Transmission of the electric fields to the low latitude ionosphere in the magnetosphere-ionosphere current circuit

The solar wind energy is transmitted to low latitude ionosphere in a current circuit from a dynamo in the magnetosphere to the equatorial ionosphere via the polar ionosphere. During the substorm growth phase and storm main phase, the dawn-to-dusk convection electric field is intensified by the southward interplanetary magnetic field (IMF), driving the ionospheric DP2 currents composed of two-cell Hall current vortices in high latitudes and Pedersen currents amplified at the dayside equator (EEJ). The EEJ-Region-1 field-aligned current (R1 FAC) circuit is completed via the Pedersen currents in midlatitude. On the other hand, the shielding electric field and the Region-2 FACs develop in the inner magnetosphere, tending to cancel the convection electric field at the mid-equatorial latitudes. The shielding often causes overshielding when the convection electric field reduces substantially and the EEJ is overcome by the counter electrojet (CEJ), leading to that even the quasi-periodic DP2 fluctuations are contributed by the overshielding as being composed of the EEJ and CEJ. The overshielding develop significantly during substorms and storms, leading to that the mid and low latitude ionosphere is under strong influence of the overshielding as well as the convection electric fields. The electric fields on the dayand night sides are in opposite direction to each other, but the electric fields in the evening are anomalously enhanced in the same direction as in the day. The evening anomaly is a unique feature of the electric potential distribution in the global ionosphere. DP2-type electric field and currents develop during the transient/short-term geomagnetic disturbances like the geomagnetic sudden commencements (SC), which appear simultaneously at high latitude and equator within the temporal resolution of 10 s. Using the SC, we can confirm that the electric potential and currents are transmitted near-instantaneously to low latitude ionosphere on both dayand night sides, which is explained by means of the light speed propagation of the TM0 mode waves in the Earth-ionosphere waveguide. © 2016 Kikuchi and Hashimoto. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Introduction This article reviews the transmission of the electric field and currents from the dynamos in the magnetosphere down to the equatorial ionosphere to better understand the ionospheric and geomagnetic disturbances at mid and low latitudes during substorms and storms. The dynamo for the convection electric field and the Region-1 field-aligned currents (R1 FACs) is reviewed in “Convection electric field and global DP2 currents” section, and that for the shielding/overshielding and the R2 FACs in “Overshielding electric field and CEJ” section. These two kinds of electric fields and currents play a crucial role in the substorm and storm as reviewed in “DP2 and CEJ during the substorm” and “Stormtime electric field and EEJ/CEJ” sections, respectively. The geomagnetic sudden commencement is briefly reviewed in “Electric field and currents during the SC” section as an introduction to the mechanism for the near-instantaneous transmission from the polar ionosphere to the equator as reviewed in detail in “Electric field transmission mechanism” section. Convection electric field and global DP2 currents The magnetospheric convection is initiated by the reconnection between the southward interplanetary magnetic Open Access *Correspondence: [email protected] 1 Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi, Japan Full list of author information is available at the end of the article Page 2 of 11 Kikuchi and Hashimoto Geosci. Lett. (2016) 3:4 field (IMF) and the Earth’s magnetic field at the magnetopause (Dungey 1961). The convection electric field is generated by the dynamo around the cusp/mantle region where the solar wind energy is converted to the thermal energy of high-pressure plasma (Tanaka 1995). Figure 1 shows dynamo currents with red lines flowing (right) across the magnetic field lines (left) in the tailward cusp/mantle region and the Region-1 field-aligned currents (R1 FACs) with black lines flowing into the polar ionosphere assumed at 3.5 Re in the simulation. The red and black colors in Fig. 1 indicate negative and positive J·E (J: current, E: electric field), respectively, referring to the generation and consumption of the electromagnetic energy. The dynamo provides the dawn-to-dusk convection electric field and the R1 FACs flowing into/up from the polar ionosphere in the morning/afternoon sector, coinciding with the satellite observations (Iijima and Potemra 1976). The dawn-to-dusk electric field propagates near-instantaneously to low latitude (Kikuchi et al. 1996), directing eastward on the dayside and westward on the nightside. The convection electric field drives the DP2 currents composed of two-cell Hall current vortices at high latitude and zonal currents at the equator (Nishida 1968). The DP2 magnetic fluctuations are well correlated with the southward IMF (Nishida 1968) and occur simultaneously at high latitude and equator (Kikuchi et al. 1996). Figure 2 shows DP2 fluctuations at high latitude (Nurmijarvi) and equator (Mokolo) with the correlation coefficient of 0.9 and no time shift greater than 25 s, suggesting near-instantaneous transmission of the convection electric field to the equator same as for the preliminary impulse (PI) of the geomagnetic sudden commencement (SC) (Araki 1977). The ionospheric currents decrease Fig. 1 Dynamo for the convection electric field and the Region-1 field-aligned currents reproduced by the global MHD simulation. The dynamo currents shown with the red lines are diamagnetic currents generated by the pressure force of hot plasma (left) poleward of the cusp region, (right) flowing across the magnetic field lines and driving the R1 FACs down the magnetic field lines into the ionosphere as shown with the black lines. The ionosphere is assumed at 3.5 Re in the simulation. (from Fig. 8 of Tanaka 1995) Fig. 2 Quasi-periodic DP2 magnetic fluctuations observed at high latitude (Nurmijarvi) and equator (Mokolo) with the high correlation (corr. coefficient = 0.9) and no time shift greater than 25 s, indicating near-instantaneous transmission of the convection electric field to the equator. (from Fig. 4 