2. Overview of lecture 9
• In this lecture we will take a high-level look at the composition of the Sun.
We will also consider:
– The solar wind
– Trapped radiation belts (i.e. Van Allen belts), which arise due to the
Earth’s protective magnetic field
– Deflections of the Earth’s magnetic field caused by variations in the solar
wind
• These effects will feature in later lectures on space weather, when we look a
little more closely at what space weather is and what impacts it has on
spacecraft and spacecraft operations
• Acknowledgements for material in this lecture:
– Dr David Alexander, Rice University
Space Environment – Space Weather
3. Composition of the Sun Space Environment – Space Weather
• The Sun provides all the heat input
and 99.9% of solar system mass
• Determines near-Earth environment
• Age: 5 × 109 years
• Power emitted = 3.8 × 1026 W
• Interior temperature ~ 20 × 106 K
• Nuclear fusion energy reaches
surface by convection
6. Photosphere Space Environment – Space Weather
• Visible surface of the Sun approximates
to a black-body at 5800 K
• Sunspots are gases ~1500 K cooler than
surrounds and having strong magnetic
fields 0.01-0.4 T
– 1000 times stronger than Earth field
• Total number of sunspots follows a
cycle of average 11.1 years
– Range from 8-16 years
7. Chromosphere Space Environment – Space Weather
• The region 2000-3000 km thick above the photosphere
• Temperature increases from 4500 K to 10000 K
8. Corona Space Environment – Space Weather
• A large, luminous halo that can be
observed during a total eclipse
• Extends many millions of kilometres
• Very low density 1015 atoms/m3
• Very hot 2 × 106 K
• Nearly all energy in UV and X-ray
comes from the corona
• Highly variable depending on
conditions of photosphere and
chromosphere
10. Solar Wind Space Environment – Space Weather
The solar wind is a stream of energized, charged particles, primarily electrons
and protons, flowing outward from the Sun
12. Solar Wind Space Environment – Space Weather
The solar wind is a stream of energized, charged particles, primarily electrons and
protons, flowing outward from the Sun:
Fast Wind Slow Wind
Source Coronal Holes Equatorial Streamers
Proton density 3×106 m-3 1.07×107 m-3
Proton speed 750 km/s 348 km/s
Proton flux ~2×1012 m-2s-1 3.66×1012 m-2s-1
Proton temperature 280,000 K 55,000 K
Electron temperature 130,000 K 190,000 K
Helium temperature 730,000 K 170,000 K
14. Radiation belts Space Environment – Space Weather
https://www.youtube.com/watch?feature=player_detailpage&v=wf1ZQM0_dWM
Data from Van Allen Probes (daily integrated fluences)
Estimate charge deposition rates behind shielding penetrated by electrons
15. Magnetic field deflection Space Environment – Space Weather
Variations in the solar wind (gusts) result in deflections of the Earth’s magnetic field
Magnetic field deflection can be measured with magnetometers
Deflection
(nT)
“Halloween storms of 2003”:
deflection* measured by
magnetometers in Kiruna, Sweden
*deflection from quiet level of
10,650 nT
http://www.spaceweatherlive.com/en/help/the-kiruna-magnetometer
16. Magnetic field deflection Space Environment – Space Weather
Geomagnetic indices:
• Magnetic field deflection (in nT) can be
converted to a K-index and an A-index
• Relationship between deflection and K-
index is location-dependent (e.g. Germany,
K = 9 corresponds to R = 500 nT or greater)
• Planetary K-index (Kp) is derived from a
weighted average of K-index values from
geomagnetic observatories
17. Overview of lecture 9
• In this lecture we took a high-level look at the composition of the Sun and
the solar wind, including:
– Trapped radiation belts (i.e. Van Allen belts), which arise due to the
Earth’s protective magnetic field
– Deflections of the Earth’s magnetic field caused by variations in the solar
wind
• These effects will feature in later lectures on space weather, when we look a
little more closely at what space weather is and what impacts it has on
spacecraft and spacecraft operations
Space Environment – Space Weather
18. Activities
• There are numerous websites focused
on understanding the Sun. Here are a
few to try:
• https://solarsystem.nasa.gov/solar-
system/sun/overview/
• https://en.wikipedia.org/wiki/Sun
• https://www.nationalgeographic.com/scienc
e/space/solar-system/the-sun/
• You can also find the book
“Understanding the Sun and Solar
System Plasmas” on the Blackboard
site
Space Environment – Space Weather
