Scaling anisotropy of the power in parallel and perpendicular components of the solar wind magnetic field

Power spectra of the components of the magnetic field parallel (Pzz) and perpendicular (Pzz+Pyy) to the local mean magnetic field direction were determined by wavelet methods from Ulysses’ MAG instrument data during eighteen 10-day segments of its first North Polar pass at high latitude at solar minimum in 1995. The power depends on frequency f and the angle θ between the solar wind direction and the local mean field, and with distance from the Sun. This data includes the solar wind whose total power (Pxx + Pyy + Pzz) in magnetic fluctuations we previously reported depends on f and the angle θ nearly as predicted by the GS95 critical balance model of strong incompressible MHD turbulence. Results at much wider range of frequencies during six evenly-spaced 10-day periods are presented here to illustrate the variability and evolution with distance from the Sun. Here we investigate the aniso tropic scaling of Pzz(f,θ) in particular because it is a reduced form of the Poloidal (pseudo-Alfvenic) component of the (incompressible) fluctuations. We also report the much larger Pxx(f,θ)+Pyy(f,θ) which is (mostly) reduced from the Toroidal (Alfvenic, i.e., perpendicular to both B and k) fluctuations, and comprises most of the total power. These different components of the total power evolve and scale differently in the inertial range. We compare these elements of the magnetic power spectral tensor with “critical balance” model predictions

Solar Orbiter SWA Observations of Electron Strahl Properties Inside 1 AU

The Solar Wind Analyser (SWA) suite on Solar Orbiter includes an Electron Analyser System (SWA-EAS) which is capable of high temporal and angular resolution measurements of solar wind electrons in the energy range ∼1 eV to ∼5 keV. In this article we report early nominal phase observations of the suprathermal electron population at energies ≥70 eV (representative of the ’strahl’ population), and use a simple fitting routine and classification system to determine the characteristics of the distributions and determine the variations in their properties as a function of heliocentric distance and solar wind properties. We find that under our classification system a significant population of radially outward moving strahl beams is identifiable in the tested samples. These are seen in across solar wind speed regimes, but, consistent with earlier observations, are slightly more prevalent in high speed wind. These beams occur at all distances examined (∼0.43 to ∼1.0 AU), but do not show significant evolution with distance, suggesting a balance between focusing and scattering processes across the distance range covered. However, the data suggests that the beams broaden on average with increasing magnetic field strength and narrow on average with increasing solar wind speed. We also identify a small population, occurring in sporadic clusters, which have deficits in phase space density in the sunward moving part of the electron distribution. These clusters occur across the distance range sampled and show some variations in average properties with radial distance, suggesting they too are influenced by competing scattering and (de-)focusing processes. The implications for the origin and evolution of these electron populations derived from these new observations are explored

Evolution of Magnetic Field Fluctuations and Their Spectral Properties within the Heliosphere: Statistical Approach

We present the first comprehensive statistical study of the evolution of compressive and noncompressive magnetic field fluctuations in the inner heliosphere. Based on Parker Solar Probe (PSP) and Solar Orbiter data at various distances from the Sun, we show the general trends and compare them with Wind observations near 1 au. The paper analyzes solar wind power spectra of magnetic field fluctuations in the inertial and kinetic ranges of frequencies. We find a systematic steepening of the spectrum in the inertial range with the spectral index of around −3/2 at closest approach to the Sun toward −5/3 at larger distances (above 0.4 au), the spectrum of the field component perpendicular to the background field being steeper at all distances. In the kinetic range, the spectral indices increase with distance from −4.8 at closest PSP approach to ≈−3 at 0.4 au and this value remains approximately constant toward 1 au. We show that the radial profiles of spectral slopes, fluctuation amplitudes, spectral breaks, and their mutual relations undergo rapid changes near 0.4 au

Non-field-aligned Proton Beams and Their Roles in the Growth of Fast Magnetosonic/Whistler Waves: Solar Orbiter Observations

The proton beam is an important population of the non-Maxwellian proton velocity distribution in the solar wind, but its role in wave activity remains unclear. In particular, the velocity vector of the proton beam and its influence on wave growth/damping have not been addressed before. Here we explore the origin and the associated particle dynamics of a kinetic wave event in the solar wind by analyzing measurements from Solar Orbiter and comparing them with theoretical predictions from linear Vlasov theory. We identify the waves as outward-propagating circularly polarized fast magnetosonic/whistler (FM/W) waves. The proton’s velocity distribution functions can destabilize FM/W waves. According to linear Vlasov theory, the velocity fluctuations of the core and the beam associated with FM/W waves render the original field-aligned background drift velocity non-field-aligned. This non-field-aligned drift velocity carrying the information of the velocity fluctuations of the core and the beam is responsible for the wave growth/damping. Specifically, for the FM/W waves we analyze, the non-field-aligned fluctuating velocity of the beam population is responsible for the growth of these unstable waves in the presence of a proton beam. In contrast, the core population plays the opposite role, partially suppressing the wave growth. Remarkably, the observed drift velocity vector between the core and the beam is not field aligned during an entire wave period. This result contrasts the traditional expectation that the proton beam is field aligned

