Global sounding of F region irregularities by COSMIC during a geomagnetic storm

. We analyse reprocessed electron density proﬁles and total electron content (TEC) proﬁles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constel-lation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3). The TEC proﬁles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the proﬁles in the magnetic polar regions show small-scale ﬂuctuations on spatial scales < 50 km. Possibly the trajectory of the tangent point inter-sects spatial electron density irregularities in the magnetic polar region. For derivation of the morphology of the electron density and TEC ﬂuctuations, a 50 km high-pass ﬁlter is applied in the s domain, where s is the distance between a reference point (bottom tangent point) and the tangent point. For each proﬁle, the mean of the ﬂuctuations is calculated for tangent point altitudes between 400 and 500 km. At ﬁrst

ΔTEC is obtained in the same manner as ΔNe where we described ΔNe in detail in Figure 1.Now we added a sentence where we inform that ΔTEC is obtained in the same manner as ΔNe.ΔTEC is not obtained as difference between two occultations.It is the difference between the TEC profile and its smoothed (filtered) TEC profile (as in Figure 1).
It is also important to better describe how the high pass filtering in the s-domain was obtained.Additionally, the authors need to be clear with the meaning of the bottom tangent point.The bottom point is located at 400 km in Figure 1? And, in this case of Figure 1, the tangent point is the point located at 500 km?
We added a sentence that the height of the bottom tangent point is usually between 50 and 150 km.
In the results, it would be important to include distributions not only referred to the longitude but also to the local time.The RO observations do not cover worldwide for every local time.Therefore, sometimes the irregularities seen in a specific location of the maps is not seen in another part because of the different local times.In the way that it is now, each pixel of the global distributions is referred for a distinct instant.The manuscript lacks a proper analysis on this.Even better would be if the authors could plot the maps in terms of magnetic latitude vs local time.Then the authors would be capable of showing a fair global distribution of irregularities.
Yes, we agree, the dependence of ΔTEC on local time is important.The revised manuscript provides a figure for the local time dependence in September 2013 (new figure 4).We find an increase of F rgion irregularities in the post-sunset equatorial ionosphere.At high latitudes the irregularities seem to be independent on local time.
One last principal question that remains about the manuscript is: Does it is possible to detect irregularities with such a low spatial resolution of the global representations?The authors said it is possible to detect small-scale fluctuations with spatial scales < 50km with RO.However, the global representation of such information is obtained with a spatial resolution of 5 • x5 • or 10 • x10 • .As far as I understood, such maps just give a general information of the number of irregularities in each pixel, but does not describe the irregularities itself.Instead of median, a more informative representation would be the number of times that the gradients of TEC are above some limit (e.g.∆TEC>0.01 or another value to be defined in the manuscript with a proper reason).Even better would be the percentage of ∆TEC above the defined limit.Then you would have a global representation of irregularities.This because, as far as I understood, the blue up to ∼green values are not irregularities, so that, you are not showing maps of irregularities.The way it is now, the irregularities are depending on the spatial resolution of the maps (compare the colorbar of Fig. 4 and 6), which has not a true meaning.
We analysed the TEC profiles having a high vertical resolution of about 1 km.From the fluctuation profiles, we derived the average global distribution of F region irregularities based on several days or a month of observations.In the past we also tried to analyse TEC gradients and the results were similar.However we think that the analysis as described by Figure 1 is most easy to understand and thus we selected this method.
A few other points: a) In the abstract, COSMIC should read COSMIC/FORMOSAT-3.
Yes, we changed it.b) Section 2 -Include that UCAR has first processed the data level 1 and level 2.
Yes, we changed it.We started a new sentence with the finding of this study.f) pg. 4 -"In the following, we average the TEC disturbances over all local times".Did you used the mean (average) or the median?It is the mean.We added a new sentence.
g) It appears to me that the colorbar of the global Figures (such as Fig. 4) is truncated.
No, the color bar is not truncated.
Thank you for your review!Referee 2: Based on my reading of the manuscript, it was not so clear if the desired emphasis of the paper is: -a demonstration of the usefulness of the dataset and the analysis technique (?), or -a highlight of the geophysical phenomena consequential to the storm event (?), or -a broad overview of the expected geospatial distribution of ionospheric irregularities under various condition (?) I would suggest that the authors emphasize one particular aspect as a focal point, and the discussion of other aspects may revolve around it.I hope this re-organization of abstract/conclusion sections would not be too much to ask.
We agree.Now we emphasize at various places of the study (e.g., end of introduction) that the focus of our study is the global behaviour of F region irregularities during a geomagnetic storm.This is the new point of the study which was not covered by Watson and Pedatella (2018).We also reformulated the whole introduction section so that the intention of our study becomes clearer.
