the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2024
The impact of particle precipitation on the ion-neutral collision frequency analyzed with EISCAT measurements
Abstract. The ion-neutral collision frequency is a key parameter for the coupling of the neutral atmosphere and the ionosphere. Especially in the mesosphere lower-thermosphere (MLT), the collision frequency is crucial for multiple processes e.g. Joule heating, neutral dynamo effects, and momentum transport due to ion drag. Very few approaches exist to directly infer ion-neutral collision frequency measurements in that altitude range. We apply the recently demonstrated difference spectrum fitting method to obtain the ion-neutral collision frequency from dual-frequency measurements with the EISCAT incoherent scatter radars in Tromsø. A 60-hour-long EISCAT campaign was conducted in December 2022. Strong variations of nighttime ionization rates were observed with electron densities at 95 km altitude varying from Ne,95 ∼ 109 − 1011 m−3 which indicates varying levels of particle precipitation. A second EISCAT campaign was conducted on 16 May 2024 capturing a Solar Energetic Particle (SEP) event, exhibiting constantly increased ionization due to particle precipitation in the lower E region Ne,95 ≳ 5 · 1010 m−3. We demonstrate that the particle precipitation significantly impacts the ion-neutral collision frequency profile. Assuming a rigid-sphere particle model, we derive neutral density profiles and show that the particle precipitation heating causes a significant uplift of neutral gas between about 90–110 km altitude. We additionally test the sensitivity of the difference spectrum method to different a priori collision frequency profiles.
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RC1: 'Comment on egusphere-2024-2708', Anonymous Referee #1, 29 Sep 2024
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This study analyzed two datasets from collaborative measurements with the EISCAT UHF and VHF radars in Tromso, Norway, during moderately active conditions characterized by hard particle precipitation. By applying the difference spectrum fitting method developed in a previous study, the height profiles of the ion-neutral collision frequency in the MLT region were derived. The ion-neutral collision frequency is a critical physical parameter for understanding the ionosphere-thermosphere coupling process; however, its features have not been fully elucidated. Based on the derivation, this study presented the impacts of precipitating particle forcing on the ion-neutral collision frequency and discussed plausible causalities that produce variations in the collision frequency. However, the explanation of the calculation results and physical mechanism requires more rigorous and comprehensive analysis of data and consideration from multiple perspectives, as the current text is biased toward only certain aspects or interprets phenomena in an overly simplistic manner. These issues are summarized in the major comments. The accuracy of the calculations and discussion is significantly low, and the results of this study do not meet the standards for publication. Since the quality is unlikely to be improved by reanalysis, as mentioned in Major comment 1, the recommendation for the editor is to reject.
[Major comments]
1. Artificial discontinuity at 100 km altitude
In examining Figure 2b, a discontinuity in the collision frequency is observed at approximately 100 km altitude. This discrepancy is more evident in the line plot presented in Figure 3. The quick-look figure of the EISCAT measurements available in the Madrigal database clearly indicates a significant difference in the noise level of the UHF-measured electron density above and below 100 km. One of the conclusions of this study addresses the differential effects of thermospheric density variations at altitudes above 100 km. Based on the results presented in Figures 4b and 6b, this study corroborates that thermospheric density decreases and increases at altitudes below and above 100 km, respectively. However, the validity of this conclusion is subject to scrutiny given the apparent disparity in the quality of the EISCAT measurements, which constitute the primary data used to calculate the collision frequency.While the manda pulse code employed in radar measurements exhibits an advantage in measuring lower-altitude electron density with high range resolution, it demonstrates reduced accuracy in measuring ionospheric parameters in the E region and above compared to other pulse codes. The analysis presented in this study does not account for this characteristic (at least, no explanation is provided in the text).
As illustrated in Figure 3, the collision frequency below 95-100 km altitude exhibits a strong dependence on a priori. The authors assert that the altitude region where the influence of the a priori is significant extends up to 95 km, and they have hatched this region with gray shadows (Figures 4 and 6). Figure 3 demonstrates that the upper altitude limit is contingent upon the choice of a priori, and the a priori should be considered dominant up to 97-100 km altitude. The noise level of the EISCAT measurements is elevated for collision frequencies above 100 km altitude, as noted above. Given these considerations, the calculated results for any altitude range are deemed unreliable for this dataset, and there appears to be no justification for the discussion and conclusions based on these ambiguous results. Consequently, the results, arguments, and conclusions drawn in this study based on the observed data are considered unreliable. It is recommended to utilize EISCAT UHF and VHF simultaneous observation data employing another pulse code rather than manda.
