the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
On mechanisms for HF pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere
Tima Sergienko
Urban Brändström
Björn Gustavsson
Michael T. Rietveld
Abstract. The EISCAT Heating facility was used to transmit powerful high frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by three stations of the Auroral Large Imaging System in northern Sweden and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I5577/I6300 ≈ 0.2 for the HF pump frequency f0 = 6.200 MHz ≈ 4.6 fe to I5577/I6300 ≈ 0.5 when f0 = 5.423 MHz 
4 fe, where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0 = 5.423 MHz 4 fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of meter-scale wavelengths that can be excited with f0 = 5.423 MHz 4 fe.
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Thomas B. Leyser et al.
Status: final response (author comments only)
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RC1: 'Comment on angeo-2023-21', Anonymous Referee #1, 15 Aug 2023
The comment was uploaded in the form of a supplement: https://angeo.copernicus.org/preprints/angeo-2023-21/angeo-2023-21-RC1-supplement.pdf
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AC1: 'Reply on RC1', Thomas Leyser, 06 Sep 2023
Reply to Referee #1
We thank the referee for the comments, which have improved the manuscript. Below is detailed response to the review comments.
Major comments
- We do not understand this comment by the referee. The displays of optical data (figures 2 and 3) as well as the radar data (figure 4) show all pump-on AND pump-off cycles during the experiment, precisely to visually show the difference between the enhancements during pump-on and the background during pump-off. Figures 5-6 and 7 show only the pump-enhanced part of the emissions. This has now been further clarified also in the captions of figures 5-6, where it was not mentioned previously. Thus, as these figures display the enhancements they would show zero emission rate without the pump wave.
- We have added a paragraph at the end of the Introduction that describes the objective and novelty of the study. More detail has also been added to the conclusions.
- Information has been added in the section Experiment setup to further describe the Heating facility, ALIS and the UHF incoherent scatter radar.
Minor comments
The line numbers below refer to those in the report of the reviewer.
Line 15-16: The choice of pump frequency and polarization is motivated by what the reviewer mentions and what also is written in the manuscript. The purpose is to get the strongest effects.
Line 102-104: We have added an explanation of the decreasing altitude in terms of that an HF wave with lower frequency will be reflected at a lower altitude in the ionosphere. In addition, a contributing altitude oscillation can be seen in the optical data, which was pointed out to us by another referee and which is now also mentioned. The mentioned intensity of the emissions is a theme of the Discussion and no changes are made in the revised manuscript in this respect.
Experiment setup: We have added additional information on the main instruments used, as mentioned above. In addition, we have given references to original publications describing the instruments, where available.
Line 115: Done.
Line 115: The long sentence has been rephrased.
Line 144: Done.
Line 168: Done.
Line 269: Done.
Line 320-321: Done.
In addition, we have clarified the conclusions by repeating more detailed explanations of several items.
Citation: https://doi.org/10.5194/angeo-2023-21-AC1
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AC1: 'Reply on RC1', Thomas Leyser, 06 Sep 2023
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RC2: 'Comment on angeo-2023-21', Savely Grach, 17 Aug 2023
The paper “On mechanisms for HF pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere” by T. B. Leyser, T. Sergienko, U. Brändström, B. Gustavsson, M. T. Rietveld presents interesting new results on artificial airglow at 630 nm (red line, level O1D) and 557.7 nm (O1S) excited by powerful HF radio waves at the EISCAT-heating. It should be noted that the results of different similar experiments are quite variable due to, first of all, variability of the ionosphere, and often it is difficult to find what parameters are responsible for different results. The presented paper contain very interesting new data, modeling and discussion which can be considered as a noticeable contribution to the artificial airglow study. So, the paper shall be published as a “regular paper”. Some aspects of the paper have to be clarified. My comments/questions. 1. Rows 174-176, the authors write:
Excitation of the O(1D) state, the source of the 630.0 nm line, has been attributed to mainly electron heating from a maxwellian electron distribution (Mantas, 1994; Mantas and Carlson, 1996), however, taking into account collisional de-excitation by collisions with molecular oxygen and nitrogen in the atmosphere (Gustavsson et al., 2001).
Comment: In the paper V. V. Klimenko, S. M. Grach, E.N. Sergeev, A.V. Shindin, Features of the ionospheric artificial airglow caused by Ohmic heating and plasma turbulence-accelerated electrons induced by HF pumping of the SURA heating facility, Radiophysics and Quantum Electronics, Vol. 60, No. 6, November, 2017 (Russian Original Vol. 60, No. 6, June, 2017) DOI 10.1007/s11141-017-9812-0 it is shown that for the approximately same ERP the red line artificial airglow is attributed mainly to the electron acceleration, but not heating. This should be discussed in the paper. By the way, H. Carlson (passed away, unfortunately) agreed with conclusions of the latter paper (private conversation).
