Letter to the Editor

Proton polytropic behavior of periodic density structures in the solar wind

¹ NASA-Goddard Space Flight Center, Greenbelt, MD, USA
e-mail: ckatsavrias@gmail.com
² Department of Physics, National and Kapodistrian University of Athens, Athens, Greece
³ Department of Space and Climate Physics, Mullard Space Science Laboratory, University College London, Dorking, Surrey RH5 6NT, UK
⁴ Physics Department, The Catholic University of America, Washington, DC, USA
⁵ Space Applications and Research Consultancy (SPARC), Athens, Greece
⁶ Princeton University, Princeton, NJ 08544, USA

Received: 2 April 2024
Accepted: 20 May 2024

Abstract

Context. In recent years, mesoscales have gained scientific interest because they have been determined to be important in a broad range of phenomena throughout heliophysics. The solar wind mesoscale structures include periodic density structures (PDSs), which are quasi-periodic increases in the density of the solar wind that range from a few minutes to a few hours. These structures have been extensively observed in remote-sensing observations of the solar corona and in in situ observations out to 1 AU, where they manifest as radial length scales greater than or equal to the size of the Earth’s dayside magnetosphere, that is, from tens to hundreds of Earth radii (R_E). While the precise mechanisms that form PDSs are still debated, recent studies confirmed that most PDSs are of solar origin and do not form through dynamics during their propagation in the interplanetary space.

Aims. We further investigate the origin of PDSs by exploring the thermodynamic signature of these structures. To do this, we estimate the values of the effective polytropic index (Y) and the entropy of protons, which in turn are compared with the corresponding values found for the solar wind.

Methods. We used an extensive list of PDS events spanning more than two solar cycles of Wind measurements (the entire Wind dataset from 1995 to 2022) to investigate the thermodynamic signatures of PDSs. With the use of wavelet methods, we classified these PDSs as coherent or incoherent, based on the shared periodic behavior between proton density and alpha-to-proton ratio, and we derive the proton polytropic index.

Results. Our results indicate that the coherent PDSs exhibit lower Y values (Ῡ≈1.54) on average and a higher entropy than the values in the entire Wind dataset (Ῡ≈1.79), but also exhibit similarities with the magnetic cloud of an interplanetary coronal mass ejection. In contrast, incoherent PDSs exhibit the same Y values as those of the entire Wind dataset.