Chapter 5 The Magnetosphere - PowerPoint Presentation

Slide1

Chapter 5 The Magnetosphere

5.1

Boryau Hsupeng

徐彭伯堯

5.2

Zhong

-You Sun

孫忠佑

5.3

Hua-Shan Shi

施驊珊

5.4

Zheng-Xian Chen

陳政憲

Slide2

5.1 Solar Wind – Magnetosphere Interaction

5.1.1 Structure of the Magnetosphere

Slide3

A sketch of the magnetosphere

modified from

Kivelson

and Russel (1995)

Slide4

 Cross section of the global magnetosphere, showing the magnetopause current system, the cusps, the magnetotail, and the magnetotail current sheet [Hughes, 1995]

Diamagnetic current

 

 

 

Slide5

5.1.2 Fundamental Physics of the Magnetosphere

Slide6

Slide7

Slide8

Slide9

Slide10

Slide11

Slide12

Slide13

Slide14

Slide15

5.1.3 Boundary of Magnetosphere (Magnetopause)

Slide16

Slide17

Slide18

Shocks : -Fast shock

-Slow shock

-Intermediate

shock

Magnetic-field

lines for

fast, intermediate, and slow

shocks

,

 

,

 

 

 

 

Slide19

Introduction: MHD Discontinuities

Discontinuities:

-Contact discontinuity -

Tangential

discontinuity -

Rotational discontinuity

 

 

 

 

 

 

 

 

Slide20

Slide21

5.1.4 Magnetic Reconnection

Slide22

Slide23

5.2 Magnetospheric Convection

Slide24

5.2.1 Magnetospheric Convection and Tail Formation

Tail Formation

The

primary factor involved in the formation of the magnetotail is the dynamic pressure exerted by the solar wind

, which

essentially determines its shape.

The magnetic

reconnections

that occur at the dayside magnetopause is the second factor.

Slide25

LobesIn expressing the physical quantities between the Sun and Earth, we use the geocentric solar

magnetospheric

coordinate system(GSM),which has its origin point the center of the Earth.

Slide26

5.2.2 Formation of Plasma Sheet and Plasma Flow

Slide27

Plasma Sheet

Located in the equatorial region of the magnetotail.

The plasma pressure within the plasma sheet is at equilibrium with magnetic pressure in the lobes at the plasma sheet boundary.

Slide28

Plasma Flow within the Plasma Sheet

The closed magnetic field lines on the Earth tend to the the most stable configuration,which is a dipole magnetic field.

This action initiates the movement of plasma toward Earth, and a flow toward Earth is always present in the plasma sheet.

Slide29

5.2.3 Drift of Hot Plasma Particles and the Plasmapause

Slide30

Drift of Charged Particles within the Magnetosphere

Assume that gyration effects around the magnetic field average out.

Normally refer to the region within about 10 earth radii as the inner magnetosphere. The magnetic field is approximated by a dipole magnetic field.

Slide31

Drift in the Equatorial Plane

The

electric field is assumed to consist of two part:

1. Uniform (E0) which runs from

dawnside

to

duskside

.

2. The co-rotational electric field associated with the rotation of the Earth.If the drift given in eq.(5.17)is expressed by an equivalent potential ,

feff

, we can write it as

Slide32

Using feff

,

the particle drift velocity V

D

is written as

Cold plasma approximation becomes:

Hot plasma approximation becomes:

Slide33

The Alfven Layer

Slide34

Plasmapause

Slide35

5.2.4 Ionospheric

Convection and Field-Aligned Current

Slide36

Low Latitude Boundary Layer

Slide37

Ionspheric Convection

Slide38

Field-Aligned Current

Slide39

5.3 Magnetospheric

Substorm

5.3.1 The Development of Auroral Currents

5.3.2 Substorms

Slide40

The c

omparison

of the geomagnetic disturbance intensity between magnetic storm and magnetic substorm

.

(up) Hawaii (down) Alaska

Stor

m

Substorms

http://www.ss.ncu.edu.tw/~lyu/

Overview

Slide41

5.3 Magnetospheric

Substorm

5.3.1 The Development of Auroral Currents

5.3.2 Substorms

Slide42

5.3.1 The Development of Auroral

Currents

Akasofu,

(2015)

Daytime:

Magnetic latitude of 75°

Night:

Extending to 65

°

A

nnual

frequency of auroral sightings if visibility

The

belt in which auroral arcs actually lie in the

geomag.

