353 research outputs found
Unique Observations of a Geomagnetic SI^+ -- SI^- Pair: Solar Sources and Associated Solar Wind Fluctuations
The paper describes the occurrence of a pair of oppositely directed sudden
impulses (SI), in the geomagnetic field (X), at ground stations, called
SI -- SI pairs, that occurred between 1835 UT and 2300 UT on 23
April 1998. The SI -- SI pair, was closely correlated with
corresponding variations in the solar wind density, while solar wind velocity
and the southward component of the interplanetary magnetic field (Bz) did not
show any correspondence. Further, this event had no source on the visible solar
disk. However, a rear-side partial halo coronal mass ejection (CME) and an M1.4
class solar flare behind the west limb, took place on 20 April 1998, the date
corresponding to the traceback location of the solar wind flows. This event
presents empirical evidence, which to our knowledge, is the best convincing
evidence for the association of specific solar events to the observations of an
SI -- SI pair. In addition, it shows that it is possible for a
rear side solar flare to propagate a shock towards the earth.Comment: The paper has just been accepted in the Journal of Geophysical
Research (Space Physics) on 20 September 2010. It is 17 pages with 4 figure
Multi-wavelength study of the star-formation in the S237 H II region
We present a detailed multi-wavelength study of observations from X-ray,
near-infrared to centimeter wavelengths to probe the star formation processes
in the S237 region. Multi-wavelength images trace an almost sphere-like shell
morphology of the region, which is filled with the 0.5--2 keV X-ray emission.
The region contains two distinct environments - a bell-shaped cavity-like
structure containing the peak of 1.4 GHz emission at center, and elongated
filamentary features without any radio detection at edges of the sphere-like
shell - where {\it Herschel} clumps are detected. Using the 1.4 GHz continuum
and CO line data, the S237 region is found to be excited by a radio
spectral type of B0.5V star and is associated with an expanding H{\sc ii}
region. The photoionized gas appears to be responsible for the origin of the
bell-shaped structure. The majority of molecular gas is distributed toward a
massive {\it Herschel} clump (M 260 M), which
contains the filamentary features and has a noticeable velocity gradient. The
photometric analysis traces the clusters of young stellar objects (YSOs) mainly
toward the bell-shaped structure and the filamentary features. Considering the
lower dynamical age of the H\,{\sc ii} region (i.e. 0.2-0.8 Myr), these
clusters are unlikely to be formed by the expansion of the H\,{\sc ii} region.
Our results also show the existence of a cluster of YSOs and a massive clump at
the intersection of filamentary features, indicating that the collisions of
these features may have triggered cluster formation, similar to those found in
Serpens South region.Comment: 21 pages, 14 figures, 1 table, Accepted for publication in The
Astrophysical Journa
INTRUSION DETECTION FOR DISCRETE SEQUENCES
Global understanding of the sequence anomaly detection problem and how techniques proposed for different domains relate to each other. Our specific contributions are as follows: We identify three distinct formulations of the anomaly detection problem, and review techniques from many disparate and disconnected domains that address each of these formulations. Within each problem formulation, we group techniques into categories based on the nature of the underlying algorithm. For each category, we provide a basic anomaly detection technique, and show how the existing techniques are variants of the basic technique. This approach shows how different techniques within a category are related or different from each other. Our categorization reveals new variants and combinations that have not been investigated before for anomaly detection. We also provide a discussion of relative strengths and weaknesses of different techniques. We show how techniques developed for one problem formulation can be adapted to solve a different formulation; thereby providing several novel adaptations to solve the different problem formulations. We highlight the applicability of the techniques that handle discrete sequences to other related areas such as online anomaly detection and time series anomaly detection
A study of density modulation index in the inner heliospheric solar wind during solar cycle 23
The ratio of the rms electron density fluctuations to the background density
in the solar wind (density modulation index, )
is of vital importance in understanding several problems in heliospheric
physics related to solar wind turbulence. In this paper, we have investigated
the behavior of in the inner-heliosphere from 0.26 to 0.82 AU.
The density fluctuations have been deduced using extensive
ground-based observations of interplanetary scintillation (IPS) at 327 MHz,
which probe spatial scales of a few hundred km. The background densities ()
have been derived using near-Earth observations from the Advanced Composition
Explorer (). Our analysis reveals that and does not vary appreciably with heliocentric distance. We
also find that declines by 8% from 1998 to 2008. We discuss the
impact of these findings on problems ranging from our understanding of Forbush
decreases to the behavior of the solar wind dynamic pressure over the recent
peculiar solar minimum at the end of cycle 23..Comment: 13 Pages, 8 Figures, Accepted for publication in Ap
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