Port Stanley and Ascension Island magnetic observatories have been in continuous operation since the early 1990's. These remote South Atlantic locations provide much needed coverage in the global network of geomagnetic observatories and help to monitor the South Atlantic Anomaly. To enhance the production of longitude-sector planetary magnetic activity indices there is a requirement for local 3-hourly K-index values from Port Stanley (PST) observatory. We describe the process followed to establish an automated routine for the derivation of the indices and we assess the congruence of the indices to those available from other suitably located observatories. A similar procedure has been followed for Ascension Island observatory although this is not shown here

Clarke, Ellen

Hamilton, Brian

Reay, Sarah

English

NERC Open Research Archive

The 3-hourly index is a range index denoted by a single digit code from 0 to 9. The
code corresponds to the class in which the greater of the two Horizontal component
ranges falls.
Before calculating the range in any 3-hour period it is first necessary to remove the
regular daily variation Sr. The algorithm used as the basis for determining this at the
BGS UK observatories (Clark, 1992) is the IAGA sanctioned Nowozynski et al (1991)
adaptive smoothing method. For BGS overseas observatories we adopt this same
algorithm. Daily Sr (denoted by ) is found by minimising the expression:
where denotes the minute mean values of or The first term represents the fit of
to and the second term represents the curvature of the estimated Sr variation. The
relative importance of the two terms is altered by changing the values of .
The implementation of the algorithm for PST is as determined by Clark (1992). First a
preliminary Sr curve is fitted using . If, in any 3-hour period, the preliminary is
greater than a threshold, is decreased during a second run ( ). Thus the estimated
Sr will not follow large irregular disturbances. Also, if the preliminary exceeds a
second threshold, the Sr curve is ignored and only the range (max-min method) is
used to compute the final . Clark observed that when comparing computed to
hand-scaled the algorithm required further adjustment to reduce bias in season and
UT. Hence weighting factors for these were also introduced for the UK observatories.
K
y
x H D.
y x
K
K
K K
K
i
i
λ
λ
λ λ
2
2
2 2
u
XIVth IAGA Workshop on Geomagnetic Observatory
Instruments, Data Acquisition and Processing
September 13 - 23, 2010 Changchun, China
New -Indices From South Atlantic Observatories:
Port Stanley And Ascension Island
K
Sarah Reay Ellen Clarke and Brian Hamilton(sjr@bgs.ac.uk),
British Geological Survey, West Mains Road, Edinburgh EH9 3LA, United Kingdom
Introduction
Acknowledgments
References
We would like to thank Dr. Michel Menvielle of ISGI for his help and advice. We also thank the operators of Hermanus, Bay
St.Louis, Tuscon, Canberra, Gnangara, Trelew and Argentine Islands observatories for providing the -indices used in this
analysis.
K
Bartels J, Heck  N.H and Johnston H.F (1939) The three-hour range index measuring geomagnetic activity, J. Geophys Res.,
44:411
Clark TDG (1992) Computer generated indices adopted by the British Geological Survey, J.Atmos.Terr.Phys, 54:447-456
Nowo y ski, K, Ernst, T, Jankowski J (1991) Adaptive Smoothing Method For Computer Derivation of -Indices Geophys.J.Int.
104:85-93
K
Kż ń
Analysis of -indicesK
Port Stanley and Ascension Island magnetic observatories have been in continuous
operation since the early 1990's. These remote South Atlantic locations provide much
needed coverage in the global network of geomagnetic observatories and help to
monitor the South Atlantic Anomaly. To enhance the production of longitude-sector
planetary magnetic activity indices there is a requirement for local 3-hourly -index
values from Port Stanley (PST) observatory. We describe the process followed to
establish an automated routine for the derivation of the indices and we assess the
congruence of the indices to those available from other suitably located observatories.
A similar procedure has been followed for Ascension Island observatory although this
is not shown here.
K
To assess if the parameters selected are
appropriate, and in the absence of any
hand-scaled indices, we have compared
PST to other observatories. We used
observatories of similar geomagnetic
latitude to test if the distribution of PST is
as expected and those of a similar time
zone to assess the UT and seasonal
weightings.
K
K
Determining : Adaptive Fitting of Srλ
2
Method
Determining an ‘ideal’ Sr curve
Analysis: Similar Time Zone
For PST we had no hand-scaled -indices to help
judge the correct and weights to use in
determining . We explored the idea of producing
an ‘ideal’ Sr curve to help compare with the Sr
produced by the algorithm.
The mean daily variation curves were derived from
all available Port Stanley hourly mean data over one
solar cycle (1998-2009).  Data from the five
international quietest days in each month were
individually de-trended and mean-subtracted. It was
then grouped by southern hemisphere seasons: summer (Jan, Feb, Nov, Dec),
equinoctial (Mar, Apr, Sep, Oct) and winter (May, Jun, Aug, Sep). These data were
