Space weather

Introduction to lectures by Dr John S. Reid

Image courtesy: http://www.astro-photography.com/ss9393.htm

Sunspot 9393

• First pass from late March to early April, 2001• See: Storms from the Sun

• http://books.nap.edu/books/0309076420/html/index.html

Animated gif, courtesy: http://antwrp.gsfc.nasa.gov/apod/ap010411.html

SatellitesSatellites on 30/10/2006

Data courtesy: http://www.astro.princeton.edu/%7Emjuric/universe/

Out there

• ‘Out there’ • EM radiation• solar wind• cosmic rays• micrometeorites

• Closer to Earth• ‘radiation’ belts

• Earth’s atmosphere provides enough protection for life• can we exist outside it? Interplanetary

magnetic field

Sun

http://nmp.jpl.nasa.gov/st5/images/hazard4.jpg

EM radiation from the Sun

• At Earth ~1366 W m-2

• Photosphere of Sun appears ~‘blackbody’ at 5780K

• Total energy o/p ~1026 W• Distribution ~ follows

Planck radiation law• Peak wavelength is

in the visible• 7% UV, 44 % visible• 37% near IR, 11% far IR• 1% radio spectrum

X-rays UV Visible γ-rays IR Micro-wave Radio

Flux hitting an astronaut on the Moon or around the Earth

• Say maximum area 2 m × 1 m = 2 m2

• Flux at distance of Moon (1 AU)• = 2 × 1366 = 2732 W

• This makes temperature control hard

Aldrin on the MoonChallenger crew

Digression on space suits

• Space suits need to:• suggestions from class

• They operate at reduced pressure, with an internal atmosphere of pure O2• several hours of acclimatisation are necessary

to remove N2 from the blood• what happens to the breathed out CO2?• how is temperature control achieved?

Hamilton Sundstrand services the Hubble Space Telescope

Reminder: Temperatures in K

• Temperature in degrees Kelvin (e.g. 300 K)• usual in physics laws• general conversion:• e.g. 20°C ≡ 293 K• e.g. 5780 K ≡ 5507°C

273+°= CK

200

293273

Celsius Kelvin

Radiant energy emitted by a hot

body

• Total radiant energy (E) emitted per m2 of surface per second for a black body at temp T

• , where σ is 5.67×10-8 W m-2 K-4

• Stefan-Boltzmann Law; σ is Stefan’s constant

• E.g. T = 5780 K, E = 63.3 MW m-2

• E.g. T = 273 K, E = 315 W m–2

4TE σ=

How hot is a body left in space?

• Body of radius r• Radiation R from

one direction• Fraction a reflected• Space all round• Radiation spread over body

by conduction and rotation• Stefan-Boltzmann law tells

us how hot the body will be

Radiation

Space

Fraction a reflected

T

R

Radius r

Energy conservation

• Radiation received = radiation re-emitted• Consider the body at distance Earth is from Sun

• incoming energy spread over a disk of area πr2

• re-radiated energy comes from area of a sphere 4πr2

• R(1 - a)πr2 = σ T4×4πr2

• T4 = R(1-a)/4σ

Total area = 4πr2Disk area πr2

Temperature at 1 AU for different fractions of incident radiation reflected

050

100150200250300

0 0.2 0.4 0.6 0.8 1

fraction reflected (a)

Tem

pera

ture

in K

The further you are from Sun, the colder it is

• At increasing distance d from the Sun, the energy passing through 1m2 decreases as 1/d2

• this is essentially a statement of the law of conservation of radiant energy

< d → 1 2 3→

1m2 4m2 9m2Source

Energy per m2

∝ 1/d2

Area ∝ d2

Example of the inverse square law in action

• As a formula: Rd = R1/d2 ; where Rd is the rate energy is received at distance d

• E.g., the Earth at 1 AU distant from the Sun receives solar radiation at a rate of 1366 W m-2

• How much radiation is received by the Venus Express probe when it is 0.6 AU from the Sun?

RVenus express= REarth/0.62

= 1366/0.62

= 3794 W m-2Venus Express, courtesy: http://www.pparc.ac.uk

How cold is the Cassini probe near Saturn?