Furthermore, I would also like to suggest that extra labels are added to some of the figures in order to improve clarity.Figure 3a: add a label "Arithmetic Mean" on the top of the colormap plot Figure 3b: add a label "Median Function" on the top of the colormap plot Figure 4a: add a label "Solar Minimum" on the top of the colormap plot Figure 4b: add a label "Solar Maximum" on the top of the colormap plot Figure 6a: add a label "Quiet Geomagnetic Condition, h=400-500 km" on the top of the colormap plot Figure 6b: add a label "Geomagnetic Storm Condition, h=400-500 km" on the top of the colormap plot Figure 7a: add a label "Quiet Geomagnetic Condition, h=200-300 km" on the top of the colormap plot Figure 7b: add a label "Geomagnetic Storm Condition, h=200-300 km" on the top of the colormap plot Good idea!We added the suggested labels in the new figures.
Recently, global distributions of topside ionospheric irregularities (above the LEO orbit) were retrieved by using in situ data of LEO satellites (Zakharenkova and Astafyeva, 2015).Hocke et al. (2002) attempted to extract the fluctuations of total electron content along the GPS-LEO link at tangent point altitudes between 400 and 600 km.However, the global coverage of the occultation events of the early GPS/MET experiment was poor, and the measurement phase was during solar minimum in 1995 when less ionospheric irregularities are expected.Another approach is the use of the ground station network of GPS and GLONASS receivers.Cherniak and Zakharenkova (2017) monitored high-latitude ionospheric irregularities during the geomagnetic storm of June 2015 and derived polar maps of the distribution of the plasma irregularities based on the observations of the ground station network.Carter et al. (2013) analysed the scintillations of the GPS signals received by COSMIC-FORMOSAT-3.They found a spatio-temporal distribution of the GPS scintillations which is similar to those of equatorial spread F and equatorial plasma bubbles.However, high-latitude F region irregularities were not found by Carter et al. (2013).Watson and Pedatella (2018) derived characteristics of medium-scale F-region plasma irregularities as observed by the COSMIC radio occultation receivers.They analysed 2 to 50 km vertical fluctuations of the observed TEC profiles.The most intense equatorial irregularities are observed around 20:00-24:00 MLT, and correspond to a decrease in the average irregularity scale-size.
The study is based on reprocessed profiles of electron density (N e ) and total electron content (TEC) from the COSMIC mission.The TEC profiles describe the total electron content along the ray path from the GPS to the LEO satellite as function of the ray path.The small bending of the ray is neglected in the ionosphere.The analysed data are level1 data (podTec) and level2 data (ionPrf) ::::: which :::: were ::::::::: processed :: by ::: the ::::::::: University :::::::::: Corporation ::: for ::::::::::: Atmospheric :::::::: Research :::::::: (UCAR) :: in ::::::: Boulder :::::: (USA).::: The :::: data ::: are : provided in the directory cosmic2013 of the COSMIC Data Analysis and Archive Center (CDAAC).The applied retrieval technique of the N e profiles is the Abel inversion which assumes local spherical symmetry.The number of electron density profiles is about 1000 per day with a good global coverage.The altitude sampling rate is about 1 km, and the tangent point moves in average 180 km through the ionosphere at altitudes from 400 to 500 km during about 5 minutes.
Thus, the profiles are usually not measured above a fixed geographical location.This means that plasma fluctuations in the horizontal, vertical and temporal dimension may contribute to the small-scale fluctuations of an electron density profile or an TEC profile.We assume that the plasma is frozen so that we do not care about temporal fluctuations.Further, we do not try to distinguish between horizontal and vertical fluctuations.Instead, we consider the fluctuation in the s-domain where s is the distance between the bottom tangent point and the tangent point.::: The :::::: height :: of ::: the :::::: bottom :::::: tangent ::::: point :: is :::::: usually ::::::: between ::: 50 ::: and ::: 150 :::: km.: In addition, we interpolate the profile to an equally spaced s-grid with a spacing of 1 km.Generally, the tangent point approximately moves along a straight line trajectory in the F region.Hence, the small-scale fluctuations are plasma fluctuations which are projected to the trajectory line of the tangent point of the occultation event.The sounding volume at the tangent point is like a cigar ::::::: cylinder with a length of about 200km in direction of the GPS-LEO ray, and about 2 km across the ray and about 1 km in altitude.Thus, small-scale fluctuations in ray direction can be smoothed out, occasionally.
We extract the fluctuations in electron density and TEC by means of high pass filtering in the s-domain.In case of TEC, we have to compute the location of the ray tangent point (height, latitude and longitude) by using the coordinates of the GPS and the LEO satellite in the podTec file.The profiles N e (s) or TEC(s) are filtered with a digital non-recursive, finite impulse response (FIR) high pass filter performing zero-phase filtering by processing the profiles in forward and reverse directions.A cutoff scale length of 50 km was selected that means that oscillations in electron density with wavelengths less than 50 km are passing the filter.The number of filter coefficients corresponds to three times of a 50 km-interval, and a Hamming window has been selected for the filter.Thus, the high pass filter has a fast response time to vertical changes in the electron density profile.
More details about the digital filtering are given by Studer et al. (2012).
The left-hand-side panel of Figure 1  The ∆N e or ∆TEC-values of the selected profiles are binned into 5 • × 5 • latitude-longitude grid cells and are averaged by the median function in order to get the global distribution of electron density irregularities.

