Read spectra and scanning maps

Hinode’s EIS measures emission lines in the extreme ultraviolet. At speeds much smaller than the speed of light, the line-of-sight Doppler relation is approximately velocity = light speed × wavelength shift ÷ reference wavelength. In this lesson, a positive shift means a longer observed wavelength and motion away from the observer. The reference and sign convention belong with the result.

For an invented example, a line with reference wavelength 20.000 nanometers is observed at 20.002 nanometers. The shift is 0.002 nanometer, or one ten-thousandth of the reference. Multiplying by about 300,000 kilometers per second gives 30 kilometers per second away. These are teaching values, not an EIS flight measurement. Motion perpendicular to the sightline is not supplied by this calculation.

A line shift measures one component of motion

A line’s central position, width and integrated intensity describe different aspects of the spectrum. A broadened profile can reflect thermal motions, unresolved motions and the instrument’s own response. A blend of nearby lines can also complicate fitting. It is therefore unsafe to label every broad line as turbulence or every wavelength displacement as an uncorrected plasma velocity.

A useful analysis compares the measured profile with an instrument calibration and a physical model. Ask which contributions have been removed, which remain uncertain and how a different reference would affect the result. A fitted number is not automatically more trustworthy because it has many decimal places. Its uncertainty and assumptions determine what distinctions it can support.

With a narrow slit, EIS builds a two-dimensional raster by recording successive positions. The completed map can look like a photograph, but different columns were observed at different times. A changing solar feature can therefore create a pattern that mixes spatial structure with evolution during the scan.

In an original simplified scan, four columns require ten seconds each with no overhead. They cover 0–10, 10–20, 20–30 and 30–40 seconds. Their midpoint times are 5, 15, 25 and 35 seconds. The exposure budget is forty seconds, and the first and last midpoint differ by thirty seconds. A flare changing over that interval should not be interpreted as though every column were simultaneous.

A raster is built column by column