the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Inferring neutral winds in the ionospheric transition region from AGWTID observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster
Florian Günzkofer
Dimitry Pokhotelov
Gunter Stober
Ingrid Mann
Sharon L. Vadas
Erich Becker
Anders Tjulin
Alexander Kozlovsky
Masaki Tsutsumi
Njål Gulbrandsen
Satonori Nozawa
Mark Lester
Evgenia Belova
Johan Kero
Nicholas J. Mitchell
Claudia Borries
Abstract. Atmospheric Gravity Waves and Traveling Ionospheric Disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward propagating atmospheric gravity waves can be simultaneously observed with the EISCAT Very High Frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multistatic measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of Traveling Ionospheric Disturbances are notably impacted by the fall transition of the mesosphere/lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere.
Florian Günzkofer et al.
Status: open (until 06 Jul 2023)

RC1: 'Comment on egusphere2023678', Stephan C. Buchert, 12 May 2023
reply
Discussion/Review of egusphere2023678
"Inferring neutral winds in the ionospheric transition region from AGWTID observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster"
by Florian Günzkofer et al.
The manuscript presents more advanced and refined methods compared to previous ones for detecting and analysing atmospheric gravity waves and traveling ionospheric disturbances with incoherent scatter and meteor radars. Both instruments seem to detect the same AGWTID activity on the in total three days used for the study. AGW parameters are found to be different depending on the seasonal phase relative to a "fall transition". The manuscript is relatively well written, but sometimes descriptions seem to be inaccurate and equations incomplete. The figures are of good quality. Before a publication I suggest a few improvements according to my comments below.
Referencing:
Our friends from the LOFAR community have in the recent years been successfully studying AGWs at small to medium scales, which should perhaps be mentioned together with other methods refered to in lines 4555. I found these papers
Boyde et al. (2022), Lensing from smallscale travelling ionospheric disturbances observed using LOFAR, https://www.swscjournal.org/articles/swsc/full_html/2022/01/swsc220042/swsc220042.html
Fallows et al. (2020), A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances, https://www.swscjournal.org/articles/swsc/full_html/2020/01/swsc190078/swsc190078.html
and a presentation at the EGU 2023: https://meetingorganizer.copernicus.org/EGU23/session/46346#Orals
Though not covering the same geographic locations and so far also not the same time periods, a general comparison between the LOFAR and here presented EISCAT methods and their advantages could be briefly attempted, e.g. in Section 5.
Lines 1516: "In the ionosphere, AGWs can be observed as MediumScale Traveling Ionospheric Disturbances (MSTIDs) from neutralion collisions (Nicolls et al., 2014)." The statement could be clearer. I think that something like > "In the ionosphere, which is coupled to the neutral atmosphere by ionneutral collisions, AGWs can be observed as MediumScale Traveling Ionospheric Disturbances (MSTIDs) (Nicolls et al., 2014)."
Lines 135147: A relation between an upward or downward propagation of the AGW and the occurence of negative vertical wavenumbers/frequencies seems to be implied, but why and how should be explained in more detail to the reader. In the 3rd quadrant both kz and f are negative, so their product is positive. The step function, Eq 1, does not remove any of the strong peaks in the 1st and 3rd quadrants, only the downward propagating "noise" in the 2nd and 4th quadrants. Is this correct?
Equation (1) is incomplete as it stands alone. What is sigma and how is the filter exactly working? The reader can guess this relatively easily, but it would be better to have the complete equations of the 2D Fourier (inverse) transform and the filter.
Lines 172173: "The obtained wave period τ = 43.1 ± 1.6 min is nearly constant with altitude which also fits previous findings and expectations (e.g., Nicolls et al., 2014)." Wouldn't be a discussion of the BruntVäisälä period be appropriate at this point. I think that the BruntVäisälä frequency is not very constant in the altitude range plotted in Figure 3 (because of the transition from molecular to atomic particles (e.g. O2 > O) and the temperature gradient). The vertical wavelength could be compared with the scale height of a hydrostatic equilibrium.
Line 185: "... for the selected gridcell at 69◦ N, 22◦ E." Why was this grid cell selected? The EISCAT VHF beam would be at 69.58° N and 19.23° E, where also one of the four meteor radars is located.
Lines 193194: "The wave period τ = 44.1±4.0 min is nearly constant with altitude and is within the uncertainties of the wave period measured with EISCAT." Again I'm a bit sceptical that this period comes out artificially because of the filtering method while physically relevant periods, for example BruntVäisälä, have different values and are not so constant over the altitude. Please discuss this.
Line 200: "The horizontal wind field is Fourier filtered at each altitude level ...", but then in line 209210 "The fit shown in Fig. 6 (bottom left) yields a horizontal wavelength λH = 230 km and a propagation direction of α = 36.9◦ ." single, not altitudedependent values are obtained. I thought you would do the whole filtering and fitting at each altitude level and then come up with zdependent λH and α.
