High-resolution, high-accuracy plasma, electric, and magnetic field measurements for discovery of kinetic plasma structures and processes in the evolving solar wind
J. J. Podesta, J. E. Borovsky, J. T. Steinberg, R. Skoug, J. Birn, S. P. Gary, S. R. Cranmer, G. Zank, G. Li, J. T Gosling, N. A. Schwadron, J. Giacalone, K. W. Ogilvie, D. A. Roberts, A. Szabo, J. A. Slavin, T. Chang, J. D. Richardson, R. P. Lin, J. Luhmann, P. J. Kellogg, C. T. Russell, L. Jian, C. W. Smith, A. Bhattacharjee, B. D. G. Chandran, O. Alexandrova, V. Pierrard, F. Sahraoui, M. L. Goldstein, M. Velli, S. D. Bale
To discover and understand the mechanisms responsible for the heating and acceleration of the solar wind is an important goal of solar and space physics that has stimulated scientific study for many years. Although significant progress has occurred in the last few decades, to achieve this goal requires the ground truth provided by an advanced generation of in situ spacecraft measurements. To solve the mystery of solar wind heating and acceleration requires direct observational knowledge of the solar wind structures, plasma waves, and wave-particle interactions that play a dominant role in these processes. Since the relevant mechanisms operate at kinetic scales, it is clear that high accuracy, high cadence plasma and wave measurements are essential to pave the way for the science that will eventually solve this mystery. Progress toward a solution of this fundamental issue requires that these next generation measurements be performed in the inner heliosphere and at 1 AU in the coming decades. Such measurements will also benefit studies of related physical processes such as shocks, including microstructure, and the acceleration of particles from thermal to suprathermal energies leading to the formation of suprathermal tails.
Next generation measurements To advance knowledge of plasma physical processes in the solar wind at 1 AU and throughout the inner heliosphere there is an urgent need for accurate high cadence particle and electromagnetic field measurements that can resolve scales from 104 Hz to 20 Hz. Such measurements will enable the discovery, analysis, and understanding of a broad range of kinetic processes including plasma heating processes, current sheet dynamics, small scale magnetic reconnection events, and the processes responsible for dissipation of MHD fluctuations in the solar wind. It is essential to resolve and characterize plasma processes throughout the important transition to kinetic scales that occurs near the proton inertial length and proton gyroradius in order to solve the challenging problem of solar wind heating and acceleration. At 1 AU the proton inertial length and proton gyroradius are typically on the order of 100 km and structures of that size are swept past the spacecraft in approximately 0.2 seconds. At present, studies of these important processes are hampered by several factors. The plasma density, bulk velocity vector, and particle distribution functions in the solar wind are usually measured with a cadence much longer than 1 second and almost never with a cadence shorter than 1 second. Therefore, existing measurements are unable to resolve plasma structures and processes that operate on scales near the proton inertial length and the proton gyroradius. Solar wind magnetic field data typically cover frequencies that are either less than a few Hz or much greater than a few Hz leaving a gap in our knowledge at the crucial point where there is a transition from MHD scales to kinetic scales, e.g., fluxgate magnetometer data is usually limited to frequencies below a few Hz and search coil magnetometer data to frequencies greater than 10 Hz so that the range around 1 Hz to 20 Hz is often poorly resolved. Electric field measurements in the range of frequencies near 1 Hz are difficult to perform either because of interference from the spacecraft spin (due to photoelectric variations of the antenna potentials), because the electric antennas are too short compared to the Debye length, or for other reasons [Kellogg, 2008]. This is a major drawback since electric field measurements in this range are essential for characterizing the waves and processes near the transition to kinetic scales where kinetic processes begin to dominate the physics. Consequently, solar wind structures, dynamics, and plasma processes at scales from seconds to milliseconds are poorly understood and largely unexplored at present. To open up this unexplored territory and to advance plasma physics and solar wind science, the development and implementation of instrumentation for sustained, simultaneous, accurate, high cadence plasma particle, electric field, and magnetic field measurements are required. Specifically, continuous coverage in the range from 104 Hz to 20 Hz is needed. The critical measurement quantities are the electric field vector, the magnetic field vector, the proton number density, proton temperature, and the proton bulk velocity vector. The number density, temperature, and bulk velocity vector of alpha particles and electrons are also desirable but not essential. In addition, full proton distribution functions (DF) with a reduced cadence (e.g., 30 seconds) are needed for the diagnosis of kinetic processes; the time interval used to measure the DF could be the same time interval used to compute the moments or a somewhat longer time interval to obtain better statistics. Higher frequency measurements of vector electric and magnetic fields in the range from 1 Hz to 300 Hz are also needed, at least in burst mode, to allow investigation of kinetic processes at electron scales. A search coil magnetometer similar to the one on board the Cluster spacecraft but with higher sensitivity is needed to provide the required high frequency magnetic field measurements [Cornilleau-Wehrlin et al. 2003].
