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Register a new userContribution to the Solar Mean Magnetic Field from Different Solar Regions
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Seven-year long seeing-free observations of solar magnetic fields with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) were used to study the sources of the solar mean magnetic field, SMMF, defined as the net line-of-sight magnetic flux divided over the solar disk area. To evaluate the contribution of different regions to the SMMF, we separated all the pixels of each SDO/HMI magnetogram into three subsets: weak (B_W), intermediate (B_I), and strong (B_S) fields. The B_W component represents areas with magnetic flux densities below the chosen threshold; the B_I component is mainly represented by network fields, remains of decayed active regions (ARs), and ephemeral regions. The B_S component consists of magnetic elements in ARs. To derive the contribution of a subset to the total SMMF, the linear regression coefficients between the corresponding component and the SMMF were calculated. We found that: i) when the threshold level of 30 Mx cm^-2 is applied, the B_I and B_S components together contribute from 65% to 95% of the SMMF, while the fraction of the occupied area varies in a range of 2-6% of the disk area; ii) as the threshold magnitude is lowered to 6 Mx cm^-2, the contribution from B_I+B_S grows to 98%, and the fraction of the occupied area reaches the value of about 40% of the solar disk. In summary, we found that regardless of the threshold level, only a small part of the solar disk area contributes to the SMMF. This means that the photospheric magnetic structure is an intermittent, inherently porous medium, resembling a percolation cluster. These findings suggest that the long-standing concept that continuous vast unipolar areas on the solar surface are the source of the SMMF may need to be reconsidered.
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We have used very high-cadence (sub-minute) observations of the solar mean magnetic field (SMMF) from the Birmingham Solar Oscillations Network (BiSON) to investigate the morphology of the SMMF. The observations span a period from 1992--2012, and the high-cadence observations allowed the exploration of the power spectrum up to frequencies in the mHz range. The power spectrum contains several broad peaks from a rotationally-modulated (RM) component, whose linewidths allowed us to measure, for the first time, the lifetime of the RM source. There is an additional broadband, background component in the power spectrum which we have shown is an artefact of power aliasing due to the low fill of the data. The sidereal rotation period of the RM component was measured as $25.23 pm 0.11$ days and suggests that the signal is sensitive to a time-averaged latitude of $sim 12^{circ}$. We have also shown the RM lifetime to be $139.6 pm 18.5$ days. This provides evidence to suggest the RM component of the SMMF is connected to magnetic flux concentrations (MFCs) and active regions (ARs) of magnetic flux, based both on its lifetime and location on the solar disc.
The magnetic network observed on the solar surface harbors a sizable fraction of the total quiet Sun flux. However, its origin and maintenance are not well known. Here we investigate the contribution of internetwork magnetic fields to the network flux. Internetwork fields permeate the interior of supergranular cells and show large emergence rates. We use long-duration sequences of magnetograms acquired by Hinode and an automatic feature tracking algorithm to follow the evolution of network and internetwork flux elements. We find that 14% of the quiet Sun flux is in the form of internetwork fields, with little temporal variations. Internetwork elements interact with network patches and modify the flux budget of the network, either by adding flux (through merging processes) or by removing it (through cancellation events). Mergings appear to be dominant, so the net flux contribution of the internetwork is positive. The observed rate of flux transfer to the network is 1.5 x 10^24 Mx day^-1 over the entire solar surface. Thus, the internetwork supplies as much flux as is present in the network in only 9-13 hours. Taking into account that not all the transferred flux is incorporated into the network, we find that the internetwork would be able to replace the entire network flux in approximately 18-24 hours. This renders the internetwork the most important contributor to the network, challenging the view that ephemeral regions are the main source of flux in the quiet Sun. About 40% of the total internetwork flux eventually ends up in the network.
We present results of a study of intermittency and multifractality of magnetic structures in solar active regions (ARs). Line-of-sight magnetograms for 214 ARs of different flare productivity observed at the center of the solar disk from January 1997 until December 2006 are utilized. Data from the Michelson Doppler Imager (MDI) instrument on-board the {it Solar and Heliospheric Observatory} (SOHO) operating in the high resolution mode, the Big Bear Solar Observatory digital magnetograph and {it Hinode} SOT/SP instrument were used. Intermittency spectra were derived via high-order structure functions and flatness functions. The flatness function exponent is a measure of the degree of intermittency. We found that the flatness function exponent at scales below approximately 10 Mm is correlated to the flare productivity (the correlation coefficient is - 0.63). {it Hinode} data show that the intermittency regime is extended toward the small scales (below 2 Mm) as compared to the MDI data. The spectra of multifractality, derived from the structure functions and flatness functions, are found to be more broad for ARs of highest flare productivity as compared to that of low flare productivity. The magnetic structure of high-flaring ARs consists of a voluminous set of monofractals, and this set is much richer than that for low-flaring ARs. The results indicate relevance of the multifractal organization of the photospheric magnetic fields to the flaring activity. Strong intermittency observed in complex and high-flaring ARs is a hint that we observe a photospheric imprint of enhanced sub-photospheric dynamics.
Dynamical changes in the solar corona have proven to be very important in inducing seismic waves into the photosphere. Different mechanisms for their generation have been proposed. In this work, we explore the magnetic field forces as plausible mechanisms to generate sunquakes as proposed by Hudson, Fisher and Welsch. We present a spatial and temporal analysis of the line-of-sight magnetic field variations induced by the seismically active 2003 October 29 and 2005 January 15 solar flares and compare these results with other supporting observations.
The emergence of dipolar magnetic features on the solar surface is an idealization. Most of the magnetic flux emergence occurs in complex multipolar regions. Here, we show that the surface pattern of magnetic structures alone can reveal the sign of the underlying magnetic helicity in the nearly force-free coronal regions above. The sign of the magnetic helicity can be predicted to good accuracy by considering the three-dimensional position vectors of three spots on the sphere ordered by their relative strengths at the surface and compute from them the skew product. This product, which is a pseudoscalar, is shown to be a good proxy for the sign of the coronal magnetic helicity.
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