2. Physical parameters to affect on the ion-neutral collision frequency
The collision frequency is proportional to the thermospheric and ionospheric-plasma densities, as demonstrated in Equation 1. However, this equation represents a simplified model and is also dependent on temperature (Prolss, Physics of the Earth's Space Environment, 2004). Given that the event was observed during periods of high geomagnetic activity, it is logical that the temperature of the thermosphere and ionosphere increased due to particle heating and Joule heating. Rather than restricting the analysis to attributing all collision-frequency variations to the density variations, it would have been more comprehensive to incorporate temperature effects in this study.The variation in thermospheric density is estimated from the increase or decrease in collision frequency (Figure 4b). As demonstrated in Equation 1, the collision frequency is a function of thermospheric density; however, it is also dependent on ionospheric plasma density. Lines 167-170 in the text briefly describe the derivation method, yet it remains unclear whether the thermospheric density was calculated considering the electron density measured by the EISCAT radar. If this factor had not been considered, it should have been incorporated into the calculations. If the collision frequency has been calculated taking this into account, the error in the atmospheric density should be determined by considering the error in the measured electron density and discussing its significance in relation to the magnitude of the thermospheric density variations.
3. Feature of the vertical motion in the lower thermosphere
It was previously mentioned that variations in collision frequency were not exclusively attributable to fluctuations in the thermospheric density. Even if density variation is presumed to be the primary factor governing the collision frequency variation, the explanation provided in the text would not align with the characteristics of vertical motion in the lower thermosphere. When examining the vertical motion of the lower thermosphere, it is essential to consider horizontal motion, particularly along isobars. In the lower thermosphere, where horizontal motion predominates, an upward displacement of the isobar in the heated region induces an apparent vertical component of the wind in geographic coordinates due to thermospheric winds flowing along the isobar. In instances of intense localized heating over brief periods, upwelling across the isobar may occur, but this phenomenon dissipates rapidly in conjunction with vertical oscillations. Upon cessation of heating, the isobar expansion terminates, and the apparent vertical component diminishes. However, during a transition process under force balance between buoyancy and gravity, the atmosphere undergoes oscillation (i.e., atmospheric gravity waves are generated), and vertical motion may persist for a duration. Nevertheless, this phenomenon does not result in an increase in the spatiotemporal mean density.Figures 4b and 6b indicate that the atmospheric density increased above 100 km altitude irrespective of the electron density level at 95 km altitude employed in this study. During periods of high geomagnetic activity, characterized by high electron density, the atmospheric density above 100 km altitude may increase. However, the intermittent increase in electron density over the two and a half days of December 13-15 suggests that it is improbable that the energy flow from the magnetosphere to the polar thermosphere/ionosphere is sustained at a sufficient level to support the density increase. Considering that particle heating should have occurred at the same location as the aurora, and given the likely structured nature of the aurora, it is implausible to assume constant upwelling in the EISCAT radar beam, although Major comments 1 and 2 elucidate the unreliability of the calculation results. Even if the calculation results capture some degree of nature, the physical interpretation of this study cannot be objectively substantiated.
[Minor comments]
L3: "momentum transport" should be revised to "momentum transfer."L13: "different a priori collision frequency profiles" may be better to say "various a priori collision frequency profiles."
L48-49: A magnetometer in Tromso, which can be checked at the IMAGE webpage, presents substorms during the high electron density periods selected in this study. The effects of Joule heating should be discussed, along with those of particle heating.
L60-61: According to the EISCAT QL from the Madrigal database, the transmitter powers of the UHF and VHF radars have not reached these numbers.
L66: "06-15 UT is analyzed" should be revised as "06-15 UT, is analyzed."
Section 5.1: The effects of the ambiguity of the beata parameter on the derived collision frequency should be evaluated in a quantitative manner.
L222-224: The explanation is incorrect, according to Figure 3.
Section 5.5: The meteor radar measures the horizontal wind, but what this study mentions is the density. To apply the meteor radar measurements, experimental evidence or theoretical support is required to link the wind and density in advance.
