# Interpreting solar event flux units

• I
• SteveA001
In summary, the number of protons hitting a 1 m^2 surface area during a solar event is #/(cm^2 s sr). The value of the solid angle to plug into the units above is 4 ##\pi## steradians.
SteveA001
Hi,

I wanted to find the number of protons striking a surface area during a solar event and the units provided are: #/(cm^2 s sr). Say I have a 1 m^2 surface area directly facing the sun, how do I use those units to discover the number of protons/s. What is the value of the solid angle to plug into the units above?

SteveA001 said:
Hi,

I wanted to find the number of protons striking a surface area during a solar event and the units provided are: #/(cm^2 s sr). Say I have a 1 m^2 surface area directly facing the sun, how do I use those units to discover the number of protons/s. What is the value of the solid angle to plug into the units above?
Hello Steve,

You probably mean photons instead of protons (*) ?

And now you want to find out how many steradians there are in 1 m2 at a distance of sun-earth.

Does it help to know that a solid angle for a whole sphere is 4 ##\pi## steradians ?

(*)  unless I am mistaken -- in which case: do you have a link to the source that provides more context ?

##\ ##

Last edited:
topsquark and berkeman
BvU said:
You probably mean photons instead of protons (*) ?
Oh, good point. I was about to move this thread from the Optics forum to the Classical Physics forum, but I guess I should wait for the OP to respond...

I meant proton flux, sorry if my question is in the wrong section. Why is proton flux given in units of particles cm-2 s-1 sr-1 and not just as particles cm-2 s-1. [A search on "solar flare proton flux" returns pages using those units as the top search results]. Given those tricky units, what is the solid angle? I'm now suspecting that cm2 sr is the same as cm^2 for general purposes, with the steradian definition being a more formal definition of the cross-sectional area of a unit sphere. Just guessing though.

SteveA001 said:
I'm now suspecting that cm2 sr is the same as cm^2 for general purposes, with the steradian definition being a more formal definition of the cross-sectional area of a unit sphere. Just guessing though.
To get an actual number one must
1. multiply by the area of the detector in cm2
2. multiply by the acceptance solid angle of the detector in steradians
A one steradian cone has a central axis angle of ~33 degrees.

So operationally the 1 cm2 and 1 steradian detector probably provides a good starting guess

I just wanted to know the number of protons hitting a 1 m^2 surface. Is the solid angle of such a 'detector', which is facing directly towards the sun, 2 PI steradians? If so, the answer I want is the particle number they give for proton flux (cm-2 s-1 sr-1) should be multiplied by 2 PI and area? My interest is more in the danger to astronauts than detectors.

In fact the incoming proton number is not isotropic and the quoted number is probably a max. The assumption of (half space) 2pi "detection" by the astronaut will therefore overestimate the exposure somewhat but is a reasonable estimate. Heck this ain't rocket science...wait

Ibix
Ok, multiplying by 3 or so it is, for rough purposes. Thanks for your help.

hutchphd
Report back if this is horribly wrong!

## 1. What are solar event flux units?

Solar event flux units are a measure of the amount of energy emitted by the sun during a specific solar event, such as a solar flare or coronal mass ejection. They are typically measured in watts per square meter (W/m²) or in solar flux units (sfu).

## 2. How are solar event flux units measured?

Solar event flux units are measured using specialized instruments such as radio telescopes or X-ray detectors. These instruments are designed to detect and measure the amount of energy emitted by the sun during a solar event.

## 3. What is the significance of solar event flux units?

Solar event flux units provide important information about the intensity and magnitude of solar events. They can help scientists understand the behavior of the sun and its impact on Earth's atmosphere and technology.

## 4. How do solar event flux units relate to space weather?

Solar event flux units are a key component in studying and predicting space weather. By monitoring changes in solar event flux units, scientists can track the occurrence and intensity of solar storms and their potential effects on Earth.

## 5. Are there different types of solar event flux units?

Yes, there are different types of solar event flux units that are used to measure different types of solar events. For example, radio flux units (rfu) are used to measure radio emissions from the sun, while X-ray flux units (xfu) are used to measure X-ray emissions. Each type of flux unit provides unique information about the characteristics of a solar event.

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