LWS CDAW Storms

GMU March 14-16 2005

Solar Sources of Large Geomagnetic Storms

During Solar Cycle 23

N. Gopalswamy (NASA/GSFC)

O. Michalek, H. Xie, S. Yashiro (CUA)

R. A. Howard (NRL)

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Objective• To identify the solar sources of large (Dst < -100 nT)

geomagnetic storms of solar cycle 23, as reported by WDC (http://swdcwww.kugi.kyotou.ac.jp/dstdir/index.htm), and

document their properties.• To identify CMEs corresponding to the magnetic clouds

reported during solar cycle 23 and study the strength of the geomagnetic storms associated with them.

• Solar sources examined: - Solar front-side coronal mass ejections (CMEs) from

SOHO/LASCO - Low-latitude coronal holes from Yohkoh/SXT; KPNO/He

10830; SOHO/EIT

CDAW WG1-WG4 Topic 2: What are the properties of the solar events that give rise to (most) majorGeomagnetic storms?

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GMU March 14-16 2005

Method

• Examine all the CMEs that occurred 1-5 days before the time of the storm.

• Identify the front-side CMEs. If more than one, pick the fastest and widest (such CMEs are likely to travel large distances from the Sun). Compile CME properties.

• For those storms without acceptable CMEs, examine low-latitude coronal holes

Flares

CMEs (height-time)

Dst

When CMEs are isolated, it is easyto associate a storm with its CMEAs in the 2003 11 21 storm

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GMU March 14-16 2005

A CME-related Storm

Largest storm of cycle 23; Dst = -472 nTGopalswamy et al. 2005 GRL

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Sometimes it is very difficult!

The extended main phase of the storm is probably due to many successive CMEs

Flares

CME height-time

Dst

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GMU March 14-16 2005

Overview• 80 intervals were identified from

Jan 1996 to December 2003. • 12 intervals had possible

fluctuation within the same storm (or extended storms due to multiple CMEs)

• 9 storms occurred during SOHO data gap

• Remaining 64 storms analyzed were analyzed

• 55 were CME-associated• 3 were probably CIR-related• 1 is being investigated for source• CIR –associated storms: Dst=-

105, -128, -117 nT• CME-related storms: Dst = -100 to

-472 nT Fig. 1 Distribution of storm strengths.The dates of top 5 storms are marked

LWS CDAW Storms

GMU March 14-16 2005

Longitudes of Storm-related CMEs

15W

N

S

WE

O Dst < - 200 nTO - 300nT < Dst < - 200 nT

O Dst < - 300 nT

East-West Asymmetry of solar sources is confirmed(Wang et al. 2002; Zhang et al. 2003)Larger storms (Dst < -200 nT) seem to occur Close to the disk center (±15 deg)

37/55 = 67%18/55 = 33%

2003/6/18 2000/4/7

2003/11/20

20001029

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East limb Event -145 nT

Only east limb event; small storm; shock running into preceding CME

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West limb Event - 288 nT

CME from W 66; sheath related storm

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Solar-cycle Variation of Storms & CMEs

The annual number of geomagnetic storms roughly tracks the number of front-side haloCMEs. However, there are moreHalo CMEs than the storms. When weakerstorms are included, the numbers become closer(Michalek et al. 2004). Also, for some asymmetric halosonly the shock arrives at Earth because the CMEsare heading almost orthogonal to the Sun-Earth line.

Year 1999 is unusual in that there were very fewintense storms even though there were many Front-side Halo CMEs.

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Properties of Storm–related CMEs

The speed distribution of storm-producing CMEs is similar to that of halo CMEs.The three near-limb CMEs appear as halos because of the disturbance above theopposite limb. At earth, the shock and sheath of these asymmetric halos are observed. The sheath contains southward B component causing the storms. The CMEs were full halos (69%) and partial halos (31%).

