effect. In the theory of Senior and Blanc [1984], a time constant of the shielding effect is derived and is typically the order of half an hour. In the simulations of Spiro et al. [1988] and Fejer et al. [1990b], the equatorial ionospheric electric field decays significantly within 10–20 min, although the enhancement of the polar cap electric field remains for 135 min. In the simulations of Peymirat et al. [2000] the polar cap electric field is kept at a fixed elevated value for 2 hours, but the low-latitude ionospheric electric field decreases significantly after ∼30 min. The shielding time constant derived by Peymirat et al. is ∼32 min. In the empirical model [Fejer and Scherliess, 1995; Scherliess and Fejer, 1998], the polar cap electric field (or magnetic activity) suddenly jumps to a large value and then remains at the elevated value for 5 hours or longer, and the penetration electric field in the low-latitude ionosphere exists for 10–30 min. Although the elevated polar cap electric field in these simulations and models is maintained for 2 hours or longer, the penetration electric field at low latitudes lasts only for ∼30 min. The decay of the low-latitude ionospheric electric field is interpreted as the consequence of the shielding effect.

[24] In contrast, our observations show that the low-latitude ionospheric electric field can be continuously enhanced without significant decay for many hours during the main phase of magnetic storms as long as the IMF remains southward and stable. The duration of the enhanced ionospheric electric field was 2.1, 7.0, 2.4, 2.5, 2.0, 1.6, and 10 hours in the cases shown in Figures 28. The case of Figure 3 can be divided into two intervals, an eastward electric field enhancement during the first interval of ∼2 hours with southward IMF and a westward electric field enhancement during the second interval of ∼5 hours with northward IMF.

[25] In the cases presented in this paper, there were no simultaneous measurements from both the Millstone Hill and Jicamarca radars, which is unfortunate. There have been a number of joint radar experiments. Since we only choose the daytime electric field penetration in order to avoid nighttime storm surges over Millstone Hill and spread F plasma bubbles over the equator, most radar chain studies of magnetic storms cannot be used for the purpose of this paper. However, there is no doubt that the electric field enhancements detected by the Jicamarca radar are caused by IEF penetration because the ionospheric electric field is clearly correlated with the IEF and because no dynamo electric field at low latitudes can be generated within the first 2–3 hours of a magnetic storm. During magnetic storms, energy input from the magnetosphere into the ionospheric auroral zone will launch atmospheric disturbances. In general, even large-scale atmospheric disturbances during intense storms will take 2–3 hours travel to low latitudes [Fuller-Rowell et al., 1994], so dynamo electric field at low latitudes can be generated 2–3 hours after sudden storm commencement (SSC). However, the enhancement of the low-latitude ionospheric electric field in our cases occurred almost immediately after IMF southward turning (or SSC); this enhancement cannot be attributed to dynamo process because atmospheric disturbances cannot travel to the equatorial ionosphere within 2 hours. Therefore the observed electric field enhancement is explained as penetration of the IEF to the low-latitude ionosphere, and the stable magnitude of the ionospheric electric field indicates that the IEF penetration is not shielded for several hours.

[26] Now the question is why the shielding process does not work effectively to prevent long-duration penetration of electric fields during the main phase of magnetic storms. This issue is not well understood. Here we propose a qualitative interpretation. A perfect shielding can be achieved only if the polarization electric field within the ring current completely cancels out the magnetospheric convection electric field [Senior and Blanc, 1984]. The efficiency of the shielding mechanism depends on the ionospheric conductivity. If the ionospheric conductivity is high enough, the polarization charges accumulated at the inner edge of the ring current will be discharged, and the shielding effect is diminished. During the main phase of magnetic storms with southward IMF, the field-aligned currents and auroral ionospheric conductivity are greatly enhanced. As a result, the polarization electric field within the ring current cannot fully develop to cancel out the magnetospheric convection electric field, and the convection electric field will continuously leak to (or penetrate to) the low-latitude ionosphere without significant decay for many hours as long as the strengthening of the magnetic activity is going on. An alternative interpretation of the ineffective shielding is the imbalance between the region 1 and region 2 currents [Kikuchi et al., 1996, 2000, 2003]. During the main phase of magnetic storms, the region 1 field-aligned current is stronger than the region 2 field-aligned current, and the unbalanced region 1 current will penetrate to the low-latitude ionosphere. It should be noticed that both interpretations are qualitative, and there is no quantitative result so far that indicates the existence of penetration electric field for many hours without shielding.

[27] The difference in the timescale of electric field penetration between theories/models and our observations may be also related to the variation of magnetic activity. The magnetic activity used for modeling penetration electric field is a step function of the auroral electrojet (AE) index [Senior and Blanc, 1984; Spiro et al., 1988; Fejer et al., 1990b; Peymirat et al., 2000; Fejer and Scherliess, 1995; Scherliess and Fejer, 1998]. The AE index in these models is assumed to suddenly jump from near zero to a large value, remains constant in the elevated value for several hours, and then suddenly drops to zero. The predicted timescale of ∼30 min for electric field penetration may be accurate for the step function of the modeled magnetic activity. However, the real magnetic activity is continuously strengthening for several hours during the main phase of magnetic storms. Figure 10 shows the different variations of the model AE index and the real AE index in the case of 13 January 1999 (Figures 2 and 9a). The step function is typical for models, and the real AE index takes ∼2 hours to grow to the maximum value. The interplanetary electric field penetrates continuously to the low-latitude ionosphere during the entire interval of increasing AE. The observations indicate that the temporal variation of the AE index may be important for modeling the low-latitude ionospheric electric field.

image

Figure 10. AE index on 13 January 1999. The thin line represents the measured AE index, and the heavy-line step function represents a model AE that is used in theories and simulations.

