Transmission of the electric fields to the low latitude ionosphere in the magnetosphere-ionosphere current circuit
© Kikuchi and Hashimoto. 2016
Received: 10 December 2015
Accepted: 5 February 2016
Published: 20 February 2016
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 day- and 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 day- and night sides, which is explained by means of the light speed propagation of the TM0 mode waves in the Earth-ionosphere waveguide.
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 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 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
Stormtime electric field and EEJ/CEJ
The auroral electrojet expands equatorward during storms, driving the DP2 currents at midlatitudes (Feldstein et al. 1997; Wilson et al. 2001; Kikuchi et al. 2008). Wilson et al. (2001) suggested that the electric field associated with the DP2 currents might have contributed to the development of the storm ring current in the inner magnetosphere. Actually, strong convection electric fields have been observed by CRRES and Akebono at L = 2–6 (Wygant et al. 1998; Shinbori et al. 2005; Nishimura et al. 2006). The electric field is as strong as 46 mV/m during the major storm on 13 March, 1989 (Shinbori et al. 2005). The convection electric field penetrates to the equatorial ionosphere, intensifying the EEJ on the dayside (Kikuchi et al. 2008).
The stormtime current circuits are composed of the R1 FAC-EEJ during the main phase and the R2 FAC-CEJ during the recovery phase, which are similar to those of the substorm growth and expansion phases, respectively. It should be noted, however, that the CEJ occurs even during the storm main phase (Fejer et al. 2007), which could have been caused by the disturbance dynamo activated by the preceding storm activities. It should be reminded that the disturbance dynamo begins to work with a time lag of several hours from the beginning of storm and continues to work for, say, 10 h (Fejer and Scherliess 1997). In contrast, the overshielding develops quickly responding to the solar wind conditions and substorm activities. The latitude and local time distribution of the ionospheric electric field would enable us to distinguish the overshielding from the disturbance dynamo.
Electric field and currents during the SC
The ionospheric currents achieved during the geomagnetic sudden commencement (SC) are similar to the DP2 currents except that the time scale of the SC is as short as a few minutes or even less. The SC is composed of the preliminary impulse (PI) and main impulse (MI) superimposed on the stepwise increase (DL) (Araki 1994). The PI and MI with typical time scales of 1 and 5 min are caused by the ionospheric currents driven by the dusk-to-dawn and dawn-to-dusk electric fields, respectively. The DL is caused by the compressional MHD waves launched by the intensified magnetopause currents (Tamao 1964). Both the PI and MI are characterized by the equatorial enhancement (Araki 1994) and the electric fields on the day- and night sides are opposite to each other except that the electric fields in the evening are in the same direction as in the day (Kikuchi 1986). The evening anomaly of the PI and MI electric fields leads to the fact that they are potential fields transmitted with the ionospheric currents, similar to the convection electric fields.
Electric field transmission mechanism
Since the TM0 mode wave has no low cutoff frequency, the propagation suffers no attenuation at all frequencies (Budden 1961). However, the TM0 mode waves suffer geometrical attenuation causing the intensity at low latitude to be less than 10 percent of the source field (Kikuchi and Araki 1979). Because of the geometrical attenuation, the ionospheric currents depending on the ionospheric conductivity are too weak to cause the PI at low latitude (Fig. 10). However, the electric field is strong enough to be detected by the HF Doppler sounder (Kikuchi 1986). The electric field associated with the ionospheric currents is transmitted by the Alfven wave upward into the F-region ionosphere and the inner magnetosphere (Kikuchi 2014), which leads to the coherent variations of the ground magnetic field and ionospheric motion at the geomagnetic equator, as observed by the HF Doppler sounders (Abdu et al. 1988) and by the Jicamarca incoherent scatter radar (Kikuchi et al. 2003). The upward transmission of the Poynting flux into the inner magnetosphere has been observed by the satellites (Nishimura et al. 2010), causing the quick development of the electric field in the inner magnetosphere (Nishimura et al. 2009), ring current (Hashimoto et al. 2002), and so on.
The convection electric field is transmitted by the Alfven wave from the dynamo in the outer magnetosphere to the polar ionosphere, accompanying the R1 FACs and driving the DP2 currents composed of ionospheric Hall currents at high latitude and the Pedersen currents amplified by the Cowling effect at the dip equator. The convection electric field is transmitted to low latitude near-instantaneously by the TM0 mode waves in the Earth-ionosphere waveguide, resulting in high correlation of the DP2 fluctuations between high latitudes and equator during storm and substorms. The electric field associated with the DP2 currents is transmitted into the F-region ionosphere and into the inner magnetosphere, causing quick response of the low latitude ionosphere and ring current development when the cross polar cap potential increases. The overshielding electric field together with the R2 FACs causes reversal of the ionospheric electric field at midlatitude and the counter electrojet at the equator during substorm expansion phase and storm recovery phase. The same current circuit is achieved in the ionosphere during the geomagnetic sudden commencements, attesting the instantaneous transmission of the electric field and currents from the polar ionosphere to the equator. The evening anomaly of the electric field directing in the same direction as in the day and enhanced magnitude is a unique feature of the electric potential in the global ionosphere, which is commonly observed during the DP2 and SC events.
The author, TK wrote the whole manuscript with his knowledge and experience on the convection electric field and its transmission mechanism. The coauthor, KH provided the author with knowledge about the substorm over-shielding based on her experience in this specific field. The selection and preparation of the figures are also due to coauthors efforts. Both authors read and approved the final manuscript.
We would like to thank T. Araki at Geophysical Institute, Kyoto University, T. Tanaka at Kyushu University, S. Fujita at Meteorological College, Y. Omura, Y. Ebihara and A. Shinbori at Research Institute for Sustainable Humanosphere, Kyoto University, Y. Nishimura at Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, and B. Veenadhari at Indian Institute of Geomagnetism for fruitful discussion on the electric field and magnetic field in the magnetosphere and ionosphere. The works of TK and KH are supported by the JSPS KAKENHI Grant Number 26400481 (KH) and the joint research programs of the National Institute of Polar Research, Tokyo. The study of TK is supported by the Grants-in-Aid for Scientific Research (15H05815) of Japan Society for the Promotion of Science (JSPS) and the joint research programs of the Institute for Space-Earth Environmental Research, Nagoya University, and the Research Institute for Sustainable Humanosphere, Kyoto University.
The authors declare that they have no competing interests.
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