Solar Wind Spectra Research Articles

Bombardment of Lunar Polar Crater Interiors by Out-of-ecliptic Ions: ARTEMIS Observations

Abstract Permanently shadowed regions (PSRs) inside lunar polar craters likely harbor volatiles which are of great interest for both science and resource utilization. The origin and evolution of this resource is in part driven by external space weathering, including the bombardment by space plasma ions. Previous work has investigated the flux of horizontal solar wind ions that are deflected into lunar polar craters by plasma wake electrostatic fields. Here, we constrain the flux of ions that travel northward or southward in the natural environment encountered by the Moon along its orbit. Using 9 years of in situ ion measurements gathered by the ARTEMIS mission, we find that all locations inside lunar craters are altered by a flux of out-of-ecliptic ions of 10−3 times the solar wind flux. In particular, the central floor of Shackleton’s crater is weathered by a non-null ion flux. The origin of northward and southward ions impacting the Moon is identified as (i) shocked thermalized solar wind ions in the terrestrial magnetosheath and (ii) the isotropic component of ion distributions in the terrestrial magnetotail. The energy spectrum of out-of-ecliptic ions is found to be flatter than the solar wind spectrum. Thus, sputtering of pure water ice irradiated by out-of-ecliptic ions would be created for more than 30% by >3 keV protons, unlike equatorial locations that are mostly sputtered by 1 keV protons. Future work may investigate the influence of the low, but nonzero flux of out-of-ecliptic ions reported here on the budget of volatiles in lunar PSRs.

Generation of kinetic Alfvén wave and whistler waves by parametric decay of high power laser in laser–plasma interaction

In this paper, we report theoretical modeling for parametric decay instability of the high-intensity elliptically polarized laser beam [pump wave, (ω0)]. A wave–wave interaction model is investigated, based on the decay of the high-intensity elliptically polarized laser (ω0) into an oblique whistler wave (OWW, (ω1)) and a kinetic Alfvén wave (KAW, (ω2)). The importance of oblique whistler waves (OWWs, (ω1)) and kinetic Alfvén waves (KAWs, (ω2)) similar to solar wind spectra [Chatterjee et al., Nat. Commun. 8, 15970 (2017)] has been pointed out, as a means to understand the turbulent magnetic field amplification, implicating electron and ion dynamics [Chatterjee et al., Nat. Commun. 8, 15970 (2017); Tzeferacos et al., Nat. Commun. 9, 591 (2018); Meinecke et al., Proc. Natl. Acad. Sci. 112, 8211 (2015); Mondal et al., Proc. Natl. Acad. Sci. 109, 8011 (2012); Romagnani et al., Phys. Rev. Lett. 122, 025001 (2019); Perri et al., Phys. Rev. Lett. 109, 191101 (2012); and Adak et al., Phys. Rev. Lett. 114, 115001 (2015)]. In the nonlinear stage, the decay instability is expected to attain the turbulent state, via a cascade process or filamentation/modulation instability (oscillating two stream instability). Therefore, in the present paper, we have considered the first part of this research, namely, the beating mechanism (ω2=ω0−ω1), induced due to the nonlinear interaction of elliptically polarized laser velocity and oblique whistler wave density perturbation. The nonlinear saturation will be conferred in future investigations. Besides turbulence, the relevance of the present work to terahertz radiation generation [Singh et al., Europhys. Lett. 104, 35002 (2013); Dewan et al., Phys. Plasmas 25, 103105 (2018); Singh et al., Phys. Plasmas 18, 022304 (2011); M. Singh and R. P. Sharma, Contrib. Plasma Phys. 53(7), 540–548 (2013); Adak et al., Phys. Rev. Lett. 114, 115001 (2015); G. Brodin and L. Stenflo, Contrib. Plasma Phys. 54, 623 (2014); L. Stenflo, Phys. Scr. T50, 15–19 (1994); Li et al., Phys. Rev. E 84, 036405 (2011); L. Stenflo, Phys. Scr. T107, 262 (2004); and R. Boyd, Nonlinear Optics, 3rd ed. (Elsevier, 2008), Chap. 2] and fast ignition laser fusion [Kumar et al., arXiv:1804.02200 (2018)] by ion heating has been emphasized. The coefficients for the nonlinear coupling pertaining to this parametric decay process and the growth rate of the decay instability are investigated.

