# 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, ##\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 ?

(*)  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 way of measuring the energy output of a solar event, such as a solar flare or coronal mass ejection. They represent the amount of energy per unit area that is emitted by the sun during a particular event.

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

Solar event flux units are typically measured using instruments called radiometers, which detect the energy emitted by the sun in different wavelengths. The measurements are then converted into flux units by taking into account the area of the sun's surface and the distance between the sun and the Earth.

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

Solar event flux units are important because they allow scientists to quantify the energy output of solar events and track changes over time. This information can help us better understand the processes happening on the sun and their potential impact on Earth.

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

Yes, there are several different types of solar event flux units, including watts per square meter, ergs per square centimeter per second, and joules per square meter per second. Each type is used to measure a specific range of energy output from the sun.

## 5. How do solar event flux units relate to other units of measurement?

Solar event flux units are related to other units of measurement, such as watts and joules, through mathematical conversions. For example, 1 watt per square meter is equal to 1 joule per square meter per second. It is important to use the correct units and conversions when working with solar event flux measurements.