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