• Saturn is 9.54 times distance of the Earth from the Sun (9.54 Astronomical Units)

• Hence the flux of energy at Saturn from the Sun is 1366/9.542 = 15.0 W m-2

• Average temperature of the Cassini probe as it spins around depends on its reflectivity

Temperature of a probe at distance of Saturn for a range of reflectivities

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

reflectivity

tem

pera

ture

(K)

Cassini probe, NASA

The electromagnetic spectrum

• Different parts of the spectrum have different historical names• Diagram shows approx wavelengths of the boundaries

• wavelengths determine the equipment used to transmit & receive• Energy, E, comes in packets (‘photons’) that depend on the

wavelength (λ) through Planck’s constant h• packets are measured in eV (‘electron volts’)• > 2 eV will break some chemical bonds• much of the UV and beyond is chemically damaging

X-rays UV Visible γ-rays IR Micro-wave Radio

eV: 105 40 3 1.5 10-3 10-5

Wavelength: 0.01 nm 30 nm 400 nm 700 nm 1000 µm 100 mm

λ==

chhfE

f is the frequency of the radiation; c the speed of travel

Visible emission

of the Sun

Courtesy:http://mesola.obspm.fr

Solar spectrum from 500 nm to 600 nm

Visible solar spectrum showing absorption lines

Joseph Fraunhofer 1787 - 1826

Broad spectrum of Sun

• Shorter than 200 nm there is much more radiation than a blackbody emits

• Where does this come from?• the outer

atmosphere of the Sun

visible

http://www.sec.noaa.gov/spacewx/Solar_Spectrum.html

Sun in X-ray and Extreme UV

• Notice the Sun doesn’t appear a uniform hard disk• soft X-ray picture on the left (0.3 to 4.5 nm)• extreme UV picture on right (30.4 nm)

Images courtesy: http://solar.physics.montana.edu/YPOP/Spotlight/Today/

X-ray →

EUV→

Sun in visible, IR and microwave

• The Sun is conspicuously uniform in visible light

White: 400 – 700 nm IR: 1083 Nm Microwave 17 mm

Images courtesy: http://solar.physics.montana.edu/YPOP/Spotlight/Today/

Radio flux from the Sun

• Substantial sunspot dependence

http://www.sec.noaa.gov/SolarCycle/index.html

X-ray monitoring

• X-ray flux monitored by geostationary satellite

• Wavelength units are Å, where 10 Å ≡1 nm

• GOES 10 Lat 135° W

• GOES 12 Lat 76 ° W

http://www.sec.noaa.gov/today.htmlX-ray

Photon flux and power density

• Photon flux in photons m-2 s-1

• Power density in W m-2

• 1W ≡ 1 J s-1

• 1 eV ≡ 1.6×10-19 J• therefore 1 J = 1/(1.6×10-19) eV = 6.25×1018 eV

• Hence 1W = 6.25×1018 eV s-1

• For X-ray photons of energy 3000 eV• 1 W ≡ 2.08×1015 photons s-1

• 10-4 W m-2 ≡ 2.08×1011 photons s-1 m-2

Photon flux W m-2

The solar wind

• Solar wind is a flux of plasma coming from the Sun• plasma is an electrical neutral ‘gas’ of positively and

negatively charged ‘particles’• solar wind:

• +ve particles are mainly protons (H+), He nuclei (He2+) and heavier element ions

• -ve particles are electrons• ‘trapped’ magnetic field, the IMF (‘interplanetary

magnetic field’)• The solar wind has a significant impact on

everyone’s use of space

http://www.starhillinn.com/images/hale-bopp-2.jpg

Positive ions in the solar wind

• The ACE probe solar wind ion composition spectrometer (SWICS) results• a second instrument, an

ion mass spectrometer (SWIMS), contributes

• Ions of elements up to Ni are measured• many have no electrons

at all

Fe+ ions observed by ACE

• Histogram of Fe+ ions in solar wind detected by ACE →• Fe has atomic mass ~56

• Variability of Fe+ ions over a period of 3 days

←

ACE

• Advanced Composition Explorer• homepage http://www.srl.caltech.edu/ACE/• ACE sits permanently between the Earth and Sun,

about 1.5×106 km from the Earth• ACE orbits around the first Lagrangian point• ACE has six instruments that monitor

particle content, speed, density, etc. and the interplanetary magnetic field

• also monitors galactic cosmic rays

Variability of the solar wind

• The solar wind and related particle flux from the Sun is the most variable component of space weather

