126 research outputs found
Hydrogen Lyman-alpha and Lyman-beta radiances and profiles in polar coronal holes
The hydrogen Lyman-alpha plays a dominant role in the radiative energy
transport in the lower transition region, and is important for the stud- ies of
transition-region structure as well as solar wind origin. We investigate the
Ly-alpha profiles obtained by SUMER in coronal holes and quiet Sun. In a subset
of these observations, also the Hi Lyman-beta, Si iii, and O vi lines were
(quasi-) simultaneously recorded. We find that the distances between the two
peaks of Ly-alpha profiles are larger in coronal holes than in the quiet Sun,
indicating a larger opacity in coronal holes. This difference might result from
the different magnetic structures or the different radiation fields in the two
regions. Most of the Ly-beta profiles in the coronal hole have a stronger blue
peak, in contrast to those in quiet-Sun regions. Whilst in both regions the
Ly-alpha profiles are stronger in the blue peak. Although the asymmetries are
likely to be produced by differential flows in the solar atmosphere, their
detailed formation processes are still unclear. The radiance ratio between
Ly-alpha and Ly-beta decreases towards the limb in the coronal hole, which
might be due to the different opacity of the two lines. We also find that the
radiance distributions of the four lines are set by a combined effect of limb
brightening and the different emission level between coronal holes and quiet
Sun.Comment: 13 pages,4 figures, 1 talbe, accepted by Ap
The detection of ultra-relativistic electrons in low Earth orbit
Aims. To better understand the radiation environment in low Earth orbit
(LEO), the analysis of in-situ observations of a variety of particles, at
different atmospheric heights, and in a wide range of energies, is needed.
Methods. We present an analysis of energetic particles, indirectly detected by
the Large Yield RAdiometer (LYRA) instrument on board ESA's Project for
On-board Autonomy 2 (PROBA2) satellite as background signal. Combining
Energetic Particle Telescope (EPT) observations with LYRA data for an
overlapping period of time, we identified these particles as electrons with an
energy range of 2 to 8 MeV. Results. The observed events are strongly
correlated to geo-magnetic activity and appear even during modest disturbances.
They are also well confined geographically within the L=4-6 McIlwain zone,
which makes it possible to identify their source. Conclusions. Although highly
energetic particles are commonly perturbing data acquisition of space
instruments, we show in this work that ultra-relativistic electrons with
energies in the range of 2-8 MeV are detected only at high latitudes, while not
present in the South Atlantic Anomaly region.Comment: Topical Issue: Flares, CMEs and SEPs and their space weather impacts;
20 pages; 7 figures; Presented during 13th European Space Weather Week, 201
Sunquake with a second bounce, other sunquakes, and emission associated with the X9.3 flare of 6 September 2017. I. Observations
Aims. The 6 September 2017 X9.3 solar flare produced very unique observations of magnetic field transients and a few seismic responses, or sunquakes, detected by the Helioseismic and Magnetic Imager (HMI) instrument aboard Solar Dynamic Observatory (SDO) spacecraft, including the strongest sunquake ever reported. This flare was one of a few flares occurring within a few days or hours in the same active region. Despite numerous reports of the fast variations of magnetic field, and seismic and white light emission, no attempts were made to interpret the flare features using multi-wavelength observations. In this study, we attempt to produce the summary of available observations of the most powerful flare of the 6 September 2017 obtained using instruments with different spatial resolutions (this paper) and to provide possible interpretation of the flaring events, which occurred in the locations of some seismic sources (a companion Paper II).
Methods. We employed non-linear force-free field extrapolations followed by magnetohydrodynamic simulations in order to identify the presence of several magnetic flux ropes prior to the initiation of this X9.3 flare. Sunquakes were observed using the directional holography and time–distance diagram detection techniques. The high-resolution method to detect the Hα line kernels in the CRISP instrument at the diffraction level limit was also applied.
Results. We explore the available γ-ray (GR), hard X-ray (HXR), Lyman-α, and extreme ultra-violet (EUV) emission for this flare comprising two flaring events observed by space- and ground-based instruments with different spatial resolutions. For each flaring event we detect a few seismic sources, or sunquakes, using Dopplergrams from the HMI/SDO instrument coinciding with the kernels of Hα line emission with strong redshifts and white light sources. The properties of sunquakes were explored simultaneously with the observations of HXR (with KONUS/WIND and the Reuven Ramaty High Energy Solar Spectroscopic Imager payload), EUV (with the Atmospheric Imaging Assembly (AIA/SDO and the EUV Imaging Spectrometer aboard Hinode payload), Hα line emission (with the CRisp Imaging Spectro-Polarimeter (CRISP) in the Swedish Solar Telescope), and white light emission (with HMI/SDO). The locations of sunquake and Hα kernels are associated with the footpoints of magnetic flux ropes formed immediately before the X9.3 flare onset.
