Currents In Solar Active Regions Research Articles

Comparison between probability density functions of vertical electric current in solar active regions based on HMI/SDO and SOT/Hinode data

Studying electric currents in solar active regions (AR) is an essential step in understanding solar activity in general and solar flares in particular. In this paper, we compare probability density functions of vertical electric current PDF(|jz|) in several active regions, using HMI/SDO and SOT/Hinode photospheric magnetic field data. We have established that at a high value (above the noise level of |jz| >9•10³ statampere/cm²) of current structures of ARs these functions are nearly identical. The main difference in PDFs for low (noise) jz≤9•10³ statampere/cm² is due to differences in sensitivity of these two instruments. We have also found that the criterion of pixel selection from magnetic field strength is inapplicable, and the similarity between PDFs is determined by high jz. For all PDF(|jz|) under study we have calculated the power law exponent of the PDF tail for the two instruments, which coincide within their errors for the current structures with current values above noise level. Thus there is no significant difference as to which instrument is used for analyzing probability density functions in high current parts of ARs where flares are localized.

Сравнение функций плотности вероятности вертикального электрического тока в активных областях Солнца по данным инструментов HMI/SDO и SOT/Hinode

Studying electric currents in solar active regions (AR) is an essential step in understanding solar activity in general and solar flares in particular. In this paper, we compare probability density functions of vertical electric current PDF(|jz|) in several active regions, using HMI/SDO and SOT/Hinode photospheric magnetic field data. We have established that at a high value (above the noise level of |jz| >9•10³ statampere/cm²) of current structures of ARs these functions are nearly identical. The main difference in PDFs for low (noise) jz≤9•10³ statampere/cm² is due to differences in sensitivity of these two instruments. We have also found that the criterion of pixel selection from magnetic field strength is inapplicable, and the similarity between PDFs is determined by high jz. For all PDF(|jz|) under study we have calculated the power law exponent of the PDF tail for the two instruments, which coincide within their errors for the current structures with current values above noise level. Thus there is no significant difference as to which instrument is used for analyzing probability density functions in high current parts of ARs where flares are localized.

Distributed Electric Currents in Solar Active Regions

Using magnetographic data provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we analyzed the structure of magnetic fields and vertical electric currents in six active regions (ARs) with different level of flare activity. We found that electric currents are well balanced over the entire AR: for all of them the current imbalance is below 0.1%, which means that any current system is closed within an AR. Decomposition of the transverse magnetic field vector into two components allowed us to reveal the existence of large-scale vortex structures of the azimuthal magnetic field component around main sunspots of ARs. In each AR, we found a large-scale electric current system occupying a vast area of an AR, which we call distributed electric current. For ARs obeying the Hale polarity law and the hemispheric helicity sign rule, the distributed current is directed upward in the leading part of an AR and it appears to be closing back to the photosphere in the following part of an AR through the corona and chromosphere. Our analysis of the time variations of the magnitude of the distributed electric currents showed that low-flaring ARs exhibit small variations of the distributed currents in the range of $\pm 20 \times 10^{12}$ A, whereas the highly flaring ARs exhibited significant slow variations of the distributed currents in the range of $30-95 \times 10^{12}$ A. Intervals of the enhanced flaring appear to be co-temporal with smooth enhancements of the distributed electric current.

Non-neutralized Electric Currents in Solar Active Regions and Flare Productivity

We explore the association of non-neutralized currents with solar flare occurrence in a sizable sample of observations, aiming to show the potential of such currents in solar flare prediction. We use the regularly produced high quality vector magnetograms by the Helioseismic Magnetic Imager and more specifically the Space weather HMI Active Region Patches (SHARP). Through a newly established method, that incorporates detailed error analysis, we calculate the non-neutralized currents contained in active regions (AR). Two predictors are produced, namely the total and the maximum unsigned non-neutralized current. Both are tested in AR time-series and a representative sample of point-in-time observations during the interval 2012-2016. The average values of non-neutralized currents in flaring active regions are by more than an order of magnitude higher than in non-flaring regions and correlate very well with the corresponding flare index. The temporal evolution of these parameters appears to be connected to physical processes, such as flux emergence and/or magnetic polarity inversion line formation that are associated with increased solar flare activity. Using Bayesian inference of flaring probabilities, it is shown that the total unsigned non-neutralized current outperforms significantly the total unsigned magnetic flux and other well established current-related predictors. Thus, it shows good prospects for inclusion in an operational flare forecasting service. We plan to use the new predictor in the framework of the FLARECAST project along with other highly performing predictors.

