Measurements of polytropic index in the January 10-11, 1997 magnetic cloud observed by WIND

The polytropic index gamma is a key parameter in the modeling of interplanetary magnetic clouds. In contrast to other models, the self-similar model of Osherovich et al [1993a] suggested gamma < 1 in clouds. The first measurements [Osherovich et al, 1993b] showed that gamma approximate to gamma e = 0.4 - 0.5 in magnetic clouds demonstrating the dominant role of electrons over protons (Te/Tp approximate to 6-7 at 1 AU). For the January 10-11, 1997 cloud observed by WIND, we found that the polytropric relation T-e= FNegamma c-1 is valid with gamma e = 0.52 +/- 0.04 and F = 1.97 10(5) +/- 0.12 10(5) (cgs units) over the number density range 1 < Ne < 10 cm(-3). The constant F is half the value found for other clouds studied at 1 AU, while gamma e is essentially the same. We believe that the small value of F should be attributed to a later stage of the self-similar evolution of this cloud as compared to other clouds studied previously at 1 AU.