Reply to comment on ‘‘The magnetic response of the ionosphere to pulsed HF heating’’ by M. T. Rietveld and P. Stubbe

[1] We want to thank Rietveld and Stubbe [2006] (hereinafter referred to as RS) for providing us with the opportunity to clarify the issues addressed by our work, including the differences in experimental results and theoretical interpretation with the early pioneering work of Rietveld et al. [1986, 1987, 1989] and Rietveld and Stubbe [1987]. We first apologize for the inadvertent omission of Rietveld et al. [1986] that occurred accidentally when the accepted manuscript was shortened to fit the GRL length guidelines. The particular figure we refer to in our paper is Figure 1 of Rietveld et al. [1986] (hereinafter referred to as R86), that was repeated as Figure 2 of Rietveld and Stubbe [1987]. With this we want to address the more substantive issues: [2] 1. We agree with RS that when disagreements between theory and experiment arise the theory should be questioned. For this reason we conducted our short pulse experiment. Figure 1a shows the magnetic response of the electrojet to 2.5 msec heating reproduced from Figure 1 of R86 along with our own experimental result (Figure 1b). The traces are similar with one critical exception: the approximately 1 pT field that follows turn on and extends from approximately 0.6 to 3 msecs in Figure 1b. RS admit that this feature was absent from their measurements and attribute it to the processing they used to convert their loop antenna measurements to a time plot for the magnetic field. This explains why Figure 1a does not represent the correct magnetic response of the electrojet to short pulse heating. Note that our experimental result shown in Figure 1b is consistent with the theoretical prediction derived using a three-dimensional Green’s function. [3] 2. In our paper we referred to Figure 1b as threecomponent with asymmetry for pulse on vs. off. This is schematically shown in Figure 2 that approximates the response as an impulse with duration To, found to be of the order of .25 msec for the experimental conditions, superimposed on a square pulse with duration equal to the pulse length and amplitude a fraction of the maximum amplitude of the pulse (in this case approximately .25). During the off time there is only an impulse with reverse polarity but no magnetic plateau component, resulting in asymmetry. This is different than the well-known heating/ cooling asymmetry explored in R86. The field structure shown in Figure 2 and revealed for the first time during the HAARP campaign is crucial in resolving the efficiency vs. frequency puzzle documented in Figure 8 of Rietveld et al. [1989], as well as revealing a possible flaw in their treatment of the issue. [4] 3. Rietveld and Stubbe’s previous work does not seem to have resolved many critical issues concerning the scaling of the efficiency with frequency. Consider their results shown in Figure 3 (corresponding to Figure 8 of Rietveld et al. [1989]). As seen, the theoretical curve (solid line) is in complete disagreement with the data. [5] Furthermore the more than ten dB lower efficiency in the range below 1 kHz cannot be accounted for theoretically. A major point of our paper was to explain the puzzling efficiency variations shown in Figure 3. Note that the amplitude peaks at about 2 kHz, due to effects attributable to the dependence of the observed waveforms on frequency or equivalently pulse length. The frequency scaling for frequencies below 2 kHz (corresponding to To > .25 msec) can be found by taking the Fourier transform of the waveform shown in Figure 2 that approximates the observed waveforms. The results are shown in Figure 4 for two values of the ratio a of the maximum amplitude of the square pulse to that of the impulse. It reproduces well both the sharp reduction below 2.0 kHz and the expected flattening at lower frequencies. For frequencies higher than 2 kHz, and neglecting the presence of minor enhancements at frequencies 2, 4, 6 and 8 kHz (that were first correctly attributed by Rietveld et al. [1989] to transit time in-phase resonance with the iono-