Reviewer #1 comments

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-- Reviewer: This paper analyses the electron, ion and neutral temperatures in the Lower Thermosphere - Ionosphere and how the energy is tranfered between these populations. It uses a large dataset, comprising both in-situ measurements from low-Earth orbit satellites and remote sensing measurements from Incoherent Scatter Radars. It shows the overall consistency of these measurements, which are based on different techniques, and then examines the cases where, contrary to the general trend, the ion temperature is lower than the neutral temperature. Potential physical mechanisms are then explored, in order to explain this "anomaly". Although there is no firm conclusion, the analysis performed by the authors provides sufficient insight on the possible causes, and shows the need for further in-situ measurements .  The analysis is careful, the results are new, the paper is clearly written, and it should be accepted for publication after some minor revision.

-- Author response: We thank the reviewer for valuable feedback and insightful comments on the submitted manuscript, which we have addressed in the revised manuscript. In the following we list our responses to these comments. Responses are marked in italics, and added text is marked in quotations.

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Specific comments:

-- Reviewer: Introduction, line 21: "It is well established that Earth’s Lower Thermosphere-Ionosphere (LTI) region is generally not in thermal equilibrium, or...": please give a reference (e.g. Pfaff, Space Sci. Rev.,  168:23-112, 2012). 

-- Author response: We agree with the reviewer that this statement on the thermal equilibrium was not supported, and have added the suggested reference in line 23 of the revised manuscript.

Pfaff R.F., The Near-Earth Plasma Environment, Space Sci. Rev.,  168:23-112, 2012, https://doi.org/10.1007/s11214-012-9872-6 

Furthermore, the following text has been added in lines 351-360 of the discussions section on the relative temperatures of electrons and ions: 

“Together with the above analyses that compares the temperatures of ions and neutrals in the ionosphere-thermosphere, the relative temperature of ions and electrons are of extreme importance to the state of the ionosphere. Whereas globally it is expected that Te > Ti is much more common due to the effects of UV radiation, at times Ti > Te has also been observed, associated with storm-time enhanced Joule heating. For example, through analyzing EISCAT ISR data, Kofman and Lathuillere (1987) have shown profiles of very high ion temperatures (greater than 8000K), observed along geomagnetic field lines, which they attributed to frictional heating between fast moving species. Similarly, Buchert and Hoz (1988) also reported observations of very high ion temperatures (on the order of 12000K), which were not accompanied by commensurate changes in the electron temperature; they also attributed such cases of Ti < Te to Joule heating. It is believed that this is less common, but is expected to be energetically significant. Such events are not analyzed statistically herein, and are the topic of a future study.”

•    Kofman, W. and Lathuillere, C.: Observation by the incoherent scatter technique of the hot spots in the auroral zone ionosphere, Geophysical Research Letters, 14, 1158–1161, 1987.

•    Buchert, S. and Hoz, C. L.: Extreme ionospheric effects in the presence of high electric fields, Nature, 333, 438–440, 1988.

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-- Reviewer: Discussion, lines 207-208: "One possible cause potentially leading to measurement errors concerns the alteration of Ti and/or Tn measurements due to interactions with a ram cloud": please develop more on how a ram cloud could lead to these measurement errors.

-- Author response: We thank the reviewer for this comment on measurement errors, that indeed requires more development. In response to this comment we expanded upon the factors affecting the accuracy of temperature determination of both Ti and/or Tn measurements, adding other effects that can influence in-situ measurements, and also listing a number of references that have addressed spacecraft-environment interactions and their effect on in-situ measurements. Also in response to a comment by Reviewer 2, these are discussed in a separate sub-section, titled “Possible sources of measurement errors”. The following text has been added to the manuscript, at line 213: 

