Final Report

Report date: 24th of January 2018.

Team members: Ferdinand Plaschke (lead), Heli Hietala (co-lead), Martin Archer (young scientist), Xóchitl Blanco-Cano, Primož Kajdič, Tomas Karlsson, Sun Hee Lee (young scientist), Nojan Omidi, Minna Palmroth, Vadim Roytershteyn, Daniel Schmid (young scientist), Victor Sergeev, David Sibeck.

Subject

Jets downstream of a shock are enhancements in dynamic pressure that are not directly driven by corresponding enhancements upstream of the shock. Near Earth, the most easily accessible shock for in-situ observations is the bow shock, at which the solar wind is decelerated and thermalized, before it flows around the magnetosphere within the magnetosheath region. That region is often permeated by jets, in particular downstream of the so-called quasi-parallel shock, which is characterized by a low angle between the upstream interplanetary magnetic field and the nominal shock-normal direction. Upstream of that shock, a foreshock region exist, in which shock-reflected particles interact with the incoming solar wind. Unsurprisingly, processes at the quasi-parallel shock and in the foreshock have been identified as a possible major source of jets.

The relevance of jets stems from their potential downstream impact on the magnetopause, the magnetosphere, and the ionosphere. When the jets hit the magnetopause, they will indent it, generate surface and inner-magnetospheric waves, and may modify the radiation belt electron populations. Furthermore, they may trigger local reconnection, have an impact on ionospheric convection, the dayside aurora, and the geomagnetic field. Thereby, jets may be considered geo-effective.

Challenges

Two major statistical studies published in 2013 made evident that jets are by no means all alike. Instead, jet characteristics and the corresponding conditions upstream of the shock can be vastly different, implying the existence of different jet types and source mechanisms, proposed in a number of case studies. All these studies, performed by separate groups, used varying definitions of jets, different terminology, methodology, and data sets, unsurprisingly finding different results on the origin, evolution, characteristics, and impacts of jets. The lack of a coordinated interdisciplinary approach in jet research, involving data analysis, simulation, and theory, made findings in this field rather look like a pile of puzzle pieces than a complete picture. Furthermore, jets in the magnetosheath and bursty bulk flows (BBFs) in the magnetotail share the fundamental property of being plasma entities that propagate through slower ambient plasma. Hence, a comparative research approach should be quite beneficial for both (jet and BBF) communities that so far were lacking any interaction.

Goals

The aim of our team was to overcome the above stated challenges by bringing together researchers with expertise in all relevant areas: theory and multi-spacecraft observations of solar wind, foreshock, bow shock, magnetosheath, magnetopause, and magnetosphere/magnetotail processes and interactions; multi-spacecraft analysis techniques; jet and BBF statistics; MHD, particle-in-cell (PIC), PIC-hybrid, and Vlasov-hybrid simulations, as well as their interpretation and comparison to observations.

Together, we sought to put all pieces of the jet puzzle together, in order to construct a full state-of-the-art picture of jet research, and based thereupon, advance our understanding of jets by:

1. Writing a review paper on jets downstream of collisionless shocks.
2. Initiating new research, involving observations, simulations, and theory, to address these science questions:

  1. What types of jets exist in terms of characteristics and generation mechanisms?
  2. How do they evolve? How do they interact with the ambient magnetosheath plasma, and with the magnetopause?
  3. What are their similarities with and differences from bursty bulk flows? What can we learn from a comparative approach in terms of jet physics?

Team work

The team proposal responded to the 2015 call for international teams in space science. After acceptance, the team started having regular teleconferences to plan the first team meeting and discuss ongoing and planned jet-related research.

The first team meeting took place between 15th and 19th of February 2016. During that meeting, we discussed and composed a current state-of-the art picture of our knowledge in the field of jets downstream of collisionless shocks. A corresponding review paper structure was agreed upon, and chapters were assigned lead and supporting authors. Furthermore, a comprehensive list of science questions and future research topics related to jets was put together, and new interdisciplinary collaborations between team members and associated research groups were initiated to address some of the identified, pending science questions and topics.

These collaborations were deepened between the first and second team meetings. Furthermore, draft versions of all chapters were prepared ahead of the second team meeting, which took place between 1st and 5th of May 2017. During that week, ongoing collaborative research was discussed, and the individual chapters of the review paper were stitched together to form a first draft version of the core manuscript part. That work on the review paper went on after the meeting, supported by regular teleconferences that have not yet been discontinued.