Storm Storm

Gopalswamy, 2004

001025 030618000404

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GMU March 14-16 2005

Dst-CME Speed Relationship

There is a reasonable correlation betweenCME speed and the strength of the DstIndex (correlation coefficient = -0.51).The scatter is very large.The single outlier with Dst ~ -100 nT is due to a limb event. The earthward speed is likely to be much smaller

Srivastava and Venkatakrishnan (2002)obtained the red line using 5 events, which does not seem to hold for the larger sample

Blue line is obtained when stormsassociated with magnetic clouds are considered (r=-0.55)

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LWS/CDAW on Geomagnetic Stormshttp://cdaw.gsfc.nasa.gov/

Three Geomagnetic Stormsassociated with Coronal holesDate of peak Dst1996/10/23 - 105 nT2002/11/21 - 128 nT2003/07/16 - 117 nT

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CHs Associated with Large (Dst < -100 nT) Geomagnetic Storms

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Microwave Enhancement

104 K disk

AR

Filament

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2003/07/15

03/07/15 19:06

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1996/10/20

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2002/11/20

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Storms Associated with Magnetic Clouds

• Relaxing the Dst < -100 nT criterion we considered magnetic cloud events alone and identified the associated front-side CMEs. There were 85 MCs observed by Wind. Only 66 had overlap with SOHO data

• The Dst – Vcme correlation similar.• In addition to CME speed, the magnetic field

and its orientation are also important. For example, the Dst-VB correlation (r=0.81) is much better than the Dst-V correlation (r = 0.55)

• Bz and total B have similar correlation with Dst

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Storms Associated with Magnetic Clouds

• Since CMEs are the near-Sun manifestations of magnetic clouds, the Dst – Vcme relationship should mimic the Dst-Vmc relationship.

• The Dst-Vmc correlation cofficient(0.74) is much larger than that for Dst-Vcme (0.55)

• However, the Dst-VmcB correlation (r=0.79) is very similar to the Dst-VcmeB correlation (r = 0.81)

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Summary• Most (95%) of the intense (Dst < -100 nT) geomagnetic storms are

associated with front-side fast and wide CMEs; the remaining (5%) are associated with corotating interaction regions. This confirms previous studies (e.g. Gosling, 1993)

• The properties of Storm-related CMEs are similar to those of halo CMEs. • Most of the severe storms (Dst < -300 nT) were associated with CMEs

originating from close to the disk center (±20 deg). • There are generally more geoeffective CMEs originating from the western

hemisphere of the Sun. However, the largest storm of cycle 23 originated from E18 (2003 11 21 storm due to the 2003 11 18 CME).

• Asymmetric halos are not very geoeffective because only the shock flanks and/or shock sheaths arrive at Earth.

• In addition to the kinematic properties of CMEs, we must consider magnetic properties for a better understanding of their geoeffectiveness. This involves devising ways of estimating B when the CMEs are still near the sun.

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Topic 2: Answers

• Location on the Sun: Center-West• Flares: Bigger on the average• CME properties: Faster & Wider on the average• SEP events: Overlap in longitude; Avg SEP-

event longitude more western• Dst-Vcme correlation: 0.55, similar to Vicme-Dst• Dst – VcmeB highest correlation 0.81• CIR storms: 3/76 ~ 4%

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References• Gopalswamy, 2004, in ASSL series, ed. G. Poletto & S. Suess,

chapter 8, in press.• Gosling, J. T., 1993, JGR, 98, 18937• Michalek, G. et al. 2004, under preparation• Srivastava, N. & Venkatakrishnan, P. V., 2002, GRL 29, 1287• Wang, Y.-M. et al., 2002, JGR, 107, 1340• Zhang, J. et al., 2003, ApJ, 582, 520

LWS CDAW Storms

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Longitudes of Storm-related CMEs

15W

N

S

WE

O Dst < - 200 nTO - 300nT < Dst < - 200 nT

O Dst < - 300 nT

East-West Asymmetry of solar sources is confirmed(Wang et al. 2002; Zhang et al. 2003)Larger storms (Dst < -200 nT) seem to occur Close to the disk center (±15 deg)

37/55 = 67%18/55 = 33%

SEP

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Newton 1943

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B and Bz have similar correlation with Dst

r = 0.74 for |Bz|

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Worst-Case Scenario• There were only 25 of

the 8000 CMEs had speed > 2000 km/s; only 4 with speed > 2500km/s

• Inferred speeds of historical events is < 2800 km/s

• CMEs probably have a speed limit of ~ 3000 km/s

• This limit arises from the maximum energy extractable from an active region (<1034 erg)

• The Sun-Earth Travel time of shocks has a limit of ~ half a day

t = a.bv + ca= 151.002, b=0.998625, and c=11.5981