Global dynamo simulation of ionospheric currents and their connection with the equatorial electrojet and counter electrojet: A case study

Abstract

The interrelationship of equatorial and planetary scale ionospheric horizontal currents on quiet days is studied by means of a global ionospheric wind dynamo simulation. This simulation aims at reproducing magnetic and radar data first for a normal quiet day, and then for the strong counterelectrojet event of January 21, 1977, which was previously studied in detail on the basis of coherent backscatter radar data for the Addis-Ababa location. For the reference quiet day (January 27, 1977), the pattern of low and middle latitude currents and electric fields can be roughly reproduced by a combination of the (1,-2) and (2,2) solar tides. Both the Sq current system, and the global electrostatic potential distribution as derived by Richmond et al. [1980] from incoherent scatter data are well simulated. Direct comparison of the computed electric field with the quiet-day averages available for each radar site also show an excellent agreement on the east-west component, but a poorer one on the northsouth component. The counterelectrojet simulation is performed by fitting the H component trace at the magnetic equator and the D trace at midlatitudes. The result appears to give a consistent solution to the problem of the electrical connection between the equatorial counterelectrojet and the planetary dynamo layer. Two horizontal current vortices of opposite directions are found to flow at low latitudes on each side of the noon sector, anticlockwise in the morning and clockwise in the afternoon. They both produce a poleward current flow at low latitudes at noon, a feature that is detected on the magnetic records. The counterelectrojet event is reproduced by a combination of the (2,2) and (2,#) solar tides, assuming that the contribution of the diurnal tide to the altitudeintegrated current flow cancels out. This result is in agreement with a previous simulation study of the counterelectrojet phenomenon. contributed to a better definition of the neutral wind source from localized observations. However, a consistent synoptic picture of these experimental data is still to be made. Most of the Sq current flow appears to originate from the S(1,-2) tide [ Stening, 1969; Tarpley, 1970b; Volland, 1971b], the dynamo effect of which is maximum in the 120-to 200-kin height range during the day [Volland, 1971a; Richmond et al., 1976; Forbes and Lindzen, 1976a]. Semidiurnal tides S(2,2) and S(2,#) also make a substantial contribution to the electric currents and fields [Richmond et al., 1976; Kirchhoff and Carpenter, 1976; Forbes and Lindzen, 1976a,b, 1977; Salah and Evans, 1977; Harper, 1977]. The solar tide S(2,2) produces much more current than the S(2,#) tide, which has a vertical wavelength half as long [Tarpley, 1970b]. Along the magnetic equator, Sq currents usually flow eastward during daytime and are at the origin of the equatorial electrojet [Martyn, 19#8; Chapman, 1951], which can be observed on the ground as a sharp amplification in the daily variation of the magnetic field horizontal component. The daytime reversal of this cu...

Relationships between pre-sunset electrojet strength, pre-reversal enhancement and equatorial spread-F onset

“…It should be noted that the present analyses did not directly exhibit spatial variations of the current system, since the pre-sunset IEEJ and the PRE strength around sunset were obtained at the same longitude at different times. Several earlier models and observational studies have proposed that the evening CEJ is caused by semi-diurnal tides (e.g., Hanuise et al, 1983;Alex and Mukherjee, 2001;Gurubaran, 2002). The semi-diurnal tides are not likely to vary dramatically within a few hours or several tenths of longitudes.…”

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Various mechanisms have been suggested to explain CEJ. Modeling studies have shown that a particular combination of tidal modes can result in a counter electrojet (Forbes and Lindzen 1976b;Takeda and Maeda 1981;Hanuise et al 1983). The study performed by Raghavarao and Anandarao (1980) showed that an upward wind with a sufficiently large magnitude (15-20 m/s) can also cause a counter electrojet.…”

Counter electrojet features in the Brazilian sector: simultaneous observation by radar, digital sounder and magnetometers

“…Later in the evening the normal EEJ conditions seems to be re-established and a weak scattering is observed between 101 and 109 km of altitude. The evidence of moderate magnetic disturbance shown by the D st index indicates that the disappearance of the irregularities around midday could be due to neutral wind circulations changes caused by the disturbance dynamo effect on the Sq system as stated by Hanuise et al (1983) and . Regarding the scattering region during CEJ event, the reversal into the Doppler velocities with respect to the normal condition indicates that both the polarizations electric field and the density gradient are downward.…”

“…The longitudinal coverage of such phenomena can not be specified in the present work, however. In order to confirm the combination of global scale tidal wind modes proposed by Hanuise et al (1983), instead of the vertical winds and gravity wave associated shearing winds proposed by Raghavarao and Anandarao (1987) we need simultaneous observations in different latitudinal sector based on clusters of instruments like those used in the present work. Moreover, checking which, among the three components, is the responsible for driving the EEJ electric field will demand accurate neutral atmosphere wind measurements.…”

“…From a detailed simulation study, Hanuise et al (1983) explained the possible reversal of the east-west electric field during the above CEJ event as due to an abnormal combination of global scale tidal wind modes, i.e., strong semidiurnal (2,2) and (2,4) modes and a weak diurnal (1,−2) mode and assuming that the contribution of the diurnal tide to the altitude-integrated current flow cancels out. Recently, a study on the CEJ was published (Gurubaran, 2002) in which the author applied the method of natural orthogonal components to the ground geomagnetic data in order to separate the normal and the abnormal field variations (Vertlib and Wagner, 1970;Faynberg, 1975).…”