then used to derive a Fourier model for each component and season with 24, 12, 8,
and 6 hour coefficients.
K
K
λ
2
Figure 1: ‘Ideal’ Sr curve for PST Horizontal component
( ) for equinoctial months (Mar, Apr, Sep, Oct)H
We first produced for a variety of with no weighting. It was clear that the 'ideal'
curve was not helpful for selecting the best as there was too much variation day-to-
day in Sr. A trial and error approach was therefore used.
We examined many quiet-day magnetograms in months and selected first
and then . Then we
examined the computed Sr and corresponding
during daytime hours and judged the appropriate
weights for each 3-hour period. The seasonal
weights were found in a similar way.
Thresholds of K>4 and K>5 were investigated for
switching to the max-min method. Optimum
parameters were selected for further analysis.
K
K
λ
λ
λ λ λ
2
2
2 2 2
equinoctial
The threshold for was set at > 0 following Clark (1992).u u K
Figure 2: Example magnetogram for a quiet day showing Sr
curve (dotted line) fit to the D and H data from Port Stanley.
Analysis: Similar Geomagnetic Latitude
Table2: List of the observatories used in this analysis. We chose those
which were similar in geomagnetic latitude or time zone.
*Corrected geomagnetic coordinates calculated from IGRF for 2010.0
Conclusions and Future Work
Figure 3: Distribution of K
values for a
geomagnetically active
(2003) and quiet (2008)
year for a number of
observatories at similar
geomagnetic latitude. Port
Stanley is in .black
Automatic derivation of PST indices is now possible and the values produced are a
reasonable match to the published indices from the two nearest observatories. In
addition, the occurrence distribution of the PST indices is comparable to those
observed at other sites where geomagnetic activity levels are likely to be similar.
This is work in progress with further adjustments to the algorithm parameters required.
In particular it should be possible to improve:
The distribution curves at =0,1,2  by modification of and
The distribution curves at =3,4,5 by modification of the max-min threshold
The UT weights by further analysis with observatories in the same time zone
Once we have determined the final parameters, the indices will be computed from
1994 (PST) and from 1992 (ASC) and made available on-line.Currently the algorithm
requires data over a full UT day without any gaps. Further improvements are planned
to adapt it to work with data in real time using the previous 24 hours and to enable it to
handle short gaps in the data.
K
K
K
K
K
=
=
=
λ λ
2 2
u
Figure 4: Comparison of difference between nearby observatories (PST-AIA and PST-TRW) for 2006.
The histograms show the % occurrence that the two sets of indices agree and % occurrence that they
differ by -2, -1, +1, +2 index values. The top panel shows the comparison for the whole year, the middle
panel is as a function of the eight 3-hour segments and the bottom is as a function of season.
Figure 5: Distribution of K-indices by 3-hour time
segment for UT-similar observatories for 2004-2007.
The top chart show poorer correlation during day-
time (15-18 UT). The bottom chart shows better
correlation during night-time (21-24UT).Direct comparison of individual indices with those from
TRW and AIA is useful in determining if there are any
unexpected biases in the PST indices. Overall the fit to both appears to be reasonably
good. However, the PST indices are more often less than those at TRW, which is not
expected, since TRW is located to the north of PST. This bias is more pronounced for
<4, indicating a possible over-fitting of the Sr curve. Overall there is no bias between
the AIA and PST indices; however there is a clear bias during UT periods 15-18 and
18-21. This is
urther investigation is required.
K
in the opposite sense expected, since AIA is to the south of PST, so
f
Looking at the distribution of PST to the other selected observatories it appears that
=0,2,4 are too low and =1,3 are too high. In particular the distribution of =1, =2
needs improvement. However it is worth noting that there is generally an increased
variation between the observatories at <4.
K K K K
K
Latitude Longitude Latitude Longitude
Port Stanley PST -51.70 302.11 -38.78 10.43 328 -4
Tuscon TUC 32.17 249.27 39.67 315.37 350 -7
Bay St.Louis BSL 30.35 270.36 40.91 341.38 350 -6
Hermanus HER -34.42 19.23 -42.45 83.23 300 -23
Gnangara GNA -31.80 116.00 -43.88 187.51 450 -16
Canberra CNB -35.32 149.36 -45.36 227.13 450 -14
Trelew TRW -43.24 294.68 -30.24 4.80 286 -4
Argentine Islands AIA -65.24 295.74 -50.57 9.13 500 -4
Hartland HAD 51.00 355.52 57.89 80.24 500 0
Name Corrected Geomagnetic Coords* Time
zone UT
Lower K9-
Limit (nT)
Geographic CoordsCode
0 1 2 3 4 5 6 7 8 9
0 3 7 13 26 46 79 131 216 328
Table1: The lower limits for each PST class. The
=9 lower limit (now supplied by ISGI) is derived
from the angular distance between the observatory
and the auroral oval and the rest are proportional to
those defined for Niemegk (Bartels et al, 1939).
K
K


New K-Indices from South Atlantic Observatories : Port Stanley and Ascension Island

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New K-Indices from South Atlantic Observatories : Port Stanley and Ascension Island

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