Magnetic fields, convection and solar luminosity variability

Livingston, W.

doi:10.1038/297208a0

Cited by 82 publications

(46 citation statements)

References 5 publications

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“…That the close packing of magnetic elements in plage could lead to a significant disturbance of the surrounding convective energy transport and thereby to lower contrasts was already proposed by , . This is supported by observational results from spectral lines indicating a weaker convective transport in regions with larger filling factors (Livingston 1982;Cavallini et al 1985;Immerschitt & Schröter 1987;Brandt & Solanki 1990). Based on Hinode/SP data, Morinaga et al (2008) recently gave evidence of the local action of the magnetic field on the flows, by observing that the vertical convection was more suppressed in locations (pixels) of larger apparent field strength.…”

Section: Inhibition Of Convective Flowssupporting

confidence: 67%

The continuum intensity as a function of magnetic field

Kobel

Solanki

Borrero

2012

A&A

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Context. To deepen our understanding of the role of small-scale magnetic fields in active regions (ARs) and in the quiet Sun (QS) on the solar irradiance, it is fundamental to investigate the physical processes underlying their continuum brightness. Previous results showed that magnetic elements in the QS reach larger continuum intensities than in ARs at disk center, but left this difference unexplained. Aims. We use Hinode/SP disk center data to study the influence of the local amount of magnetic flux on the vigour of the convective flows and the continuum intensity contrasts. Methods. The apparent (i.e. averaged over a pixel) longitudinal field strength and line-of-sight (LOS) plasma velocity were retrieved by means of Milne-Eddington inversions (VFISV code). We analyzed a series of boxes taken over AR plages and the QS, to determine how the continuum intensity contrast of magnetic elements, the amplitude of the vertical flows and the box-averaged contrast were affected by the mean longitudinal field strength in the box (which scales with the total unsigned flux in the box).Results. Both the continuum brightness of the magnetic elements and the dispersion of the LOS velocities anti-correlate with the mean longitudinal field strength. This can be attributed to the "magnetic patches" (here defined as areas where the longitudinal field strength is above 100 G) carrying most of the flux in the boxes. There the velocity amplitude and the spatial scale of convection are reduced. Due to this hampered convective transport, these patches appear darker than their surroundings. Consequently, the average brightness of a box decreases as the the patches occupy a larger fraction of it and the amount of embedded flux thereby increases. Conclusions. Our results suggest that as the magnetic flux increases locally (e.g. from weak network to strong plage), the heating of the magnetic elements is reduced by the intermediate of a more suppressed convective energy transport within the larger and stronger magnetic patches. This, together with the known presence of larger magnetic features, could explain the previously found lower contrasts of the brightest magnetic elements in ARs compared to the QS. The inhibition of convection also affects the average continuum brightness of a photospheric region, so that at disk center, an area of photosphere in strong network or plage appears darker than a purely quiet one. This is qualitatively consistent with the predictions of 3D MHD simulations.

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Section: Inhibition Of Convective Flowssupporting

confidence: 67%

The continuum intensity as a function of magnetic field

Kobel

Solanki

Borrero

2012

A&A

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“…These convective phenomena can be found all over the stellar surface, except in active regions where convection is significantly lower (e.g. Dravins 1982;Livingston 1982;Brandt & Solanki 1990;Gray 1992;Meunier et al 2010). The amplitudes A&A 525, A140 (2011) of granulation phenomena are similar to the ones observed for pressure modes (e.g.…”

Section: Introductionsupporting

confidence: 62%

Planetary detection limits taking into account stellar noise

Dumusque

Udry

Lovis

et al. 2010

A&A

284

311

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Context. Stellar noise produced by oscillations, granulation phenomena (granulation, mesogranulation, and supergranulation), and activity affects radial velocity measurements. The signature of the corresponding effect in radial velocity is small, around the meterper-second, but already too large for the detection of Earth-mass planets in habitable zones. Aims. We address the important role played by observational strategies in averaging out the radial velocity signature of stellar noise. We also derive the planetary mass detection limits expected in the presence of stellar noise. Methods. We start with HARPS asteroseismology measurements for four stars (β Hyi, α Cen A, μ Ara, and τ Ceti) available in the ESO archive and very precise measurements of α Cen B. This sample covers different spectral types from G2 to K1 and different evolutionary stages, from subgiant to dwarf stars. Since data span between 5 and 8 days, only stellar noise sources with timescales shorter than this time span will be extracted from these observations. Therefore, we are able to study oscillation modes and granulation phenomena without being significantly affected by activity noise present on longer timescales. For those five stars, we generate synthetic radial velocity measurements after fitting the corresponding models of stellar noise in Fourier space. These measurements allow us to study the radial velocity variation due to stellar noise for different observational strategies as well as the corresponding planetary mass detection limits. Results. Applying three measurements per night of 10 min exposure each, 2 h apart, seems to most efficiently average out the stellar noise considered. For quiet K1V stars such as α Cen B, this strategy allows us to detect planets of about three times the mass of Earth with an orbital period of 200 days, corresponding to the habitable zone of the star. Moreover, our simulations suggest that planets smaller than typically 5 M ⊕ can be detected with HARPS over a wide range of separations around most non-active solar-type dwarfs. Since activity is not yet included in our simulation, these detection limits correspond to a case, which exists, where the host star has few magnetic features and stellar noise is dominated by oscillation modes and granulation phenomena. For our star sample, a trend between spectral type and surface gravity and the level of radial velocity variation is also identified by our simulations.

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“…Observations show that solar line bisectors smaller lower velocity spans and convective blueshifts in active regions (Livingston 1982;Brandt & Solanki 1990). This could explain the observed variations in the shape of the CCF as a function of the activity level of the star.…”

Section: Activity and Ccf Parametersmentioning

confidence: 99%

Do stellar magnetic cycles influence the measurement of precise radial velocities?

Santos

Silva

Melo

2010

A&A

105

118

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The ever increasing level of precision achieved by present and future radial-velocity instruments is opening the way to discovering very low-mass, long-period planets (e.g. solar-system analogs). These systems will be detectable as low-amplitude signals in radial-velocity (RV). However, an important obstacle to their detection may be the existence of stellar magnetic cycles on similar timescales. Here we present the results of a long-term program to simultaneously measure radial-velocities and stellar-activity indicators (CaII, H α , He i) for a sample of stars with known activity cycles. Our results suggest that all these stellar activity indexes can be used to trace the stellar magnetic cycle in solar-type stars. Likewise, we find clear indications that different parameters of the HARPS cross-correlation function (BIS, FWHM, and contrast) are also sensitive to activity level variations. Finally, we show that, although in a few cases slight correlations or anti-correlations between radial-velocity and the activity level of the star exist, their origin is still not clear. We can, however, conclude that for our targets (early-K dwarfs) we do not find evidence of any radial-velocity variations induced by variations of the stellar magnetic cycle with amplitudes significantly above ∼1 m/s.

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Magnetic fields, convection and solar luminosity variability

Cited by 82 publications

References 5 publications

The continuum intensity as a function of magnetic field

The continuum intensity as a function of magnetic field

Planetary detection limits taking into account stellar noise

Do stellar magnetic cycles influence the measurement of precise radial velocities?

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