Relating the Solar Wind Turbulence Spectral Break at the Dissipation Range with an Upstream Spectral Bump at Planetary Bow Shocks

Abstract At scales much larger than the ion inertial scale and the gyroradius of thermal protons, the magnetohydrodynamic (MHD) theory is well equipped to describe the nature of solar wind turbulence. The turbulent spectrum itself is defined by a power law manifesting the energy cascading process. A break in the turbulence spectrum develops near-ion scales, signaling the onset of energy dissipation. The exact mechanism for the spectral break is still a matter of debate. In this work, we use the 20 Hz Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) magnetic field data during four planetary flybys at different heliocentric distances to examine the nature of the spectral break in the solar wind. We relate the spectral break frequencies of the solar wind MHD turbulence, found in the range of 0.3–0.7 Hz, with the well-known characteristic spectral bump at frequencies ∼1 Hz upstream of planetary bow shocks. Spectral breaks and spectral bumps during three planetary flybys are identified from the MESSENGER observations, with heliocentric distances in the range of 0.3–0.7 au. The MESSENGER observations are complemented by one Magnetospheric Multiscale observation made at 1 au. We find that the ratio of the spectral bump frequency to the spectral break frequency appears to be r- and B-independent. From this, we postulate that the wavenumber of the spectral break and the frequency of the spectral bump have the same dependence on the magnetic field strength ∣B∣. The implication of our work on the nature of the break scale is discussed.

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Solar cycle changes in the geo-effectiveness of small-scale solar wind turbulence measured by Wind and ACE at 1 AU

Annales Geophysicae ◽

10.5194/angeo-25-1183-2007 ◽

2007 ◽

Vol 25 (5) ◽

pp. 1183-1197 ◽

Cited By ~ 3

Author(s):

M. L. Parkinson ◽

R. C. Healey ◽

P. L. Dyson

Keyword(s):

Solar Wind ◽

Solar Cycle ◽

Solar Minimum ◽

Large Scale ◽

Small Scale ◽

Scaling Exponent ◽

Mhd Turbulence ◽

Scaling Exponents ◽

Solar Wind Turbulence ◽

Wind Turbulence

Abstract. Multi-scale structure of the solar wind in the ecliptic at 1 AU undergoes significant evolution with the phase of the solar cycle. Wind spacecraft measurements during 1995 to 1998 and ACE spacecraft measurements during 1997 to 2005 were used to characterise the evolution of small-scale (~1 min to 2 h) fluctuations in the solar wind speed vsw, magnetic energy density B2, and solar wind ε parameter, in the context of large-scale (~1 day to years) variations. The large-scale variation in ε most resembled large-scale variations in B2. The probability density of large fluctuations in ε and B2 both had strong minima during 1995, a familiar signature of solar minimum. Generalized Structure Function (GSF) analysis was used to estimate inertial range scaling exponents aGSF and their evolution throughout 1995 to 2005. For the entire data set, the weighted average scaling exponent for small-scale fluctuations in vsw was aGSF=0.284±0.001, a value characteristic of intermittent MHD turbulence (>1/4), whereas the scaling exponents for corresponding fluctuations in B2 and ε were aGSF=0.395±0.001 and 0.334±0.001, respectively. These values are between the range expected for Gaussian fluctuations (1/2) and Kolmogorov turbulence (1/3). However, the scaling exponent for ε changed from a Gaussian-Kolmogorov value of 0.373±0.005 during 1997 (end of solar minimum) to an MHD turbulence value of 0.247±0.004 during 2003 (recurrent fast streams). Changes in the characteristics of solar wind turbulence may be reproducible from one solar cycle to the next.

