V.N. Lutsenko, Space Research Institute, Russian Acad. of Sci., Profsojuznaja 84/32,

117810 Moscow, Russia.

K. Kudela (PI), Institute of Experimental Physics, Slovak Acad. of Sci., Watsonowa 47

04353 Kosice, Slovakia.

E.T. Sarris, Demokritos University of Thrace, Department of Electrical Engineering,

67100 Xanthi, Greece.

Correspondence to: V.N. Lutsenko. Phone: 7-095- 333-33-56,

Fax: 7-095-310-70-23, E-mail:

The study of energetic particles acceleration and transport within the magnetosphere and on its boundaries is the main goal of DOK-2 experiment on Tail Probe in a frame of INTERBALL project. Two particle spectrometers: DOK-2 and its small version - DOK-2S were installed on the main satellite and small subsatellite. Both function successfully since launch on 2 August, 1995. DOK-2 experiment continue our studies of energetic particle population on Prognoz 10 satellite in 1985 (Intershock project). Due to much higher energy, angular and time resolution, better separation of electrons and ions it is expected that DOK-2 experiment will allow to re-examine in more details energetic particle population and related processes in magnetosphere regions covered by the spacecraft orbit.

DOK-2 uses two pairs of energetic particle telescopes. Each telescope has a single, passively cooled, totally depleted, surface-barrier silicon detector. One detector of each pair (1e,2e) is of 0.3 mm thick. Thin foil in front of detector absorbes protons with energies E<400 keV so this detector measures electron spectrum in 20 - 420 keV range. The second detector (1p,2p) of 0.15 mm thick is supplied with a broom magnet, deflecting electrons up to 1500 keV . It measures spectrum of ions (protons) in 20-850 keV range. Full aperture angles and geometric factors of electron telescopes are of 27° and 0.066 . For proton telescopes these values are 12.7° and 0.015 . Particle flux angular distribution is measured using spacecraft rotation with a ~2 min period around the axis directed to the Sun (the angle ) and mechanical scanning of the second telescope pair in a plane containing the rotation axis (the angle ). So 3-dimensional particle distribution function is measured during each 2 min rotation period. About seven full scanning cycles are carried out for every rotation. The first detector pair (1e,1p) is fixed in the antisolar direction. Moving detector pair (2e,2p) can either scan from =45° to 180° (to a Sun direction) or stay in one of 4 positions: 45, 90, 135 and 180°.

Analog electronics for each of 4 detectors consists of a charge sensitive preamplifier (in a cooled part of the instrument), a shaping pulse amplifier and 56-57 channel amplitude analyser AA (logarithmic ADC). The measurements base is formed of "elementary measurement" (EM) - one full 56-57 channel spectrum measured every 1 sec for each of 4 telescopes. This information is available only during the most informative Real Time mode of DOK-2. In storing modes there are two kinds of output information formed from EM's :

- Full 56-57 channel spectra accumulated during time intervals from 10 s to 1464 s depending on a particle intensity.

- Three time profile (TP) parameters for each detector. TP-parameters are counting rates in 3 narrow (<30%) energy intervals. Energy values of TP-parameters are shown in Table 1. An accumulation time for TP is variable in basic Monitoring modes (1-260 s). It depends on variability of particle intensities. Special algorithm ensure time resolution of 1 s when intensity change is statistically valuable. In Fast modes accumulation time interval is 1 s .

The mechanical scanning occurs only in Fast and Real Time modes. Transition from Monitoring to more informative Fast modes occurs automatically when particle flux variability is high enough or after a sharp increase of fluxes. Return to Monitoring mode is also automatic.

Spectrometer DOK-2 was switched on 5 hours after the launch on 3. 08.1995 and is in a continuous operation up today exept 3-4 hours swich-off in the radiation belts once on each orbit. In flight calibrations show that energetic resolution of detectors is 5-7 keV. Some problems were with 2e-detector ADC, which was blocked from 11.09.1995 to 31.12.1995 and from 3.03.1996 up today so as with a mechanically scanning system. From 1.11.1995 the moving detector pair stay at =62°. All other systems both hardware and software are working normally.

DOK-2 data are processed in the Space Research Institute RAS, in the Institute of Experimental Physics SAS and in the Demokritos University of Thrace, Greece. DOK-2S data are processed in the Institute of Experimental Physics.

Due to high energy and time resolution of DOK-2 spectrometer we got many new data concerning energetic particle population in the geotail, magnetosheath and upstream of the bow shock during first year. The systematic analysis of these data together with magnetometer and plazma instruments data is starting now. Below we give some examples of DOK-2 data.

Fig. 1 and 2. give examples of DOK-2 responce by a passage of some magnetic structures in the geotail where magnetic field vector is about parallel to the spin axis. Fig. 1. show TP-parameters modulation during plazmoid structure passage (between two vertical red lines). Energy independed spin modulation (T=2min) in 2p-detector is due to magnetic field direction change. In Fig. 2. the amplitude of the spin modulation is different for different energies. It increase with energy (and gyroradius) of protons. The modulation here is caused by violation of axial symmetry of particle angular distribution relative to magnetic field vector. This is due to approaching of a boundary of some region with higher particle density (plasma sheet). The variation of the modulation amplitude reflects the change of the distance to the boundary. After the moment marked by red line the spacecraft crossed this boundary and enters plasma sheet.

Figure 3 showes examples of electron and proton spectra in the magnetotal plazma sheet. The spectra are averaged over 1-3 hour time intervals. It is seen that experimental spectra (hystograms) for electrons can be approximated (smooth lines) with a high accuracy by exponential law with a characteristic energy 22-24 keV, while proton spectra - by power law with the exponent values - 4.0...- 4.2. Statistical uncertainties bar are plotted for each 5-th channel but they are too small to be seen except the highest channels. These approximations are valid in 4-5 orders of magnitude range and in energy ranges 20-250 keV (electrons) and 30-700 keV (protons) .

Figure 4 illustrate the variability of spectra shapes in different parts of the magnetosphere: velosity dispersion effects, build up of proton spectrum with a peak and spectra with peaks. Upper right spectra were measured in the plasma sheet, other in the magnetosheath.

1.10.1996