GIGeoscientific Instrumentation, Methods and Data SystemsGIGeosci. Instrum. Method. Data Syst.2193-0864Copernicus PublicationsGöttingen, Germany10.5194/gi-5-253-2016Forecasting auroras from regional and global magnetic field measurementsKauristieKirstikirsti.kauristie@fmi.fiMyllysMinnaPartamiesNooraViljanenAriPeitsoPyryJuusolaLiisahttps://orcid.org/0000-0003-0864-5949AhmadzaiShabanaSinghVikramjitKeilRalfMartinezUnaiLugininAlexejGloverAlexiNavarroVicenteRaitaTeroFinnish Meteorological Institute, Helsinki, FinlandUniversity of Helsinki, Helsinki, FinlandThe University Centre in Svalbard, Svalbard, NorwayAalto University, Espoo, FinlandEuropean Space Agency, ESOC, Darmstadt, Germanyetamax space GmbH, Darmstadt, GermanySodankylä Geophysical Observatory, University of Oulu, Oulu, FinlandKirsti Kauristie (kirsti.kauristie@fmi.fi)28June2016512532624December201518January201626May201631May2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://gi.copernicus.org/articles/5/253/2016/gi-5-253-2016.htmlThe full text article is available as a PDF file from https://gi.copernicus.org/articles/5/253/2016/gi-5-253-2016.pdf
We use the connection between auroral sightings and rapid geomagnetic field
variations in a concept for a Regional Auroral Forecast (RAF) service. The
service is based on statistical relationships between near-real-time alerts
issued by the NOAA Space Weather Prediction Center and magnetic time
derivative (dB/dt) values measured by five MIRACLE magnetometer
stations located in Finland at auroral and sub-auroral latitudes. Our database
contains NOAA alerts and dB/dt observations from the years
2002–2012. These data are used to create a set of conditional probabilities,
which tell the service user when the probability of seeing auroras exceeds the
average conditions in Fennoscandia during the coming 0–12 h. Favourable
conditions for auroral displays are associated with ground magnetic field
time derivative values (dB/dt) exceeding certain latitude-dependent
threshold values. Our statistical analyses reveal that the probabilities of
recording dB/dt exceeding the thresholds stay below 50 % after NOAA
alerts on X-ray bursts or on energetic particle flux enhancements.
Therefore, those alerts are not very useful for auroral forecasts if we
want to keep the number of false alarms low. However, NOAA alerts on global
geomagnetic storms (characterized with Kp values > 4) enable
probability estimates of > 50 % with lead times of 3–12 h.
RAF forecasts thus rely heavily on the well-known fact that bright auroras
appear during geomagnetic storms. The additional new piece of information
which RAF brings to the previous picture is the knowledge on typical storm
durations at different latitudes. For example, the service users south of
the Arctic Circle will learn that after a NOAA ALTK06 issuance in night,
auroral spotting should be done within 12 h after the alert, while at
higher latitudes conditions can remain favourable during the next night.
Introduction
According to Lilensten et al. (2008):
Space weather is the physical and
phenomenological state of natural space environments. The associated
discipline aims, through observations, monitoring, analysis and modelling, at
understanding and predicting the state of the Sun, the interplanetary and
planetary environments, and the solar and non-solar driven perturbations
that affect them, and also at forecasting and nowcasting the potential
impacts on biological and technological systems.
Auroras are a harmless,fascinating feature of ionospheric space weather. They are an important
factor in the business of nature tourism in polar areas. In this context
there is a demand to obtain auroral forecasts with long lead times – hours, days
or even weeks.
The original energy source for space weather phenomena is the Sun, which
emits a wide spectrum of electromagnetic waves and a continuous flow of
charged particles (solar wind) to its surroundings. Rapid variations in
space weather conditions (space weather storms) are associated with large-scale dynamic phenomena (coronal holes, flares and mass ejections) taking
place in the solar atmosphere (corona). The first signs of solar eruptions
are X-ray flares and extreme ultraviolet and radio wave bursts which reach the Earth
surroundings with ∼ 8 min delay after their initiation. The
next sign is the enhancements in energetic particle fluxes as observed, for example, at
the geostationary orbit (with a few hours' delay). X-ray flares often
generate coronal mass ejections (CMEs), which are huge, massive bubble-like
structures in the solar wind. It takes typically 1–2 days for a CME to
propagate from its origin region to the Earth distance.