Conclusions. For the first time we present the detection of the largest sunquake ever recorded with the first and second bounces of acoustic waves generated in the solar interior, the ripples of which appear at a short distance of 5–8 Mm from the initial flare location. Four other sunquakes were also detected, one of which is likely to have occurred 10 min later in the same location as the largest sunquake. Possible parameters of flaring atmospheres in the locations with sunquakes are discussed using available temporal and spatial coverage of hard X-ray, GR, EUV, hydrogen Hα-line, and white light emission in preparation for their use in an interpretation to be given in Paper II
Simultaneous Observations of the Chromosphere with TRACE and SUMER
Using mainly the 1600 angstrom continuum channel, and also the 1216 angstrom
Lyman-alpha channel (which includes some UV continuum and C IV emission),
aboard the TRACE satellite, we observed the complete lifetime of a transient,
bright chromospheric loop. Simultaneous observations with the SUMER instrument
aboard the SOHO spacecraft revealed interesting material velocities through the
Doppler effect existing above the chromospheric loop imaged with TRACE,
possibly corresponding to extended non-visible loops, or the base of an X-ray
jet.Comment: 14 pages, 10 figures, accepted by Solar Physic
The quiet Sun average Doppler shift of coronal lines up to 2 MK
The average Doppler shift shown by spectral lines formed from the
chromosphere to the corona reveals important information on the mass and energy
balance of the solar atmosphere, providing an important observational
constraint to any models of the solar corona. Previous spectroscopic
observations of vacuum ultra-violet (VUV) lines have revealed a persistent
average wavelength shift of lines formed at temperatures up to 1 MK. At higher
temperatures, the behaviour is still essentially unknown. Here we analyse
combined SUMER/SoHO and EIS/Hinode observations of the quiet Sun around disk
centre to determine, for the first time, the average Doppler shift of several
spectral lines formed between 1 and 2 MK, where the largest part of the quiet
coronal emission is formed. The measurements are based on a novel technique
applied to EIS spectra to measure the difference in Doppler shift between lines
formed at different temperatures. Simultaneous wavelength-calibrated SUMER
spectra allow establishing the absolute value at the reference temperature of 1
MK. The average line shifts at 1 MK < T < 1.8 MK are modestly, but clearly
bluer than those observed at 1 MK. By accepting an average blue shift of about
(-1.8+/-0.6) km/s at 1 MK (as provided by SUMER measurements), this translates
into a maximum Doppler shift of (-4.4+/-2.2) km/s around 1.8 MK. The measured
value appears to decrease to about (-1.3+/-2.6) km/s at the Fe XV formation
temperature of 2.1 MK. The measured average Doppler shift between 0.01 and 2.1
MK, for which we provide a parametrisation, appears to be qualitatively and
roughly quantitatively consistent with what foreseen by 3-D coronal models
where heating is produced by dissipation of currents induced by photospheric
motions and by reconnection with emerging magnetic flux.Comment: 9 pages, 10 figures. Astronomy and Astrophysics (in press
Chromospheric jets around the edges of sunspots
Aims. Evidence is beginning to be put forward that demonstrates the role of the chromosphere in supplying energy and mass to the corona. We aim to asses the role of chromospheric jets in active region dynamics.
Methods. Using a combination of the Hinode/SOT Ca II H and TRACE 1550 Å and 1600 Å filters we examine chromospheric jets situated at the edge of a sunspot.
Results. Analysis reveals a near continuous series of jets, that raise chromospheric material into the low corona above a sunspot. The jets have average rise speeds of 30 km/s
and a range of 10−100 km/s. Enhanced emission observed at the jets leading edge suggests the formation of a shock front. Increased emission in TRACE bandpasses above the sunspot and the disappearance of the jets from the Ca II filter suggests that some of the chromospheric jet material is at least heated to ∼ 0.1 MK. The evidence suggests that the jets could be a mechanism which provides a steady, low-level heating for active region features
Relativistic treatment of harmonics from impurity systems in quantum wires
Within a one particle approximation of the Dirac equation we investigate a
defect system in a quantum wire. We demonstrate that by minimally coupling a
laser field of frequency omega to such an impurity system, one may generate
harmonics of multiples of the driving frequency. In a multiple defect system
one may employ the distance between the defects in order to tune the cut-off
frequency.Comment: 9 pages Latex, 8 eps figures, section added, numerics improve
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