143 research outputs found
Causes for the broadening of hydrogen lines in the solar spectrum
Broadening of H alpha and H beta lines due to intrinsic pressure and Stark effec
Comparison of the mean photospheric magnetic field and the interplanetary magnetic field
Polarity comparison of solar magnetic field and interplanetary magnetic fiel
Is the Polar Region Different from the Quiet Region of the Sun?
Observations of the polar region of the Sun are critically important for
understanding the solar dynamo and the acceleration of solar wind. We carried
out precise magnetic observations on both the North polar region and the quiet
Sun at the East limb with the Spectro-Polarimeter of the Solar Optical
Telescope aboard Hinode to characterize the polar region with respect to the
quiet Sun. The average area and the total magnetic flux of the kG magnetic
concentrations in the polar region appear to be larger than those of the quiet
Sun. The magnetic field vectors classified as vertical in the quiet Sun have
symmetric histograms around zero in the strengths, showing balanced positive
and negative flux, while the histogram in the North polar region is clearly
asymmetric, showing a predominance of the negative polarity. The total magnetic
flux of the polar region is larger than that of the quiet Sun. In contrast, the
histogram of the horizontal magnetic fields is exactly the same between the
polar region and the quiet Sun. This is consistent with the idea that a local
dynamo process is responsible for the horizontal magnetic fields. A
high-resolution potential field extrapolation shows that the majority of
magnetic field lines from the kG-patches in the polar region are open with a
fanning-out structure very low in the atmosphere, while in the quiet Sun,
almost all the field lines are closed.Comment: Accepted for publication in AP
The optical flare
Optical observationd now present considerable information on the flare process. It is always associated with filaments and with simplification of existing magnetic connections, and it arises from the emergence and expansion of new flux.
The optical flare divides into impulsive phase, with multiple flashes along the neutral line, and thermal phase, with two-ribbon expansion. The former bears some resemblance to tearing mode phenomena.
The appearance of loops in emission requires very high densities in those phenomena.
The ratios of the hydrogen lines, the excitation of HeII 4686, and the relation of vertical to horizontal structure all remain to be explained
Proper orthogonal decomposition of solar photospheric motions
The spatio-temporal dynamics of the solar photosphere is studied by
performing a Proper Orthogonal Decomposition (POD) of line of sight velocity
fields computed from high resolution data coming from the MDI/SOHO instrument.
Using this technique, we are able to identify and characterize the different
dynamical regimes acting in the system. Low frequency oscillations, with
frequencies in the range 20-130 microHz, dominate the most energetic POD modes
(excluding solar rotation), and are characterized by spatial patterns with
typical scales of about 3 Mm. Patterns with larger typical scales of 10 Mm, are
associated to p-modes oscillations at frequencies of about 3000 microHz.Comment: 8 figures in jpg in press on PR
Neutral Solar Wind Generated by Lunar Exospheric Dust at the Terminator
We calculate the flux of neutral solar wind observed on the lunar surface at
the terminator due to solar wind protons penetrating exospheric dust grains
with (1) radii greater than 0.1 microns and (2) radii greater than 0.01
microns. For grains with radii larger than 0.1 microns, the ratio of the
neutral solar wind flux produced by exospheric dust to the incident ionized
solar wind flux is estimated to be about 10^-4-10^-3 for solar wind speeds in
excess of 800 km/s, but much lower (less than 10^-5) at average to slow solar
wind speeds. However, when the smaller grain sizes are considered, this ratio
is estimated to be greater than 10^-5 at all speeds, and at speeds in excess of
700 km/s reaches about 10^-3. These neutral solar wind fluxes are easily
measurable with current low energy neutral atom instrumentation. Observations
of neutral solar wind from the surface of the Moon would provide independent
information on the distribution of very small dust grains in the lunar
exosphere that would complement and constrain optical measurements at
ultraviolet and visible wavelengths.Comment: in press in J. Geophys. Re
A spatio-temporal description of the abrupt changes in the photospheric magnetic and Lorentz-force vectors during the 2011 February 15 X2.2 flare
The active region NOAA 11158 produced the first X-class flare of Solar Cycle
24, an X2.2 flare at 01:44 UT on 2011 February 15. Here we analyze SDO/HMI
magnetograms covering a 12-hour interval centered at the time of this flare. We
describe the spatial distributions of the photospheric magnetic changes
associated with this flare, including the abrupt changes in the field vector,
vertical electric current and Lorentz force vector. We also trace these
parameters' temporal evolution. The abrupt magnetic changes were concentrated
near the neutral line and in two neighboring sunspots. Near the neutral line,
the field vectors became stronger and more horizontal during the flare and the
shear increased. This was due to an increase in strength of the horizontal
field components near the neutral line, most significant in the horizontal
component parallel to the neutral line but the perpendicular component also
increased in strength. The vertical component did not show a significant,
permanent overall change at the neutral line. The increase in total flux at the
