A critical note on the IAGA-endorsed Polar Cap (PC) indices: excessive excursions in the real-time index values

A critical note on the IAGA-endorsed Polar Cap indices

A critical note on the IAGA-endorsed Polar Cap (PC) indices: excessive excursions in the real-time index valuesA critical note on the IAGA-endorsed Polar Cap indicesPeter Stauning

Peter StauningPeter Stauning Peter Stauning

Danish Meteorological Institute, Copenhagen, Denmark

Danish Meteorological Institute, Copenhagen, Denmark

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 http://pcindex.org holding prompt (real-time) as well as
archival index values. The present note provides the first reported
examination of the validity of the IAGA-endorsed method to generate
real-time PC index values. It is demonstrated that features of the
derivation procedure defined by Janzhura and Troshichev (2011) may cause considerable excursions in the
real-time PC index values compared to the final index values. In examples
based on occasional downloads of index values, the differences between
real-time and final values of PC indices were found to exceed 3 mV m^{−1}, which
is a magnitude level that may indicate (or hide) strong magnetic storm
activity.

Keywords. Magnetospheric physics (solar wind–magnetosphere interactions; polar cap phenomena) – ionosphere (modelling and forecasting)

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).

Figure 1H-component data (20 min avg.) from Qaanaaq for days
135–235 of year 2001 in the blue line. Smoothed H-component median values in the red
line. Smoothed IMF B_{Y} values in the magenta line on the right scale.

where ΔF_{PROJ} is the horizontal polar magnetic variation vector
($\mathrm{\Delta}\mathit{F}=\mathit{F}-{\mathit{F}}_{\mathrm{QL}})$ counted from a reference
quiet level (F_{QL}) and projected to a specific optimum
direction in space considered to be perpendicular to the transpolar forward
(noon to midnight) plasma convection direction. The optimum direction is
defined through its angle, φ, with the E–W meridian, while the
slope, α, and the intercept, β, are calibration parameters.
The parameters (φ, α, and β) are tabulated for each
minute through the year (e.g. http://pcindex.org). They are
invariant over years.

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
(http://www.iaga-aiga.org/resolutions). In the resolution “IAGA
… recommends use of the PC index by the international scientific
community in its near-real time and definitive forms”. The IAGA approval was
given on the basis of documentation summarized in Appendix A (2014) referring
to the publications Troshichev et al. (2006), Janzhura and Troshichev (2008,
2011), and Troshichev and Janzhura (2012). Index values derived by the
IAGA-approved method are distributed from the PC index web site
http://pcindex.org and from the web portal
http://isgi.unistra.fr/data_download.php of the International Service
of Geomagnetic Indices (ISGI). A description of the PC indices and their
derivation is provided in Troshichev (2011).

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
(φ, α, and β) are those used for the final index
values. A special procedure was developed to derive the actual QL from past
(pre-event) magnetic data only (J&T2011). As post-event data become
available, the procedure includes the added data to recalculate QL and PC
index values, which are gradually turned into archival values. The available
documentation is rather sparse in the description of the procedure and does
not provide examples of real-time QL or PC index values. The real-time
procedure and examples from occasional downloads of actual PC index values
are discussed here in order to identify and quantify the problems with the
IAGA-endorsed methodology.

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 http://pcindex.org are
termed “near-real-time” values. These definitions shall be maintained in
posterior data processing attempting to emulate downloads of prompt PC
indices.

2 Quiet reference level and solar wind sector term

The quiet reference level for the horizontal magnetic field vector,
F_{QL}, (H_{QL}, D_{QL}) or (X_{QL}, Y_{QL}), could be
considered built from the secularly varying base level, F_{BL},
adding the daily variation, F_{QDC}, observed during quiet days
(quiet day curve, QDC).

