The problem of solar wind data correlation for the observations onboard several spacecraft is very important for three points of view:

• the methodical one: intercalibration of the several instruments for checking of their correctness
• the scientific one: study of the solar wind space structures and their variations
• the pragmatical one: the verification of the possibility to use the measurements on the spacecraft far ahead from the Earth (about 200 Re) for space weather predictions

This problem is studied now in detail by systematic determination of ion flux and ion velocity data correlations by INTERBALL-1 (IKI group), WIND and IMP 8 (both - MIT group).

At the Fig. 1,2,3,4 one of examples of such comparing for the 6-hours interval on April 3, 1996 is presented.

Fig. 1 shows the positions of three spacecraft at GSE-coor dinates in the projection to the ecliptic plane. WIND was ahead of the Earth at about 75 Re along the Xse; INTERBALL and IMP 8 were separated by about 50 Re along Yse.

Fig. 2 shows the comparing of simultaneous ion flux measurements onboard three s/c with time resolution 1-1.5 min. Data for WIND and IMP 8 are shifted by the time of solar wind propagation to the INTERBALL-1 position. It can be seen the very good coincidence of these data including many fine features.

Fig. 3 shows the dependence of cross-correlation coefficients (INTERBALL-1 / WIND and INTERBALL-1 / IMP 8) on the time lag. For both comparings correlation coefficient is very high - up to 0.96 (!) - and its maximum is near zero lag. It means that in this event: a) the solar wind structures aren't changed significantly during about 20 min. (propagation time from WIND to INTERBALL-1); b) correlation length is more than 50 Re in the direction perpendicular the Xse.

Fig. 4 shows the similar comparing for solar wind velocities. Also the data from three spacecraft coincide very well.

This work is in progress now and it's possible to present as a preliminary result the hystogramm of cross-correlation coefficients for comparing of INTERBALL-1 and WIND ion flux data in 76 events (each event contents 6 hours of measurements - about 330 points) during April, 1996.

Such distribution is shown at Fig. 5. It can be seen, that in the 50% of events the cross-correlation coefficient is very high - more than 0.82. The average value of correlation coefficient for this period (one month) is 0.76. But there are a few events (about 20%) with low correlation (coefficient less than 0.6). The reasons of the low correlation events observed sometimes now is one of the targets of our study.

Referencies

• Paularena K.I., G.N. Zastenker, A.J. Lazarus, and P.A. Dalin. Solar wind plasma correlation lengths from IMP 8, WIND and INTERBALL-1. EOS Trans. AGU, 77, S220, 1996.
• Dalin P.A., G.N. Zastenker, K.I. Paularena, A.J. Lazarus. Correlation of solar wind observations onboard several spacecraft: INTERBALL-1, IMP 8 and WIND data. WDS'96, Proceedings of contribution papers (Physics of plasmas and ionized media), Faculty of Mathematics and Physics, Charles University, Prague, 1996.
• Paularena K.I., J.D. Richardson, A.J. Lazarus, G.N. Zastenker, P. A. Dalin. IMP 8, WIND and INTERBALL observations of the solar wind. Physics and Chemistry of the Earth, 1997 (in press).
• Dalin P.A., G.N.Zastenker, K.I. Paularena, A.J. Lazarus. Solar wind plasma correlation lengths from INTERBALL-1, IMP 8 and WIND data. Proceedings of International Conference "Problems of Geocosmos", University of Sankt-Petersburg, 1997 (in press).

Contact persons:

• K.Paularena, MIT, Center for Space Research, Cambridge, USA
• G.Zastenker, Space Research Institute, Moscow, Russia