Information about ECHO
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ECHO measures the properties of the solar oscillations. These oscillations are acoustic waves trapped below the surface of the Sun which set up globally coherent modes of oscillation. Observation of the surface manifestation of these modes allows the inference of the structural and rotational properties of the solar interior. This process is analogous to the seismological study of the earth and is therefore called "helioseismology".
At any given time, the Sun oscillates in millions of oscillation modes with different surface patterns all of which propagate to different depths into the Sun. The periods of the oscillations are approximately 5 minutes. The modes are specified by the number of nodal planes around the Sun, which is called the mode degree and usually denoted by the letter L. There is a relationship between the mode degree and the depth of propagation in the sense that low-degree modes travel to the solar core, while high degree modes are confined closer to the surface.
The name of the first instrument, LOWL, was derived from the fact that it was optimized to observe the deeply penetrating LOW degree (L) modes. The LOWL was located at the Mauna Loa Solar Observatory on Mauna Loa, Hawaii and operated from Feb. 24, 1994 to Sep. 2000. The first ECHO instrument is hosted at the Observatorio del Teide, located at Izana, Tenerife, Spain. It has been operating since Dec. 15, 1999. The LOWL instrument on Mauna Loa was upgraded to an ECHO instrument during Sep. 2000 bringing the ECHO network online. The ECHO network was built and is operated by the High Altitude Observatory, of the National Center for Atmospheric Research in Boulder, Colorado.
ECHO observes the oscillations as perturbations of the radial velocity over the surface of the Sun. This is accomplished by taking images in narrow bandpass filters displaced slightly redward and blueward of a solar absorption line. A velocity image is obtained from the difference between the two intensity images. Modes are separated in the data by projecting the velocity images onto spherical harmonic functions. The time series of each mode coefficient is then Fourier transformed in time to yield a power spectrum which has discrete peaks at the mode eigenfrequencies. Accurately determined eigenfrequencies are the basic data product of this instrument. The internal structure and rotational characteristics of the Sun can be determined from these frequencies.
A more detailed explanation of the instrument is given in the paper, Instrument to Observe Low-Degree Solar Oscillations, which is available in the Recent Publications section of this Homepage.
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