Browsing by Subject "LOW-FREQUENCY WAVES"

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  • Ala-Lahti, Matti; Kilpua, Emilia K. J.; Soucek, Jan; Pulkkinen, Tuija; Dimmock, Andrew P. (2019)
    We report on a statistical analysis of the occurrence and properties of Alfven ion cyclotron (AIC) waves in sheath regions driven by interplanetary coronal mass ejections (ICMEs). We have developed an automated algorithm to identify AIC wave events from magnetic field data and apply it to investigate 91 ICME sheath regions recorded by the Wind spacecraft. Our analysis focuses on waves generated by the ion cyclotron instability. AIC waves are observed to be frequent structures in ICME-driven sheaths, and their occurrence is the highest in the vicinity of the shock. Together with previous studies, our results imply that the shock compression has a crucial role in generating wave activity in ICME sheaths. AIC waves tend to have their frequency below the ion cyclotron frequency, and, in general, occur in plasma that is stable with respect to the ion cyclotron instability and has lower ion beta(parallel to) than mirror modes. The results suggest that the ion beta anisotropy beta(perpendicular to)/beta(parallel to) > 1 appearing in ICME sheaths is regulated by both ion cyclotron and mirror instabilities.
  • Turc, L.; Roberts, O. W.; Archer, M. O.; Palmroth, M.; Battarbee, M.; Brito, T.; Ganse, U.; Grandin, M.; Pfau-Kempf, Y.; Escoubet, C. P.; Dandouras, I. (2019)
    The foreshock, extending upstream of Earth's bow shock, is a region of intense electromagnetic wave activity and nonlinear phenomena, which can have global effects on geospace. It is also the first geophysical region encountered by solar wind disturbances journeying toward Earth. Here, we present the first observations of considerable modifications of the foreshock wave field during extreme events of solar origin called magnetic clouds. Cluster's multispacecraft data reveal that the typical quasi-monochromatic foreshock waves can be completely replaced by a superposition of waves each with shorter correlation lengths. Global numerical simulations further confirm that the foreshock wave field is more intricate and organized at smaller scales. Ion measurements suggest that changes in shock-reflected particle properties may cause these modifications of the wave field. This state of the foreshock is encountered only during extreme events at Earth, but intense magnetic fields are typical close to the Sun or other stars.
  • Plaschke, Ferdinand; Hietala, Heli; Archer, Martin; Blanco-Cano, Xochitl; Kajdic, Primoz; Karlsson, Tomas; Lee, Sun Hee; Omidi, Nojan; Palmroth, Minna; Roytershteyn, Vadim; Schmid, Daniel; Sergeev, Victor; Sibeck, David (2018)
    The magnetosheath flow may take the form of large amplitude, yet spatially localized, transient increases in dynamic pressure, known as "magnetosheath jets" or "plasmoids" among other denominations. Here, we describe the present state of knowledge with respect to such jets, which are a very common phenomenon downstream of the quasi-parallel bow shock. We discuss their properties as determined by satellite observations (based on both case and statistical studies), their occurrence, their relation to solar wind and foreshock conditions, and their interaction with and impact on the magnetosphere. As carriers of plasma and corresponding momentum, energy, and magnetic flux, jets bear some similarities to bursty bulk flows, which they are compared to. Based on our knowledge of jets in the near Earth environment, we discuss the expectations for jets occurring in other planetary and astrophysical environments. We conclude with an outlook, in which a number of open questions are posed and future challenges in jet research are discussed.
  • Kilpua, E. K. J.; Hietala, H.; Koskinen, H. E. J.; Fontaine, D.; Turc, L. (2013)