“4.1 Possible sources of measurement errors

There are several potential sources of uncertainty that can lead to measurements errors of in-situ ion and neutral temperature. The removal or correction of such errors has been the topic of multiple studies over the past decades (e.g., De-Forest, 1972; Whipple, 1981; Hastings, 1995; Hanley et al., 2021). These errors are primarily due to the high spacecraft velocity and the interaction of the spacecraft with the surrounding plasma and neutral environment. For example, factors that affect the accuracy of measuring ion temperatures include the acceleration of plasma by a charged surface, the generation of a complex plasma cloud that surrounds the spacecraft and interacts with the environment, and impact ionization and reflection of particles off of the spacecraft and the subsequent inclusion of reflected ions in the measurements (e.g., Heelis and Hanson, 1998; Ergun et al. 2020; Hanley et al., 2021). In particular, Ergun et al. (2021) addressed spacecraft motion effects due to the creation of a wake in the Martian ionosphere, and demonstrated the recalibration of MAVEN instrumentation on the Mars Atmosphere and Volatile EvolutionN (MAVEN) spacecraft (Jakosky et al. (2015)) with the aid of kinetic solutions and published results from laboratory experiments, through which they achieved a significant improvement in the systematic uncertainty of Te measurements. Similarly, Hanley et al. (2021) discussed a series of rigorous processes that they employed to identify and correct various sources of uncertainties in measurements of Ti arising from the supersonic velocities of MAVEN; these include altitude-dependent systematic errors as well as random errors from statistical fluctuations and uncertainties in spacecraft potential. Factors affecting the accuracy of neutral temperature measurements include the applicability of the kinetic theory used in extracting neutral temperatures, in particular at lower altitudes where a shorter mean-free-path of the measured particles might affect the measurements, and gas surface interactions, which are also dependent on altitude and neutral density (e.g., Spencer et al., 1973).”

The following references were added:

•    DeForest,  S.  E.  (1972).  Spacecraft  charging  at  synchronous  orbit.  Journal  of  Geophysical  Research, 77(4),  651–659.  https://doi.org/10.1029/ja077i004p00651 

•    Ergun, B., et al, Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956 

•    Hastings, D. E. (1995). A review of plasma interactions with spacecraft in low Earth orbit. Journal of Geophysical Research, 100(A8), 14457. https://doi.org/10.1029/94ja03358

•    Hanley et al., J Geophys Res Space Phys. https://doi.org/10.1029/2021JA029531

•    Jakosky, B. M., et al., The Mars Atmosphere and Volatile Evolution (MAVEN) Mission, Space Sci Rev (2015) 195:3–48, https://doi.org/10.1007/s11214-015-0139-x

•    Spencer, N. W., H. B. Niemann, G. R. Carignan, The Neutral Atmosphere Temperature Instrument, Radio Science, Volume 8, Number 4, pages 287-296, April 1973, https://doi.org/10.1029/RS008i004p00287 

•    Heelis and Hanson, Measurement Techniques in Space Plasmas, in Geophys. Monogr. Ser., Vol. 102 AGU, Washington, D.C., 1998, p. 61.

•    Whipple, E. C. (1981). Potentials of surfaces in space. Reports on Progress in Physics, 44(11), 1197–1250.  https://doi.org/10.1088/0034-4885/44/11/002 

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-- Reviewer: Summary, line 340: "Several potential causes have been identified that can explain": -> "Several potential causes have been identified that could explain". 

-- Author response: We agree with the reviewer that “could explain” is a more suitable expression, due to the uncertainty of the actual cause; a correction has been made in the manuscript in line 374.

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-- Reviewer: Several figure labels are too small (e.g. Figures 4 and A1, A2, A3 axes labels): please resize. 

-- Author response: Indeed, we agree that the labels were too small in these figures; we thank the reviewer pointing it out. The relevant figure labels and legends have been resized.

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Reviewer #2 Comments

Comments on: Analysis of in-situ measurements… by Pirnaris and Sarris

W.K. Peterson, July 2023

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-- Reviewer: This paper presents a re-analysis of co-temporal, co-spatial observations of electron, ion, and neutral temperatures (Te, Ti, Tn) in the lower thermosphere-ionosphere from the Atmosphere Explorer (AE) and Dynamics Explorer (DE) satellites.  The authors show that with the exception reports from one rocket, these are the only observations available to stress test models of lower thermosphere dynamics. 

The published observations of AE and DE Te, Ti, and Tn do not include estimates of uncertainty. The authors use nearly co-temporal and co-spatial observations of Te and Ti from incoherent scatter radars to examine the validity of the AE and DE Te and Ti observations.

They then find an astonishingly large number of observations in the AE data base where Ti < Tn, contrary to the text book results of Schunk and Nagy.  An analysis of the geographic and altitude dependence of these observations shows that is unlikely that a single instrumental feature or geophysical mechanism could explain the diverse distribution of observations of Ti < Tn.

This is a very important result. It identifies a large gap in our understanding of the dynamics of the lower thermosphere-ionosphere and the need for new measurements that incorporate new technology and report uncertainties in co-temporal, co-spatial observations of electron, ion, and neutral temperatures.

I recommend its publication.

-- Author response: We thank the reviewer for the time and effort in producing this review report. We also thank the reviewer for a thorough review of the manuscript and for the constructive comments and suggestions for changes, which we have implemented, as described below. In the following, replies to comments are marked in italics, and text that has been added to the manuscript is marked in quotations.

