SEE Level 2B EGS Occultation Data

Summary of EGS Solar Occultation Data

Top two figures show the atmospheric transmission as a function of wavelength from 27 nm to 190 nm. The color in these figures represent the tangent altitude of the occultation as shown in the third plot. The bottom plot shows the location on Earth for each measurement.


Detailed Level 2B EGS Occultation Description

README_EGS_L2B_OCC_011.TXT - information for using EGS_L2B occultation data

The EUV Grating Spectrograph (EGS) aboard SEE performs measurements of the Sun for about 3 minutes every orbit (97 minutes) which usually gives 14-15 measurements per day. 4x per year, in-between direct solar measurements, the SEE instrument has opportunities to observe the solar irradiance transmitted through the atmosphere. The Level 2B EGS Occultation data is time averaged over each measurement and divided by the average of the previous and next solar observations. The EGS resolution is about 0.4 nm, but the data product is sampled at 0.1 nm with valid data ranging from ~27-190 nm. Missing values are filled with -1.

A poster (Eparvier 2005) containing preliminary analysis results was presented at the fall 2005 AGU meeting and contains additional useful information.


Reading the Data with IDL

With read_netcdf.pro individual data files may be read using an IDL command such as:

The data will then reside in the returned structure "d", with each tag in the structure defined in the attributes array "att".


SEE Level 2B EGS Occultation Software

  • TIMED-SEE NetCDF README - Described IDL source code and gives examples using read_netcdf.pro and write_netcdf.pro
  • read_netcdf.pro - IDL procedure for reading NetCDF files as a structure
  • write_netcdf.pro - IDL procedure for writing a data structure (or array of structures) to a NetCDF file
  • plot_occ_alt.pro - IDL procedure for plotting a time series of EGS Level 2B transmission data near a certain date (requires the mission merged file).

SEE Level 2B EGS Occultation Data Format

The data file contains a structure with 1 elements

L2B_EGS_DATA    STRUCT    = ->  Array[1]

The L2B_EGS_DATA structure has the following form:

** Structure <585f98>, 12 tags, length=32375368, data length=32375368, refs=1:
DATE LONG Array[1196]
TIME FLOAT Array[1196]
LS_TIME FLOAT Array[1196]
TAN_PT_LAT FLOAT Array[1196]
TAN_PT_ALT FLOAT Array[1196]
WAVE FLOAT Array[1690]
TRANSMISSION FLOAT Array[1690, 1196]
ERR_TOT FLOAT Array[1690, 1196]
ERR_MEAS FLOAT Array[1690, 1196]
STDEV_MEAS FLOAT Array[1690, 1196]
VER STRUCT -> Array[1]

Nested structure VER ** Structure <587e88>, 4 tags, length=64, data length=64, refs=2:
CAL STRING '09.01'

NetCDF file attributes


Title = SEE Level 2B EGS Occultation Time Series Data Product - EGS occultation ratio
Source = SEE POC at LASP / CU
Mission = TIMED
Data_product_type = SEE EGS Level 2B Occultation Time Series
Data_product_version = 011
Data_product_revision = 01
Software_version = 11.01
Software_name = write_netCDF.pro called from time_series_egsl2b_add.pro
Calibration_version = 09.01
Description = Fractional Solar Irradiance transmitted during occultation
History = 03/10/06 D. L. Woodraska Original file creation
Filename = /titus/timed/data/merged/see__egs_L2B_merged_2012205_011.ncdf
Date_generated = Wed Aug 22 09:51:30 2012
Description = UT date of observation in year and day of year format
Units = UT seconds of day
Description = Center time of the measurement that spans about 3 minutes.
Units = hours
Description = Local mean solar time, Universal time + east longitude expressed in hours. Calculation is {([(tan_pt_long+360) mod 360]*24/360) + (time*24/86400) + 24} mod 24. Values are clustered around 6 and 18 hours corresponding to sunrise and sunset observations.
Units = degrees
Description = The mean earth latitude of the tangent ray point
Units = degrees
Description = The mean earth longitude of the tangent ray point (convention is -180 to +180)
Units = kilometers
Description = The mean earth altitude above the WGS84 ellipsoid at the tangent ray point location. The altitude during the transmission measurements changes from about ~10km for low altitudes and decreases to ~7km for higher altitudes. Values range from approximately from 50-450 km.
Units = nanometers
Description = Wavelength array of 0.1 nm bin centers that match the transmission measurements.
Units = None
Description = Atmospheric transmission through the tangent ray point (Note that due to finite and variable instrument precision, this value may exceed 1.) Transmissions are calculated by averaging 6-8 transmission measurements and dividing by the average of approximately 20 direct solar measurements from the previous orbit with 20 additional direct solar measurements from the next orbit.
Missing_value = -1
Units = Relative
Description = NIST combined relative standard uncertainty - a relative fractional uncertainty including all known sources of uncertainty that appear in the measurement equation. This is typically very high in regions that have degraded, or where the calibration is less accurate.
Missing_value = -1
Units = Relative
Description = NIST combined relative standard uncertainty - a relative fractional measurement uncertainty including only sources of uncertainty based on counts from the numerator of the measurement equation.
Missing_value = -1
Units = Relative
Description = NIST combined relative standard uncertainty - a propagated standard deviation of all the contributing integrations from normal (direct) solar observations and atmospheric transmission measurements.
Missing_value = -1
Units = None
Description = SEE calibration version number
Units = None
Description = SEE software version number
Units = None
Description = SEE file format version number (3-digits)
Units = None
Description = SEE data product version number (3-digits)


Revised: November 2007
+ Questions or comments?