Review of "Ionospheric Upwelling and the Level of Associated Noise at Solar Minimum" by David et al.

The authors present a statistical study into the occurrence and characteristics of noise in incoherent scatter radar observations. Noise frequently accompanies ISR altitude profiles, and, as researchers in the field well know, one must be aware of its characteristics to properly interpret ISR data. The work is a timely scrutiny of measurement noise based on a large dataset of ISR data. The authors provide a well written introduction to ISR operation and explain how noise is defined. The results are presented in a structured concise manner. I quite enjoyed reading this paper.

I believe that publishing the paper as is is justifiable. Nevertheless, there are potential points of improvement that bear promise should the authors wish to pursue them. The below list is meant to provide some ideas, and I do not expect or demand that the authors pursue all of these points.

• The authors are rather vague on the mechanisms that produce noise to begin with. Are low ionization levels alone enough to cause local winter noise to dominate in this way? Does noise always appear with the same essential characteristics or is it possible to discern certain physical traits in the noise?
• How does the noise proportion during local winter respond to the onset of geomagnetic storms? A simple superposed epoch analysis of storm-time onsets (or the onset of other geomagnetic index-excursions) which may show when or whether the signal rises above the noise during such events.
• The radar in question is well positioned to observe the cusp, where ion outflows are highly characteristc, as well as elevated ionization rates. This sector, as well as the midnight sector, see a dip in the noise occurrence in Figure 4. Notably, the soft electron ionization that is characteristic for the cusp provides abundant F-region ionization. A short discussion of why noise is suppressed in these sectors may be enlightening.

Sentence 20: "(...) common high-latitude phenomena, and are frequent during local summer."

Sentence 25: should it read "(...) altitudinal increase of the ionosphere"?

Sentence 80: Perhaps the authors can offer preliminary suggestions as to whether and/or why a deep solar minimum is associated with increased levels of noise. 

I am positive to the authors' replies and would welcome a new version in order that I can write my recommendation to publish.

This paper examines and classifies the variability of noise occurrence in the ISR ion velocities data. This is useful work that is potentially worth publishing. However, the present version needs a major revision because of rather poor structuring and lack of necessary information mainly related to the introduction and discussion.

Comments.

Ll. 39-40. “The main focus of this paper …” This basic statement does not seem to adequately reflect what is actually being done. First, the statistical occurrence of noise is studied, rather than noise in terms of its inherent properties. Secondly, not only seasonal variability is presented, but also the dependence on LT. Please formulate your goal more precisely.

It is also not clear how the present study is placed into context. The sentence preceding "the main focus", with reference to earlier work by Wannberg et al. (1997), lists possible sources of the noise occurrence. And after this, a reader may expect a brief overview of what has been done (or not done) in the past to evaluate the noise and what remains unexplored. More references and explanations are needed here. Otherwise, the purpose of this study does not seem sufficiently justified.

The statement that noise is associated with non-physical velocities (ll. 43-44) hardly needs so many references. And they all seem rather formal, since the papers mentioned are actually in-depth studies of various aspects of radar observations, naturally using only physically meaningful values.  

To avoid confusion and ambiguity, it would be much better to make the introduction as a separate section and add more relevant information there. The next section should be Instrumentation&data. The classification of fluxes should certainly be moved to this second section.

Although section 2 is titled Results and Discussion, this reviewer did not find any discussion. Only the two last sentences can be considered somewhat related to the discussion. And it is too few. The discussion should be expanded or the word “discussion” should be removed from the title. The results without discussion seem not a good idea though, especially if the introduction is too brief. There can be different ways to have an interesting discussion, e.g. implementation of the results obtained (for EISCAT 3-D?), their physical meaning, comparison with previous results.