Abstract
Several two-ribbon solar flares observed on the disk, notably including the Bastille flare of 2000 July 14, show an extended ridge of plasma running along the loop tops of the ...post-reconnection arcade. In that and two more recent examples, the ridge is visible in emission by Fe
xxiv
at roughly 17 MK, with a high, steadily increasing emission measure suggesting an expanding column of very dense plasma. We find that ridges are consistent with overhead views of long, vertical plasma sheets, such as seen above certain limb flares. Those vertical features show enhanced temperature and density over their entire lengths, making explanations in terms of termination shocks and evaporation collision seem less plausible. We use observations of several ridge events to argue in favor of compression and heating by slow magnetosonic shocks in the reconnection outflow. In this scenario, the ridge is built up as retracting flux piles hot, compressed plasma atop the post-flare arcade. Thanks to the overhead perspective offered by the ridge observations, we are able to measure the reconnection rate and show it to be consistent with the rate of increase in column emission measure across the ridge. This consistency supports the hypothesis that slow shocks and retraction compress the plasma seen in ridges, vertical plasma sheets, and possibly the high-temperature fans through which post-reconnection downflows are observed. Such a unified picture of these diverse features enhances our understanding of the role played by magnetic reconnection in solar flares.
We apply a new method to determine the magnetic field in coronal loops, using observations of coronal loop oscillations. We analyze seven Doppler-shift oscillation events detected by SUMER in the hot ...flare line Fe XIX to obtain the oscillation periods of these events. The geometry, temperature, and electron density of the oscillating loops are measured from coordinated multichannel soft X-ray imaging observations from SXT. All the oscillations are consistent with standing slow waves in their fundamental mode. These parameters are used to calculate the magnetic field of coronal loops based on MHD wave theory. For the seven events, the plasma b is in the range 0.15-0.91 with a mean of 0.33 c 0.26, and the estimated magnetic field varies between 21 and 61 G with a mean of 34 c 14 G. With background emission subtracted, the estimated magnetic field is reduced by 9%-35%. The maximum background subtraction gives a mean of 22 c 13 G in the range 12-51 G. We discuss measurement uncertainties and the prospect of determining coronal loop magnetic fields from future observations of coronal loops and Doppler-shift oscillations.
Issue Title: Sun-Earth Events Topical Issue. Guest Editor: Louise. K. Harra Observations of the large two-ribbon flare on 7 November 2004 made using SOHO and TRACE data are interpreted in terms of a ...three-dimensional magnetic field model. Photospheric flux evolution indicates that -1.4×10^sup 43^ Mx^sup 2^ of magnetic helicity was injected into the active region during the 40-hour buildup prior to the flare. The magnetic model places a lower bound of 8×10^sup 31^ ergs on the energy stored by this motion. It predicts that 5×10^sup 21^ Mx of flux would need to be reconnected during the flare to release the stored energy. This total reconnection compares favorably with the flux swept up by the flare ribbons, which we measure using high-time-cadence TRACE images in 1600 Å. Reconnection in the model must occur in a specific sequence that would produce a twisted flux rope containing significantly less flux and helicity (10^sup 21^ Mx and -3×10^sup 42^ Mx^sup 2^, respectively) than the active region as a whole. The predicted flux compares favorably with values inferred from the magnetic cloud observed by Wind. This combined analysis yields the first quantitative picture of the flux processed through a two-ribbon flare and coronal mass ejection. PUBLICATION ABSTRACT
We explore the relationship between the total reconnection flux q sub(rec) estimated from flare observations and the velocity V sub(CME) of coronal mass ejections (CMEs) observed with the Large Angle ...and Spectrometric Coronagraph (LASCO) Experiment. Our study includes 13 events with varying magnetic configurations in source regions. It is shown that V sub(CME) is proportional to q sub(rec), with a linear cross-correlation of 89% and confidence level greater than 99.5%. This result confirms the importance of magnetic flux transferred by magnetic reconnection in the early stage of fast CMEs. On the other hand, the CME velocity and kinematic energy are probably independent of magnetic configurations of source regions.
In order to better understand the solar genesis of interplanetary magnetic clouds (MCs), we model the magnetic and topological properties of four large eruptive solar flares and relate them to ...observations. We use the three-dimensional
Minimum Current Corona
model (Longcope,
1996
,
Solar Phys.
