Comment on “Identification of the IMF sector structure in near-real time by ground magnetic data” by Janzhura and Troshichev (2011)

The only published description of the solar wind sector (SS) term used for the reference level in the post-event and real-time derivation of the Polar Cap (PC) indices, PCN (Polar Cap North) and PCS (Polar Cap South), in the version endorsed by the International Association for Geomagnetism and Aeronomy (IAGA) is found in the commented publication, Janzhura and Troshichev: Identification of the IMF sector structure in near-real time by ground magnetic data, Annales Geophysicae, 29, 1491–1500, 2011. Actually, the publication has served as a basis for the index endorsement by IAGA in 2013. However, neither the illustrations nor the results presented there have been derived by the specified near real-time method. Figures 1, 6, 7, and 8 display values derived by post-event calculations based on daily medians smoothed over 7 d centred on the day of interest. Figures 2, 3, and 4 display observed values smoothed over 7 d, while the remaining Fig. 5 displays averages over 4 months. In summary, there are strong disagreements between indications in the title, abstract, and statements in the text compared to the actual results and their illustrations.


Introduction
The derivation of the Polar Cap (PC) indices, PCN (Polar Cap North) based on Qaanaaq data and PCS (Polar Cap South) based on Vostok data, in the versions endorsed by the International Association for Geomagnetism and Aeronomy (IAGA) in Resolution no. 3 (2013) is to a large extent based on the methods described in Janzhura and Troshichev (2011): Identification of the IMF sector structure in near-real time by ground magnetic data (hereinafter J&T2011) (and its replicate in Janzhura, 2012, hereinafter T&J2012). This work provides the only published description of the solar wind sector (SS) term related to the Y component, IMF B Y , of the interplanetary magnetic field (IMF). The SS terms are derived from daily median values of the recorded magnetic field components and added to the index reference level in the post-event or near real-time versions. For post-event PC index calculations, the SS terms are derived from 7 d averaging of daily median values of the recorded magnetic data. For the near real-time calculations, the SS terms are derived from cubic spline-based forward extrapolation of past median values.
However, the method is invalid since it assumes that the IMF B Y -related effects originating at the dayside cusp region can be compensated for by using a daily median-based SS term at all local hours. Instead, the addition of this singular term to the index reference level generates unfounded positive or negative PC index contributions at different observatory positions along its daily path with respect to the polar cap ionosphere. The solar wind sector "compensation", typically, generates unfounded contributions to the PC indices at the nightside although the real IMF B Y effects on polar magnetic fields at the night side are usually very small. Correspondingly, the compensation might have little effect on PC indices at the dayside although the cusp-related IMF B Y effects maximize there.
An example case gave an unfounded change of 2.45 mV/m (magnetic storm level according to Troshichev and Sormakov, 2018) at local midnight and hardly any effect at noon at 4 nT amplitude in smoothed IMF B Y values, which is a common occurrence (Stauning, 2015).   Keeping in mind this specification, the 3 d smoothing averages of the median values were subjected to the interpolation procedure including the following steps: 2. piecewise polynomial form of the cubic spline interpolant for r1, r2, r3, and r4 segments is determined; 3. termination of this form related to day n = 0 is examined as representative of the SS effect for the current day, even if this day is disturbed.
The procedure is repeated each subsequent day. Results of the procedure -the variation of the reconstructed magnetic H component is presented by the magenta line in the same Fig. 6, the reconstructed H -component curve being shifted by 50 nT to a lower position.
Thus, it is stated (p. 1497) that this procedure was used to derive the smoothly varying display of the H -component SS term (magenta line) in their Fig. 6 using magnetic data from Qaanaaq (THL) for the interval 1-30 June 2001, here reproduced in Fig. 1 and recalculated in Fig. 2. However, the statement concluding the quoted procedure is incorrect. The SS term (H SS ) displayed in Fig. 6 of J&T2011 could not have been generated by the quoted near real-time procedure. The smoothed magenta curve for H SS is not a real-time version   Values of the solar wind sector term, H SS , derived by adhering rigorously to the above-quoted "near-real time" procedure (including the cubic spline-based forward projection) are displayed by the jagged curve in magenta line in Fig. 3.
The similarity between the H SS curves in Figs. 1 and 2 and the large difference with respect to the simulated realtime H SS values in Fig. 3 derived from cubic spline-based extrapolation of past median values implies that the display in Fig. 6 of J&T2011 (Fig. 4.15 of T&J2012), contrary to the statement in pp. 1496-1497, was actually generated by using post-event calculations with smoothed averages of 7 d daily median values.

