A spatio-temporal description of the abrupt changes in the photospheric magnetic and Lorentz-force vectors during the 2011 February 15 X2.2 flare

The active region NOAA 11158 produced the first X-class flare of Solar Cycle 24, an X2.2 flare at 01:44 UT on 2011 February 15. Here we analyze SDO/HMI magnetograms covering a 12-hour interval centered at the time of this flare. We describe the spatial distributions of the photospheric magnetic changes associated with this flare, including the abrupt changes in the field vector, vertical electric current and Lorentz force vector. We also trace these parameters' temporal evolution. The abrupt magnetic changes were concentrated near the neutral line and in two neighboring sunspots. Near the neutral line, the field vectors became stronger and more horizontal during the flare and the shear increased. This was due to an increase in strength of the horizontal field components near the neutral line, most significant in the horizontal component parallel to the neutral line but the perpendicular component also increased in strength. The vertical component did not show a significant, permanent overall change at the neutral line. The increase in total flux at the neutral line was accompanied by a compensating flux decrease in the surrounding volume. In the two sunspots near the neutral line the azimuthal flux abruptly decreased during the flare but this change was permanent in only one of the spots. There was a large, abrupt, downward vertical Lorentz force change during the flare, consistent with results of past analyses and recent theoretical work. The horizontal Lorentz force acted in opposite directions along each side of neutral line, with the two sunspots at each end subject to abrupt torsional forces. The shearing forces were consistent with field contraction and decrease of shear near the neutral line, whereas the field itself became more sheared as a result of the flux collapsing towards the neutral line from the surrounding volume.Comment: DOI 10.1007/s11207-012-0071-0. Accepted for publication in Solar Physics SDO3 Topical Issue. Some graphics missing due to 15MB limi

Evolution of active and polar photospheric magnetic fields during the rise of Cycle 24 compared to previous cycles

The evolution of the photospheric magnetic field during the declining phase and minimum of Cycle 23 and the recent rise of Cycle 24 are compared with the behavior during previous cycles. We used longitudinal full-disk magnetograms from the NSO's three magnetographs at Kitt Peak, the Synoptic Optical Long-term Investigations of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM), the Spectromagnetograph and the 512-Channel Magnetograph instruments, and longitudinal full-disk magnetograms from the Mt. Wilson 150-foot tower. We analyzed 37 years of observations from these two observatories that have been observing daily, weather permitting, since 1974, offering an opportunity to study the evolving relationship between the active region and polar fields in some detail over several solar cycles. It is found that the annual averages of a proxy for the active region poloidal magnetic field strength, the magnetic field strength of the high-latitude poleward streams, and the time derivative of the polar field strength are all well correlated in each hemisphere. These results are based on statistically significant cyclical patterns in the active region fields and are consistent with the Babcock-Leighton phenomenological model for the solar activity cycle. There was more hemispheric asymmetry in the activity level, as measured by total and maximum active region flux, during late Cycle 23 (after around 2004), when the southern hemisphere was more active, and Cycle 24 up to the present, when the northern hemisphere has been more active, than at any other time since 1974. The active region net proxy poloidal fields effectively disappeared in both hemispheres around 2004, and the polar fields did not become significantly stronger after this time. We see evidence that the process of Cycle 24 field reversal has begun at both poles.Comment: Accepted for publication in Solar Physic

Recovering Joys Law as a Function of Solar Cycle, Hemisphere, and Longitude

Bipolar active regions in both hemispheres tend to be tilted with respect to the East West equator of the Sun in accordance with Joys law that describes the average tilt angle as a function of latitude. Mt. Wilson observatory data from 1917 to 1985 are used to analyze the active-region tilt angle as a function of solar cycle, hemisphere, and longitude, in addition to the more common dependence on latitude. Our main results are as follows: i) We recommend a revision of Joys law toward a weaker dependence on latitude (slope of 0.13 to 0.26) and without forcing the tilt to zero at the Equator. ii) We determine that the hemispheric mean tilt value of active regions varies with each solar cycle, although the noise from a stochastic process dominates and does not allow for a determination of the slope of Joys law on an 11-year time scale. iii) The hemispheric difference in mean tilt angles, 1.1 degrees + 0.27, over Cycles 16 to 21 was significant to a three-sigma level, with average tilt angles in the northern and southern hemispheres of 4.7 degrees + 0.26 and 3.6 degrees + 0.27 respectively. iv) Area-weighted mean tilt angles normalized by latitude for Cycles 15 to 21 anticorrelate with cycle strength for the southern hemisphere and whole-Sun data, confirming previous results by Dasi-Espuig, Solanki, Krivova, et al. (2010, Astron. Astrophys. 518, A7). The northern hemispheric mean tilt angles do not show a dependence on cycle strength. vi) Mean tilt angles do not show a dependence on longitude for any hemisphere or cycle. In addition, the standard deviation of the mean tilt is 29 to 31 degrees for all cycles and hemispheres indicating that the scatter is due to the same consistent process even if the mean tilt angles vary.Comment: 13 pages, 4 figures, 3 table