For illustration, Fig. 1 presents the H component of magnetic data (blue
line) measured at Qaanaaq (Thule). The interval of data, days 135–235 of
year 2001, is much the same as that used in Fig. 1 of J&T2011. The base
level, H_{BL}, is marked by the solid horizontal line. The excursions from
the base level relate to the combination of quiet day variations to be
omitted and magnetic disturbances to be included at PC index calculations.
The varying transverse component, IMF B_{Y}, of the interplanetary
magnetic field (IMF) displayed in a smoothed version (the magenta line) at
the bottom of Fig. 1 appears to impose systematic variations on the recorded
level.

Figure 2Qaanaaq (Thule) H-component data averaged through the 10 quietest
hours of the summer months in 2000–2002 for groups of data with
−5 < IMF B_{Y}<−3 nT (green dotted line), −2 < IMF
B_{Y}<+2 nT (solid blue line), and +3 < IMF
B_{Y}<+5 nT (red dashed line), respectively. H_{QL,SS}
for 22 June 2001 in “final” (solid magenta line) and “real-time” (magenta
line with dots) versions (see text). Local solar and magnetic noon are at around 16 UT.

The H-component daily median values have been Gaussian smoothed through 7
days (e-folding period = 2 days). The resulting median values presented by
the wavy curve (the red line) in Fig. 1 are modulated much like the smoothed
IMF B_{Y} values, which have a dominant period at or close to the 27 days
solar rotation period. The systematic modulation of the IMF B_{Y} patterns
relates to shifts between sustained away and toward solar magnetic field
directions within solar wind sectors (e.g. Svalgaard, 1973). The effects on
the magnetic data are caused by IMF B_{Y}-related changes in the polar
plasma convection patterns particularly close to the dayside cusp region
located near noon in magnetic local time (MLT). The IMF B_{Y}-related
effects are enhanced by high ionospheric conductivities at local time (LT)
noon, in the summer season, and at solar maximum (e.g. Friis-Christensen et
al., 1985).

Figure 1 conveys the impression that the recorded H-component level varies
systematically, both day and night, by a slowly varying amount related to IMF
B_{Y}. In an attempt to compensate for such level changes,
Troshichev (2011) introduced a solar wind sector (SS) term, which for each
component of the recorded magnetic field is the difference between the long
term average (horizontal line in Fig. 1) and the daily median value with
adequate smoothing (superimposed wavy curve in Fig. 1). For the resulting
reference QL, it is stated in J&T2011 (p. 1499) that “this level of
reference can be derived if the SS effect is taken into account prior to the
QDC derivation”.

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, F_{QL,SS}, could be considered built from the
secularly varying base level, F_{BL}, the quiet daily
variation, F_{QDC,SS}, and a solar wind sector term,
F_{SS}, according to Eq. (2):

The IAGA-endorsed QDC procedure (function qday_db in Appendix A, 2014) is
described in Janzhura and Troshichev (2008, hereafter J&T2008). An initial
QDC for each component is calculated by superposition of samples of quiet
data collected through an interval of 30 days to determine the daily
variation for one day at a time. Depending on the distribution of quiet
samples, this day is usually positioned at the middle of the interval. A
series of initial QDCs is calculated by shifting the 30-day period by 10 days
at a time. The final QDCs for all days are now found by smoothing
interpolation through the series of initial QDCs

To provide an illustration of the quiet daily variation, Fig. 2 presents QDCs
made from hourly averages of the H component of the magnetic field measured
at Qaanaaq during the 10 quietest hours of each month of the summer periods
(May–August) of 2000–2002. The data are grouped according to the IMF
B_{Y} level. Data for the interval −5 < IMF B_{Y}<−3 nT
are represented by the green dotted curve, −2 < IMF B_{Y}<+2 nT by the solid blue curve, and
+3 < IMF B_{Y}<+5 nT by the dashed red curve.

