67 research outputs found
Large amplitude oscillatory motion along a solar filament
Large amplitude oscillations of solar filaments is a phenomenon known for
more than half a century. Recently, a new mode of oscillations, characterized
by periodical plasma motions along the filament axis, was discovered. We
analyze such an event, recorded on 23 January 2002 in Big Bear Solar
Observatory H filtergrams, in order to infer the triggering mechanism
and the nature of the restoring force. Motion along the filament axis of a
distinct buldge-like feature was traced, to quantify the kinematics of the
oscillatory motion. The data were fitted by a damped sine function, to estimate
the basic parameters of the oscillations. In order to identify the triggering
mechanism, morphological changes in the vicinity of the filament were analyzed.
The observed oscillations of the plasma along the filament was characterized by
an initial displacement of 24 Mm, initial velocity amplitude of 51 km/s, period
of 50 min, and damping time of 115 min. We interpret the trigger in terms of
poloidal magnetic flux injection by magnetic reconnection at one of the
filament legs. The restoring force is caused by the magnetic pressure gradient
along the filament axis. The period of oscillations, derived from the
linearized equation of motion (harmonic oscillator) can be expressed as
, where represents the Alfv\'en speed based on the
equilibrium poloidal field . Combination of our measurements with
some previous observations of the same kind of oscillations shows a good
agreement with the proposed interpretation.Comment: Astron. Astrophys., 2007, in pres
Combined STEREO/RHESSI study of CME acceleration and particle acceleration in solar flares
Using the potential of two unprecedented missions, STEREO and RHESSI, we
study three well observed fast CMEs that occurred close to the limb together
with their associated high energy flare emissions in terms of RHESSI HXR
spectra and flux evolution. From STEREO/EUVI and STEREO/COR1 data the full CME
kinematics of the impulsive acceleration phase up to 4 Rs is measured with a
high time cadence of less equal 2.5 min. For deriving CME velocity and
acceleration we apply and test a new algorithm based on regularization methods.
The CME maximum acceleration is achieved at heights h < 0.4 Rs, the peak
velocity at h < 2.1 Rs (in one case as small as 0.5 Rs). We find that the CME
acceleration profile and the flare energy release as evidenced in the RHESSI
hard X-ray flux evolve in a synchronized manner. These results support the
standard flare/CME model which is characterized by a feed-back relationship
between the large-scale CME acceleration process and the energy release in the
associated flare.Comment: accepted for Ap
Morphology and density of post-CME current sheets
Eruption of a coronal mass ejection (CME) drags and "opens" the coronal
magnetic field, presumably leading to the formation of a large-scale current
sheet and the field relaxation by magnetic reconnection. We analyze physical
characteristics of ray-like coronal features formed in the aftermath of CMEs,
to check if the interpretation of this phenomenon in terms of reconnecting
current sheet is consistent with the observations. The study is focused on
measurements of the ray width, density excess, and coronal velocity field as a
function of the radial distance. The morphology of rays indicates that they
occur as a consequence of Petschek-like reconnection in the large scale current
sheet formed in the wake of CME. The hypothesis is supported by the flow
pattern, often showing outflows along the ray, and sometimes also inflows into
the ray. The inferred inflow velocities range from 3 to 30 km s,
consistent with the narrow opening-angle of rays, adding up to a few degrees.
The density of rays is an order of magnitude larger than in the ambient corona.
