Electron Gyroharmonic Effects in Ionization and Electron Acceleration during High-Frequency Pumping in the Ionosphere

Optical emissions and incoherent scatter radar data obtained during high-frequency electromagnetic pumping of the ionospheric plasma from the ground give data on electron energization in an energy range from 2 to 100 eV. Optical emissions at 4278 Å from $\mathrm{N}_2{}^{+}$ that require electrons with energies above the 18 eV ionization energy give the first images ever of pump-induced ionization of the thermosphere. The intensity at 4278 Å is asymmetric around the ionospheric electron gyroharmonic, being stronger above the gyroresonance. This contrasts with emissions at 6300 Å from $\mathrm{O}({}^1\mathrm{D})$ and of electron temperature enhancements, which have minima at the gyroharmonic but have no apparent asymmetry. This direct evidence of pump-induced ionization contradicts previous indirect evidence, which indicated that ionization is most efficiently produced when the pump frequency was below the gyroharmonic.

Figure 1

Experimental results for March 7, 2002. (a) The HF-pump frequency $f_0$ for each pulse (black) and the double resonance frequency (dots, red online) versus time. The subsequent panels show (b) the temporal evolution of $T_e$, (c) emission at 6300 Å, while the bottom panel (d) displays emissions at 5577 Å until 1820:30 UT and at 4278 Å thereafter.

Figure 2

Experimental results for November 8, 2002. (a)– (c) display the same quantities as in Fig. 1, and (d) shows emissions at 4278 Å.