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[28] The penetration electric field during magnetic storms can cause significant disturbances in the low-latitude ionosphere. Basu et al. [2001] reported on the satellite and ground-based measurements of ionospheric electron density variations and scintillations at the equatorial and anomaly latitudes during a severe magnetic storm. When the IMF turned southward, the penetration electric field uplifted the equatorial ionosphere, and the ion density measured by the DMSP satellite at 840-km altitude over a latitudinal range of ∼10° across the equator was several orders of magnitude smaller than that at the anomaly latitudes. The ionospheric scintillations in these disturbed regions were enhanced remarkably. Kelley et al. [2004] examined the behavior of total electron content in this storm and presented a quantitative explanation of storm-enhanced density. In their scenario the penetrating zonal electric field drives the equatorial anomaly poleward. This low-latitude source is maintained into the nightside, but the plasma is advected westward by a penetrating poleward field. The buildup of plasma in a narrow channel results in the formation of storm-enhanced density that is connected to the plasmaspheric tails.

[29] The long-duration enhancements of the ionospheric electric field also have profound effects on the midlatitude ionospheric electron density. An eastward electric field on the dayside will move the F region plasma to higher altitudes through E × B drift, and lower recombination rate at higher altitudes will cause large increases in the F region electron density. In the case shown in Figure 8, the electric field enhancement lasted for ∼10 hours, and the dayside ionospheric F region TEC was increased by ∼100%. Huang et al. [2005] suggest that the long-duration enhancement of the ionospheric electric field is responsible for the generation of the long-duration ionospheric positive storm. M. Swisdak et al. (Simulations of a positive storm phase observed at Millstone Hill, submitted to Geophysical Research Letters, 2005), using the SAMI2 model, simulated the effect of electric fields on the midlatitude ionospheric electron density in the case of Huang et al. [2005]. Swisdak et al. used the electric field and neutral wind measured by the Millstone Hill radar as input to SAMI2 and reproduced the ionospheric electron density enhancement that is in good agreement with the radar measurement.

[30] The IMF is generally strongly southward for many hours during the main phase of super magnetic storms (e.g., with a minimum Dst of smaller than −200 nT). If the interplanetary electric field during superstorms (at least for the first 2–3 hours) can penetrate to the low-latitude ionosphere without shielding, it will cause significant redistribution of the global ionospheric electron density. At low latitudes, an enhanced eastward electric field will uplift the equatorial ionospheric plasma particles, making the equatorial F region almost empty and the anomaly-latitude plasma density much higher. At middle latitudes, the E × B drift will move the ionospheric plasma upward and cause great increases of the plasma density. At higher latitudes, the enhanced electric field can cause storm enhanced density in the dusk sector and subsequent transport to the polar ionosphere. Studies of ionospheric electric fields during magnetic storms are very important for understanding global ionospheric disturbances and for space weather applications.

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[31] We have presented the observations of ionospheric electric field enhancements at middle and low latitudes in association with IMF reorientations. When the IMF turns southward and remains stably southward for several hours, the dayside eastward ionospheric electric field at low latitudes is enhanced throughout the entire interval of southward IMF. A similar enhancement of the westward ionospheric electric field is observed when the IMF turns northward and remains stably northward. In the cases analyzed, the ionospheric electric field enhancement lasts for longer than 1 hour without significant decay, so it is termed the long-duration enhancement of the ionospheric electric field. More specifically, the duration of the ionospheric electric field enhancements without decay in the seven cases is 2.1, 7.0, 2.4, 2.5, 2.0, 1.6, and 10 hours, respectively.

[32] All cases are related to magnetic storms. One storm is weak with a minimum Dst of −35 nT, two storms are moderate with a minimum Dst between −50 and −100 nT, and four storms are intense with a minimum Dst of less than −100 nT. The long-duration enhancements of the eastward ionospheric electric field occur during the main phase of magnetic storms with continuously southward IMF, and the enhancement of the westward ionospheric electric field occurs during continuously northward IMF. The enhancements of the ionospheric electric field in all seven cases are explained as the consequence of interplanetary electric field penetration.

[33] The observations show that the interplanetary electric field penetrates to the low-latitude ionosphere for many hours without decay, which implies that the ring current does not shield the electric field under the storm conditions studied. We suggest that the polarization electric field in the inner magnetosphere is not large enough to cancel out the magnetospheric convection electric field during the main phase of magnetic storms with southward IMF. The convection electric field will continuously penetrate to the low-latitude ionosphere for many hours as long as the strengthening of the magnetic activity is going on.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References

[34] We thank Bela Fejer of Utah State University for helpful discussion. Work at MIT Haystack Observatory was supported by a NSF cooperative agreement with Massachusetts Institute of Technology. The Jicamarca Radio Observatory is operated by the Instituto Geofisico del Peru, with support from the NSF Cooperative Agreement ATM-0432565 through Cornell University, and we thank Jorge Chau for providing the radar and magnetometer data. We acknowledge the CDAWeb for access to the ACE, Wind, and Geotail data. The SYM-H and AE data are provided by the World Data Center for Geomagnetism at Kyoto University.

[35] Lou-Chuang Lee thanks Takashi Kikuchi and another referee for their assistance in evaluating this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Observations
  5. 3. Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References