62 research outputs found
Chromospheric Velocities of a C-class Flare
We use high spatial and temporal resolution observations from the Swedish
Solar Telescope to study the chromospheric velocities of a C-class flare
originating from active region NOAA 10969. A time-distance analysis is employed
to estimate directional velocity components in H-alpha and Ca II K image
sequences. Also, imaging spectroscopy has allowed us to determine flare-induced
line-of-sight velocities. A wavelet analysis is used to analyse the periodic
nature of associated flare bursts. Time-distance analysis reveals velocities as
high as 64 km/s along the flare ribbon and 15 km/s perpendicular to it. The
velocities are very similar in both the H-alpha and Ca II K time series.
Line-of-sight H-alpha velocities are red-shifted with values up to 17 km/s. The
high spatial and temporal resolution of the observations have allowed us to
detect velocities significantly higher than those found in earlier studies.
Flare bursts with a periodicity of approximately 60 s are also detected. These
bursts are similar to the quasi-periodic oscillations observed at hard X-ray
and radio wavelength data. Some of the highest velocities detected in the solar
atmosphere are presented. Line-of-sight velocity maps show considerable mixing
of both the magnitude and direction of velocities along the flare path. A
change in direction of the velocities at the flare kernel has also been
detected which may be a signature of chromospheric evaporation.Comment: Accepted for publication in Astronomy and Astrophysics, 5 figure
Solar Flare Impulsive Phase Emission Observed with SDO/EVE
Differential emission measures (DEMs) during the impulsive phase of solar
flares were constructed using observations from the EUV Variability Experiment
(EVE) and the Markov-Chain Monte Carlo method. Emission lines from ions formed
over the temperature range log T = 5.8 - 7.2 allow the evolution of the DEM to
be studied over a wide temperature range at 10s cadence. The technique was
applied to several M- and X-class flares, where impulsive phase EUV emission is
observable in the disk-integrated EVE spectra from emission lines formed up to
3 - 4 MK, and we use spatially-unresolved EVE observations to infer the thermal
structure of the emitting region. For the nine events studied the DEMs
exhibited a two component distribution during the impulsive phase, a low
temperature component with peak temperature of 1 - 2 MK, and a broad high
temperature one from 7 - 30 MK. A bimodal high temperature component is also
found for several events, with peaks at 8 and 25 MK during the impulsive phase.
The origin of the emission was verified using AIA images to be the flare
ribbons and footpoints, indicating that the constructed DEMs represent the
spatially-average thermal structure of the chromospheric flare emission during
the impulsive phase.Comment: 18 pages, 6 figures, accepted for publication in Ap
Radiative hydrodynamic modelling and observations of the X-class solar flare on 2011 March 9
We investigated the response of the solar atmosphere to non-thermal electron
beam heating using the radiative transfer and hydrodynamics modelling code
RADYN. The temporal evolution of the parameters that describe the non-thermal
electron energy distribution were derived from hard X-ray observations of a
particular flare, and we compared the modelled and observed parameters. The
evolution of the non-thermal electron beam parameters during the X1.5 solar
flare on 2011 March 9 were obtained from analysis of RHESSI X-ray spectra. The
RADYN flare model was allowed to evolve for 110 seconds, after which the
electron beam heating was ended, and was then allowed to continue evolving for
a further 300s. The modelled flare parameters were compared to the observed
parameters determined from extreme-ultraviolet spectroscopy. The model produced
a hotter and denser flare loop than that observed and also cooled more rapidly,
suggesting that additional energy input in the decay phase of the flare is
required. In the explosive evaporation phase a region of high-density cool
material propagated upward through the corona. This material underwent a rapid
increase in temperature as it was unable to radiate away all of the energy
deposited across it by the non-thermal electron beam and via thermal
conduction. A narrow and high-density ( cm) region at
the base of the flare transition region was the source of optical line emission
in the model atmosphere. The collision-stopping depth of electrons was
calculated throughout the evolution of the flare, and it was found that the
compression of the lower atmosphere may permit electrons to penetrate farther
