HF-Induced Modifications of the Electron Density Profile in the Earth’s Ionosphere Using the Pump Frequencies near the Fourth Electron Gyroharmonic

Round 1

Reviewer 1 Report

Review of ”HF Induced Modifications of the Electron Density Profile in the Earth's Ionosphere Using the Pump Frequencies Near the Fourth Electron Gyroharmonic” by A. V. Shindin et al.

 

The authors present results from an experiment involving HF pumping of ionospheric plasma with different frequencies near the fourth harmonic of the ionospheric electron gyro frequency. The experiments were performed using the HAARP facility in Alaska, USA, and the pumping of the plasma was studied by multifrequency Doppler sounding (MDS) and detection of stimulated electromagnetic emissions (SEE) on the ground. The MDS was done by transmission of sufficiently short HF pulses by HAARP, implying pulses of a wide frequency range that enabled detailed sounding of the pump-plasma interaction.

 

However, whereas the MDS technique seems potentially useful, no new results are stated in the manuscript, at least not clearly and in relation to the results from several similar previous experiments. The authors present their results in detail, but it is difficult to see what is actually new and is an advance in current knowledge. In addition, the results emanate from two rather short experiments that are classified into two ionospheric height regions, but it is not known whether the data is representative of a larger data set, that is, whether the claimed differences between the two height regions are reliable or not. Further, the manuscript is submitted to Remote Sensing, but the manuscript topic is far from what one usually understands belongs to the field of Remote Sensing (although strictly speaking, the employed diagnostic methods are ”remote sensing” in the sense that ionospheric plasma is studied remotely from the ground). Also, readers interested in the topic of the manuscript, which can broadly be categorized as (ionospheric) plasma physics, would not naturally look for this in the journal Remote Sensing. To the best of my understanding the manuscript has nothing to do with the section Remote Sensing in Geology, Geomorphology and Hydrology, nor Geodetic Observations for Earth System. Therefore, I cannot recommend publication of the manuscript in its present form. Below are some additional comments to assist the authors in developing their work.

 

L 45-46: SI units should be used in the definition of plasma parameters.

 

L 50: "EISCAT" is an acronym, which therefore is written in capitals. But is "SURA" an acronym? If it is, then it needs to be defined.

 

L 85-89: The authors mention in general terms a first study from the 1990s, but only vaguely say that many question were left open. The authors should be more specific about gaps in knowledge that their study addresses particularly in the context of the rather many studies employing MDS during HF-pumping of ionospheric F-region plasma.

 

L 119: If I understand the sentence correctly, I suggest to replace "at 20th of the 30 ms pauses"  by something like "after 20 ms in the 30-ms pump-off period”.

 

L 222-229: These two paragraph should be deleted!

 

Equation (2): Is Delta Z the same as Delta z (lower-case z)?

 

L 282-284: I do not understand what the figure displays, concerning the 300-m shifts between the reflection heights. Does the abscissa not display the real altitude difference for the black, blue and red lines?

 

L 297-298: This seems to be an incomplete sentence.

 

Figure 5c: More than one tick mark on the ordinate should be given to provide information.

 

L 314: I do not see where the Doppler shifts become positive just after QCW pump-on. Could the authors explain or indicate this in the figures?

 

L 326: It is a small decrease: what is the uncertainty in the plotted curves? No error bars are given.

 

L 347: I do not understand where f_di<0. Figures 1a and 3a are more red during QCW pumping than before and after.

 

L 359-360: Does this sentence mean that the UHR for the two pump frequencies was similar, that is, that the background ionospheric density profile changed between the two cases?

 

L 361: Do the authors mean that the AA "decreased" with time, or that the AA was smaller than the compared case?

 

L 371: I guess the authors now mean figure 2c or 4c, whereas the preceding sentences concerned figure 1 or 3. Is this correct?

 

L 381: How do the authors know that the DM peak frequency gets into the double resonance, and not for example the pump frequency or perhaps the high-frequency flank of the DM?

 

Section 4: The data is presented in detail. The question arises what actually are repeatable results, since only two cases are presented which are differentiated in two altitude regions (figure 1 and 3). It is little use to present experimental results in great detail without any assessment of whether the results are consistent or due to random fluctuations. Without this, differences between the two altitude regions cannot be known, similarities are, however, more credible.

