The Polar Cap (PC) indices were approved by the International Association
for Geomagnetism and Aeronomy (IAGA) in 2013 and made available at the web
portal

The Polar Cap (PC) indices, PCN (North) based on magnetic data from Qaanaaq (Thule) and PCS (South) based on Vostok data, reflect the transpolar convection of plasma and magnetic fields. They have important applications for space weather analyses and forecasting and have been used in many publications (e.g. Stauning, 2013a, and references therein). The PC indices could be used, among others, to indicate the energy transfer from the solar wind to the magnetosphere–ionosphere–thermosphere system (e.g. Troshichev et al., 2014).

PC index values are calculated from the basic formula (e.g. Troshichev et
al., 2006) by

The PC indices in the formulation suggested by the suppliers, the Arctic and
Antarctic Research Institute (AARI) in St. Petersburg, Russia, and DTU Space
in Lyngby, Denmark, were approved by the International Association for
Geomagnetism and Aeronomy (IAGA) by Resolution no. 3, 2013
(

An essential element of the calculation of PC index values is the derivation of the quiet reference level (QL), from which the disturbance amplitudes are counted. The QL derivation method described in Janzhura and Troshichev (2011, hereafter J&T2011) used for calculation of archival (final) index values was discussed in Stauning (2013b, 2015). This method uses data recorded approximately one month before and after the day in question to derive the relevant daily varying quiet level.

For calculation of real-time PC index values, the scaling parameters
(

The transitions between the various states of processing of PC index values
are not defined in the documentation supplied for the IAGA endorsement.
Here, the designation “archival” (or “final”) values shall be used on PC
indices retrieved one or more years after the index date assuming that the
magnetic data of importance and their processing have been finalized. The
term “prompt” values shall be used for downloads of one month's worth of
PC indices up to and including the actual “real-time” values. Within this
interval, the database for QL derivation is definitely changing. PC indices
from the two days of current data on display at

The quiet reference level for the horizontal magnetic field vector,

For illustration, Fig. 1 presents the

Qaanaaq (Thule)

The

Figure 1 conveys the impression that the recorded

A comprehensive description of the derivation of the QL reference level, from
which the disturbances are counted, is not available. The computer program
description (function pc_db in Appendix A, 2014) in the PC index
documentation supplied to IAGA shows that the solar wind sector term is added
as a specific contribution to the quiet level as well as taken into account
in the QDC derivation. Thus, in the IAGA-endorsed version, the quiet
reference level,

To provide an illustration of the quiet daily variation, Fig. 2 presents QDCs
made from hourly averages of the

These three curves in Fig. 2 represent the expected daily variation,

For the J&T2008 QDC procedure, the contributions from the IMF

This is illustrated in Fig. 2 by the solid magenta curve where, as an
example, the

The QL derivation for archival (post-event) data in the IAGA-endorsed
procedure is discussed in Stauning (2013b, 2015). The main problem for the
procedure is the incorrect assumption that the IMF

Calculation of

The derivation of the reference QL in real time poses further challenges. As indicated in Eq. (2), the QL (IAGA-endorsed version) comprises a QDC and an additional solar wind sector (SS) term. The QDC procedures described in J&T2008 comprise a real-time option, where the most recent completed QDCs are projected forward in time by using the seasonal trend obtained from stored QDCs derived at corresponding times in past year(s). It is not clear how to derive the proper trend if past QDCs are based on data corrected for the SS effects, which could not be taken to repeat a year later. This issue, left unresolved, is considered a minor problem compared to the derivation of the SS term in real time.

The J&T2011 publication describes the SS-related contribution to the
reference quiet level (cf. Eq. 2), from which the magnetic variations used in
the derivation of PC index values are counted. The SS term relies on median
values of the recorded data. The daily medians display large fluctuations
from day to day. In the post-event processing, these fluctuations are reduced
according to J&T2011 by smoothing the daily medians over 7 days centred at
the day in question. Such smoothing is not possible at real time applications
where, in addition to missing the median value for the present day (which may
not have ended), median values for 3 future days are lacking. The procedure
for deriving the real-time SS terms, as defined in J&T2011 (1496–1497),
is quoted below.

“Keeping in mind this specification, the 3-day smoothing averages of the median values were subjected to the interpolation procedure including the following steps:

median values for magnetic components

piecewise polynomial form of the cubic spline interpolant for

termination of this form related to day

The procedure is repeated each subsequent day. Results of the procedure, the
variation of the reconstructed magnetic

However, there must be
an error in the presentation by J&T2011 of the procedure and its results.
As will be shown, the smooth

In J&T2011 and in the following sections here, the median values are
presented by their deviations from the base level. Representative results are
displayed in Figs. 3 and 4. In these figures, the green curve using the left
scale reproduces the 3-day median values shown by the green curve in Fig. 6b of J&T2011. Calculations
of

For clarity and to store the results, the

Figure 4 presents the corresponding calculations to define the solar sector
term

In Fig. 4, the

The cubic spline construction operating on four points leaves no room for
smoothing. The

The magenta curve marked by dots in Fig. 2 presents the

From top: solar wind electric field (blue line, left scale) and IMF

PCS indices for 7 to 11 November 2014 from downloads on 11 November 2014 (red line) and 25 October 2017 (blue line). The prompt values shown by the red curve terminate in the real-time PCS value at the time of download. The displayed values are 5 min averages of the 1 min data.