In the range 20 Hz, an important goal is to improve measurement accuracy by roughly an order of magnitude over existing measurements since higher frequencies are usually associated with smaller changes in the physical variables. Magnetometer measurements at frequencies up to a few Hz have an absolute accuracy of approximately 0.1 nT [Lepping et al. 1995; Acuna et al. 2008], limited by the spacecraft field. The goal for the next generation of instrumentation, for fluxgates or other designs, is to improve the accuracy by a factor of between 3 and 10 to obtain an accuracy <0.033 nT for each orthogonal componentideally, the desired accuracy is <0.01 nTand to reduce interference from spacecraft fields so as to achieve solar wind measurements with this improved accuracy.
Measurements of proton bulk velocity obtained using, for example, the Faraday cups on board the Wind spacecraft, have achieved an uncertainty as low as 0.16% in magnitude (1 km/s) and 3 in direction [Kasper et al. 2006]. Previous solar wind measurements have achieved an accuracy of 1.5 in direction [Feldman et al. 1977]. The proton density is generally not measured very accurately and its calibration varies with bulk speed and temperature [Petrinec and Russell, 1993]. The goal for the next generation instrumentation is to improve the accuracy by a factor of between 3 and 10 for the proton speed, direction, and density at a cadence up to 20 Hz20 Hz desiredwith similar order of magnitude improvements for the proton temperature. This can be accomplished through marginal or modest increases in various design factors such as collecting area, number of sensors simultaneously deployed, power consumption, etc.
Three axis electric field measurements with an uncertainty <3 V/m for each orthogonal component provide the desired improvement over existing measurements at 20 Hz, however, interference from the spacecraft must also be minimized to allow accurate solar wind measurements at these levels. All of the abovementioned requirements will stretch the limits of existing instrument techniques and spacecraft technologies, but are achievable without the development of new measurement techniques, electronic components, or capabilities.
Some Key Science Questions Current sheets and magnetic reconnection. It is known that the solar wind contains magnetic discontinuities (current sheets) of all amplitudes and thicknesses [Siscoe et al., 1968; Vasquez et al., 2007; Neugebauer and Giacalone 2010] and that the frequency of occurrence of these structures increases as the amplitude decreases. The discontinuities could be long-lived fossils from the Sun [Burlaga, 1969; Borovsky, 2008], long-lived structures generated by processes in the solar wind [Vasquez and Hollweg, 1999; Parker, 2004], or short-lived current sheets forming in the wind [Greco et al., 2008]. Computer simulations show that under the action of compression or expansion, a current sheet in a plasma will form distinct substructure [Schindler and Hesse, 2008]. Diagnosis of this fine-scale structure may provide information about the age and history of the current sheets. Why are some discontinuities diamagnetic [Burlaga, 1968] while others are not? High-time-resolution density, temperature, and anisotropy measurements are required to characterize the internal plasma of the current sheet and find hints as to the physical mechanisms that have produced them. Computer simulations indicate that MHD turbulence will produce thin current sheets at the turbulence dissipation scale [Dmitruk et al., 1998; Maron and Goldreich, 2001], which is the ion inertial length or ion gyroradius. Reconnection at these fine-scale current sheets could be a major dissipation mechanism for the turbulence and for the heating of the solar-wind plasma [Servido et al., 2010]. Reconnection exhausts have been identified in 3 s solar wind plasma and magnetic field data down to spatial scales of 25 ion inertial lengths and there are indications in 0.1 s magnetic field observations that many smaller reconnection exhausts are yet unresolved by present plasma measurements [Gosling and Szabo, 2008]. High-resolution high-accuracy measurements are required to see these current sheets and to determine how often they undergo reconnection. Some key questions: What are the characteristics and frequency of occurrence of magnetic discontinuities and current sheets in the solar wind at scales near the proton inertial length and proton gyroradius? Is fast reconnection taking place at these scales and, if so, then what role does this process play in the heating of solar wind plasma and the dissipation of MHD turbulence?