On the 1/f Spectrum in the Solar Wind and Its Connection with Magnetic Compressibility

Abstract We discuss properties of Alfvénic fluctuations with large amplitude in plasmas characterized by low magnetic field compression. We note that in such systems power laws cannot develop with arbitrarily steep slopes at large scales, i.e., when becomes of the order of the background field . In such systems there is a scale l 0 at which the spectrum has to break due to the condition of weak compressibility. A very good example of this dynamics is offered by solar wind fluctuations in Alfvénic fast streams, characterized by the property of constant field magnitude. We show here that the distribution of in the fast wind displays a strong cutoff at , as expected for fluctuations bounded on a sphere of radius . This is also associated with a saturation of the rms of the fluctuations at large scales and introduces a specific length l 0, above which the amplitude of the fluctuations becomes independent on the scale l. Consistent with that, the power spectrum at l > l 0 is characterized by a −1 spectral slope, as expected for fluctuations that are scale-independent. Moreover, we show that the spectral break between the 1/f and inertial range in solar wind spectra indeed corresponds to the scale l 0 at which . Such a simple model provides a possible alternative explanation of magnetic spectra observed in interplanetary space, also pointing out the inconsistency for a plasma to simultaneously maintain at arbitrarily large scales and satisfy a Kolmogorov scaling.

Three-dimensional simulations of solar wind turbulence with the hybrid code CAMELIA

We investigate the spectral properties of plasma turbulence from fluid to sub-ion scales by means of high-resolution three-dimensional (3D) numerical simulations performed with the hybrid particle- in-cell (HPIC) code CAMELIA. We produce extended turbulent spectra with well-defined power laws for the magnetic, ion bulk velocity, density, and electric fluctuations. The present results are in good agreement with previous two-dimensional (2D) HPIC simulations, especially in the kinetic range of scales, and reproduce several features observed in solar wind spectra. By providing scaling tests on many different architectures and convergence studies, we prove CAMELIA to represent a very efficient, accurate and reliable tool for investigating the development of the turbulent cascade in the solar wind, being able to cover simultaneously several decades in wavenumber, also in 3D.

INERTIAL RANGE TURBULENCE OF FAST AND SLOW SOLAR WIND AT 0.72 AU AND SOLAR MINIMUM

We investigate Venus Express (VEX) observations of magnetic field fluctuations performed systematically in the solar wind at 0.72 Astronomical Units (AU), between 2007 and 2009, during the deep minimum of the solar cycle 24. The Power Spectral Densities (PSD) of the magnetic field components have been computed for the time intervals that satisfy data integrity criteria and have been grouped according to the type of wind, fast and slow defined for speeds larger and respectively smaller than 450 km/s. The PSDs show higher levels of power for the fast than for the slow wind. The spectral slopes estimated for all PSDs in the frequency range 0.005-0.1 Hz exhibit a normal distribution. The average value of the trace of the spectral matrix is -1.60 for fast solar wind and -1.65 for slow wind. Compared to the corresponding average slopes at 1 AU, the PSDs are shallower at 0.72 AU for slow wind conditions suggesting a steepening of the solar wind spectra between Venus and Earth. No significant time variation trend is observed for the spectral behavior of both slow and fast wind.