The density-excess measurements are compared with the results of the analytical
model in which the Petschek-like reconnection geometry is applied to the
vertical current sheet, taking into account the decrease of the external
coronal density and magnetic field with height. The model results are
consistent with the observations, revealing that the main cause of the density
excess in rays is a transport of the dense plasma from lower to larger heights
by the reconnection outflow
A new method of measuring Forbush decreases
Forbush decreases (FDs) are short-term depressions in the galactic cosmic ray
flux and one of the common signatures of coronal mass ejections (CMEs) in the
heliosphere. They often show a two-step profile, the second one associated with
the CMEs magnetic structure (flux rope, FR), which can be described by the
recently developed model ForbMod. The aim of this study is to utilise ForbMod
to develop a best-fit procedure to be applied on FR-related FDs as a convenient
measurement tool. We develop a best-fit procedure that can be applied to a data
series from an arbitrary detector. Thus, the basic procedure facilitates
measurement estimation of the magnitude of the FR-related FD, with the
possibility of being adapted for the energy response of a specific detector for
a more advanced analysis. The non-linear fitting was performed by calculating
all possible ForbMod curves constrained within the FR borders to the designated
dataset and minimising the mean square error (MSE). In order to evaluate the
performance of the ForbMod best-fit procedure, we used synthetic measurements
produced by calculating the theoretical ForbMod curve for a specific example
CME and then applying various effects to the data to mimic the imperfection of
the real measurements. We also tested the ForbMod best-fit function on the real
data, measured by detector F of the SOHO-EPHIN instrument on a sample
containing 30 events, all of which have a distinct FD corresponding to the CMEs
magnetic structure. Overall, we find that the ForbMod best-fit procedure
performs similar to the traditional algorithm-based observational method, but
with slightly smaller values for the FD amplitude, as it is taking into account
the noise in the data. Furthermore, we find that the best-fit procedure has an
advantage compared to the traditional method as it can estimate the FD
amplitude even when there is a data gap at the onset of the FD.Comment: 14 pages, 10 figures, A&
The chaotic solar cycle II. Analysis of cosmogenic 10Be data
Context. The variations of solar activity over long time intervals using a
solar activity reconstruction based on the cosmogenic radionuclide 10Be
measured in polar ice cores are studied. Methods. By applying methods of
nonlinear dynamics, the solar activity cycle is studied using solar activity
proxies that have been reaching into the past for over 9300 years. The
complexity of the system is expressed by several parameters of nonlinear
dynamics, such as embedding dimension or false nearest neighbors, and the
method of delay coordinates is applied to the time series. We also fit a damped
random walk model, which accurately describes the variability of quasars, to
the solar 10Be data and investigate the corresponding power spectral
distribution. The periods in the data series were searched by the Fourier and
wavelet analyses. The solar activity on the long-term scale is found to be on
the edge of chaotic behavior. This can explain the observed intermittent period
of longer lasting solar activity minima. Filtering the data by eliminating
variations below a certain period (the periods of 380 yr and 57 yr were used)
yields a far more regular behavior of solar activity. A comparison between the
results for the 10Be data with the 14C data shows many similarities. Both
cosmogenic isotopes are strongly correlated mutually and with solar activity.
Finally, we find that a series of damped random walk models provides a good fit
to the 10Be data with a fixed characteristic time scale of 1000 years, which is
roughly consistent with the quasi-periods found by the Fourier and wavelet
analyses.Comment: 8 pages, 11 figure
Temporal comparison of nonthermal flare emission and magnetic-flux change rates
To test the standard flare model (CSHKP-model), we measured the magnetic-flux
change rate in five flare events of different GOES classes using
chromospheric/photospheric observations and compared its progression with
observed nonthermal flare emission. We calculated the cumulated positive and
negative magnetic flux participating in the reconnection process, as well as
the total reconnection flux. Finally, we investigated the relations between the
total reconnection flux, the GOES class of the events, and the linear velocity
of the flare-associated CMEs. Using high-cadence H-alpha and TRACE 1600 A image
time-series data and MDI/SOHO magnetograms, we measured the required
observables (newly brightened flare area and magnetic-field strength inside
this area). RHESSI and INTEGRAL hard X-ray time profiles in nonthermal energy
bands were used as observable proxies for the flare-energy release rate. We
detected strong temporal correlations between the derived magnetic-flux change
rate and the observed nonthermal emission of all events. The cumulated positive
and negative fluxes, with flux ratios of between 0.64 and 1.35, were almost
equivalent to each other. Total reconnection fluxes ranged between 1.8 x 10^21
Mx for the weakest event (GOES class B9.5) and 15.5 x 10^21 Mx for the most
energetic one (GOES class X17.2). The amount of magnetic flux participating in
the reconnection process was higher in more energetic events than in weaker
ones. Flares with more reconnection flux were associated with faster CMEs.Comment: 12 pages, 13 figure
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