Lesson 5 of 6 · auroras
When particles make the atmosphere glow
10 minute lesson · 3 checks along the way
Your goal for this lessonConnect auroral light to excitation and evaluate what a camera measurement can establish.
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The light is made near Earth
An is light produced in an atmosphere after energetic particles transfer energy to its constituents. At Earth, electrons often provide that energy. Collisions excite atoms and molecules; as they relax to lower energy states, they can emit photons. The visible curtain is therefore light made high above Earth, rather than a colored beam of sunlight painted onto the night sky.
Many auroral electrons belong to populations in Earth's and gain energy through processes there. Solar-wind interactions help power the larger system, but tracing every glowing patch to electrons traveling directly from the Sun would be misleading. Electric fields and waves can accelerate particles toward the atmosphere. Magnetic geometry guides their motion and helps organize auroral activity into changing regions around the magnetic poles.
Color contains information, with conditions
Oxygen can produce prominent green and red emissions, while nitrogen and its ions contribute other colors. Which transitions emit efficiently depends on particle energies, the gases present, and atmospheric density. Altitude matters because competing collisions can interrupt a long-lived excited state before it emits. A color is therefore evidence about a physical process, not a simple thermometer painted across the sky.
A camera further changes what you see through its wavelength response, exposure time, and processing. Long exposures can reveal light too faint for an observer to distinguish clearly. Colors may overlap along the line of sight, and daylight or clouds can hide an active display. A photograph cannot by itself give the altitude, energy spectrum, or total power of all the particles producing the aurora.
Check 1 of 3
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Work through a decision
Separate the signal from the background
Consider a fictional calibrated camera measuring one auroral emission band. It records 900 counts in 30 seconds. A matching background measurement gives 300 counts over the same duration. Subtracting the background leaves 600 counts, corresponding to a mean signal of 20 counts per second. The answer is a band-specific detector signal; it is not yet a count of incoming electrons.
On the next exposure the total is 1,200 counts over 60 seconds, with a matched background of 600 counts. The net signal is again 600 counts, but now only 10 counts per second. Equal accumulated counts did not mean equal brightness under otherwise matched calibration and geometry. To infer an electron flux, you would also need a model of , emission, collection efficiency, and atmospheric conditions.
From excitation to a measured camera signal
Original schematic and invented camera measurements. The process is not drawn to physical scale. Rates are compared only with matched calibration and viewing geometry; neither count total is a direct count of auroral electrons.
Open full-size diagram in a new tab: From excitation to a measured camera signalCheck 2 of 3
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Join observations across the system
THEMIS combined measurements in space with ground observations to study connections between magnetic activity and auroras. Such comparisons can test timing: did a measured disturbance precede a change in the light, and can a physical process connect the locations? Timing must allow for travel and instrument delays. A shared pattern is informative, but a single simultaneous brightening does not identify every step of causation.
For an optional nighttime observation, record location, time, viewing direction, cloud conditions, and camera settings from a safe familiar place. A report of no visible aurora under thick cloud cannot establish that none occurred overhead. Forecasts express changing possibilities, not a promise for one observer. Keep the distinction between the atmosphere's emission and your view of it; both are needed to interpret a photograph or a personal observation responsibly.
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Put it together
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