Evidence for Partial Taylor Relaxation from Changes in Magnetic Geometry and Energy during a Solar Flare

Solar flares are powered by energy stored in the coronal magnetic field, a portion of which is released when the field reconfigures into a lower energy state. Investigation of sunspot magnetic field topology during flare activity is useful to improve our understanding of flaring processes. Here we investigate the deviation of the non-linear field configuration from that of the linear and potential configurations, and study the free energy available leading up to and after a flare. The evolution of the magnetic field in NOAA region 10953 was examined using data from Hinode/SOT-SP, over a period of 12 hours leading up to and after a GOES B1.0 flare. Previous work on this region found pre- and post-flare changes in photospheric vector magnetic field parameters of flux elements outside the primary sunspot. 3D geometry was thus investigated using potential, linear force-free, and non-linear force-free field extrapolations in order to fully understand the evolution of the field lines. Traced field line geometrical and footpoint orientation differences show that the field does not completely relax to a fully potential or linear force-free state after the flare. Magnetic and free magnetic energies increase significantly ~ 6.5-2.5 hours before the flare by ~ 10^31 erg. After the flare, the non-linear force-free magnetic energy and free magnetic energies decrease but do not return to pre-flare 'quiet' values. The post-flare non-linear force-free field configuration is closer (but not equal) to that of the linear force-free field configuration than a potential one. However, the small degree of similarity suggests that partial Taylor relaxation has occurred over a time scale of ~ 3-4 hours.Comment: Accepted for Publication in Astronomy & Astrophysics. 11 pages, 11 figure

Conditions for electron-cyclotron maser emission in the solar corona

Context. The Sun is an active source of radio emission ranging from long duration radio bursts associated with solar flares and coronal mass ejections to more complex, short duration radio bursts such as solar S bursts, radio spikes and fibre bursts. While plasma emission is thought to be the dominant emission mechanism for most radio bursts, the electron-cyclotron maser (ECM) mechanism may be responsible for more complex, short-duration bursts as well as fine structures associated with long-duration bursts. Aims. We investigate the conditions for ECM in the solar corona by considering the ratio of the electron plasma frequency {\omega}p to the electron-cyclotron frequency {\Omega}e. The ECM is theoretically possible when {\omega}p/{\Omega}e < 1. Methods. Two-dimensional electron density, magnetic field, plasma frequency, and electron cyclotron frequency maps of the off- limb corona were created using observations from SDO/AIA and SOHO/LASCO, together with potential field extrapolations of the magnetic field. These maps were then used to calculate {\omega}p/{\Omega}e and Alfven velocity maps of the off-limb corona. Results. We found that the condition for ECM emission ({\omega}p/{\Omega}e < 1) is possible at heights < 1.07 R_sun in an active region near the limb; that is, where magnetic field strengths are > 40 G and electron densities are greater than 3x10^8 cm-3. In addition, we found comparatively high Alfv\'en velocities (> 0.02 c or > 6000 km s-1) at heights < 1.07 R_sun within the active region. Conclusions. This demonstrates that the condition for ECM emission is satisfied within areas of the corona containing large magnetic fields, such as the core of a large active region. Therefore, ECM could be a possible emission mechanism for high-frequency radio and microwave bursts.Comment: 4 pages, 3 figure

Observation of a resonant four-body interaction in cold cesium Rydberg atoms

Cold Rydberg atoms subject to long-range dipole-dipole interactions represent a particularly interesting system for exploring few-body interactions and probing the transition from 2-body physics to the many-body regime. In this work we report the direct observation of a resonant 4-body Rydberg interaction. We exploit the occurrence of an accidental quasi-coincidence of a 2-body and a 4-body resonant Stark-tuned Forster process in cesium to observe a resonant energy transfer requiring the simultaneous interaction of at least four neighboring atoms. These results are relevant for the implementation of quantum gates with Rydberg atoms and for further studies of many-body physics.Comment: 5 pages, 5 figure

Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976

It is still unknown how magnetic field-generation mechanisms could operate in low-mass dwarf galaxies. Here, we present a detailed study of a nearby pure-disk dwarf galaxy NGC 2976. Unlike previously observed dwarf objects, this galaxy possesses a clearly defined disk. For the purpose of our studies, we performed deep multi-frequency polarimetric observations of NGC 2976 with the VLA and Effelsberg radio telescopes. Additionally, we supplement them with re-imaged data from the WSRT-SINGS survey. The magnetic field morphology discovered in NGC 2976 consists of a southern polarized ridge. This structure does not seem to be due to just a pure large-scale dynamo process (possibly cosmic-ray driven) at work in this object, as indicated by the RM data and dynamo number calculations. Instead, the field of NGC 2976 is modified by past gravitational interactions and possibly also by ram pressure inside the M 81 galaxy group environment. The estimates of total (7 muG) and ordered (3 muG) magnetic field strengths, as well as degree of field order (0.46), which is similar to those observed in spirals, suggest that tidally generated magnetized gas flows can further enhance dynamo action in the object. NGC 2976 is apparently a good candidate for the efficient magnetization of its neighbourhood. It is able to provide an ordered (perhaps also regular) magnetic field into the intergalactic space up to a distance of about 5 kpc. Tidal interactions (and possibly also ram pressure) can lead to the formation of unusual magnetic field morphologies (like polarized ridges) in galaxies out of the star-forming disks, which do not follow any observed component of the interstellar medium (ISM), as observed in NGC 2976. These galaxies are able to provide ordered magnetic fields far out of their main disks.Comment: 16 page

Determination of the Rb ng-series quantum defect by electric-field-induced resonant energy transfer between cold Rydberg atoms

Resonant energy transfer between cold Rydberg atoms was used to determine Rydberg atom energy levels, at precisions approaching those obtainable in microwave spectroscopy. Laser cooled Rb atoms from a magneto-optical trap were optically excited to 32d Rydberg states. The two-atom process 32d(j=5/2) + 32d(j=5/2) -> 34p(j=3/2) + 30g is resonant at an electric field of approximately 0.3 V/cm. This process is driven by the electric dipole-dipole interaction, which is allowed due to the partial f character that the g state acquires in an electric field. The experimentally observed resonant field, together with the Stark map calculation is used to make a determination of the Rb ng-series quantum defect: delta_g (n=30) = 0.00405(6)

Observation of blue-shifted ultralong-range Cs2_{2} Rydberg molecules

We observe ultralong-range blue-shifted Cs$_{2}$ molecular states near $ns_{1/2}$ Rydberg states in an optical dipole trap, where $31\leq n\leq34$. The accidental near degeneracy of $(n-4)l$ and $ns$ Rydberg states for $l>2$ in Cs, due to the small fractional $ns$ quantum defect, leads to non-adiabatic coupling among these states, producing potential wells above the $ns$ thresholds. Two important consequences of admixing high angular momentum states with $ns$ states are the formation of large permanent dipole moments, $\sim 15-100\,$Debye, and accessibility of these states via two-photon association. The observed states are in excellent agreement with theory. Both projections of the total angular momentum on the internuclear axis are visible in the experiment

Photoionization Rates of Cs Rydberg Atoms in a 1064 nm Far Off-Resonance Trap

Experimental measurements of photoionization rates of $nD_{5/2}$ Rydberg states of Cs ($50 \leq n \leq 75$) in a 1064 nm far off-resonance dipole trap are presented. The photoionization rates are obtained by measuring the lifetimes of Rydberg atoms produced inside of a 1064 nm far off-resonance trap and comparing the lifetimes to corresponding control experiments in a magneto-optical trap. Experimental results for the control experiments agree with recent theoretical predictions for Rydberg state lifetimes and measured photoionization rates are in agreement with transition rates calculated from a model potential.Comment: 12 pages, 4 figure