Furthermore, I use data from the ground-based GONG network to examine the solar cycle dependence of the damping widths, amplitudes, as well as several physical quantities of solar p modes as functions of the level of solar activity. Finally, I employ techniques from quasi-degenerate perturbation theory to derive the analytical expression for the coupling strengths of oscillation eigenmodes under the influence of a superposition of zonal toroidal magnetic fields. These coupling strengths are then used to compute the effect of six magnetic field models on p-mode multiplet frequencies.
Results and Conclusions. I detect significant frequency shifts in time for 23 of the 24 investigated solar-like stars. For six of them, I link these shifts to magnetic activity. I also find that the amplitude of the frequency shifts decreases with stellar age and rotation period.
These results show that magnetic activity can be routinely observed in the oscillation parameters for solar-like stars. This opens up the possibility of placing the solar activity cycle in the context of other stars by asteroseismology. From my analysis of 22 years of GONG data, I find that the amplitude of the parameter variation changes with mode frequency and harmonic degree.
Damping widths are correlated with the level of solar activity, mode energy and mean square velocity are anti-correlated with it. Generally, I can confirm previous measurements of the variation of mode parameters with the solar cycle. Two main results of this thesis are the general matrix elements for a superposition of zonal toroidal fields: first, the general matrix element for the direct effect - that is the coupling of modes conveyed by the magnetic field - and second, the indirect effect - that is the coupling of modes due to a change in the stellar structural quantities due to the introduced perturbation.
From forward calculations with these matrix elements, it can be seen that the magnetic field affects the multiplet frequencies in a way that depends on its location and the geometry inside the Sun. By comparison of my theoretical results and observed shifts, I conclude that strong tachocline fields cannot be responsible for the observed frequency shifts of p modes over the solar cycle.
I also find that part of the surface effect in helioseismic oscillation frequencies might be attributed to magnetic fields in the outer layers of the Sun. Files A star with a magnetic field will generate a magnetosphere that extends outward into the surrounding space. Field lines from this field originate at one magnetic pole on the star then end at the other pole, forming a closed loop.
The magnetosphere contains charged particles that are trapped from the stellar wind , which then move along these field lines. As the star rotates, the magnetosphere rotates with it, dragging along the charged particles. As stars emit matter with a stellar wind from the photosphere, the magnetosphere creates a torque on the ejected matter. This results in a transfer of angular momentum from the star to the surrounding space, causing a slowing of the stellar rotation rate.
Rapidly rotating stars have a higher mass loss rate, resulting in a faster loss of momentum. As the rotation rate slows, so too does the angular deceleration. By this means, a star will gradually approach, but never quite reach, the state of zero rotation. A T Tauri star is a type of pre—main sequence star that is being heated through gravitational contraction and has not yet begun to burn hydrogen at its core. They are variable stars that are magnetically active.
The magnetic field of these stars is thought to interact with its strong stellar wind, transferring angular momentum to the surrounding protoplanetary disk. This allows the star to brake its rotation rate as it collapses. Small, M-class stars with 0. These fluctuations are hypothesized to be caused by flares, although the activity is much stronger relative to the size of the star. Straddling the boundary between stars that undergo nuclear fusion in their cores and non-hydrogen fusing brown dwarfs are the ultracool dwarfs.
These objects can emit radio waves due to their strong magnetic fields. Planetary nebulae are created when a red giant star ejects its outer envelope, forming an expanding shell of gas. However it remains a mystery why these shells are not always spherically symmetrical. One hypothesis for the formation of a non-spherical shape is the effect of the star's magnetic field. Instead of expanding evenly in all directions, the ejected plasma tends to leave by way of the magnetic poles. Observations of the central stars in at least four planetary nebulae have confirmed that they do indeed possess powerful magnetic fields.
After some massive stars have ceased thermonuclear fusion , a portion of their mass collapses into a compact body of neutrons called a neutron star. These bodies retain a significant magnetic field from the original star, but the collapse in size causes the strength of this field to increase dramatically. The rapid rotation of these collapsed neutron stars results in a pulsar , which emits a narrow beam of energy that can periodically point toward an observer.
Compact and fast-rotating astronomical objects white dwarfs , neutron stars and black holes have extremely strong magnetic fields. The magnetic field of a newly born fast-spinning neutron star is so strong up to 10 8 teslas that it electromagnetically radiates enough energy to quickly in a matter of few million years damp down the star rotation by to times. Matter falling on a neutron star also has to follow the magnetic field lines, resulting in two hot spots on the surface where it can reach and collide with the star's surface.
These spots are literally a few feet about a metre across but tremendously bright. Their periodic eclipsing during star rotation is hypothesized to be the source of pulsating radiation see pulsars. An extreme form of a magnetized neutron star is the magnetar.
Solar and Stellar Magnetic Activity - PDF Free Download
These are formed as the result of a core-collapse supernova. Jets of relativistic plasma are often observed along the direction of the magnetic poles of active black holes in the centers of very young galaxies. In , a team of astronomers first described how as the exoplanet orbiting HD A reaches a certain place in its orbit, it causes increased stellar flaring.
In , a different team found that every time they observe the exoplanet at a certain position in its orbit, they also detected X-ray flares. Theoretical research since suggested that an exoplanet very near to the star that it orbits may cause increased flaring due to the interaction of their magnetic fields , or because of tidal forces. In , astronomers combined data from Arecibo Observatory , MOST , and the Automated Photoelectric Telescope, in addition to historical observations of the star at radio, optical, ultraviolet, and X-ray wavelengths to examine these claims.
Their analysis found that the previous claims were exaggerated and the host star failed to display many of the brightness and spectral characteristics associated with stellar flaring and solar active regions , including sunspots. They also found that the claims did not stand up to statistical analysis, given that many stellar flares are seen regardless of the position of the exoplanet, therefore debunking the earlier claims.
The magnetic fields of the host star and exoplanet do not interact, and this system is no longer believed to have a "star-planet interaction. From Wikipedia, the free encyclopedia. Play media.
Star portal. The Astrophysics Spectator. Retrieved July 8—13, Cambridge, England: Cambridge University Press. Science Daily. Bibcode : Natur. Space Science Reviews. Bibcode : SSRv Surveys in Geophysics.
Bibcode : SGeo Astrophysics and Space Science. University of Rochester. Bibcode : Sci January 18, Living Reviews.
Related Solar and Stellar Magnetic Activity
Copyright 2019 - All Right Reserved