- Question: Authors compare the altitude of the volume emission rate maximum with the altitude where f0~ 4fe (which is reasonable), but do not compare these altitudes with the pump wave upper hybrid altitude. Why? This altitude is known to be the altitude where the pump wave energy contribution to the ionospheric plasma is maximum.
- Rows 199-203. The authors write:
Stimulated electromagnetic emissions can also be used to estimate the vicinity of f0 to sfe (Leyser, 2001), but in our experiment the emissions were generally too weak for spectral structure to be identified. However, for the fourth HF pulse at f0 = 5.423 MHz, a weak broad upshifted maximum (BUM) can be identified. In Fig. 4 it can be seen that the electron temperature is slightly more enhanced in the fourth (and last) HF pulse than in the preceding pulses at f0 = 5.423 MHz, which indicates a stronger plasma excitation in the last pulse. Etc.
Comment 1. The BUM peak position in the SEE spectrum strongly depends on f0-4fe, so the appearance in the paper a figure with the SEE spectra would be reasonable.
Rows 315 -319:
From tomography-like reconstruction of the altitude distribution of the optical volume emission rates, we conclude that f0 = 6.200 MHz~ 4.6fe and f0 = 5.423 MHz < 4fe in the height regions with the largest optical enhancements.
Comment 2. The absence of the SEE in the used pump wave frequencies can be related to belonging of these frequencies to specific ranges (“the weak emission range” for 6.2 MHz, and the “below harmonic range” (s=4) for 5.423 MHz, see Leyser, 2001 and Sergeev, E.N., Frolov, V.L., Grach, S.M., Kotov, P.V., 2006, On the morphology of stimulated electromagnetic emission spectra in a wide pump wave frequency range. Adv. Space Res. 38, 2518–2526). These ranges are known to have a weak SEE. Dependence of SEE spectra on f0-sfe (which does not have an adequate interpretation yet) points to the dependence of the efficiency of the PW energy contribution to the formation of the plasma wave spectra and efficiency of electron heating and electron acceleration on f0-sfe.
Finally. I am not agreeing with some points in the Section Discussion. However, this is not a barrier for the paper publication, but the question of further experiments and discussions.
Sincerely, Savely M. Grach
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AC2: 'Reply on RC2', Thomas Leyser, 06 Sep 2023
Reply to Referee #2
We thank the referee for the comments, which have improved the manuscript. Below is detailed response to his comments.
Comment: We thank the referee for bringing the paper by Klimenko & al. (2017) to our attention. We now discuss it in the manuscript. In short, we agree with the results which concern assumption of moderate electron temperature enhancements. And our conclusion of that electron heating is the dominant source of the red-line emissions in our case too is consistent with the results of Klimenko & al. as we measure larger electron temperatures above 3000 K. In addition, we comment on that whereas Klimenko & al. observed red-line emissions only for foF2 > f_0 + 0.5 MHz, we had the opposite case with foF2 < f_0 + 0.5 MHz throughout the experiment.
Question: We have not provided information on the upper hybrid resonance altitude (UHR) since that altitude gives only a rough indication of where HF energy may be deposited. As the referee is aware of, HF pump excitations can occur both below the UHR with freely propagating upper hybrid waves and above with upper hybrid waves localized in density depletions, as well as with Langmuir waves at all angles. In a paper of ours (”Electron heating by HF pumping of high-latitude ionospheric F-region plasma near magnetic zenith”, by Leyser & al., Ann. Geo., 38, 297, 2020, doi: 10.5194/angeo-38-297-2020) we found that the maximum heating source can be even above the plasma resonance altitude for a pump beam in the magnetic zenith direction at EISCAT (as in the present experiment). This is consistent with pump wave propagation in the L mode parallel to the geomagnetic field, which can reach well above the plasma resonance height.
Comment 1: We have added a figure with an SEE spectrum and a spectrum of the noise level as the SEE is weak and may be difficult to observe for the general reader. A paragraph discussing the SEE has also been added.
Comment 2: We agree with the referee. However, for 5.423 MHz we seem to have f_0 close to 4 f_e, rather than in the ”below harmonic range” as becomes clearer with the now included SEE spectrum and discussion (the BUM appears close to its so called cut off frequency). It is a bit puzzling that the SEE is weak, although electron temperature enhancements are relatively large. But we do not want to speculate on reasons for this in more detail as this would not add to the main theme of the manuscript. Here we simply wish to relate f_0 to 4 f_e.
Citation: https://doi.org/10.5194/angeo-2023-21-AC2
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RC3: 'Comment on angeo-2023-21', Anonymous Referee #3, 21 Aug 2023
The manuscript is about an interesting experiment that consists in heating the ionosphere by electromagnetic waves transmitted by the EISCAT facility. The aim was to increase the intensity of the red and green lines emitted by atomic oxygen according to the frequency of the electromagnetic waves at 6.200 MHz and 5.423 MHz emitted by EISCAT. The excitation process was studied. The experiment also used cameras from the Auroral Large Imaging System (ALIS) to obtain the volume emission rate. Ionospheric parameters such as electron density, electron temperature and ion temperature were observed by the EISCAT UHF radar.