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Statistical Research of the Intermittency and Cascade of Solar Wind Turbulence Based on Analysis of PSP Measurements

10.5194/egusphere-egu2020-13249 ◽

2020 ◽

Author(s):

Ying Wang ◽

Jiansen He ◽

Die Duan ◽

Xingyu Zhu

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Break Point ◽

Inertial Range ◽

Scaling Exponent ◽

The Sun ◽

Peak Point ◽

Solar Wind Turbulence ◽

Wind Turbulence ◽

Inner Heliosphere

By analyzing the turbulent magnetic field data from PSP, we find that: the solar wind turbulence in the inner heliosphere close to the Sun has formed the transition from multifractal intermittency at MHD scales to monofractal intermittency at kinetic scales. The order-dependent scaling exponent of the multi-order structure function shows a concave profile indicating the multifractal property at MHD scales, while its counterpart at kinetic scales shows a linear trend suggesting the monofractal property. We also find that, the closer to the sun, the more obvious the concave profile of the scaling exponent in the inertial range, which indicates that the multifractal characteristic of the magnetic field turbulence intermittency is also more evident when getting closer to the Sun.Based on the Castaing description of the probability distribution function(PDF) of the disturbance difference, the key parameters(&#956; & &#955;^2) of the Castaing function are estimated as a function of scale. We find that: (1) when close to the sun (R~0.17 AU), the break point of &#956; is about 0.2 second, and the peak point of &#955;^2 is about 0.6 second, the two of which are about three times different in scale; (2) when far from the sun (R~0.8 AU), the break point of &#956; is about 1 second and the peak point of &#955;^2 is about 3 seconds, the two of which are also about three times different in scale. We also point out that the profiles (including the break/peak position) of both the parameters (&#956; & &#955;^2) along with the scale together determine the profile (including the spectral breaks) of the power spectrum.Following the PP98 model function of incompressible MHD turbulent cascade rate (&#949;Z), we first compared the cascade rate &#949;Z with &#949;B=<&#948;B^3>/&#964; at the distance close to the sun, we find that the two trends over scales are in good agreement with one another. We therefore suggest that, to some extent (e.g. in the inertial region), &#949;B=<&#948;B^3>/&#964; can be used as a proxy of the cascade rate &#949;Z. For the first time, by statistical analysis, we obtained that &#949;B satisfies the following relation with the scale and the heliocentric distance: &#949;B=((&#964;/&#964;0)^&#945;)((r/r0)^&#946;). In the inertial range, &#945; changes from about -0.5 to about 0.5 as r increases from 0.17 AU to 0.81 AU, and &#946; is about 6.4; in the kenetic range, when r increases from 0.17 AU to 0.25 AU, &#945; keeps at about 2, and &#946; is about 12.8. The &#949;B(&#964;,r) expression given in this work, is believed to help understanding the transport and cascade processes of solar wind turbulence in the inner heliosphere.&#160;Corresponding author: Jiansen HE, [email protected]Acknowledgements: We would like to thank the PSP team for providing the data of PSP to the public.

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Scale dependent anisotropy of electric field fluctuations in solar wind turbulence

10.5194/egusphere-egu21-11968 ◽

2021 ◽

Author(s):

Deepali Deepali ◽

Supratik Banerjee

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Electric Field ◽

Electric Power ◽

Flow Direction ◽

Power Spectra ◽

Solar Wind Turbulence ◽

Wind Turbulence ◽

Field Fluctuations ◽

Spacecraft Data

We study the variation of average powers and spectral indices of electric field fluctuations with respect to the angle between average flow direction and the mean magnetic field in solar wind turbulence. Cluster spacecraft data from the years 2002 and 2007 are used for the present analysis. We perform a scale dependent study with respect to the local mean magnetic field using wavelet analysis technique. Prominent anisotropies are found for both the spectral index and power levels of the electric power spectra. Similar to the magnetic field fluctuations, the parallel (or antiparallel) electric fluctuation spectrum is found to be steeper than the perpendicular spectrum. However the parallel (or antiparallel) electric power is found to be greater than the perpendicular one. Below 0.1 Hz, the slope of the parallel electric power spectra deviates substantially from that of the total magnetic power spectra, supporting the existence of Alfv&#233;nic turbulence.

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