The very slow solar wind: Properties, origin and variability

Sanchez-Diaz, Eduardo; Rouillard, A. P.; Lavraud, B.; Segura, Kevin; Tao, Chihiro; Pinto, Rui; Sheeley, N. R.; Plotnikov, Illya

doi:10.1002/2016ja022433

Cited by 50 publications

(48 citation statements)

References 52 publications

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“…We conclude that, for most of their propagation, CDSs are therefore associated with density variations in the background slow solar wind. Recent analysis of Helios observations near 0.3 AU has revealed that the densest solar wind is associated with the heliospheric plasma sheet and propagates with a speed of less than 300 km s −1 in over 8 % of all observations (Sanchez-Diaz et al, 2016). Additionally, SIR/CIRs are rarely well formed at these heights.…”

Section: Discussionmentioning

confidence: 99%

Long-Term Tracking of Corotating Density Structures Using Heliospheric Imaging

et al. 2016

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The systematic monitoring of the solar wind in high-cadence and high-resolution heliospheric images taken by the Solar-Terrestrial Relation Observatory (STEREO) spacecraft permits the study of the spatial and temporal evolution of variable solar wind flows from the Sun out to 1 AU, and beyond. As part of the EU Framework 7 (FP7) Heliospheric Cataloguing, Analysis and Techniques Service (HELCATS) project, we have generated a catalog listing the properties of 190 corotating structures well-observed in images taken by the Heliospheric Imager (HI) instruments onboard STEREO-A (ST-A). Based on this catalog, we present here one of very few long-term analyses of solar wind structures advected by the background solar wind. We concentrate on the subset of plasma density structures clearly identified inside corotating structures. This analysis confirms that most of the corotating density structures detected by the heliospheric imagers comprises a series of density inhomogeneities advected by the slow solar wind that eventually become entrained by stream interaction regions. We have derived the spatial-temporal evolution of each of these corotating density structures by using a well-established fitting technique. The mean radial propagation speed of the corotating structures is found to be 311 ± 31 km s −1 . Such a low mean value corresponds to the terminal speed of the slow solar wind rather than the speed of stream interfaces, which is typically intermediate between the slow and fast solar wind speeds (∼ 400 km s −1 ). Using our fitting technique, we predicted the arrival time of each corotating density structure at different probes in the inner heliosphere. We find that our derived speeds are systematically lower by ∼ 100 km s −1 than those measured in situ at the predicted impact times. Moreover, for cases when a stream interaction region is clearly detected in situ at the estimated impact time, we find that our derived speeds are lower than the speed of the stream interface measured in situ by an average of 55 km s −1 at ST-A and 84 km s −1 at STEREO-B (ST-B). We show that the speeds of the corotating density structures derived using our fitting technique track well the long-term variation of the radial speed of the slow solar wind during solar minimum years (2007 -2008). Furthermore, we demonstrate that these features originate near the coronal neutral line that eventually becomes the heliospheric current sheet.

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Section: Discussionmentioning

confidence: 99%

Long-Term Tracking of Corotating Density Structures Using Heliospheric Imaging

et al. 2016

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“…We note that this velocity is about four times lower than that of the very slow solar wind (e.g., Sanchez-Diaz et al 2016), or the wind of the K-type host star HD 189733b (as derived by Bourrier & Lecavelier des Etangs 2013 from transit observations of its evaporating hot Jupiter companion).…”

Section: Simulations Of Hydrogen Exospheresmentioning

confidence: 93%

Strong H i Lyman-α variations from an 11 Gyr-old host star: a planetary origin?

Bourrier

Ehrenreich

Allart

et al. 2017

A&A

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Kepler-444 provides a unique opportunity to probe the atmospheric composition and evolution of a compact system of exoplanets smaller than the Earth. Five planets transit this bright K star at close orbital distances, but they are too small for their putative lower atmosphere to be probed at optical/infrared wavelengths. We used the Space Telescope Imaging Spectrograph instrument onboard the Hubble Space Telescope to search for the signature of the planet's upper atmospheres at six independent epochs in the Lyman-α line. We detect significant flux variations during the transits of both Kepler-444 e and f (∼20%), and also at a time when none of the known planets was transiting (∼40%). Variability in the transition region and corona of the host star might be the source of these variations. Yet, their amplitude over short time scales (∼2-3 hours) is surprisingly strong for this old (11.2±1.0 Gyr) and apparently quiet main-sequence star. Alternatively, we show that the in-transits variations could be explained by absorption from neutral hydrogen exospheres trailing the two outer planets (Kepler-444 e and f). They would have to contain substantial amounts of water to replenish such hydrogen exospheres, which would reveal them as the first confirmed ocean-planets. The outof-transit variations, however, would require the presence of a yet-undetected Kepler-444 g at larger orbital distance, casting doubt on the planetary origin scenario. Using HARPS-N observations in the sodium doublet, we derived the properties of two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This allowed us to reconstruct the stellar Lyman-α line profile and to estimate the XUV irradiation from the star, which would still allow for a moderate mass loss from the outer planets after 11.2 Gyr. Follow-up of the system at XUV wavelengths will be required to assess this tantalizing possibility.

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“…Dividing this number by the area of that sphere, we derive a proton flux of 1.7 × 10 11 m −2 s −1 due to blobs during the period of study. If we assume that the proton flux of the solar wind at 1 au is 2.5 × 10 12 m −2 s −1 (Wang 2010;Sanchez-Diaz et al 2016), and that 40% to 50% of the solar wind is SSW during solar maximum (Tokumaru et al 2010), we estimate that the proton flux released into the solar wind in the form of blobs would represent around 15% of the bulk of the whole SSW at solar maximum. Based on in situ measurements, Kasper et al (2007) and McGregor (2011) suggest that there are two sources of SSW.…”

Section: Contribution Of Blobs To the Total Mass Flux Of The Slow Solmentioning

confidence: 99%

“…Provided that the total proton flux of the SSW is known from in situ measurements at 1 au (Wang 2010;Sanchez-Diaz et al 2016), the fraction of the SSW mass flux transported by blobs alone may be obtained by exploiting heliospheric imagery to retrieve the sizes, release rate, and spatial distribution of blobs. The tracking of blobs to 1 au, and their identification in situ, is only possible for certain spacecraft configurations.…”

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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The very slow solar wind: Properties, origin and variability

Cited by 50 publications

References 52 publications

Long-Term Tracking of Corotating Density Structures Using Heliospheric Imaging

Long-Term Tracking of Corotating Density Structures Using Heliospheric Imaging

Strong H i Lyman-α variations from an 11 Gyr-old host star: a planetary origin?

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

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