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9 pages, 1342 KiB

Open AccessArticle

Investigation of Alpha-Proton Drift Speeds in the Solar Wind: WIND and HELIOS Observations

by Vamsee Krishna Jagarlamudi, Roberto Bruno, Rossana De Marco, Raffaella D’Amicis, Denise Perrone, Daniele Telloni and Nour E. Raouafi

Universe 2023, 9(1), 21; https://doi.org/10.3390/universe9010021 - 29 Dec 2022

Viewed by 982

Abstract

In this paper, we present an analysis of how alpha–proton drift speeds (the difference between the magnitudes of alpha and bulk proton speeds) are constrained in the inner heliosphere using observations from the WIND and twin HELIOS spacecraft. The solar wind is separated [...] Read more.

In this paper, we present an analysis of how alpha–proton drift speeds (the difference between the magnitudes of alpha and bulk proton speeds) are constrained in the inner heliosphere using observations from the WIND and twin HELIOS spacecraft. The solar wind is separated based on its bulk proton speed into the fast wind (>600 km/s) and slow wind (<400 km/s). The slow wind is again separated based on its normalized cross-helicity; slow wind intervals with average absolute normalized cross-helicity greater than 0.6 are considered Alfvénic, and those less than 0.6 are considered non-Alfvénic. Analysis of different types of wind intervals between 0.3 to 1 au have shown that the alpha-proton drift speeds are very much constrained by the angle between the B and V vectors for fast and slow Alfvénic wind intervals. Depending on the polarity of the magnetic field, there is a clear correlation or anti-correlation between the drift speeds and the angle between the B and V vectors. Interestingly, we did not observe any such relation in the non-Alfvénic slow wind intervals. Large-amplitude Alfvénic fluctuations present in the fast and slow Alfvénic winds control the drift between the alpha and proton core in the Alfvénic solar wind. The drift speeds can be modeled using the equation

+ / - V_{A} \sqrt{r_{A}} cos θ_{B V}

, where

V_{A}

is the Alfvén speed and

r_{A}

is the Alfvén ratio. Because the observations of drift speed constrained by the angle between the B and V vector for the fast and slow Alfvénic wind intervals are observed throughout the inner heliosphere, it is possible to consider this observed behavior to be a universal phenomenon of Alfvénic wind above the Alfvénic surface. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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15 pages, 12295 KiB

Open AccessArticle

Solar Orbiter SWA Observations of Electron Strahl Properties Inside 1 AU

by Christopher J. Owen, Joel Baby Abraham, Georgios Nicolaou, Daniel Verscharen, Philippe Louarn and Timothy S. Horbury

Universe 2022, 8(10), 509; https://doi.org/10.3390/universe8100509 - 28 Sep 2022

Cited by 4 | Viewed by 1702

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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20 pages, 7176 KiB

Open AccessArticle

Plasma Turbulence in the Near-Sun and Near-Earth Solar Wind: A Comparison via Observation-Driven 2D Hybrid Simulations

by Luca Franci, Emanuele Papini, Daniele Del Sarto, Petr Hellinger, David Burgess, Lorenzo Matteini, Simone Landi and Victor Montagud-Camps

Universe 2022, 8(9), 453; https://doi.org/10.3390/universe8090453 - 30 Aug 2022

Cited by 1 | Viewed by 1871

Abstract

We analyse two high-resolution 2D hybrid simulations of plasma turbulence with observation-driven initial conditions that are representative of the near-Sun and the near-Earth solar wind. The former employs values of some fundamental parameters that have been measured by the Parker Solar Probe at [...] Read more.

We analyse two high-resolution 2D hybrid simulations of plasma turbulence with observation-driven initial conditions that are representative of the near-Sun and the near-Earth solar wind. The former employs values of some fundamental parameters that have been measured by the Parker Solar Probe at 0.17 au from the Sun, while, in the latter, they are set to average values typically observed at 1 au. We compare the spatial and spectral properties of the magnetic, ion velocity, and density fluctuations, as well as the time evolution of magnetic reconnection events that occur spontaneously as the result of the development of turbulence. Despite some differences due to the different plasma conditions, some key features are observed in both simulations: elongated ion-scale Alfvénic structures form in between vortices whenever the orientation of the magnetic field lines is the same, i.e., magnetic reconnection via the formation of an X point cannot occur; the magnetic and density fluctuations at sub-ion scales are governed by force balance; the magnetic compressibility at sub-ion scales is compatible with isotropic magnetic field components; the characteristic time of the formation of current sheets is the eddy turnover at the energy injection scale, while the characteristic time for their disruption via reconnection is compatible with the Alfvén time of the background turbulence. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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14 pages, 959 KiB

Open AccessArticle

Depletion of Heavy Ion Abundances in Slow Solar Wind and Its Association with Quiet Sun Regions

by Liang Zhao, Enrico Landi, Susan T. Lepri and Daniel Carpenter

Universe 2022, 8(8), 393; https://doi.org/10.3390/universe8080393 - 27 Jul 2022

Cited by 3 | Viewed by 1099

Abstract

The exact coronal origin of the slow-speed solar wind has been under debate for decades in the Heliophysics community. Besides the solar wind speed, the heavy ion composition, including the elemental abundances and charge state ratios, are widely used as diagnostic tool to [...] Read more.