neutral line was accompanied by a compensating flux decrease in the surrounding
volume. In the two sunspots near the neutral line the azimuthal flux abruptly
decreased during the flare but this change was permanent in only one of the
spots. There was a large, abrupt, downward vertical Lorentz force change during
the flare, consistent with results of past analyses and recent theoretical
work. The horizontal Lorentz force acted in opposite directions along each side
of neutral line, with the two sunspots at each end subject to abrupt torsional
forces. The shearing forces were consistent with field contraction and decrease
of shear near the neutral line, whereas the field itself became more sheared as
a result of the flux collapsing towards the neutral line from the surrounding
volume.Comment: DOI 10.1007/s11207-012-0071-0. Accepted for publication in Solar
Physics SDO3 Topical Issue. Some graphics missing due to 15MB limi
A Statistical Study on Photospheric Magnetic Nonpotentiality of Active Regions and Its Relationship with Flares during Solar Cycles 22-23
A statistical study is carried out on the photospheric magnetic
nonpotentiality in solar active regions and its relationship with associated
flares. We select 2173 photospheric vector magnetograms from 1106 active
regions observed by the Solar Magnetic Field Telescope at Huairou Solar
Observing Station, National Astronomical Observatories of China, in the period
of 1988-2008, which covers most of the 22nd and 23rd solar cycles. We have
computed the mean planar magnetic shear angle (\bar{\Delta\phi}), mean shear
angle of the vector magnetic field (\bar{\Delta\psi}), mean absolute vertical
current density (\bar{|J_{z}|}), mean absolute current helicity density
(\bar{|h_{c}|}), absolute twist parameter (|\alpha_{av}|), mean free magnetic
energy density (\bar{\rho_{free}}), effective distance of the longitudinal
magnetic field (d_{E}), and modified effective distance (d_{Em}) of each
photospheric vector magnetogram. Parameters \bar{|h_{c}|}, \bar{\rho_{free}},
and d_{Em} show higher correlation with the evolution of the solar cycle. The
Pearson linear correlation coefficients between these three parameters and the
yearly mean sunspot number are all larger than 0.59. Parameters
\bar{\Delta\phi}, \bar{\Delta\psi}, \bar{|J_{z}|}, |\alpha_{av}|, and d_{E}
show only weak correlations with the solar cycle, though the nonpotentiality
and the complexity of active regions are greater in the activity maximum
periods than in the minimum periods. All of the eight parameters show positive
correlations with the flare productivity of active regions, and the combination
of different nonpotentiality parameters may be effective in predicting the
flaring probability of active regions.Comment: 20 pages, 5 figures, 4 tables, accepted for publication in Solar
Physic
Svestka's Research: Then and Now
Zdenek Svestka's research work influenced many fields of solar physics,
especially in the area of flare research. In this article I take five of the
areas that particularly interested him and assess them in a "then and now"
style. His insights in each case were quite sound, although of course in the
modern era we have learned things that he could not readily have envisioned.
His own views about his research life have been published recently in this
journal, to which he contributed so much, and his memoir contains much
additional scientific and personal information (Svestka, 2010).Comment: Invited review for "Solar and Stellar Flares," a conference in honour
of Prof. Zden\v{e}k \v{S}vestka, Prague, June 23-27, 2014. This is a
contribution to a Topical Issue in Solar Physics, based on the presentations
at this meeting (Editors Lyndsay Fletcher and Petr Heinzel
Wind structure of late B supergiants I. Multi-line analyses of near-surface and wind structure in HD 199 478 (B8 Iae)
We provide a quantitative analysis of time-variable phenomena in the
photospheric, near-star, and outflow regions of the late-B supergiant (SG) HD
199478. The analysis is based primarily on optical spectroscopic datasets
secured between 1999 and 2000 from the Bulgarian NAO, Tartu, and Ritter
Observatories. The temporal behaviour of HD 199478 is characterised by three
key empirical properties: (i) systematic central velocity shifts in the
photospheric absorption lines, including C II and He I, over a characteristic
time-scale of abou 20 days; (ii) extremely strong, variable H alpha emission
with no clear modulation signal, and (iii) the occurrence in 2000 of a (rare)
high-velocity absorption (HVA) event in H alpha, which evolved over about 60
days, showing the clear signature of mass infall and outflows. In these
properties HD 199478 resembles few other late-B SGs with peculiar emission and
HVAs in H alpha (HD 91619, HD 34085, HD 96919). Non-LTE line synthesis
modelling is conducted using FASTWIND for these late-B SGs to constrain and
compare their fundamental parameters within the context of extreme behaviour in
the H alpha lines. Our analysis indicate that at the cooler temperature edge of
B SGs, there are objects whose wind properties, as traced by H alpha, are
inconsistent with the predictions of the smooth, spherically symmetric wind
approximation. This discordance is still not fully understood and may highlight
the role of a non-spherical, disk-like, geometry, which may result from
magnetically-driven equatorial compression of the gas. Ordered dipole magnetic
fields may also lead to confined plasma held above the stellar surface, which
ultimately gives rise to transient HVA events.Comment: 12 pages. To be published in Astronomy and AStrophysic
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