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Shock wave formation heights using

2D density and Alfvén maps of the corona

ABSTRACT


Coronal

shock

waves

can

produce

decametric

radio

emission

known

Type

II

bursts,

which

are

effected

by

plasma

density

and

Alfvén

speed

variations

in

the

solar

corona
.

To

date,

1
D

radial

models

of

densities

and

Alfvén

speeds

have

been

used,

but

these

are

not

appropriate

for

non
-
radial

shocks

along

the

flanks

of

CMEs
.



Here,

2
D

density

maps

were

derived

using

images

from

the

Solar

Dynamic

Observatory/Atmospheric

Imaging

Assembly

(SDO/AIA)

and

the

Solar

and

Heliospheric

Observatory/Large

Angle

and

Spectrometric

Coronagraph

(SOHO/LASCO)
.

These

were

then

combined

with

potential

field

source

surface

(PFSS)

extrapolations

to

construct

2
D

Alfvén

speed

maps

of

a

region

of

the

corona

where

a

CME

and

Type

II

radio

burst

was

observed
.



We

show

that

the

a

shock

was

formed

along

the

leading

flank

of

the

CME

at

a

height

where

the

speed

of

the

CME

front

becomes

super
-
Alfvénic
.

INTRODUCTION

Type

II

radio

bursts

are

signatures

of

coronal

shocks
.

They

appear

as

slow

drifting

features

moving

towards

lower

frequencies

in

dynamic

spectrum,

which

results

from

the

plasma

emission

generated

by

the

super
-
Alfvénic

shocks
.


To

date,

1
D

radial

density

models

such

as

Newkirk

(
1961
)

and

Mann

et

al
.

(
1999
)

have

been

used

to

calculate

Type

II

kinematics
.

However,

due

to

the

solar

corona

variability,

actual

density

observations

are

crucial

to

obtain

the

correct

shock

kinematics
.


In

this

work,

a

new

independent

observational

method

to

observe

the

mean

coronal

density

values

and

the

Alfvén

speed

in

the

two

dimensional

plane
-
of
-
sky

(POS)

is

presented
.

These

2
D

maps

are

then

used

to

calculate

the

kinematics

and

the

direction

of

propagation

of

a

Type

II

radio

burst

observed

on

2011

September

22

(Fig
.
1
)
.

Pietro Zucca, Eoin P. Carley, D. Shaun Bloomfield, Peter T. Gallagher

Figure

4

Height

time

at

different

propagation

angles

of

the

EUV

feature

observed

in

SDO/AIA

running

difference

images

(red)

and

of

Type

II

radio

burst

from

different

non
-
radial

density

profiles

(blue)
.

Insets

show

profiles

separated

by

10

,

wi t h

pr of i l e

used

mar ked

wi t h

in

red
.

(a)

Height

time

for

a

“low
-
flank”

region

(
20

from

limb

tangent)
.

( b )

An

“upper
-
flank”

region

(
60

from

limb

tangent)
.

(c)

Radial

profile

(
90

from

the

tangent)
.

Trinity College Dublin

School of Physics, Trinity College Dublin, Dublin 2, Ireland

www.rosseobservatory.ie

90
15
2.5
0.5
500
2500
5000
1200
600
300
150
0.1
(a)
(b)
(c)
1.0e+05
6.0e+05
4.0e+06
2.5e+07
1.5e+08
1.0e+09
E
l
e
c
t
r
o
n

D
e
n
s
i
t
y

[
c
m
-
3
]

M
a
g
n
e
t
i
c

F
i
e
l
d

[
G
]

A
l
f
v
é
n

S
p
e
e
d

[
k
m

s
-
1
]

F
igure

3

(a)

2
D

electron

density

map

measured

on

2011

September

22
.

(b)

Magnetic

field

strength

from

PFSS

and

on

(c)

Alfvén

speed

map

obtained

from

the

electron

density

and

magnetic

field

strength

values
.

DATA ANALYSIS


Density

maps

were

derived

from

SDO/AIA

(
1

1
.
3

R

)

and

SOHO/LASCO

(
2
.
5

5

R

)
.

For

1
.
3

2
.
5

R


a

combined

plane
-
parallel

and

spherically
-
symmetric

model

was

used

(
Fig
.

2
)
.

An

Alfvén

speed

map

was

obtained

from

PFSS

magnetic

fields

within

+/
-

5

degrees

of

the

POS

and

the

2
D

density

map

(Fig
.

3
)
.


The

Type

II

radio

burst

kinematics

were

calculated

using

non
-
radial

density

profiles

from

the

2
D

map
.

The

CME

kinematics

were

obtained

from

running

difference

height
-
time

plots

from

SDO/AIA

(
131

Å
)
.

A

comparison

of

the

CME

and

Type

II

kinematics

for

3

different

propagation

angles

is

shown

in

Fig
.

4
.


Figure

2

Coronal

density

model

constrained

by

SDO/AIA

and

SOHO/LASCO

electron

densities
.

The

plane

parallel

solution

(blue)

models

active

region

densities

while

the

spherically

symmetric

solution

(red)

models

the

quiet

Sun
.

The

combined

solution

(orange)

is

used

to

model

densities

at

1
.
3
-
2
.
5

R

.

RESULTS AND CONCLUSIONS


A

new

method

to

derive

2
D

coronal

density

and

Alfvén

speeds

is

presented
.



These

were

used

to

find

evidence

for

a

shock

formed

along

the

“upper
-
flank”

of

a

CME
.



In

addition,

temporal

agreement

with

the

start

of

the

radio

emission

signature

and

the

time

where

the

propagating

feature

reaches

a

super
-
Alfvénic

speed

was

found,

suggesting

that

the

Type

II

emission

was

triggered

by

the

erupting

CME
.


REFERENCES

Aschwanden

M. J.,
Boerner

P.,
Schrijver

C. J.
,
Malanushenko

A
.,
Solar Physics
,

Volume
283
, Issue 1, pp.5
-
30, 2011

Benz A.O.,
Monstein

C., Meyer H.,
Solar Physics
,

Volume
226
, Issue 1, pp.143
-
151, 2005

Newkirk
, G., Jr
.,

Astrophysical

Journal
,

Volume
133
, p.
983, 1961

Mann
, G., Jansen, F.,
MacDowall
, R. J., Kaiser, M. L., &
Stone
, R. G.
,
Astronomy

and
Astrophysics
,
Volume
348
, p.614
-
620, 1999

Zucca P., Carley

E
. P
., McCauley

J
., Gallagher

P. T.
,
Monstein

C.
,
McAteer

R. T. J
.,
Solar Physics
, Volume
280,

Issue 2, pp.591
-
602

Figure

1

Dynamic

spectrum

observed

with

e
-
Callsito

(Benz

et

al
.

2005
)

at

the

Rosse

Solar

Terrestrial

Observatory

(RSTO
;

Zucca

et

al
.

2012
)

of

the

2011

September

22

Type

II

radio

burst
.

This

burst

shows

fundamental

(F)

and

harmonic

(H)

emission
.

Electron Density

Magnetic Field

Alfven Speed