Rebecca Centeno Elliott - Emergence Of Small-Scale Magnetic Fields On The Sun
Magnetic fields play a key role in the structure and dynamics of the solar atmosphere. Their manifestation in the form of big active regions with sunspots represents only the tip of the iceberg. The quiet-sun (this is, the 90% of the solar surface apparently free of magnetic activity) stores an unknown amount of magnetic energy, which we recently have begun to appreciate with the help of state-of-the-art instrumentation. The rate of magnetic flux emergence driven to the surface by the convective motions (granulation) of the solar plasma is thought to be altogether greater than that appearing in the form of bipolar active regions averaged over the whole 11-year solar magnetic cycle.
Fig 1: Image of a sunspot in the solar photosphere as seen by Hinode. The small inset in the bottom-left corner shows a blow-up of the convection pattern (granulation) on the solar surface. This “quiet Sun” pattern occupies, approximately, 90% of the solar surface and stores an unknown amount of magnetic energy that is thought to be larger than that appearing in the form of sunspots throughout the whole solar cycle. One arcsecond corresponds to ~750 km of the solar surface.
The nature of these small-scale magnetic fields is one of the big puzzles in current solar physics research. The question of whether it is strong fields (of kiloGauss strengths) occupying a very small areas or weak fields occupying a significant portion of the surface that produce the observed signals, is still unresolved. While both scenarios lead to the same magnetic flux, the magnetic energy in those two cases is very different. But regardless of their intrinsic field strength, their origin seems to be related to the convective processes taking place underneath the solar surface. The motions of solar granulation can either generate magnetic fields through local dynamo processes or drag them towards the surface from the underlying convection zone. It is currently unknown how much of the apparent change in this quiet Sun magnetic field is due to the emergence of new fields and/or simply due to the shuffling of the existing field by the convection.
Part of my work at NCAR has been devoted to understanding the origin of these magnetic fields. The access to state-of-the-art observational facilities has allowed me to achieve a deeper insight on the subject. Using the Spectro-Polarimeter instrument on board the Japanese/US/UK space mission Hinode, we found observational evidence for the emergence of a small-scale magnetic loop in the quiet-sun photosphere.
Fig. 2 follows, in four consecutive steps with a cadence of ~2 minutes, the evolution of an emergence event from the first hint of a magnetic signal until a magnetic loop was fully developed. The gray-scale background shows the convective pattern of the solar plasma, where white is correlated with upflows (movements of the plasma directed towards the observer) and black with downflows. Red and green contours represent the positive and negative circular polarization signals, that correspond to vertical magnetic fields pointing out and into the image respectively. The linear (orange) polarization signal reveals magnetic fields parallel to the surface. The center-left part of each image (inside the ellipse) captures the emergence of a magnetic loop structure in four steps from the moment when there is barely any signal (snapshot 1) until a full magnetic loop is developed (snapshot 3).
Fig 2: Example of the emergence of a small-scale magnetic loop in the solar photosphere measured in four consecutive steps. The gray-scale background represents the solar granulation. Red, green and orange contours represent the location of the positive circular, negative circular and linear polarization signals, which we can use to determine the magnetic field topology of the loop.
The upward plasma motions inside the granule pull the magnetic field -stored underneath the solar photosphere- towards the surface. The upper part of the loop (horizontal magnetic field) comes first, followed by its footpoints (vertical field) some minutes later. As time goes on, the traces of the horizontal field disappear while the the vertical dipoles drift -carried away by the convective plasma motions- towards the surrounding intergranular (dark) lanes, where they stay trapped for a while and aggregate to other magnetic field concentrations, resulting in larger flux elements.
This is a step forward in our comprehension of the origin of magnetic fields in the “quiet Sun”. Determining the role that these fields play in the total energy budget of the solar atmosphere is of crucial importance towards understanding the big picture.
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ASP Spotlight January 2008
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