Inherent length‐scales of periodic solar wind number density structures

Viall, N. M.; Kepko, L.; Spence, Harlan E.

doi:10.1029/2007ja012881

Cited by 51 publications

(131 citation statements)

References 15 publications

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“…This could be a manifestation of finite-size wave trains or (perhaps more plausibly) an indication that they are plasmoids that have been released individually and therefore have their own physical integrity via the tension force (Sheeley et al 1999). With the additional smaller density blobs identified in this new data set, the blobs collectively might form a significant fraction of the solar wind, which is consistent with the observation of Viall et al (2008) that periodic blobs with scales of 5-30 minutes comprised 80% of the in situ slow solar wind during a solar-maximum observation. The visibly variable component of the wind, as observed in Figure 12, sums to approximately 10% of the overall measured coronal brightness ("K") at each radius.…”

Section: The Temporally Structured Outer Coronasupporting

confidence: 74%

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The Highly Structured Outer Solar Corona

DeForest

Howard

Velli

et al. 2018

ApJ

137

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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Section: The Temporally Structured Outer Coronasupporting

confidence: 74%

“…This method has been applied both to time series of solar-wind density data and white light imaging data (Viall et al 2008(Viall et al , 2010Viall & Vourlidas 2015). We plotted the power spectra and results of the significance tests in Figure 17.…”

Section: Analysis Of Blobsmentioning

confidence: 99%

The Highly Structured Outer Solar Corona

DeForest

Howard

Velli

et al. 2018

ApJ

137

111

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“…In a separate study, Viall et al. () showed that the observations were better organized by the radial length scale of the periodic density structures, rather than apparent frequency (which depends on the velocity of the solar wind). This growing body of work has established that periodic density structures in the solar wind are a significant source of discrete, magnetospheric pulsations, particularly at f <∼4 mHz (Hartinger et al., ) and that these solar wind structures occur more frequently at certain scale sizes (and apparent frequencies) than others.…”

Section: Introductionmentioning

confidence: 99%

The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions

Kepko

Viall

2019

JGR Space Physics

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We present several examples of magnetospheric ultralow frequency pulsations associated with stream interaction regions and demonstrate that the observed magnetospheric pulsations were also present in the solar wind number density. The distance of the solar wind monitor ranged from just upstream of Earth's bow shock to 261 RE, with a propagation time delay of up to 90 min. The number density oscillations far upstream of Earth are offset from similar oscillations observed within the magnetosphere by the advection timescale, suggesting that the periodic dynamic pressure enhancements were time stationary structures, passively advecting with the ambient solar wind. The density structures are larger than Earth's magnetosphere and slowly altered the dynamic pressure enveloping Earth, leading to a quasi‐static and globally coherent “forced‐breathing” of Earth's dayside magnetospheric cavity. The impact of these periodic solar wind density structures was observed in both magnetospheric magnetic field and energetic particle data. We further show that the structures were initially smaller‐amplitude, spatially larger, structures in the upstream slow solar wind and that the higher‐speed wind compressed and amplified these preexisting structures leading to a series of quasiperiodic density structures with periods typically near 20 min. Similar periodic density structures have been observed previously at L1 and in remote images, but never in the context of solar wind shocks and discontinuities. The existence of periodic density structures within stream interaction regions may play an important role in magnetospheric particle acceleration, loss, and transport, particularly for outer zone electrons that are highly responsive to ultralow frequency wave activity.

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“…The latter could relate to the smaller density structures observed in coronal images (Viall et al 2010;Viall & Vourlidas 2015). According to in situ measurements, 70% to 80% of the SSW is made up of quasi-periodic structures (Viall et al 2008).…”

Section: Introductionmentioning

confidence: 99%

The Temporal and Spatial Scales of Density Structures Released in the Slow Solar Wind During Solar Activity Maximum

Sanchez-Diaz

Rouillard

Davies

et al. 2017

ApJ

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In a recent study, we took advantage of a highly tilted coronal neutral sheet to show that density structures, extending radially over several solar radii (R s ), are released in the forming slow solar wind approximately 4-5 R s above the solar surface. We related the signatures of this formation process to intermittent magnetic reconnection occurring continuously above helmet streamers. We now exploit the heliospheric imagery from the Solar Terrestrial Relation Observatory (STEREO) to map the spatial and temporal distribution of the ejected structures. We demonstrate that streamers experience quasi-periodic bursts of activity with the simultaneous outpouring of small transients over a large range of latitudes in the corona. This cyclic activity leads to the emergence of welldefined and broad structures. Derivation of the trajectories and kinematic properties of the individual small transients that make up these large-scale structures confirms their association with the forming slow solar wind (SSW). We find that these transients are released, on average, every 19.5 hr, simultaneously at all latitudes with a typical radial size of 12 R s . Their spatial distribution, release rate, and three-dimensional extent are used to estimate the contribution of this cyclic activity to the mass flux carried outward by the SSW. Our results suggest that, in interplanetary space, the global structure of the heliospheric current sheet is dominated by a succession of blobs and associated flux ropes. We demonstrate this with an example event using STEREO-A in situ measurements.

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Inherent length‐scales of periodic solar wind number density structures

Cited by 51 publications

References 15 publications

The Highly Structured Outer Solar Corona

The Highly Structured Outer Solar Corona

The Source, Significance, and Magnetospheric Impact of Periodic Density Structures Within Stream Interaction Regions

The Temporal and Spatial Scales of Density Structures Released in the Slow Solar Wind During Solar Activity Maximum

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