The brightest and strongest auroras and disturbances in the geomagnetic
field are typically caused by CMEs. The term geoefficiency is used to characterize the
capability of a structure to generate variations in the near-Earth space.
Besides solar wind speed and density also the magnetic field topology of the
solar wind structure is a critical factor controlling geoefficiency.
Structures whose magnetic field points in the opposite direction to Earth's
magnetic field at dayside magnetopause are particularly good in generating
beautiful and extensive auroras. Reliable information about the magnetic
topology can be achieved only by in situ measurements. For this purpose
continuous solar wind measurements have been conducted at the Lagrange 1
point (L1) 1.5 million kilometres from Earth at the Sun–Earth line since the 1980s. A
typical CME propagation time from L1 to Earth is 1 h, which is – with
our current scientific knowledge – also the upper limit for the lead time of
reliable auroral forecasts.
Several space weather monitoring and predicting services publish alerts on
X-ray flares and earthward-directed CMEs (see e.g. the service of Space
Weather Prediction Center of the US National Oceanic and Atmospheric
Administration (NOAA), http://www.swpc.noaa.gov/, and the Solar Influences
Data Center service in Belgium, http://sidc.oma.be/). Near-real-time (NRT)
information about geostationary energetic particle fluxes and global
magnetic activity is also available for public use. These services thus
provide useful background information for the attempts to monitor and
forecast regional auroral occurrence rates.
Observations of auroral ionospheric phenomena were started in Sodankylä
already during the first International Polar Year 1882–1883 (Seppinen and Pellinen, 2009).
The Sodankylä Geophysical Observatory was established in 1913 by the
Finnish Academy of Science and Letters (Sucksdorff et al., 2001). The
Finnish Meteorological Institute started regular auroral observations in
Sodankylä and in some other sites in Lapland during the International
Geophysical Year (1957–1958). In 1975 Finland became a member of the
EISCAT scientific association, which built and started to operate a system of
incoherent scatter radars with antennas in Tromsø, Kiruna and
Sodankylä. This triggered space research groups in Sodankylä
Geophysical Observatory, Oulu University and Finnish Meteorological
Institute to start a collaboration in order to conduct systematic
ionospheric observations with versatile instrumentation in the surroundings
of the EISCAT radars. Today's progeny from these activities is the MIRACLE
network of magnetometers and auroral cameras, whose data records have been
used in several studies on statistical auroral occurrence rates (Nevanlinna
and Pulkkinen, 2001; Partamies et al., 2015) and on ionospheric
electrodynamics linking auroras with ionospheric electric currents and
geomagnetic variations (Amm et al., 2005).
In this paper we describe a concept for an auroral forecast service
(hereafter called Regional Auroral Forecast, RAF), which is based on
archived NOAA space weather alerts and regional magnetic field and auroral
recordings. The archives are used to create a set of conditional
probabilities, which tell the service user when the probability of seeing
auroras exceeds the average conditions in Fennoscandia during the coming
0–12 h. The data archives and methodology used in the development of RAF
are described in Sect. 2. Results and a case study on the service
performance are presented in Sect. 3. Concluding remarks and future
prospects are discussed in Sect. 4.
Data and methodologiesMagnetic field data and their connection with auroral activity
Auroral activity is associated with variations in the geomagnetic field.
During strong geomagnetic storms the amplitude of these variations can be
even 4–5 % (2000 nT) of the strength of the main field in the
Fennoscandian area (roughly 50 000 nT). Typical timescales of the
disturbances vary from days (duration of a storm; Gonzalez et al., 1994) to
a few seconds (magnetic pulsations; Fukunishi et al., 1981). Magnetic
variations are coupled with visible auroras: electron precipitation, which
causes the auroral emissions by collisions with atmospheric particles,
enhances also the conductivity and electric currents in the ionosphere. The
ionospheric current system – according to the Biot–Savart law – generates
magnetic perturbations which are measurable with ground-based magnetometers.
Stations of the MIRACLE network. The fields of view of
auroral cameras are shown with black circles and the observing area of the
bistatic STARE radar (operated 1997–2005) with the rectangle (dashed
lines). Magnetometer stations used in the RAF and Auroras Now! services are
shown with the small red and orange circles, respectively.
