Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind

DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer, Steven R.

doi:10.3847/0004-637x/828/2/66

Cited by 87 publications

(93 citation statements)

References 65 publications

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“…Another intriguing possibility is that R b could correspond to the distance recently identified in DeForest et al (2016) where a transition from relatively steady and laminar radial flow to sheared and turbulently mixed flow is remotely observed. Using an improved analysis of remote observations from the Heliospheric Imager on STEREO, the authors identified a region R 40 80 s -from the solar surface in which the smooth radial expansion of the slow solar wind appears to fragment and break up.…”

Section: Discussionmentioning

confidence: 84%

A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun

Kasper

Klein

Weber

et al. 2017

ApJ

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The extreme temperatures and nonthermal nature of the solar corona and solar wind arise from an unidentified physical mechanism that preferentially heats certain ion species relative to others. Spectroscopic indicators of unequal temperatures commence within a fraction of a solar radius above the surface of the Sun, but the outer reach of this mechanism has yet to be determined. Here we present an empirical procedure for combining interplanetary solar wind measurements and a modeled energy equation including Coulomb relaxation to solve for the typical outer boundary of this zone of preferential heating. Applied to two decades of observations by the Wind spacecraft, our results are consistent with preferential heating being active in a zone extending from the transition region in the lower corona to an outer boundary 20-40 solar radii from the Sun, producing a steady-state super-massproportional α-to-proton temperature ratio of 5.2-5.3. Preferential ion heating continues far beyond the transition region and is important for the evolution of both the outer corona and the solar wind. The outer boundary of this zone is well below the orbits of spacecraft at 1 au and even closer missions such as Helios and MESSENGER, meaning it is likely that no existing mission has directly observed intense preferential heating, just residual signatures. We predict that the Parker Solar Probe will be the first spacecraft with a perihelion sufficiently close to the Sun to pass through the outer boundary, enter the zone of preferential heating, and directly observe the physical mechanism in action.

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Section: Discussionmentioning

confidence: 84%

A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun

Kasper

Klein

Weber

et al. 2017

ApJ

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“…To reduce radial blurring, we smoothed in a moving coordinate system, as in DeForest et al (2016). To do that, we measured wind flow using an autocorrelation of the L5 images.…”

Section: Datamentioning

confidence: 99%

“…(An animation of this figure is available.) the bottom panel of Figure 5 is scaled by the cube of the apparent radius from the Sun, following DeForest et al (2016).…”

Section: Datamentioning

confidence: 99%

“…To distinguish the coronal signal from noise, we analyzed the second-order structure function of the Figure 8 image, as described by DeForest et al (2016).…”

Section: Universal Fine-scale Structurementioning

confidence: 99%

“…Because the difference between nearby pixels is squared, S 2 may be averaged or cut over one or more of its four dimensions to explore the structure of an image. The second-order structure function is developed in more detail by DeForest et al (2016) and references therein. Figure 9 shows vertical and horizontal cuts through the lag axes of S 2 that correspond to the frame in Figure 8, from the L3 and L7 steps of analysis.…”

Section: Universal Fine-scale Structurementioning

confidence: 99%

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The Highly Structured Outer Solar Corona

DeForest

Howard

Velli

et al. 2018

ApJ

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129

111

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We report on the observation of fine-scale structure in the outer corona at solar maximum, using deep-exposure campaign data from the Solar Terrestrial Relations Observatory-A (STEREO-A)/COR2 coronagraph coupled with postprocessing to further reduce noise and thereby improve effective spatial resolution. The processed images reveal radial structure with high density contrast at all observable scales down to the optical limit of the instrument, giving the corona a "woodgrain" appearance. Inferred density varies by an order of magnitude on spatial scales of 50 Mm and follows an f −1 spatial spectrum. The variations belie the notion of a smooth outer corona. They are inconsistent with a well-defined "Alfvén surface," indicating instead a more nuanced "Alfvén zone"-a broad trans-Alfvénic region rather than a simple boundary. Intermittent compact structures are also present at all observable scales, forming a size spectrum with the familiar "Sheeley blobs" at the large-scale end. We use these structures to track overall flow and acceleration, finding that it is highly inhomogeneous and accelerates gradually out to the limit of the COR2 field of view. Lagged autocorrelation of the corona has an enigmatic dip around 10 R e , perhaps pointing to new phenomena near this altitude. These results point toward a highly complex outer corona with far more structure and local dynamics than has been apparent. We discuss the impact of these results on solar and solar-wind physics and what future studies and measurements are necessary to build upon them.

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The effects of inhomogeneous proton‐α drifts on the heating of the solar wind

2017

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Previous modeling studies have demonstrated that waves and super‐Alfvénic drift can lead to perpendicular preferential heating of the α particles with respect to protons. Using 2.5‐D hybrid model of the solar wind α‐proton plasma, we study the effects of inhomogeneous (across the magnetic field) background streaming focusing on the fast solar wind. We explore the effects of an initial relative, inhomogeneous ion drift on the perpendicular ion heating and cooling and consider the effects of solar wind expansion. We study the spectrum of the magnetic fluctuations in the inhomogeneous background solar wind and demonstrate the generation of oblique waves and their effects on enhanced resonant anisotropic ion heating. The model reproduces the typical ion temperature anisotropy values seen in observations. Using our model, we find that inhomogeneous super‐Alfvénic ion drift in the plasma generates significant power of oblique waves in the solar wind plasma, in addition to enhanced heating compared to the nondrifting populations. We demonstrate the effects of various inhomogeneity profiles and regions of the drift on the ion anisotropic heating in super‐Alfvénic and near‐Alfvénic drifts. We find that the cooling effect due to the solar wind expansion is not significant when super‐Alfvénic drifts are considered.

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Fading Coronal Structure and the Onset of Turbulence in the Young Solar Wind

Cited by 87 publications

References 65 publications

A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun

A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun

The Highly Structured Outer Solar Corona

The effects of inhomogeneous proton‐α drifts on the heating of the solar wind

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