Leon Ofman

Determination of the coronal magnetic field by coronal loop oscillations

We develop a new method for the determination of the absolute value of the magnetic field strength in coronal closed magnetic structures, based on the analysis of flare-generated oscillations of coronal loops. Interpretation of the oscillations observed in terms of global standing kink waves allows to connect the period of the oscillations and the loops length with the magnetic field strength in the loops. For loop oscillations observed with TRACE on 14th July 1998 and 4th July 1999, we estimate the magnetic field strength as 4-30 G. Using TRACE 171 A and 195 A images of the loop, taken on 4th July 1999 to determine the plasma density, we estimate the magnetic field in the loop as 13 +- 9 G. Improved diagnostic of the loop length, the oscillation period, and the plasma density in the loop will significantly improve the methods precision.

Slow Magnetosonic Waves in Coronal Plumes

Recent observations of polar plumes in the southern solar coronal hole by the Extreme-Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft show signatures of quasi-periodic compressional waves with periods of 10-15 minutes. The relative wave amplitude was found to increase with height in the plumes up to about 1.2 R☉. Using a one-dimensional linear wave equation for the magnetosonic wave, we show that the waves are propagating and that their amplitude increases with height. The observed propagation velocity agrees well with the expected sound velocity inside the plumes. We present the results of the first nonlinear, two-dimensional, magnetohydrodynamic (MHD) simulation of the magnetosonic waves in plumes for typical coronal conditions consistent with observations and gravitationally stratified solar corona. We find numerically that outward-propagating slow magnetosonic waves are trapped, and nonlinearly steepen in the polar plumes. The nonlinear steepening of the magnetosonic waves may contribute significantly to the heating of the lower corona by compressive dissipation.

Hot coronal loop oscillations observed by SUMER: Slow magnetosonic wave damping by thermal conduction

Recently, strongly damped Doppler shift oscillations of hot (T > 6 MK) coronal loops were observed with the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) spectrometer on board the Solar and Heliospheric Observatory. The oscillations are interpreted as signatures of slow-mode magnetosonic waves excited impulsively in the loops. Using a one-dimensional MHD code, we model the oscillations and the damping of slow magnetosonic waves in a model coronal loop. We find that because of the high temperature of the loops, the large thermal conduction, which depends on temperature as T2.5, leads to rapid damping of the slow waves on a timescale comparable to observations (5.5-29 minutes). The scaling of the dissipation time with period agrees well with SUMER observations of 35 cases in 17 events. We also find that the decay time due to compressive viscosity alone is an order of magnitude longer than the observed decay times.

ULTRAVIOLET CORONAGRAPH SPECTROMETER OBSERVATIONS OF DENSITY FLUCTUATIONS IN THE SOLAR WIND

Recent Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel (WLC) observations on board the Solar and Heliospheric Observatory (SOHO) indicate quasi-periodic variations in the polarized brightness (pB) in the polar coronal holes. This is the first observation of possible signatures of compressional waves high above the limb (at heliocentric distances in the range 1.9-2.45 R☉). The Fourier power spectrum of the pB time series at 1.9 R☉ shows significant peak at about 6 minutes and possible fluctuations on longer timescales (20-50 minutes). The observation at 1.9 R☉ is the only currently available WLC data set with sufficient cadence to resolve the 6 minute period. These preliminary observations may result from density fluctuations caused by compressional waves propagating in polar coronal holes. We stress that our results are preliminary, and we plan future high-cadence observations in both plume and interplume regions of coronal holes. Recently, Ofman & Davila used a 2.5 D MHD model and found that Alfven waves with an amplitude of 20-70 km s-1 at the base of the coronal hole can generate nonlinear, high-amplitude compressional waves that can contribute significantly to the acceleration of the fast solar wind. The nonlinear solitary-like waves appear as fluctuations in the density and the radial outflow velocity and contribute significantly to solar wind acceleration in open magnetic field structures. The motivation for the reported observations is the MHD model prediction.

Interaction of EIT Waves with Coronal Active Regions

Large-scale coronal waves associated with flares were first observed by the Solar and Heliospheric Observatory (SOHO) Extreme ultraviolet Imaging Telescope (EIT). We present the first three-dimensional MHD modeling of the interaction of the EIT waves with active regions and the possibility of destabilization of an active region by these waves. The active region is modeled by an initially force-free, bipolar magnetic configuration with gravitationally stratified density. We include finite thermal pressure and resistive dissipation in our model. The EIT wave is launched at the boundary of the region, as a short time velocity pulse that travels with the local fast magnetosonic speed toward the active region. We find that the EIT wave undergoes strong reflection and refraction, in agreement with observations, and induces transient currents in the active region. The resulting Lorentz force leads to the dynamic distortion of the magnetic field and to the generation of secondary waves. The resulting magnetic compression of the plasma induces flows, which are particularly strong in the current-carrying active region. We investigate the effect of the magnetic field configuration and find that the current-carrying active region is destabilized by the impact of the wave. Analysis of the threedimensional interaction between EIT waves and active regions can serve as a diagnostic of the active region coronal magnetic structure and stability. Subject headings: MHD — Sun: corona — Sun: magnetic fields — waves On-line material: color figures, mpeg animations

Damping Time Scaling of Coronal Loop Oscillations Deduced from Transition Region and Coronal Explorer Observations

The damping mechanism of recently discovered coronal loop transverse oscillations provides clues to the mechanism of coronal heating. We determine the scaling of the damping time with the parameters of the loops observed in extreme ultraviolet by the Transition Region and Coronal Explorer. We find excellent agreement of the scaling power to the power predicted by phase mixing and poor agreement with the power predicted by the wave leakage or ideal decay of the cylindrical kink mode mechanisms. Phase mixing leads to rapid dissipation of the Alfven waves due to the variation of the Alfven speed across the wave front and formation of small scales. Our results suggest that the loop oscillations are dissipated by phase mixing with anomalously high viscosity.

