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1.
Observations of the flare on 7 January 1992 are interpreted using a topological model of the magnetic field. The model, developed here, applies a theory of three-dimensional reconnection to the inferred magnetic field configuration for 7 January. In the model field a new bipole ( 1021 Mx) emerges amidst pre-existing active region flux. This emergence gives rise to two current ribbons along the boundaries (separators) separating the distinct, new and old, flux systems. Sudden reconnection across these boundary curves transfers 3 ×1020 Mx of flux from the bipole into the surrounding flux. The model also predicts the simultaneous (sympathetic) flaring of the two current ribbons. This explains the complex two-loop structure noted in previous observations of this flare. We subject the model predictions to comparisons with observations of the flare. The locations of current ribbons in the model correspond closely with those of observed soft X-ray loops. In addition the footpoints and apexes of the ribbons correspond with observed sources of microwave and hard X-ray emission. The magnitude of energy stored by the current ribbons compares favorably to the inferred energy content of accelerated electrons in the flare. 相似文献
2.
We present a quantitative model of the magnetic energy stored and then released through magnetic reconnection for a flare on 26 February 2004. This flare, well observed by RHESSI and TRACE, shows evidence of non-thermal electrons for only a brief, early phase. Throughout the main period of energy release there is a super-hot (T?30 MK) plasma emitting thermal bremsstrahlung atop the flare loops. Our model describes the heating and compression of such a source by localized, transient magnetic reconnection. It is a three-dimensional generalization of the Petschek model, whereby Alfvén-speed retraction following reconnection drives supersonic inflows parallel to the field lines, which form shocks: heating, compressing, and confining a loop-top plasma plug. The confining inflows provide longer life than a freely expanding or conductively cooling plasma of similar size and temperature. Superposition of successive transient episodes of localized reconnection across a current sheet produces an apparently persistent, localized source of high-temperature emission. The temperature of the source decreases smoothly on a time scale consistent with observations, far longer than the cooling time of a single plug. Built from a disordered collection of small plugs, the source need not have the coherent jet-like structure predicted by steady-state reconnection models. This new model predicts temperatures and emission measure consistent with the observations of 26 February 2004. Furthermore, the total energy released by the flare is found to be roughly consistent with that predicted by the model. Only a small fraction of the energy released appears in the super-hot source at any one time, but roughly a quarter of the flare energy is thermalized by the reconnection shocks over the course of the flare. All energy is presumed to ultimately appear in the lower-temperature (T?20 MK) post-flare loops. The number, size, and early appearance of these loops in TRACE’s 171 Å band are consistent with the type of transient reconnection assumed in the model. 相似文献
3.
We present a simplified analytic model of a quadrupolar magnetic field and flux rope to model coronal mass ejections. The model magnetic field is two-dimensional, force-free and has current only on the axis of the flux rope and within two current sheets. It is a generalization of previous models containing a single current sheet anchored to a bipolar flux distribution. Our new model can undergo quasi-static evolution either due to changes at the boundary or due to magnetic reconnection at either current sheet. We find that all three kinds of evolution can lead to a catastrophe, known as loss of equilibrium. Some equilibria can be driven to catastrophic instability either through reconnection at the lower current sheet, known as tether cutting, or through reconnection at the upper current sheet, known as breakout. Other equilibria can be destabilized through only one and not the other. Still others undergo no instability, but they evolve increasingly rapidly in response to slow steady driving (ideal or reconnective). One key feature of every case is a response to reconnection different from that found in simpler systems. In our two-current-sheet model a reconnection electric field in one current sheet causes the current in that sheet to increase rather than decrease. This suggests the possibility for the microscopic reconnection mechanism to run away. 相似文献
4.
The coronal magnetic field above a particular photospheric region will vanish at a certain number of points, called null points.
