Mid-latitude neutral wind responses to  sub-auroral polarization streams

Billett, Daniel D.; McWilliams, Kathryn A.; Kerr, Robert B.; Makela, Jonathan J.; Chartier, Alex T.; Ruohoniemi, J. Michael; Kapali, Sudha; Migliozzi, Mike A.; Riccobono, Juanita

doi:https://doi.org/10.5194/angeo-40-571-2022

Articles | Volume 40, issue 5

https://doi.org/10.5194/angeo-40-571-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/angeo-40-571-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 40, issue 5

Regular paper

|

28 Sep 2022

Regular paper |

| 28 Sep 2022

Mid-latitude neutral wind responses to sub-auroral polarization streams

Daniel D. Billett, Kathryn A. McWilliams, Robert B. Kerr, Jonathan J. Makela, Alex T. Chartier, J. Michael Ruohoniemi, Sudha Kapali, Mike A. Migliozzi, and Juanita Riccobono

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Final revised paper (published on 28 Sep 2022)
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Preprint (discussion started on 06 Jul 2022)
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Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-593', Anonymous Referee #1, 07 Aug 2022

This paper conducted a case study of the mid-latitude thermospheric neutral wind response to a SAPS event using FPI and SuperDARN measurements. The authors found that different neutral wind responses can be attributed to the varying balance of pressure gradient, ion drag, and Coriolis forces. This paper is well-written with good logic, which is useful to advance the current understanding of the ion-neutral coupling effects of SAPS and is potentially suitable for publication in this journal. However, this reviewer has one concern about some of the results and explanations. Please see the comment below.

Figure 6b and line 265 show that “the correlation of the zonal disturbance wind is highest when there is no lag and steadily decreases up to a lag of 180 mins”. This “no-lag” phenomenon is quite surprising and needs to be carefully checked again. Even if the MH FPI is right within the SAPS channel with the ion drag being the dominant force, there should still be some time lag between the plasma drift and neutral wind response, as shown in previous observational and/or modeling results (e.g., Zhang et al., 2015; Aa et al., 2021; Ferdousi et al., 2019; Zou et al., 2022). Can you add similar panels in Figure 3 or somewhere else to show the actual temporal variation of the fitted zonal/meridional plasma drifts at VT and MH so that readers can make a comparison between plasma and neutral wind response? Did the enhanced westward plasma drift (SAPS) and zonal neutral wind reach their peak value at the same time between 04-06 UT with no delay? Whether the cross-correlation coefficient was calculated using neutral wind (FPI) or (FPI-HWM)? This might cause different results. Anyway, it is hard to understand the physical mechanism of “no lag” between plasma drift and neutral wind. More explanation is needed if it were proved true.

Citation: https://doi.org/10.5194/egusphere-2022-593-RC1
- AC2:
  'Reply on RC1', Daniel Billett, 22 Aug 2022
  Thank you for your review,
  We agree that "no lag" on neutral winds does not make sense physically, assuming that all other forces barring that directly caused by enhanced plasma (in this case, the SAPS) is removed. For Figure 6b, we believe that correlation is highest when the lag=0 due to a limitation of the cross-correlation technique itself. That shortcoming being that a large part of the neutral wind timeseries is being compared to a large part of the plasma velocity timeseries and only a single number (the correlation coefficient) is being produced. Therefore, peaks and troughs in the data are being considered as a whole, which in this case, happens to cause the correlation to be highest when lag=0.
  Below is the timeseries' used to perform the cross-correlation in Figure 6, which in a revised manuscript we will add to Figure 6. It is the zonal winds at Millstone Hill which gives high correlation with the fitted convection plasma at zero lag. However, these new panels show that the lag for the very first wind enhancement, beginning around 03:00UT for the Millstone zonal winds, is probably closer to 30 minutes.
  The cross-correlation looks at entire windows of data, so peaks and troughs at times other that the first wind enhancement don’t correlate as well as at lag=0. This could be for a few different reasons:
  There are uncertainties in the fitted plasma velocity timeseries. The velocity is set to zero at all times before ~03:00 because the convection fit does not expand into the latitude where the average (the box in Figure 5) was taken. This might not truly be the case, but the fit is being constrained by data elsewhere to a higher latitude.
  
