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1
Relation of РС index to Solar Wind
parameters and to AL and Dst indices
in course of Magnetic Storms
О.A.Troshichev and D.A.Sormakov
Arctic and Antarcrtic Research Institute, St.Petersburg
Outline:
1. Introduction
2. Magnetic storms taken for the analysis.
3. Choice of the smoothing window width.
4. Correlation of the PC index with interplanetary electric field EKL and with
magnetic activity indices AL and Dst.
5. Three basic types of the magnetic storms development.
6. Solar wind drivers (ICME and SIR) and types of magnetic storms.
7. Relationship between PC index and magnetic storms (statistical results).
8. Summary
1. Introduction: PC index as indicator of the polar cap magnetic activity
generated by the solar wind
Experimental data are evidence that the field-aligned currents
permanently presented on the poleward boundary of the auroral
oval (Region 1 FAC system) are strongly dependent on
southward IMF (Langel, 1975; McDiarmid et al.,1977; Iijima &
Potemra, 1982) and interplanetary electric field (Bythrow &
Potemra, 1983).
PC index has been introduced [Troshichev et al., 1988] to
characterize the polar cap DP2 magnetic disturbances
related to the interplanetary electric field EKL [Kan and Lee,
1979] EKL=VSW BT2sin2(θ/2)
As the experimental results (Troshichev et al., JGR,
2014; Troshichev and Sormakov, EPSP, 2015) show, the
PC index can be regarded as a proxy of the solar
wind energy that entered into the magnetosphere.
These field-aligned currents generate the potential difference across the
polar caps and the appropriate polar cap DP2 magnetic disturbances
(Troshichev and Tsyganenko, 1979; Troshichev et al., 1979).
4
Resolutions of XXII Scientific Assembly of International Geomagnetism and
Aeronomy Association (12th IAGA), Merida, Меxico, August 2013
No. 3: Polar Cap (PC) index
IAGA,
• noting that polar cap magnetic activity is not yet described by existing IAGA geomagnetic indices,
• considering that the Polar Cap (PC) index constitutes a quantitative estimate of geomagnetic activity at polar latitudes and serves as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling,
• emphasising that the usefulness of such an index is dependent on having a continuous data series,
• recognising that the PC index is derived in partnership between the Arctic and Antarctic Research Institute (AARI, Russian Federation) and the National Space Institute, Technical University of Denmark (DTU, Denmark)
• recommends use of the PC index by the international scientific community in its near-real time and definitive forms, and
• urges that all possible efforts be made to maintain continuous operation of all geomagnetic observatories contributing to the PC index.
The PC index serves as a proxy for energy that enters into the magnetosphere.
Therein lies a principal distinction of the PC index from
• various coupling functions, which are characteristics of the solar wind arriving to the Lagrange point L1, and
• from AL and Dst indices, which are characteristics of the energy realized in form of magnetospheric substorm and magnetic storms.
The 1-min PCN and PCS indices, which calculated on-line by magnetic data from
near-pole stations Thule and Vostok, are presented at web site: http://pcindex.org
The PC index used in our analyses is average of values PCN and PCS.
2. Magnetic storms taken for the analysis
1998 2000 2002 2004 2006 2008 2010 2012 2014 2016
0
5
10
15
20
Numb
er of
even
ts
Years
Dst=30-60 nT
Dst=60-90 nT
Dst=90-120 nT
Dst=120-200 nT
Dst=200-400 nT
The following parameters were used to characterize magnetic storm:
• magnetic storm beginning as a beginning of the geomagnetic field depression (T0 moment) ,
• magnetic storm intensity as a maximum of the geomagnetic field depression (Dstmin)
• storm growth phase as a time interval between the moments of storm beginning and storm intensity.
For analysis were taken magnetic storms detected in 1998-2015 (N=568) answering to the following criteria:
intensity higher than = -30nT, duration ΔΤ longer than 12 hours.
The storms were divided by their intensity into 5 categories : 30-60nT, 60-90nT, 90-120nT, 120-200nT, 200-
400nT. Distribution of storms of different intensity over period 1998-2015 is shown in Figure.
