scholarly journals Processes in the magnetized chromosphere of the Sun

2008 ◽  
Vol 4 (S257) ◽  
pp. 121-131
Author(s):  
S. S. Hasan
Keyword(s):  
Magnetic Fields ◽  
Large Field ◽  
The Sun ◽  
Physical Processes ◽  
Flux Tubes ◽  
Weak Magnetic Fields ◽  
Space Experiments ◽  
New Space ◽  
Magnetic Network

AbstractWe review physical processes in magnetized chromospheres on the Sun. In the quiet chromosphere, it is useful to distinguish between the magnetic network on the boundaries of supergranules, where strong magnetic fields are organized in mainly vertical flux tubes and internetwork regions in the cell interiors, which have traditionally been associated with weak magnetic fields. Recent observations from Hinode, however, suggest that there is a significant amount of horizontal magnetic flux in the cell interior with large field strength. Furthermore, processes that heat the magnetic network have not been fully identified. Is the network heated by wave dissipation and if so, what is the nature of these waves? These and other aspects related to the role of spicules will also be highlighted. A critical assessment will be made on the challenges facing theory and observations, particularly in light of the new space experiments and the planned ground facilities.

scholarly journals Models of coronal heating, turbulence and fast reconnection

2015 ◽  
Vol 373 (2042) ◽  
pp. 20140262 ◽  
Author(s):  
M. Velli ◽  
F. Pucci ◽  
F. Rappazzo ◽  
A. Tenerani
Keyword(s):  
Energy Transfer ◽  
Magnetic Fields ◽  
Mass Loss ◽  
Coronal Heating ◽  
The Sun ◽  
X Ray ◽  
Magnetically Confined ◽  
Small Scales ◽  
Fast Reconnection

Coronal heating is at the origin of the EUV and X-ray emission and mass loss from the sun and many other stars. While different scenarios have been proposed to explain the heating of magnetically confined and open regions of the corona, they must all rely on the transfer, storage and dissipation of the abundant energy present in photospheric motions, which, coupled to magnetic fields, give rise to the complex phenomenology seen at the chromosphere and transition region (i.e. spicules, jets, ‘tornadoes’). Here we discuss models and numerical simulations which rely on magnetic fields and electric currents both for energy transfer and for storage in the corona. We will revisit the sources and frequency spectrum of kinetic and electromagnetic energies, the role of boundary conditions, and the routes to small scales required for effective dissipation. Because reconnection in current sheets has been, and still is, one of the most important processes for coronal heating, we will also discuss recent aspects concerning the triggering of reconnection instabilities and the transition to fast reconnection.

scholarly journals Irradiance Effects of Small-Scale Magnetic Fields on the Sun

1994 ◽  
Vol 143 ◽  
pp. 226-235 ◽  
Author(s):  
Sami K. Solanki
Keyword(s):  
Magnetic Fields ◽  
Time Scales ◽  
Magnetic Feature ◽  
Small Scale ◽  
The Sun ◽  
Theoretical Understanding ◽  
Flux Tubes ◽  
Solar Luminosity ◽  
Long Time ◽  
Observational Knowledge

Small-scale magnetic fields affect the solar luminosity mainly on long time scales. To understand their contribution to solar luminosity variations we must know and understand the contribution of a typical small-scale magnetic feature. In this review I briefly outline our theoretical understanding of the processes leading to the enhancement (or reduction) of the brightness of flux tubes. I also present a brief overview of our observational knowledge.

scholarly journals The magnetic field of the nearest cool star A Paradigm for Stellar Activity

1996 ◽  
Vol 176 ◽  
pp. 1-16
Author(s):  
Carolus J. Schrijver
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Differential Rotation ◽  
Stellar Atmospheres ◽  
The Sun ◽  
Flux Tubes ◽  
Cool Star ◽  
Temperature Regimes ◽  
The Magnetic Field ◽  
Selection Of

Looking at the Sun forges the framework within which we try to interpret stellar observations. The stellar counterparts of spots, plages, flux tubes, chromospheres, coronae, etc., are readily invoked when attempting to interpret stellar data. This review discusses a selection of solar phenomena that are crucial to understand stellar atmospheric activity. Topics include the interaction of magnetic fields and flows, the relationships between fluxes from different temperature regimes in stellar atmospheres, the photospheric flux budget and its impact on the measurement of the dynamo strength, and the measurement of stellar differential rotation.