An easy way to characterize the intensity of space weather variations is to
use a proxy, which describes the strength of ionospheric and magnetospheric
currents and is based on measurements by a global and/or local network of
ground-based magnetometers. The global Kp index is one of the most widely
used proxies in this area. It is defined to be the mean value of the
disturbance levels in the horizontal magnetic field component observed at
13 selected, sub-auroral stations (Bartels et al., 1939). The index has 3 h
time resolution, and its value is given in a range 0–9 according to a station-specific, quasi-logarithmic scale. While Kp describes nicely the overall
space weather activity, observations of the local magnetic field time
derivative (dB/dt) with high time resolution are a more useful way
to support regional auroral monitoring services. This linkage is utilized in
an already existing public auroral monitoring system Auroras Now!
(http://aurora.fmi.fi), which was designed as a Space Weather Applications
Pilot Project with some support of the European Space Agency (ESA) in the early 2000s.
The service has become popular with thousands of daily visitors during wintertime.
The Auroras Now! service is based on NRT data from the
Magnetometers–Ionospheric Radars–All-sky Cameras Large Experiment (MIRACLE) network of
auroral cameras and magnetometers (http://space.fmi.fi/MIRACLE,
cf. Fig. 1 and Table 1). In the original version of Auroras Now!
dB/dt values from two observatories – Nurmijärvi (NUR,
sub-auroral latitudes) and Sodankylä (SOD, auroral latitudes) – were
monitored continuously. Enhanced opportunity to see auroras is empirically
defined to take place when the hourly maximum of dB/dt exceeds
0.3 nT s-1 in Nurmijärvi and 0.5 nT s-1 in Sodankylä. More exactly, the
hourly maxima of time derivatives of x and y components (geographic north
and east components with 1 min time resolution) are calculated and the
larger one is compared with the threshold. The performance of Auroras Now!
has been evaluated by comparing Sodankylä auroral and magnetometer
observations during the season from 1 November 2003 to 31 March 2004
(Mälkki et al., 2006). The analysis shows that in 86 % of the cases
when the dB/dt threshold was exceeded also auroras were observed.
In the 13 % of the cases when Auroras Now! failed to spot the auroras, the
intensities were typically dim or even below the sensitivity of human eye.
RAF uses the same empirical rules between auroral occurrence and
dB/dt that were used in Auroras Now! The threshold values for the
magnetometer stations depend on the magnetic latitudes; for additional
stations used in RAF they are determined by linear inter- and extrapolation
from the corresponding values of Nurmijärvi and Sodankylä. The
statistical study of Finnish all-sky camera recordings from years 1973–1997
by Nevanlinna and Pulkkinen (2001) shows that assuming a linear trend in the
auroral occurrence probability according latitude is a good approximation at
magnetic latitudes 63–70∘. At latitudes below
63∘ the evidence for a linear trend is less clear, but as all-sky
observations from these latitudes are scarce in the analysed database, we
use the linear relationship there also as the first approximation. The RAF
stations with their coordinates and dB/dt threshold values are
listed in Table 1. Stations KEV and MUO are at latitudes poleward of the
Arctic Circle (66.56∘ N) and under the average auroral oval during
moderate activity levels. Stations OUJ, HAN and NUR are at sub-auroral
latitudes where high dB/dt values are recorded only during space
weather storms.
Magnetometer stations used in the Auroras Now! and RAF services and
the corresponding dB/dt threshold for enhanced probability of
aurora occurrence. Magnetic latitude (MLAT) is given in the frame of
corrected geomagnetic coordinates.
CodeNameGeographicalMLATdB/dtcoordinatesdegree NthresholdNURNurmijärvi60.50∘ N, 24.65∘ E56.9∘0.30 nT s-1HANHankasalmi62.25∘ N, 26.60∘ E58.7∘0.35 nT s-1OUJOulujärvi64.52∘ N, 27.23∘ E61.0∘0.42 nT s-1SODSodankylä67.37∘ N, 26.63∘ E63.9∘0.50 nT s-1MUOMuonio68.02∘ N, 23.53∘ E64.7∘0.52 nT s-1KEVKevo69.76∘ N, 27.01∘ E66.3∘0.57 nT s-1Statistical relationship between regional magnetic field variations and space weather alerts
Forecasts of auroral activity in RAF are based on statistical relationships
between space weather alerts which describe solar and global activity and
dB/dt values measured at the RAF magnetometer stations. In the
development work we used archives of NRT alerts by NOAA, Halo-CME alerts by
SIDC and Finnish Meteorological Institute's (FMI's) alerts for enhanced
magnetic variability based on ACE data (available with the Auroras Now!
service). We concentrate on the results based on NOAA alerts (issued
2002–2012, cf. Table 2) as they appeared to be most useful for prediction purposes.