  There is no time history in the convection fitting, causing a certain degree of jumpiness between sequential convection patterns. This is most prominent with the convection velocity spike at ~03:00, which may not be realistic. In reality, the convection velocity would evolve more smoothly.
  
  We assume that quiet time winds are fully removed to leave the disturbance winds using HWM14, but this is unlikely the case because it is only a climatology. Therefore, there is probably at least a small amount of forcing on the disturbance winds from non-SAPS related sources (e.g. solar heating pressure gradients, Coriolis, etc).
  
  In summary, in a revised manuscript we will add these new timeseries panels to Figure 6. We will also include additional discussion about the caveats surrounding cross-correlation and the meaning of “zero lag”, as described in our points above. We still use cross-correlation, as it is a good way to give an empirical lag value for the event as a whole, despite its shortcomings.
  
  Citation: https://doi.org/10.5194/egusphere-2022-593-AC2
RC2:
'Comment on egusphere-2022-593', Anonymous Referee #2, 08 Aug 2022
This study focuses on a Sub-Auroral Polarization Stream (SAPS) occurrence from November 9^th, 2013, and aims to understand how this event impacts the behaviour of mid-latitude thermospheric neutral winds. The authors utilise four mid-latitude SuperDARN radars to observe the SAPS flow and two Fabry-Perot Interferometers (FPIs) to study the response of the neutral winds. They find that the winds show different responses depending on where they were located compared to the SAPS flow. The authors suggest that the difference is due to a combination of the pressure gradient force, Coriolis force, and ion-drag force. This paper is well written, with a clear structure and results clearly shown and discussed. This paper is a good contribution to the field of Ionosphere-Thermosphere coupling. There are some technical corrections, which are identified below.

The surname of one of the co-authors is misspelled: Chariter should be Chartier.

Line 40: The digit ‘4’ is used here when referring to the SuperDARN radars used but in the rest of the paper ‘four’ has been spelled out.

Line 40: 'FPIs' is incorrectly written: ‘that the FPIFPIss both observed’

Throughout the paper the Wallops Island SuperDARN radar is referred to as ‘Wallops’ only, including in the relevant figures and their captions.

Line 99: ‘the “main phase” of geomagnetic’ change to ‘the “main phase of [a/the] geomagnetic’

(Optional) Line 103: When mentioning the plots in Figure 2 of the SuperDARN data, they can specifically be referred to as 2c-f.

Line 115: The units of this line do not need to be in italics: ‘where they reach approximately 30ms^-1’.

Line 150: When defining what SSUSI stands for, the first S stands for ‘Special’.

Line 151: The D in DMSP is spelled ‘Defense’.

Line 168: ‘exist’ not ‘exists’.

(Optional) Line 196: For consistency with the rest of the paper, 11 UT could be written as 11:00 UT. Line 201: 03 UT could be written as 03:00 UT

Line 205: ‘lower-latitude winds is may…’, the ‘is’ should be removed.

Line 270: ‘Polarization’ with a ‘z’
Citation: https://doi.org/10.5194/egusphere-2022-593-RC2
- AC1: 'Reply on RC2', Daniel Billett, 22 Aug 2022
  
  Thank you for your review.
  When submitting a revised manuscript, we will include all of your corrections and suggestions. Thank you for spotting them!
  
  Citation: https://doi.org/10.5194/egusphere-2022-593-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish as is (09 Sep 2022) by Steve Milan

AR by Daniel Billett on behalf of the Authors (09 Sep 2022) Author's response Manuscript

Short summary

Sub-auroral polarisation streams (SAPSs) are very fast plasma flows that occur at mid-latitudes, which can affect the atmosphere. In this paper, we use four ground-based radars to obtain a wide coverage of SAPSs that occurred over the USA, along with interferometer cameras in Virginia and Massachusetts to measure winds. The winds are strongly affected but in different ways, implying that the balance forces on the atmosphere is strongly dependent on proximity to the disturbance.