3. Choice of the smoothing window width
0.0 0.2 0.4 0.6 0.8 1.0
0
15
30
45
60
75
Num
ber
of e
vent
s
Smooth_120
N=187
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
Num
ber
of e
vent
s
Smooth_90
N=187
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
Num
ber
of e
vent
s
Smooth_60
N=187
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
Num
ber
of e
vent
s
Smooth_30
N=187
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
Delay time, min
N=169
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
N=165
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
N=162
0.0 0.2 0.4 0.6 0.8 1.0
0
15
30
45
60
75
Num
ber
of e
vent
s
Correlation
Smooth_180
N=187
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
N=153
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
N=158
0 20 40 60 80 100 120 140 160 180
0
5
10
15
20
N=160
0.0 0.2 0.4 0.6 0.8 1.0
0
10
20
30
40
Num
ber
of e
vent
s
Smooth_15
N=187
Correlation between PC and Dst indices
To reveal conditions of best correlation between
the solar wind parameters and magnetic
activity indices, all parameters and indices
were smoothed using the averaging running
window of different width (from 0 to 180 min).
Correlation between diverse quantities was calculated
with different time shifts between them. The shift
value providing the best correlation was taken
as a delay time ΔT between quantities.
Left column in Figure demonstrates, as a example,
occurrence of events with different correlation
between the smoothed SymH and PC indices in
period of 1998-2001 under conditions of
different smoothing window width (15, 30, 60,
90, 120 and 180 minutes). Right column shows
occurrence of events over delay times ΔΤ (when
correlation is essential, R>0.5)
Results:
Correlation obviously grows while widening the
smoothing interval, but maximum at certain
ΔΤ is also progressively smoothed.
Taking into account these two contrary tendencies,
the running window with 30 min width was
taken as optimal one.
4. Correlation of the PC index with interplanetary electric field EKL and with
magnetic activity indices AL and Dst in course of magnetic storms
Correlation between the 30-min smoothed quantities Ekl, PC, AL, SymH has been estimated within the intervals (Tmax ± 2
days), where Tmax is moment of maximal storm intensity.
Correlation between EKL and PC index (R > 0.5) is observed for 558 events of 568 (98%) with delay time ~ 20-30 min.
Correlation between EKL and AL index (R > 0.5) is observed for 536 events of 568 (94%) with delay time ~ 30-50 min.
Correlation between EKL and SymH index (R > 0.5) is observed for 339 events of 564 (94%) with delay time ~ 70-100 min.
Correlation between PC and AL indices (R > 0.5) is observed for 567 events of 568 (99.8%) with delay time ~ 0-10 min.
Correlation between PC and SymH indices (R > 0.5) is observed for 426 events of 566 (75%) with delay time ~ 30-90 min.
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
0
40
80
120
160
200
N=564
0 20 40 60 80 100 120
0
40
80
120
160
Delay time, hours
N=339
PC / AL PC / SymH
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
0
40
80
120
160
200
N=556
0 20 40 60 80 100 120
0
40
80
120
160
Delay time, hours
N=426
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
0
40
80
120
160
200
N=568
0 20 40 60 80 100 120
0
40
80
120
160
Delay time, hours
N=536
-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
0
40
80
120
160
200
N=568
-20 0 20 40 60 80 100 120
0
40
80
120
160
Delay time, hours
N=567
Ekl / PC Ekl / AL Ekl / SymH PC / AL PC / SymH
Ekl / PC Ekl / AL Ekl / SymH
Correlation
Delay time
0 20 40 60 80 100 120
0
40
80
120
160
Num
ber
of e
vent
s
Delay time, hours
N=558
0,0 0,2 0,4 0,6 0,8 1,0
0
40
80
120
160
200
Num
ber
of e
vent
s
CorrelationCorrelation CorrelationCorrelationCorrelation
N=568
5. Three basic types of the magnetic storm development
-8 0 8 16 24 32
-2
0
2
4
6
8
-8 0 8 16 24 32
-1400
-1200
-1000
-800
-600
-400
-200
0
0 16 32 48 64
-100
-80
-60
-40
-20
0
20
0 16 32 48 64
-1400
-1200
-1000
-800
-600
-400
-200
0
0 16 32 48 64
-2
0
2
4
6
8
-16 -8 0 8 16 24 32
-100