Lines 226227: "We can rewrite this by introducing the wind velocity along the propagation direction of the wave U∥ = (kH · UH ) / kH ." With this definition U∥ is a scalar, not a vector with a direction. It is a wind "speed". The subscript ∥ normally refers to the magnetic field direction, but here this is not meant. Rather it is a horizontal direction. The AGW has horizontal velocity and wave (or k) vectors. Both havve different directions, and I think that you want to project the wind onto the wave vector direction. Please describe more precisely (also the equation).
Lines 233235: "Equation 6 is solved for the optimum wind velocity U∥ applying a nonlinear leastsquares fit using a LevenbergMarquardt algorithm." Again I think that this is incomplete and a bit confusing. Equation 6 has no parameter U∥ that could be optimized. Please describe more comprehensively which expression is minimized with LM, which parameters are observed or are filtered observations, which parameters are from a model, and whether parameters are z dependent ot assumed to be constant.
Line 240: "... approximately 140 km altitude. Above that, the fit of the nonviscous dispersion relation no longer converges." but Figure 7 shows a curve for the fit of the nonviscous dispersion relation also above 140 km. The curve seems to be at exactly 0 m/s which probably indicates no convergence. It would be better to not plot the curve at altitudes where the LM did not converge.
Line 597: "... https://doi.org/110.1029/2017JD027970 ..." > "... https://doi.org/10.1029/2017JD027970 ..."
Citation: https://doi.org/10.5194/egusphere2023678RC1 
AC1: 'Reply on RC1', Florian Günzkofer, 02 Jun 2023
reply
Discussion/Review of egusphere2023678
"Inferring neutral winds in the ionospheric transition region from AGWTID observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster"
by Florian Günzkofer et al.
Reviewer:
The manuscript presents more advanced and refined methods compared to previous ones for detecting and analysing atmospheric gravity waves and traveling ionospheric disturbances with incoherent scatter and meteor radars. Both instruments seem to detect the same AGWTID activity on the in total three days used for the study. AGW parameters are found to be different depending on the seasonal phase relative to a "fall transition". The manuscript is relatively well written, but sometimes descriptions seem to be inaccurate and equations incomplete. The figures are of good quality. Before a publication I suggest a few improvements according to my comments below.
Authors:
We thank the referee for their feedback.
Reviewer:
Referencing:
Our friends from the LOFAR community have in the recent years been successfully studying AGWs at small to medium scales, which should perhaps be mentioned together with other methods refered to in lines 4555. I found these papers
Boyde et al. (2022), Lensing from smallscale travelling ionospheric disturbances observed using LOFAR, https://www.swscjournal.org/articles/swsc/full_html/2022/01/swsc220042/swsc220042.html
Fallows et al. (2020), A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances, https://www.swscjournal.org/articles/swsc/full_html/2020/01/swsc190078/swsc190078.html and a presentation at the EGU 2023: https://meetingorganizer.copernicus.org/EGU23/session/46346#Orals.
Though not covering the same geographic locations and so far also not the same time periods, a general comparison between the LOFAR and here presented EISCAT methods and their advantages could be briefly attempted, e.g. in Section 5.
Authors:
The LOFAR studies have been added to the list of previous measurements in the introduction and the capabilities and results are briefly discussed in comparison to EISCAT in Section 5.
Reviewer:
Lines 1516: "In the ionosphere, AGWs can be observed as MediumScale Traveling Ionospheric Disturbances (MSTIDs) from neutralion collisions (Nicolls et al., 2014)." The statement could be clearer. I think that something like > "In the ionosphere, which is coupled to the neutral atmosphere by ionneutral collisions, AGWs can be observed as MediumScale Traveling Ionospheric Disturbances (MSTIDs) (Nicolls et al., 2014)."
Authors:
Changed as suggested.
Reviewer:
Lines 135147: A relation between an upward or downward propagation of the AGW and the occurrence of negative vertical wavenumbers/frequencies seems to be implied, but why and how should be explained in more detail to the reader. In the 3rd quadrant both kz and f are negative, so their product is positive. The step function, Eq 1, does not remove any of the strong peaks in the 1st and 3rd quadrants, only the downward propagating "noise" in the 2nd and 4th quadrants. Is this correct?
Authors:
That is correct. Downward phase progression is usually an indication that the AGWTID wave is propagating upward. Strong background wind shear could reverse the phase behavior; however, this is presumably not the case for the shown measurements.
The fact that downward phase progression corresponds to  f * k_z  > 0 in a 2D Fourier spectrum is a mathematical property of the Fourier transform which can easily be seen from an artificial example wave pattern (see attached pdf). The explanation in the manuscript has been extended and clarified.
Reviewer:
Equation (1) is incomplete as it stands alone. What is sigma and how is the filter exactly working? The reader can guess this relatively easily, but it would be better to have the complete equations of the 2D Fourier (inverse) transform and the filter.
Authors:
As suggested, Eq. 1 has been rewritten to a full equation showing how the filter function is applied to the spectrum to give a filtered Fourier spectrum.