Figure 3(a) Calculation of H_{SS} from 3-day median
values (green curve), based on data available up to and including 13 June
(from Fig. 6b of J&T2011), using cubic spline interpolation (black line)
on 3-day medians between 6 and 12 June (dots on green curve) and
extrapolation to define H_{SS}=91 nT on 14 June (large black dot).
Resulting H_{SS} values for June (so far) are displaced 60 nT
downward and connected by a dashed line using the lower right scale.
(b) cubic spline interpolation on 3-day median samples between 9
and 15 June to define H_{SS}=21 nT on 17 June. (c) cubic spline
interpolation on samples between 14 and 20 June to define
H_{SS}=112 nT on 22 June. (d) Cubic spline interpolation
between 19 and 25 June to define ${H}_{\mathrm{SS}}=-\mathrm{70}$ nT on 27 June.
Smoothed IMF B_{Y} values are displayed at the top of the
diagrams using the upper right
scale.

These three curves in Fig. 2 represent the expected daily variation,
H_{QDC}, with sustained IMF B_{Y} levels within the defined limits
and for the epoch considered. Local (LT and MLT) noon at Qaanaaq is at around
16 UT. The night H_{QDC} values (00–08 UT) are not changed much with differing IMF
B_{Y}, while the daytime H_{QDC} values (12–20 UT), and thus the
amplitude in the daily variation, change considerably with the varying IMF
B_{Y} level. For the three cases, the IMF B_{Z} conditions are about the same
with average values ranging between −0.1 and +0.1 nT.

For the J&T2008 QDC procedure, the contributions from the IMF
B_{Y}-related positive and negative level shifts over the 30-day interval
considered at a time tend to balance each other (cf. Fig. 1). For the
examined interval, days 135–235 of 2001, as an example, the average IMF
B_{Y} over any 30-day subsections range between −0.7 and 1.2 nT. Thus,
the quiet daily variation for the SS-corrected data, H_{QDC,SS}, is
most likely close to the result obtained for the case −2 < IMF
B_{Y}<+2 nT (the solid blue curve in Fig. 2). Consequently, the
sector effect on the QL is mainly provided by the addition of the slowly
varying term, H_{SS}, (cf. Eq. 2) to the daily course found at IMF
B_{Y}≈0. The addition will change the daytime and the nighttime
parts of the QLs by similar amounts without changing the amplitude in the
daily variation.

This is illustrated in Fig. 2 by the solid magenta curve where, as an
example, the H_{SS} term (64 nT) in the final version from
J&T2011 for day 173 (22 June) of 2001 has been added to the
H_{QDC} for IMF B_{Y}≈0 (blue solid line) to provide the
H_{QL,SS} variations for the day according to Eq. (2). On this day,
IMF B_{Y} (smoothed) ≈4 nT (see Fig. 1). The expected QL based on
quiet samples only is represented by
the H_{QDC} curve for 3 < IMF B_{Y}< 5 nT (red dashed
curve) in Fig. 2. It is seen that the H_{QL,SS} resulting from
Eq. (2) matches the expected H_{QDC} level at daytime, while large
differences appear at night causing PC index changes that may not be
justified by the actual magnetic variations. In order to ease comparisons of
real-time and post-event QL methods, the curve marked by dots (in magenta) in
Fig. 2 displays the H_{QL,SS} variations on 22 June obtained by
addition of the real-time H_{SS}=112 nT for this day (cf.
Fig. 3c) to the H_{QDC} for IMF B_{Y}≈0.

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 B_{Y}-related level
changes are the same day and night. As seen in Fig. 2 for the quiet samples,
in Fig. 5 of J&T2011 for all data samples, or by separate displays of the
daytime and nighttime data (Stauning, 2015), the IMF B_{Y}-related
effects on the H components are strong during daytime only. The addition of the
same term, H_{SS}, to daytime as well as nighttime QLs causes the
nighttime reference levels to step up or down with IMF B_{Y} instead of
remaining steady. The problem is aggravated by the still larger
H_{SS} amplitudes found by using the real-time QL version.

Figure 4Calculation of H_{SS}=21 nT for 30 June from 3-day
medians from 22, 24, 26, and 28 June. The 3-day median values (from Fig. 6b
of J&T2011) are shown in green line. The H_{SS} values from
Fig. 6b of J&T2011 are shown by the smooth solid magenta line on the scale
to the right, while the H_{SS} values calculated here by cubic spline
extrapolation are shown on the same scale by dots connected by the
dashed magenta line.

