Cynthia López-Portela and Xochitl Blanco-Cano Instituto de Geofísica, UNAM A brief introduction:...

15
Cynthia López-Portela and Xochitl Blanco-Cano Instituto de Geofísica, UNAM A brief introduction: Magnetic Clouds’ characteristics The study: Event types in this study and their geoeffectiviness Δt, -Bzmax, and (-VBz)max in relation with the Dst index Correlation between (- VBz)max and SHINE WORKSHOP 2005 STUDENT’S SESSION Magnetic Clouds and their Geoeffectiviness

Transcript of Cynthia López-Portela and Xochitl Blanco-Cano Instituto de Geofísica, UNAM A brief introduction:...

Cynthia López-Portela and Xochitl Blanco-Cano

Instituto de Geofísica, UNAM

A brief introduction:•Magnetic Clouds’ characteristics The study:•Event types in this study and their geoeffectiviness•Δt, -Bzmax, and (-VBz)max in relation with the Dst index•Correlation between (-VBz)max and Dst index•Solar cycle 23•Conclusions SHINE WORKSHOP 2005

STUDENT’S SESSION

Magnetic Clouds and their Geoeffectiviness

Magnetic Clouds (MCs) are perturbations in the interplanetary medium often observed in situ by spacecraft. These perturbations satisfy the next three characteristics:

(1)the magnetic field is higher than the ambient solar wind field

(2)the magnetic field rotates smoothly on a plane, and

(3) the temperature is lower than in the solar wind.

MCs are good candidates to study their geoeffectiviness

MC

Shock

-104 nT

In this event the storm is caused by a MC which has a long-lasting and well order magnetic field in the south Bz component

STORM CAUSED BY MC

B (nT)

Lat. angle

Temperature (K)

Density (p/cm3)

Speed (km/s)

Pressure (nPa)

Plasma β

Dst (nT)

OMNIWeb,Wind (20-23/04/2001)

STORM CAUSED ONLY BY SHEATH

REGION

MC

-110 nT

In our data set, sheath regions with south Bz component (9 events) generate Dst dips of around -127nT

In this event the storm is caused by the sheath region and not by the MC

ShockB (nT)

Lat. angle

Density (p/cm3)

Temperature (K)

Speed (km/s)

Pressure (nPa)

Plasma β

Dst (nT)

OMNIWeb, Wind (05-10/11/1997)

MC

-43 nT-68 nT

TWO- STEP STORMS EVENT

A two-step storm event has the next three conditions (Kamide et al., 1998):

(1) the first dip must be less than -30nT, and be separated from the second dip by more than 3 hours

(2) the later dip must be greater than the previous one, and

(3) there must be no storm sudden commencement (SSC) between the two dips

In this event sheath and MC are geoeffective causing a two-step storm event.

ShockB (nT)

Lat. angle

Temperature (K)

Density (p/cm3)

Speed (km/s)

Pressure (nPa)

Plasma β

Dst (nT)

OMNIWeb, Wind (18-22708/1998)

Two-step storm events

Dst caused by sheath Dst caused by MC MF Distribution -55 -68 S S S -105 -150 S S S -30 -83 S N S -70 -105 S N S -43 -68 S S N -120 -152 S S N

In the table the first S indicates that the sheath has a south Bz component, the second letter indicates the MF distribution in the leading part of MC, and the last letter indicates the MF distribution in the trailing part of MC.

Two-step storms do not show a prefence related to the magnetic field (MF) distribution.

0

1

2

3

4

5

6

7

8

9

0 -30 -60 -90 -120 -150 -180 -210

Dst (nT)

# E

VE

NT

S

Magnetic Clouds with south Bz

0

1

2

3

4

5

6

7

8

9

0 -30 -60 -90 -120 -150 -180 -210 -240

Dst (nT)

# E

VE

NT

S

Magnetic Clouds with south-north Bz

Dst frequency

Averaged storm intensity (minimum Dst) = -90 nT

MCs with a south Bz configuration are more geoeffective than MCs with south-north Bz configuration.

Averaged storm intensity (minimum Dst) = -81 nT

15 events 24 events

0 -29

9 eventsMCs with South-North Bz + Sheath with North Bz

0

1

2

3

4

5

6

0 -30 -60 -90 -120 -150 -180 -210 -240

Dst (nT)

# E

VE

NT

S

12 events

0

1

2

3

4

5

6

0 -30 -60 -90 -120 -150 -180 -210 -240

Dst (nT)

# E

VE

NT

S

Average Dst = -58.3 nT Average Dst = -106.2 nT

Sheath regions with south Bz configuration enhance the MC-magnetosphere interaction. This is more notorious MCs that have a S-N configuration.

