the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Nov 2023
Calibrating estimates of ionospheric long-term change
Abstract. Long-term change in the height of the ionospheric F2 layer, hmF2, is predicted to result from increased levels of tropospheric greenhouse gases. Sufficiently long sequences of ionospheric data exist to investigate this long-term change, recorded by a global network of ionosondes. However, direct measurements of ionospheric layer height with these instruments is not possible. As a result, most estimates of hmF2 rely on empirical formulae based on parameters routinely scaled from ionograms. Estimates of trends in hmF2 using these formulae show no global consensus. We present an analysis in which data from the Japanese ionosonde station at Kokubunji were used to estimate monthly median values of hmF2 using an empirical formula. These were then compared with direct measurements of the F2 layer height determined from Incoherent Scatter measurements made at the Shigaraki MU observatory, Japan. Our results reveal that the formula introduces diurnal, seasonal and long-term biases in the estimates of hmF2 of ≈ ±10 % (±25 km an altitude of 250 km). These can be explained by the presence of underlying F1 layer ionisation not accounted for in the formula. We demonstrate, that for Kokobunji, the ratio of F2/F1 peak electron concentrations is strongly controlled by changes in geomagnetic activity represented by the am index. Changes in thermospheric composition in response to geomagnetic activity have been shown to be highly localised. We conclude that localised changes in thermospheric composition modulate the F2/F1 peak ratio, leading to differences in hmF2 trends. We further conclude that the influence of thermospheric composition on the underlying ionosphere needs to be accounted for in these empirical formulae if they are to be applied to studies of long-term ionospheric change.
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CC1: 'Comment on egusphere-2023-2599', Claudia Borries, 01 Dec 2023
This manuscript written by C. Scott et al. addresses the limitations of using the height of the F2-layer maximum electron density (hmF2), which is a derived quantity from ionosonde data, for studying long-term changes in the ionosphere. This is an important topic because there have been already many publications using hmF2 for long-term studies and this work helps evaluating these results and using hmF2 more careful in future. From my point of view the manuscript is written excellently. It contains a very good overview of the state of the art at the beginning and the applied analysis and presentation of the results is adequate and well understandable. The authors detect a relation between the occurrence and strength of the F1 layer and the accuracy of the hmF2 layer (derived with one of the common approaches), which has not been described before. The results are discussed with respect to numerous related studies and the conclusions are logically derived from the results. The manuscript also contains some relevant results and discussions on the potential impact of climate change on the ionosphere. I evaluate the manuscript very good and I have some questions and remarks which may be considered before publication.
Questions and remarks:
- Section 3.5 provides corrections for the time delay in the ISR data. Why has this correction not been applied earlier in the study?
- line 482: Geomagnetic activity correlates to Joule heating driven by solar wind. How do the authors evaluate the potential that changes in thermosphere due to greenhouse effects may affect the magnitude of geomagnetic activity?
- The authors use the ratio of foF2 and F10.7 as a composition proxy and refer to Wright and Conkright (2001). Such a proxy sounds very favourable, but reading Wright and Conkright (2001) I cannot see a justification that the ration of foF2 and F10.7 is a proxy for thermosphere composition. Wright and Conkright (2001) worked with a sunrise extrapolation index SRCC, which is related indirectly to foF2. Wright and Conkright (2001) correlate the ratio log(SRCC/F10.7) with log(O/N2) and find a rather moderate correlation. The authors describe in their conclusion that they intended to provide a composition index, but the morphology of the proposed one differs from that of [O/N2]. If the ratio [foF2/10.7] is used by Scott et al. as a proxy for thermosphere composition, they need to provide better justification.
Minor issues
- line 75 “Data from a such …”
- lines 92 and 96 error in citing
- line 243 “… function of The Earth …”
- line 296 “In the ionosphere, While the …”
- line 371 “… in to the …”
- line 389 “… formulae tends introduces …”
- line 413 “(\pm 25 km at 250)”. Add “km altitude” after the 250
- line 416: Using am index is very reasonable. However, since it is not yet very popular to use, I recommend adding some justification, why this is used instead of the more frequently used kp/ ap indices.
- line 417: “… between the two, …” it is not immediately clear what are the two parameters that are correlated. Accordingly, it is not clear for the correlation values in lines 423 and 426, too.
- line 430: What is meant with “longitude sector near to the geomagnetic pole (\approx 48-50N)”? How can a longitude sector be close to the pole and why does it have latitude (north) coordinates?
- line 438: I get confused with the description. Chilton does not have a semiannual variation in foF2?
- line 438-439 “such as is seen”: Where is it seen? Is there a figure or paper?
- line 437: What means far enough away? Does it just need to be outside the auroral oval or even further away?
- line 440 “at these stations”: here the two station are addressed and in the second part of the sentence only Chilton. This is confusing.