The exact coronal origin of the slow-speed solar wind has been under debate for decades in the Heliophysics community. Besides the solar wind speed, the heavy ion composition, including the elemental abundances and charge state ratios, are widely used as diagnostic tool to investigate the coronal origins of the slow wind. In this study, we recognize a subset of slow speed solar wind that is located on the upper boundary of the data distribution in the O

^{7 +}

/O

^{6 +}

versus C

^{6 +}

/C

^{5 +}

plot (O-C plot). In addition, in this wind the elemental abundances relative to protons, such as N/P, O/P, Ne/P, Mg/P, Si/P, S/P, Fe/P, He/P, and C/P are systemically depleted. We compare these winds (“upper depleted wind” or UDW hereafter) with the slow winds that are located in the main stream of the O-C plot and possess comparable Carbon abundance range as the depletion wind (“normal-depletion-wind”, or NDW hereafter). We find that the proton density in the UDW is about 27.5% lower than in the NDW. Charge state ratios of O

^{7 +}

/O

^{6 +}

, O

^{7 +}

/O, and O

^{8 +}

/O are decreased by 64.4%, 54.5%, and 52.1%, respectively. The occurrence rate of these UDW is anti-correlated with solar cycle. By tracing the wind along PFSS field lines back to the Sun, we find that the coronal origins of the UDW are more likely associated with quiet Sun regions, while the NDW are mainly associated with active regions and HCS-streamer. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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14 pages, 445 KiB

Open AccessArticle

Investigating Alfvénic Turbulence in Fast and Slow Solar Wind Streams

by Raffaella D’Amicis, Denise Perrone, Marco Velli, Luca Sorriso-Valvo, Daniele Telloni, Roberto Bruno and Rossana De Marco

Universe 2022, 8(7), 352; https://doi.org/10.3390/universe8070352 - 27 Jun 2022

Cited by 2 | Viewed by 1588

Abstract

Solar wind turbulence dominated by large-amplitude Alfvénic fluctuations, mainly propagating away from the Sun, is ubiquitous in high-speed solar wind streams. Recent observations performed in the inner heliosphere (from 1 AU down to tens of solar radii) have proved that also slow wind [...] Read more.

Solar wind turbulence dominated by large-amplitude Alfvénic fluctuations, mainly propagating away from the Sun, is ubiquitous in high-speed solar wind streams. Recent observations performed in the inner heliosphere (from 1 AU down to tens of solar radii) have proved that also slow wind streams show sometimes strong Alfvénic signatures. Within this context, the present paper focuses on a comparative study on the characterization of Alfvénic turbulence in fast and slow solar wind intervals observed at 1 AU where degradation of Alfvénic correlations is expected. In particular, we compared the behavior of different parameters to characterize the Alfvénic content of the fluctuations, using also the Elsässer variables to derive the spectral behavior of the normalized cross-helicity and residual energy. This study confirms that the Alfvénic slow wind stream resembles, in many respects, a fast wind stream. The velocity-magnetic field (v-b) correlation coefficient is similar in the two cases as well as the amplitude of the fluctuations although it is not clear to what extent the condition of incompressibility holds. Moreover, the spectral analysis shows that fast wind and Alfvénic slow wind have similar normalized cross-helicity values but in general the fast wind streams are closer to energy equipartition. Despite the overall similarities between the two solar wind regimes, each stream shows also peculiar features, that could be linked to the intrinsic evolution history that each of them has experienced and that should be taken into account to investigate how and why Alfvénicity evolves in the inner heliosphere. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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15 pages, 817 KiB

Open AccessArticle

Contrasting Scaling Properties of Near-Sun Sub-Alfvénic and Super-Alfvénic Regions

by Tommaso Alberti, Simone Benella, Vincenzo Carbone, Giuseppe Consolini, Virgilio Quattrociocchi and Mirko Stumpo

Universe 2022, 8(7), 338; https://doi.org/10.3390/universe8070338 - 21 Jun 2022

Cited by 3 | Viewed by 1256

Abstract

Scale-invariance has rapidly established itself as one of the most used concepts in space plasmas to uncover underlying physical mechanisms via the scaling-law behavior of the statistical properties of field fluctuations. In this work, we characterize the scaling properties of the magnetic field [...] Read more.