MCs with South-North Bz + Sheath with South Bz

0

1

2

3

4

5

6

0 -30 -60 -90 -120 -150 -180 -210 -240Dst (nT)

# E

VE

NTS

0

1

2

3

4

5

6

0 -30 -60 -90 -120 -150 -180 -210 -240

Dst (nT)

# E

VE

NTS

Dst frecuency

Average Dst = -84.4 nT Average Dst = -101 nT

MCs with South Bz + Sheath with North Bz MCs with South Bz + Sheath with South Bz

7 events 7 events

0

2

4

6

8

10

0 5 10 15 20 25 30 35

time (hour)

# E

VEN

TS

-100 nT < Dst < = -20 nT

0

2

4

6

8

10

0 5 10 15 20 25 30 35time (hour)

# E

VE

NT

S

Dst < = -100 nT

Δt is the temporal interval where the MCs and sheath regions have a south Bz component.

Duration of south Bz component does not determine alone the strength of the geomagnetic storm.

Δt parameter for MCs and sheath regions

0

5

10

15

20

0 5 10 15 20 25 30-Bzmax

# E

VE

NT

S

-100 nT < Dst < = -20 nT

0

5

10

15

20

0 5 10 15 20 25 30-Bzmax (nT)

# E

VE

NT

S

Dst < = -100 nT

-Bzmax has higher values for extreme storms and lower values for low to high intensity storms.

-Bzmax parameter for MCs and sheath regions

0

5

10

15

0 5 10 15 20 25

(-VBz)max (mV/m)

# E

VE

NT

S

0

5

10

15

0 5 10 15 20 25

(-VBz)max (mV/m)

# E

VE

NT

S

-100 nT < Dst < = -20 nT

Dst < = -100 nT

High values for (-VBz)max are related to extreme storms and low values are related to medium storms.This is in agreement with previous results, i.e. R. K. Burton et al., 1975; Yuming Wang et al., 2003; and X. H. Xue et al., 2004.

(-VBz)max for MCs and sheath regions

MCs

CC = -0,69

-300

-250

-200

-150

-100

-50

0

0 2 4 6 8 10 12 14 16 18 20 22 24

(-VBz)max (mV/m)

Dst

(n

T)

It shows that the most intense storms are related to high values of (–VBz)max, and vice versa.

The correlation shows that the most intense storms are related to high values of (-VBz)max, and vice versa.

Sheath regions for the 2-step storms events

CC = -0,707

-300

-250

-200

-150

-100

-50

0

0 2 4 6 8 10 12 14 16 18 20 22 24

(-VBz)max (mV/m)

Dst

(nT)

Correlation between (-VBz)max and Dst index

Sheath regions

0.0

25.0

50.0

75.0

100.0

125.0

150.0

175.0

Ene-96 Dic-96 Dic-97 Dic-98 Dic-99 Dic-00 Dic-01 Dic-02

Su

nsp

ot

Nu

mb

er

Jan-96

Solar cycle 23

Comparison between the sunspot numberand temporal evolution of the Dst index for geomagnetic storms caused by MCs and sheath regions.

Jan-96

-300

-250

-200

-150

-100

-50

0

Ene-96 Dic-96 Dic-97 Dic-98 Dic-99 Dic-00 Dic-01 Dic-02

Dst

(n

T)

Jan-96 Dec-96 Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02

MCs and sheath regions

Jan-96 Dec-96 Dec-97 Dec-98 Dec-99 Dec-00 Dec-01 Dec-02

CONCLUSIONS

• MCs with south Bz configuration are more geoeffective than MCs with s-n Bz configuration.

• Sheath regions with south Bz enhance the MC-magnetosphere interaction. • In some cases the sheath is the only region which interacts geoeffectivily.

• Two-step storm events are generated by a sheath region with south Bz followed by a MC with south Bz somewhere inside the cloud. • When MC or a sheath region has a south Bz component for a long time it does not mean it will cause a storm of extreme intensity.

• –Bzmax has high values for extreme storms and low values for moderate storms.

• High values of (-VBz)max are related to extreme storms and low values are related to moderate storms.

• The correlation between (–VBz)max and Dst index for MCs is CC = -0.69 and for sheath regions is CC = -0.71.

• The most intense storms caused by MC and sheath regions occur during the solar maximum.

EVENT TYPES IN THIS STUDY:

1) MC with south or south-north Bz

2) MC with south Bz + sheath region

3) MC with south-north Bz + sheath region