169
, 91) and observations of pre-flare photospheric magnetic field and flare ribbons to derive values of reconnected magnetic flux, flare energy, flux rope helicity, and orientation of the flux-rope poloidal field. We compare model predictions of those quantities to flare and MC observations, and within the estimated uncertainties of the methods used find the following: The predicted model reconnection fluxes are equal to or lower than the reconnection fluxes inferred from the observed ribbon motions. Both observed and model reconnection fluxes match the MC poloidal fluxes. The predicted flux-rope helicities match the MC helicities. The predicted free energies lie between the observed energies and the estimated total flare luminosities. The direction of the leading edge of the MC’s poloidal field is aligned with the poloidal field of the flux rope in the AR rather than the global dipole field. These findings compel us to believe that magnetic clouds associated with these four solar flares are formed by low-corona magnetic reconnection during the eruption, rather than eruption of pre-existing structures in the corona or formation in the upper corona with participation of the global magnetic field. We also note that since all four flares occurred in active regions without significant pre-flare flux emergence and cancelation, the energy and helicity that we find are stored by shearing and rotating motions, which are sufficient to account for the observed radiative flare energy and MC helicity.
An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate ...of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well- observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field E sub(rec) inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred E sub(rec) is ~5.8 V cm super(-1) and the peak mass acceleration is ~3 km s super(-2), while for the M1.0 flare event E sub(rec) is ~0.5 V cm super(-1) and the peak mass acceleration is 0.2-0.4 km s super(-2).
We propose a mechanism for quasi-periodic oscillations of both coronal mass ejections (CMEs) and flare loops as related to magnetic reconnection in eruptive solar flares. We perform two-dimensional ...numerical MHD simulations of magnetic flux rope eruption, with three different values of the global Lundquist number. In the low Lundquist number run, no oscillatory behavior is found. In the moderate Lundquist number run, on the other hand, quasi-periodic oscillations are excited both at the bottom of the flux rope and at the flare loop top. In the high Lundquist number run, quasi-periodic oscillations are also excited; in the meanwhile, the dynamics become turbulent owing to the formation of multiple plasmoids in the reconnection current sheet. In high and moderate Lundquist number runs, thin reconnection jets collide with the flux rope bottom or flare loop top and dig them deeply. Steep oblique shocks are formed as termination shocks where reconnection jets are bent (rather than decelerated) in the horizontal direction, resulting in supersonic backflows. The structure becomes unstable, and quasi-periodic oscillations of supersonic backflows appear at locally confined high-beta regions at both the flux rope bottom and flare loop top. We compare the observational characteristics of quasi-periodic oscillations in erupting flux ropes, post-CME current sheets, flare ribbons, and light curves with corresponding dynamical structures found in our simulation.
We use the Michelson Doppler Imager and TRACE observations of photospheric magnetic and velocity fields in NOAA 10759 to build a three-dimensional coronal magnetic field model. The most dramatic ...feature of this active region is the 34 deg rotation of its leading polarity sunspot over 40 hr. We describe a method for including such rotation in the framework of the Minimum Current Corona model. We apply this method to the buildup of energy and helicity associated with the eruptive flare of 2005 May 13. We find that including the sunspot rotation almost triples the modeled flare energy (1.0 X 1031 erg) and flux rope self-helicity (-7.1 X 1042 Mx2). This makes the results consistent with observations: the energy derived from GOES is 1.0 X 1031 erg, the magnetic cloud helicity from WIND is -5 X 1042 Mx2. Our combined analysis yields the first quantitative picture of the helicity and energy content processed through a flare in an active region with an obviously rotating sunspot and shows that rotation dominates the energy and helicity budget of this event.
We report observations of a two-stage coronal dimming in an eruptive event of a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming persists for more than half an hour ...before the onset of the two-ribbon flare and the fast rise of the CME. It is followed by abrupt rapid dimming. The two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the two flare ribbons, and the flare onset marks the transition between the two stages of dimming. At the onset of the two-ribbon flare, transient brightenings are also observed inside the dimming regions, before rapid dimming occurs at the same places. These observations suggest that the CME structure, most probably anchored at the twin dimming regions, undergoes a slow rise before the flare onset, and its kinematic evolution has significantly changed at the onset of flare reconnection. We explore diagnostics of the CME evolution in the early phase with analysis of the gradual dimming signatures prior to the CME eruption.
As the observational signature of the footprints of solar magnetic field lines open into the heliosphere, coronal holes provide a critical measure of the structure and evolution of these lines. Using ...a combination of
Solar and Heliospheric Observatory/Extreme ultraviolet Imaging Telescope
(SOHO/EIT),
Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA), and
Solar Terrestrial Relations Observatory/Extreme Ultraviolet Imager
(STEREO/EUVI A/B) extreme ultraviolet (EUV) observations spanning 1996 – 2015 (nearly two solar cycles), coronal holes are automatically detected and characterized. Coronal hole area distributions show distinct behavior in latitude, defining the domain of polar and low-latitude coronal holes. The northern and southern polar regions show a clear asymmetry, with a lag between hemispheres in the appearance and disappearance of polar coronal holes.
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