6.2.4 Solar Wind and Shock Physics
Artemis provides a unique opportunity to address long standing questions concerning the physics of the solar wind and of collisionless shocks. In particular, the two Artemis spacecraft can be used to study turbulence in a new region of parameter space. They can also be used to study in new detail the diffusive shock acceleration process, and resolve long standing questions concerning how collisionless shocks accelerate particles to high energies. Finally, Artemis will provide excellent observations of solar wind transient structure – for example the nature of small scale reconnection jets in the solar wind, and the structure of interplanetary shocks.
6.2.4.1 Solar wind turbulence
The solar wind is an excellent laboratory for the study of turbulence. Most investigations have used a single spacecraft in combination with the Taylor hypothesis (that the fluctuations evolve slowly with respect to the time it takes for the plasma to pass the instrument) to study solar wind turbulence, and only recently have multi spacecraft measurements been used to examine turbulent fluctuations across space without the assumption of frozen in flow. For example, Cluster and other solar wind spacecraft data have been used to determine the magnetic correlative and Taylor microscale lengths [ Mattheaus et al.
, 2005; Weygand et al.
2007a] Reliable knowledge about the correlative and Taylor scale values allows the effective magnetic
Reynolds number in the solar wind to be determined, which is important for magnetohydrodynamic modeling of the solar wind and may provide constraints on kinetic theories of dissipation in space plasmas.
These studies used spacecraft separations as small as 150 km (much smaller that the Taylor micro scale of about 2400 km), and as large as 2.3x10
6 km (much larger than the correlative scale at about 1.3x10
6 km).
However, they were unable to obtain cross correlation values for separations between about 10,000 km and
110,000 km, since there was no data from appropriately separated spacecraft. Measurements within this range of separations are crucial, because they help improve our understanding of the turbulent correlative scale and Taylor microscale value, and to understand how the turbulent cascade links into the dissipative scales.
Furthermore, while correlative and Taylor scales have been determined over certain separations, the variation of these scales with respect to the mean magnetic field direction has not been examined using multiple spacecraft. It is fundamentally important to know how scales vary with respect to the magnetic field direction, and how the correlation and Taylor scales vary with solar wind speed. Single spacecraft analysis suggests that the correlative scale does vary with respect to the mean magnetic field direction
[ Matthaeus et al.
, 1986; 1990]. Preliminary work by Weygand et al.
[2007b] and Osman et al.
[2007] suggests that the correlative scale varies significantly, but the Taylor scale remained relatively constant.
The work of Osman et al.
is different from Weygand et al.
in the fact that they only looked at scales smaller than 40,000 km. Both studies, however, lacked measurements within the 40,000 to 110,000 km range and lacked cross correlation measurements for separation along the mean magnetic field direction. If that preliminary study is correct, then the implication is that the effective magnetic Reynolds number varies with respect to the mean magnetic field direction. Additional two point cross correlation measurements with in the range of 10,000 to 110,000 km combined with the already existing database of separations could be used to provide the first multi spacecraft study of the variation of the correlative scale and Taylor scale with respect to the magnetic field direction. The Artemis spacecraft, identically instrumented and in orbit around the Moon, will make long timeseries measurements of the solar wind, at new separations and will thus be able to discover how these turbulence scales vary with solar wind conditions and with respect to the magnetic field.
A second target is to use the electric and magnetic field measurements together to determine how turbulence is dissipated on the smallest scales [ Bale et al.
, 2005]. The power spectra of magnetic and electric fluctuations have the same index in the cascade region, but diverge on small scales (high frequencies) in the dissipation range. The difference between the magnetic and electric spectra can be used to determine whether the energy is deposited in whistler waves or alfven ion-cyclotron waves, a question that is still not completely resolved. By measuring in 3 dimensions both the dc electric and magnetic field, it will be possible to explore this question in new detail and assess the relative importance of these two wave modes in the dissipation of collisionless plasma turbulence as a function of solar wind conditions.

6.2.4.2 Shock physics – diffusive transport in the upstream region
As well as mediating the flow of supermagnetosonic plasma, collisionless shocks also act as sites for particle acceleration [e.g. reviews by Terasawa , 2003; Burgess , 2007]. For certain magnetic field geometries a portion of the inflowing plasma returns to the upstream region rather than being processed by the shock, and the combination of the inflowing plasma with this backstreaming component in the upstream region leads to wave generation and particle acceleration [ Eastwood et al.
, 2005]. The way in which collisionless shocks produce energetic particles is a problem of extremely broad importance. It is important in understanding, for example the production of Solar Energetic Particles (SEPs) in so-called gradual events. The Earth’s bow shock and foreshock is one of the best laboratories we have for studying the basic physical processes that govern shock particle acceleration [ Burgess , 2007].
