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

olegtro@aari.ru

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

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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

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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

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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

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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.

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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

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-16 -8 0 8 16 24 32 40 48

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40

-16 -8 0 8 16 24 32 40 48 56

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0

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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)

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4

8

12

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0

4

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0

50

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0

4

8

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20

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-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.

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20

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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!