H.L. Rowland, Beam Physics Branch, Plasma Physics Division, Naval Research Laboratory, Washington, D.C.
By modulating the ambient current flowing in the ionosphere, e.g., the auroral electrojet, it is possible to generate extremely low frequency (ELF) and very low frequency (VLF) radiation. This ionospheric modification technique can provide such waves for probing both the Earth and the ionosphere- magnetosphere. The modification occurs in the lower D-region and can provide information about the ambient conditions in one of the least diagnosed regions of the ionosphere.
The electrojet is modulated by using a high frequency heater (a few MHZ) with the power modulated at the desired ELF/VLF frequency to heat the ionospheric electrons in the lower D-region. Figure 1a shows a sketch of the heater and heated region. The heated region is typically at 75 km (though this depends upon the carrier frequency) and can be 30 km in diameter and a few km thick. Viewed from above (see Figure 1b) the heated region is a roughly circular patch. The smoothness of the heated region depends upon the antenna radiation pattern as well as D-region conditions. The heating increases the electron-neutral collision rate which changes the conductivities. Since on ELF time scales the ambient electric field is constant, modulating the conductivity produces a current modulated at the same frequency. At these altitudes the conductivity change is predominantly in the Hall conductivity. If the ambient electric field, E, is in ±y direction, a time varying current perturbation is generated, j, in the ±x direction (Fig. 1b). The time varying current launches waves both up and down the Earth's magnetic field. In the simulations shown here, we start with a time-varying current and study the downward propagating waves and how they couple into the Earth-ionospheric wave guide.
The animations show 5 different representations of the same simulation. The simulation uses a time-varying current perturbation (1 kHz) in the D-region at 75 km. The current is in the magnetic east-west direction. The Earth's magnetic field is vertical. The simulation box is 1800 by 1800 by 120 km. Isosurfaces are shown for the absolute value of the horizontal magnetic field ABSB and of the vertical electric field ABSEZ. Also shown is the east-west magnetic field in the near-field BX1 and in the far-field BX2. Since the field amplitude falls off with distance, BX1 uses a order-of-magnitude larger isosurface value than BX2 to emphasize the field close to the site. The north- south magnetic field is shown in BY1 and BY2. These plots look slightly different from the absolute value plots where both the positive and negative surfaces were shown. Also BX and BY do have a different orientation of the their radiation patterns. The direction of the radiation is determined by the total horizontal field shown in ABSB and by the vertical electric field shown in ABSEZ. The radiation pattern in the earth-ionosphere waveguide is a combination of a linear dipole antenna and a right-hand circular antenna. At ELF frequencies because of low D-region absorption the dipole is dominant. The dipole radiates in the magnetic east-west direction.
Because 1 kHz is below cutoff the mode in the waveguide is a TEM mode. The mode consists of a horizontal B field perpendicular to the direction of propagation and a vertical electric field. With perfect conductors, the mode is uniform in the vertical direction. As the wave propagates in the waveguide, the top of the wave is approximately at the bottom of the ionosphere. Above the heated region, waves are also launched along the Earth's magnetic field. In the near-field ( BX1 and BY1) one can see the pulse being radiated downward. It strikes the ground and reflects back up to the ionosphere. Part of the energy propagates up the field lines into the ionosphere. This is the bubble seen rising up. The D-region is highly collisional and damps this wave. Looking at BX2 and BY2 one can see that the energy mainly stays in the waveguide. If one looks closely at the top of the wave in the waveguide the wave appears to be curved. The waveguide mode is coupling into the bottom of the D-region and driving a whistler mode up the field lines. The whistlers have a much lower velocity than the waveguide mode and can only propagate along the field lines. This acts to curve the top of the waves. These waves help form the bubble that propagates up the field line. Because of this, the diameter of the bubble is much larger than the heated region.
Above the heated region in ABSEZ one can see a pair of coils revolving around each other. These are the currents that flow up and down the Earth's magnetic field forming the current loops associated with the waves propagating up the field lines. Finally, EZ1 is a blow-up of the high-altitude portion of the vertical electric field for positive values of the electric field; the current loop is more clearly seen.
More details can be found in Rowland et al., JGR, 101, 27027, 1996 and Rowland, JGR, 104, 4319, 1999.
This work is supported by the Office of Naval Research and, in part, by a grant of HPC time from the DoD High Performance Computing Center at the Army Research Laboratory, Aberdeen Proving Ground.