scholarly journals Complexity of magnetic fields on red dwarfs

2019 ◽  
Vol 629 ◽  
pp. A83 ◽  
Author(s):  
N. Afram ◽  
S. V. Berdyugina
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Circular Polarization ◽  
Large Fraction ◽  
The Sun ◽  
M Dwarfs ◽  
Magnetic Studies ◽  
Molecular Lines ◽  
Magnetic Features ◽  
M Dwarf

Context. Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature- and Zeeman-sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Aims. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how “surface” inhomogeneities are distributed in height – the dimension that is usually neglected in stellar magnetic studies. Methods. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1–M7). Our χ2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Results. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3–6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma β is less than one, implying highly magnetized stars. Conclusions. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.

Emergence of Twisted Magnetic Flux Related Sigmoidal Brightening

2000 ◽  
Vol 179 ◽  
pp. 263-264
Author(s):  
K. Sundara Raman ◽  
K. B. Ramesh ◽  
R. Selvendran ◽  
P. S. M. Aleem ◽  
K. M. Hiremath
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Ultra Violet ◽  
Active Regions ◽  
Morphological Properties ◽  
Energy Storage And Conversion ◽  
The Sun ◽  
X Rays ◽  
The Magnetic Field

Extended AbstractWe have examined the morphological properties of a sigmoid associated with an SXR (soft X-ray) flare. The sigmoid is cospatial with the EUV (extreme ultra violet) images and in the optical part lies along an S-shaped Hαfilament. The photoheliogram shows flux emergence within an existingδtype sunspot which has caused the rotation of the umbrae giving rise to the sigmoidal brightening.It is now widely accepted that flares derive their energy from the magnetic fields of the active regions and coronal levels are considered to be the flare sites. But still a satisfactory understanding of the flare processes has not been achieved because of the difficulties encountered to predict and estimate the probability of flare eruptions. The convection flows and vortices below the photosphere transport and concentrate magnetic field, which subsequently appear as active regions in the photosphere (Rust & Kumar 1994 and the references therein). Successive emergence of magnetic flux, twist the field, creating flare productive magnetic shear and has been studied by many authors (Sundara Ramanet al.1998 and the references therein). Hence, it is considered that the flare is powered by the energy stored in the twisted magnetic flux tubes (Kurokawa 1996 and the references therein). Rust & Kumar (1996) named the S-shaped bright coronal loops that appear in soft X-rays as ‘Sigmoids’ and concluded that this S-shaped distortion is due to the twist developed in the magnetic field lines. These transient sigmoidal features tell a great deal about unstable coronal magnetic fields, as these regions are more likely to be eruptive (Canfieldet al.1999). As the magnetic fields of the active regions are deep rooted in the Sun, the twist developed in the subphotospheric flux tube penetrates the photosphere and extends in to the corona. Thus, it is essentially favourable for the subphotospheric twist to unwind the twist and transmit it through the photosphere to the corona. Therefore, it becomes essential to make complete observational descriptions of a flare from the magnetic field changes that are taking place in different atmospheric levels of the Sun, to pin down the energy storage and conversion process that trigger the flare phenomena.

scholarly journals Optical and Radio Observations of Large Scale Magnetic Fields on the Sun

1971 ◽  
Vol 43 ◽  
pp. 609-615 ◽  
Author(s):  
G. Daigne ◽  
M. F. Lantos-Jarry ◽  
M. Pick
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Interplanetary Medium ◽  
Coronal Magnetic Field ◽  
The Sun ◽  
Radio Observations ◽  
Active Centers ◽  
Field Patterns

It is possible to deduce information concerning large scale coronal magnetic field patterns from the knowledge of the location of radioburst sources.As the method concerns active centers responsible for corpuscular emission, the knowledge of these structures may have important implications in the understanding of corpuscular propagation in the corona and in the interplanetary medium.

About the Relation Between the Limb Effect of the Redshift on the Sun and the Large-Scale Distribution of Solar activity

1976 ◽  
Vol 71 ◽  
pp. 113-118
Author(s):  
P. Ambrož
Keyword(s):  
Magnetic Field ◽  
Solar Activity ◽  
Magnetic Fields ◽  
Interplanetary Magnetic Field ◽  
Large Scale ◽  
The Sun ◽  
Limb Effect

The measurement of the magnitude of the limb effect was homogenized in time and a recurrent period of maxima of 27.8 days was found. A relation was found between the maximum values of the limb effect of the redshift, the boundaries of polarities of the interplanetary magnetic field, the characteristic large-scale distribution of the background magnetic fields and the complex of solar activity.

