Abstract
The properties of the solar wind fraction that exhibits an Interplanetary Magnetic Field (IMF) orientation orthogonal to the classical Parker spiral (so-called ortho-Parker) are investigated. We make use of a solar wind plasma categorization scheme, using 10 years of OMNI data, and show that the fractions of the different plasma origins (streamer-belt-origin plasma, coronal-hole-origin plasma, sector-reversal-region plasma and ejecta) identified by this scheme are rather constant when expressed as a function of the IMF orientation whereas the Alfvén Mach number significantly depends on this orientation. This has direct implication on the magnetosheath dynamics and, as an example, the stability of the mirror mode in this compressed plasma is studied thanks to Rankine-Hugoniot anisotropic relations. This study sheds light on previously reported, yet unexplained, observations of a larger occurrence of mirror mode in the magnetosheath downstream of ortho-Parker IMF.
Highlights
If the Parker model predicts the orientation of the interplanetary magnetic field (IMF) (Parker, 1963) which commonly adopts the configuration of an Archimedean spiral, observations show that a large fraction of orientations significantly departs from this model, even in the regular solar wind i.e., when solar wind transients such as Coronal Mass Ejections (CME), Co-rotating Interactiob Region (CIR) or magnetic clouds are absent
We have studied 10 years of OMNI solar wind data and characterized the orientations of the IMF at 1 AU with respect to their origin at the Sun
The proportions of the different plasma types are similar in the Parker or orthoParker populations with a slight increase for plasmas originating from sector reversal regions
Summary
If the Parker model predicts the orientation of the interplanetary magnetic field (IMF) (Parker, 1963) which commonly adopts the configuration of an Archimedean spiral, observations show that a large fraction of orientations significantly departs from this model, even in the regular solar wind i.e., when solar wind transients such as Coronal Mass Ejections (CME), Co-rotating Interactiob Region (CIR) or magnetic clouds are absent. For instance Borovsky (2010) proposed the “braiding” concept which explains how small magnetic field deviations (from the Parker direction) close to the Sun are augmented during the solar wind propagation (see Lockwood et al, 2019 for a related study with the heliocentric distance). These uncommon orientations of the IMF have singular effects on planetary magnetospheres, and in first instance, their shock, and magnetosheath. All analysis have been done on public data with publicly available software to ensure full reproducibility
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