119 research outputs found
Slow magnetoacoustic waves in coronal loops : EIT and TRACE
On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition Region And Coronal Explorer) instruments produced simultaneous high cadence image sequences of the same active region (AR 8218). TRACE achieved a 25 s cadence in the FeIX (171 Å) bandpass while EIT achieved a 15 s cadence (operating in "shutterless mode", SoHO JOP 80) in the FeXII (195 Å) bandpass. These high cadence observations in two complementary wavelengths have revealed the existence of weak transient disturbances in an extended coronal loop system. These propagating disturbances (PDs) seem to be a common phenomenon in this part of the active region. The disturbances originate from small scale brightenings at the footpoints of the loops and propagate along the loops. The projected propagation speeds roughly vary between 65 and 150 km s-1 for both instruments which is close to and below the expected sound speed in the coronal loops. The measured slow magnetoacoustic propagation speeds seem to suggest that the transients are sound (or slow) wave disturbances. This work differs from previous studies in the sense that it is based on a multi-wavelength observation of an entire loop bundle at high cadence by two EUV imagers. The observation of sound waves along the same path shows that they propagate along the same loop, suggesting that loops contain sharp temperature gradients and consist of either concentric shells or thin loop threads, at different temperatures
Fe XIII coronal line emission in cool M dwarfs
We report on a search for the Fe xiii forbidden coronal line at 3388.1 \AA in
a sample of 15 M-type dwarf stars covering the whole spectral class as well as
different levels of activity. A clear detection was achieved for LHS 2076
during a major flare and for CN Leo, where the line had been discovered before.
For some other stars the situation is not quite clear. For CN Leo we
investigated the timing behaviour of the Fe xiii line and report a high level
of variability on a timescale of hours which we ascribe to microflare heating.Comment: 13 pages, 10 figure
Dust in the Local Interstellar Wind
The gas-to-dust mass ratios found for interstellar dust within the Solar
System, versus values determined astronomically for the cloud around the Solar
System, suggest that large and small interstellar grains have separate
histories, and that large interstellar grains preferentially detected by
spacecraft are not formed exclusively by mass exchange with nearby interstellar
gas. Observations by the Ulysses and Galileo satellites of the mass spectrum
and flux rate of interstellar dust within the heliosphere are combined with
information about the density, composition, and relative flow speed and
direction of interstellar gas in the cloud surrounding the solar system to
derive an in situ value for the gas-to-dust mass ratio, . Hubble observations of the cloud surrounding the solar system
yield a gas-to-dust mass ratio of Rg/d=551+61-251 when B-star reference
abundances are assumed. The exclusion of small dust grains from the heliosheath
and heliosphere regions are modeled, increasing the discrepancy between
interstellar and in situ observations. The shock destruction of interstellar
grains is considered, and comparisons are made with interplanetary and presolar
dust grains.Comment: 87 pages, 9 figures, 6 tables, accepted for publication in
Astrophysical Journal. Uses AASTe
Evolution of an eruptive flare loop system
<p><b>Context:</b> Flares, eruptive prominences and coronal mass ejections are phenomena where magnetic reconnection plays an important role. However, the location and the rate of the reconnection, as well as the mechanisms of particle interaction with ambient and chromospheric plasma are still unclear.</p>
<p><b>Aims:</b> In order to contribute to the comprehension of the above mentioned processes we studied the evolution of the eruptive flare loop system in an active region where a flare, a prominence eruption and a CME occurred on August 24, 2002.</p>
<p><b>Methods:</b> We measured the rate of expansion of the flare loop arcade using TRACE 195 Å images and determined the rising velocity and the evolution of the low and high energy hard X-ray sources using RHESSI data. We also fitted HXR spectra and considered the radio emission at 17 and 34 GHZ.</p>
<p><b>Results:</b> We observed that the top of the eruptive flare loop system initially rises with a linear behavior and then, after 120 mn from the start of the event registered by GOES at 1–8 Å, it slows down. We also observed that the heating source (low energy X-ray) rises faster than the top of the loops at 195 Å and that the high energy X-ray emission (30–40 keV) changes in time, changing from footpoint emission at the very onset of the flare to being coincident during the flare peak with the whole flare loop arcade.</p>