scholarly journals Sun-like Stars: magnetic fields, cycles and exoplanets

2015 ◽  
Vol 11 (A29A) ◽  
pp. 360-364
Author(s):  
Rim Fares
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Observational Constraints ◽  
The Sun ◽  
Field Generation ◽  
Large Scale Magnetic Field ◽  
Planetary Orbits ◽  
Activity Cycles ◽  
Magnetic Cycles

AbstractIn Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how “normal” the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

Magnetic fields in the solar photosphere

2008 ◽  
Vol 366 (1884) ◽  
pp. 4465-4476 ◽  
Author(s):  
Paul J Bushby
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Active Regions ◽  
Theoretical Models ◽  
Solar Photosphere ◽  
Complex Field ◽  
Theoretical Understanding ◽  
Magnetic Features ◽  
Localized Magnetic Field ◽  
Made In

Recent high-resolution observations of the surface of the Sun have revealed the fine structure of a vast array of complex photospheric magnetic features. Observations of these magnetic field structures have already greatly enhanced our theoretical understanding of the interactions between magnetic fields and turbulent convection, and future photospheric observations will inevitably present new theoretical challenges. In this review, I discuss recent progress that has been made in the modelling of photospheric magnetic fields. In particular, I focus upon the complex field structures that are observed within the umbrae and the penumbrae of sunspots. On a much smaller scale, I also discuss models of the highly localized magnetic field structures that are observed in less magnetically active regions of the photosphere. As the spatial resolution of telescopes has improved over the last few years, it has now become possible to observe these features in detail, and theoretical models can now describe much of this behaviour. In the last section of this review, I discuss some of the remaining unanswered questions.

scholarly journals Electrical conductivity of plasma in magnetic configurations of the sunspots

2019 ◽  
pp. 29-34
Author(s):  
V. Krivodubskij
Keyword(s):  
Magnetic Field ◽  
Electrical Conductivity ◽  
Magnetic Fields ◽  
Electric Field ◽  
Energy Release ◽  
Electric Circuit ◽  
The Sun ◽  
Turbulent Environment ◽  
Strong Magnetic Fields ◽  
Electromagnetic Models

The main problem of electromagnetic models of flares on the Sun is that in conditions of high electrical conductivity of the solar plasma it is difficult to provide an effective energy release as a result of Joule dissipation of currents in the “kernel of the flare”. In order to explain the rapid dissipation of electric currents in the “kernel of the flare”, we, within the framework of macroscopic magnetohydrodynamics, have considered the effect of reducing the electrical conductivity in a turbulent environment. The idea of redistribution of the electrical conductivity in groups of sunspots with complex magnetic field configuration is proposed. The proposed concept for the redistribution of electrical conductivity is based on the following physical effects and well-known observational conditions in the solar atmosphere. 1. Decreasing of the electrical conductivity (increase in the resistivity) in a turbulent environment. 2. Magnetic inhibition of the turbulence under the influence of magnetic fields. 3. Excitation of a large-scale electric field by macroscopic movements of the plasma in the photosphere in the presence of a weak general magnetic field of the Sun (photosphere dynamo). 4. Observed spatial heterogeneous structure of magnetic configurations in the vicinity of groups of sunspots, which leads to the formation of the current layers with the zero (neutral) magnetic fields. In the places of the zero magnetic field in the photosphere (which correspond to the “kernel of the flare”), where there is no suppression of turbulence by magnetism, the conductivity is turbulent in the nature. At the same time, in the vicinity of the sunspots outside the “kernel of the flare”, turbulent motions are largely suppressed by strong magnetic fields (B ≈ 3000 G), which almost alleviates the effect of the influence of turbulence on the conductivity of the plasma. Therefore, the electrical conductivity here will be gas-kinetic in the nature, the value of which greatly exceeds the turbulent conductivity. The turbulent conductivity calculated by us in the photosphere σ T ≈ 5 ⋅ 108 CGSE turned out to be 2-3 orders of magnitude smaller than the gaskinetic conductivity σ ≈ 1011 CGSE (in the places of strong magnetic fields). The discovered areas of the abnormal reduced turbulent conductivity in the places of the zero magnetic lines of complex configurations of the sunspot groups can contribute to the efficient dissipation of the electric currents, which provides efficient thermal energy release of the flares. The problem of circulation of two currents in the electric circuit of the corona-photosphere is briefly considered. According to the model of the photosphere dynamo, the convective movements on the photosphere level excite an electric field of magnitude E0 ≈ 10-4 CGSE. In this case, in external areas (in relation to the region of the “kernel of the flare”) of the electric circuit of the corona-photosphere in the places of strong magnetic fields, where the turbulence is almost suppressed, the value of the current will be ja = σ E0 ≈ 107 CGSE. At the same time, in the area of the “kernel of the flare”, where neutral magnetic fields do not affect turbulence, the current value will be much smaller: jT ≈ σ T E0 ≈ 5 ⋅ 104 CGSE. The existence of two sections with different currents in the electric circle of the corona-photosphere may contribute to the spatial division of charges, which in turn may be useful in the further development of the electromagnetic models of the flare.

