the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Aug 2026
A Compact Fabry-Perot Interferometer with a Solid Etalon for Measuring Upper Atmospheric Wind and Temperature
Abstract. A compact solid etalon Fabry-Perot interferometer (SEFPI) with an effective aperture diameter of 70 mm has been developed for ground-based measurements of upper atmospheric winds and temperatures at an altitude of approximately 250 km. Unlike conventional FPIs employing air-spaced etalons, the SEFPI uses a solid etalon, enabling a more compact, lightweight, and cost-effective instrument design, while requiring stringent thermal stabilization. Laboratory validation using an acousto-optic frequency shifter (AOFS) to generate controlled Doppler shifts demonstrated the SEFPI’s high measurement accuracy, with an average wind uncertainty of 0.42 m/s and a root mean square error (RMSE) of 0.61 m/s. Field comparison validation was conducted via coincident ground-based measurements with two interferometry techniques. The first employed a Dual-Channel Optical Interferometer (DCOI), while the second utilized a well-established FPI employing a 100 mm diameter air-spaced etalon. The SEFPI measurements exhibited strong agreement with both instruments, yielding Pearson correlation coefficients exceeding 0.84. Statistical analysis yielded an overall mean wind difference of 1.88 m/s relative to the co-located DCOI instrument and 5.58 m/s relative to the conventional FPI located 502 km away. Temperature comparisons between the two FPIs revealed a correlation coefficient of 0.86 and an average difference of 62.38 K.
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RC1: 'Comment on egusphere-2026-2475', Anonymous Referee #1, 14 Sep 2026
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AC1: 'Reply on RC1', Tiancai Wang, 25 Sep 2026
Dear Referee #1,
We sincerely thank you for the careful and constructive evaluation of our manuscript entitled “A Compact Fabry-Perot Interferometer with a Solid Etalon for Measuring Upper Atmospheric Wind and Temperature.” We greatly appreciate your positive assessment of the scientific relevance and instrumental contribution of this work. The comments have helped us improve the clarity, rigor, and presentation of the manuscript.
We have carefully considered all the comments and have revised the manuscript accordingly. In particular, we have clarified the physical basis of the solid-etalon instrument function, provided additional information on the fitting parameters, moderated the interpretation of the temperature intercomparison, clarified the temporal matching procedure, added statistical metrics, and unified several terms and definitions throughout the manuscript. Our detailed responses are provided below. The reviewer’s comments are reproduced in bold. Our responses and descriptions of the corresponding revisions are in normal.
Comment 1: The manuscript states that the conventional FPI instrumental function is modified to account for the solid etalon, but only the final equation is presented. Since the solid etalon is the central methodological novelty of this study, the physical basis of the modified Airy function deserves a more explicit explanation. In particular, the manuscript should clarify how the phase term is modified when the etalon is filled by fused silica. In addition, in the derivation of the instrument function, Equation (1) includes three parameters: I0, I1, and I2. The text only briefly describes these as “constant coefficients”. Please clearly specify the specific physical meanings or mathematical fitting roles of these three coefficients within the model to help readers better understand the retrieval algorithm.
Response: We thank the reviewer for this important comment. We agree that the physical basis of the modified Airy function and the roles of the fitting coefficients should be described more explicitly, particularly because the solid etalon is a key component of the SEFPI.
In the revised manuscript, we have expanded the description of the instrumental function and clarified that, for the solid fused-silica etalon, the phase term is determined by the optical path difference inside the solid medium. Specifically, the phase term is expressed as a function of the refractive index (n), the etalon thickness (t), the wavelength (λ), and the internal incidence angle (θ). Thus, compared with an air-spaced etalon, the refractive index of fused silica explicitly contributes to the optical phase and consequently to the transmission profile of the etalon.
We have also clarified the roles of I0, I1, and I2. These coefficients are treated as fitting parameters in the spectral forward model and account for the average intensity level, the linear falloff of intensity, and the quadratic falloff of intensity, respectively.
The corresponding description and equation have been revised in Section 3 of the manuscript. We believe that these additions make the forward model and the subsequent retrieval procedure clearer and more reproducible.
Comment 2: The temperature comparison between the SEFPI at SIZW and the reference FPI at XLO shows a correlation coefficient of 0.86, but also a mean temperature difference of 62.38 K and a standard deviation of 69.89 K. The regression slope is only 0.77, with an intercept of 255.72 K. Although the authors provide a reasonable discussion of possible causes, including differences in spatial sampling and OH contamination, the magnitude of the systematic temperature difference is relatively large. More importantly, the two FPIs are separated by approximately 502 km. Therefore, this comparison should not be interpreted as a direct validation of the absolute temperature accuracy of the SEFPI, because the two instruments do not observe the same atmospheric volume and may experience different thermospheric conditions. The authors are encouraged to revise the relevant discussion and describe this result more appropriately as a temperature intercomparison, temperature consistency, or preliminary temperature validation. The limitation associated with the spatial separation should also be explicitly acknowledged.
Response: We sincerely thank the reviewer for this insightful and constructive comment. We completely agree that given the ~502 km spatial separation between the SEFPI at SIZW and the reference FPI at XLO, the two instruments do not observe the identical atmospheric volume and are likely subject to different local thermospheric conditions. Unfortunately, the XLO station lacks the necessary infrastructure to support the co-located installation and simultaneous observation of both the SEFPI and the reference FPI. Consequently, we had to rely on observations taken at a distance of 502 km for this initial field campaign. However, given the high correlation (R=0.86) and the generally consistent temporal evolution observed on most nights, we believe this comparison provides a valuable reference for the temperature measurement consistency of the SEFPI. Furthermore, we completely share your perspective on the necessity of spatial colocation. Therefore, we explicitly outlined in the "Conclusion and discussion" section of our manuscript that our future work will focus on conducting co-located simultaneous observations to thoroughly verify the reliability of the temperature measurements.