Diffusive shock acceleration [ Axford et al.
, 1977; Bell , 1978a,b; Blandford and Ostriker , 1978] is widely cited as the process by which ions are accelerated to high energies at shocks. As noted by Burgess et al.
[2005], the diffusive shock acceleration mechanism has often been challenged by the proposition that all upstream ion enhancements at Earth are exclusively of magnetospheric origin [ Sarris et al.
, 1987]. Trattner et al.
[1994] determined statistically with a single spacecraft an energetic ion flux e-folding distance varying from 3.2+/-0.2 Re at 10 keV to 9.3+/-1.0 Re at 67 keV, providing evidence for shock diffusion.
Mobius et al.
, [1986] however reported the presence of energetic magnetospheric oxygen in the foreshock.
Kis et al.
[2004] using two spacecraft Cluster measurements found the e-folding distance to vary from
0.5Re at 11 keV to 2.8 Re at 27 keV. It is not clear why these values are not in agreement with those of
Trattner et al.
One explanation may be that most analysis is based on linear theories. At the bow shock, there is evidence that magnetic field fluctuations do not present a power law spectrum (as assumed by the models) over the required frequency range [ Terasawa , 1995]; although linear theory has been extended to the quasi-linear regime, where the energy flows from the particles to the waves, but the waves themselves are given by linear theory [ Lee , 1983], tests of quasi-linear theories of diffusive shock acceleration at the
Earth’s bow shock have not been carried out [ Burgess et al.
, 2007]. In one study of note, Kennel et al.
[1987] used ISEE-3 observations of an interplanetary shock to test the Lee [1983] quasi-linear theory of diffusive shock acceleration. They observed a number of discrepancies which may be explained for example, by variations in upstream conditions, but since only single spacecraft data were available these problems could not be resolved. Consequently, the discrepancies between previous observations, theories and the extent to which the magnetosphere influences the foreshock energetic ion population are questions that remain open.
Kis et al.
[2004] demonstrated the diffusive shock acceleration process to be operating in the foreshock, but only in one event, limiting the conclusions to high Mach number solar wind (M ~ 7) at limited energies
(ions up to 32keV), in a limited location (a few Re upstream of the shock) and for limited times (8 hours of data) due to the orbit of the spacecraft. Since diffusive shock acceleration is required to account for particle energies extending above several hundred keV [ Lin et al.
, 1974], it is clear that investigations are incomplete.
The trajectories of the Artemis spacecraft, tied to the moon, mean that they will make extensive observations of the upstream region where particle acceleration to high energies is occurring. It should be noted that they will not cross the sub-solar bow shock unless there are extreme solar wind conditions
[ Farris and Russell , 1994] which would result in serendipitous science discovery. The presence of two spacecraft allows correlation lengths parallel and perpendicular to the field to be studied without the confounding effects of upstream variability – indeed observations can be quantified according to upstream conditions. It will also help to establish the homogeneity of the upstream wave field. The orbit means that there will be continuous sampling of the foreshock particles and waves for many days, providing excellent statistics (multi-spacecraft missions such as Cluster have been limited by their Earth orbit, leading to observations of a few hours (8 hours) [ Kis et al.
, 2004]). The spacecraft will sample a variety of distances from the shock (much further than the 19.6Re apogee of Cluster) and with a variety of separations (also much greater than the 1.5Re maximum studied by Cluster [ Kis et al.
, 2004]). The instrumentation is appropriate: the SST instruments are capable of measuring ion energies above several MeV, EFI can

measure the 3d electric fields, and burst mode operations can be targeted to study regions with large scale magnetic fluctuations, for example.
Consequently, an extremely versatile and rich dataset can be collected to address these problems. It should be noted that the THEMIS probes will also encounter the foreshock during the prior mission phases, but that by orbiting the Moon, a much wider range of spacecraft separations is achieved, and the foreshock is sampled continuously for much longer. Furthermore, the combination of Artemis, making measurements in the foreshock, together with the Themis spacecraft at the magnetopause and in the magnetosphere measuring changes in energetic ions and the state of the magnetosphere, will allow the magnetospheric input of energetic particles to the shock to be deconvolved from the diffusive shock acceleration.
6.2.4.3 Solar wind transient structure
Solar wind reconnection jets: Although reconnection has been observed on extremely large scales in the solar wind [ Phan et al.
, 2006], there are many more short duration reconnection events. Thus far, these events have only been observed by Wind, which unlike ACE, is capable of measuring the ion plasma on timescales (3s) necessary to capture them [ Gosling et al.
