Solar Temperature and Density Diagnostics Based Upon Observations by the SOHO Extreme-ultraviolet Imaging Telescope

by Jeffrey Newmark

DANGER!! ENTER AT YOUR OWN RISK!!

 

• Goals - Can we use EIT to estimate temperatures and electron densities (Emission Measures) in regions of interest (e.g. prominences, active regions, polar plumes, coronal holes, quiet sun network, etc...)

• Input data - Atomic Physics = CHIANTI, Instrument parameters = Jim Lemen

• CHIANTI: CHIANTI consists of a database of atomic data and IDL procedures for calculating spectroscopic emission line intensities from optically thin astrophysical spectra as a function of density and temperature. A paper describing the database is Astronomy and Astrophysics Supplement Series, 7.2.1997, authored by K. P. Dere (NRL), E. Landi (U. Florence), H. E. Mason (U. Cambridge), B. Monsignori Fossi (Arcetri) and P. R. Young (U. Cambridge).

• Instrument Data - Presently this reads the Instrument Data from the EIT GENX files on the SSW as provided by J. Lemen. This section is modularized so as to facilitate updates or even new instruments (TRACE?).

  Effective area for Clear Filter:
  defined as: (Mirror Reflectivity)*(Filter Transmission)*(QE)*(Aperature)

• Main driver: EIT_TEMP -> returns temperature (K) and emission measure (cm^-5)

  inputs: EIT processed (e.g. EIT_PREP) data to obtain either FeXII/FeIX,X or FeXV/FeXII ratio. Data can be whole images, single pixels, or vectors.

  *Pre-flight Instrument Calibration- warning error source

  /pre_calc option - use previously calculated spectra for speed

  Please read header (doc_library,'eit_temp') for this routine

  Example: Use the ratio of FeXII/FeIX,X to obtain the temperature and emission measure. The data chosen are a FeXII image taken on 7-Jul-1997 22:03:19 and a FeIX,X image on 7-Jul-1997 22:00:14. These are both subfields contained in the same Level zero file.

  sswidl> eit_prep,'efz19970707.220014',hdr171,img171,image_no=0,/normalize, /response
  sswidl> eit_prep,'efz19970707.220014',hdr195,img195,image_no=2,/normalize, /response
  sswidl> temp_map = eit_temp(dn171=img171,dn195=img195,/pre_calc,dem=em_map)
  sswidl> print,minmax(temp_map)
  987684. 1.51475x10⁰⁶
  sswidl> print,minmax(em_map)
  1.10257x10²⁷ 9.41696x10²⁸
  

• Lower level routines:

  ISOTHERMAL: Calculate synthetic isothermal line spectra based upon synthetic in CHIANTI (units=photons s^-1 sr^-1 cm³ or ergs...).

  CHIANTI_SPEC: Calculate synthetic isothermal spectra (units=photons s^-1 sr^-1 cm³ or ergs...). Shell routine around isothermal and conflx (this is a continuum flux routine written by J. Lemen based upon Mewe, Lemen, and van den Oord (Paper VI, 1986, A. A. Suppl., 65, 511.).

  EIT_FLUX: EIT specific flux calculations:

  Specify Te + filter + DEM ==> compute DN/s/pixel

  Specify Te + filter + DN ==> compute DEM cm^{-5} = INTEGRAL(N_e²)dh

• Examples of Isothermal Spectra, n_e=1x10⁹, Temp=70,000, 1.2x10⁶, 1.6x10⁶, 2x10⁶

• Example Response Curves: Given an Emission Measure distribution, e.g. for active region, quiet sun, and an intrinsic electron density (affects the level populations) Note: Response Curves in Solar Physics paper assume a constant emission measure with temperature.

• Computation of electrons/pixel/sec - based upon note of Ken Dere - "Spectral Response of the EIT"
  $e\; =\; A\; a$_pix INTEGRAL{(T R² 13.6eV 911)/(f² 3.65eV lam) q P} dlam

  where

  e	electrons s-1 pixel-1
  A	aperture area in cm2
  apix	pixel area (cm2)
  T	transmission of aluminum filter(s)
  R	reflectivity of a single mirror
  f	telescope focal length in cm
  lam	wavelength in Angstroms
  q	CCD quantum efficiency (<= ~1.0)
  P	solar spectrum in photons cm-2 s-1 sr-1

  
  

• Emission measure:
  Define solar spectrum P = Isothermal spectrum * Line-of-sight Emission measure
  P = photons cm³ s^-1 sr^-1 * cm^-5
  Therefore if input observed DN's you can invert the above equation to solve for the emission measure.

A) Temperature calculations:
  1)  non-unique solutions 
  2) EIT relative response changes - 
     we have an initial calibration, however this overcompensates any
     long term Solar trends
  3) Line of Sight effects versus feature of interest - is background
     subtraction being used, source of error
  4) We are just beginning to use cross checks with CDS NIS/GIS

B) Emission measure calculations:
  1) Instrument calibration: pre-launch values concerning:
     bandpass, mirror reflectivities, filter transmission, quantum efficiency
  2) CCD response - time variability - physical changes?

Since EIT is sensitive to a narrow temperature range and of observations have 3 filters, the true temperature can have fairly good constraints.

No serious error analysis yet -> probably better than 1-2x10⁵ K.

Jeffrey Newmark
newmark@midas.nrl.navy.mil, +1 301 286-3163
NASA Goddard Space Flight Center,
Code 682, Greenbelt MD 20771 USA