Kinetic Features of Alpha Particles in a Pestchek-like Magnetic Reconnection Event in the Solar Wind Observed by Solar Orbiter

The acceleration and heating of solar wind particles by magnetic reconnection are important mechanisms in space physics. Although alpha particles (4He2+) are the second most abundant population of solar wind ions, their kinetic behavior in solar wind magnetic reconnection is not well understood. Using the high-energy (1500–3000 eV) range of the Solar Wind Analyser/Proton–Alpha Sensor instrument on board Solar Orbiter, we study the kinetic features of alpha particles in an exhaust region of a Pestchek-like solar-wind reconnection event with a weak guide field. A pair of back-to-back compound discontinuities is observed in the exhaust region. We find that the plasma in the magnetic exhaust region is heated and bounded by slow shocks (SSs), while the accelerated reconnection jet is bounded by rotational discontinuities (RDs). The SSs are outside the RDs, which is not expected from the magnetohydrodynamical prediction. We suggest this different location of the discontinuities is due to the enhanced parallel temperature Tp∥ > Tp⊥, which reduces the local Alfvén speed in the exhaust region, allowing the SSs to propagate faster than the RDs. Inside the exhaust region, the guide field is dominant. We find a two-population distribution of the alpha particles. These two populations are field aligned downstream the SSs and shift to have a perpendicular offset in the reconnection jet, suggesting that the change of the magnetic field at the RDs has similar timescales with the proton gyroperiod, but faster than those of the alpha particles, such that the alpha particles behave like pickup ions

The gamma-ray giant flare from SGR1806-20: Evidence for crustal cracking via initial timescales

We report here on serendipitous observations of the intense gamma-ray flare from SGR 1806-20 that occured on 27 December 2004. Unique data from the Cluster and Double Star-2 satellites, designed to study the Earth's magnetosphere, provide the first observational evidence of three separate timescales within the early (first 100ms) phases of this class of events. These observations reveal that, in addition to the initial very steep (<0.25ms) X-ray onset, there is firstly a 4.9ms exponential rise timescale followed by a continued exponential rise in intensity on a timescale of 70ms. These three timescales are a prominent feature of current theoretical models including the timescale (several ms) for fracture propagation in the crust of the neutron star.Comment: 10 pages including 2 figures Ap J Letters in press, May 200

Three-dimensional modelling of the shock-turbulence interaction

The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully-developed plasma turbulence, employing a novel combination of magnetohydrodynamic (MHD) and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed.We find that the behaviour of shock ripples, i.e., shock surface fluctuations with short (a few ion skin depths, $d_i$) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multi-spacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterised by the presence of shock waves interacting with plasma turbulence.Comment: Submitted to MNRA

Anisotropic Scaling of Magnetohydrodynamic Turbulence

We present a quantitative estimate of the anisotropic power and scaling of magnetic field fluctuations in inertial range magnetohydrodynamic turbulence, using a novel wavelet technique applied to spacecraft measurements in the solar wind. We show for the first time that, when the local magnetic field direction is parallel to the flow, the spacecraft-frame spectrum has a spectral index near 2. This can be interpreted as the signature of a population of fluctuations in field-parallel wavenumbers with a k║-2 spectrum but is also consistent with the presence of a "critical balance" style turbulent cascade. We also find, in common with previous studies, that most of the power is contained in wavevectors at large angles to the local magnetic field and that this component of the turbulence has a spectral index of 5/3

The plasma structure of coronal hole solar wind: Origins and evolution

Whereas slow solar wind is known to be highly structured, the fast (coronal hole origin) wind is usually considered to be homogeneous. Using measurements from Helios 1 + 2, ACE, Wind, and Ulysses, structure in the coronal hole origin solar wind is examined from 0.3 AU to 2.3 AU. Care is taken to collect and analyze intervals of “unperturbed coronal hole plasma.” In these intervals, solar wind structure is seen in the proton number density, proton temperature, proton specific entropy, magnetic field strength, magnetic field to density ratio, electron heat flux, helium abundance, heavy‐ion charge‐state ratios, and Alfvenicity. Typical structure amplitudes are factors of 2, far from homogeneous. Variations are also seen in the solar wind radial velocity. Using estimates of the motion of the solar wind origin footpoint on the Sun for the various spacecraft, the satellite time series measurements are converted to distance along the photosphere. Typical variation scale lengths for the solar wind structure are several variations per supergranule. The structure amplitude and structure scale sizes do not evolve with distance from the Sun from 0.3 to 2.3 AU. An argument is quantified that these variations are the scale expected for solar wind production in open magnetic flux funnels in coronal holes. Additionally, a population of magnetic field foldings (switchbacks, reversals) in the coronal hole plasma is examined: this population evolves with distance from the Sun such that the magnetic field is mostly Parker spiral aligned at 0.3 AU and becomes more misaligned with distance outward.Key PointsCoronal hole origin solar wind is structured as seen by density, field strength, helium abundance, entropy, strahl, Alfvenicity, and so onThe structure exhibits several variations per supergranule; the structure does not evolve with distance from the Sun from 0.3 AU to 2.3 AUThe structure scale sizes might be consistent with scale sizes of the open flux funnels emanating from coronal holesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/133555/1/jgra52697.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/133555/2/jgra52697_am.pd