Results
First at all, we like to compare the global maps for ∆N e and ∆TEC.Figure 2a) shows the result of ∆N e , and Figure 2b) shows the result for ∆TEC both for September 2013 where the COSMIC satellites collected about 30'000 occultation events.
Both images have quite similar patterns with enhanced fluctuations in the magnetic polar regions.The coordinates of the geomagnetic and magnetic pole were provided by the World Data Center for Geomagnetism in Kyoto.Generally, the ∆TEC values are a bit enhanced compared to the ∆N e values.We suppose that the ∆TEC values are more reliable than the ∆N e values since the ∆N e values require the Abel inversion and the assumption of local spherical symmetry of the ionosphere.
Thus, we provide in the following only the results for the ∆TEC values.
Another question is the influence of the averaging method on the retrieved global map. Figure 3a) shows the ∆TEC map of September 2013 for the case that the arithmetic average is applied to the binned values in the grid cells.On the other hand, Figure 3b) shows the result if the median function is applied to the ∆TEC values.Generally, the arithmetic mean leads to higher values.Especially at the equator, there are some strong fluctuations which may result from the sporadic appearance of equatorial plasma bubbles in the F 2 -region.However, in case of monthly global maps we prefer the median-function since it reduces the effect of outliers in the data.In case of daily maps, we only have about 1000 occultation events for the globe, and here it seems to be better to apply the arithmetic mean.It is not good to apply the median function if only a few values are present.
Further the temporal evolution of the Kp index is shown in the bottom panel.For the data analysis, two days-intervals are indicated by the vertical lines for the quiet phase and the storm phase in Fig. 6 .During the storm phase, positive anomalies occur in BX and BY while a negative BZ anomaly is present.That means BZ is southward and the interplanetary magnetic field can reconnect with the magnetospheric field lines which results in a high geomagnetic activity during the storm.
Finally, we like to know how the geomagnetic storm acts on the lower ionosphere where we perform the same analysis but for TEC with ray tangent points from 200 to 300 km altitude.Figure 8 shows the result of ∆TEC for h=200-300 km, and it can be compared to Fig. 7 which showed the results for ray tangent points at h=400-500 km in the upper ionosphere.The number of analysed occultation events is 1176 during the quiet phase and 1603 during the storm phase.The number of occultation events is a bit smaller in Fig. 8 than in Fig. 7 since some occultations did not reach down to 100 km ray tangent point height c) pg. 3 -change the word cigar to cylinder.Yes, we changed it.d) pg. 4 -NASA should read National Aeronautics and Space Administration (NASA) Yes, we changed it.e) pg. 4 -Citation of Zakharenkova and Astafyeva (2015) is lost in the middle of the text.

Figure 1 .Figure 2
Figure 1.Example of a disturbed electron density profile from COSMIC (blue line in the left panel).The red line are the 50 km low pass filtered data.The filtering is applied in the s-domain where s is the distance between the bottom tangent point and the tangent point.The right panel shows the electron density fluctuations filtered with the 50 km high pass filter.The study is focused on the altitude region h=400-500 km (with exception of Fig. 8) .

Figure 3 .
Figure 3. a) Global map of f ∆TEC during September 2013 obtained by the arithmetic mean of the values in the binned cells.b) Global map of ∆TEC during September 2013 obtained by the median of the values in the binned cells.The geomagnetic (magnetic) poles are indicated by the magenta (cyan) star symbols respectively.

Figure 5 .
Figure 5. a) Global map of ∆TEC during September 2008 (solar minimum).b) Global map of ∆TEC during September 2013 (solar maximum).The geomagnetic (magnetic) poles are indicated by the magenta (cyan) star symbols respectively.

Figure 6 .
Figure 6.BX, BY and BZ of the interplanetary magnetic field (upper panels) and Kp index of the geomagnetic activity (bottom panel).Two days-intervals during the quiet phase before the geomagnetic storm of 15 July 2012 and during the storm phase are indicated by the vertical lines.

Figure 7 .
Figure 7. a) Global map of ∆TEC (tangent points at h=400-500 km) during the quiet phase, and b) during the storm phase of the geomagnetic storm of 15 July 2012.The arithmetic mean is applied to the values of the binned cells (10 • ×10 • in latitude and longitude).The geomagnetic (magnetic) poles are indicated by the magenta (cyan) star symbols respectively.

Figure 8 .
Figure 8. a) Global map of ∆TEC (tangent points at h=200-300 km) during the quiet phase, and b) during the storm phase of the geomagnetic storm of 15 July 2012.The arithmetic mean is applied to the values of the binned cells (10 • ×10 • in latitude and longitude).The geomagnetic (magnetic) poles are indicated by the magenta (cyan) star symbols respectively.