Use of the solar wind sector (SS) term in reference level for PC indices
The IMF B Y -related solar wind sector effects on the convection patterns generate changes in the PC index response to the merging electric fields. The solar wind sector (SS) term was implemented in the derivation of PC index reference levels by J&T2011. The SS term from their Fig. 6 has been added to the quiet day variation (QDC) with slowly (seasonally) varying amplitude calculated by the method published in Janzhura and Troshichev (2008) to generate the June section (days 152-182) of the reference level displayed by the solid line superimposed on the 1 min H -component values displayed in their Fig. 1. The remaining part of the reference levels has no doubt been generated by the same method reproduced here in Fig. 4 (including caption) and recalculated in Fig. 5. However, using the near real-time H SS version gen-   Recalculated values are displayed in Fig. 8 where the postevent symbols (magenta diamonds) merge to form a continuous broad trace of H SS values. The scattered near real-time symbols (black diamonds) have been calculated by using rigorously the J&T2011 near real-time procedure quoted above.  Fig. 12a and b.
The similarity between the diagrams of Fig. 11 definitely constructed by post-event calculations (at DTU Space) and that of Fig. 10 indicates beyond doubt that the latter have been derived by post-event methods. From the post-event processing displayed in Figs. 10 and 11, the Dand Hcomponent solar wind sector terms appear highly correlated (r = 0.95, 0.97) with the daily mean IMF B Y values. Thus, they could be used to estimate past IMF B Y levels and signs with good probability from archived data. However, the objective according to the title and abstract of the paper was to estimate IMF B Y in near real time.
Compared to the D SS and H SS solar wind sector terms derived by post-event calculations displayed in Figs. 10 and 11, the corresponding solar wind sector terms generated by using real-time processing are much less well correlated with the daily average IMF B Y values (r = 0.67, 0.71). The relations displayed in Fig. 12 by the scattered D SS and H SS values derived by using near real-time methods could hardly be used to determine the actual IMF B Y magnitude level and sign with any certainty, which was the main scope of the J&T2011 publication.

Summary and conclusion
The commented paper, J&T2011, and its replica in Troshichev and Janzhura (2012) are significant since, along with the publications Troshichev et al. (2006) and Troshichev et al. (2011) held in chapter 4 of Troshichev and Janzhura (2012), they form the basis for the derivation procedures (Matzka, 2014;Nielsen and Willer, 2019)  In summary, there is strong disagreement between indications in the title, abstract, statements in the text, and captions, as well as in the presentation of results, compared to recalculations by rigorous use of the presented near real-time procedure. Thus, it is concluded to caution against uncritical use of the methods and results presented in the commented publication by Janzhura and Troshichev (2011).
Data availability. Geomagnetic data from Qaanaaq (THL) were supplied from the INTERMAGNET data service web portal at https://intermagnet.org/data-donnee/download-eng.php, last access: 10 April 2021. Solar wind plasma and magnetic field data based on data from the ACE, IMP, GEOTAIL, and WIND space missions were supplied from the OMNIWeb data service at http://omniweb.gsfc.nasa.gov/form/omni_min.html, last access: 10 April 2021. Interim values of solar wind sector, H SS and D SS , and quiet day, QDC, values from PCN calculations for 2001 were supplied by the index providers at DTU Space (https://doi.org/10.11581/DTU:00000057).
Competing interests. The author declares that there is no conflict of interest.