-80
-60
-40
-20
0
20
-16 -8 0 8 16 24 32
-1400
-1200
-1000
-800
-600
-400
-200
0
-16 -8 0 8 16 24 32
-2
0
2
4
6
8
-16 -8 0 8 16 24 32
-1400
-1200
-1000
-800
-600
-400
-200
0
-16 -8 0 8 16 24 32
-2
0
2
4
6
8
-16 -8 0 8 16 24 32
-100
-80
-60
-40
-20
0
20
-8 0 8 16 24 32
-100
-80
-60
-40
-20
0
20
PC index
Ekl field
Storm duration, hours
08 November 1999
PC index
Ekl field
Storm duration, hours
31 October 2005
PC index
Ekl field
AL
ind
ex
(n
T)
PC
ind
ex,
Ekl
fie
ld (
mV
/m)
PC index
Ekl field
Classical storms (long growth phase)
Pulsed storms Combined storms
Different types of magnetic storm development
(short growth phase)
Classical storms
Sym
H ii
nd
ex
(n
T)
Storm duration, hours
23 July 2015
Storm duration, hours
26 September 2001
Three basic types of magnetic storms have been determined taking peculiarities of the Dst time evolution as a principle:
- “classical storms”, which demonstrate the main phase with one clearly expressed maximum of depression (42%),
- “pulsed storms”, which contain a series of periodically repeating depressions, lasting during many hours (27%),
- “combined storms”, which are superposition, with different weight, of classical and pulsed storms (31%).
It is significant that the time evolution of Dst index in all cases is predetermined by the PC index behavior.
6. Types of the magnetic storms and the solar wind drivers (ICME and SIR)
Conclusion:
Classical storms are related mainly to Interplanetary Coronal Mass Ejections (59%); the powerful magnetic storms (Dst>120 nT) are generated exclusively by ICME .
Pulsed storms are related (64%) to Stream Interaction Regions (SIRs) or Corotating Interaction Regions (CIRs), it is especially evident in case of weak magnetic storms (Dst<90 nT),
Combined storms are produced by joint action of ICME (32%) and SIR (44%), the weak storms (Dst<60 nT) being mainly related to SIR, the strong storms (Dst>120 nT) being related to ICME.
30-60 60-90 90-120 120-200 >200 nT 30-60 60-90 90-120 120-200 >200 nT
3
0
-
6
0
6
0
-
30-60 60-90 90-120 120-200 >200 nT
Storm intensity Storm intensity
Storm intensity
Analysis of magnetic storms for 1998-2009 was carried out basing on data presented in “List of Near-
Earth Interplanetary Coronal Mass Ejections (ICME) for 1997-2015” by I. Richardson and H.Cane
and “List of Stream Interaction Regions (SIR) for 1995-2009 by L. Jian.
20 40 60 80 100 120 140 160
0
10
20
30
40
50
60
20 40 60 80 100 120 140 160
0
10
20
30
40
50
60
20 40 60 80 100 120 140 160
0
10
20
30
40
50
60
Nu
mb
er
of e
ve
nts
Storm intensity, nT
ICME
Unknown
Total
Storm intensity, nT
SIR i
Unknown
Total
Storm intensity, nT
ICME
SIR
Unknown
Total
Classical storms Pulsed storms Combined storms
7. Relationship of the PC index to storm development
(classical storms with intensity Dstmax = 30-60 nT)
Development of classical magnetic storm is determined exclusively by time evolution of the PC index:
• Magnetic storm starts (T=0) when PC index steadily exceeds the threshold level PC~1.5 mV/m.
• Magnetic storm continues till PC index stays higher the threshold level, as a result, the storm growth phase duration is determined by time period with PC>1.5 mV/m.
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-60
-40
-20
0
20
40
Sym
H,
nT
Storm duration, hours
PC
, m
V/m
Storm duration, hours Storm duration, hours Storm duration, hours
PC
, m
V/m
Sym
H,
nT
Storm duration, hours
Classical storms with intensity Dstmax
= 30-60 nT
Storm duration, hours Storm duration, hoursStorm duration, hours
Growth phase 0-1 hour Growth phase 2-4 hours Growth phase 4-6 hours Growth phase 6-8 hours
Growth phase 8-10 hours Growth phase 10-12 hours Growth phase 12-14 hours Growth phase 14-16 hours
Relationship of the PC index to storm development (classical storms with intensity Dstmax = 60-90 nT)
Development of classical magnetic storm is determined by time evolution of the PC index:
• The maximal depression of magnetic field (storm intensity) follows, with time delay of ~ 1 hour,
to maximum of the 30-min smoothed РС index, irrespective of storm category and intensity.