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-- Reviewer: Below are minor comments and suggestions that I believe can strengthen the paper.

Minor comments

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-- Reviewer: Line 35. Observations of Tn in the lower thermosphere from UV instruments such as GUVI on the TIMED satellite are available after the time of the AE and DE satellites.  Peterson et al. 2023 showed that co-temporal, co-spatial observations these neutral temperatures and Ti and Te from incoherent scatter radars are possible.  For completeness the possibility of this method for obtaining co-temporal, co-spatial observations of Te, Ti, and Tn should be mentioned, perhaps in the discussion or conclusion section. 

-- Author response: We thank the reviewer for this comment; this is indeed a dataset that should be further exploited, and this has been pointed out in the conclusions section of the revised manuscript, lines 393-400, as follows: 

“It is noted that observations of Tn in the lower thermosphere have also been provided from space-born UV instruments, such as GUVI on the TIMED satellite (Christensen et al., 2003), which could be combined with remote sensing of Te and Ti from ISR measurements; for example, recently, Peterson et al. (2023) analyzed these measurements and presented examples of co-temporal, co-spatial observations of Tn from GUVI as well as Ti and Te observations from incoherent scatter radars. In their conclusions, they point out that the error bars on the presented temperature profile observations do not allow a strong conclusion to be drawn; however a systematic statistical investigation of these combined datasets could yield more insight into the conditions leading to observations of ΔΤin < 0.”

Christensen, A. B., et al., Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellitemission,J. Geophys. Res.,108(A12), 1451, doi:10.1029/2003JA009918, 2003.

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-- Reviewer: Line 53.  Do you mean …between 130 and 140 km? 

-- Author response: Indeed, this was a typo and has been corrected in line 56.

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-- Reviewer: Line 80. Do you mean, With the exception of one rocket observation (Sasaki and Kawashima, 1975) the only in co-temporal, co-spatial observations of electron, ion, and neutral temperatures were obtained ….? 

-- Author response: Indeed, this sentence was not very well formulated, and has been rephrased in line 84.

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-- Reviewer: Discussion of Figure 4:   Please include in the figure caption and/or text a description of the color bar used to encode the data points.  

-- Author response: In response to the reviewer’s comment, and have added a description of the color bar in the figure caption of Figures 4 as follows:

“The color scale of the data points represents the neutral density, as obtained from the addition of N2 and O in-situ density measurements”

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-- Reviewer: Figure 5: This is the most important figure in the paper.  The reader should not have to go searching for a description of the quantity encoded by the color bar. It should be included in the figure caption. 

-- Author response: Indeed, the color scale is a critical aspect of this figure; the following sentence has been added in the figure caption of Figure 5 in response to the Reviewer’s comment:

“The color scale represents the normalized PDF ranging from 0 (corresponding to the lowest likelihood for the observation of Ti < Tn), which is marked as blue, to 1 (corresponding to the highest likelihood), which is marked as red.”

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-- Reviewer: To help guide the reader, the discussion section starting at line 206 needs a short introduction and sub sections addressing: 1) The possible sources of measurement errors and 2) The possible physical mechanisms that could lead to extended intervals of Ti<Tn

-- Author response: We thank the reviewer for this suggestion, which greatly helps the structure of the discussions and the differentiation between measurement effects and physical mechanisms. We have followed the reviewer’s suggestion, and have included a brief introductory statement, followed by subsections 4.1 and 4.2, as follows:

“In the following, we discuss possible reasons leading to the appearance and distribution of the occurrences of ∆Tin < 0. Both potential instrumental or measurement effects and physical processes are discussed, including implications for our current understanding of LTI processes.

4.1 Possible sources of measurement errors

4.2 Possible physical mechanisms that could lead to observations of Ti < Tn”

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-- Reviewer: Regarding measurement errors. Much has been learned from the instruments on the Mars Maven instrument about the measurements of Ti, Te, and Tn and the role of space craft charging as reported by, for example

•    Hanley et al., J Geophys Res Space Phys. doi: 10.1029/2021JA029531

•    Ergun et al, Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956

Perhaps references to these and paper(s) addressing the ‘ram cloud’ hypothesis can be included in the sub-section on the sources of measurement errors.

