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AGU June 2000 -- Poster
SYNOPTIC MAPS FROM THE LASCO AND EIT EXPERIMENTS ON SOHO
N. B. R.1 2
K. P. D.1
R. A. Howard (PI)1
Summary
Synoptic (or Carrington) Maps are an efficient method of presenting an overview of both short term and long term coronal evolution. Presented here are east and west limb maps from the C2 and C3 instruments of the Large Angle Spectrometric Coronagraph, and also meridian maps from the Extreme-ultraviolet Imaging Telescope on the SOHO spacecraft. Data is available for approximately 51 solar rotations spanning 4 years, from January 1996 to the present. Presented here is a subset of this data in Carrington format, giving an overview of the evolution of coronal structure in the rising phase of the solar cycle. The maps demonstrate the evolution of the corona from a relatively simple, two-sectored structure to an increasingly complex multisectored structure. Distortions and discontinuities in the streamer belt visible in the limb maps are associated with active regions on the sun, the shape of the current sheet, and coronal mass ejections. Meridian maps for EIT illustrate the corresponding structures and long-term changes in the lower atmosphere.
PROCEDURE FOR MAKING LASCO CARRINGTON MAPS
Compile list of images from database.
Take a slice of each image at each specified altitude, 181 values per slice. (see Slices Example)
Insert slice as a column in a 360x181 or 720x181 array.
If there are one or more columns skipped, interpolate from the last column; if gap is more than 1.5 days, leave blank and fill with a constant value.
If images are closer than one column, skip the image.
Scale display to best visibility of structure (scale is not constant between displayed images).
PROCEDURE FOR MAKING EIT CARRINGTON MAPS
Compile list of images from database.
Based on time of previous and next image, compute the amount of rotation to be covered by the current image.
At the equator, take a horizontal segment corresponding to the rotation distance to be covered by the current image, centered on the current time.
For each increment in latitude, take the horizontal component of the rotation distance at that latitude and expand to the width of the component at the equator. This results in a Mercator projection of the heliosphere. (See EIT figures.)
Use a maximum of one image per column. EIT maps are 720x512 pixels.
ABOUT LASCO CARRINGTON MAPS...
Time moves from right to left.
Each rotation of the sun is 27.3 days; begin and end times correspond to when the central meridian is on the east/west limb, so east/west limb maps correspond to each other as well as conventional Carrington maps (such as Kitt Peak magnetograms).
White bars at top of map indicate missing data.
C2 and C3 maps in the FITS files have a longitudinal resolution of up to 0.5°, depending on the image cadence. Latitudinal resolution is 1.0°.
Horizontal streaks may be due to interpolation.
Pylon obscures field in lower half of east limb in C3 (see individual image).
Data Gaps are filled with a constant value if over 1.5 days.
The LASCO images are processed by taking a ratio using a 4-week average "model" to cancel the dust corona and instrumental stray light. To create this model, the minimum for each pixel is taken from 28 daily median images covering the same DOY over 1 year (C3 Oct. 1998 - 2000), 2 years (C3 1996 - June 1998), or 4 years (C2). Click here for example.
The images used are pre-calibration (level 0.5)
ABOUT EIT CARRINGTON MAPS...
Longitudinal resolution in the FITS files is 0.5° at the equator. Latitudinal resolution is 4.6 arcseconds.
USES OF SYNOPTIC MAPS
Gives an overview of coronal activity
Locate major events (CMEs, active regions) quickly
Gives position of the streamer belt
East and west limb maps can be used to identify simultaneous global activity
Comparing east and west limb maps show evolution of corona on a 14-day time scale
AVAILABLE FORMATS
All Carrington maps are made in FITS format. LASCO C2 east and west limb maps are available at radii of 2.5, 3.0, 4.5 and 5 Rsun. Carrington maps for LASCO C3 are in the process of being computed and are being made for Rsun=5, 7, 10, 15, 20, 25. Carrington maps for EIT are being made for wavelengths 171 Å, 195 Å, 284 Å and 304 Å. EIT limb maps are possible. To work with the FITS data, please contact one of the authors. Carrington maps for EIT and LASCO are also available for viewing in GIF or PS format at
http://lasco-www.nrl.navy.mil/carr_maps
.
FEATURES IN SYNOPTIC MAPS
Coronal Mass Ejections
Appear as discontinuities and/or vertical stripes
CMEs sometimes disrupt the streamer belt, which may or may not return to its original state.
Small CMEs may not be apparent in maps.
Streamer Structure
The same features are usually apparent on both limbs, but features may also evolve on time scales less than 14 days (half a rotation).
The maps show the structure of the corona becoming more complex from 1996 to 2000 as solar maximum approaches.
Activity in the corona persists at certain longitudes (such as 270º) over many rotations, particularly in 1996.
Other Features
Visible in Synoptic Maps
Planets: Mostly in C3
Proton Events: such as 21 April 1998
EIT
Active regions
Coronal holes