Auroras: neon signs in the sky

The phenomenon of the aurora australis (and its northern counterpart the aurora
borealis) is one of nature’s wonders. The majestic displays of the aurora—vast
curtains of undulating green, red or blue light hundreds of kilometres high—can be
seen in night skies in Antarctica and sometimes as far north as Tasmania, and are
the result of a complex interaction between three major elements.
The first of these, Earth’s atmosphere, is the collection of gases that surround
the planet, mainly nitrogen and oxygen. This gas envelope begins at the planet’s
surface and extends upwards more than 700 km, becoming less dense with
increasing altitude. Also enveloping the planet is a strong magnetic field called the
magnetosphere, which arises from deep within Earth’s core and spreads along
invisible ‘field lines’. The magnetosphere causes charged particles from space to be
deflected around the planet. This function is made important by the third element in
the equation: the solar wind. This ‘wind’ is actually a plasma composed of charged
particles (protons and electrons) ejected from the Sun at high velocity by its intense
nuclear fusion activity.
High in Earth’s atmosphere, at the border between the denser gaseous regions
and outer space, lies a zone known as the ionosphere where the aurora occurs.
Here, the high-energy charged particles of the solar wind become captured by
the magnetosphere and are driven into collision with the gas particles of the
atmosphere. As gas atoms absorb energy from collisions with the solar-windborne
particles, the atmospheric gases become ‘excited’, or at even higher energies,
‘ionised’ (positively charged). These atoms release light (photons) when they fall
back out of their excited or ionised states. Much like the gas contained in a neon
sign, which glows as a current is passed through it, the particles in the ionosphere
glow as they return to an unexcited state.
But why the differing colours of the auroras, and why do they only happen near the
poles? The colours are explained by the different spectra emitted by different gases
at different levels of excitation—lower-energy oxygen yields different (reddish) hues
compared to higher-energy nitrogen (greenish). As for the phenomenon’s polar
locations, interactions between magnetic fields and charged particles are simply
stronger where the magnetic field itself is stronger—near the planet’s magnetic poles.

The difference between the aurora australis and the aurora borealis is that
they are produced by different gases.
they occur in different parts of the world.
they are created from different chemical compounds.
they vary significantly in their intensity

• Name the three major elements that interact to produce auroras.

• In paragraph 2, why is the word ‘wind’ in inverted commas?
  to emphasise that solar wind is not a wind in the usual sense of the word
  to indicate that the word has an alternative meaning that is specific to the
  study of physics
  to signal that from that point on in the text, solar wind will be referred to
  as wind
  to stress that not all winds can be described as solar winds

• In paragraph 3, the comparison between the aurora and a neon sign is used to
  explain a concept with a poetic image.
  call on the reader’s existing scientific knowledge.
  make an unfamiliar process seem more familiar.
  extend a scientific explanation with greater detail.

• The first and last paragraphs of this text
  are both expressed in non-scientific language.
  both refer to the auroras’ appearance and location.
  both suggest there is still much to learn about auroras.
  both use figurative language to describe auroras