Auroral Zone

Auroral

Oval

Slide43

Diffuse

Aurora

(

擴散極光

)

http://spaceweather.com/

Discrete

Aurora

(

分立極光

)

Bright

D

efinite

lower

border

Stretching

high into the sky like

curtains

Moving

fast

Before midnight

Quite faint

D

iffuse glows

Spreading

over a wide

area

Little motion

Midnight & lasts into the morning hours

Slide44

http://www.ss.ncu.edu.tw/~lyu/

Van Allen Radiation Belt

Slide45

Van Allen R

adiation Belt

https://en.wikipedia.org/wiki/Van_Allen_radiation_belt

Slide46

2 4

Fig. 5.11 (c) Illustrates the convection structure in the polar ionosphere. (P. 143)

 

 

 

 

Substorm Current System

Westward

Auroral Current

Eastward

Auroral

Current

Slide47

Eastward

Electrojet

Westward

Electrojet

http://www.ss.ncu.edu.tw/~lyu/

Partial Ring Current

Slide48

Slide49

Fig. 5.14 Auroral breakup and field-aligned current [Obayashi, 1970] (P.146)

Substorm:

Growth Phase

Expansion Phase

Recovery Phase

Slide50

Plasma Sheet

Slide51

Fig. 5.14 Auroral breakup and field-aligned current [Obayashi, 1970] (P.146)

1 km/s

Strong Westward Current

Upward Current

→

Lead

the

surge

up

Downward Current

→

Following the surge

(wide area)

Slide52

5.3 Magnetospheric

Substorm

5.3.1 The Development of Auroral Currents

5.3.2 Substorms

Slide53

5.3.2

Substorms

Baumjohan

&

Treumann

, 1996

Slide54

5.3.2

Substorms-Growth Phase

Southward Interplanetary

Magnetic Field

Magnetopause

Magnetic

Reconnection

http://www.ss.ncu.edu.tw/~lyu/

Magnetic Tail

Stronger dawn-dusk magnetic field

Thinner Plasma Sheet

Stronger cross-tail

current

Slide55

Fig. 5.16 Formation of a near-earth neutral line (P.149)

Plasma flow

Formation of the Near Earth Neutral Line (NENL)

Plasmoid

A

coherent structure of plasma and magnetic fields.

20

 

DNL (Distant Neutral Line)

80~140

 

Slide56

Fig.

5.15 Magnetotail current structure during

Substorm expansion phase (P.148)

Field-aligned

Currents

Inner edge

of tail current

Current Disruption Model

65

°

~70

°

???

Slide57

L-value (L-shell)

 

L-value often describes the set of magnetic field lines which cross the Earth's magnetic equator at a number of Earth-radii equal to the L-value.

https://en.wikipedia.org/wiki/L-shell

Slide58

Fig.

5.15 Magnetotail current structure during

Substorm expansion phase (P.148)

Field-aligned

Currents

Inner edge

of tail current

Current Disruption Model

L = 6 ~ 8

L = 6 ~ 8

 

Slide59

http://themis.ss.ncu.edu.tw/CD_and_MR.htm

Auroral breakup

Magnetic reconnection

Current Disruption Model

Slide60

Fig.

5.15 Magnetotail current structure during

Substorm expansion phase (P.148)

Field-aligned

Currents

Inner edge

of tail current

Current Disruption Model

L = 6 ~ 8

Dipolarization

Substorm current wedge

One million amperes (A

)

Anti-sunward / Earthward flow

Electrical resistance of the plasma sheet becomes very high

Slide61

Ground Level

Geostationary orbit

Near-tail

Mid-tail

Far-tail

Growth

Phase

Oval

expansion,

Quiet arc,

DP-2

(Hall current↑)

Magnetic field line stretching

Plasma sheet grows thinnerEarthward flow (bursty bulk flow)Plasma sheet grows thinnerFlow in anti-sunward direction

Expansion PhaseAuroral breakup

Development of westward auroral current,DP-1 (Substorm current wedge)Auroral Bulge, WTSDipolarization of the magnetic field

Increase in particle flux (injection)Breakup, Earthward flow (bursty bulk flow)Large B fluctuation

NENL formation, Plasmoid developmentFlow in anti-sunwardRecovery Phase

Double oval

Increase in particle flux

Plasma sheet grows thicker

Plasma sheet grows thicker

Plasmoid passing

Table 5.1 Substorm phases and regional effect (P.147)

Slide62

Theta

Aurora (

Transpolar Arc)

http://www.sci-news.com/space/science-high-latitude-theta-aurora-02361.html

Theta

aurora over Antarctica extends

to

the polar cap

from local midnight, across the polar cap

and joins with the auroral oval at local noon.

