In 2000, Doug Braun and Charlie Lindsey used the technique of helioseismic
holography to image, for the first time, active regions on the
backsi... er, far surface of the Sun. The figure on the right (click for a
high-resolution version) shows some examples of far-side seismic images
obtained from MDI observations over a ten day period, and compares
them with front-side
magnetograms obtained two weeks later (as the regions moved into direct
view).
The approach exploits
the horizontally extended acoustic travel-time reductions
recently discovered in plages and
moats surrounding sunspots.
Large active regions
are readily apparent in seismic images
reconstructed using observations of
global p-modes which can propagate long distances through
the Sun without significant loss of coherence.
The first images of the far side of the Sun were published in
the March 10, 2000 issue of
Science
(reprint here)
and reported in a ``Space Science Update'' at NASA Headquarters.
Starting in the fall of 2000, Lindsey and Braun began a collaboration with
Phil Scherrer and the SOI-MDI team to produce daily images of the
solar far side. In April 2001, the
web site
was officially announced by NASA and ESA.
Here is the
most recent far-side image.
The initial images of the far side were
limited to a region of the Sun within a heliocentric
angle less than about 50 degrees from the
antipode of disk center. In 2001, Braun and Lindsey demonstrated in this
ApJ Letter how
holographic procedures may be employed to produce
diffraction limited images of the entire far surface (and poles)
of the Sun as shown in the Figure to the left.
The top four panels show
composite images of the near-side magnetic flux
density and far-side acoustic travel-time
perturbations for
four succesive days, and the
bottom panel shows a synoptic magnetogram
for the following solar rotation.
Here is a
nice mpeg,
courtesy of Phil Scherrer, showing a sequence
which compares full-hemisphere MDI farside images with the previous
images.
During 2001, the newly upgraded GONG+ network began returning high resolution
images. Using some of the first 24 hours of continuous data available, a comparison
of far side images made from GONG+ and MDI was made (see figure to the right). The images
are nearly identical, demonstrating the feasibility of continuous montioring
of the solar far side using the GONG+ network. See the
GONG farside imaging site
for the daily image from GONG, as well as more information.
Below are links to some of the press releases and media coverage of far-side imaging as well as graphics and movies prepared for public dissemination. Note that most of the outdated links to the media coverage were removed.
Links to far-side imaging services and related items:
SOI-MDI far-side synoptic maps
spaceweather.com (now includes daily far-side image)
SWAN far side site (another interesting way to monitor the solar far side)
Links to press releases and media coverage:
Press release on first far-side image from GONG+
ESA release on initiation of routine far-side service
NASA's Space Science Update: press releases and graphics
National Science Foundation's press release
MDI-SOI site at Stanford with links to more movies and media coverage
This research uses solar observations obtained primarily from the Solar Oscillations Investigations - Michelson Doppler Imager on SOHO and the Global Oscillations Network Group (GONG), and is funded by the National Science Foundation and the National Aeronautics and Space Administration.