The number of NOAA alerts used in the study. For Kp= 4–8 the four
values in the given sums are the number of the events which took place in the
local time bins of dawn, dusk, night and noon (for more details see text).
In the statistical analysis we sought answers to questions such as the following: what is
the probability of measuring dB/dt>A at station B with
the alert of type C issued T hours earlier? Here values of A and
corresponding stations B are those listed in Table 1. The value T varies in
the range 1–48, and the different alert types (C) are described below. In
practice the analysis was conducted in the following steps:
Constructing a summary matrix on the NOAA alerts: each row in the matrix
corresponds to 1 h during the years 2002–2012. Each alert type has one
dedicated column in the row. If that alert is issued during the hour
of the row, the variable in the column is 1; otherwise it is zero.
Constructing a summary matrix on the hourly maxima in dB/dt values
recorded at the RAF magnetometer stations: also this matrix has values 1 (in
the case of dB/dt threshold excess) or 0 (no threshold excess).
Determining statistical relationships between the parameters in the two
matrices described above: for each alert type the hours of issuance were
searched and the values in the dB/dt matrix for the following 48 h
were inspected. For these 48 h and for each RAF magnetometer
stations the ratio W/V was determined, where W is the number of hours when
the threshold for auroras was exceeded and V is the total number of hours in
the analysis (i.e. the number of issuances of the analysed alert type during
the 10-year period). The combined effect of subsequent alerts was ignored
in the analysis as alerts with less than 48 h separation were handled as
independent separate cases.
Identifying those NOAA alert types which yield W/V values equal to or
larger than 0.5.
Refining the analysis of step 3 by binning the data points according to
magnetic local time (MLT) of the RAF stations at issuance moment and by
studying the combined effect of some of the most influential alerts. Four
bins were used in the local time binning: noon (06:00–12:00 UT), night
(18:00–24:00 UT), dawn (00:00–06:00 UT) and dusk (12:00–18:00 UT). (Note: for the MIRACLE local time
sector magnetic local time ∼ UT + 2.5 h.)
The NOAA archives contain the following types of alerts:
Solar X-ray flare alerts (ALTXMF) are issued when the solar X-ray flux exceeds
the M5 level (5 × 10-5 W m-2, at wavelengths 0.1–0.8 nm and
measured at the geostationary distances).
Alerts on enhanced proton fluxes at the geostationary distances
(ALTPX1–ALTPX4) are issued when the integral flux of protons with
energies above 10 MeV exceeds values 10, 100, 1000, or 10 000 pfu (particle flux units).
Alerts on enhanced electron fluxes at the geostationary
distances (ALTEF3) are issued when the integral flux of electrons with energies above 10 MeV
exceeds a value 1000 pfu.
Solar Radio Burst alerts (ALTTP2, ALTTP4) are issued in the cases of
enhancements in Type II or Type IV radio emissions with frequencies < 15 MHz.
Emissions are caused by accelerated electrons in the context of solar wind shocks and CMEs.
Alerts on enhanced global geomagnetic activity (ALTK04–ALTK09) are
issued when the Kp estimate by the Wing Kp model (Wing et al., 2005) exceeds values 4–9.
In the following discussion we use the W/V value (in %) as a proxy for
the auroral occurrence probability, although strictly speaking this value
represents the probability of dB/dt excess above the given
threshold. Figure 2 is an example plot on the W/V value for stations KEV and
NUR during the next 48 h after the NOAA ALTK04 and ALTK06 issuance
times. According to this plot the probability of enhanced auroral
occurrence is above 50 % at KEV during ∼ 10 h (0 h)
after the issuance of ALTK06 (ALTK04). At the sub-auroral station NUR the
probability stays above 50 % only for the first hour after the ALTK06 issuance time.
W/V values (in %) for stations KEV (thick lines) and
NUR (thin lines) during 48 h after the issuance of ALTK06 (solid lines)
and ALTK04 (dashed lines). W is the number of cases with dB/dt
excess above the threshold for enhanced auroral occurrence. V is number of
ALTK06 (195) and ALTK04 (995) issued during the years 2002–2012.