, 2007]. The extent of these small, low shear current sheets along the X-line is unknown, and cannot be established with a single spacecraft such as
Wind alone. Artemis thus presents an excellent opportunity to accumulate the statistics of these small reconnection events, and establish whether their X-line extent is comparable to the long duration events, or if it scales with current sheet width as observed in 3d simulations of reconnection [e.g. Shay et al.
, GRL
2003].
Structure of Interplanetary Shocks: A useful tool for the study of Coronal Mass Ejection propagation is type II radio emission. This emission is thought to be patchily generated in the upstream region of CMEdriven interplanetary shocks [ Bale et al.
, 1999, Pulupa and Bale , 2008]. It is still unclear whether the structuring of Type II emission is due to the curvature of the shock on scales of 10s of Earth radii, or due to curvature of the magnetic field, although it is known that upstream turbulence alone cannot explain the dimensions of the acceleration regions (inferred from a single spacecraft) [ Pulupa and Bale , 2008].
Furthermore, the ultimate cause of such rippled shock structure is unclear, especially as a function of upstream plasma beta, Mach number (also related to the CME speed) and magnetic field orientation. Since the emission is controlled by the shock structure, to better understand how type II emission is produced and thus improve its utility in tracking CMEs, it is important to study how shock structure varies with upstream conditions.
The two point measurements of Artemis allow the curvature of interplanetary shocks (on the relevant scales
[ Pulupa and Bale , 2008]) to be determined as a function of upstream conditions and the overall structure of the foreshock to be established. The plasma instruments operating at high time resolution (3s) with targeted burst mode operations will resolve the electron foreshock (via measurements of the electron foreshock beams which establish connection geometries) providing new detail about the upstream structure. Burst mode operations can be targeted to capture the interplanetary shock crossing at high time resolution, including the 3d electric field, to better understand how the electron foreshock beams are created.
Monitoring of upstream conditions: The Artemis spacecraft, in lunar orbit, will be an excellent upstream monitor of solar wind conditions. They will be closer to the Earth than ACE or Wind and thus provide a more precise measurement of the exact solar wind input to the magnetosphere. Unlike Earth orbiting satellites, they will remain outside of the magnetosphere for extended periods of time, and will, for example, be able to see the entire passage of a CME. This will help to improve models that advect L1 monitoring data to the Earth. A second advantage, and an opportunity for discovery, is that Artemis will provide unprecedented observations of solar wind transient features interacting with the Moon itself – for example, it will be possible to discover how rapid changes in solar wind density and pressure at an interplanetary shock change the lunar Mach cone and the electrostatic structure of the wake. By virtue of its magnetic field measurements (absent from Lunar Reconnaissance Orbiter) together with particle data
(measuring ion energies up to 6 MeV), Artemis will also be able to examine how the moon affects the propagation of energetic particles associated with CME-driven shocks.

References
Axford, I. A., E. Leer and G. Skadron (1977), The acceleration of cosmic rays by shock waves,
Proceedings of the 15th International Cosmic Ray Conf.
, 11 , 132
Bale, S. D., M. J. Reiner, J. L. Bougeret, M. L. Kaiser, S. Krucker, D. E. Larson, and R. P. Lin (1999), The source region of an interplanetary type II radio burst, Geophys. Res. Lett.
, 26 , 1573-1576
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
Bell, A. R. (1978), The acceleration of cosmic rays in shock fronts I, Mon. Not. R. Astron. Soc.
, 182 , 147-
156
Bell, A. R. (1978), The acceleration of cosmic rays in shock fronts II, Mon. Not. R. Astron. Soc.
, 182 , 443-
455
Blandford, R. D., and J. P. Ostriker (1978), Particle acceleration by astrophysical shocks, Astrophys. J.
,
221 , L29
Burgess, D., et al. (2005), Quasi-parallel shock structure and processes, Space Sci. Rev.
, 118 , 205–222
Burgess, D. (2007), Particle Acceleration at the Earth’s Bow Shock, Lect. Notes Phys.
, 725 , 161-190
Eastwood, J. P., E. A. Lucek, C. Mazelle, K. Meziane, Y. Narita, J. Pickett and R. A. Treumann (2005),
The foreshock, Space Sci. Rev.
, 118 , 41-94
Farris, M. H., and C. T. Russell (1994), Determining the standoff distance of the bow shock: Mach number dependence and use of models, J. Geophys. Res.