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 H and D are derived for 4 intervals of preceding days with the exception of the current day (n=0):

r1=F[for interval from n−3 day to n−1 day]

r2=F[for interval from n−5 day to n−3 day]

r3=F[for interval from n−7 day to n−5 day]

r4=F[for interval from n−9 day to n−7 day];

piecewise polynomial form of the cubic spline interpolant for r1, r2, r3, and r4 segments is determined;

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, are presented by the
magenta line in the same Fig. 6, the reconstructed H-component curve being
shifted by 50 nT to a lower position.”

However, there must be
an error in the presentation by J&T2011 of the procedure and its results.
As will be shown, the smooth H_{SS} variation represented by the
magenta line in their Fig. 6b (reproduced by the smooth magenta curve in
Fig. 5 here) could not have been derived by using the above real-time
procedure. In order to demonstrate the correct result, the 3-day median
values were read from Fig. 6b of J&T2011 (shown in Appendix A here) to
provide a series of values for June 2001. These values were processed
strictly according to steps 1–3 of the quoted procedure.

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 H_{SS} start here with the value on 9 June in order to have
enough prior 3-day median values available for the cubic spline construction.
Figure 3a presents an example of the cubic spline function (the black line)
defined from the four 3-day median values (shown by black dots) for days 6 (spanning days 5–7), 8
(7–9), 10 (9–11), and 12 (11–13) of June 2001. The extrapolation from 12
to 14 June defines the resulting H_{SS} value, marked by a large dot
at 91 nT on the left scale, for 14 June.

For clarity and to store the results, the H_{SS} values (including
the 14 June value) are subsequently shifted downward by 60 nT (shown by a
downward arrow) and displayed by dots connected by the dashed line using the
lower right scale. Figure 3b, c, d display corresponding constructions of
further H_{SS} values by similar cubic spline inter- and
extrapolations. All H_{SS} values have been derived from data in the
past relative to their own time as illustrated in the figures. For
information on the solar wind sector conditions, smoothed IMF B_{Y} values
are displayed at the top of the diagrams using the upper right scale.

Figure 4 presents the corresponding calculations to define the solar sector
term H_{SS}=21 nT for 30 June 2001 by cubic spline extrapolation
based on 3-day medians from 22 (21–23), 24 (23–25), 26 (25–27), and 28
(27–29) June. The diagram presents available data up to and including
29 June.

In Fig. 4, the H_{SS} values read from the magenta line in
Fig. 6b of J&T2011 are shown by the smooth solid magenta line on the scale
to the right. Their values range between −35 nT (5 June) and +64 nT
(22 June). These values could also be deduced from Fig. 1 by the differences
between the median values (red wavy line) and the base level (horizontal
line) on the appropriate days.

The cubic spline construction operating on four points leaves no room for
smoothing. The H_{SS} values calculated by following the procedure
defined in J&T2011 to the letter are shown (on the same scale as the
smoothed values) by the dots connected by the dashed magenta line. With their
large excursions, these values are not reproducing the smooth “reconstructed
H component presented by the magenta line” as claimed in the above
procedure quoted from J&T2011. The H_{SS} values calculated by
the real-time cubic spline extrapolation procedure range between −70 nT
(on 27 June) and 112 nT (on 22 June). The differences, ΔH_{SS}, between the real-time and the final H_{SS} values
range between −70 nT (on 27 June) and +72 nT (on 14 June).

The magenta curve marked by dots in Fig. 2 presents the
H_{QL} values for 22 June 2001 using H_{SS}=112 nT (cf.
Fig. 3c) determined by the real-time method quoted from J&T2011. This
curve aggravates the differences, seen particularly at night (00–08 UT),
between the H_{QL,SS} level derived by using H_{SS}=64 nT
(from Fig. 6 of J&T2011 on 22 June) and the H_{QL} (quiet) values
defined for the same IMF B_{Y} level (≈4 nT) and corresponding
seasonal conditions (summer, solar max), but from quiet samples only (red
dashed line in Fig. 2).