In Eq. (2) the SS term,

It should be noted that the 3-day median values displayed in Fig. 6b of
J&T2011 are possibly smoothed by the authors. At true real-time
conditions, the smoothing is not possible for the most recent 3-day median
values. Hence, the potential fluctuations might generate still larger
excursions in the

Apparently, the real-time PC index values exist only at the time of their
presentation at

The last value of the PCS (prompt) data in the second panel from the top of
Fig. 5 is the real-time value at the time of the download (11 November 2014,
09:41 UT). Further data in this panel are “prompt” values that include the
“near-real-time” values. The average, rms, and peak differences between the
final and the prompt values for the span of data displayed in Fig. 5 are
noted in the bottom panel. It is seen that the prompt values deviate from the
final values by up to 3.67 mV m

Figure 6 holds a more detailed display through the days 7 to 11 November 2014 of the PCS prompt (near-real-time) values (from download 11 November 2014) in the red line and final values (from download 25 October 2017) in the blue line. Note in Fig. 6 that the differences between the final and the prompt PCS values vary between (mostly) positive values at local daytime (local MLT noon at Vostok is at around 13 UT) and negative values at night at twice the amplitude.

The differences between real-time and final values need not be that large. Figure 7 presents PCS values based on the same Vostok magnetic data as those used for the PCS indices displayed in Figs. 5 and 6, but processed according to the methods suggested in Stauning (2016). The quiet reference levels (QLs) for the prompt values from a simulated download on 11 November 2014 were calculated using the “solar rotation weighted” (SRW) QDC method (Stauning, 2011) on Vostok data extending up to the date and time of the download of the PCS values presented in Figs. 5 and 6.

With the SRW method, the QL is constructed by weighted superposition of quiet
samples for corresponding times of the day from an interval of

Prompt and final PCS index values based on Vostok data for the dates
and in the format of Fig. 5. The

For the case presented in Fig. 7, the maximum difference between prompt and
final values is just 0.43 mV m

The example of large differences between prompt (real-time) and final PC
index values presented here in Figs. 5 and 6 agree with the indications
presented in Figs. 3 and 4 of the possible effects (large excursions) of
using the real-time cubic spline extrapolation to derive the (daily) solar
wind sector (SS) term,

It should be noted that the example presented in Figs. 5 and 6 just represents one occasional download of PC indices including the real-time value at the time of the download. Further cases not recorded may display still larger deviations between the real-time index values supplied at download times and the final values downloaded at later times and considered to represent the best possible values. The cubic spline extrapolation method to estimate the solar wind sector term for the real-time QLs is vulnerable to the configuration of the four involved 3-day median values, as evident in Figs. 3a–d and 4, and probably also highly sensitive to irregularities in the supply of magnetic data.

The magnitude of the peak differences between prompt and final values found
here,

The present study provides the first reported validity analyses of the
IAGA-endorsed method used to generate the quasi-real-time PC index values
made available at the web portal

The inclusion of a solar sector term may change the reference quiet
level, particularly at local night, from the level determined from quiet
samples recorded during similar IMF

The observed excessive deviations between real-time and final PC index values agree with expectations based on using here the cubic spline procedure and the example data provided in Janzhura and Troshichev (2011) to determine the solar wind sector terms included in the reference quiet levels (QL) used in the IAGA-endorsed calculations of real-time PC index values.

In an example based on the download of PC index data on 11 November 2014,
differences between the real-time and later downloaded final PCS index values
were found to range up to 3.67 mV m

Results were presented from using different methods (Stauning, 2016) for
processing the Vostok data used in the example. Now, the deviations between
real-time and final PCS index values were below 0.44 mV m

Near-real-time PC index values and PCN and PCS index series
derived by the IAGA-endorsed procedure are available through the web site:

Geomagnetic data from Qaanaaq and Vostok were supplied from the INTERMAGNET
data service center at

Solar wind OMNI BSN data from combined ACE, WIND, IMP8, and Geotail
interplanetary satellite measurements were provided from the OMNIweb data
service at the Goddard Space Flight Center, NASA, at

The “DMI” PC index version is documented in the report SR-16-22 (Stauning,
2016) available at the DMI web site:

Appendix A (2014): The web site

IAGA PC_index_description_main_document.pdf (12 February 2014)

IAGA PC_index_description_appendix_A.pdf (27 January 2014)

IAGA PCS October–November 2014 prompt data: pcnpcs2014.zip (download 11 November 2014 09:41)

IAGA PCS October–November 2014 final data: pcnpcs2014.zip (download 25 October 2017 11:32)

DMI PCS October–November 2014 prompt data: PCS14C.5QP

DMI PCS October–November 2014 final data: PCSU2014.5MQ

The supplement related to this article is available online at:

The author declares that he has no conflict of interest.

The staff at the observatories in Qaanaaq and Vostok and their supporting
institutes are gratefully acknowledged for providing high-quality geomagnetic
data for this study. The excellent service at the OMNIweb data center
(