These points can be found directly in a potential field extrapolation or their density can be estimated from the Fourier spectrum
of the magnetogram. The spectral estimate, in which the extrapolated field is assumed to be random and homogeneous with Gaussian
statistics, is found here to be relatively accurate for quiet Sun magnetograms from SOHO’s MDI. The majority of null points
occur at low altitudes, and their distribution is dictated by high wavenumbers in the Fourier spectrum. This portion of the
spectrum is affected by Poisson noise, and as many as five-sixths of null points identified from a direct extrapolation can
be attributed to noise. The null distribution above 1500 km is found to depend on wavelengths that are reliably measured by
MDI in either its low-resolution or high-resolution mode. After correcting the spectrum to remove white noise and compensate
for the modulation transfer function we find that a potential field extrapolation contains, on average, one magnetic null
point, with altitude greater than 1.5 Mm, above every 322 Mm2 patch of quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two latest solar minima shows that the null point
density is relatively constant with roughly 10% day-to-day variation. At heights above 1.5 Mm, the null point density decreases
approximately as the inverse cube of height. The photospheric field in the quiet Sun is well approximated as that from discrete
elements with mean flux 〈|φ|〉=1.0×1019 Mx distributed randomly with density n=0.007 Mm−2. 相似文献
5.
Maria D. Kazachenko Richard C. Canfield Dana W. Longcope Jiong Qiu 《Solar physics》2012,277(1):165-183
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. 相似文献
6.
Time-dependent magneto-hydrodynamic simulations of active region coronal magnetic field require the underlying photospheric magnetic footpoint velocities. The minimum energy fit (MEF) is a new velocity inversion technique to infer the photospheric magnetic footpoint velocities using a pair of vector magnetograms, introduced by Longcope (2004). The MEF selects the smallest overall flow from several consistent flows by minimizing an energy functional. The inferred horizontal and vertical flow fields by the MEF can be further constrained by incorporating the partial or imperfect velocity information obtained through independent means. This hybrid method is expected to give a velocity close to the true magnetic footpoint velocity. Here, we demonstrate that a combination of the MEF, the local correlation tracking (LCT) and Doppler velocity is capable of inferring the velocity close to the photospheric flow. 相似文献
7.
Magnetic field enters the corona from the interior of the Sun through isolated magnetic features on the solar surface. These features correspond to the tops of submerged magnetic flux tubes, and coronal field lines often connect one flux tube to another, defining a pattern of inter-linkage. Using a model field, in which flux tubes are represented as point magnetic charges, it is possible to quantify this inter-linkage. If the coronal field were current-free then motions of the magnetic features would change the inter-linkage through implicit (vacuum) magnetic reconnection. Without reconnection the conductive corona develops currents to avoid changing the flux linkage. This current forms singular layers (ribbons) flowing along topologically significant field lines called separators. Current ribbons store magnetic energy as internal stress in the field: the amount of energy stored is a function of the flux tube displacement. To explore this process we develop a model called the minimum-current corona (MCC) which approximates the current arising on a separator in response to displacement of photospheric flux. This permits a model of the quasi-static evolution of the corona above a complex active region. We also introduce flaring to rapidly change the flux inter-linkage between magnetic features when the internal stress on a separator becomes too large. This eliminates the separator current and releases the energy stored by it. Implementation of the MCC in two examples reveals repeated flaring during the evolution of simple active regions, releasing anywhere from 1027–1029 ergs, at intervals of hours. Combining the energy and frequency gives a general expression for heat deposition due to flaring (i.e., reconnection). 相似文献
8.
In the mean, bipolar active regions are oriented nearly toroidally, according to Hale's polarity law, with a latitude-dependent tilt known as Joy's Law. The tilt angles of individual active regions deviate from this mean behavior and change over time. It has been found that on average the change is toward the mean angle at a rate characteristic of 4.37 days (Howard, 1996). We show that this orientational relaxation is consistent with the standard model of flux tube emergence from a deep dynamo layer. Under this scenario Joy's law results from the Coriolis effect on the rising flux tube (D'Silva and Choudhuri, 1993), and departures from it result from turbulent buffeting of the tubes (Longcope and Fisher, 1996). We show that relaxation toward Joy's angle occurs because the turbulent perturbations relax on shorter time scales than the perturbations from the Coriolis force. The turbulent perturbations relax more rapidly because they are localized to the topmost portion of the convection zone while the Coriolis perturbations are more widely distributed. If a fully-developed active region remains connected to the strong toroidal magnetic field at the base of the convection zone, its tilt will eventually disappear, leaving it aligned perfectly toroidally. On the other hand, if the flux becomes disconnected from the toroidal field the bipole will assume a tilt indicative of the location of disconnection. We compare models which are connected and disconnected from the toroidal field. Only those disconnected at points very deep in the convection zone are consistent with observed time scale of orientational relaxation. 相似文献
9.
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