Reply to comment by R. Lukianova on ‘‘Unified PCN and PCS indices: Method of calculation, physical sense, dependence on the IMF azimuthal and northward components’’ Oleg Troshichev, 1 Alexander Janzhura, 1 and Peter Stauning 2 Received 29 August 2006; revised 14 January 2007; accepted 8 February 2007; published 7 July 2007. Citation: Troshichev, O., A. Janzhura, and P. Stauning (2007), Reply to comment by R. Lukianova on ‘‘Unified PCN and PCS indices: Method of calculation, physical sense, dependence on the IMF azimuthal and northward components,’’ J. Geophys. Res., 112, A07205, doi:10.1029/2006JA012029. 1. General Remarks [1] The comment by Renata Lukianova [Lukianova, 2007, hereinafter referred to as CRL] on the paper by Troshichev et al. [2006, hereinafter referred to as TJS] have three stated goals. The first is a delivery of missing datasets for a comparative analysis between ‘‘former’’ and ‘‘new’’ parameters. The second goal is a reminder of previous results [Lukianova et al., 2002, hereinafter referred to as LTL]. The third goal is caution against a simplification of the problem of PC response to the interplanetary ‘‘merging’’ electric field (MEF). To bring the unprepared reader up to date of the discussion, we shall start from general remarks concerning the PC index problems and then examine the particular points of the commentator. [2] As stated in TJS, the method of the PC index derivation was formulated about 20 years ago [Troshichev and Andrezen, 1985; Troshichev et al., 1988], and from that time the main principles of the method remain invariable. The PC index is by definition proportional to the intensity of the polar cap magnetic disturbance and calibrated with respect to the merging electric field (MEF = V SW B T sin 2 (q/2)) [Kan and Lee, 1979] penetrating into the magnetosphere from the solar wind. Hence the value of the PC index should be adequate to the merging electric field (MEF) measured in a suitable unit (mV/m). The procedure to derive PC index values include: (1) determination of the level of reference for estimation of the polar cap magnetic disturbance dF related to the MEF at the near-pole stations Thule and Vostok, (2) determination of the statistically most represen- tative (optimum) direction for the magnetic disturbances dF (angle F) perpendicular to the equivalent transpolar current, and (3) calculation of coefficients a (slope) and b (inter- cept), characterizing on a solid statistical basis a linear relationship between dF, projected to the optimum direction, and the MEF. With these parameters, which vary with season, UT, and hemisphere, the PC index is now calculated from actual values of dF as documented in TJS (i.e., PC = (dF proj À b)/a). [3] The calculation procedures, adopted in AARI for the PCS index and in DMI for the PCN index, turned out to be divergent in certain details, the determination of the quiet day reference level and the derivation of coefficients a (slope) and b (intercept) being the most important of them. The difference in the calculation procedures caused discrepancy of the PCN and PCS values, and that led, in turn, to questionable physical interpretation of the PC index series (see TJS for more details). The authors of the paper (TJS), who are responsible for production of the PCN and PCS indices in DMI and AARI, respectively, consented to develop a fully unified calculation technique for the PCN and PCS indices. The following concepts were adopted as principal guide lines while elaborating the unified tech- nique: (1) the PC index should, first of all, be equal (as closely as possible) to the merging electric field, and (2) the procedure of the PC index calculation should be workable in the online regime. In consequence of (1), values of the PCN and PCS indices would be similar under ordinary conditions. [4] The first concept deserves a special comment. Right from the start, the PC index was intended to characterize the merging electric field proceeding from ground measure- ments of the polar cap geomagnetic variations. These variations are generated by the ionospheric electric currents, which are determined, in turn, by the ionospheric conduc- tance and the electric field applied to the ionosphere. Under ordinary conditions the ionospheric conductance is regulated by the intensity of the solar UV light. Hence its daily and seasonal variations can easily be taken into account, except during extraordinary enhancements of the ionospheric conductance in the polar caps, for instance, in response to bombardment of the polar caps by energetic particles from solar flares. [5] The electric field in the near-pole region is determined by the cross-polar cap voltage, which under ordinary conditionsis mainly controlled by the interplanetary merg- ing electric field, but can be affected also by the solar wind dynamic pressure pulses and specific current systems related to magnetic substorms. Thus as stated in CRL, other factors than the MEF can influence the actual values of dF. [6] There are two possible approaches to the problem. The first is to examine each individual event with allowance JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, A07205, doi:10.1029/2006JA012029, 2007 1 Department of Geophysics, Arctic and Antarctic Research Institute, St. Petersburg, Russia. 2 Danish Meteorological Institute, Copenhagen, Denmark. Copyright 2007 by the American Geophysical Union. 0148-0227/07/2006JA012029 A07205 1 of 5