Dissipation of MHD turbulence and plasma heating. Solar wind fluctuations generate a turbulent energy cascade from large to small scales and the dissipation of this energy cascade is believed to be a primary source of heating of solar wind plasma at heliocentric distances below 5 AU or so [Matthaeus et al. 1999; Smith et al. 2001]. This process is also believed to be a viable mechanism for the heating of the solar corona [Cranmer et al. 2007; Verdini et al. 2010]. However, the physics of the dissipation process in collisionless space plasmas is not understood. Theory and simulations suggest that the predominantly Alfvenic fluctuations at MHD scales have wavevectors oriented obliquely to and that this Alfven wave cascade excites a kinetic Alfven wave cascade (KAW cascade) at the proton gyroradius scale which dissipates and heats the plasma through Landau damping and possibly other mechanisms [Howes et al. 2008; Schekochihin et al. 2009]. Tentative evidence for such a KAW cascade in the solar wind has been obtained by Bale et al. [2005] and by Sahraoui et al. [2009, 2010]. Nonlinear processes are expected to be important because wave amplitudes at the proton gyroscale are not negligible, with [Lau & Siregar 1996, Markovskii et al. 2008]. However, the details of these nonlinear mechanisms are unknown. To elucidate the physics of turbulent dissipation and plasma heating in the solar wind requires high cadence plasma and field measurements capable of resolving proton and ion kinetic scales. Among the key science questions: Are kinetic Alfven waves the dominant wave modes at proton kinetic scales and what other wave modes are present, if any? What is the wavevector spectrum of the different wave modes? What is the relationship, if any, between the observed waves and plasma structures such as current sheets? How do ions and electrons interact with the observed wave fields and what are the principle wave-particle interactions or other mechanisms responsible for plasma heating?
Recently it has been shown how the energy cascade rate of MHD turbulence in the solar wind can be determined using in situ measurements [MacBride et al. 2008]. Measurement of the energy cascade rate in the solar wind is experimentally challenging because it requires large amounts of data acquired under relatively steady and homogeneous conditions. Measurement techniques based on the Politano and Pouquet relations are now well established. However, existing measurements have uncertainties that may be on the order of 100% [Podesta et al. 2009; Podesta, 2010]. High cadence solar wind plasma and magnetic field measurements are needed to make practically useful measurements with uncertainties less than 20% or 30%. Accurate measurements of the energy cascade rate are crucial for assessing the role of turbulent dissipation in solar wind heating. Some key questions are: What is the energy cascade rate of MHD turbulence in the solar wind? How does the energy cascade rate (the heating rate) depend on the normalized cross-helicity of the fluctuations and how does the cross-helicity behave at the transition to proton kinetic scales? What is the nature of the anisotropy of the fluctuations in the kinetic range of scales and what role does this anisotropy play in the turbulent heating of the plasma?
References
Acuna, M. H., et al. (2008), The STEREO/IMPACT Magnetic Field Experiment, Space Sci. Rev., 136, 203–226.
Bale, S. D., P. J. Kellogg, F. S. Mozer, T. S. Horbury, and H. Reme (2005), Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence, Phys. Rev. Lett. 94, 215002.
Borovsky, J. E. (2008), The flux-tube texture of the solar wind: Strands of the magnetic carpet at 1 AU?, J. Geophys. Res., 113, A08110.
Burlaga, L. F. (1968), Micro-scale structures in the interplanetary medium, Solar Phys., 4, 67.
Burlaga, L. F. (1969), Directional discontinuities in the interplanetary magnetic field, Solar Phys., 7, 54.
Cranmer, S. R., A. A. van Ballegooijen, and R. J. Edgar (2007), Astrophys. J. Suppl., 171, 520. Cornilleau-Wehrlin, N., G. Chanteur, S. Perraut, L. Rezeau, P. Robert, et al. (2003), First results obtained by the Cluster STAFF experiment, Annales Geophysicae 21, 437–456.
Dmitruk, P., D. O. Gomez, and E. E. DeLuca (1998), Magnetohydrodynamic turbulence of coronal active regions and the distribution of nanoflares, Astrophys. J., 505, 974.
Feldman, W. C., and J. R. Asbridge, S. J. Bame, and J. T. Gosling (1977), Plasma and Magnetic Fields from the Sun, O. R. White (ed.), The Solar Output and its Variation, 351-382.
Gosling, J. T., and A. Szabo (2008), Bifurcated current sheets produced by magnetic reconnection in the solar wind, J. Geophys. Res., 113, A10103.
Greco, A., P. Chuychai, W. H. Matthaeus, S. Servidio, and P. Dmitruk (2008), Intermittent MHD structures and classical discontinuities, Geophys. Res. Lett. 35, L19111.
Howes, G. G., S. C. Cowley, W. Dorland, G. W. Hammett, E. Quataert, and A. A. Schekochihin (2008), A model of turbulence in magnetized plasmas: Implications for the dissipation range in the solar wind, J. Geophys. Res., 113, A05103.
Kasper, J. C., A. J. Lazarus, J. T. Steinberg, K. W. Ogilvie, and A. Szabo (2006), Phsyics-based tests to identify the accuracy of solar wind ion measurements: A case study with the Wind Faraday Cups, J. Geophys. Res., 111, A03105.