Reviewer:
Lines 172173: "The obtained wave period τ = 43.1 ± 1.6 min is nearly constant with altitude which also fits previous findings and expectations (e.g., Nicolls et al., 2014)." Wouldn't be a discussion of the BruntVäisälä period be appropriate at this point. I think that the BruntVäisälä frequency is not very constant in the altitude range plotted in Figure 3 (because of the transition from molecular to atomic particles (e.g. O2 > O) and the temperature gradient). The vertical wavelength could be compared with the scale height of a hydrostatic equilibrium.
Authors:
The BruntVäisälä frequency is not constant at these altitudes. However, for k_H << k_z (which is definitely the case here), the observed wave frequency can be much smaller than the buoyancy frequency (see Fritts and Alexander 2003 and Nicolls et al. 2014). A brief summary/discussion has been added to our manuscript.
Reviewer:
Line 185: "... for the selected gridcell at 69◦ N, 22◦ E." Why was this grid cell selected? The EISCAT VHF beam would be at 69.58° N and 19.23° E, where also one of the four meteor radars is located.
Authors:
In Figure 6 (left, top) you can see that the wave amplitude maximizes towards the east of the EISCAT location. The same wave mode can also be detected at 69.5° N 19.5°E but with reduced amplitude and therefore we decided to show the vertical crosssection of the wave at a position of strong wave amplitude rather than at the exact EISCAT location.
Reviewer:
Lines 193194: "The wave period τ = 44.1±4.0 min is nearly constant with altitude and is within the uncertainties of the wave period measured with EISCAT." Again, I'm a bit skeptical that this period comes out artificially because of the filtering method while physically relevant periods, for example BruntVäisälä, have different values and are not so constant over the altitude. Please discuss this.
Authors:
We refer to Vadas and Becker, 2018 especially Eq. 8, where it can be seen that only the change in time of the background wind and intrinsic wave frequency can change the observed wave period (see Vadas and Fritts, 2005). So, if the buoyancy frequency changes in altitude but not in time (which is usually the case over time periods of a few hours or less), then the observed wave period will be constant in height. The explanation in the paper has been clarified.
Reviewer:
Line 200: "The horizontal wind field is Fourier filtered at each altitude level ...", but then in lines 209210 "The fit shown in Fig. 6 (bottom left) yields a horizontal wavelength λH = 230 km and a propagation direction of α = 36.9◦ ." single, not altitude dependent values are obtained. I thought you would do the whole filtering and fitting at each altitude level and then come up with zdependent λH and α.
Authors:
The fitting at several altitude levels is only to confirm that lambda_H and alpha are approximately constant with altitude. This has to be assumed up to the EISCAT altitudes. Since the assumptions could be confirmed for the meteor radar altitudes, constant values are applied for the dispersion relation fit throughout all altitudes. This has been clarified in the text.
Reviewer:
Lines 226227: "We can rewrite this by introducing the wind velocity along the propagation direction of the wave U∥ = (kH · UH ) / kH ." With this definition, U∥ is a scalar, not a vector with a direction. It is a wind "speed". The subscript ∥ normally refers to the magnetic field direction, but here this is not meant. Rather it is a horizontal direction. The AGW has horizontal velocity and wave (or k) vectors. Both have different directions, and I think that you want to project the wind onto the wave vector direction. Please describe more precisely (also the equation).
Authors:
U_parallel is the neutral wind speed along the direction of the horizontal wave vector. We edited the manuscript to carefully distinguish between "velocity" and "speed" The explanation has been clarified.
Reviewer:
Lines 233235: "Equation 6 is solved for the optimum wind velocity U∥ applying a nonlinear leastsquares fit using a LevenbergMarquardt algorithm." Again I think that this is incomplete and a bit confusing. Equation 6 has no parameter U∥ that could be optimized. Please describe more comprehensively which expression is minimized with LM, which parameters are observed or are filtered observations, which parameters are from a model, and whether parameters are z dependent or assumed to be constant.
Authors:
Eq. 6 has been extended to show the parameter U_parallel and the explanation has been extended.
Reviewer:
Line 240: "... approximately 140 km altitude. Above that, the fit of the nonviscous dispersion relation no longer converges." but Figure 7 shows a curve for the fit of the nonviscous dispersion relation also above 140 km. The curve seems to be at exactly 0 m/s which probably indicates no convergence. It would be better to not plot the curve at altitudes where the LM did not converge.
Authors:
The plot has been modified according to your suggestion.
Reviewer:
Line 597: "... https://doi.org/110.1029/2017JD027970 ..." > "... https://doi.org/10.1029/2017JD027970 ..."
Authors:
Changed, thank you.
References:
Fritts and Alexander, 2003, https://doi.org/10.1029/2001RG000106
Vadas and Fritts, 2005, https://doi.org/10.1029/2004JD005574
Nicolls et al., 2014, https://doi.org/10.1002/2013JA018988
Vadas and Becker, 2018, https://doi.org/10.1029/2017JD027970

AC1: 'Reply on RC1', Florian Günzkofer, 02 Jun 2023
reply
Florian Günzkofer et al.
Florian Günzkofer et al.
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