Forecast of Geomagnetic Storm based on CME and IP condition

R.-S. Kim1, K.-S. Cho2, Y.-J. Moon3, Yu Yi1, K.-H. Kim3

1Chungnam National University2Korea Astronomy and Space Science Institute3Kyunghee University

IHY WB 2-3 Sep. 23, 2009

Geomagnetic storm

What is a geomagnetic storm? Disturbances in the geomagnetic

field caused by gusts in the solarwind that blows by Earth.

Large negative perturbations of Dst index are indicative of a geomagnetic storm.

Causes of a geomagnetic storm Main origin: Coronal Mass ejection (CME)

Circumstance: Interplanetary condition

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Forecast of geomagnetic storm

Forecasts of a geomagnetic storm based on, IP condition for urgent warning

CME parameters for 2~3 days early warning

We use a two-step prediction for the storm forecast capability.

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CME and geomagnetic storm

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What parameters of CMEs control their geoeffectiveness? Only a small portion of the CMEs result in the geomagnetic

storms.

For front-side and large angular width events (1997~2003),

Source location (L) Earthward direction (D)

Initial speed (V) Magnetic field orientationof CME source region (M)

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Speed The CME speeds are roughly

correlated with the strength

of geomagnetic storms, but

even slow CMEs can trigger

geomagnetic storms.

Geoeffectiveness of CME parameters

Location The source locations of

geoeffective CMEs are

asymmetrical in longi-tude.

The offset 15° to the west gives the best

results.

Dst vs. distance from the offset

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Magnetic field orientation |Θ| ≤ 90° southward |Θ| > 90° northward

All CMEs associated with the super storms (Dst ≤ -200

nT) have southward mag-

netic field orientations.

Direction parameter The ratio of distance between

the shorter CME front and

the solar center to that of

the longer CME front. The direction parameter has

better correlation than the

other parameters.

Geoeffectiveness of CME parameters

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Geomagnetic storm prediction model

Comparison of their correlations with the Dst index

Direction parameter has the best correlation, but magnetic field orientation has the worst correlation.

We divide the CMEs into two groups according to their magnetic field orientation.

Empirical geomagnetic storm prediction model Formula to predict the geomagnetic storm strength (Dst index)

For southward events, For northward events,

Parameter cc

Location 0.25

Speed -0.29

Direction parame-ter

-0.60

MFO -0.12

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Evaluation of the storm prediction model

Forecast based on the storm prediction model The relationship between observed Dst index and predicted Dst

index for northward events (cc=0.81) is better than for southward events (cc=0.67).

Evaluation of the storm prediction model

Forecast based on the storm prediction model For 64 halo or partial halo CMEs associated with M and X class so-

lar flares, ‘yes’ prediction: predicted Dst ≤ -50 nT ‘yes’ observation: the occurrence of a geomagnetic storm The mean probability of geomagnetic storm is about 63%

(40/64) and 44 events are correctly forecasted (69%).

To improve the forecast capability of our model, we examine IP condition.

ObservedPredicted Yes No Total

Yes 36 16 52

No 4 8 12

Total 40 24 64

IP Condition of geomagnetic storm

Interplanetary parameters (Echer et al., 2008) What IP parameter has the strongest relation with storm strength

among the IP condition such as the magnetic field, electric field, solar wind speed and dynamic pressure.

Most strong storms (Dst ≤ -100 nT) have peak Bs between 10–20 nT, and peak Ey between 5–10 mV/m.

IP Condition of geomagnetic storm

Gonzalez -Tsurutani empirical criteria (1987) Bs ≥ 10 nT or Ey ≥ 5 mV/m for t ≥ 3 h

For the storms with Dst > -150 nT, 50% of the storms are sat-isfied.

For the storms with Dst ≤ -150 nT, 93% of the stronger storms are satisfied.

Our storm criteria is Dst ≤ -50 nT

We need to modify these criteria.

IP Condition of the 64 CME

Data Interplanetary Bz and Ey

ACE Magnetic Field 1-Hour Level 2 Data (B)

ACE/SWEPAM Solar Wind Experiment 1-Hour Level 2 Data (V)

E=-V×B Ey=-BxVz+BzVx

Bz minimum and Ey maximum

Bz ≤ -5 nT, Ey ≥ 3 mV/m

IP Condition for 64 CME

Duration time of Bz, Ey criteria t ≥ 2h

Forecast using IP criteria

IP criteria We select the criteria for moderate storms (Dst ≤ -50 nT)

Bz ≤ -5 nT or Ey ≥ 3 mV/m for t ≥ 2 h

For 64 events, 90% of the storms are in the IP criteria.

80% are correctly forecasted (51/64) (cf. CME parameter: 69%)

ObservedPredicted

Yes No Total

Yes 36 9 45

No 4 15 19

Total 40 24 64

ObservedPredicted

Yes No Total

Yes 36 16 52

No 4 8 12

Total 40 24 64

Forecast using CME and IP condition

For 64 events CME criteria: storm prediction formulae

IP criteria: Bz ≤ -5 nT or Ey ≥ 3 mV/m for t ≥ 2 hour

CME criteria IP criteria Forecast

YesYes Yes

No No

NoYes No

No No

ObservedPredicted Yes No Total

Yes 32 5 37

No 8 19 27

Total 40 24 64

80% are correct

CME criteria IP criteria Forecast

YesYes Yes

No Yes

NoYes Yes

No No

ObservedPredicted Yes No Total

Yes 40 20 60

No 0 4 4

Total 40 24 64

69% are correct

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Conclusions

Empirical geomagnetic storm prediction model Formulae to predict the geomagnetic storm strength (Dst index)

based on CME parameters For southward events, For northward events,

Empirical IP criteria For more better forecasts, we consider the IP conditions, since the

CME characteristics can change during its propagation.

Our empirical IP criteria: Bz ≤ -5 nT or Ey ≥ 3 mV/m for t ≥ 2 h 90% of the storms satisfy the IP criteria. For 20 exceptional events, 15 cases can be explained by the

IP conditions.

Forecast using CME and IP conditions We found that all geomagnetic storms occur when the CME condi-

tions or IP conditions are satisfied.

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CME parameters

Earthward direction parameter (D)

Advantages The direction parameter can be directly estimated from the

coronagraph observation

it can reduce the ambiguity of location caused by occulting disk.

It includes both of the CME propagation and angular effect of cone model.

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CME parameters

Magnetic field orientation angle θ (Song et al., 2006)

Magnetic reconnections between southward interplanetary mag-netic field and the northward directed geomagnetic field occur at the day side of magnetopause and then transport energy from the solar wind into the magnetosphere.

If we assume that the magnetic field orientation of a CME is pre-served during its interplanetary transit to Earth, we can expect that a CME with southward field orientation will cause a geomag-netic storm.

Forecast using IP condition

Limitation of the forecast using CME parameter We assumed that,

The effective acceleration ceases at some distance less than 1 AU and then CME travels with a constant speed to Earth (Gopalswamy et al, 2001).

The direction of the CME propagation (at C2 or C3 region) does not change through its travel to the Earth.

The magnetic field orientation of ICME has the same direction as in the CME source region.

The changes of CME characteristics increase the ambiguity in the storm forecast.

We used the plane-of-the-sky speed Error in predicted storm oc-currence time.

We use the IP condition to increase the storm forecast capabil-ity.