Kellogg, P. J. (2008), Measuring Electric Field and Density Turbulence in the Solar Wind, Particle Acceleration and Transport in the Heliosphere and Beyond 7th Annual International Astrophysics Conference, Kauai, Hawaii, G. Li, Q. Hu, O. Verkhoglyadova, G. P. Zank, R. P. Lin, J. Luhmann (eds), AIP Conference Proceedings 1039, 87-92.
Lau, Y.-T. and E. Siregar (1996), Alfven wave propagation in the solar wind, Astrophys. J. 465, 451-461.
Lepping, R. P. et al. (1995), The Wind Magnetic Field Investigation, Space Sci. Rev., 71, 207–229.
MacBride, B. T., C. W. Smith, and M. Forman (2008), The turbulent cascade at 1 AU: Energy transfer and the third- order scaling for MHD, Astrophys. J., 679, 1644.
Markovskii, S. A., B. J. Vasquez, and C. W. Smith (2008), Statistical Analysis of the High-Frequency Spectral Break of the Solar Wind Turbulence at 1 AU. Astrophys. J., 675, 1576–1583.
Maron, J., and P. Goldreich (2001), Simulations of incompressible magnetohydrodynamic turbulence, Astrophys. J., 554, 1175.
Matthaeus, W. H., G. P. Zank, C. W. Smith, and S. Oughton (1999): Turbulence, Spatial Transport, and Heating of the Solar Wind, Phys. Rev. Lett., 82, 3444–3447.
Neugebauer, M., and J. Giacalone (2010), Progress in the study of interplanetary discontinuities, AIP Conf. Proc., 1216, 194.
Parker, E. N. (2004), Tangential discontinuities in untidy magnetic topologies, Phys. Plasmas, 11, 2328.
Petrinec, S.M. and C.T. Russell (1993), Intercalibration of solar wind instruments during the international magnetospheric study, J. Geophys. Res. 98, 18963–18970.
Podesta, J. J., M. A. Forman, C. W. Smith, D. C. Elton, Y. Malecot, and Y. Gagne (2009), Accurate estimation of third- order moments from turbulence measurements, Nonlin. Processes Geophys., 16, 99–110.
Podesta, J. J. (2010), Comment on “Turbulent Cascade at 1 AU in High Cross-Helicity Flows,” Phys. Rev. Lett. 104, 169001.
F. Sahraoui, M. L. Goldstein, P. Robert, Y. Khotyaintsev (2009), Evidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale, Phys. Rev. Lett. 102, 231102 F. Sahraoui, M. L. Goldstein, G. Belmont, P. Canu & L. Rezeau (2010), Three dimensional k-spectra of turbulence at sub-proton scales in the solar wind, Phys. Rev. Lett., 105, 131101
Schekochihin, A. A., S. C. Cowley, W. Dorland, G. W. Hammett, G. G. Howes, E. Quataert, and T. Tatsuno (2009), Astrophysical Gyrokinetics: Kinetic and fluid turbulent cascades in magnetized weakly collisional plasmas, Astrophys. J. Supp., 182, 310.
Schindler, K., and M. Hesse (2008), Formation of thin bifurcated current sheets by quasisteady compression, Phys. Plasmas, 15, 042902.
Servido, S., W. H. Mathaeus, M. A. Shay, P. Dmitruk, P. A. Cassak, and M. Wan (2010), Statistics of magnetic reconnection in two-dimensional magnetohydrodynamic turbulence, Phys. Plasmas, 17, 032315.
Siscoe, G. L., L. Davis, P. J. Coleman, E. J. Smith, and D. E. Jones (1968), Power spectra and discontinuities of the interplanetary magnetic field: Mariner 4, J. Geophys. Res., 73, 61.
Smith, C. W., W. H. Matthaeus, G. P. Zank, N. F. Ness, S. Oughton, and J. D. Richardson: Heating of the low-latitude solar wind by dissipation of turbulent magnetic fluctuations, J. Geophys. Res., 106, 8253–8272
Vasquez, B. J., and J. V. Hollweg (1999), Formation of pressure-balanced structures and fast waves from nonlinear Alfven waves, J. Geophys. Res. 104, 4681.
Vasquez, B. J., V. I. Abramenko, D. K. Haggerty, and C. W. Smith (2007), Numerous small magnetic field discontinuities of Bartels rotation 2286 and the potential role of Alfvenic turbulence, J. Geophys. Res., 112, A11102.
Verdini, A., M. Velli, W. H. Matthaeus, S. Oughton, and P. Dmitruk (2010), A Turbulence-Driven Model for Heating and Acceleration of the Fast Wind in Coronal Holes, Astrophys. J., 708, L116.