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
2
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?)
3
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
4
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)
5
Space Weather Magnetic Storms
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
6
Space Weather Magnetic Storms
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.
7
Space Weather Magnetic Storms
Coronal Mass Ejections
SOHO observations of a CME ejection
Artistic rendition of a flare and CME
Computer simulation of a CME
8
Space Weather Magnetic Storms
• 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
)
9
Space Weather Magnetic Storms
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 magneto- spheric 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.
10
Space Weather Magnetic Storms
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
11
Space Weather Magnetic Storms
• 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.
12
Space Weather Magnetic Storms
• 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
13