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

• Published in: Geophysical research letters 1998-08, Vol.25 (15), p.3003-3006
• Main Authors: Osherovich, Vladimir A., Fainberg, J., Stone, R. G., Fitzenreiter, R., Viñas, A. F.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Interplanetary space; Solar system; Solar wind plasma
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/98GL00547

The polytropic index γ 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 γ < 1 in clouds. The first measurements [Osherovich et al, 1993b] showed that γ ≈ γe = 0.4 ‐ 0.5 in magnetic clouds demonstrating the dominant role of electrons over protons (Te/Tp ≈ 6‐7 at 1 AU). For the January 10‐11, 1997 cloud observed by WIND, we found that the polytropric relation Te = FNeγe−1 is valid with γe = 0.52 ± 0.04 and F = 1.97 105 ± 0.12 105 (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 γe 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.