Observations of Plasma Line Splitting in the Ionospheric Incoherent Scatter Spectrum

Wide-bandwidth ionospheric incoherent scatter (IS) spectra obtained using the Arecibo IS radar show the occurrence of a split in the plasma line (i.e., two plasma lines) when the plasma frequency is close to the second harmonic of the electron gyrofrequency. This split is predicted in the IS theory for a magnetized plasma, but observations have never been reported. Here we present the experimental results and theoretical calculations supporting the observations. These results may assist in understanding the behavior of Langmuir waves in the magnetized plasma and are a validation of what historically was a somewhat controversial aspect of the IS theory.

Figure 1

FTI plot showing the plasma line from 2.5 MHz to $\sim 2.25\mathrm{MHz}$, jumping to $\sim 1.9\mathrm{MHz}$ and disappearing at $\sim 1.4\mathrm{MHz}$. The box inserted in the figure shows individual spectra for the period of 1726 to 1742 LT at an interval of approximately 1 min, and the frequency offset from 1.5 to 2.5 MHz. The color bar indicates relative intensity of the spectra.

Figure 2

FTI plot from data obtained during the morning of November 22, 2006 for an altitude of 217 km. The sunrise at this altitude was at 0602 LT. The plasma line starts at 1.5 MHz at 0600 LT, splits at $\sim 2\mathrm{MHz}$ and reappears at $\sim 2.2\mathrm{MHz}$ at 0700 LT. Another density layer appears just before 0700 LT at $\sim 1.6\mathrm{MHz}$, splits at $\sim 2\mathrm{MHz}$ and reappears at $\sim 2.3\mathrm{MHz}$ at 0737 LT, which is clearer in the inserted subplot. The color bar indicates relative intensity of the spectra.

Figure 3

Theoretical spectra calculated for two different values of electron temperature, varying electron density, and $\alpha =60°$. The electron density increases from bottom to top. The split is shown by the dashed line.

Figure 4

Theoretical spectra calculated for different values of $\alpha$. These spectra are calculated for $T_e=1000\mathrm{K}$ and $N_e=4.0\times {10}^{10}{\mathrm{m}}^{-3}$. For Arecibo parameters, the split occurs only in the range of $55°\le \alpha \le 75°$.