V. Heating and Dynamics of Chromosphere and Corona

1988 ◽  
Vol 20 (1) ◽  
pp. 100-102
Author(s):  
G.E. Brueckner
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Acoustic Waves ◽  
Convection Zone ◽  
Flux Tubes ◽  
Magnetic Structures ◽  
Convective Motions ◽  
Field Lines ◽  
The Magnetic Field

The crucial role of magnetic fields in any mechanism to heat the outer solar atmosphere has been generally accepted by all authors. However, there is still no agreement about the detailed function of the magnetic field. Heating mechanisms can be divided up into 4 classes: (I) The magnetic field plays a passive role as a suitable medium for the propagation of Alfvén waves from the convection zone into the corona (Ionson, 1984). (II) In closed magnetic structures the slow random shuffling of field lines by convective motions below the surface induces electric currents in the corona which heat it by Joule dissipation (Heyvaerts and Priest, 1984). (Ill) Emerging flux which is generated in the convection zone reacts with ionized material while magnetic field lines move through the chromosphere, transition zone and corona. Rapid field line annihilation, reconnection and drift currents result in heating and material ejection (Brueckner, 1987; Brueckner et al., 1987; Cook et al., 1987). (IV) Acoustic waves which could heat the corona can be guided by magnetic fields. Temperature distribution, wave motions and shock formation are highly dependent on the geometry of the flux tubes (Ulmschneider and Muchmore, 1986; Ulmschneider, Muchmore and Kalkofen, 1987).

scholarly journals Study of magnetism cyclicity of the Sun in the framework of the macroscopic magnetohydrodynamics theory

2019 ◽  
pp. 22-32
Author(s):  
V. Krivodubskij
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lower Half ◽  
Dynamo Model ◽  
The Sun ◽  
Solar Plasma ◽  
Radial Gradient ◽  
Magnetic Buoyancy ◽  
Global Magnetic Field

Since the mid-70s of the last century, a new direction in theoretical studies of the evolution of the global magnetism of the Sun in the framework of macroscopic MHD has been launched at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv. The paper presents the results of a study of the processes of generation and restructuring of a large-scale (global) magnetic field based on the αΩ-dynamo model, taking into account new turbulent effects discovered in the theory of macroscopic MHD and data of helioseismological experiments on the internal rotation of the Sun. It was established that a sharp radial gradient of turbulent velocity in the lower half of the solar convective zone (SCZ) leads to a change in the sign of the azimuthal component of the helicity parameter α, resulting in the formation of a relatively thin layer of negative α-effect near the bottom of the SCZ. It was found that the layer of negative α-effect, together with the sign of the radial gradient of the angular velocity, detected in helioseismological experiments, makes it possible to explain the direction of migration of dynamo-waves on the solar surface. The magnetic saturation of the α-effect (alpha-quenching) in the deep layers of the SCZ was calculated. An explanation of the protracted duration of the 23rd solar cycle of about 13 years is proposed. For this, we used the observed data on a significant increase of the annual module of the magnetic fields of sunspots in the 23rd cycle. The calculated north-south asymmetry of the structure of the global magnetic field provides an opportunity to explain the phenomenon of the seeming magnetic “monopole”, which is observed during reversal of polar magnetism. It was found that the values of turbulent electrical conductivity and turbulent magnetic permeability of the solar plasma are significantly less than the corresponding gas-kinetic parameters. Therefore, the turbulent dissipation of solar magnetic fields is enhanced by 4–9 orders of magnitude compared with classical ohmic dissipation. Macroscopic turbulent diamagnetism of solar plasma was investigated. It has been found that in the lower part of the SCZ, turbulent diamagnetism acts against magnetic buoyancy, thus fulfilling the role of “negative magnetic buoyancy”. As a result of the balance of the effects of magnetic buoyancy and turbulent diamagnetism, a layer of blocked magnetic field of magnitude ≈ 3000 G is formed in the depths of the SCZ. The turbulent advection of a magnetic field in an inhomogeneous plasma density of the SCZ was studied. It was found that in the lower half of the SCZ of the equatorial domain, turbulent advection is directed upwards. As a result of the combined action of magnetic buoyancy and turbulent advection, deep strong toroidal fields are carried to the surface of the Sun in the latitudinal “royal zone” of sunspots. The role of horizontal turbulent diamagnetism in ensuring the long-term stability of sunspots was noted. To explain the observed phenomenon of double maxima of the solar spot cycle, a scenario was developed containing the generation of a magnetic field in the tachocline at the bottom of the SCZ and subsequent removal of this magnetic field from the depth layers to the surface in the latitudinal “royal zone”. The role of the radial omega-effect in the radiant zone in explaining the observed asymmetry in the amplitude of two neighbouring 11-years sunspot cycles was noted.