Citation: https://doi.org/10.5194/egusphere-2023-2599-CC1 -
RC1: 'Comment on egusphere-2023-2599', Anonymous Referee #1, 24 Dec 2023
Review report of the manuscript entitled, “Calibrating estimates of ionospheric long-term change” authored by Christopher Scott, Matthew Wild, Luke Barnard, Bingkun Yu, Tatsuhiro Yokoyama, Michael Lockwood, Cathryn Mitchel, John Coxon, and Andrew Kavanagh.
Investigations on the ionospheric trends is an important area of research. Researchers have been using hmF2 and foF2 from ionograms to address this issue. The authors of this work dwell on the corrections for possible uncertainties that may be introduced in the use of these parameters for assessing the long term trends given the fact that there is ambiguity in the rates of changes in the hmF2 in different studies reported in the literature. Thus, the current manuscript attempt to investigate the efficacy of deriving long-term ionospheric trends in hmF2 using empirical formulae and to investigate the extent to which this can reconcile the difference in trends derived from the global network of ionospheric monitoring stations.
Thus, the authors suggest using foF2/foF1 ratio to qualify the values of hmF2 for a mid-latitude location. For that they use common data from the MU radar (from 1986-2020) and the ionosonde at Kokubunji (1957 – 2020) to arrive at the frequency rations of F2 to F1 regions to “calibrate” the hmF2 values from ionosonde.
This is an interesting effort which needs to be encouraged. There are, however, several loose ends, incomplete information, which need to be addressed before this article can be considered further.
- The formation of F1 layer has seasonal dependence. So, in order to provide an appropriate correction, which is more “accurate”, why not use the data from only that season? It will reduce the number of points, however, the values considered for such a season over 35 years may provide a different (better?) correction?
- Line 20: How much is the role of radiative cooling by CO2 in the thermospheric cooling?
- Line nos: 92 and 96, correct the citations.
- Is there any study that describe the relationship between stratospheric cooling with ionosphere? If it is there then provide the reference.
- The title of section 1.2 can be different as it does not match with the text.
- Line 126: define M3000F2.
- In equation 2, what is M?
- In eq. 5, define Mo.
- Line 236: Insert ‘and’ before ΔM.
- Equation 4 has been used for the estimation of hmF2. According to Bradley and Dudeney, (1973) this relation is not valid for the values of xE < 1.7. Then, how the estimation has been carried out? In addition, what about those ionograms when the signal of E-layer is insignificant even in daytime?
- Mention the R square values in Figure-2.
- Correct the caption of Figure 2, such as, write +/- at the place of pm, and change the symbol by < for the case of SZA values.
- In Fig. 1, there is a significant spread in the hmF2 values derived from ISR values which is not present in ionosonde derived values. Can author comment on this?
- Line 341 – 342: daytime is when SZA < 900. In these sentences it is mentioned incorrectly.
- 5 is incomplete. The text points to the ISR data as well (388 – 389), but it does not exist in the Figure. The figure caption too alludes to this. ISR hmF2 also to be plotted in this figure, similar to the format in Fig. 3.
- Lines 411-412: Explain how the percentage error is reconstructed using gradient and offset.
- Lines 438- 439: This statement may be substantiated by an appropriate reference.
- Lines 462 – 463: In the estimation of range bias, why inospheric profile is integrated only up to hmF2 as topside ionosphere also affects the radio wave propagation?
- Lines 467-468: can one carryout a simple subtraction like this?
- One can expect that the neutral wind changes with the magnitude of geomagnetic activity. Can author comment on this aspect?
- According to these results, the composition effect that varies with geomagnetic activities, should be taken care of for the long-term study. It is mentioned at the line number 119 that regression model is not significant alone to remove the effect of geomagnetic activities. But, in Fig. 8, am index shows clear proportional behaviour with composition proxy. Then, how do author think that correcting the compositional variations in the hmF2 variation will give better result? Could the authors compare their results with the study of Xu et al., (2004) to validate/assess their method?
=== End of review ====
Citation: https://doi.org/10.5194/egusphere-2023-2599-RC1 -
RC2: 'Comment on egusphere-2023-2599', Anonymous Referee #2, 20 Feb 2024
Scott et al provide a compelling case that previous efforts to discern climate-driven lowering of the ionosphere have been hampered by a reliance on semi-empirical equations which incorrectly assume any F1 layer present has an insignificant effect.
They show F1 layers can cause biases in height estimates which risk masking any climate trends, or causing misattributions.
They attribute this to thermospheric composition effects, driven by geomagnetic activity.The manuscript is of very high quality, and only needs a few revisions - should be minor in effort - to exclude some minor remaining doubts about potentially confounding effects, and make their case unarguable. As well as to make some of their reasoning a little more explicit, improving their work's accessibility to a wider audience. A more detailed breakdown of general and specific comments, and technical corrections is attached.
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