Figure 5From top: solar wind electric field (blue line, left scale) and IMF
B_{Y} component (red line, right scale), PCS (prompt), PCS (final), and (in
bottom panel) differences between final and prompt PCS values. Average, rms,
and peak differences are noted. The presented values are 5 min averages of
1 min data.

Figure 6PCS 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, F_{SS}, added to derive the quiet
level, from which the projected variations, ΔF_{PROJ}, are
counted, is a vector comprising the H component (shown in Figs. 1–4) as
well as the D component (not presented in J&T2011). With specification
of the quiet level defined in Eq. (2), the expression for the PC index in
Eq. (1) could be written as

The projection angle varies through 360^{∘} each day. Hence, the
projected term, F_{SS,PROJ}, equals the H_{SS} component in
magnitude two times a day (at night and in the day). Thus, the effect on the
PC index value can be derived for these two cases (whether real-time or final
values) through

The H_{SS} term is derived once a day. With typical daily variations
in the slope, α, the ΔPC values at night would be around
twice the corresponding values at daytime although the real SS effects are
much smaller at night than in daytime (see Fig. 2). Typical values for the
slope in June are α=32 nT (mV m^{−1})^{−1} at night and
α=65 nT (mV m^{−1})^{−1} in the day (see
http://pcindex.org). Accordingly, the changes in PC index values for
differences of 70 nT between the real-time and the final H_{SS}
values range between 1.1 mV m^{−1} in the day and 2.2 mV m^{−1} with
opposite sign at night. Corresponding deviations between real-time and final
values of D_{SS} could only increase (not reduce) the amplitudes in
the daily oscillations (like those seen in Figs. 5 and 6) of the differences
between real-time and final PC index values.

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 H_{SS} values and in the derived real-time QL and
PC index values than demonstrated here.

4 Recorded differences between the real-time and final PC index values

Apparently, the real-time PC index values exist only at the time of their
presentation at http://pcindex.org. It seems that they are not kept for
further analyses of their validity. Hence the only available examples are
those recorded at occasional downloads of PC index values. Figure 5 presents
an example based on the download of PC indices on 11 November 2014 at 09:41 UT.
The data define the prompt PCS index values extending up to the real-time
value provided at the time of the download. On the provision that the
magnetic data are not changed from their real-time values, the current
recalculations with added data turn the prompt PC index data into final
values in 1–2 months. The download of final values of PCS for 2014 used here
took place on 25 October 2017.

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^{−1} in this example.

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 ±40 days
from the day in question. In the superposition, the samples are weighted to
give preference to dates close to (and including) the day in question and to
dates with the same view of the sun in its 27-day rotation. In real-time
calculations of PC indices, the QL estimates use data recorded 40 days prior
to the date and time in question. In post-event calculations, the QL estimates
are gradually improved as samples from up to 40 days past the day in question
become available. In contrast to the IAGA-endorsed QL method, the SRW method
provides adequate QL values at night and preserves the IMF B_{Y}-related
differences in daytime QL amplitudes (cf. Fig. 2). For QL calculations in
real-time applications, the SRW method is inherently more robust to data gaps
and other irregularities in the stream of incoming magnetic data than the
cubic-spline-based extrapolation method.

Figure 7Prompt and final PCS index values based on Vostok data for the dates
and in the format of Fig. 5. The E_{M} and IMF B_{Y} data in the top
panel are the same as those presented in Fig. 5. The PCS prompt and final
indices have been processed from Vostok data by using the “DMI” methods
(Stauning, 2016).

For the case presented in Fig. 7, the maximum difference between prompt and
final values is just 0.43 mV m^{−1}. At the start of the interval
presented in Fig. 7, the date (15 October) is 27 days from the date
(11 November) of simulated download. Thus, just a small part (13/80) of the
total amount of samples from the usual ±40 days interval are missing at
the start and their weights are low. The derived QDC is now close to its
final value. As the date for PC index calculations approaches the simulated
download date, more samples ahead of the day in question are missing such
that the QDCs may differ more and more from their final values.

