ScotlandPhysicsSyllabus dot point

How do charged particles from space move in magnetic fields and produce aurorae?

Cosmic rays and the solar wind, the motion of charged particles in magnetic fields, and the formation of aurorae.

An SQA Advanced Higher Physics answer on particles from space, covering cosmic rays and the solar wind, the helical motion of charged particles in magnetic fields, and how charged particles interacting with the Earth's magnetic field and atmosphere produce aurorae.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-14

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section

What this key area is asking
Cosmic rays and the solar wind
Motion of charged particles in magnetic fields
How aurorae form
Examples in context
Try this

What this key area is asking

The SQA wants you to describe the nature and origin of cosmic rays and the solar wind, analyse the motion of charged particles in magnetic fields, and explain qualitatively how charged particles from the Sun interacting with the Earth's magnetic field and atmosphere produce aurorae.

Cosmic rays and the solar wind

Both are streams of fast charged particles, and because they carry charge they interact strongly with magnetic fields. When cosmic rays strike the upper atmosphere they produce showers of secondary particles. The solar wind varies with solar activity, becoming much stronger during solar storms, which drives space weather and intensifies the aurora.

Motion of charged particles in magnetic fields

Because this force is always perpendicular to the velocity, it does no work and cannot change the particle's speed; it only changes its direction. A particle moving perpendicular to a uniform field therefore travels in a circle, with the magnetic force providing the central force: $qvB = \frac{mv^2}{r}$ , so $r = \frac{mv}{qB}$ . If the particle also has a velocity component along the field, that component is unaffected, and the combined motion is a helix spiralling along the field lines. This helical guiding is exactly what channels space particles towards the poles.

How aurorae form

The Earth's magnetic field acts as a shield, deflecting most solar-wind particles, but it funnels some along field lines towards the magnetic poles, which is why aurorae are seen at high latitudes (the aurora borealis and aurora australis). The colours come from the specific energy-level transitions of the atmospheric gases: green and red from oxygen, blue and purple from nitrogen. This ties directly to the quantised energy levels of the previous key area.

Radius of a proton's circular path

A proton ( $m = 1.67 \times 10^{-27}\ \text{kg}$ , $q = 1.6 \times 10^{-19}\ \text{C}$ ) moves at $4.0 \times 10^{6}\ \text{m s}^{-1}$ perpendicular to a field of $0.20\ \text{T}$ . Find the radius of its circular path.

step 1 Equate the magnetic force to the central force

$qvB = \dfrac{mv^2}{r}$ , which rearranges to $r = \dfrac{mv}{qB}$ .

step 2 Substitute the values

$r = \dfrac{1.67 \times 10^{-27} \times 4.0 \times 10^{6}}{1.6 \times 10^{-19} \times 0.20}$ .

step 3 Evaluate

Numerator $= 6.68 \times 10^{-21}$ ; denominator $= 3.2 \times 10^{-20}$ ; so $r = 0.21\ \text{m}$ .

Examples in context

Space weather forecasting tracks the solar wind to warn of geomagnetic storms that can disrupt satellites and power grids. The Van Allen belts are regions where charged particles are trapped on helical paths along the Earth's field lines. Particle detectors at high altitude and underground study cosmic-ray showers. The aurora borealis over northern Scotland, sometimes visible during strong solar activity, is a direct local example of this key area.

Try this

Q1. State the relationship for the force on a charge $q$ moving at speed $v$ perpendicular to a field $B$ . [1 mark]

Cue. $F = qvB$ .

Q2. State why a magnetic force cannot change the speed of a charged particle. [1 mark]

Cue. It acts perpendicular to the velocity, so it does no work.

Q3. State where on the Earth aurorae are most commonly seen and why. [1 mark]

Cue. Near the magnetic poles, because the field guides charged particles there.

Exam-style practice questions

Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

SQA AH style4 marksA proton of charge

1.6 \times 10^{-19}\ \text{C}

enters a magnetic field of flux density

0.50\ \text{T}

at right angles, moving at

3.0 \times 10^{6}\ \text{m s}^{-1}

. Calculate the magnetic force on it.

Show worked answer →

The force on a charge moving at right angles to a magnetic field is $F = qvB$ .

Substitute: $F = 1.6 \times 10^{-19} \times 3.0 \times 10^{6} \times 0.50$ .

Multiply: $F = 1.6 \times 10^{-19} \times 1.5 \times 10^{6} = 2.4 \times 10^{-13}\ \text{N}$ .

This force is always perpendicular to the velocity, so it changes direction but not speed, curving the proton into a circular path.

Markers reward the correct relationship, the value with unit, and the point that the force is perpendicular to the motion.

SQA AH style4 marksExplain how charged particles from the Sun produce the aurora.

Show worked answer →

The Sun emits a stream of charged particles called the solar wind.

When these particles reach the Earth they are deflected by the Earth's magnetic field and guided towards the magnetic poles, spiralling along the field lines.

There they collide with atoms and molecules high in the atmosphere, exciting them; as the excited atoms return to lower energy levels they emit light, producing the glowing aurora.

Markers reward naming the solar wind, the guiding effect of the Earth's magnetic field towards the poles, and the excitation and de-excitation of atmospheric atoms emitting light.

Related dot points

Sources & how we know this

SQA Advanced Higher Physics Course Specification — SQA (2019)