排序方式： 共有32条查询结果，搜索用时 15 毫秒
1.
Zaitsev and Stepanov (1991, 1992) proposed a mechanism for energy release in solar flares that involves the intrusion of dense prominence material into a coronal loop. The resulting non-steady state conditions are claimed to increase the resistance of the loop by 8–10 orders of magnitude. It is shown here that the dramatic increase in resistance calculated by Zaitsev and Stepanov depends on a gross overestimate of the of the magnitude of the magnetic force in the loop prior to the flare trigger. A more realistic estimate of the increase due to the mechanism suggests that it is by no more than about four orders of magnitude. As a consequence, the prominence-loading mechanism does not provide a tenable flare model. 相似文献
2.
A time-dependent model for the energy of a flaring solar active region is presented based on an existing stochastic jump-transition
model (Wheatland and Glukhov in Astrophys. J.
494, 858, 1998; Wheatland in Astrophys. J.
679, 1621, 2008 and Solar Phys.
255, 211, 2009). The magnetic free energy of an active region is assumed to vary in time due to a prescribed (deterministic) rate of energy
input and prescribed (random) jumps downwards in energy due to flares. The existing model reproduces observed flare statistics,
in particular flare frequency – size and waiting-time distributions, but modeling presented to date has considered only the
time-independent choices of constant energy input and constant flare-transition rates with a power-law distribution in energy.
These choices may be appropriate for a solar active region producing a constant mean rate of flares. However, many solar active
regions exhibit time variation in their flare productivity, as exemplified by NOAA active region (AR) 11029, observed during
October – November 2009 (Wheatland in Astrophys. J.
710, 1324, 2010). Time variation is incorporated into the jump-transition model for two cases: (1) a step change in the rates of flare transitions,
and (2) a step change in the rate of energy supply to the system. Analytic arguments are presented describing the qualitative
behavior of the system in the two cases. In each case the system adjusts by shifting to a new stationary state over a relaxation
time which is estimated analytically. The model exhibits flare-like event statistics. In each case the frequency – energy
distribution is a power law for flare energies less than a time-dependent rollover set by the largest energy the system is
likely to attain at a given time. The rollover is not observed if the mean free energy of the system is sufficiently large.
For Case 1, the model exhibits a double exponential waiting-time distribution, corresponding to flaring at a constant mean
rate during two intervals (before and after the step change), if the average energy of the system is large. For Case 2 the
waiting-time distribution is a simple exponential, again provided the average energy of the system is large. Monte Carlo simulations
of Case 1 are presented which confirm the estimate for the relaxation time and the expected forms of the frequency – energy
and waiting-time distributions. The simulation results provide a qualitative model for observed flare statistics in AR 11029. 相似文献
3.
M. S. Wheatland 《Solar physics》2006,236(2):313-324
Solar flare sympathy is the triggering of a flare in one active region by a flare in another. Statistical tests for flare
sympathy have returned varying results. However, existing tests have relied on flaring rates in active regions being constant
in time, or else have attempted to model the rate variation, which is a difficult task. A simple test is described which is
independent of flaring rates. The test generalizes the approach of L. Fritzová-Švestkova, R.C. Chase, and Z. Švestka [Solar Phys.
48, 275, 1976], and examines the distribution of flare coincidences in pairs of active regions as a function of coincidence
interval τ. The test is applied to available soft X-ray and Hα flare event listings. The soft X-ray events exhibit a deficit
of flare coincidences for τ≤;20 min, which is most likely due to an event-selection effect whereby the increased soft X-ray
emission due to one flare prevents a second flare being identified. The Hα events show an excess of flare coincidences for
τ≤; 10 min, suggesting flare sympathy. The number of Hα event pairs occurring within 10 min of one another is higher than
that expected on the basis of random coincidence by a fraction 0.12± 0.02. Nearby active regions (spatial separation <50˚)
show a greater excess of coincidences for τ≤; 10 min than do active regions which are far apart (spatial separation ≥50˚).
However, the active regions which are far apart still show some evidence for an excess of coincidences at very short coincidence
intervals (τ≤; 2 min), which appears to exclude the possibility of a coronal disturbance propagating from one region to another. 相似文献
4.
Thomas R. Metcalf Marc L. DeRosa Carolus J. Schrijver Graham Barnes Adriaan A. van Ballegooijen Thomas Wiegelmann Michael S. Wheatland Gherardo Valori James M. McTtiernan 《Solar physics》2008,247(2):269-299
We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional,
and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied
to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model.
When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s
free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric
boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small
spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric
boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive
to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are
well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though,
like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent
to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least
a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to
solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various
codes will not necessarily converge to the correct, or even the same, solution.
On 07/07/2007, the NLFFF team was saddened by the news that Tom Metcalf had died as the result of an accident. We remain grateful
for having had the opportunity to benefit from his unwavering dedication to the problems encountered in attempting to understand
the Sun’s magnetic field; Tom had completed this paper several months before his death, leading the team through the many
steps described above. 相似文献
5.
