Inferring neutral winds in the ionospheric transition region from AGW-TID observations with the EISCAT VHF radar and the Nordic Meteor Radar Cluster
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 multi-static 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 egusphere-2023-678', Stephan C. Buchert, 12 May 2023
- AC1: 'Reply on RC1', Florian Günzkofer, 02 Jun 2023 reply
Florian Günzkofer et al.
Florian Günzkofer et al.
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Discussion/Review of egusphere-2023-678
"Inferring neutral winds in the ionospheric transition region from AGW-TID 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 AGW-TID 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.
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 45-55. I found these papers
Boyde et al. (2022), Lensing from small-scale travelling ionospheric disturbances observed using LOFAR, https://www.swsc-journal.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.swsc-journal.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 15-16: "In the ionosphere, AGWs can be observed as Medium-Scale Traveling Ionospheric Disturbances (MS-TIDs) from neutral-ion 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 ion-neutral collisions, AGWs can be observed as Medium-Scale Traveling Ionospheric Disturbances (MS-TIDs) (Nicolls et al., 2014)."
Lines 135-147: 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 2-D Fourier (inverse) transform and the filter.
Lines 172-173: "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 Brunt-Väisälä period be appropriate at this point. I think that the Brunt-Väisälä 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 grid-cell 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 193-194: "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 Brunt-Väisälä, 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 209-210 "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 z-dependent λH and α.
Lines 226-227: "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 233-235: "Equation 6 is solved for the optimum wind velocity U∥ applying a nonlinear least-squares fit using a Levenberg-Marquardt 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 non-viscous dispersion relation no longer converges." but Figure 7 shows a curve for the fit of the non-viscous 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 ..."