Interpreting solar event flux units

[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?

 

[BvU]

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,

$\qquad$ !​

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 ?

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

$\$

 

[berkeman]

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...

 

[SteveA001]

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.

 

[hutchphd]

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

 

[SteveA001]

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.

 

[hutchphd]

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

 

[SteveA001]

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

 

[hutchphd]

Report back if this is horribly wrong!