Evolution of Relative Drifts in the Expanding Solar Wind: Helios Observations

Ďurovcová, Tereza; Šafránková, Jana; Němeček, Zdenek

doi:10.1007/s11207-019-1490-y

Cited by 36 publications

(29 citation statements)

References 27 publications

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“…Extending this instability analysis to large sets of measurements that characterize the proton core, proton beam, and α components at 1 au, in the inner heliosphere (e.g. the recently reprocessed ion dataset from Durovcová et al (2019)), and at much closer radial distances (e.g. with Parker Solar Probe (Fox et al 2015)), will be necessary to improve our understanding of waveparticle interactions in the inner heliosphere.…”

Section: Discussionmentioning

confidence: 99%

Linear Stability in the Inner Heliosphere: Helios Re-evaluated

Klein

Martinović

Stansby

et al. 2019

ApJ

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Wave-particle instabilities driven by departures from local thermodynamic equilibrium have been conjectured to play a role in governing solar wind dynamics. We calculate the statistical variation of linear stability over a large subset of Helios I and II fast solar wind observations using a numerical evaluation of the Nyquist stability criterion, accounting for multiple sources of free energy associated with protons and helium including temperature anisotropies and relative drifts. We find that 88% of the surveyed intervals are linearly unstable. The median growth rate of the unstable modes is within an order of magnitude of the turbulent transfer rate, fast enough to potentially impact the turbulent scale-to-scale energy transfer. This rate does not significantly change with radial distance, though the nature of the unstable modes, and which ion components are responsible for driving the instabilities, does vary. The effect of ion-ion collisions on stability is found to be significant; collisionally young wind is much more unstable than collsionally old wind, with very different kinds of instabilities present in the two kinds of wind.

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Section: Discussionmentioning

confidence: 99%

Linear Stability in the Inner Heliosphere: Helios Re-evaluated

Klein

Martinović

Stansby

et al. 2019

ApJ

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“…On the other hand, the constant temperature profile can be consistent with the solar wind turbulent heating. Under assumptions that 1) the heat needed for the non-adiabatic cooling of the solar wind is provided by the turbulent cascade and 2) the cooling rate does not depend on the temperature, the amount of the heat needed for the observed cooling rate T(r) ∝ r 0.8 [37,50] should increase with a temperature. The temperature increases across the shock by a factor of 4 (Figure 10) and thus the amount of heat that should be provided by the turbulent cascade to keep the same rate of non-adiabatic cooling should increase by a similar factor.…”

Section: Figure 9 |mentioning

confidence: 99%

Turbulence Upstream and Downstream of Interplanetary Shocks

et al. 2021

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The paper reviews the interaction of collisionless interplanetary (IP) shocks with the turbulent solar wind. The coexistence of shocks and turbulence plays an important role in understanding the acceleration of particles via Fermi acceleration mechanisms, the geoeffectiveness of highly disturbed sheaths following IP shocks and, among others, the nature of the fluctuations themselves. Although our knowledge of physics of upstream and downstream shock regions has been greatly improved in recent years, many aspects of the IP-shock/turbulence interaction are still poorly known, for example, the nature of turbulence, its characteristics on spatial and temporal scales, how it decays, its relation to shock passage and others. We discuss properties of fluctuations ahead (upstream) and behind (downstream) of IP shock fronts with the focus on observations. Some of the key characteristics of the upstream/downstream transition are 1) enhancement of the power in the inertial range fluctuations of the velocity, magnetic field and density is roughly one order of magnitude, 2) downstream fluctuations are always more compressible than the upstream fluctuations, and 3) energy in the inertial range fluctuations is kept constant for a significant time after the passage of the shock. In this paper, we emphasize that–for one point measurements–the downstream region should be viewed as an evolutionary record of the IP shock propagation through the plasma. Simultaneous measurements of the recently launched spacecraft probing inner parts of the Solar System will hopefully shed light on some of these questions.

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“…Figure 2 demonstrates the proton beam effect on the temperature determination. The left-hand panel shows 2D cut through the fit of Helios measurements [12] on 11 March 1976 at 0302 UT. The velocities of the proton core (615 km/s) and beam (665 km/s) are distinguished by a small diamond and a large rectangle, respectively.…”

Section: Temperature Determinationmentioning

confidence: 99%

“…Since their T and T ⊥ are not a function the magnetic field direction, we can believe that their parameters are good estimates of real T and T ⊥ of the proton core. Furthermore, also Durovcova et al [12] reprocessed the VDFs measured by Helios and discussed evolutions of relative drifts between three dominant components-the proton core, proton beam, and α-particle core at different distances from the Sun. The processing is based on assumptions that partial VDFs of all these components can be approximated by a bi-Maxwellian distribution.…”

Section: Introductionmentioning

confidence: 99%

Spectra of Temperature Fluctuations in the Solar Wind

et al. 2021

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Turbulent cascade transferring the free energy contained within the large scale fluctuations of the magnetic field, velocity and density into the smaller ones is probably one of the most important mechanisms responsible for heating of the solar corona and solar wind, thus the turbulent behavior of these quantities is intensively studied. The temperature is also highly fluctuating quantity but its variations are studied only rarely. There are probably two reasons, first the temperature is tensor and, second, an experimental determination of temperature variations requires knowledge of the full velocity distribution with an appropriate time resolution but such measurements are scarce. To overcome this problem, the Bright Monitor of the Solar Wind (BMSW) on board Spektr-R used the Maxwellian approximation and provided the thermal velocity with a 32 ms resolution, investigating factors influencing the temperature power spectral density shape. We discuss the question whether the temperature spectra determined from Faraday cups are real or apparent and analyze mutual relations of power spectral densities of parameters like the density, parallel and perpendicular components of the velocity and magnetic field fluctuations. Finally, we compare their spectral slopes with the slopes of the thermal velocity in both inertial and kinetic ranges and their evolution in course of solar wind expansion.

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Evolution of Relative Drifts in the Expanding Solar Wind: Helios Observations

Cited by 36 publications

References 27 publications

Linear Stability in the Inner Heliosphere: Helios Re-evaluated

Linear Stability in the Inner Heliosphere: Helios Re-evaluated

Turbulence Upstream and Downstream of Interplanetary Shocks

Spectra of Temperature Fluctuations in the Solar Wind

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