Estimating Electric Current Densities in Solar Active Regions

Electric currents in solar active regions are thought to provide the energy released via magnetic reconnection in solar flares. Vertical electric current densities J z at the photosphere may be estimated from vector magnetogram data, subject to substantial uncertainties. The values provide boundary conditions for nonlinear force-free modelling of active region magnetic fields. A method is presented for estimating values of J z taking into account uncertainties in vector magnetogram field values, and minimising \(J_{z}^{2}\) across the active region. The method is demonstrated using the boundary values of the field for a force-free twisted bipole, with the addition of noise at randomly chosen locations.

DISTRIBUTION OF ELECTRIC CURRENTS IN SOLAR ACTIVE REGIONS

There has been a long-lasting debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.

Photospheric and Coronal Currents in Solar Active Regions

Using photospheric line-of-sight magnetograms from the National Solar Observatory (NSO) Kitt Peak and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we have determined the value of the constant α of the linear force-free field model (∇ × = α) that gives the best visual fit to the overall coronal X-ray structure (αc) of 34 flare-productive active regions of relatively simple bipolar morphology. Vector magnetograms for 24 of these active regions are available from the Haleakala Stokes Polarimeter at Mees Solar Observatory. For each of them, we determine the single best-fit value of α in the photosphere (αp) by three quite different methods and show that these methods give statistically consistent values. By combining this data set with that of the NSO and SXT, we are able to compare for the first time quantitatively and statistically the observed values of α in the photosphere and corona of these regions. We find that the distribution of αp and αc values is fully consistent with the hypothesis that the overall twist density of the magnetic fields of active regions, as measured by the linear force-free field parameter α, is the same in the photosphere and the corona. We therefore conclude that the electric currents that create the nonpotential structure of such solar coronal active regions are of subphotospheric origin and pass without significant modification through the photosphere.

Are Electric Currents in Solar Active Regions Neutralized?

Models for energy storage in the solar corona due to the twisting of coronal magnetic fields by subphotospheric flows imply that coronal currents should be neutralized (i.e., observed currents over a given polarity of the magnetic field should sum to zero). The neutralized current hypothesis is quantitatively tested by examining vector magnetic field data from 21 active regions observed by the Solar Magnetic Field Telescope of the Huairou Solar Observing Station of Beijing Astronomical Observatory. For each active region, the current over the positive polarity of the field, I+, is estimated, as well as the current over the negative polarity, I-, and the total current over both polarities, Itot. In no case is the total current Itot significantly different from zero. The currents I+ and I- are found to be significantly different from zero (at the 3 σ level) in more than half of the active regions studied, implying that large-scale currents in active regions are typically unneutralized. The implications of this result and the relationship of this study to related studies (e.g., of current helicity) are discussed.

Estimating horizontal electric current in solar active regions

Within the framework of nonlinear force-free field models, we have developed a relaxation scheme for estimating horizontal electric currents in solar active regions. The test case which uses an analytical solution shows that at each iteration step the average Lorentz force decreases rapidly and the values of partial derivative B-x/partial derivative z and partial derivative By/partial derivative z converge well toward analytical ones. As a result the computed horizontal electric currents and analytical ones show good agreement in both distribution and magnitude. This scheme has avoided mathematical singularities along the inversion line B-z = 0 which appeared in the previous extrapolating methods. Then this newly developed scheme is used to analyze the vector magnetogram of a Rare-prolific region NOAA 6659. The computational results show the following: (1) The distribution of horizontal electric current and vertical electric current share a similar configuration. (2) Near the separator of a quadrupole field configuration of this active region there is a confined region of strong horizontal electric currents, near which a white-light flare took place on the same day.