<p><b>Conclusions:</b> The evolution of the loop system and of the X-ray sources allowed us to interpret this event in the framework of the Lin & Forbes model (2000), where the absolute rate of reconnection decreases when the current sheet is located at an altitude where the Alfvén speed decreases with height. We estimated that the lower limit for the altitude of the current sheet is km. Moreover, we interpreted the unusual variation of the high energy HXR emission as a manifestation of the non thermal coronal thick-target process which appears during the flare in a manner consistent with the inferred increase in coronal column density.</p>
Properties of Flares-Generated Seismic Waves on the Sun
The solar seismic waves excited by solar flares (``sunquakes'') are observed
as circular expanding waves on the Sun's surface. The first sunquake was
observed for a flare of July 9, 1996, from the Solar and Heliospheric
Observatory (SOHO) space mission. However, when the new solar cycle started in
1997, the observations of solar flares from SOHO did not show the seismic
waves, similar to the 1996 event, even for large X-class flares during the
solar maximum in 2000-2002. The first evidence of the seismic flare signal in
this solar cycle was obtained for the 2003 ``Halloween'' events, through
acoustic ``egression power'' by Donea and Lindsey. After these several other
strong sunquakes have been observed. Here, I present a detailed analysis of the
basic properties of the helioseismic waves generated by three solar flares in
2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2
flare of January 15, 2005, the helioseismology observations are compared with
simultaneous observations of flare X-ray fluxes measured from the RHESSI
satellite. These observations show a close association between the flare
seismic waves and the hard X-ray source, indicating that high-energy electrons
accelerated during the flare impulsive phase produced strong compression waves
in the photosphere, causing the sunquake. The results also reveal new physical
properties such as strong anisotropy of the seismic waves, the amplitude of
which varies significantly with the direction of propagation. The waves travel
through surrounding sunspot regions to large distances, up to 120 Mm, without
significant decay. These observations open new perspectives for helioseismic
diagnostics of flaring active regions on the Sun and for understanding the
mechanisms of the energy release and transport in solar flares.Comment: 12 pages, 4 figures, submitted to Ap
The Influence of Solar Flares on the Lower Solar Atmosphere: Evidence from the Na D Absorption Line Measured by GOLF/SOHO
Solar flares presumably have an impact on the deepest layers of the solar
atmosphere and yet the observational evidence for such an impact is scarce.
Using ten years of measurements of the Na D and Na D Fraunhofer
lines, measured by GOLF onboard SOHO, we show that this photospheric line is
indeed affected by flares. The effect of individual flares is hidden by solar
oscillations, but a statistical analysis based on conditional averaging reveals
a clear signature. Although GOLF can only probe one single wavelength at a
time, we show that both wings of the Na line can nevertheless be compared. The
varying line asymmetry can be interpreted as an upward plasma motion from the
lower solar atmosphere during the peak of the flare, followed by a downward
motion.Comment: 13 pages, 7 figure
Direct medical costs associated with schizophrenia relapses in health care services in the city of São Paulo
Review of Coronal Oscillations - An Observer's View
Recent observations show a variety of oscillation modes in the corona. Early
non-imaging observations in radio wavelengths showed a number of fast-period
oscillations in the order of seconds, which have been interpreted as fast
sausage mode oscillations. TRACE observations from 1998 have for the first time
revealed the lateral displacements of fast kink mode oscillations, with periods
of ~3-5 minutes, apparently triggered by nearby flares and destabilizing
filaments. Recently, SUMER discovered with Doppler shift measurements loop
oscillations with longer periods (10-30 minutes) and relatively short damping
times in hot (7 MK) loops, which seem to correspond to longitudinal slow
magnetoacoustic waves. In addition, propagating longitudinal waves have also
been detected with EIT and TRACE in the lowest density scale height of loops
near sunspots. All these new observations seem to confirm the theoretically
predicted oscillation modes and can now be used as a powerful tool for
``coronal seismology'' diagnostic.Comment: 5 Figure
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