The manuscript has a good introduction about similar experiments done earlier. But it doesn´t describe the experimental setup well, especially to the optical apparatus.
A better description of the optical setup is needed to clarify the observation of OI557.7 nm and OI630 nm intensities. The cameras of ALIAS take only 7 s of exposure time. Normally, depending on the system (optical components and sensor), the integration takes about 90 s for both emissions.
The intensity of the green line from ~97 km altitude is higher than that one from the F region. It is estimated to be ~5x (Silverman, 1970). The cameras observed the total green (and red) line intensity. How did you remove this background from the mesospheric altitude? It is important because the excitation process is different at ~97 km and in the F region. The total background of the green line intensity can be estimated during the off period (85 s). But this total, as noted above, comes from the mesosphere and the F region.
Could the presence of the aurora in the thermosphere interfere with the experiment?
In figures 5 and 6, the colored lines could be a little thicker. The same goes for the label color, maybe black would be better.
Line 15 – Kvammen et al., 2009, pumped with right-handed circular polarization. In this work you pumped with left-handed polarization. Is there a special reason for this?
Line 75 – “To keep fo below the decreasing ….was 116MW”. Why didn´t you use the same power for fo = 6.200 MHz? Other works also used the same criterium to the power. This seems to be an important condition for the success of the experiment. Could you comment on this?
Line 104 – “But again the altitude region of the emission decreased as fo was lowered”. Now, in Figures 2 and 3, it appears to have a 40-minute oscillation with a maximum altitude at 250 km near 40 minutes after 16 UT (or 16:40). It could be a dynamic effect, not necessarily the change of frequency. Could you open this discuss in a better way?
Line 168 – “both”, not “b oth”;
The manuscript must be publish, it has a good contribution to this area. But some improvements should be made in order to make the work clear to new readers who want to enter in the area of geophysics. It is not too much. Maybe some lines added to the work could be better to understand the experiment and the results. Some corrections need to be made.
Citation: https://doi.org/10.5194/angeo-2023-21-RC3 -
AC3: 'Reply on RC3', Thomas Leyser, 06 Sep 2023
Reply to Referee #3
We thank the referee for the comments, which have improved the manuscript. Below is detailed response to his comments. The line numbers below refer to those of the reviewer’s report.
On the experimental setup: We have added information on the EISCAT Heating and UHF facilities, as well as ALIS. Further, no integration of the optical data is done, other than that during the 7-s exposure. This is now clarified in the text.
On the background emissions: Our concern in this paper is the pump-enhancement of the emissions in the F region. We have added a description of how the background nightglow was subtracted to get the pump-enhanced emissions. It is not necessary for us to discriminate between airglow from the mesosphere and F region. We only need the total background emissions to be subtracted.
On presence of aurora: Yes, aurora could interfere. In order to perform successful experiments one needs a clear sky without clouds, no (or only weak) aurora and a sufficiently high ionospheric electron density in the F region after sunset. Attempts are commonly made many nights before such conditions occur. This difficulty has been noted several times in the literature, e.g., Stubbe & al. Ionospheric modification experiments in northern Scandinavia. J. Atmos. Terr. Phys., 44, 1025-1041, 1982. We therefore see no need to repeat this in the present manuscript.
Figures 5 and 6: The colored lines will be thicker in the revised manuscript and labels will have slightly larger fonts. Thank you also for noting the label color which now has been changed to black in all figures.
Line 15: This is an unfortunate mix of definitions of the polarization. It is the same polarization. We use the standard textbook definition for the electromagnetic L mode (see, for example, Waves in Plasmas, by Stix, 1992) in which the rotational direction of the electric field is given relative to the ambient magnetic field and which is downward in the high-latitude northern hemisphere. We have now added a definition of the handedness. Kvammen et al. used a definition of the rotational direction relative to the direction of propagation which is upward upon transmission (although downward after reflection in the ionosphere and thereby ”opposite polarization”). This definition is more common in the optics community where usually effects of a magnetic field and reflection in a plasma is not of interest.
Line 75: The different ERP levels depend primarily on different antenna gains at the different frequencies. We have added a note on this in the revised manuscript. However, to only use the same ERP at different frequencies does not give more information on the physics in the F region. In order to study the power dependence it is important to measure and take into account also the linear D-region absorption, in addition to the ERP. But we did not measure the D-region absorption in the discussed experiment.
Line 104: Interesting, thank you for noticing this and bringing it to our attention! It is now commented in the revised text.
Line 168: Done.
Citation: https://doi.org/10.5194/angeo-2023-21-AC3
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AC3: 'Reply on RC3', Thomas Leyser, 06 Sep 2023
Thomas B. Leyser et al.
Thomas B. Leyser et al.
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