Scale-invariance has rapidly established itself as one of the most used concepts in space plasmas to uncover underlying physical mechanisms via the scaling-law behavior of the statistical properties of field fluctuations. In this work, we characterize the scaling properties of the magnetic field fluctuations in a sub-alfvénic region in contrast with those of the nearby super-alfvénic zone during the ninth Parker Solar Probe perihelion. With our observations, (i) evidence of an extended self-similarity (ESS) for both the inertial and the sub-ion/kinetic regimes during both solar wind intervals is provided, (ii) a multifractal nature of field fluctuations is observed across inertial scales for both solar wind intervals, and (iii) a mono-fractal structure of the small-scale dynamics is reported. The main novelty is that a universal character is found at the sub-ion/kinetic scale, where a unique rescaling exponent describes the high-order statistics of fluctuations during both wind intervals. Conversely, a multitude of scaling symmetries is observed at the inertial scale with a similar fractal topology and geometrical structures between the magnetic field components in the ecliptic plane and perpendicular to it, in contrast with a different level of intermittency, more pronounced during the super-alfvénic interval rather than the sub-alfvénic one, along the perpendicular direction to the ecliptic plane. The above features are interpreted in terms of the possible underlying heating and/or acceleration mechanisms in the solar corona resulting from turbulence and current sheet formation. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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10 pages, 1998 KiB

Open AccessArticle

Modulation of Solar Wind Impact on the Earth’s Magnetosphere during the Solar Cycle

by Francesco Carbone, Daniele Telloni, Emiliya Yordanova and Luca Sorriso-Valvo

Universe 2022, 8(6), 330; https://doi.org/10.3390/universe8060330 - 14 Jun 2022

Cited by 1 | Viewed by 1473

Abstract

The understanding of extreme geomagnetic storms is one of the key issues in space weather. Such phenomena have been receiving increasing attention, especially with the aim of forecasting strong geomagnetic storms generated by high-energy solar events since they can severely perturb the near-Earth [...] Read more.

The understanding of extreme geomagnetic storms is one of the key issues in space weather. Such phenomena have been receiving increasing attention, especially with the aim of forecasting strong geomagnetic storms generated by high-energy solar events since they can severely perturb the near-Earth space environment. Here, the disturbance storm time index

D s t

, a crucial geomagnetic activity proxy for Sun–Earth interactions, is analyzed as a function of the energy carried by different solar wind streams. To determine the solar cycle activity influence on

D s t

, a 12-year dataset was split into sub-periods of maximum and minimum solar activity. Solar wind energy and geomagnetic activity were closely correlated for both periods of activity. Slow wind streams had negligible effects on Earth regardless of their energy, while high-speed streams may induce severe geomagnetic storming depending on the energy (kinetic or magnetic) carried by the flow. The difference between the two periods may be related to the higher rate of geo-effective events during the maximum activity, where coronal mass ejections represent the most energetic and geo-effective driver. During the minimum period, despite a lower rate of high energetic events, a moderate disturbance in the

D s t

index can be induced. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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12 pages, 10548 KiB

Open AccessArticle

The Dipolar Solar Minimum Corona

by Daniele Telloni

Universe 2021, 7(12), 507; https://doi.org/10.3390/universe7120507 - 20 Dec 2021

Cited by 1 | Viewed by 1867

Abstract

The large-scale configuration of the UV solar corona at the minimum activity between solar cycles 22 and 23 is explored in this paper. Exploiting a large sample of spectroscopic observations acquired by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory in [...] Read more.

The large-scale configuration of the UV solar corona at the minimum activity between solar cycles 22 and 23 is explored in this paper. Exploiting a large sample of spectroscopic observations acquired by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory in the two-year period of 1996–1997, this work provides the first-ever monochromatic O vi 1032 Å image of the extended corona, and the first-ever two-dimensional maps of the kinetic temperature of oxygen ions and the O vi

1037 / 1032

Å doublet intensity ratio (a proxy for the outflow velocity of the oxygen component of the solar wind), statistically representative of solar minimum conditions. A clear dipolar magnetic structure, both equator- and axis-symmetric, is distinctly shown to shape the solar minimum corona, both in UV emission and in temperature and expansion rate. This statistical approach allows for robust establishment of the key role played by the magnetic field divergence in modulating the speed and temperature of the coronal flows, and identification of the coronal sources of the fast and slow solar wind. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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Review

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27 pages, 1489 KiB

Open AccessReview

Parametric Instability: An Evolutive Mechanism for the Alfvénic Turbulence in the Solar Wind

by Francesco Malara, Leonardo Primavera and Pierluigi Veltri

Universe 2022, 8(8), 391; https://doi.org/10.3390/universe8080391 - 23 Jul 2022

Cited by 3 | Viewed by 1090

Abstract

Fluctuations in fast streams or in slow Alfvénic streams of the solar wind, and in the high-latitude wind, are characterized by high cross-helicity and a low level of compressions. Such properties, which are typical of Alfvénic fluctuations, tend to decline with increasing heliocentric [...] Read more.

Fluctuations in fast streams or in slow Alfvénic streams of the solar wind, and in the high-latitude wind, are characterized by high cross-helicity and a low level of compressions. Such properties, which are typical of Alfvénic fluctuations, tend to decline with increasing heliocentric distance. Parametric decay, where the energy of an initial Alfvén wave is progressively transferred to both backward-propagating Alfvén and compressive modes, has been proposed as a mechanism responsible for such a behavior. Over the years, the parametric process has been studied, both analytically and numerically, in many configurations, from monochromatic waves to increasingly complex situations which include broad-band turbulent configurations with one- and two-dimensional spectra. In this paper, we give a brief review of this theoretical development, discussing its relevance in the context the evolution of Alfvénic turbulence in the solar wind. Full article

(This article belongs to the Special Issue Advances in Solar Wind Origin and Evolution)

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