into a flaring atmosphere compared to a quiet Sun atmosphere.Comment: 12 pages, 12 figure
Hα and EUV observations of a partial CME
We have obtained Hα high spatial and time resolution observations of the upper solar chromosphere and supplemented these with multi-wavelength observations from the Solar Dynamics Observatory (SDO) and the Hinode Extreme-ultraviolet Imaging Spectrometer. The Hα observations were conducted on 2012 February 11 with the Hydrogen-Alpha Rapid Dynamics Camera instrument at the National Solar Observatory's Dunn Solar Telescope. Our Hα observations found large downflows of chromospheric material returning from coronal heights following a failed prominence eruption. We have detected several large condensations ("blobs") returning to the solar surface at velocities of â200 km sâ1 in both Hα and several SDO Atmospheric Imaging Assembly band passes. The average derived size of these "blobs" in Hα is 500 by 3000 km2 in the directions perpendicular and parallel to the direction of travel, respectively. A comparison of our "blob" widths to those found from coronal rain, indicate that there are additional, smaller, unresolved "blobs" in agreement with previous studies and recent numerical simulations. Our observed velocities and decelerations of the "blobs" in both Hα and SDO bands are less than those expected for gravitational free-fall and imply additional magnetic or gas pressure impeding the flow. We derived a kinetic energy of â2 orders of magnitude lower for the main eruption than a typical coronal mass ejection, which may explain its partial nature.Publisher PDFPeer reviewe
Discovery of a White Dwarf Companion (MS0354.6-3650 = EUVE J0356-366) to a G2V Star
We present x-ray, ultraviolet, and optical observations of the mysterious EUV/soft x-ray source EUVE J0356-336 (= MS0354.6-3650). Initial Einstein observations identified this source with a cluster of galaxies, but the relatively high source count rate in the Extreme Ultraviolet Explorer (EUVE) 100 Ă
band and the lack of variability hinted that EUVE J0356-3650 might be a white dwarf; the UK Schmidt plate of the field surrounding this object found a 12.45 magnitude G2V star that could hide a compact companion. This hypothesis was confirmed in an IUE ultraviolet spectrum that shows the definite signature of a hydrogen-rich white dwarf (DA). A model atmosphere analysis shows that the DA star is hot (Teffâ„52,000 K) and may have a low abundance of heavy elements. We show that the G2V star and the DA star form a physical pair at a distance of 400 pc and therefore add to the list of white dwarf plus luminous main sequence star binaries discovered in EUV surveys
Observational Evidence for Gentle Chromospheric Evaporation During the Impulsive Phase of a Solar Flare
Observational evidence for gentle chromospheric evaporation during the
impulsive phase of a C9.1 solar flare is presented using data from the Reuven
Ramaty High-Energy Solar Spectroscopic Imager and the Coronal Diagnostic
Spectrometer on board the Solar and Heliospheric Observatory. Until now,
evidence for gentle evaporation has often been reported during the decay phase
of solar flares, where thermal conduction is thought to be the driving
mechanism. Here we show that the chromospheric response to a low flux of
nonthermal electrons (>=5x10^9 ergs cm^-2 s^-1) results in plasma upflows of
13+/-16, 16+/-18, and 110+/-58 km s^-1 in the cool He I and O V emission lines
and the 8 MK Fe XIX line. These findings, in conjunction with other recently
reported work, now confirm that the dynamic response of the solar atmosphere is
sensitively dependent on the flux of incident electrons.Comment: 5 page
Faint Sources in the EUVE Survey: Identification of White Dwarfs, Active LateâType Stars, and Galactic Nuclei
We report the classification of 21 new extreme-ultraviolet sources from the recent catalog of Lampton et al. The optical spectra presented identify the objects as 14 active late-type stars (including two double active stars and a possible T Tauri star), three white dwarfs, and six active galactic nuclei (a Seyfert galaxy, the BL Lac object 1ES 1028+511 [=EUVE J1031+508], and four quasi-stellar objects). We have detected Ca II absorption lines in the BL Lac object and measured its redshift. Two of the white dwarfs are unusually massive (M \u3e 1.1 Mâ). Our sample of late-type stars includes five previously known high proper motion objects (EUVE J1004+503, J2244-332A,B, J1802+642, and J1131-346), of which one is the well-known flare star TX PsA (EUVE J2244-332B). We report an unusually high level of activity for the primary component of the TX PsA system (EUVE J2244-332A), which may indicate flare activity. The group of late-type stars is on average almost 3 mag fainter (m â 13) than the typical member of the Extreme Ultraviolet Explorer (EUVE) all-sky survey catalog. All Galactic and extragalactic objects were also detected in the ROSAT Position Sensitive Proportional Counter survey, and most are at the faint limit of the EUVE detectors. These new identifications substantially increase the total number of EUV-selected extragalactic sources
