Typical observations at the 60' Solar Tower are obtained in
a continuous manner by which 2 images are taken every minute for up
to 12 hours/day. One group of 12 images is taken in the early morning
hours before the continuous observations are started. From this set
of 12 images, a magnetogram, dopplergram, intensitygram, and velocity map
are computed and added to this database on a daily basis (when the
weather cooperates). In some cases it may not be possible to compute
this series due to inclimate weather in the early morning hours, but the
continuous data may still be obtainable for the rest (or portion) of the
day. The opposite may also be true, whereby, the 12 image set is obtained
but the weather has worsened for continuous observations of the sun.
Presently, all images are taken in the chromospheric Sodium D-line
region of the solar atmosphere with the Magneto-Optical-Filter. The
orientation of the images is north at the top and west to the right.
The 12 image set consists of 3 sets of 4 image groups. The 4 image
group is taken over a course of 15 seconds, each image 5 seconds apart,
starting at the top of the minute. The 4 images can be indentified as red+,
red-, blue+, and blue-. The 2nd and 3rd sets are taken at 90
second intervals from the start of the 1st image set. By the time the 12th
image is taken, 3 minutes 15 seconds have elapsed.
Below is an explanation of how the four image types are computed.
A filtergram is simply the addition of two images (a red
and blue shifted component of a specific wavelength)
A dopplergram is a difference of a blue shifted
image of the sun from a red shifted image, divided by the sum of the red
and blue components. The dopplergram shows regions of the sun that are
rising or sinking, and rotation of the sun has not been subtracted out (as
can be seen by the large light to dark gradient across the disk of the sun).
Since the sun rotates to the west and the darkest portion is on the right
hand side, dark regions are those which are moving away from the earth and
light regions are those moving toward earth. Two dopplergrams are computed
(but only one is displayed), one in the first image group and one in the
3rd image group.
A velocity map is the difference between two dopplergrams
separated by 3 minutes (this is roughly 1/2 of the 5-minute oscillation
cycle). This procedure subtracts out the rotation of the sun and emphasizes
the numerous up and down motions of the sun scattered across the disk.
Color has been added to represent regions moving toward earth (blue) and
those regions moving away from earth (red). The time labeled for the velocity
map is the start time for the first dopplergram. The second dopplergram
from which the velocity map is computed will always be 3 minutes later.
NOTE: As of September 27, 1999 the velocity map images will be
discontinued. On a day-to-day basis, these images show no change, and
take up precious disk space. The older velocity images will remain on
disk for access through the image-database, but no future images will be
provided. Eventually, the existing velocity images may be unavailable
through this URL to save on disk space. Velocity images will continue
to be processed but only available through request.
A magnetogram is computed from the entire set of 12 images
(3 sets of 4) taken in the Sodium-D line spectrum of the sun. Each set of 4
images is used to compute 1 magnetogram. These 3 magnetograms are then
integrated together to obtain an averaged image of the sun over a course
of 3 minutes, 15 seconds. The time identified and labeled for the
magnetogram is the start time for the entire set.
Fits format images are provided as courtesy to the scientific
community. The fits header keywords are provided below as an example.
Additional keywords may be added as they become useful or needed. If you
see a reason to have another keyword added, please contact Perry Rose.
SIMPLE = T / Converted by mw2fits
BITPIX = 16 / 16 bit/pixel
NAXIS = 2 /
NAXIS1 = 1024 /
NAXIS2 = 1024 /
DATE = '25/05/99' / Observation date
TIME = '06:06:00' / Observation time, PST
KTIME = 1249236005 / Observation time, 100th sec since...
STATUS = 0 / Observation status
ORIGIN = Mt. Wilson / Telescope location
TELESCOP= MWO_60 Ft / Telescope name
OBS_MODE= HR_Na-Magnetogram / Image type
ORIENT = SESW / Image orientation
For more information on this project, or how to obtain additional
images and/or data, contact
Dr. Edward Rhodes of USC.