1,749 research outputs found
Solar cycle changes of large-scale solar wind structure
In this paper, I present the results on large-scale evolution of density
turbulence of solar wind in the inner heliosphere during 1985 - 2009. At a
given distance from the Sun, the density turbulence is maximum around the
maximum phase of the solar cycle and it reduces to ~70%, near the minimum
phase. However, in the current minimum of solar activity, the level of
turbulence has gradually decreased, starting from the year 2005, to the present
level of ~30%. These results suggest that the source of solar wind changes
globally, with the important implication that the supply of mass and energy
from the Sun to the interplanetary space has significantly reduced in the
present low level of activity.Comment: 3 pages, 2 figure
Three-dimensional Evolution of Solar Wind during Solar Cycles 22-24
This paper presents the analysis of 3D evolution of solar wind density
turbulence and speed at various levels of solar activity between solar cycles
22 and 24. The solar wind data has been obtained from interplanetary
scintillation (IPS) measurements made at the Ooty Radio Telescope. Results show
that (i) on the average, there was a downward trend in density turbulence from
the maximum of cycle 22 to the deep minimum phase of cycle 23; (2) the
scattering diameter of the corona around the Sun shrunk steadily towards the
Sun, starting from 2003 to the smallest size at the deepest minimum, and it
corresponded to a reduction of ~50% in density turbulence between maximum and
minimum phases of cycle 23; (3) The latitudinal distribution of solar wind
speed was significantly different between minima of cycles 22 and 23. At the
minimum phase of solar cycle 22, when the underlying solar magnetic field was
simple and nearly dipole in nature, the high-speed streams were observed from
poles to ~30 deg. latitudes in both hemispheres. In contrast, in the long-decay
phase of cycle 23, the sources of high-speed wind at both poles, in accordance
with the weak polar fields, occupied narrow latitude belts from poles to ~60
deg. latitudes. Moreover, in agreement with the large amplitude of heliospheric
current sheet, the low-speed wind prevailed the low- and mid-latitude regions
of the heliosphere. (4) At the transition phase between cycles 23 and 24, the
high levels of density and density turbulence were observed close to the
heliospheric equator and the low-speed speed wind extended from equatorial- to
mid-latitude regions. Results are consistent with the onset of the current
cycle 24, from middle of 2009 and it has almost reached near to the maximum
phase at the northern hemisphere of the Sun, but activity not yet developed in
the southern hemisphere.Comment: 14 pages, 9 figures, Accepted for Publication in The Astrophysical
Journa
Evolution of Solar Magnetic Field and Associated Multi-wavelength Phenomena: Flare events on 20 November 2003
We analyze H-alpha images, soft X-ray profiles, magnetograms, extreme
ultra-violet images and radio observations of two homologous flare events
(M1.4/1N and M9.6/2B) on 20 November 2003 in the active region NOAA 10501 and
study properties of reconnection between twisted filament systems, energy
release and associated launch of coronal mass ejections (CMEs). During both
events twisted filaments observed in H-alpha approached each other and
initiated the flare processes. However, the second event showed the formation
of cusp as the filaments interacted. The rotation of sunspots of opposite
polarities, inferred from magnetograms likely powered the twisted filaments and
injection of helicity. Along the current sheet between these two opposite
polarity sunspots, the shear was maximum, which could have caused the twist in
the filament. At the time of interaction between filaments, the reconnection
took place and flare emission in thermal and non-thermal energy ranges attained
the maximum. The radio signatures revealed the opening of field lines resulting
from reconnection. The H-alpha images and radio data provide the inflow speed
leading to reconnection and the scale size of particle acceleration region. The
first event produced a narrow and slow CME, whereas the later one was
associated with a fast full halo CME. The halo CME signatures observed between
Sun and Earth using white-light and scintillation images and in-situ
measurements indicated the magnetic energy utilized in the expansion and
propagation. The magnetic cloud signature at the Earth confirmed the flux rope
ejected at the time of filament interaction and reconnection.Comment: 22 pages, 16 figures, Accepted for the publication in Astrophysical
Journal (APJ
High Energy Solar Particle Events and their Associated Coronal Mass Ejections
Intense solar energetic particle (SEP) events data, associated with ground
level enhancements (GLEs), occurred during 1989 to 2006 have been obtained from
the spectrometers on board GOES spacecraft in the energy range 10-100 MeV. The
interplanetary effects of these events and their associated coronal mass