Electric current circuits in astrophysics

Cosmic magnetic structures have in common that they are anchored in a dynamo, that an external driver converts kinetic energy into internal magnetic energy, that this magnetic energy is transported as Poynting fl ux across the magnetically dominated structure, and that the magnetic energy is released in the form of particle acceleration, heating, bulk motion, MHD waves, and radiation. The investigation of the electric current system is particularly illuminating as to the course of events and the physics involved. We demonstrate this for the radio pulsar wind, the solar flare, and terrestrial magnetic storms

An investigation of the topology and structure of constant-α force-free fields

Coronal field structures corresponding to specified normal fields on the photosphere are investigated using a force-free model. The force-free parameter is insufficient to determine the field uniquely, and so a second parameter representing the contribution of the complementary Green's function is included. For isolated loops represented by a single source-sink pair, the influence of the two parameters on field structure is comparable. Effects on the topology (as measured by field line connectivity) of multiple source configurations are also found to be comparable. It is notable that some configurations are more stable than others to variation of either parameter. The results of this study are compared with previous magnetic charge topology studies involving only the force-free parameter

Steady 2D prominence-like solutions of the MHD equations with field-aligned compressible flow

An exact two-dimensional solution of the ideal magnetohydrodynamic (MHD) equations with compressible flow in a uniform gravitational field is presented and applied to solar prominences. The solution is constructed via a systematic nonlinear separation of variables method used to calculate several classes of MHD equilibria in Cartesian geometry and uniform gravity. This simple model of steady plasma flow along the dipped field lines of a solar prominence is the first 2D MHD model with a nonisothermal temperature distribution which selfconsistently also examines the required heating. Although the model is 2D, a third magnetic/velocity vector field component is included and the highly sheared fields observed in prominences are reproduced. A description is given of the balance of gas pressure gradient, gravity, Lorentz and inertial forces acting along and across the prominence. It is found that the flow may significantly influence the energy balance as in a similar application of this class of solutions to solar coronal loops

A solar active region loop compared with a 2D MHD model

We analyzed a coronal loop observed with the Normal Incidence Spectrometer (NIS), which is part of the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO). The measured Doppler shifts and proper motions along the selected loop strongly indicate unidirectional flows. Analysing the Emission Measure Curves of the observed spectral lines, we estimated that the temperature along the loop was about 380 000 K. We adapted a solution of the ideal MHD steady equations to our set of measurements. The derived energy balance along the loop, as well as the advantages/disadvantages of this MHD model for understanding the characteristics of solar coronal loops are discussed. © ESO 2005

The tilted solar dipole as observed and modeled during the 1996 Solar Minimum

We examine the tilt of the solar magnetic dipole away from the rotational axis near the 1996 solar minimum. A persistent tilted dipole may result from an MHD instability acting on the toroidal bands in the solar interior. Nonaxisymmetric eruption of sunspots has been predicted by dynamo theory and observed in sunspot location patterns. The decay of follower spots and the poleward migration of flux could create polar caps that are slightly misaligned with the north-south rotational axis. To investigate this, we analyze the coronal streamer geometry observed with LASCO-C2 and the center of gravity of the polar caps defined by coronal hole boundaries in EIT images and the unipolar magnetic regions in KPVT magnetograms.We model the coronal hole boundaries and neutral line locations by potential field source surface (PFSS) modeling using Kitt Peak magnetograms. Our results are consistent with an observed tilt of 5-10 in the heliospheric current sheet at solar minimum and the idea of persistent off-axis magnetic polar caps for CRs 1911-1919. The coronal holes show a stable azimuthal angle for CRs 1911-1919 with a rotation rate slightly less than the Carrington rate. The PFSS modeling is able to recreate the observed coronal hole geometry and predict the maximum extent of the heliospheric current sheet as observed by streamer locations.A6 observed tilt of the polar caps during this time is consistent with the analytical value provided from the PFSS dipole terms. However, the determination of a tilt of the magnetic polar caps is dominated by noise. The LASCO coronal streamer geometry traces out an 10 tilt of the solar dipole from the equatorial plane during CRs 1915-1919