• The higher the PCmax value, the larger is magnetic storm intensity (Dstmin).
-16 -8 0 8 16 24
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
10
-16 -8 0 8 16 24
-2
0
2
4
6
8
10
-16 -8 0 8 16 24
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
10
-16 -8 0 8 16 24
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24
-2
0
2
4
6
8
10
-16 -8 0 8 16 24
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32
-2
0
2
4
6
8
10
-16 -8 0 8 16 24 32
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32 40
-2
0
2
4
6
8
10
-16 -8 0 8 16 24 32 40
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32 40 48
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32 40 48 56
-2
0
2
4
6
8
10
Sym
H in
dex,
nT
Storm duration, hours
PC
inde
x, m
V/m
Storm duration, hours Storm duration, hours Storm duration, hours
Growth phase 10-12 hours Growth phase 12-14 hours Growth phase 14-16 hours
Growth phase 2-4 hours Growth phase 4-6 hours Growth phase 6-8 hours Growth phase 8-10 hours
Classical storms with intensity Dstmax
= 60-90 nTP
C in
dex,
mV
/mS
ymH
inde
x, n
T
Storm duration, hours Storm duration, hours Storm duration, nT
Relationship of the PC index to storm development (classical storms with intensity Dstmax = 90-120, 120-200 and >200 nT)
-16 -8 0 8 16 24
0
4
8
12
16
-16 -8 0 8 16 24 32
-120
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32
-120
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32
0
4
8
12
16
-16 -8 0 8 16 24 32
-120
-100
-80
-60
-40
-20
0
20
40
-16 -8 0 8 16 24 32
0
4
8
12
16
-16 -8 0 8 16 24 32
-120
-100
-80
-60
-40
-20
0
20
40
60
-16 -8 0 8 16 24 32
0
4
8
12
16
-16 -8 0 8 16 24 32
0
4
8
12
16
-16 -8 0 8 16 24 32
-200
-150
-100
-50
0
50
-16 -8 0 8 16 24 32
0
4
8
12
16
-16 -8 0 8 16 24 32
-200
-150
-100
-50
0
50
-16 -8 0 8 16 24 32
0
4
8
12
16
20
-16 -8 0 8 16 24 32
-400
-350
-300
-250
-200
-150
-100
-50
0
50
-16 -8 0 8 16 24 32 40
0
4
8
12
16
20
-16 -8 0 8 16 24 32 40
-400
-350
-300
-250
-200
-150
-100
-50
0
50
PC
inde
x, m
V/m
Sym
H in
dex,
nT
Storm duration, hours Storm duration, hours Storm duration, hours Storm duration, hours
PC
inde
x, m
V/m
Sym
H in
dex,
nT
Storm duration, hours Storm duration, hours Storm duration, hours Storm duration, hours
Growth phase 4-6 hours Growth phase 8-10 hours Growth phase 5-8 hours Growth phase 12-16 hours
Growth phase 4-6 hours Growth phase 6-8 hours Growth phase 8-10 hours Growth phase 10-12 hours
Classical storms, Dstmax
= 90-120 nT
Dstmax
= 120-200 nT Dstmax
>200 nT
• Both, the shorter action of the higher PC value and the longer action of the lower PC value, seem to be followed by magnetic storm of same intensity.
• In case of strong magnetic storm the storm beginning moment (T0) is often delayed owing to strong effect of the solar wind dynamic pressure
Relationship between the magnetic storm intensity DstMIN and preceding maximal
value РСMAX
The magnetic storm intensity (DstMIN) and preceding maximal value PCMAX are
connected by linear relationship irrespective of the storm power and length of the
storm main phase.
