About the Sun II
The fact that the Sun has a strong magnetic field is certainly not a matter of debate --- magnetic field manifests itself in the solar atmosphere in many ways: sunspots, the bright loops of magnetically confined plasma observed by, e.g., the TRACE satellite (see Figure 4), coronal mass ejections observed by the LASCO instrument on SOHO, the solar cycle, streamer belts, and the list goes on. But how is this magnetic field generated? Can we understand the cycles of magnetic activity observed on the Sun and the consequence of this activity here on earth?
Figure 4: An image of the corona taken by Transition Region and Coronal Explorer in the 171 Angstrom bandpass that shows coronal plasma of approximately 1 million degrees K. Image courtesy of Lockheed Martin Solar and Astrophysics Labs.
It is widely believed that the Sun's strong magnetic field is generated via a dynamo mechanism (a means of turning mechanical energy into electromagnetic energy, like a generator) located at or near the interface between the turbulent, differentially rotating plasma of the convection zone, and the convectively stable layers of the radiative interior. Magnetic field can also be generated within the turbulent convection zone itself; if this were not the case, it would be difficult to understand how cool, fully-convective stars (lacking a stable layer) can be magnetically active (for example, the M-dwarf star AD-Leo routinely exhibits flares that can dramatically increase it's brightness for a short time).
Figure 5: An image of ~1.3 million degree K coronal plasma taken by the EIT instrument on SOHO. Image courtesy of the SOHO/EIT consortium.
Nevertheless, observations of the geometric orientation of active regions at the photosphere (and the persistence of these patterns over many solar cycles) support the notion that a strong toroidal layer of magnetic flux must exist in the stable layers near the base of the convection zone. Portions of this layer can become unstable, allowing strong magnetic fields to buoyantly rise toward the surface where they emerge into the corona structuring the visible plasma around active regions into the bands of activity seen in Figure 5.
Active region magnetic fields can substantially disrupt the global magnetic configuration of the quiet (non-active) corona, which is not much different from a dipole --- the same type of field you infer from placing a bar magnet on a page full of iron filings. The disruption can be quite significant, affecting the orientation of streamer belts (see Figure 3), and the global structure and speed of the solar wind (the constant stream of charged particles that escape the solar atmosphere along field lines). Coronal mass ejections --- spectacular eruptions of material outward away from the Sun --- are often associated with the presence of large active regions, and are of particular interest since they are among the principal drivers of "space weather" events here at earth (see, e.g., the NOAA Space Environment Center or SpaceWeather.com). They are magnetically driven phenomena and are thought to originate in the low corona (see Figure 6). Their initiation mechanism, is still a matter of great debate, and the modeling and prediction of these events is currently a very vibrant cross-disciplinary research area.
Figure 6: A Coronal Mass Ejection (CME) observed over an eight hour period on 5-6 August 1999 by LASCO C3. The dark disk blocks the Sun so that the LASCO instrument can observe the structures of the corona in visible light. The white circle represents the size and position of the Sun. Image courtesy of the SOHO/LASCO consortium.
Of course, this introduction just scratches the surface of several of the many research areas in solar physics. Other interesting topics include helioseismology, the neutrino problem, the solar cycle, long-term changes in solar irradiance (and its effect on climate change), and the lifecycle (birth and death) of the Sun. To do justice to these topics would require a books worth of material, and my purpose here is simply to define a few terms, and introduce concepts relevant to my own limited areas of research so that it might be easier for those who are not solar physicists to appreciate some of the other, more technical, pages on my site. For more complete, easy-to-understand expositions on the Sun, check out the Center for Science Education 's solar pages, or HAO's education pages.