Surges of the weak magnetic field in the photosphere of the Sun

2021 ◽  
Author(s):  
Dmitrii Baranov ◽  
Elena Vernova ◽  
Marta Tyasto
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Summation Method ◽  
Solar Observatory ◽  
Primary Data ◽  
Solar Photosphere ◽  
The Sun ◽  
National Solar Observatory ◽  
Weak Magnetic Fields ◽  
One Year

<p>The properties of the magnetic fields of the solar photosphere are investigated, in particular, the distribution of fields of different polarity over the solar surface. As primary data, synoptic maps of the photospheric magnetic field of the Kitt Peak National Solar Observatory for 1978-2016 were used. Using the vector summation method, the non-axisymmetric component of the magnetic field is determined. It was found that the nonaxisymmetric component of weak magnetic fields B < 5 G changes in antiphase with the flux of these fields. Magnetic fields of B < 5 G constitute a significant part of the total magnetic field of the Sun, since they occupy more than 60% of the area of the photosphere. The fine structure of the distribution of weak fields can  be observed by setting the upper limit to the strength of the  fields  included in the time–latitude diagram. This allows to eliminate the contribution of the strong fields of sunspots.</p><p>On the time-latitude diagram for weak magnetic fields (B < 5 G), bands of differing colors correspond to the streams of the magnetic fields moving in the direction to the Sun’s poles.. These streams or surges show the alternation of the dominant polarity - positive or negative - which is clearly seen in all four cycles. The slopes of the bands indicate the velocity of the fields movement towards the poles. The surges can be divided into two groups. The surges of the first group belong to the so-called Rush-to-the-Poles. These are bands with the width of about three years, which begin at approximately 40° of latitude and have the same polarity as the trailing sunspots. They reach high latitudes and cause the polarity reversal of the polar field. However, in addition to these surges, for most of the solar  cycle (the descending phase, the minimum and the ascending phase), there are narrower surges of both polarities (with the width less than one year), which extend from the equator almost to the poles. These surges are most clearly visible in the southern hemisphere when the southern pole is positive. Consideration of the latitude-time diagrams separately for positive and negative polarities showed that the alternating dominance of one of the polarities is associated with the antiphase development  of the positive and negative fields of the surges. The widths of surges and the periodicity of their appearance vary significantly for the two hemispheres and from one solar cycle to the other. The mean period of the polarity alternation is about 1.5 years.</p>

scholarly journals Magnetic fields in astrophysical objects

2008 ◽  
Vol 366 (1884) ◽  
pp. 4453-4464 ◽  
Author(s):  
L.J Silvers
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Accretion Discs ◽  
Weak Magnetic Fields ◽  
Large Scale Magnetic Field ◽  
Astrophysical Objects ◽  
Recent Developments ◽  
The Magnetic Field

Magnetic fields are known to reside in many astrophysical objects and are now believed to be crucially important for the creation of phenomena on a wide variety of scales. However, the role of the magnetic field in the bodies that we observe has not always been clear. In certain situations, the importance of a magnetic field has been overlooked on the grounds that the large-scale magnetic field was believed to be too weak to play an important role in the dynamics. In this article I discuss some of the recent developments concerning magnetic fields in stars, planets and accretion discs. I choose to emphasize some of the situations where it has been suggested that weak magnetic fields may play a more significant role than previously thought. At the end of the article, I list some of the questions to be answered in the future.

scholarly journals Circulation conservation and vortex breakup in magnetohydrodynamics at low magnetic Prandtl number

2018 ◽  
Vol 857 ◽  
pp. 38-60 ◽  
Author(s):  
D. G. Dritschel ◽  
P. H. Diamond ◽  
S. M. Tobias
Keyword(s):  
Magnetic Fields ◽  
Prandtl Number ◽  
Scaling Laws ◽  
Vortex Patch ◽  
Magnetic Prandtl Number ◽  
Weak Magnetic Fields ◽  
Dimensional Parameters ◽  
Hydrodynamical Problem ◽  
And Diffusion

In this paper we examine the role of weak magnetic fields in breaking Kelvin’s circulation theorem and in vortex breakup in two-dimensional magnetohydrodynamics for the physically important case of a fluid with low magnetic Prandtl number (low  $Pm$ ). We consider three canonical inviscid solutions for the purely hydrodynamical problem, namely a Gaussian vortex, a circular vortex patch and an elliptical vortex patch. We examine how magnetic fields lead to an initial loss of circulation $\unicode[STIX]{x1D6E4}$ and attempt to derive scaling laws for the loss of circulation as a function of field strength and diffusion as measured by two non-dimensional parameters. We show that for all cases the loss of circulation depends on the integrated effects of the Lorentz force, with the patch cases leading to significantly greater circulation loss. For the case of the elliptical vortex, the loss of circulation depends on the total area swept out by the rotating vortex, and so this leads to more efficient circulation loss than for a circular vortex.