Alfvén waves play three related roles in the impulsive phase of a solar flare: they transport energy from a generator region to an acceleration region; they map the cross-field potential (associated with the driven energy release) from the generator region onto the acceleration region; and within the acceleration region they damp by setting up a parallel electric field that accelerates electrons and transfers the wave energy to them. The Alfvén waves may also be regarded as setting up new closed-current loops, with field-aligned currents that close across field lines at boundaries. A model is developed for large-amplitude Alfvén waves that shows how Alfvén waves play these roles in solar flares. A picket-fence structure for the current flow is incorporated into the model to account for the “number problem” and the energy of the accelerated electrons. 相似文献
6.
A model is presented to explain the observed frequency distribution of flare energies, based on independent flaring at a number of distinct topological structures (separators) within active-region magnetic fields. The model is a modification and generalization of a recent model due to Craig (2001), and reconciles that model with the observed flare waiting-time distribution, and the observed absence of a flare waiting-time versus energy relationship. The basic assumptions of the model are that flares of energy E
2 occur at separators of length
, and that the frequency of flaring at a separator is defined by the Alfvén transit time of the structure. To reproduce the observed distribution of flare energies the model requires a probability distribution P(
)
–1 of separator lengths
within active regions. This prediction of the model is in principle testable. A theoretical origin for this distribution is also discussed. 相似文献
7.
Combined and Comparative Analysis of Power Spectra 总被引：1，自引：0，他引：1
P.?A.?SturrockEmail author J.?D.?Scargle G.?Walther M.?S.?Wheatland 《Solar physics》2005,227(1):137-153
In solar physics, especially in exploratory stages of research, it is often necessary to compare the power spectra of two or more time series. One may, for instance, wish to estimate what the power spectrum of the combined data sets might have been, or one may wish to estimate the significance of a particular peak that shows up in two or more power spectra. One may also on occasion need to search for a complex of peaks in a single power spectrum, such as a fundamental and one or more harmonics, or a fundamental plus sidebands, etc. Visual inspection can be revealing, but it can also be misleading. This leads one to look for one or more ways of forming statistics, which readily lend themselves to significance estimation, from two or more power spectra. We derive formulas for statistics formed from the sum, the minimum, and the product of two or more power spectra. A distinguishing feature of our formulae is that, if each power spectrum has an exponential distribution, each statistic also has an exponential distribution. The statistic formed from the minimum power of two or more power spectra is well known and has an exponential distribution. The sum of two or more powers also has a well-known distribution that is not exponential, but a simple operation does lead to an exponential distribution. Concerning the product of two or more power spectra, we find an analytical expression for the case n = 2, and a procedure for computing the statistic for n > 2. We also show that some quite simple expressions give surprisingly good approximations. 相似文献
8.
We present a code for solving the nonlinear force-free equations in spherical polar geometry, with the motivation of modeling the magnetic field in the corona. The code is an implementation of the Grad–Rubin method. Our method is applicable to a spherical domain of arbitrary angular size. The implementation is based on a global spectral representation for the magnetic field that makes no explicit assumptions about the form of the magnetic field at the transverse boundaries of the domain. We apply the code to a bipolar test case with analytic boundary conditions, and demonstrate the convergence of the Grad–Rubin method and the self-consistency of the resulting numerical solution. 相似文献
9.
Since the early 1980s, decimetric spike bursts have been attributed to electron cyclotron maser emission (ECME) by the electrons that produce hard X-ray bursts as they precipitate into the chromosphere in the impulsive phase of a solar flare. Spike bursts are regarded as analogous to the auroral kilometric radiation (AKR), which is associated with the precipitation of auroral electrons in a geomagnetic substorm. Originally, a loss-cone-driven version of ECME, developed for AKR, was applied to spike bursts, but it is now widely accepted that the measured distribution function is horseshoe-like (an isotropic distribution with a one-sided loss cone), and that a horseshoe-driven version of ECME applies to AKR. We explore the implications of the assumption that horseshoe-driven ECME also applies to spike bursts. We develop a 1D model for the acceleration of the electrons by a parallel electric field, and show that under plausible assumptions it leads to a horseshoe distribution of electrons in a solar flare. A second requirement for horseshoe-driven ECME is an extremely low plasma density, referred to as a density cavity. We argue that a coronal density cavity should develop in association with a hard X-ray burst, and that such a density cavity can overcome a long-standing problem with the escape of ECME through the second-harmonic absorption layer. Both the horseshoe distribution and the associated coronal density cavity are highly localized, and could not be resolved in the statistically large number of local precipitation regions needed to explain a hard X-ray burst. The model highlights the “number problem” in the supply of the electrons needed to explain a hard X-ray burst. 相似文献
10.
John Leibacher Cristina H. Mandrini Lidia van Driel-Gesztelyi Michael S. Wheatland 《Solar physics》2017,292(1):19
We are pleased to acknowledge, with sincere thanks, the following referees who supported the community by refereeing articles for Solar Physics during 2016. 相似文献