Question E3C10:
What does the 304A solar parameter measure?  

Choices:  
A: The ratio of X-ray flux to radio flux, correlated to sunspot number  
B: UV emissions at 304 angstroms, correlated to the solar flux index  
C: The solar wind velocity at an angle of 304 degrees from the solar equator, correlated to geomagnetic storms  
D: The solar emission at 304 GHz, correlated to X-ray flare levels    

Correct Answer is  B: UV emissions at 304 angstroms, correlated to the solar flux index  

Correct answer: B

Why B is correct
- “304A” refers to the solar EUV emission at a wavelength of 304 angstroms (He II line), i.e., $304\,\text{Å} = 30.4\,\text{nm}$.
- This emission is a proxy for ionizing extreme ultraviolet radiation that drives ionospheric density. It tends to rise and fall with the Solar Flux Index (F10.7), so it is correlated with SFI and HF propagation conditions.

Why the other choices are incorrect
- A: The 304A parameter is not a ratio of X-ray to radio flux. It is a specific UV wavelength measurement; “304A” indicates angstroms, not a flux ratio.
- C: It is not a solar wind velocity at some angle. Solar wind is measured in km/s and not labeled by an angle like “304 degrees.” The “304” here denotes a wavelength.
- D: It is not an emission at 304 GHz. 304 in this context is angstroms (wavelength), not gigahertz (frequency). 304 GHz is a millimeter-wave frequency and not the standard solar index referenced here.

Useful formulas and rules of thumb
- Angstrom to nanometer:
  $$1\,\text{Å} = 0.1\,\text{nm} \quad\Rightarrow\quad 304\,\text{Å} = 30.4\,\text{nm}$$
- Wavelength–frequency conversion:
  $$f = \frac{c}{\lambda}$$
  For $304\,\text{Å}$ ($\lambda = 30.4\times 10^{-9}\,\text{m}$):
  $$f \approx \frac{3.00\times 10^{8}}{30.4\times 10^{-9}} \approx 9.9\times 10^{15}\,\text{Hz} \;(\text{EUV})$$
- F10.7 (Solar Flux Index) reference:
  - F10.7 is measured at $10.7\,\text{cm}$ ($2.8\,\text{GHz}$) in solar flux units (SFU), where $1\,\text{SFU} = 10^{-22}\,\text{W}\,\text{m}^{-2}\,\text{Hz}^{-1}$.
  - Rule of thumb: Higher 304 Å emission and higher F10.7 usually indicate stronger ionospheric ionization and higher MUF for HF propagation.