Space Weather: Magnetic Storms 31 October 2011

Space Weather: Magnetic Storms

31 October 2011

William J. Burke

Air Force Research Laboratory/Space Vehicles Directorate

Boston College Institute for Scientific Research

DMSPC/NOFS

CRESS

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Space Weather Course Overview

• Lecture 1: Overview and Beginnings

• Lecture 2: The Aurorae

• Lecture 3: Basic Physics (painlessly administered)

• Lecture 4: The Main Players

• Lecture 5: Solar Wind Interactions with the Earth’s Magnetic Field

• Lecture 6: Magnetic Storms

• Lecture 7: Magnetic Substorms

• Lecture 8: Magnetosphere – Ionosphere Coupling

• Lecture 9 The Satellite Drag Problem

• Lecture 10: Verbindung (to help make up for your rash decision not to take

Wollen Sie Deutch Sprechen?)

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Space WeatherMagnetic Storms

• Last week we looked at the Sun as the source of Earth’s space weather.

• Pressure gradients in the corona drive a H+/e- supersonic solar wind

– Typical densities: ~ 5 cm-3

– Typical speeds: ~ 400 km/s

– Earth’s magnetic field acts like a cavity in solar wind

– Bow shock stands in front of the Earth

• The solar wind carries a weak magnetic field away from the Sun into interplanetary space called the interplanetary magnetic field (IMF)

– Dungey (1961) argued that when the IMF has a southward component it should interact strongly with the Earth’s field to drive magnetic disturbances.

– Experimental studies over intervening 50 years overwhelmingly confirm Dungey’s hypothesis: magnetic activity is always preceded by southward turning of the IMF.

Overview

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Space Weather Magnetic Storms

• In preparing this this presentation it seemed useful to concentrate on a very simple, but very intense magnetic storm that occurred in November 2003.

• ACE was at the first Lagrange point L1 where measured the solar wind density and speed as well as the interplanetary magnetic field (IMF).

• The GRACE satellite was in circular polar orbit near 490 km. - An onboard accelerometer measured the atmospheric drag on the spacecraft. - From the accelerometer measurements we inferred globally-averaged mass densities in the thermosphere and its total energy content

• We compare interplanetary forcing and thermospheric responses with variations of the stormtime disturbance Dst index - Dst measured as N-S magnetic variations observed at 4 widely spaced stations around globe - Reported at 1-hour cadence as spatial and temporal average BNS - Linearly proportional to energy in the ring current (Dessler-Parker –Sckopke)

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SunEarth

Clo

sed

Fie

ld l

ines

Interp

lanetary F

ield lin

es

Open Field lines

Open Field lines

Magnetic merging at dayside magnetopause

Magnetopause current sheet

Solar Wind

Three Magnetic Topologies- IMF: two feet in solar wind- Closed: two feet on Earth- Open: one foot on Earth and one in the solar wind

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Earth

Northern Lobe

Southern Lobe

Plasma Injection

Plasma Ejection

Magnetic Reconnection in the magnetotail

Near Earth X-line

(activated during

substorms)

DistantX-line

Dayside merging

site

Dungey’s picture provide a rationalfor the existence and dynamics of the plasma sheet, the then undiscovered storage region from which auroral particles are drawn.

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Coronal Mass Ejections

SOHO observations of a CME ejection

Artistic rendition of a flare and CME

Computer simulation of a CME

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• Concrete example: consider November 19 - 23, 2003 Storm - X-28 class X-ray flare - Coronal mass ejection - No solar energetic particles

• Largest magnetic storm of last

solar cycle

• The plots to the right show measurements from ACE at L1: - Solar wind speed (top) - Solar wind density (blue) and dynamic pressure (red) - IMF BZ component (bottom)

-48

-24

0

24

48

323 324 325 326 327

IM

F B

Z (n

T)

JD 2003

0

5

10

15

20

25

30

0

5

10

15

20

25

30

Sola

r W

ind

Den

sity

(cm

-3)

So

lar W

ind

Pressu

re (n

Pa)

300

400

500

600

700

800

So

lar W

ind

Sp

eed

(k

m/s

)

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0

1

2

3

-500

-400

-300

-200

-100

0

100

323:00 324:00 325:00 326:00 327:00

VS (

mV

/m)

Dst (n

T)

JD 2003

-48

-24

0

24

48

IM

F B

Z (n

T)

0

5

10

15

20

25

30

0

5

10

15

20

25

30

Sola

r W

ind

Den

sity

(cm

-3)

So

lar W

ind

Pressu

re (n

Pa)

• The two top plots repeat ACE measurements of the solar wind density and pressure as well as IMF BZ.

• The bottom plot shows the magnetospheric response in the form of the Dst index which indicates the growth and decay of the stormtime ring current

• The symbol VS represents the magnetospheric electric field in the equatorial plane

• The storm’s main phase (negative Dst slope) began when VS turned on.

• The storm’s recovery phase (positive Dst slope) began when VS turned off.

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10-16

10-15

Mass

Den

sity

(g

ram

s/cc)

GRACE Altitude 490 km

0

1

2

3

-500

-400

-300

-200

-100

0

100

323:00 324:00 325:00 326:00 327:00

VS (

mV

/m)

Dst (n

T)

JD 2003

6 1017

6.5 1017

7 1017

Th

erm

osp

heric

En

erg

y (

Jou

les)

• Slide shows thermosphere’s response to magnetic storm driving

• The trace in the top panel shows that the globally-averaged thermospheric density at 490 km increased by a factor of 6 from 5 ∙10-16 to 3 ∙10-15 grams/cc .

• The trace in the middle panel indicates that the total energy of the thermosphere rose from 6.2 ∙1017 to 6.8 ∙1017 (Eth ~ 6 ∙ 1016 ) Joules.

• Since the rise occurred in ~12 hour the average power into the thermosphere was ~ 1.4 ∙1012 Watts.

• The energy of the ring current estimated via D-P-S relation ERC (Joules) 3.87 ∙ 1013 ∙ |Dst (nT)|

• Minimum Dst -475 nT ERC 1.9 ∙ 1016 Joules

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• This slide shows southern auroral ionospheric response to storm driving on November 20, 2003

• False color EUV image of ionosphere acquired by NASA’s Polar satellite at an altitude of ~ 9 RE. Red indicates most intense auroral emissions.

• During large storms auroral particle fluxes into the ionosphere are their most intense.

• Based on particle and optical measurements from AF, NOAA and NASA satellites, electron and ion energy precipitation rates and can reach ~ 100 GW.

• This is about a factor of 10 less than the electromagnetic power needed to heat the global thermosphere.

• Electric and magnetic field measurements from AF and NASA satellites agree with thermospheric power estimates.

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• In looking at the magnetic superstorm November 20, 2003 we have been exposed to a wide sampling of what the Sun can throw at us.

• Be warned however, it does not represent the full spectrum of consequences: - Halloween storm 2003: severe MeV particle fluxes generated in the solar flare destroyed ability of ACE to measure solar wind characteristics - March 1989 storm: brought down Hydro Quebec electric grid. AFSPC lost ~3500 space objects that it was tracking. - March 1991 storm: created a new radiation belt in about two minutes.

• In next weeks lecture we will turn our attention to substorms the other geomagnetic disturbance that occurs after southward turnings of the IMF.

• This type of event involves activations of the near-Earth X line and have much shorter lifetimes than storms, but they can have deadly consequences for satellites in geostationary orbit.

Summary and Conclusions

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Documents

Space Weather: Magnetic Storms 31 October 2011