Hinode observations of transverse waves with flows in coronal loops

Aims. We report the first evidence for transverse waves in coronal multithreaded loops with cool plasma ejected from the chromosphere flowing along the threads. These observations are good candidates for coronal seismology. Methods. We analyzed observations made with Solar Optical Telescope (SOT) on board the Hinode satellite in the Ca II H line filter. Results. The oscillations are visible for about 3 periods, with a period lasting about 2 min, with weak damping. We see the oscillations in thin threads (∼0.5 �� ) of cool plasma flowing in the coronal loops with speeds in the range 74−123 km s −1 . Conclusions. Observations indicate that the waves exhibit different properties in the various threads. In some threads, the waves are nearly standing fundamental kink modes with a phase speed of about 1250 km s −1 , whereas the dynamics of other threads is consistent with propagating fast magnetosonic waves. Based on the observed wave and loop properties and the assumed active region loop density in the range (1−5) × 10 9 cm −3 , the estimated energy flux is sufficient to heat the loops to coronal temperatures, and the average magnetic field in the threads is estimated as 20 ± 7G .

Dissipation of Slow Magnetosonic Waves in Coronal Plumes

Recently, slow magnetosonic waves were identified in polar plumes, at heights up to about 1.2 R☉ using the Extreme Ultraviolet Imaging Telescope (EIT) observations of quasi-periodic EUV intensity fluctuations, and higher in the corona using the Ultraviolet Coronagraph Spectrometer (UVCS) white-light channel. First, we derive the linear dispersion relation for the slow waves in the viscous plasma. Next, we derive and solve an evolutionary equation of the Burgers type for the slow waves, incorporating the effects of radial stratification, quadratic nonlinearity, and viscosity. Finally, we model the propagation and dissipation of slow magnetosonic waves in polar plumes using one-dimensional and two-dimensional MHD codes in spherical geometry. The waves are launched at the base of the corona with a monochromatic source. We find that the slow waves nonlinearly steepen as they propagate away from the Sun into the solar wind. The nonlinear steepening of the waves leads to enhanced dissipation owing to compressive viscosity at the wave fronts. The efficient dissipation of the slow wave by compressive viscosity leads to damping of the waves within the first solar radii above the surface. We investigate the parametric dependence of the wave properties.

Advances in Observing Various Coronal EUV Waves in the SDO Era and Their Seismological Applications (Invited Review)

Global extreme-ultraviolet (EUV) waves are spectacular traveling disturbances in the solar corona associated with energetic eruptions such as coronal mass ejections (CMEs) and flares. Over the past 15 years, observations from three generations of space-borne EUV telescopes have shaped our understanding of this phenomenon and at the same time led to controversy about its physical nature. Since its launch in 2010, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) has observed more than 210 global EUV waves in exquisite detail, thanks to its high spatio–temporal resolution and full-disk, wide-temperature coverage. A combination of statistical analysis of this large sample, more than 30 detailed case studies, and data-driven MHD modeling, has been leading their physical interpretations to a convergence, favoring a bimodal composition of an outer, fast-mode magnetosonic wave component and an inner, non-wave CME component. Adding to this multifaceted picture, AIA has also discovered new EUV wave and wave-like phenomena associated with various eruptions, including quasi-periodic fast propagating (QFP) wave trains, magnetic Kelvin–Helmholtz instabilities (KHI) in the corona and associated nonlinear waves, and a variety of mini-EUV waves. Seismological applications using such waves are now being actively pursued, especially for the global corona. We review such advances in EUV wave research focusing on recent SDO/AIA observations, their seismological applications, related data-analysis techniques, and numerical and analytical models.

UVCS WLC OBSERVATIONS OF COMPRESSIONAL WAVES IN THE SOUTH POLAR CORONAL HOLE

Recent SOHO Ultraviolet Coronagraph Spectrometer (UVCS) white light channel (WLC) observations of the south polar coronal hole plumes and interplume regions produce signatures of quasi-periodic variations in the polarized brightness (pB) at a heliocentric distance of 1.9 solar radii (R☉). The Fourier power spectrum of the pB time series shows significant peaks at about 1.6-2.5 mHz and additional smaller peaks at longer and shorter timescales. Wavelet analysis of the pB time series shows that the coherence time of the fluctuations is about 30 minutes. The new observations strongly suggest that the fluctuations are compressional wave packets propagating in the coronal hole high above the limb. The presence of compressional waves may have important implications that help to explain the heating of coronal holes and the fast solar wind acceleration.