From nico.gunnarsson@live.se Fri Feb 14 10:43:58 2025
Date: Fri, 14 Feb 2025 10:44:52 +0100
From: Nicolaas Gunnarsson <nico.gunnarsson@live.se>
To: Maria Hamrin <Maria.Hamrin@space.umu.se>
Subject: Fwd: VS: Advice for baseline method





-------- Forwarded Message --------
Subject:
VS: Advice for baseline method
Date:
Fri, 14 Feb 2025 07:44:10 +0000
From:
Juusola Liisa (FMI) <Liisa.Juusola@fmi.fi>
To:
Nicolaas Gunnarsson <nico.gunnarsson@live.se>

Hello Nicolaas

If you are only interested in calculating differential equivalent currents, you
could simply bypass the baseline subtraction and use the magnetic field at epoch
t1 as a baseline for t2. This will produce the same equivalent current
distribution for t2 as first subtracting a (constant) baseline from the magnetic
field data at t1 and t2, calculating the equivalent currents, and then
subtracting them.

If you want to calculate the non-differential equivalent currents, you need a
baseline. In case you are only interested in very active periods (current
densities of several hundred to thousands A/km), the baseline selection is not
so critical. You could use, for example, the quiet-time method you mention. For
more quiet periods, more care is needed with the baseline.

What I'm myself currently using is a sliding 10-day median. I calculate the
median for the midnight of each day and then use a simple cosine curve to
interpolate over the day. The 10-day median is long enough not to produce
significant depressions during active periods but still seems to follow the
instrument drifts sufficiently. Because I'm often interested in time
derivatives, I need a baseline that does not produce large, artificial time
derivatives, such as a step-change at midnight between baseline levels. The
interpolation helps with that. Of course, the median window could be slid over
the entire day, but this is quite slow when you have a lot of data to process.

I'd say that the main difference between Max's more complicated method and my
simple 10-day median method is that Max's method removes the diurnal variation
whereas mine does not. Not removing the diurnal variation suits my purposes, but
you may have a different need.

Implementing the baseline method is not such a big task. The most time-consuming
part is cleaning the magnetic field data from spikes, jumps, and bad periods. I
have a database of cleaned and baseline-subtraced IMAGE data from 1994 until
August 2024. For the years 1994-2014, Max's baseline method has been used, and
after that, mine. I'm planning to go back and reprocess the earlier years as
well, to be consistent, but it's slow work. How much data do you need? If you
like, I can share mine.

Also, please note that the matlab code you are using and the old online code
Audrey used are not identical. Your code, I assume, interprets the magnetic
field in terms of ionospheric and telluric equivalent currents. The online
method interprets the horizontal part of the observed magnetic field in terms of
ionospheric equivalent currents only, and uses truncated SVD to smooth the
solution.

I hope this helps!

Kind regards,
Liisa

________________________________________________________________________________
Lähettäjä: Nicolaas Gunnarsson <nico.gunnarsson@live.se>
Lähetetty: torstai 13. helmikuuta 2025 10.13
Vastaanottaja: Juusola Liisa (FMI) <Liisa.Juusola@fmi.fi>
Aihe: Advice for baseline method  
Hello Liisa.

My name is Nicolaas Gunnarsson and I am currently working on a master thesis
for Maria Hamrin where I will investigate the ionosphere equivalent current
from dB/dt-spikes during a non-stormy time period. I believe Maria has been
mailing you earlier, in late december, asking about the easiest way to
compute
the differential equivalent currents.

Maria has spoken well about you and your talents about computing
equivalent currents together with the spherical elementary current systems
(SECS) and I was wondering if you could help me in my current work.

I am trying to recreate some of the plots in one of Audrey Schillings
articles called "Signatures of wedgelets over Fennoscandia during
the St Patrick’s Day Storm 2015" (https://doi.org/10.1051/swsc/2023018) by
using a MATLAB code, that I see you are one of the creators of, that
Maria provided me with. More specifically the plots in figure 3 and
figure 4.

I can somewhat recreate the differential plots in figure 4, but I am
having problem figuring out the best way to remove the data baseline to
be able
to recreate figure 3 that shows the non-differential equivalent currents.

I have found an article from M. van de Kamp where he speaks of
determining the
baseline (https://doi.org/10.5194/gi-2-289-2013). In this article he writes
"A traditional method of removing a baseline from magne-
tometer data of a particular day is to look for a magnetically
quiet day near the day of interest, and calculate the average
value of the magnetic field of this day.".

M. van de Kamp later writes that this method provides four different
inaccuracies where the first one is:
"There may not be any day in the entire month which
is completely free from disturbances. In this case, the
“quiet” day is not really “quiet” and the data, and even
the average value, will still be affected by some distur-
bance effects." The remaining three inaccuracies also seems to be
somewhat related
to the first inaccuracy.

 From my understanding M. van de Kamp further states that
scientists use this method in some applications today mainly because of its
simplicity even though it leads to inaccuracies. Instead of using this
method
M. van de Kamp introduces a way of determining quiet days which is
interpolated
to create a long-term baseline.

Why I am mailing you is to ask for advice about which method I should pick.

Is it necessary take my time to create an accurate dataset from each
magnetometer stations measured data by using M. van de Kamps method? Or
can I use the
"simple and inaccurate"-method and still have a somewhat reliable
equivalent
current since I will be performing this study towards a non-stormy time?

I hope you the very best and thank you for taking the time to read this
mail.

Best regards,
Nicolaas


