For reference, the number density per cubic cm for molecules in Earth's atmosphere at
sea level is 2e19 (or 2e25 /m3).  The mass density, assuming N2 molecules (N14),
is 2e25particles/m3*28amu/particle*1.67e-27kg/amu = 0.94 kg/m3.

Date: Mon, 22 Oct 2007 12:15:08 -0400
From: "Joseph B. Gurman" 
Subject: Re: another solar question
To: Barbara Thompson , Gerard Williger 
Cc: Therese Kucera 

     By the way, I should never read old papers out of the office: 
optical depth unity in the 500 nm continuum in the VAL3c is down 
around lines 49 - 50 in Table 12 of Vernazza, Avrett. and Loeser 
(1981 --- attached), and the hydrogen number density is ~ 10^17, so 
the mass densities (that number times 1.66 x 10^-24 gm) are ~ 10^-7 
gm cm^-3.If, as good pedagogues, they were using SI units, that would 
be 10^-4 kg m^-3, so the typos is "number density" instead of "mass 
density." (The top line of the table is the top of the model, 
referred to euphemistically as the "base of the corona" but really 
the middle of the transition region, before the density really starts 
nosediving.)

     n_e is a lot closer to n_h in the transition region than in the 
photosphere, where not much is ionized, and generally n_e is used in 
the corona for number densities, since the hydrogen number density 
depends on radiative transfer (resonant scattering) and flow fields 
(Doppler dimming). 10^8 - 9 is a reasonable figure for the very 
bottom of the corona, but not for a few radii out, as per my earlier 
message (Vourlidas et al. paper).

     Sorry to have steered you wrong earlier, and sorry to be such an 
old curmudgeon that I have to think in cgs units and convert to SI.

     Best,

						Joe


HERE ARE REAL VALUES FOR CORONAL AND PHOTOSPHERIC DENSITIES.  REMEMBER TO
MULTIPLY BY 1 MILLION TO CONVERT FROM PER CUBIC CM TO PER CUBIC M!

Date: Sat, 20 Oct 2007 00:00:45 -0400
From: "Joseph B. Gurman" , NASA Goddard Space Flight Center
Subject: Re: another solar question
To: Gerard Williger 

     FWIW, Table 12 of Vernazza, Avrett, and Loeser (1981), the 
"VAL3C" standard solar atmospheric model gives a number density for 
hydrogen of 10^9 at a height corresponding to optical depth unity in 
the 500 nm continuum, the generally accepted definition of "the 
photosphere." I doubt any other quiet Sun models would differ by more 
than 20 - 30%.

     A recent coronal _electron_ density model can be found at:
Hayes et al. 2001, ApJ,  548, 1081
 
http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v548n2/52619/52619.html 
Corona electron density: 
solar radii, /cm3   2.    3.    4.   5.   
-----------------------------------------------
polar              3.5e5 4.8e4 4.e4 1.5e4
equatorial         2.5e6 4.e5  1.e5 5.e4


     Best,

						Joe Gurman