Accomplishments

1. The review paper was finalized last year and has been submitted on 20th of December 2017 to Space Science Reviews. It is currently under review. The paper is the expression of our understanding of the state-of-the-art in the field (as published by submission date), and comprises chapters on:

  • Different definitions of jets and the associated terminology that can be found in literature, as well as a detailed comparison.
  • The occurrence of jets (in space and time), including favorable shock-upstream conditions and latest findings on impact rates of geo-effective jets on the dayside magnetopause.
  • The properties of jets, with respect to plasma parameters, scale sizes, and morphology, including a detailed comparison of the findings that were obtained in a number of case and statistical studies.
  • Generation mechanisms of jets, which includes an extensive introduction to shock and foreshock physics and phenomena, deemed to be important in jet generation.
  • The multitude of downstream consequences of jets, when impacting the magnetopause.
  • BBFs in the tail of the magnetosphere, including the first comparison between jets and BBFs.
  • Jets observed/expected in other plasma environments: at interplanetary shocks, shocks at other solar system bodies, the termination shock, or in laboratory plasmas.
  • Numerous open questions and topics related to jets that should be addressed in the future.

The last chapter is an invitation to researchers reading the review paper, to join our efforts in exploring the origins, nature, consequences of jets.

2. The efforts in conducting new research initiated and/or supported by our ISSI team meetings has materialized in a number of papers. So far (24th of January 2018), 8 papers have been published with ISSI team acknowledgment:

  1. Plaschke, F., H. Hietala, V. Angelopoulos, and R. Nakamura (2016), Geoeffective jets impacting the magnetopause are very common, J. Geophys. Res. Space Physics, 121, 3240–3253, doi:10.1002/2016JA022534.
  2. Karlsson, T., E. Liljeblad, A. Kullen, J. M. Raines, J. A. Slavin, and T. Sundberg (2016), Isolated magnetic field structures in Mercury’s magnetosheath as possible analogues for terrestrial magnetosheath plasmoids and jets, Planet. Space Sci., 129, 61–73, doi:10.1016/j.pss.2016.06.002.
  3. Schmid, D., R. Nakamura, M. Volwerk, F. Plaschke, Y. Narita, W. Baumjohann, W. Magnes, D. Fischer, H. U. Eichelberger, R. B. Torbert, C. T. Russell, R. J. Strangeway, H. K. Leinweber, G. Le, K. R. Bromund, B. J. Anderson, J. A. Slavin, and E. L. Kepko (2016), A comparative study of dipolarization fronts at MMS and Cluster, Geophys. Res. Lett., 43, 6012–6019, doi:10.1002/2016GL069520.
  4. Han, D.-S., H. Hietala, X.-C. Chen, Y. Nishimura, L. R. Lyons, J.-J. Liu, H.-Q. Hu, and H.-G. Yang (2017), Observational properties of dayside throat aurora and implications on the possible generation mechanisms, J. Geophys. Res. Space Physics, doi:10.1002/2016JA023394.
  5. Dorfman S., H. Hietala, P. Astfalk, and V. Angelopoulous (2017), Growth Rate Measurement of ULF Waves in the Ion Foreshock, Geophys. Res. Lett., 44, doi:10.1002/2017GL072692.
  6. Plaschke, F., T. Karlsson, H. Hietala, M. Archer, Z. Vörös, R. Nakamura, W. Magnes, W. Baumjohann, R. B. Torbert, C. T. Russell, and B. L. Giles (2017), Magnetosheath high-speed jets: internal structure and interaction with ambient plasma, J. Geophys. Res., 122, doi:10.1002/2017JA024471.
  7. Kajdic, P., X. Blanco-Cano, N. Omidi, D. Rojas-Castillo, D. G. Sibeck, and L. Billingham (2017), Traveling foreshocks and transient foreshock phenomena, J. Geophys. Res., 122, 9148–9168, doi:10.1002/2017JA023901.
  8. Kajdic, P., H. Hietala, and X. Blanco-Cano (2017), Different Types of Ion Populations Upstream of the 2013 October 8 Interplanetary Shock, Astrophys. J. Lett., 849, 2, L27.

In addition, there are several papers in preparation or under revision addressing:

  1. The plasma flow patterns produced by jets as they traverse slower magnetosheath plasma.
  2. The anatomy of jets.
  3. Magnetopause reconnection induced by jet impacts.
  4. Different definitions of jets by application to global Vlasov-hybrid simulations.