-- Author response: We completely agree with the reviewer that the extensive recent experience from MAVEN is a key resource on, among multiple other topics, spacecraft-plasma interactions. This was indeed a key missing element of the paper, and we thank the reviewer for pointing it out. In response to this comment, the following sentences have been added in line 216 (see also response to reviewer 1):

“Factors that affect the accuracy of measuring electron and ion temperatures include the acceleration of plasma by a charged surface, the generation of a complex plasma cloud that surrounds the spacecraft and interacts with the environment, and impact ionization and reflection of particles off of the spacecraft and the subsequent inclusion of reflected ions in the measurements (e.g., Heelis and Hanson, 1998; Ergun et al., 2021; Hanley et al., 2021). In particular, Ergun et al. (2021) addressed spacecraft motion effects due to the creation of a wake in the Martian ionosphere, and demonstrated the recalibration of MAVEN instrumentation on the Mars Atmosphere and Volatile EvolutionN (MAVEN) spacecraft (Jakosky et al., 2015) with the aid of kinetic solutions and published results from laboratory experiments, through which they achieved a signifcant improvement in the systematic uncertainty of Te measurements. Similarly, Hanley et al. (2021) discussed a series of rigorous processes that they employed to identify and correct various sources of uncertainties in measurements of Ti arising from the supersonic velocities of MAVEN; these include altitude-dependent systematic errors as well as random errors from statistical fluctuations and uncertainties in spacecraft potential.”

The following references were added:

•    Hanley et al., J Geophys Res Space Phys. https://doi.org/10.1029/2021JA029531

•    Ergun et al, Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956 

•    Jakosky, B. M., et al., The Mars Atmosphere and Volatile Evolution (MAVEN) Mission, Space Sci Rev (2015) 195:3–48, https://doi.org/10.1007/s11214-015-0139-x

Reviewer #2 Comments

Comments on: Analysis of in-situ measurements… by Pirnaris and Sarris

W.K. Peterson, July 2023

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-- Reviewer: This paper presents a re-analysis of co-temporal, co-spatial observations of electron, ion, and neutral temperatures (Te, Ti, Tn) in the lower thermosphere-ionosphere from the Atmosphere Explorer (AE) and Dynamics Explorer (DE) satellites.  The authors show that with the exception reports from one rocket, these are the only observations available to stress test models of lower thermosphere dynamics. 

The published observations of AE and DE Te, Ti, and Tn do not include estimates of uncertainty. The authors use nearly co-temporal and co-spatial observations of Te and Ti from incoherent scatter radars to examine the validity of the AE and DE Te and Ti observations.

They then find an astonishingly large number of observations in the AE data base where Ti < Tn, contrary to the text book results of Schunk and Nagy.  An analysis of the geographic and altitude dependence of these observations shows that is unlikely that a single instrumental feature or geophysical mechanism could explain the diverse distribution of observations of Ti < Tn.

This is a very important result. It identifies a large gap in our understanding of the dynamics of the lower thermosphere-ionosphere and the need for new measurements that incorporate new technology and report uncertainties in co-temporal, co-spatial observations of electron, ion, and neutral temperatures.

I recommend its publication.

-- Author response: We thank the reviewer for the time and effort in producing this review report. We also thank the reviewer for a thorough review of the manuscript and for the constructive comments and suggestions for changes, which we have implemented, as described below. In the following, replies to comments are marked in italics, and text that has been added to the manuscript is marked in quotations.

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-- Reviewer: Below are minor comments and suggestions that I believe can strengthen the paper.

Minor comments

-----------------------------------------------------------------------------

-- Reviewer: Line 35. Observations of Tn in the lower thermosphere from UV instruments such as GUVI on the TIMED satellite are available after the time of the AE and DE satellites.  Peterson et al. 2023 showed that co-temporal, co-spatial observations these neutral temperatures and Ti and Te from incoherent scatter radars are possible.  For completeness the possibility of this method for obtaining co-temporal, co-spatial observations of Te, Ti, and Tn should be mentioned, perhaps in the discussion or conclusion section. 

-- Author response: We thank the reviewer for this comment; this is indeed a dataset that should be further exploited, and this has been pointed out in the conclusions section of the revised manuscript, lines 393-400, as follows: 

“It is noted that observations of Tn in the lower thermosphere have also been provided from space-born UV instruments, such as GUVI on the TIMED satellite (Christensen et al., 2003), which could be combined with remote sensing of Te and Ti from ISR measurements; for example, recently, Peterson et al. (2023) analyzed these measurements and presented examples of co-temporal, co-spatial observations of Tn from GUVI as well as Ti and Te observations from incoherent scatter radars. In their conclusions, they point out that the error bars on the presented temperature profile observations do not allow a strong conclusion to be drawn; however a systematic statistical investigation of these combined datasets could yield more insight into the conditions leading to observations of ΔΤin < 0.”

Christensen, A. B., et al., Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellitemission,J. Geophys. Res.,108(A12), 1451, doi:10.1029/2003JA009918, 2003.

