Why does H alpha give good solar detail?

[spaceman_spiff]

Hi there,

Apologies if this has been posted before but I have been wondering why narrow band (>1nm bandpass) give so much solar detail.

Is it because the magnetic field of the Sun shifts H alpha emission towards/away from the filter bandpass - giving contrast in a filtered image? Or is there something else going on.

Many thanks!

[Stub Mandrel]

As I understand it, it 'maps' the distribution of areas of  'relatively cold' hydrogen on the surface of the sun which absorbs more than it gives out.

[spaceman_spiff]

Thanks Stub Mandrel, so it's more to do with temperature than magnetism, I guess these things are themselves related.

[jabeoo1]

Hydrogen is primarily what we are staring at when we are looking at the sun, and most of the action is taking place in one of the possible energy states an electron can occupy in the hydrogen atom.  A hydrogen atom can only permit an electron to exist in one of 3 states.  One of those energy states involves an electron jump from the 3rd shell downwards towards the second shell towards the nucleus.  This event makes up near 50% of the energy state changes overall.  This specific jump will emit a package of energy in the form of a photon at the visible red end of the electromagnetic spectrum at a precise 656.281 nm wavelength.  

Having a Ha scope taps into this precise photon package that is the dominant event and enriches the view by rejecting the other energy state events that make up the other parts of the show.  There are 3 other visible lines in the Balmer series and these are filtered out allowing a richer view of the more dominating one Ha Balmer series line.  

A scope capable of a 0.7Å bandpass is just saying it is capable of tuning down to an accuracy 0.07 nm, pretty close to the dominating 656.28nm energy package.  

The plasma state is in constant state of flux, sometimes rapid movements can doppler shift the apparent wavelengths of the this (and other) Balmer series lines.  By accurately tuning into either of the wings (either side) of the 656.281nm wavelength we can compensate for this, enriching the view in favour of a high flux event such as a CME.  Most of the surface will appear rich by tuning into the central part of the line as most is not significantly doppler shifted.  

I am guessing based on all this rambling that the contrast must come from a mixture of factors such as:  

The more accurate the scope (the lower the Å value) is at tuning into 656.281nm the better the view is, as this is simply exactly where most the action takes place, whilst conversely,

the areas being ignored by the rejection fitters are essentially leaving gaps in the view and giving space for the Ha line to dominate.  

Doppler shifts where rapid plasma movement is tuning in and out on the wings of the scopes current tuned wavelength would give variation. 

The sun spots are visible as black spots even when the scope is detuned way off band so I assume they are permanent sharp voids in the view. 

I may be totally wrong on this and am not a solar expert or physicist.  I just have a Lunt 50 and use it & also like yourselves wonder how this stuff works. 

[Uplooker]

Its because the details that you are observing are emitting light in the near infra-red i.e. 653 nm. The etalon / filters isolate this wavelength of light so that you may view it.

[spaceman_spiff]

Thanks, I didn't consider a doppler effect, but this does explain why strong contrast can be seen around 'active' regions (such as CMEs, edges of convective zones...etc). 

[PeterW]

Don't forget the near UV Calcium detail either... Another strong absorption line where you get contrast.

Peter

[spaceman_spiff]

Don't forget the near UV Calcium detail either... Another strong absorption line where you get contrast.

Peter

Thanks Peter,

Does imaging at that wavelength require the same narrow bandwidths (<1nm) as H alpha?

[Peter Drew]

Apparently not. 

[ollypenrice]

Depending on the filter you use, you are imaging or observing at different depths within the sun's outer layers.

Olly

[spaceman_spiff]

Depending on the filter you use, you are imaging or observing at different depths within the sun's outer layers.

Olly

Thanks Olly, good to know