ResultsAnalysis of W/V curves
We begin the investigation of the W/V curves with the ALTXMF case because
X-ray flares give the first signs of forthcoming space weather activity, and
thus they have potential to support forecasts with the longest feasible lead
times. Figure 3 shows the probability curve of ALTXMF for stations KEV, OUJ
and NUR. In this case we extend the axis of delay times up to 120 h in
order to take into account also the impact of slowly propagating CMEs. Error
bars in Fig. 3 (and in the subsequent similar figures) are determined with
the standard deviation for Poisson distribution, i.e. ε=((W))-1 (100 W/V).
ALTXMF appears not to be a reliable enough way
to forecast enhanced auroral occurrence as all probability values in Fig. 3
are below 50 %. The impact of CMEs is visible as a moderate increase in
W/V values (∼ 15 % units) for delay times 37–80 h in
the curves of sub-auroral stations OUJ and NUR, where the average level of
magnetic variability is low. At KEV the baseline level of W/V is so high
(∼ 20–30 %) that no specific CME signatures can be
distinguished from the background activity. In general, the feature of
ALTXMF W/V curves staying at values < 50 % can be explained with
the different propagation speeds of CMEs and two factors limiting their
geoefficiency: not all flares generate CMEs, which are directed towards the
Earth, and not all CMEs have the correct magnetic topology to generate high
dB/dt values.
W/V values (in %) for stations KEV (black), OUJ
(green) and NUR (blue) during 120 h after the issuance of ALTXMF. W is
the number of cases with dB/dt excess above the threshold for
enhanced auroral occurrence. V is number of ALTXMF (159) issued during the
years 2002–2012.
The W/V curves of solar radio bursts (ALTTP2, ALTTP4) and those for
energetic proton and electron enhancements (ALTEF3, ALTPX1–ALTPX4)
gave similar results as those of ALTXMF (no values exceeding
50 %). The alerts on global geomagnetic activity (ALTK04–ALTK09),
however, yielded more promising results. As explained in
Sect. 2.2, further improvement is achieved by binning the alerts according to
their issuance times. The response at RAF stations depends on their local
time sector. High W/V values are achieved for those delay times which
correspond to the situation where RAF stations are around midnight.
UT binning was applied only for ALTK04-ALTK06; for ALTK07 the total number
of alerts is too small to allow MLT binning for meaningful statistical
analysis. Also for ALTK08 and ALTK09 we still need longer data archives
before any W/V curves can be derived, but on the other hand the curves of
ALTK07 already can give a relatively good picture of the case of
exceptionally strong space weather storms. Thus in the operational RAF
service probability curves from the combined ALTK07, ALTK08 and ALTK09 are used.
Like mentioned in Sect. 2.2, the NOAA K-alerts and consequently also the
RAF forecasts are based on the Wing Kp model, which in some cases may result
in too optimistic estimates on auroral occurrence rates. Bala and Reiff (2014)
have tested the performance of Wing Kp 1 h forecast with real-time
output values collected during a test period of 22 months (April 2011–February 2013).
This study shows that the Wing Kp approach has some tendency
to overestimate Kp values during enhanced activity. In the test data set of
15 960 time instants the Wing approach claimed the Kp to be equal to or more
than 4 in 1222 cases. A check against the official Kp values reveals that
335 of these were false alarms (i.e. the real Kp was < 4).
Figures 4 and 5 show the W/V curves of MUO and HAN for ALTK04–ALTK06
(for the night bin) and for ALTK07 (all points). The W/V curves of
MUO and KEV are mainly similar (latter not shown), and they describe the
dB/dt activity at auroral latitudes: the threshold of 50 % is
exceeded already after ALTK04 although only for the first hour. In the case
of ALTK05, occurrence of high dB/dt values lasts some 7 h after
the alert; for ALTK06 high dB/dt values were recorded with
50 % probability for the delay hours 1–3 and 26–30. After ALTK07, enhanced
dB/dt activity lasts some 26 h. The W/V curves of HAN have the
same features as those of NUR (not shown). At the sub-auroral latitudes
occurrence rates of high dB/dt values with auroral occurrence
probablity > 50 % appear only for ALTK06 or higher and for
delays of 1–11 h. In the case of ALTK07, enhanced activity persists for
13–15 h. The W/V curves of OUJ (not shown) are similar to those of HAN
and NUR otherwise, but the 50 % threshold of occurrence of high
dB/dt values is exceeded already at the activity level of ALTK05,
although only during the first hour after the alert. The most important
conclusion from Figs. 4 and 5 is that at auroral latitudes the occurrence
rates for high dB/dt are close to 50 % still during the next
night after the issuance of ALTK06 or ALTK07, while at the sub-auroral
stations the W/V values drop below 50 % already after a delay of 12–16 h.