, 99 , 17681-17689
Gosling, J. T., T. D. Phan, R. P. Lin, and A. Szabo (2007), Prevalence of Magnetic Reconnection at Small
Field Shear Angles in the Solar Wind, Geophys. Res. Lett., 34, L15110, doi:10.1029/2007GL030706
Kennel, C. F., F. V. Coroniti, F. L. Scarf, W. A. Livesey, C. T. Russell., E. J. Smith, K. P. Wenzel, and M.
Scholer (1986), A test of Lee’s quasi-linear theory of ion acceleration by interplanetary traveling shocks, J.
Geophys. Res.
, 91 , 11917-11928
Kis, A., M. Scholer, B. Klecker, E. Mobius, E. Lucek, H. Reme, J. M. Bosqued, L. M. Kistler, and H.
Kucharek (2004), Multispacecraft observations of diffuse ions upstream of Earth’s bow shock, Geophys.
Res. Lett. 31 , L20801, doi:10.1029/2004GL020759
Krymsky, G. F. (1977), A regular mechanism for the acceleration of charged particles in front of a shock wave, Dokl. Akad. Nauk SSSR , 23 , 327.
Lee, M. A. (1983), Coupled hydromagnetic wave excitation and ion acceleration at interplanetary traveling shocks, J. Geophys. Res.
, 88 , 6109
Lin, R. P., C.-I. Meng, and K. A. Anderson (1974), 30- to 100 keV protons upstream from the Earth’s bow shock, J. Geophys. Res.
, 79 , 489-498
Matthaeus, W.H., M.L. Goldstein, and J.H. King (1986), An interplanetary magnetic field ensemble at 1
AU, J. Geophys. Res ., 91 , 59-69
Matthaeus, W.H., M.L. Goldstein, and D.A. Roberts (1990), Evidence of the presencen of quasi-twodimensional nearly incompressible fluctuations in the solar wind, J. Geophys. Res ., 95 , 20673-20683

Matthaeus, W. H., S. Dasso, J. M. Weygand, L. J. Milano, C. W. Smith and M. G. Kivelson (2005), Spatial
Correlation of solar wind turbulence from two point measurements, Phys. Rev. Lett.
, 95 , 231101
Mobius, E., D. Hovestadt, B. Klecker, M. Scholer, F. M. Ipavich, C. W. Carlson, R. P. Lin (1986), A burst of energetic O+ ions during an upstream particle event, Geophys. Res. Lett.
, 13 , 1372-1375
Osman, K.T., and T.S. Horbury (2007), Multispacecraft measurement of anisotropic correlation functions in solar wind turbulence, Astrophys. J.
, 654 , L103–L106
Phan, T. D., et al. (2006), A magnetic x-line extending more than 390 Earth radii in the solar wind, Nature ,
439 , 175
Pulupa, M., and S. D. Bale (2008), Structure on Interplanetary shock fronts: Type II radio burst source regions, submitted to Astrophys. J .
Sarris, E. T., G. C. Anagnostopoulos, and S. M. Krimigis (1987), Simultaneous measurements of energetic ion (50 keV and above) and electron (220 keV and above) activity upstream of Earth’s bow shock and inside the plasma sheet - Magnetospheric source for the November 3 and December 3,
1977 upstream events, J. Geophys. Res., 92 , 12083–12096
Shay, M. A., J. F. Drake, M. Swisdak, W. Dorland, and B. N. Rogers (2003), Inherently three-dimensional magnetic reconnection: A mechanism for bursty bulk flows?, Geophys. Res. Lett.
, 30 , 1345, doi:10.1029/2002GL016267
Terasawa, T., Astrophysical Particle Acceleration (2003), Prog. Theor. Phys. Supp.
, 151 , 95-104
Trattner, K. J., E. Mobius, M. Scholer, B. Klecker, M. Hilchenbach, and H. Luehr (1994), Statistical analysis of diffuse ion events upstream of the Earth’s bow shock, J. Geophys. Res., 99 , 13389
Terasawa, T. (1995), Ion acceleration, Adv. Space Res.
, 15 (8-9), 53-62
Weygand, J.M., W.H. Matthaeus, M.G. Kivelson, S. Dasso, and R.J. Walker (2007a), Taylor scale and effective magnetic Reynolds number detereminations from plasma sheet and solar wind magnetic field fluctuations, J. Geophys. Res.
, 112 , A10201,doi:10.1029/2007JA012484
Weygand, J.M., M.G. Kivelson, W.H. Matthaeus, S. Dasso, L.M. Kistler (2007b), Correlative scale and effective magnetic Reynolds number determination from plasma sheet and solar wind magnetic field fluctuations, STAMMS-2 conference presentation, Orleans, France September 2007