of Kikuchi et al. 1996) Page 3 of 11 Kikuchi and Hashimoto Geosci. Lett. (2016) 3:4 with decreasing latitude because of the geometrical attenuation, but increased at the dayside equator where the currents are intensified by the Cowling effect (EEJ) (Hirono 1952; Baker and Martyn 1953). The enhanced EEJ is an important feature of the equatorward extension of ionospheric currents from the polar ionosphere. Figure 3 shows a schematic diagram of the R1 FACs-EEJ circuit via the polar ionosphere achieved when the R1 FACs dominates over the R2 FACs under the southward IMF condition (Kikuchi et al. 1996). A current circuit is completed by the midlatitude Pedersen currents carried by the TM0 mode wave in the Earth-ionosphere waveguide (Kikuchi and Araki 1979). It should be noted that the DP2 electric fields in the evening are significantly enhanced having the same direction as those in the day (Abdu et al. 1988). Figure 4 shows that the eastward electric field in the afternoon-evening hours (upward drift velocity indicated with the thick solid curves in Fig. 4, top) is well correlated with the DP2 currents at the dayside equator (ALCANTARA in the middle) and afternoon high latitude (NURMIJARVI in the bottom). The evening anomaly is a unique feature of the global distribution of the electric potential as calculated by the potential solver with an input of the field-aligned currents in the polar ionosphere (Senior and Blanc 1984; Tsunomura and Araki 1984). Figure 5 shows the electric field calculated for the equator with the model of Tsunomura and Araki (1984) (T and A), Senior and Blanc (1984) (S and B) and Tsunomura (1999) (New), reproducing the evening anomaly with the same direction as in the day and enhanced magnitude. The diurnal magnetic variation at the geomagnetic equator is often depressed substantially during disturbed periods (Matsushita and Balsley 1972). Matsushita and Balsley (1972) critically discussed that the DP2 fluctuations should be measured negative from the quiettime diurnal variation. However, the good correlation between the DP2 fluctuations at high and equatorial latitudes (Kikuchi et al. 1996) are in favor of measuring positive as was done by Nishida (1968). The depression of the diurnal variation must be caused by a westward electric field due to the disturbance dynamo (Blanc and Richmond Fig. 3 A schematic diagram of the DP2 ionospheric currents composed of the two-cell Hall currents at high latitudes driven by the dawn-to-dusk convection electric field under the southward IMF condition and the Pedersen currents flowing into the equatorial ionosphere where the current is enhanced by the Cowling effect (EEJ). A current circuit is completed between the EEJ and R1 FACs via the midlatitude Pedersen currents carried by the TM0 mode wave in the Earth-ionosphere waveguide. (from Fig. 9 of Kikuchi et al. 1996) Fig. 4 (top, thick solid curves) Vertical drift of the F-region ionosphere observed with the HF Doppler sounder over the equator (KODAIKANAL), which is well correlated with the DP2 magnetic fluctuations at the (middle) dayside equator (ALCANTARA) and (bottom) afternoon high latitude (NURMIJARVI). The electric field deduced from the drift velocity is significantly enhanced in the evening with the same direction as in the day. (from Fig. 2 of Abdu et al. 1988) Page 4 of 11 Kikuchi and Hashimoto Geosci. Lett. (2016) 3:4 1980), which is activated in the midlatitude thermosphere/ionosphere by the westward thermospheric wind having traveled from the disturbed polar thermosphere. Overshielding electric field and CEJ The enhanced convection electric field drives an earthward motion of plasma in the plasma sheet, generating the partial ring current and Region-2 field-aligned currents (R2 FACs) in the inner magnetosphere (Vasyliunas 1972). The partial ring current builds up the shielding electric field with an opposite direction to the convection electric field, which intensifies the electric field at auroral latitude but reduces it at the mid and low latitudes. The time constant of the growth of the shielding has been estimated as 20 min from the magnetometer observations (Somayajulu et al. 1987) and 20–30 min from the theoretical calculations (Peymirat et al. 2000). When the convection electric field reduces abruptly because of the northward turning of the IMF, the electric field reverses its direction at mid-equatorial latitudes, causing the equatorial counter electrojet (CEJ) (Rastogi 1977). The reversal of the electric field was confirmed by the Jicamarca incoherent scatter radar at the equator, which was identified as the overshielding electric field (Kelley et al. 1979; Gonzales et al. 1979). DP2 and CEJ during the substorm The substorm growth phase is initiated by the southward turning of the IMF, which causes the DP2 currents in the ionosphere (McPherron 1970; Kamide et al. 1996). Kikuchi et al. (2000) separated out the convection and shielding electric fields during the substorm as shown in Fig. 6 where the solid/dashed curves indicate the convection/shielding electric fields at (top) auroral and (bottom) subauroral latitudes. The convection electric field dominates during the growth phase before the onset identified with the midnight Pi2 (vertical dotted line); however, the shielding electric field develops after the onset, leading to the overshielding when the shielding electric field dominates over the convection electric field in the recovery of the substorm as shown in Fig. 6 (bottom). The overshielding electric field drives the equatorial CEJ, causing the equatorial enhancement of the negative bay (Kikuchi et al. 2000). Figure 7 indicates a schematic diagram of the substorm currents composed of the partial ring current, R2 FACs, and the equatorial CEJ with the Hall currents surrounding the R2 FACs causing reversal of the ionospheric currents at midlatitude (Kikuchi et al. 2003). It is to be noted that the overshielding electric field starts to increase at the onset of the substorm and continues Fig. 5 Electric fields at the equator calculated by the three different potential solvers [T and A (Tsunomura and Araki 1984), S and B (Senior and Blanc 1984), New (Tsunomura 1999)] with an input of fieldaligned currents in the polar ionosphere. All the model calculations show the evening anomaly of the electric field in the same direction as in the day with significant enhancement in magnitude. (from Fig. 5