The Solar System

CHAPTER 8 | THE SUN 163

Magnetic reconnections can release enough
energy to blow large amounts of ionized gas
outward from the corona in coronal mass ejections
(CMEs). If a CME strikes Earth, it can produce
especially violent disturbances in Earth’s magnetic field.

The solar wind, enhanced by eruptions on the sun,

interacts with Earth’s magnetic field and can create

electrical currents up to a million megawatts. Those currents

flowing down into a ring around Earth’s magnetic poles excite

atoms in Earth’s upper atmosphere to emit photons

as shown below. Seen from Earth’s surface,

the gas produces glowing clouds and

curtains of aurora.

At right, waves rush outward at 50 km/sec

from the site of a solar flare 40,000 times

stronger than the 1906 San Francisco earthquake.

The biggest solar flares can be a billion times

more powerful than a hydrogen bomb.

SOHO/MDI, ESA, and NASA

NSSDC, Holzworth and Meng

Yohkoh/ISAS/NASA

Hinode JAXA/NASA

This multiwavelength

image shows a

sunspot interacting

with a neighboring

magnetic field to

produce a solar flare.

Auroras occur about 130 km

above the Earth’s surface.

Ring of aurora

around the north

magnetic pole

Coronal mass

ejection

Coronal hole

Helioseismology

image

2c X-ray image

2b

2a

3

2 Solarflaresrise to maximum in minutes and decay in an hour. They
occur in active regions where oppositely directed magnetic fields meet
and cancel each other out in what astronomers call reconnections. Energy
stored in the magnetic fields is released as short-wavelength photons and
as high-energy protons and electrons. X-ray and ultraviolet photons reach
Earth in 8 minutes and increase ionization in our atmosphere, which can
interfere with radio communications. Particles from flares reach Earth hours
or days later as gusts in the solar wind, which can distort Earth’s magnetic
field and disrupt navigation systems. Solar flares can also cause surges in
electrical power lines and damage to Earth satellites.

Much of the solar wind comes from

coronal holes,where the magnetic field

does not loop back into the sun. These open

magnetic fields allow ionized gas in the corona

to flow away as the solar wind. The dark area

in this X-ray image at right is a coronal hole.