scholarly journals The role of the convective zone in the excitation of the magnetic activity of the Sun

2018 ◽  
pp. 31-41
Author(s):  
V. Krivodubskij
Keyword(s):  
Energy Transfer ◽  
Magnetic Fields ◽  
Convection Zone ◽  
Magnetic Activity ◽  
Toroidal Field ◽  
Dynamo Model ◽  
The Sun ◽  
Deep Layers ◽  
Cyclic Changes

The sources of energy of solar activity are analyzed. The primary source of solar energy is the core of the Sun, where as a result of the reactions of thermonuclear fusion, energy is released in the form of γ-quanta and neutrino particles that propagate outward. At approaching the surface, the temperature is rapidly decreasing and at the same time the opacity of the substance of the radiation zone steadily increases, resulting in the creation of conditions for the emergence of a convective energy transfer at a distance from surface of about 0.3 radius of the Sun. Above this boundary lies a layer called the convection zone. The existence and localization of the convection zone of the Sun is determined by two reasons: the first – the structural (radiative) temperature gradient increases due to increased opacity when the temperature drops; the second – the adiabatic gradient of the temperature of the floating elements reduces its value in the zones of partial ionization of hydrogen and helium. It is the convection zone that plays the role of the landfill, where the main processes are born, which are responsible for the cyclic manifestations of the Sun’s activity. However, part of the convective flow of energy coming from the interior of the Sun, accumulates and is carried upwards in the “magnetic form”. An important specific property of magnetic energy transfer is manifested in cyclic changes in most of the phenomena generated by magnetic fields, which are called magnetic activity of the Sun. The main mechanism providing the cyclic nature of the fluctuations of magnetic activity is the turbulent dynamo, localized in the convection zone. The most favorable place for the generation of a toroidal magnetic field, on which the intensity of spot formation depends, are the deep layers near the bottom of the convection zone, covering the layer of permeable convection (convective overshoot layer) and the tachocline. Overshoot creates the necessary conditions for the formation of a layer of long retention maintenance of magnetic fields, whereas in the tachocline, due to the sharp decrease in angular velocity in the presence of a weak poloidal field, a powerful toroidal field is effectively generated. Parker buoyancy of this field dominates over the effects of anti-buoyancy. Therefore, eventually, toroidal field rises to the surface and forms magnetic bipolar groups of sunspots. An important factor of physical processes in the deep layers is also the meridional flow directed to the equator, which, within the framework of the hydromagnetic dynamo model, provides the migration of toroidal fields from high latitudes to low ones. The author’s recent studies on the role of the deep layers of the solar convection zone in explaining the observed phenomenon of double peaks of the cycle of sunspots are noted.

scholarly journals Inevitable consequences of ion–neutral damping of intermediate MHD waves in Sun-like stars

2020 ◽  
Vol 498 (2) ◽  
pp. 2018-2029
Author(s):  
Philip G Judge
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Transition Region ◽  
Mhd Waves ◽  
The Sun ◽  
Solar Transition Region ◽  
Solar Transition ◽  
Sufficient Energy ◽  
Intermediate Mode

ABSTRACT In the context of the solar atmosphere, we re-examine the role of neutral and ionized species in dissipating the ordered energy of intermediate-mode MHD waves into heat. We solve conservation equations for the hydrodynamics and for hydrogen and helium ionization stages, along closed tubes of magnetic field. First, we examine the evolution of coronal plasma under conditions where coronal heating has abruptly ceased. We find that cool (<105K) structures are formed lasting for several hours. MHD waves of modest amplitude can heat the plasma through ion–neutral collisions with sufficient energy rates to support the plasma against gravity. Then we examine a calculation starting from a cooler atmosphere. The calculation shows that warm (>104) K long (> several Mm) tubes of plasma arise by the same mechanism. We speculate on the relevance of these solutions to observe properties of the Sun and similar stars whose atmospheres are permeated with emerging magnetic fields and stirred by convection. Perhaps this elementary process might help to explain the presence of ‘cool loops’ in the solar transition region and the production of broad components of transition region lines. The production of ionized hydrogen from such a simple and perhaps inevitable mechanism may be an important step towards finding the more complex mechanisms needed to generate coronae with temperatures in excess of 106K, independent of a star’s metallicity.

Sign in / Sign up

Export Citation Format

Share Document