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, F_{SS}. In the IAGA-endorsed procedure
(Appendix A, 2014), the SS term is part of the quiet reference level,
(F_{QL,SS}), from which the magnetic variations included in
the PC index calculations are counted (cf. Eq. 2). F_{QL,SS},
furthermore, includes the quiet daily variation, F_{QDC,SS},
calculated around one month earlier and projected forward to the actual date
by using past year(s) seasonal trend (J&T2008). The validity of this
process is difficult to assess when operating on data corrected for the SS
term. The SS effects may not repeat in successive years. However, it is
assumed that possible differences between real-time and final
F_{QDC,SS} values are relatively small compared to variations
in the F_{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, ΔPC(max) = 3.67 mV m^{−1}, corresponds to the
anticipated PC index levels at strong substorms since PC index values above
2 mV m^{−1} usually indicate magnetic storm and substorm activity (e.g.
Troshichev et al., 2014). Thus, in a real-time monitoring application, the
distorted PC index values may indicate ongoing substorm activity during calm
conditions or indicate quiet conditions hiding an actual magnetic storm or
substorm event.

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 http://pcindex.org. It should be noted
that the presented and further similar cases are built on occasional
downloads of index values. Systematic recordings of the real-time index
values supplied from the PC web portal are not available to document the
differences between real-time and final PC index values on a comprehensive
statistical basis.

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 B_{Y} and seasonal conditions. The
aggravated effects by using the real-time procedure to estimate the solar
wind sector term by cubic spline forward projection of IMF B_{Y}-related
variations may cause substantial differences between PC index values
determined in real time and those calculated posterior.

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^{−1}, which is at an alarming level
considering that PC index values above 2 mV m^{−1} usually indicate
magnetic storm conditions. The example may not even represent the most
extreme cases.

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^{−1}. Elements
from this procedure, particularly the QL estimate, might be used in possible
future modifications of the IAGA-endorsed PC index calculation methods.

Near-real-time PC index values and PCN and PCS index series
derived by the IAGA-endorsed procedure are available through the web site:
http://pcindex.org. The web site, furthermore, holds PCN and PCS index
coefficients. QDC values are not included. The web site includes the document
“Polar Cap (PC) Index” (Troshichev, 2011). Prompt and final PCS data used
in the present paper are provided in the Supplement.

Geomagnetic data from Qaanaaq and Vostok were supplied from the INTERMAGNET
data service center at http://intermagnet.org.

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
http://omniweb.gsfc.nasa.gov

Appendix A (2014): The web site ftp://ftp.space.dtu.dk/WDC/indices/pcn/
includes documentation forwarded to IAGA for endorsement of the PC indices,
among others the documents:
PC_index_description_main_document.pdf and
PC_index_description_Appendix_A.pdf, and a directory,
PC_index_description_Appendix_A___file_archive, with
program transcripts and data files. The documents referred to in the present
work are available in the Supplement.

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
(http://omniweb.gsfc.nasa.gov) to provide processed solar wind
satellite data, the efficient provision of geomagnetic data from the
INTERMAGNET data center (http://intermagnet.org), and the efficient
performance of the PC index portal (http://pcindex.org) are greatly
appreciated. The author gratefully acknowledges the good collaboration and
many rewarding discussions in the past with Oleg A. Troshichev and
Alexander S. Janzhura at the Arctic and Antarctic Research Institute in St.
Petersburg, Russia. The topical editor,
Anna Milillo, thanks Alan Rodger for help in evaluating this paper.

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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. The PC indices could be used, among others, to indicate the energy transfer from the solar wind to the magnetosphere–ionosphere–thermosphere system in space weather monitoring applications. The present IAGA-endorsed methods to derive PC indices in real time are found to generate inconsistent index values.

The Polar Cap (PC) indices, PCN (North) based on magnetic data from Qaanaaq (Thule) and PCS...