 

L 482: Do the authors really think that heating is the main process (although it is the end result for the different kinds of pump-ionosphere interactions)?

 

L 487: Most if not all of the results presented have been obtained previously. The summary of results should be accompanied by comments and references on what has been observed and concluded previously. What is actually new in this study? This should be clearly stated.

 

L 499: But what do the results imply for the interpretation? Is the result consistent or inconsistent with current understanding?

 

L 535: What do the authors mean by "again"? This case is the only one displayed with two clear minima.

 

L 536: Again, no error bars are given or no uncertainty is commented on. Since the differences between the depletions at the PR and UHR are relatively small the question arises whether they are real.

 

L 567: The conclusions reached by the authors have been found in previous studies. The authors should clearly state the findings of the present study in relation to previous work.

 

L 591-593: This needs one or several references.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The study can be considered unique both in terms of the formulation of the experiment, and in terms of the quantity and quality of the data obtained. The article consistently and fully describes the concept of the conducted research, justifies the research methods used, describes in detail the methodology of processing the experimental data obtained.

The article contains a fairly complete review of the results of previous studies, convincingly shows the relevance of the conducted research in terms of the development of heating experiments and the study of ionospheric physics in general. The authors of the article are well-known experimenters, which provided a high level of presentation of the results, and their analysis convincingly shows the reliability of the conclusions made.

The work uses the concepts and results of research conducted on other heating stands. A complete reproduction of these experiments at other experimental facilities is unlikely, but the concepts developed in the work will certainly be used by other researchers.

When reading the text of the article, the following inaccuracies and shortcomings were noticed:

- For a number of acronyms (PW, DW, SEE, AA) the expansion is given several times, which is hardly necessary.

- Section 2.1. "Experimental setup" ends with two paragraphs that probably got there from the MDPI Article Template? (lines 222-229)

As for the list of references: Of the 41 references, 28 works were published more than 20 years ago. I think that there is no need to use these works, since their main provisions are considered in later cited sources.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

From my observation, the manuscript has been well written from abstract to the end of the manuscript by covering all the points and citing the exiting knowledge on this particular issue. Hence, I believe the present work is one-step further to the existing literature that would be of the reader's interests.

Author Response

Thank you for your opinion.

Round 2

Reviewer 1 Report

Review of ”HF Induced Modifications of the Electron Density Profile in the Earth's Ionosphere Using the Pump Frequencies Near the Fourth Electron Gyroharmonic” by A. V. Shindin et al.

 

The authors have improved the manuscript, including having added clear statements on what is new and how the results fit it on previously obtained results by other authors. The work is interesting and important, and should be published. I only have a few remaining comments for clarification.

 

Lines 119-122: I agree that the used ERP for the diagnostic pulses of an average of 80 kW is likely ok for the purposes of the present experiments, where actual time scales of the evolving processes is not the main concern. However, the authors might be interested to know that 80 kW has been enough to excite SEE in a preconditioned ionosphere (see, e.g., Cheung & al., PRL 80, 4891, 1998), which may indicate that even such a low ERP level may affect time scales for the thermal processes.

 

L 155: Thanks to the authors response, I now understand what figure 2a and 4a display. I suggest to replace "temporal variations of the reflection heights" by something like "temporal variations of the change in reflection heights (from the start of QCW pumping)”.

 

L 297-301: I do not understand the reasoning of the authors: Positive doppler shifts usually mean a velocity towards the observer, meaning that reflection heights should decrease, not increase as now written. But heating should give increasing reflection heights, as the authors observed. This should mean negative doppler shifts. Also, I would read ”enlargements” to mean positive doppler shifts, not negative as I think the authors mean in the separate reply to the reviewer. Please clarify.

 

I now realize that my initial confusion is related to that the color scale of the doppler shifts employed by the authors in figures 1 and 3 appears opposite to what is usual in physics, which might lead to unnecessary misunderstanding also in other readers. Negative doppler shifts are commonly associated with red shifts and positive doppler shifts with blue shifts, which is opposite to what the authors use.

Author Response

Please see the attachment.

Author Response File: Author Response.docx