RELATIONSHIP BETWEEN THE SHELL-AVERAGED ENERGY SPECTRUM AND THE FREQUENCY SPECTRUM MEASURED BY A SINGLE SPACECRAFT IN THE SOLAR WIND

The relationship between the shell-averaged energy spectrum E(k) and the frequency spectrum P(f) measured by a single spacecraft is needed in studies of solar wind turbulence to allow comparisons between theory and experiment. This relationship is derived for a three-dimensional (scalar) wavevector spectrum of the power-law form that is either isotropic or cylindrically symmetric about the mean magnetic field. In the latter case, it is assumed that the power-law exponents in directions parallel and perpendicular to the mean magnetic field are the same, an assumption that allows the analysis to be performed analytically. The results show that the effects of anisotropy are small when the angle θ BV between the solar wind velocity and the mean magnetic field is between approximately 35° and 90°. The largest effects occur near 0° where a significant correction factor is needed compared to the isotropic case due to the lower energy in the k ∥ direction compared to the k ⊥ direction. For solar wind spectra with an unknown degree of anisotropy, measurements of E(k) obtained when 35° ≤ θ BV ≤ 90° should be reasonably accurate for most purposes since even if the spectrum is assumed to be isotropic and no corrections are made for spectral anisotropy, the resulting errors are typically less than 10% or 20%.

Comparison of single-site interplanetary scintillation solar wind speed structure with coronal features

Interplanetary Scintillation (IPS) Observations were made during the period 1984–1990 using a single radio telescope at 103 MHz situated at Thaltej (Ahmedabad), India. Solar wind speeds were estimated using a recently developed method based on matching the observed IPS spectra with model solar wind spectra for Kolmogorov turbulence. The best-fit speeds derived are traced back to a source surface, and average velocity maps are made for each year, averaging over a number of Carrington rotations. It is found that the resulting single-site, large-scale IPS speed structure agrees well with that derived from 3-site observations from earlier workers. The IPS speed structure during this period was compared with other coronal features. Nearly 85% of the observed high-speed regions were associated with coronal holes. At solar minimum, in 1986, a quasi-sinusoidal, narrow belt of slow solar wind was observed which matched well with the neutral line structure of the solar magnetic field and the belt of active centers. Near solar maximum, in 1990, the speed structure was chaotic, similar to that of the neutral line, with low speed regions appearing all over the source surface.

Modifications of magnetohydrodynamics as applied to the solar wind

The effect of including the Braginskii viscous stress tensor in magnetohydrodynamics is remarked upon. It is shown that semiquantitative agreement with a recently observed anisotropy in the turbulent solar wind spectrum can be achieved in this way. The modifications of the dynamical equations are simple enough to permit their inclusion in numerical codes. The effects of large “ion parallel viscosities” also may be significant for plasmas in quite different regimes than the solar wind.

Long‐term variation of radar‐auroral backscatter and the interplanetary sector structure

Recurrent variation of geomagnetic activity at the ∼27‐day solar rotation period and higher harmonics is a well‐documented phenomenon. Auroral radar backscatter data from the Sweden and Britain Radar‐Auroral Experiment (SABRE) radar provide a continuous time series from 1981 to the present which is a highly sensitive monitor of geomagnetic activity. In this study, Maximum Entropy Method (MEM) dynamic power spectra of SABRE backscatter data from 1981 to 1989, concurrent interplanetary magnetic field (IMF) and solar wind parameters from 1981 to 1987, and the Kp index since 1932 are examined. Data since 1977 are compared with previously published heliospheric current sheet measurements mapped out from the solar photosphere. Strong periodic behavior is observed in the radar backscatter during the declining phase of solar cycle 21, but this periodicity disappears at the start of solar cycle 22. Similar behavior is observed in earlier solar cycles in the Kp spectra. Details of the radar backscatter, IMF, and solar wind spectra indicate that the solar wind momentum density is the dominant parameter in determining the backscatter periodicity. The temporal evolution of two‐ and four‐sector structures, as predicted by SABRE backscatter spectra, throughout solar cycle 21 generally still agree well with heliospheric current sheet measurements. For one interval, however, there is evidence that evolution of the current sheet has occurred between the photospheric source surface and the Earth.

