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Description of the Objective

Solar-wind turbulence is characterised by broad-band fluctuations in the magnetic field and the velocity field. Energy cascades from larger scales to smaller scales until it eventually dissipates on the smallest scales of the turbulent spectrum. The foot-point motion of magnetic field lines at the bottom of the corona contains a significant amount of energy that is apparently sufficient to explain the heating and acceleration of the solar wind (De Pontieu et al., 2007; McIntosh et al., 2011). It is unclear, however, through which processes and in which form this large-scale energy is transferred to smaller scales where it actually heats and dissipates.

The fluctuations observed in the solar wind typically show Alfvénic correlations between the fluctuations in each velocity component and the fluctuations in each magnetic-field component (Belcher & Davis, 1971; Tu & Marsch, 1995). This fact leads to the notion of “wave turbulence”, the presence of certain linear-mode properties even in strong and fully nonlinear turbulence (Schekochihin et al., 2009). In addition, the solar wind shows small-scale fluctuations consistent with other wave modes, which are likely created by local instabilities (Bale et al., 2009; Jian et al., 2010; Wicks et al., 2016).

A key missing piece in our understanding of the origins of waves and turbulence in the solar wind is the actual understanding of the nature of the fluctuations in the solar wind. It is critical to determine the correct description of the fluctuations on the broad range of scales and then to answer the question regarding their origin.


Needed Observations

A unique science opportunity for Solar Orbiter is the exploitation of linkage science between remote-sensing observations and in-situ measurements of the waves and turbulence. Remote-sensing observations can study the foot-point motion in the photosphere. Linked in-situ measurements of the magnetic-field fluctuations will then allow us to study the propagation, convection, and spectral modification of these fluctuations with distance from the Sun (see, for example, Matthaeus & Goldstein, 1986; Bruno & Carbone, 2005; Matthaeus et al., 2007; Bello González et al., 2010).  It is especially important to determine the polarisation properties of the fluctuations by simultaneously measuring fluctuations in the fields and particle properties on the same scales in the spectrum. Solar Orbiter will be able to achieve the time resolution necessary to determine fluctuations in density, velocity, pressure, and higher moments of the distribution function on small scales. This will help us to understand the nature of the small-scale fluctuations in the solar wind (cf Verscharen et al., 2017; Wu et al., 2018).

For our understanding of kinetic plasma instabilities as wave sources, it is crucial to understand the non-thermal configuration of the plasma distribution function and how this deviation from equilibrium triggers instabilities (Hellinger et al., 2006; Matteini et al., 2010). The stability of the plasma based on the actually measured distribution function can be determined with numerical tools like ALPS, the Arbitrary Linear Plasma Solver (Verscharen et al., 2018). Combining this tool with Nyquist’s method to determine the stability (Klein et al., 2017) and high-resolution Solar Orbiter data will allow us to quantify the unstable time intervals as well as the role and importance of instabilities as the source of fluctuations in the solar wind. A similar previous study based on the bi-Maxwellian assumption for the background distribution revealed that the solar wind is unstable in the majority of times (Klein et al., 2018).

With the help of high-frequency radio measurements, Solar Orbiter will also have access to fluctuations beyond the characteristic proton scales. RPW can detect Langmuir waves, for example, and drive studies of their conversion to electromagnetic waves (Bale et al., 1998; Kellogg et al., 1999; Farrell et al., 2004; Ergun et al., 2008). Coordinated observations between RPW and SWA/EAS will allow us to connect Langmuir-wave activity with features in the electron distribution function like the halo electrons (e.g., Yoon et al., 2016). It is important to study the energy partition between electron properties, Langmuir waves, and radio waves at different heliocentric distances to understand their global interplay.

Other processes in addition to waves and turbulence can create fluctuations in the solar wind. These include solitons (Rees et al., 2006), mirror modes (Stevens & Kasper, 2007), and draped field lines from the interaction with planetary bodies (Jones et al., 2003). These features are not fully understood and require further studies, again combining the knowledge of fields and particle properties at the same scale.


  • Quantify the undisturbed waves and relate the wave power and other characteristics to the source regions (by measuring photospheric motion in the regions from which the plasma originated) (Matthaeus and Goldstein, 1986; Bruno and Carbone, 2005; Matthaeus et al., 2007; Bello González et al., 2010).
  • Identify and characterize the waves associated with the plasma instabilities that isotropize and heat the solar wind (Hellinger et al., 2006; Matteini et al., 2010).
  • Resonant absorption and emission by thermal particle distributions: role of the high-frequency cyclotron waves.
  • Ion energization processes in the solar wind (study of the electric fluctuations near the ion cyclotron frequencies).
  • Ion cyclotron resonance damping of the high-frequency part of the Alfvén spectrum (e.g. Cranmer, 2002).
  • Solve the problem of the mode conversion from Langmuir to electromagnetic waves (Bale et al., 1998; Kellogg et al. 1999; Farrell et al., 2004; Ergun et al., 2008). Characterize the energy balance between electron beams, Langmuir waves and electromagnetic radio waves at several heliocentric distances.
  • How do variations and structure in the solar wind affect low-frequency radio wave propagation?
  • Small scale structures such as solitons (Rees et al., 2006), mirror modes (Stevens and Kasper, 2007), and draped fields and - in the case of dust trail signatures (Jones et al., 2003) - confirm or refute their correlation with predicted trails. 
  • Study inbound waves in the corona (Verdini et al., 2009; DeForest et al., 2014).


Low cadence context would be beneficial to this objective. And if possible connectivity.

SoloHI, METIS and IS to coordinate for defining interesting configurations. Not that important for SAP.


YZ: Separate clearly IS-only objectives from the others. All of them are IS-only except first and last bullet?


Other remarks:

  • Turbulence anisotropy makes apparent power very dependent on magnetic field direction, a significant complicating factor. Also, Taylor hypothesis is not well satisfied, so it will be hard to determine the plasma frame power levels. Comparison of Solar Orbiter / SPP in same kinds of stream at same distance is important because they will have different velocities relative to the flow due to SPP's large speed.