Solar Dynamo

2020 ◽  
Author(s):  
Robert Cameron
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Differential Rotation ◽  
Large Scale ◽  
Toroidal Field ◽  
Solar Dynamo ◽  
Small Scale ◽  
The Sun ◽  
The Magnetic Field

The solar dynamo is the action of flows inside the Sun to maintain its magnetic field against Ohmic decay. On small scales the magnetic field is seen at the solar surface as a ubiquitous “salt-and-pepper” disorganized field that may be generated directly by the turbulent convection. On large scales, the magnetic field is remarkably organized, with an 11-year activity cycle. During each cycle the field emerging in each hemisphere has a specific East–West alignment (known as Hale’s law) that alternates from cycle to cycle, and a statistical tendency for a North-South alignment (Joy’s law). The polar fields reverse sign during the period of maximum activity of each cycle. The relevant flows for the large-scale dynamo are those of convection, the bulk rotation of the Sun, and motions driven by magnetic fields, as well as flows produced by the interaction of these. Particularly important are the Sun’s large-scale differential rotation (for example, the equator rotates faster than the poles), and small-scale helical motions resulting from the Coriolis force acting on convective motions or on the motions associated with buoyantly rising magnetic flux. These two types of motions result in a magnetic cycle. In one phase of the cycle, differential rotation winds up a poloidal magnetic field to produce a toroidal field. Subsequently, helical motions are thought to bend the toroidal field to create new poloidal magnetic flux that reverses and replaces the poloidal field that was present at the start of the cycle. It is now clear that both small- and large-scale dynamo action are in principle possible, and the challenge is to understand which combination of flows and driving mechanisms are responsible for the time-dependent magnetic fields seen on the Sun.

scholarly journals Solar and Stellar Magnetic Fields and Atmospheric Structures: Theory

1989 ◽  
Vol 104 (1) ◽  
pp. 271-288
Author(s):  
E. N. Parker
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Dynamical Behavior ◽  
Convective Zone ◽  
Coronal Heating ◽  
The Sun ◽  
Current Sheets ◽  
Spontaneous Formation ◽  
Stellar Magnetic Fields ◽  
The Magnetic Field

AbstractThis presentation reviews selected ideas on the origin of the magnetic field of the Sun, the dynamical behavior of the azimuthal field in the convective zone, the fibril state of the field at the photosphere, the formation of sunspots, prominences, the spontaneous formation of current sheets in the bipolar field above the surface of the Sun, coronal heating, and flares.

scholarly journals Analog Video Magnetograms in Real Time

1971 ◽  
Vol 43 ◽  
pp. 76-83 ◽  
Author(s):  
R. C. Smithson ◽  
R. B. Leighton
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Solar Magnetic Field ◽  
Solar Spectrum ◽  
Zeeman Splitting ◽  
The Other ◽  
Line Of Sight ◽  
Zero Field ◽  
The Sun ◽  
Solar Magnetic Fields

For many years solar magnetic fields have been measured by a variety of techniques, all of which exploit the Zeeman splitting of lines in the solar spectrum. One of these techniques (Leighton, 1959) involves a photographic subtraction of two monochromatic images to produce a picture of the Sun in which the line-of-sight component of the solar magnetic field appears as various shades of gray. In a magnetogram made by this method, zero field strength appears as neutral gray, while magnetic fields of one polarity or the other appear as lighter or darker areas, respectively. Figure 1 shows such a magnetogram.

scholarly journals Magnetic field observations of low-mass stars

2008 ◽  
Vol 4 (S259) ◽  
pp. 339-344
Author(s):  
Ansgar Reiners
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Measurements ◽  
Polarized Light ◽  
M Dwarfs ◽  
Magnetic Field Generation ◽  
Low Mass Stars ◽  
Strong Magnetic Fields ◽  
Direct Measurements ◽  
Low Mass

AbstractDirect measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.

Sign in / Sign up

Export Citation Format

Share Document