W/V values (in %) of station MUO for ALTK04 (cyan),
ALTK05 (red), ALTK06 (blue) and for ALTK07 (black). The curve for ALTK07 is
based on all data points, while for the other activity levels only the
points of nighttime bin were used (for the number of data points, see
Table 2). The dashed lines represent smoothed curves (seven-point running
averages) for ALTK06 and ALTK07, which are used in the operational RAF service.
Figure 6 demonstrates the effect of UT binning in W/V curves for MUO after
ALTK06. Again, similar behaviour appears in the W/V curves of KEV. The
curves of night and dusk sector issuance times suggest that for the coming
night V/W values are well above 50 %. ALTK06 issuance around noon also
indicates ≥ 60 % auroral probabilities for the coming night (curves
not shown). In the case of dawn sector issuances the ongoing night is
clearly more favourable for auroral spotting than the following night. In
other words if there is already high magnetic activity in the beginning of
the dark time, it will likely continue during the nearest night hours. On
the other hand, high morning activity does not strongly indicate that the
next night ∼ 12 h later will still show auroral displays.
W/V values (in %) of station HAN for ALTK04 (cyan),
ALTK05 (red), ALTK06 (blue) and for ALTK07 (black). The curve for ALTK07 is
based on all data points, while for the other activity levels only the
points of nighttime bin were used (for the number of data points, see Table 2).
W/V values (in %) of station MUO for ALTK06 and the UT bins of dawn
(red), dusk (blue) and night (black).
RAF forecasts on auroral occurrence probability (a, b) for a couple of
time instants around midnight on 7–8 September 2015 and an example image
from the Sodankylä auroral camera station from the same time period (c). The
forecasts were published at (a) 15:17 UT and (b) 17:02 UT. Cyan (green)
colour gives regions with > 50 % (> 70 %) probability of auroral sightings.
Description of the operational RAF service
The RAF service has been developed with ESA funding in the space weather
segment of ESA's Space Situational Awareness programme during years
2013–2015. The service has two parts: the nowcast service which
characterizes prevailing auroral occurrence probability with the same
approach as Auroras Now! and the forecast service which uses the above
described RAF approach. In both parts the regions of enhanced auroral
occurrence probabilities are shown as bands of cyan (W/V> 50 %)
or green (W/V> 70 %) colour overlaid on the map of Fennoscandia.
These bands are positioned at the latitudes of ±2∘ around the RAF
stations where the forecast dB/dt exceeds the threshold of enhanced
probability of auroral occurrence (for an example, see Fig. 7). The forecast
service checks the latest NOAA alerts every 15 min. If alerts of the correct
type (ALTK04-09, ALTPX) were issued during the previous 15 min, the
service would check the corresponding W/V curves with UT binning for delays of
T0+ 3, T0+ 6, T0+ 9 and T0+ 12 (where T0 is the alert issuance hour) and
draws the forecast maps accordingly.
Figure 7 presents an example of RAF performance on the evening of 7 September 2015.
On that day Kp values started to increase after noon so that the
values for the 3 h periods ending at UT times 15:00, 18:00, 21:00, and 24:00
were 4.67, 6.33, 5.67, and 6.33, respectively. The first maps forecasting auroral
activity appeared to the RAF service at 15:17 UT (at 18:17 LT – local time). The
maps for T0+ 3, T0+ 6, and T0+ 9 (i.e. until 00:17 UT) showed bands of
cyan colour above KEV and MUO stations (cf. Fig. 7a). Roughly
2 h later at 17:02 UT, RAF made a radical correction in its forecasts:
the forecast maps showed auroras to all latitudes for all lead times
(T0+ 3–T0+ 12), and even with > 70 % probability
for latitudes above KEV, MUO and OUJ until 02:02 UT (cf. Fig. 7b).
This time the correction was successful: beautiful auroras were observed
at several sites all over Finland. The photograph archives maintained by the
Finnish Ursa Association of amateur astronomers (http://www.taivaanvahti.fi/observations/browse/list/1120892/observation_start_time)
contain photos of auroral displays until 00:30 UT (03:30 LT) on 8 September 2015. The auroral camera of MIRACLE network
in Sodankylä also captured spectacular auroras for several hours during
that night (Fig. 7c).
