Solar Physics

, Volume 289, Issue 4, pp 1371–1378

Long-Term Variability of the Polytropic Index of Solar Wind Protons at 1 AU


DOI: 10.1007/s11207-013-0401-x

Cite this article as:
Nicolaou, G., Livadiotis, G. & Moussas, X. Sol Phys (2014) 289: 1371. doi:10.1007/s11207-013-0401-x


Spacecraft data from the OMNI database were used to calculate the value of the polytropic index of the solar wind by fitting the logarithms of proton density and temperature in selected time intervals from 1 January 1995 to 30 June 2012. Bernoulli’s integral and the correlation coefficient were used to filter the results. An alternative method based on the maximization of the correlation coefficient was employed to confirm our results. The long-term behavior of the polytropic index we obtained is found to be virtually identical for both methods. We noticed a characteristic behavior of the estimated polytropic index values, particularly from 1995 to 2006, which tends to have a periodicity of about one year. The distribution of the polytropic index is best described by a κ-Gaussian distribution with mean ≈ 1.8 and standard deviation ≈ 2.4. We finally examined the possible correlation between the polytropic index values and solar activity.


Polytropic index Solar cycle Thermodynamics Variations 

Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  1. 1.Department of Physics and AstronomyUniversity of Texas at San AntonioSan AntonioUSA
  2. 2.Southwest Research InstituteSan AntonioUSA
  3. 3.Department of PhysicsNational and Kapodistrian University of AthensAthensGreece

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