621 research outputs found
The Origin of the Solar Flare Waiting-Time Distribution
It was recently pointed out that the distribution of times between solar
flares (the flare waiting-time distribution) follows a power law, for long
waiting times. Based on 25 years of soft X-ray flares observed by Geostationary
Operational Environmental Satellite (GOES) instruments it is shown that 1. the
waiting-time distribution of flares is consistent with a time-dependent Poisson
process, and 2. the fraction of time the Sun spends with different flaring
rates approximately follows an exponential distribution. The second result is a
new phenomenological law for flares. It is shown analytically how the observed
power-law behavior of the waiting times originates in the exponential
distribution of flaring rates. These results are argued to be consistent with a
non-stationary avalanche model for flares.Comment: 7 pages, 3 figures, accepted by ApJ Letter
Time-dependent Stochastic Modeling of Solar Active Region Energy
A time-dependent model for the energy of a flaring solar active region is
presented based on a stochastic jump-transition model (Wheatland and Glukhov
1998; Wheatland 2008; Wheatland 2009). The magnetic free energy of the model
active region varies in time due to a prescribed (deterministic) rate of energy
input and prescribed (random) flare jumps downwards in energy. The model has
been shown to reproduce observed flare statistics, for specific
time-independent choices for the energy input and flare transition rates.
However, many solar active regions exhibit time variation in flare
productivity, as exemplified by NOAA active region AR 11029 (Wheatland 2010).
In this case a time-dependent model is needed. Time variation is incorporated
for two cases: 1. a step change in the rates of flare jumps; 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 new model retains
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. 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 analytic estimates. The simulation results provide a
qualitative model for observed flare statistics in active region AR 11029.Comment: 25 pages, 9 figure
Reconciliation of Waiting Time Statistics of Solar Flares Observed in Hard X-Rays
We study the waiting time distributions of solar flares observed in hard
X-rays with ISEE-3/ICE, HXRBS/SMM, WATCH/GRANAT, BATSE/CGRO, and RHESSI.
Although discordant results and interpretations have been published earlier,
based on relatively small ranges ( decades) of waiting times, we find that
all observed distributions, spanning over 6 decades of waiting times ( hrs), can be reconciled with a single distribution
function, , which
has a powerlaw slope of at large waiting times ( hrs) and flattens out at short waiting times \Delta t \lapprox
\Delta t_0 = 1/\lambda_0. We find a consistent breakpoint at hours from the WATCH, HXRBS, BATSE, and RHESSI data.
The distribution of waiting times is invariant for sampling with different flux
thresholds, while the mean waiting time scales reciprocically with the number
of detected events, . This waiting time
distribution can be modeled with a nonstationary Poisson process with a flare
rate that varies as . This flare rate distribution represents a highly
intermittent flaring productivity in short clusters with high flare rates,
separated by quiescent intervals with very low flare rates.Comment: Preprint also available at
http://www.lmsal.com/~aschwand/eprints/2010_wait.pd
Toward a reconnection model for solar flare statistics
A model to account for observed solar flare statistics in terms of a superposition of independent random flaring elements (assumed to be sites of magnetic reconnection in the coronal magnetic field and hence termed āseparatorsā) is described. A separator of length is assumed to flare as a Poisson process in time, with a rate v(l) inversely proportional to the AlfvĆ©n transit time for the structure. It is shown that a relationship Ī¾ālk between the mean energy of events Ī¾ at a separator and the separator length implies a relationship EāTk between individual waiting times Ļ and energies E of events at the separator. The most plausible K=2 model is found to be compatible with simple pictures for magnetohydrodynamic energy storage prior to magnetic reconnection in a current sheet with anomalous (turbulent) resistivity. Formal inversion of the observed flare frequency-energy distribution is shown to imply a distribution P(l) āl-1 of the separator lengths in active regions. A simulation confirms the basic results of the model. It is also demonstrated that a model comprising time-dependent separator numbers N=N(t) can reproduce an observed power-law tail in the flare waiting-time distribution, for large waiting times
Modeling sunspot and starspot decay by turbulent erosion
Disintegration of sunspots (and starspots) by fluxtube erosion, originally proposed by Simon and Leighton, is considered. A moving boundary problem is formulated for a nonlinear diffusion equation that describes the sunspot magnetic field profile. Explicit expressions for the sunspot decay rate and lifetime by turbulent erosion are derived analytically and verified numerically. A parabolic decay law for the sunspot area is obtained. For moderate sunspot magnetic field strengths, the predicted decay rate agrees with the results obtained by Petrovay and Moreno-Insertis. The new analytical and numerical solutions significantly improve the quantitative description of sunspot and starspot decay by turbulent erosion
Time-energy correlations in solar flare occurrence
The existence of time-energy correlations in flare occurrence is still an
open and much debated problem. This study addresses the question whether
statistically significant correlations are present between energies of
successive flares as well as energies and waiting times. We analyze the GOES
catalog with a statistical approach based on the comparison of the real catalog
with a reshuffled one where energies are decorrelated. This analysis reduces
the effect of background activity and is able to reveal the role of
obscuration. We show the existence of non-trivial correlations between waiting
times and energies, as well as between energies of subsequent flares. More
precisely, we find that flares close in time tend to have the second event with
large energy. Moreover, after large flares the flaring rate significantly
increases, together with the probability of other large flares. Results suggest
that correlations between energies and waiting times are a physical property
and not an effect of obscuration. These findings could give important
information on the mechanisms for energy storage and release in the solar
corona
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