Test versions of RAF have been operated at the servers of ESA and the
Finnish Meteorological Institute since May 2014. Validation studies with
auroral observations from the Ursa service and by auroral cameras of
Japanese and Finnish research groups have revealed that the performance of
RAF is on a satisfactory level in the case of strong, extensive auroras
(activity also at sub-latitudes), but it can miss auroral displays occurring
at high latitudes during moderate activity. The W/V curves of KEV in
Figs. 2 and 3 help in understanding this result. In both figures the baseline
level of high dB/dt occurrence rate, i.e. the level where W/V values
settle at long delay times, is around 20–30 % for KEV. This means
that at auroral latitudes nice auroral displays can take place relatively
often, although no significant global activity is ongoing. Giving
case-by-case forecasts of such displays is challenging since they most
likely manifest the stochastic part of solar wind–geospace interactions
related to turbulence in the solar wind (Pulkkinen et al., 2006). Anyways, it is
possible to estimate the locations of the average auroral oval
boundaries with statistical oval models. Sigernes et al. (2011) present a
method for deriving the oval location for different Kp levels. The method is
based on oval models derived from optical and particle precipitation data
(Starkov, 1994; Zhang and Paxton, 2008). We have compared the oval locations
by the dB/dt approach used in RAF to those by the Starkov oval with
data from a test period (5 May–28 October 2014). This comparison study suggests
that these two approaches complement each other nicely: the tool by Sigernes
et al. (2011) guides users to appropriate latitudes during moderate activity,
while RAF gives a more realistic representation on oval dynamics during
strong Kp activity.
W/V values (in %) of station KEV (black) and NUR (green) for the
special case of ALTPX* preceding ALTK06 (thick lines) and for the case of
all ALTK06 events. The number of data points in the bin of special cases is 69.
Concluding remarks and future prospects
We have used the connection between auroral sightings and rapid geomagnetic
field variations in the development of the Regional Auroral Forecast (RAF)
service. The service is based on statistical relationships between NRT
alerts issued by the NOAA Space Weather Prediction Center and
dB/dt values measured by five MIRACLE magnetometer stations located in Finland at
auroral and sub-auroral latitudes. Our database contains NOAA alerts and
dB/dt observations from the years 2002–2012. Magnetometer data have
been used instead of direct auroral observations when constructing the
statistics because processing numerical data is simpler than recognizing
auroras from images, whose quality can occasionally suffer from cloudiness
and moonlight contamination. The close linkage of auroral and magnetic
activity has been utilized also in the NRT service by Johnsen (2013), which
associates the latitudes of enhanced auroral occurrence rates with regions
where auroral electrojets are strongest. Another way to overcome the
complications in statistical analysis of auroral images is to construct the
oval model with the help of auroral particle precipitation measurements by
polar-orbiting satellites. This pathway has been used in the OVATION Prime
empirical model, which is available, for example, on the home page of NOAA Space
Weather Prediction Center and is based on particle data from the Defense
Meteorological Satellite Program (DMSP).
Our statistical analyses reveal that NOAA alerts on X-ray bursts or on
energetic particle flux enhancements cannot be used in the forecasts if
only probability values above 50 % for successful auroral spotting are
used in the service. However, NOAA alerts on global geomagnetic storms
(characterized with Kp values > 4) enable probability estimates
of > 50 % with lead times of 1–12 h. RAF forecasts thus
rely heavily on the well-known fact that bright auroras appear during
geomagnetic storms. The additional new piece of information which RAF brings
to the previous picture is the knowledge on typical storm durations at
different latitudes. For example, the service users southward of the Arctic
Circle will learn that after a NOAA ALTK06 issuance, auroral spotting should
be done within 12 h after the alert, while at higher latitudes conditions
can remain favourable still during the next night.
Occurrence probability of auroras at MIRACLE auroral camera
stations with the following magnetic latitudes: HAN 59∘; SOD 64∘; KIL 66∘; LYR 75∘.
Probabilities are based on visual inspections of quick look data
(keograms, one image per night). The annual number of nights with auroras
was normalized with the number of nights when the camera was
operational (updated version of Fig. 9 by Pulkkinen et al., 2011).