• The solar wind can be a hazard to man and instrumentation

http://www.srl.caltech.edu/ACE/ASC/DATA/browse-plots/4day_plot.html

Output from ACE probe• Bz is magnetic

field in 10-9 T (a unit called gamma (γ))

• Phi is azimuthalangle (see next slide)

• Density: particles per cm3

••• SpeedSpeedSpeed km s-1

• Temp in K

Coordinate systems

• What exactly are you measuring?• There are various useful coordinate systems• ACE data reports results in GSE coordinates

• “Geocentric solar ecliptic”• X-direction is Earth – Sun line• Z direction is ecliptic north pole• A magnetic field B in diagram

• Bx, By, Bz

• or B, θ, ϕ

Plane of ecliptic

To Sun

Z

X

YEarth

Phi (ϕ)

Theta (θ) B

Particle density and flux

• Density is quoted in particles cm-3

• e.g. 4 particles cm-3

• Flux is quoted in particles cm-2 s-1

• in 1 second all the particles in a cylinder of length v pass through unit area

• if v = 500 km s-1 ≡ 5×107 cm s-1

• 4 particles cm-3 ≡ flux of 2 ×108 particles cm-2 s-1

• ≡ flux of 2 ×1012 particles m-2 s-1

• Fluence is quoted in particles cm-2

• radiation damage depends on fluence

cm3

cm2

Velocity v

Temperature and thermal speed

• If particles are in ‘thermal equilibrium’ with their surroundings, their average KE = thermal energy

• Temperature T is given by

• E.g. coronal proton, m = 1.67×10-27 kg, v = 2.5×105 ms-1

• T = 2.52×106 K• Calculation fails if particles aren’t in thermal equilibrium

• temperature is a concept that applies to systems in equilibrium

123-2 JK101.3807 constant,sBoltzmann'is,23

21 −×= kkTmv

vm

kmvT3

2

=

Particles in thermal equilibrium

v2 is the average square speed

Temperature of a stream of particles

• Normally gas particles are spread around an average speed of zero →

• Temperature is related to the spread of their speed, the average of v2

• If the same particles are all given a speed (say 10) then their temperature stays the same

• The spread of v2 about the average is the same ←

Schematic distribution of velocities in a beam of particles of average speed 10

00.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8 10 12 14

velocity

rela

tive

num

ber o

f par

ticle

s

Schematic distribution of particle velocities

0

0.2

0.4

0.6

0.8

1

-3 -2 -1 0 1 2 3

velocity

rela

tive

num

ber o

f par

ticle

s

Solar wind temperatures

• Just moving a box of particles at speed v0doesn’t change its temperature

• What counts is the speed of the wind particles once the average motion has been subtracted• ACE’s measurements show T ~ 105 K• the spread of particle velocities of the protons is

~50 km s-1

• electron temperatures are comparable to proton temperatures

V0

Additional solar wind indicators

• Dials and the auroral oval give a quick overview of the solar wind on the Earth • dials show ‘real-time’

display and history loop

http://www.sec.noaa.gov/pmap/pmapN.html

http://www.sec.noaa.gov/SWN/index.html

Geostationary satellite environment

• Example of fluctuating environment at height of geostationary satellite

• Kp is the ‘planetary K index’, a measure of the fluctuations in the Earth’s magnetic field in range 0 to 9

http://www.sec.noaa.gov/ftpdir/plots/satenv/20030813_satenv.gif

Public prediction of Kp index from ACE’s solar wind data

• High index means high geomagnetic abnormality

http://sec.noaa.gov/rpc/costello/

http://www.n3kl.org/sun/images/noaa_kp_3d.gif

Tremendous aurora