Comment 3: The field comparison is one of the important validation components of the paper, but the manuscript does not provide sufficient details on how measurements from different instruments were paired in time. The SEFPI integration time is 300 s, while the DCOI has a temporal resolution of approximately 26 min and the XLO FPI has a complete observation cycle of approximately 31 min. The authors should specify: the temporal matching criterion, whether linear interpolation was used, whether the different integration times were taken into account.
Response: Thank you for your helpful suggestion. We agree that the temporal matching procedure should be described explicitly because the three instruments have substantially different observation and integration times. In particular, the SEFPI and DCOI measurements had an integration time of 5 min and a temporal resolution of approximately 26 min, while the FPI used an integration time of 100 s for each of its three channels and a temporal resolution of approximately 31 min. In the revised manuscript, we have added a detailed description of the temporal matching procedure used for the field comparisons. The comparison was performed based on the observation timestamps of the measurements. We have clarified the temporal matching criterion and specified how the measurements with different integration periods were handled.
Comment 4: In Fig. 7 and 9, the RMSE can also be provided, together with mean bias μ.
Response: We thank the reviewer for this suggestion. Accordingly, we have added the root-mean-square error (RMSE) to the statistical analysis of the field comparisons. For the wind comparison between the SEFPI and DCOI at the SIZW station, the RMSEs of the meridional and zonal wind components are 29.4 m/s and 19.88 m/s, respectively. For the comparison between the SEFPI at SIZW and the FPI at XLO, the corresponding RMSEs are 18.33 m/s and 17.91 m/s for the meridional and zonal wind components, respectively. For the temperature comparison, the RMSE is 96.96 K.
The RMSE provides a quantitative measure of the overall discrepancy between the SEFPI measurements and the reference observations, while the mean difference characterizes the average systematic offset. Together, these metrics provide a more comprehensive assessment of the agreement and measurement differences among the instruments. The corresponding RMSE values have been added to the relevant panels in Figures 7 and 9 of the manuscript.
Comment 5: The abstract states an “overall mean wind difference of 1.88 m/s”, whereas the main text describes this quantity as a “mean absolute difference.” To avoid ambiguity, I suggest using the same terminology throughout the manuscript and revising the abstract to “an overall mean absolute difference of 1.88 m/s”.
Response: We thank the reviewer for identifying this inconsistency. We agree that the terminology should be unified to accurately describe the statistical quantity. We have therefore revised the Abstract and the corresponding descriptions in the main text to consistently use “an overall mean absolute difference of 1.88 m/s”.
Comment 6: The abstract refers to an “effective aperture diameter of 70 mm,” whereas Table 1 lists the solid etalon diameter as “Φ= 70 mm.” It is recommended that the authors clarify whether the reported 70 mm refers to the physical diameter, clear aperture, or effective optical aperture of the solid etalon. Consistent terminology should then be used throughout the manuscript.
Response: Thank you for your insight suggestion. We agree that “effective aperture diameter” and “physical diameter” should not be used interchangeably. In the revised manuscript, we have clarified that the reported 70 mm refers to the physical diameter of the solid fused-silica etalon, rather than an independently determined effective optical aperture. We have therefore removed or revised the term “effective aperture diameter” where appropriate and use “70 mm-diameter solid etalon” consistently throughout the manuscript.
The above are our responses to all of your comments. For more details regarding our responses, please review the supplementary materials we have provided. We hope this addresses your concerns.
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AC1: 'Reply on RC1', Tiancai Wang, 25 Sep 2026
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RC2: 'Comment on egusphere-2026-2475', Anonymous Referee #2, 04 Oct 2026
The manuscript can be acceptable for publication.
Citation: https://doi.org/10.5194/egusphere-2026-2475-RC2 -
AC2: 'Reply on RC2', Tiancai Wang, 04 Oct 2026
Dear Referee #2,
We are grateful to the reviewer for the careful assessment of our manuscript entitled “A Compact Fabry-Perot Interferometer with a Solid Etalon for Measuring Upper Atmospheric Wind and Temperature.” We sincerely appreciate the reviewer’s positive evaluation and support for the publication of this work.
Citation: https://doi.org/10.5194/egusphere-2026-2475-AC2
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AC2: 'Reply on RC2', Tiancai Wang, 04 Oct 2026
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This manuscript presents the development and experimental validation of a compact Fabry-Perot interferometer employing a solid fused-silica etalon for ground-based measurements of thermospheric neutral winds and temperatures near 250 km. The instrument combines a 70 mm solid etalon with a narrow field sky scanner, a temperature-controlled chamber, and a cooled CCD detector. The authors evaluate the instrument performance using an AOFS-based laboratory Doppler-shift simulation and further validate the instrument through field comparisons with a co-located DCOI at SIZW and an air-spaced etalon FPI at XLO. The work is timely, relevant, and addresses a practical need for more affordable and deployable ground-based optical instruments. Overall, this is a well-written paper that makes a compelling case for the proposed approach. The manuscript can be considered publication after the authors address the few points below.