Suprathermal electrons at Earth's bow shock

A hot, suprathermal population of electrons is often present near the Earth's bow shock. The overall morphology of this hot population as observed with the Los Alamos/Garching fast plasma experiments on ISEE 1 and 2 in the energy range from 1 to 20 keV is much as described previously by others working primarily with more energetic particle measurements. In particular, at energies below 20 keV the flux of suprathermal electrons is most intense immediately downstream from the shock and decreases in intensity with increasing penetration into the magnetosheath. Upstream fluxes of electrons in this energy range are generally considerably weaker than are the downstream fluxes. The major new results from our ISEE measurements are as follows: (1) The suprathermal electrons are commonly found downstream from perpendicular and quasi‐perpendicular portions of the shock but not downstream from quasi‐parallel portions. (2) The suprathermal electron spectrum extends smoothly out of the shocked solar wind spectrum generally as a power law in energy with an exponent in the range from −3 to −4. (3) Angular distributions for the suprathermal electrons are generally isotropic immediately downstream of the shock ramp, but an anisotropy perpendicular to the magnetic field usually develops with increasing penetration into the magnetosheath. (4) Suprathermal electrons are often first observed within the shock layer itself as a field‐aligned beam escaping upstream. We interpret these observations in terms of a qualitative model wherein the suprathermal electrons are accelerated out of the solar wind thermal and halo populations as the solar wind convects across perpendicular portions of the shock. Subsequently, the suprathermal electrons leak back upstream along the magnetic field to form the backstreaming fluxes of energetic electrons commonly observed in the Earth's foreshock region. Although the mechanism of acceleration is presently uncertain, we believe it is unlikely that these electrons are accelerated by magnetic mirroring of upstream electrons at the shock or that they result from a simple adiabatic mapping of electron distributions from upstream to downstream.

Numerical simulation of the generation of turbulence from cometary ion pick‐up

Observations of magnetic field fluctuations near comet Halley have revealed a rapid development of a Kolmogoroff‐like turbulence spectrum extending from below 10−2 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler‐shifted ion‐cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. We solved the equations of incompressible MHD using a two‐dimensional 256×256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modelled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy‐turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.

The characterization of lunar surface impact erosion and solar wind sputter processes on the lunar surface

A wide variety of processes have been instrumental in the evolutionary progression of the lunar surface towards its present state. During the early history of the Moon, chemical differentiation, volcanism and, particularly for the mare areas, infilling by magma flow have been major mass transport factors but, since further cooling some 3 x 10 to the power of 9 years ago, the development of surface features has been primarily determined by impact erosion from remnants of the primeval stellar condensation and matter associated with asteroids and comets. As a result of the evolutionary decrement of interplanetary matter impact erosion is time dependent but even so at a particular epoch the relative importance of impact erosion to solar wind sputter erosion in the determination of the topology of a feature will be dependent on the scale of the feature examined. An impact crater formed on the lunar surface is normally exposed to the 400 km s-1 solar wind spectrum which will result in sputter erosion of surface atoms. Under varying angles of incidence during the lunar cycle, features exposed to the solar wind will be gradually smoothed and will eventually disappear. A similar erosion effect is the abrasion of large craters due to the formation within them of much smaller craters, i.e. a process similar to sand blasting. We also have the possibility during these quasi-continuous processes of single and complete obliteration by an impacting particle at least as large as that which caused the original crater. Thus we may characterize any particular crater with three erosion lifetimes: (1) the sputter lifetime, (2) the (small particle) abrasion lifetime, and (3) the (large particle) erasure lifetime.

The transition at a periodT∼ 1 Day in solar wind spectra

The sharp change in behavior at periods above and below approximately 1 day in the solar wind spectra, reported by Goldstein and Siscoe, is discussed in terms of a damping-growth transition for ion acoustic waves in a density gradient recently observed in laboratory experiments. The region immediately surrounding the sun (∼ 10 RS radial extent) is envisaged as a low-pass filter for compressional perturbations, with a sharply defined upper frequency of ∼ 10−5 s−1.