Relationship of the PC index to storm development
in case of pulsed and combined storms
In case of pulsed magnetic storms with periodically repeated PC and Dst oscillations, only relationship between the mean values (levels) of PC and Dst can be estimated.
In case of combined storms the main phase length is evidently controlled by duration of the increased PC value (PC>2 mV/m), but the relationship between the PCmax and Dstmin can be revealed only if magnitude of smoothed PC variation (ICME effect) essentially exceeds the oscillation amplitude (SIR effect); otherwise the relationship between the mean PC and Dst values is estimated.
-16 -8 0 8 16 24 32
-2
0
2
4
6
-16 -8 0 8 16 24 32 40
-2
0
2
4
6
-16 -8 0 8 16 24 32 40
-60
-40
-20
0
20
-16 -8 0 8 16 24 32 40 48
-2
0
2
4
6
-16 -8 0 8 16 24 32 40 48
-60
-40
-20
0
20
-16 -8 0 8 16 24 32 40 48 56
-2
0
2
4
6
-16 -8 0 8 16 24 32 40 48 56
-60
-40
-20
0
20
-16 -8 0 8 16 24 32 40
-2
0
2
4
6
8
-16 -8 0 8 16 24 32 40
-60
-40
-20
0
20
-16 -8 0 8 16 24 32 40 48 56
-2
0
2
4
6
8
-16 -8 0 8 16 24 32 40 48 56
-60
-40
-20
0
20
-16 -8 0 8 16 24 32 40 48 56
-2
0
2
4
6
8
-16 -8 0 8 16 24 32 40 48 56
-60
-40
-20
0
20
-16 -8 0 8 16 24 32
-60
-40
-20
0
20
-16 -8 0 8 16 24 32
-60
-40
-20
0
20
-16 -8 0 8 16 24 32
-2
0
2
4
6
8P
C in
dex,
mV
/m
Pulsed storms with intensity Dstmax
= 30-60 nT
Steady intensity (T<32h) Steady intensity (T>48h) Falling intensity (T<48h) Falling intensity (T>48h)
Duration T<16 h Duration T=24-28 h Duration T = 28-32 h DurationT = 36-40 h
Combined storms with intensity Dstmax
= 30-60 nT
Storm duration, hours Storm duration, hours Storm duration, hours
Storm duration, hours Storm duration, hoursStorm duration, hours
Sym
H,
nT
Storm duration, hours
Sym
H,
nT
Storm duration, hours
PC
inde
x, m
V/m
Summary of results
In course of magnetic storms the PC index strongly follows the time evolution of interplanetary electric field EKL (correlation R > 0.5 in 98% of events) with delay time ΔT ~ 20-30 min.
AL and SymH indices much better correlate with PC index than with EKL field (correlation between PC and AL indices is > 0.5 in 99.8% of events, with ΔT ~ 0-10 min, correlation between PC and SymH indices is > 0.5 in 75% of events, with ΔT ~ 30-90 min).
The necessary and sufficient condition for the storm beginning (i.e. for development of the geomagnetic field depression) is steady excess of PC above the threshold level, like to case of magnetic substorms. Just this moment controls the magnetic storm beginning.
The basic types of magnetic storms are the following: “classical storms”, related to ICME impact, with clearly expressed maximum of depression, “pulsed storms”, related to SIR impact, with periodically repeating PC and SymH oscillations, and “combined storms”, which seem to be effect of simultaneous ICME and SIR action.
In course of magnetic storms the SymH index generally follows the time evolution of the 30-min smoothed PC index irrespective of type and intensity of magnetic storms.
In case of classical storms the the maximal depression of geomagnetic field (i.e magnetic storm intensity Dstmin), follows, with delay ~60 min, the maximal value of smoothed PC (PCmax), the values PCmax and Dstmin being connected by linear relationship.
Conclusions:
The PC index is an adequate ground-based indicator of the solar wind energy incoming
into the magnetosphere. The PC index might be useful to monitor the space weather
and real state of the magnetosphere and to keep check whether or not the solar wind
fixed in Lagrange point L1 actually encounters the magnetosphere.
The historical PC indices (sets of data for 1997-2015) and current PCN and PCS indices
calculated on-line by magnetic data from stations Thule and Vostok are presented at
. web site: http://pcindex.org
Thank you for attention!