Magnetic flux in the solar convective envelope
Owing to proximity, the sun influences the earth's climate and environment. Overwhelming evidence is building up that the sunspot cycle and related activity phenomena are correlated with the earth's global climate and temperature, the sea surface temperatures of the three (Atlantic, Pacific and Indian) main ocean basins, the earth's albedo, the galactic cosmic ray flux that in turn is correlated with the earth's cloud cover and, Indian monsoon rainfall (Hiremath and Mandi, New Astronomy, 9, 651, 2004 and references there in; Hiremath, K. M., ILWS workshop, 2006; Hiremath, K. M., JAA, 27, 367, 2006).
Figure 1: (a)The left figure illustrates variation of magnetic field strength of the bipolar spots for different life spans τ during their first observation on the solar disc. (b)The right figure illustrates the inferred strength of the magnetic field at different anchoring depths of the sunspots in the convective envelope.
The sunspots with strong magnetic field structures are cool and dark compared to the ambient photosphere and are most interesting activities of the sun. The sunspots have been observed since the invention of telescope. Understanding of their evolution and their origin ultimately may give clue to the sun's internal dynamo mechanism that is supposed to be sustaining the solar cycle and the activity phenomena. On the surface though sunspots' dynamical and morphological properties are well understood, recently only helioseismic investigations reveal the jelly fish like structure below the surface consistent with Parker's (ApJ, 230, 905, 1979) idea.
On the surface, sunspots erupt and are oriented in the east-west direction nearly parallel to the solar equator suggesting that they are supposed to be formed by the perturbation of the underlying diffused toroidal magnetic field structure. In the convective envelope, such a toroidal field structure may be prone to dynamical instabilities due to buoyancy. However, it is not clear whether instability of the underlying toroidal field structure that represents the dynamo activity occurs near base or occurs everywhere in the convective envelope as recently proposed by Brandenburg (ApJ, 625, 339, 2005).
Present consensus is that the sunspots originate below the solar surface. In the convective envelope, owing to differential rotation and cyclonic turbulence, the dynamo mechanism is supposed to wind the poloidal magnetic field structure into a toroidal magnetic field structure leading to formation of the sunspot structures. It is believed that the solar cycle and the activity phenomena are produced and maintained by such a turbulent dynamo mechanism. Moreover it is a unsettled problem whether sunspots are formed due to conventional turbulent dynamo mechanism or formed due to the perturbation of a diffused toroidal field structure in the convective envelope (Hughes, D. W., in "Sunspots", p. 371., 1992). If somehow sunspots are formed below the surface, still the following questions are to be answered. (i) The magnitude of the magnetic field or magnetic flux at the sites of sunspots' anchoring depths. (ii) The rate of emergence of the magnetic flux at at different anchoring depths and, (iii) is the dynamo activity is distributed in the entire convection zone or confined to near region of the base of the convective envelope?
In order to address these questions, one should need consecutive observations of the well developed sunspots when they appear very first on the surface, a sunspot (flux tube) model and, a life-span anchoring depth relationship in the solar convective envelope. For different life spans and by considering seven years of SOHO/MDI magnetograms (Scherrer, P. H. et. al.1995, Sol. Phys., 162, 129) we measure consecutive sunspots' magnetic flux during their very first appearances on the surface. We use Parker's (ApJ, 121, 491, 1955) flux tube model and Hiremath's (A&A, 386, 674, 2002) anchoring depth-life span relationship in the solar convective envelope. Important findings are : (i) majority of the spot groups that have first observation on the surface are bipolar, (ii) irrespective of their sizes, the bipolar spots with different life spans have average magnetic field strengths of ∼500 G (Fig 1 (a)) during their first observation, (iii) the average field strength at the site of anchoring depths (Fig 1 (b)) of the sunspots is estimated to be ∼106G near base of the convective envelope and ∼104G near the surface, (iv) the dynamo-a source of sunspot activity- is distributed through out the convective envelope and, (v) the rate of emergence of initial magnetic flux of such a distributed dynamo near base of the convection zone is ∼6 X 1019 Mx/day and is 40% higher than the the rate of emergence of initial magnetic flux near the surface.
It is interesting to note that the estimated strength of magnetic field at different anchoring depths of the sunspots are strikingly consistent with the helioseismic inversions and the MHD calculations (Hiremath, K. M., Bull. Astron. Soc. India., 29, 169, 2001).
The details of this study will appear in the Astrophysical Journal.
(Hiremath, K. M and Lovely, M. R)