ejections (CMEs) have been provided by the LASCO/SOHO coronagraph images in the
field of view of 2-30 {\rsun} and the interplanetary scintillation images from
the Ooty Radio Telescope in the heliocentric distance range of 40-250
R. The comparison between the radial evolution of the CME and its
associated particle spectrum shows that the spectrum is soft at the onset of
the particle event. A flat spectrum is observed at the peak of the particle
event and the spectrum becomes steeper as the CME moves farther out into the
inner heliosphere. However, the magnitude of change in spectral slopes differs
from one CME to the other, suggesting the difference in energy available within
the CME to drive the shock. The spectral index evolution as a function of
initial speed of the CME at different parts of the particle profile has also
been compared. The result shows that the change in particle flux with time is
rather quick for the high-energy portion of the spectrum than that of the
low-energy part, which makes the steepening of the energy spectrum with
time/distance from the Sun. It indicates that the acceleration of particles by
a CME-driven shock may be efficient at low energies (30 MeV) and the
efficiency of the shock decreases gradually towards the high-energy side of the
spectrum.Comment: 14 pages, 9 figures, 1 table; Submitted to Advances in Geosciences
(Solar-Terrestrial Sciences
Eruption of a plasma blob, associated M-class flare, and large-scale EUV wave observed by SDO
We present a multiwavelength study of the formation and ejection of a plasma
blob and associated EUV waves in AR NOAA 11176, observed by SDO/AIA and STEREO
on 25 March 2011. SDO/AIA images clearly show the formation and ejection of a
plasma blob from the lower solar atmosphere at ~9 min prior to the onset of the
M1.0 flare. This onset of the M-class flare happened at the site of the blob
formation, while the blob was rising in a parabolic path with an average speed
of ~300 km/s. The blob also showed twisting and de-twisting motion in the lower
corona, and the blob speed varied from ~10-540 km/s. The faster and slower EUV
wavefronts were observed in front of the plasma blob during its impulsive
acceleration phase. The faster EUV wave propagated with a speed of ~785 to 1020
km/s, whereas the slower wavefront speed varied in between ~245 and 465 km/s.
The timing and speed of the faster wave match the shock speed estimated from
the drift rate of the associated type II radio burst. The faster wave
experiences a reflection by the nearby AR NOAA 11177. In addition, secondary
waves were observed (only in the 171 \AA channel), when the primary fast wave
and plasma blob impacted the funnel-shaped coronal loops. The HMI magnetograms
revealed the continuous emergence of new magnetic flux along with shear flows
at the site of the blob formation. It is inferred that the emergence of twisted
magnetic fields in the form of arch-filaments/"anemone-type" loops is the
likely cause for the plasma blob formation and associated eruption along with
the triggering of M-class flare. Furthermore, the faster EUV wave formed ahead
of the blob shows the signature of fast-mode MHD wave, whereas the slower wave
seems to be generated by the field line compression by the plasma blob. The
secondary wave trains originated from the funnel-shaped loops are probably the
fast magnetoacoustic waves.Comment: A&A (in press), 22 pages, 13 figure
Multi-frequency scatter broadening evolution of pulsars - I
We present multi-wavelength scatter broadening observations of 47 pulsars,
made with the Giant Metre-wave Radio Telescope (GMRT), Ooty Radio Telescope
(ORT) and Long Wavelength Array (LWA). The GMRT observations have been made in
the phased array mode at 148, 234, and 610 MHz and the ORT observations at 327
MHz. The LWA data sets have been obtained from the LWA pulsar data archive. The
broadening of each pulsar as a function of observing frequency provides the
frequency scaling index, . The estimations of have been
obtained for 39 pulsars, which include entirely new estimates for 31 pulsars.
This study increases the total sample of pulsars available with
estimates by 50\%. The overall distribution of with the
dispersion measure (DM) of pulsar shows interesting variations, which are
consistent with the earlier studies. However, for a given value of DM a range
of values are observed, indicating the characteristic turbulence along
each line of sight. For each pulsar, the estimated level of turbulence,
, has also been compared with and DM. Additionally, we
compare the distribution of with the theoretically predicated model to
infer the general characteristics of the ionized interstellar medium (ISM).
Nearly 65\% of the pulsars show a flatter index (i.e., ) than
that is expected from the Kolmogorov turbulence model. Moreover, the group of
pulsars having flatter index is typically associated with an enhanced value of
than those with steeper index.Comment: 13 pages, 4 figures, 3 tables. Accepted for publication in Ap
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