Citation: https://doi.org/10.5194/egusphere-2024-2708-RC1 -
AC1: 'Reply on RC1', Florian Günzkofer, 02 Oct 2024
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We thank the referee for taking the time to read and evaluate our paper. We agree that the presented analysis underlies considerable uncertainties which are, however, mostly discussed in the paper. We would be happy to extend the discussion to cover the points raised by the referee. As this paper presents a completely new approach to assessing the impact of particle precipitation in the lower thermosphere, we still think that the study is worth publication. Below, we address the major comments and some of the minor comments provided by the referee.
Major comments
1. EISCAT data quality
The low signal-to-noise ratio (SNR) of EISCAT measurements in the lower thermosphere is indeed a major concern of this study and partly responsible for the considerable uncertainties of the results. However, as the referee states, the low SNR and the consequent impact of the a priori profile are discussed in the paper. It should be noted, that the noise level that the referee sees in the madrigal quicklooks above 100 km is limited to times of low ionization. These are also the conditions under which the discontinuity is observed in our collision frequency profiles at around 100 km altitude. It is therefore likely that increased noise levels cause the discontinuity. This is equivalent to the explanation given in the paper where we attribute the discontinuity to an increased SNR level. However, this does not invalidate the results obtained under particle precipitation conditions which are the main focus of this paper. The supplement zip folder contains plots of the UHF spectra at 95 km, 100 km, and 105 km for average and high Ne,95 values during the December 2022 campaign. It can be seen that despite the considerable noise in the spectrum wings, the ion line peak can be identified at all altitudes. The spectra therefore appear to have sufficient quality to perform the difference spectrum analysis for average ionization conditions. Due to the large statistics of multiple hours of measurements, single time points with high-noise measurements at all altitudes do not disturb the analysis result significantly.
It should also be noted that the “difference in noise level above and below 100 km” that the referee noticed in the quicklooks is caused by changes in the settings of a priori parameters for the GUISDAP analysis of plasma parameters. For the UHF analysis, the plasma temperatures are taken from the a priori model below 100 km altitude though the exact altitude can be set differently in GUISDAP (for the VHF analysis the a priori temperatures are fixed even up to 120 km). However, for the difference spectrum method, the GUISDAP software is only applied to convert auto-correlation functions into incoherent scatter spectra. The a priori parameters and chosen settings do not affect the result of the difference spectrum analysis.
The manda pulse code is, according to the EISCAT experiment documentation, designed to perform measurements in the lower thermosphere and would therefore be ideal for the presented analysis. We agree that the high altitude resolution of manda measurements and the consequently lower signal quality makes this mode presumably less suitable for difference spectrum measurements compared to other modes. Since this was not tested before, the presented measurements do have scientific value and this can be added to the conclusions of the paper following the referee's line of argument.
2. Parameters that impact the collision frequency
The referee is correct in pointing out that other parameters than the neutral density can potentially impact the collision frequency. However, as described by (Ieda, A. (2020) “Ion‐neutral collision frequencies for calculating ionospheric conductivity”), only the resonant ion-neutral collisions are temperature dependent. Non-resonant collisions are not temperature-dependent and dominate below 600 K (which is usually the case at the investigated altitudes, see Figure of EISCAT UHF ion temperature on May 16, 2024 in the supplement). However, we agree that the T<600 K condition does not trivially hold during intensified particle precipitation. We thank the referee for pointing this out to us. We will incorporate a quantitative analysis/discussion of the influence of resonant O2 + - O2 collisions on the calculated neutral densities in our revision.
The plasma density impact on the ion-neutral collision frequency is apparent from Equation 1. However, it is clearly stated that since the plasma density is by far lower than the neutral density, it does not significantly impact the collision frequency and has therefore been neglected. A brief quantitative discussion of this can be added to the manuscript.
In general, ion-neutral collisions can be described in multiple ways, e.g. as rigid-sphere collisions (Chapman (1956) “The Electrical Conductivity of the Ionosphere: a Review.”) which results in Equation 1, or as non-resonant Maxwell collisions (Schunk and Walker (1971) “Transport Processes in the E region of the Ionosphere”). As shown in our previous publication on the difference spectrum method (Günzkofer et al., 2023), these two approaches result in very similar neutral density profiles. (Thomas et al. (2024) “D-region ion-neutral collision frequency observed by incoherent scatter spectral width combined with LIDAR measurements”) in turn showed that the non-resonant collision frequency according to Ieda (2020) is nearly equivalent to the Schunk and Walker (1971) equation. As stated above, other parameters become significant for resonant ion-neutral collision frequencies (dominant at T>600 K).
