The DIFOS-M Experiment

Observations of Solar Eigen Oscillations aboard the CORONAS-F Satellite

Spherical harmonics describe the brightness distribution over the solar surface due to solar oscillations. They are characterized by its degree l, measuring the total wave number and its azimuthal order m. For each value of l and m many modes may exist, which have for different radial orders n different frequencies.

The photometer responds to each oscillation mode only by the integral value of the brightness distribution over the entire solar disk. This response decreases very quickly with increasing of l because all spherical harmonics are averaged over the disk. Therefore the DIFOS-M photometer working without any spatial resolution can record only modes with low l (< 3). But even in low l the oscillation spectrum is very complicated due to the frequency splitting by the solar rotation. A sophisticated frequency resolution needs continuous observation during at least two weeks. The CORONAS-F satellite will have a synchronous orbit with an altitude of 500 km and 83° inclination. This orbit permits observational periods up to 20 days without interruption by the Earth's shadow.

Planned activities:

Continuous measurements of the solar flux intensity and its fluctuations with high quality in 6 channels.
Determination of frequencies, amplitudes and phases for solar p-mode oscillations in the frequency range of 1 to 10 mHz.
Determination of the dependence or independence of these parameters from the 11-year solar cycle.
Experiments about registration of g-modes of solar oscillations.
Using the found characteristics of oscillations as entries for:
- model design of the solar structure,
- determination of the sound speed value,
- determination of flux and rotation speed of the inner solar layers.

The amplitude of the solar brightness variations is very small due to the eigen oscillations and consists of some millions of particles of the full flux. For their observation we need a high-stable instrument with high resolution to seperate so small signals from background of the full solar flux. To realize these listed tasks the solar photometer DIFOS-M was designed by I.M. KOPAEV et al. at IZMIRAN (Russia).

Main parameters of the DIFOS photometer designed for the continuous registration of the solar flux intensity from the whole Sun:

Spectral ranges:	6 spectral bands: 
			350, 500, 650, 850, 1100, 1500 nm  
			(spectral characteristic of the photosensors,
			optical glass filters with a band-pass width of 10%)

Accuracy:		The relative resolution of the solar flux is
                   	10&sup-6; I&subo; for an integration time of
                   	16 s during each measuring cycle

Visual field:		2°
	 
Pointing accuracy:	Pointing to the Sun within an accuracy of < 10'
			supported by the orientation system of the satellite
	
Orientation sensor:	The photometer is additionally equipped by an 
			orientation system with a resolution of 5"

Calibration:		It is done by the sensitiveness of the photosensors
			in the optical bands 650 nm and 850 nm
	
Data output:		16 bit binary code 
	
Temperature control:	Thermostatic control of the box of photosensors 
			within 1°
	     
Power supply:		25 W 

Mass:			-- Box of photosensors:	5.1 kg
			-- Box of electronics:	2.8 kg

Size:			-- Box of photosensors:	130 × 130 × 510 mm³
			-- Box of electronics:	120 × 230 × 210 mm³