Credit:

Dynamics Explorer-1 / University of

Iowa

Some controversy concerns the cause of

this

auroral configuration, which

occurs at very high latitudes, when the IMF is northward.

Slide63

Fear

et al

.

,

Science (

2014

)

“

The

excellent

correspondence

between the transpolar arc and the trapped closed flux at high altitudes provides very strong evidence of the trapping mechanism as the cause of transpolar arcs

.“

Slide64

State transition models of the Substorm onset

The thermal catastrophe model (

Goertz

& Smith, 1989)

→

Alfvén

wave

provides

the

heating of plasma sheet

particles.

Alfvén

layer model (Atkinson, 1991)→ Magnetosphere-ionosphere coupling

→ Northward re‐turning of the IMF

MHD model (Tanaka, 2000)→

Northward re‐turning of the IMF

→ The flux ejected from the NENL

And more...

Slide65

5.4 Geomagnetic Storms

Slide66

L value

The L value is defined as:

r

eq

(

wikipedia

)

θ

Slide67

Dst index

Main phase

(P.150 Fig 5.17)

Recovery phase

Slide68

Ring current

(http://www.angelfire.com/rnb/pp0/magnetosphere.html)

Slide69

(5.8)

In

eq

(5.8), setting d/

dz

= 0 leads to -

▽

p + J X B = 0. which indicates a balance between the pressure gradient and J x B. In the regions of L > 3, plasma pressure decreases with increasing distance from the Earth. Since the geomagnetic

field

is pointing northward, the equatorial ring current flows

westward, and this ring current causes a drop in the geomagnetic field strength. In regions where L < 3, the pressure gradient is reversed, causing an eastward current, but the overall scale of this current is smaller. and becomes overwhelmed by the aforementioned westward current. As a result,

Dst, values become negative.

Slide70

The effect of magnetic field to extend into the middle and lower latitude. Some time after the commencement of the magnetic storm, the plasma density in the F region starts to decrease in a process that can last for several days. This decrease in density affects shortwave and other communications in what is referred to as a

communications storm

.

Each magnetic storm has its own set of characteristics, but in some storms the Earth becomes enveloped in a thick plasma cloud. In such cases, the intensity of the cosmic rays reaching ground level abruptly weaken. These events are generally known as

cosmic ray storms

, and their characteristics depend on the phase of the magnetic storm.

Slide71

(https://www.swpc.noaa.gov/products/planetary-k-index)

K index

Slide72

Kp

index

A index

(https://www.swpc.noaa.gov/products/station-k-and-indices)

K

0

1

2

3

4

5

6

7

8

9

a

p037

15274880

140240400

Slide73

AE = AU-AL

AO = (AU+AL)/2

AE index

(http://wdc.kugi.kyoto-u.ac.jp/ae_realtime/201810/index.html)

Slide74

Radiation Belt

Inner Belt (L~1.5-2.5):

The primary species in that belt is the high-energy protons (>50

MeV

). With regards to the electrons in the inner belt, very few exceed energy levels of 5

MeV

, and most range from 1 to 5

MeV

.Outer Belt (L~4-6):

The primary species is the electrons with energies of over 1 MeV. the electron radiation peak exists at about L = 4. The intensity of the electrons in the outer belt decrease with an increase of L.

(http://fp7-spacecast.eu/)

Slide75

Effect of Magnetic Storms

(P.152 Fig 5.18)

Slide76

(5.24)

A number of attempts have been made to describe the anomalous increase in the outer belt electrons during a magnetic storm. One theory is that the convective electric

field

in the magnetosphere injects high-energy particles into the near-earth region. However, as these particles approach the Earth, the gradient of the magnetic field causes an increase in drift motion. The ratio of gradient drift velocity (V0) to electric

field

drift velocity (VF) permits the evaluation of this effect.