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-- Reviewer: Line 53.  Do you mean …between 130 and 140 km? 

-- Author response: Indeed, this was a typo and has been corrected in line 56.

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-- Reviewer: Line 80. Do you mean, With the exception of one rocket observation (Sasaki and Kawashima, 1975) the only in co-temporal, co-spatial observations of electron, ion, and neutral temperatures were obtained ….? 

-- Author response: Indeed, this sentence was not very well formulated, and has been rephrased in line 84.

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-- Reviewer: Discussion of Figure 4:   Please include in the figure caption and/or text a description of the color bar used to encode the data points.  

-- Author response: In response to the reviewer’s comment, and have added a description of the color bar in the figure caption of Figures 4 as follows:

“The color scale of the data points represents the neutral density, as obtained from the addition of N2 and O in-situ density measurements”

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-- Reviewer: Figure 5: This is the most important figure in the paper.  The reader should not have to go searching for a description of the quantity encoded by the color bar. It should be included in the figure caption. 

-- Author response: Indeed, the color scale is a critical aspect of this figure; the following sentence has been added in the figure caption of Figure 5 in response to the Reviewer’s comment:

“The color scale represents the normalized PDF ranging from 0 (corresponding to the lowest likelihood for the observation of Ti < Tn), which is marked as blue, to 1 (corresponding to the highest likelihood), which is marked as red.”

-----------------------------------------------------------------------------

-- Reviewer: To help guide the reader, the discussion section starting at line 206 needs a short introduction and sub sections addressing: 1) The possible sources of measurement errors and 2) The possible physical mechanisms that could lead to extended intervals of Ti<Tn

-- Author response: We thank the reviewer for this suggestion, which greatly helps the structure of the discussions and the differentiation between measurement effects and physical mechanisms. We have followed the reviewer’s suggestion, and have included a brief introductory statement, followed by subsections 4.1 and 4.2, as follows:

“In the following, we discuss possible reasons leading to the appearance and distribution of the occurrences of ∆Tin < 0. Both potential instrumental or measurement effects and physical processes are discussed, including implications for our current understanding of LTI processes.

4.1 Possible sources of measurement errors

4.2 Possible physical mechanisms that could lead to observations of Ti < Tn”

-----------------------------------------------------------------------------

-- Reviewer: Regarding measurement errors. Much has been learned from the instruments on the Mars Maven instrument about the measurements of Ti, Te, and Tn and the role of space craft charging as reported by, for example

•    Hanley et al., J Geophys Res Space Phys. doi: 10.1029/2021JA029531

•    Ergun et al, Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956

Perhaps references to these and paper(s) addressing the ‘ram cloud’ hypothesis can be included in the sub-section on the sources of measurement errors.

-- Author response: We completely agree with the reviewer that the extensive recent experience from MAVEN is a key resource on, among multiple other topics, spacecraft-plasma interactions. This was indeed a key missing element of the paper, and we thank the reviewer for pointing it out. In response to this comment, the following sentences have been added in line 216 (see also response to reviewer 1):

“Factors that affect the accuracy of measuring electron and ion temperatures include the acceleration of plasma by a charged surface, the generation of a complex plasma cloud that surrounds the spacecraft and interacts with the environment, and impact ionization and reflection of particles off of the spacecraft and the subsequent inclusion of reflected ions in the measurements (e.g., Heelis and Hanson, 1998; Ergun et al., 2021; Hanley et al., 2021). In particular, Ergun et al. (2021) addressed spacecraft motion effects due to the creation of a wake in the Martian ionosphere, and demonstrated the recalibration of MAVEN instrumentation on the Mars Atmosphere and Volatile EvolutionN (MAVEN) spacecraft (Jakosky et al., 2015) with the aid of kinetic solutions and published results from laboratory experiments, through which they achieved a signifcant improvement in the systematic uncertainty of Te measurements. Similarly, Hanley et al. (2021) discussed a series of rigorous processes that they employed to identify and correct various sources of uncertainties in measurements of Ti arising from the supersonic velocities of MAVEN; these include altitude-dependent systematic errors as well as random errors from statistical fluctuations and uncertainties in spacecraft potential.”

The following references were added:

•    Hanley et al., J Geophys Res Space Phys. https://doi.org/10.1029/2021JA029531

•    Ergun et al, Journal of Geophysical Research: Space Physics, 126, e2020JA028956. https://doi.org/10.1029/2020JA028956 

•    Jakosky, B. M., et al., The Mars Atmosphere and Volatile Evolution (MAVEN) Mission, Space Sci Rev (2015) 195:3–48, https://doi.org/10.1007/s11214-015-0139-x