We are aware that we apply a simplified equation to describe the collision frequency and agree with the referee that this should be discussed in the manuscript in more detail. However, the applied collision frequency approach is in line with common literature in this field. We are happy to extend the discussion regarding the collision frequencies to address the referee’s concerns within our revision.
3. Vertical motion in the lower thermosphere
The referee states that localized heating can cause upwelling across the isobars (increasing thermosphere density) which diminishes shortly after the heating ceases. Therefore, the heating does not cause a general, persistent increase in the thermospheric density. We agree with the referee and see this as the main conclusion of our paper.
“Figures 4b and 6b indicate that the atmospheric density increased above 100 km altitude irrespective of the electron density level at 95 km altitude employed in this study.”
We would argue that the opposite is the case. Figures 4b and 6b show that the atmospheric density above 100 km altitude is increased for Ne,95 > 2*1010 (7*1010) m-3 compared to Ne,95 <1010 m-3.
“During periods of high geomagnetic activity, characterized by high electron density, the atmospheric density above 100 km altitude may increase. However, the intermittent increase in electron density over the two and a half days of December 13-15 suggests that it is improbable that the energy flow from the magnetosphere to the polar thermosphere/ionosphere is sustained at a sufficient level to support the density increase.”
As argued in the manuscript, the majority of Ne,95 > 2*1010 m-3 conditions during December 13-15 2022 occur during the night from Dec 14 to 15 where the particle precipitation heating sustains over a longer time interval. As shown in Figure 4 a, the increase of atmospheric density is restricted to these conditions and significantly lower densities are found for non-heating conditions. This is equivalent to the referee's statement that the upwelling of the atmosphere ceases quickly after the heating stops.
“…, it is implausible to assume constant upwelling in the EISCAT radar beam”
We agree with this statement and do not see how our results would indicate a constant upwelling of the atmosphere. Our results explicitly show that the upwelling is only found for strong heating conditions, quantified by the electron density at 95 km altitude. The duration of the auroral precipitation events presented in (Grandin et al. (2024) “Statistical comparison of electron precipitation during auroral breakups occurring either near the open-closed field line boundary or in the central part of the auroral oval”) of roughly 20 min is within the range of the required atmosphere reaction time discussed in Section 5.7 of our paper.
Minor comments
The substorm conditions mentioned are also visible from the SME data shown in Figure 1. We agree with the referee that the Joule heating impact during this time should be discussed. However, as shown in (Baloukidis et al. (2023), “A Comparative Assessment of the Distribution of Joule Heating in Altitude as Estimated in TIE-GCM and EISCAT Over One Solar Cycle”) and (Günzkofer et al. (2024) “Evaluation of the Empirical Scaling Factor of Joule Heating Rates in TIE‐GCM With EISCAT Measurements”), the maximum of Joule heating occurs at 120-130 km altitude. Below 110 km altitude, the Joule heating only reaches significant values for Kp>4 conditions which needs to be considered in the presented case. We thank the referee for pointing this out and would be happy to add a discussion on this to the paper within our revision.
The meteor radar measurements were applied to assess the tidal activity during the time of the December 2022. Since the tidal activity was considerably low in the horizontal wind, we followed that tidal waves do not affect the neutral density either. However, we would be happy to look into neutral density variations following (Stober et al. (2011) “Neutral air density variations during strong planetary wave activity in the mesopause region derived from meteor radar observations”).
Regarding the remaining minor comments, we are happy to adjust the manuscript according to the referee’s suggestions.
In summary, we would like to thank the referee again for the thorough review and for bringing up several points that indeed require a more detailed discussion in the manuscript. However, we strongly disagree with the statement that the results are completely unreliable. Though the results underly considerable uncertainties, they still provide relevant qualitative insight into the processes of the lower thermosphere. Given the low number of direct measurements of neutral atmosphere processes at these altitudes, the presented results are a relevant increment. The mentioned uncertainties of the results are (mostly) dutifully reported and discussed in the manuscript. As mentioned before, we are happy to address all of the referee’s points with a thorough revision.
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AC1: 'Reply on RC1', Florian Günzkofer, 02 Oct 2024
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