We have handled the different NOAA alert types as separate independent
cases, which is a limitation to be overcome in future studies with longer
records of NOAA alerts. It is very likely that sequences of several
subsequent Kp alerts or their combinations (e.g. with alerts on enhanced
energetic particle fluxes) produce different probability curves for high
dB/dt values than single alerts. The probability curves of Fig. 8
support this anticipation: the probabilities for the special case, where
ALTK06 has been preceded (within 24 h) by an alert on enhanced proton
fluxes (ALTPX*), are larger than those for the case of all ALTK06 alerts.
This feature is taken into account in RAF, but obviously accounting also for
other alert combinations would improve the performance of the service as
soon as enough archived alert data have been accumulated to test this hypothesis.
Our approach is not very useful in the attempts to forecast auroras at high
latitudes during non-storm times. Statistical analysis of MIRACLE all-sky
camera data shows that during the best years of auroral activity (some
2–3 years after sunspot maxima) the occurrence rates are 60–75 % at stations
under the auroral oval (i.e. at magnetic latitudes 64–75∘; stations SOD, KIL
and LYR in Fig. 9). Comparing these values to the threshold which we use
in RAF for enhanced auroral activity (occurrence probability > 50 %)
reveals that cloudiness forecasts provide at auroral latitudes more
useful information for auroral spotting than RAF statistics. At sub-auroral
latitudes an announcement of enhanced probability by RAF can be interpreted
to represent conditions which prevail at auroral latitudes during the most
favourable years in the solar cycle. With the latitudinal coverage of MIRACLE
all-sky and magnetometer observations (Fennoscandian sector), we conclude
that the auroral oval latitudes in this context correspond roughly to
magnetic latitudes 64–75∘, while latitudes below MLAT 61 represent
sub-auroral regions.
The threshold values which we use for dB/dt as an implication of
enhanced auroral activity may be adjusted in the future, when we have
gathered more experience in aurora data analysis with advanced machine-learning methods (Rao et al., 2014; Syrjäsuo and Partamies, 2011).
Finding optimal values for automatic recognition will not be
straightforward since there is some variability in the user requirements
(photographing versus naked-eye observations). The threshold values used in
RAF come as legacy from the Auroras Now! service, which was designed during
the years 2003–2005. These thresholds usually deserve their place as the
first approximation, but as nowadays the user community includes more
auroral photographers with high-end camera equipment than 10 years ago, the
detection threshold values may need some lowering in the future RAF
upgradings. Long, homogeneous and validated records of ionospheric
observations, like those provided by the Sodankylä research station and the
surrounding MIRACLE network, will be crucial input for such upgrading work.
Data availability
Space weather alerts are available via the service of Space Weather Prediction
Center of the US National Oceanic and Atmospheric Administration (NOAA)
(http://www.swpc.noaa.gov/, old archives at
http://legacy-www.swpc.noaa.gov/alerts/archive.html) and via the Solar
Influences Data Center service in Belgium (http://sidc.oma.be/). The
NOAA OVATION auroral forecast is available at
http://www.swpc.noaa.gov/products/aurora-30-minute-forecast. The
auroral forecast service maintained by Kjell Henriksen Observatory (Svalbard,
Norway) is available at http://kho.unis.no. The Auroras Now! service for
NRT monitoring of auroras in Finland is available at
http://aurora.fmi.fi. The Ursa Astronomical Association (Finland) maintains a browsing system for auroral photos at
http://www.taivaanvahti.fi/observations/browse/list/1120892/observation_start_time.
Auroral and magnetometer data by MIRACLE network can be requested from
http://space.fmi.fi/MIRACLE.
Acknowledgements
The authors thank the Space Weather Prediction Center of NOAA and SIDC for
providing access to their archived space weather alerts.
The MIRACLE network is operated as an international collaboration under the
leadership of the Finnish Meteorological Institute. The IMAGE magnetometer
data are collected as a joint European collaboration. INAF-IAPS (Italy) and
the University of Oulu (Finland) maintain the ITACA ASCs and the ASC in
Sodankylä. National Institute on Polar Research (Japan) is acknowledged
for their service of auroral images which was used in RAF testing.
A. Ketola, L. Häkkinen, S. Mäkinen, P. Posio, K. Pajunpää
and A. Koistinen (all in FMI Observation Unit) are acknowledged for their
persistent and professional work for MIRACLE observations. P. Janhunen (FMI)
gave valuable advice in the analysis of W/V curves.
Edited by: J. Pulliainen
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