Multi-wavelength observations of the 2014 June 11 M3.9 flare:Temporal and spatial characteristics
We present multi-wavelength observations of an M-class flare (M3.9) that
occurred on 2014 June 11. Our observations were conducted with the Dunn Solar
Telescope (DST), adaptive optics, the multi-camera system ROSA (Rapid
Oscillations in Solar Atmosphere) and new HARDcam (Hydrogen-Alpha Rapid
Dynamics) camera in various wavelengths, such as Ca~II~K, Mg~I~b (at 5172.7
Ang), and H narrow-band, and G-band continuum filters. Images were
re-constructed using the Kiepencheuer-Institut Speckle Interferometry Package
(KISIP) code, to improve our image resolution. We observed intensity increases
of 120-150% in the Mg, Ca~K and H narrow band filters during
the flare. Intensity increases for the flare observed in the SDO EUV channels
were several times larger, and the GOES X-rays increased over a factor of 30
for the harder band. Only a modest delay is found between the onset of flare
ribbons of a nearby sympathetic flare and the main flare ribbons observed in
these narrow-band filters. The peak flare emission occurs within a few seconds
for the Ca~K, Mg, and H bands. Time-distance techniques find
propagation velocities of 60 km/s for the main flare ribbon and as
high as 300 km/s for smaller regions we attribute to filament eruptions. This
result and delays and velocities observed with SDO (100 km/s) for
different coronal heights agree well with the simple model of energy
propagation versus height, although a more detailed model for the flaring solar
atmosphere is needed. And finally, we detected marginal quasi-periodic
pulsations (QPPs) in the 40--60 second range for the Ca~K, Mg and H
bands, and such measurements are important for disentangling the detailed
flare-physics.Comment: 16 Pages, 7 Figures, 1 Table (1 video in on-line journal); Accepted
in Research in Astronomy and Astrophysic
The Velocity Distribution of Solar Photospheric Magnetic Bright Points
We use high spatial resolution observations and numerical simulations to
study the velocity distribution of solar photospheric magnetic bright points.
The observations were obtained with the Rapid Oscillations in the Solar
Atmosphere instrument at the Dunn Solar Telescope, while the numerical
simulations were undertaken with the MURaM code for average magnetic fields of
200 G and 400 G. We implemented an automated bright point detection and
tracking algorithm on the dataset, and studied the subsequent velocity
characteristics of over 6000 structures, finding an average velocity of
approximately 1 km/s, with maximum values of 7 km/s. Furthermore, merging
magnetic bright points were found to have considerably higher velocities, and
significantly longer lifetimes, than isolated structures. By implementing a new
and novel technique, we were able to estimate the background magnetic flux of
our observational data, which is consistent with a field strength of 400 G.Comment: Accepted for publication in ApJL, 12 pages, 2 figure
The Search for Super-saturation in Chromospheric Emission
We investigate if the super-saturation phenomenon observed at X-ray
wavelengths for the corona, exists in the chromosphere for rapidly rotating
late-type stars. Moderate resolution optical spectra of fast rotating EUV- and
X-ray- selected late-type stars were obtained. Stars in alpha Per were observed
in the northern hemisphere with the Isaac Newton 2.5 m telescope and IDS
spectrograph. Selected objects from IC 2391 and IC 2602 were observe in the
southern hemisphere with the Blanco 4m telescope and R-C spectrograph at CTIO.
Ca II H & K fluxes were measured for all stars in our sample. We find the
saturation level for Ca II K at log(L_CaK/L_bol) = -4.08. The Ca II K flux does
not show a decrease as a function of increased rotational velocity or smaller
Rossby number as observed in the X-ray. This lack of "super-saturation"
supports the idea of coronal-stripping as the cause of saturation and
super-saturation in stellar chromospheres and corona, but the detailed
underlying mechanism is still under investigation.Comment: 28 Pages, 6 figures, 1 Table; Accepted in ApJ for Aug 201
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