416,203 research outputs found
Vainu Bappu Memorial Lecture: What is a sunspot?
Sunspots have been known in the West since Galileo Galilei and Thomas Harriot
first used telescopes to observe the Sun nearly four centuries ago; they have
been known to the Chinese for more than two thousand years. They appear as
relatively dark patches on the surface of the Sun, and are caused by
concentrations of magnetism which impede the flow of heat from deep inside the
Sun up to its othewise brilliant surface. The spots are not permanent: the
total number of spots on the Sun varies cyclically in time, with a period of
about eleven years, associated with which there appear to be variations in our
climate. When there are many spots, it is more dangerous for spacecraft to
operate. The cause of the spots is not well understood; nor is it known for
sure how they die. Their structure beneath the surface of the Sun is in some
dispute, although much is known about their properties at the surface,
including an outward material flow which was discovered by John Evershed
observing the Sun from Kodaikanal a hundred years ago. I shall give you a
glimpse of how we are striving to deepen our understanding of these fascinating
features, and of some of the phenomena that appear to be associated with them.Comment: Lecture delivered at the Indian Institute of Astrophysics, December
2008 Typing errors corrected To appear in Magnetic Coupling between the
Interior and the Atmosphere of the Sun, ed. S.S. Hasan & R.J. Rutten, Astr.
Sp. Sci. Pro
Cryptanalysis of Sun and Cao's Remote Authentication Scheme with User Anonymity
Dynamic ID-based remote user authentication schemes ensure efficient and
anonymous mutual authentication between entities. In 2013, Khan et al. proposed
an improved dynamic ID-based authentication scheme to overcome the security
flaws of Wang et al.'s authentication scheme. Recently, Sun and Cao showed that
Khan et al. does not satisfies the claim of the user's privacy and proposed an
efficient authentication scheme with user anonymity. The Sun and Cao's scheme
achieve improvement over Khan et al.'s scheme in both privacy and performance
point of view. Unfortunately, we identify that Sun and Cao's scheme does not
resist password guessing attack. Additionally, Sun and Cao's scheme does not
achieve forward secrecy
Helicity at Photospheric and Chromospheric Heights
In the solar atmosphere the twist parameter has the same sign as
magnetic helicity. It has been observed using photospheric vector magnetograms
that negative/positive helicity is dominant in the northern/southern hemisphere
of the Sun. Chromospheric features show dextral/sinistral dominance in the
northern/southern hemisphere and sigmoids observed in X-rays also have a
dominant sense of reverse-S/forward-S in the northern/southern hemisphere. It
is of interest whether individual features have one-to-one correspondence in
terms of helicity at different atmospheric heights. We use UBF \Halpha images
from the Dunn Solar Telescope (DST) and other \Halpha data from Udaipur Solar
Observatory and Big Bear Solar Observatory. Near-simultaneous vector
magnetograms from the DST are used to establish one-to-one correspondence of
helicity at photospheric and chromospheric heights. We plan to extend this
investigation with more data including coronal intensities.Comment: 5 pages, 1 figure, 1 table To appear in "Magnetic Coupling between
the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten,
Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg,
Berlin, 200
Interplanetary Consequences of a Large CME
We analyze a coronal mass ejection (CME) which resulted from an intense flare
in active region AR486 on November 4, 2003. The CME propagation and speed are
studied with interplanetary scintillation images, near-Earth space mission
data, and Ulysses measurements. Together, these diverse diagnostics suggest
that the internal magnetic energy of the CME determines its interplanetary
consequences.Comment: 5 pages, 9 figures, To appear in "Magnetic Coupling between the
Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten,
Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg,
Berlin, 200
Evaluation of a ground-based sky camera system for use in surface irradiance measurement
This paper describes the evaluation of a ground-based sky camera system for studying the effect of clouds on the level of the ambient ultraviolet radiation. The system has been developed for research in the characterization of the effect of clouds around the sun. It is the first sky camera system to be used for the assessment of cloud conditions in the vicinity of the sun, rather than a whole-sky assessment. The system features a sun-tracking sky camera with an integrated measurement of horizontally received radiation at the same location. The image-processing algorithm uses solar radiation readings to reduce reflections from the sun on the camera system being mistaken for cloud in the images. Cloud amount was estimated in an angular region of between 12.5°and 37.5° around the sun. The algorithm also estimates the amount of solar obstruction by cloud (sun not covered, partially or totally covered). The system was evaluated during September 1997 at Toowoomba, Australia (27.6°S latitude). Compared to manual assessment of 592 images, 76.5% were identified correctly by the algorithm for the degree of solar obstruction and 81.9% for cloud amount. The behavior of ultraviolet radiation levels with cloud conditions is discussed
Sun tracking solar energy collector
A parabolic reflector is supported so that it can track the sun. The support for this reflector comprises an azimuth frame supported on two wheels and a central pivotal point which are positioned in a substantially triangular configuration. On top of the azimuth frame, there is provided an elevation frame. The reflector rides on wheels captured within curved rails. The wheels of the azimuth frame are driven by an azimuth actuator. The reflector structure is counterbalanced about its elevation axis by a pendulum cable system which is driven by a motor. At the focal point of the parabolic reflector, a heat engine or receiver is mounted independently on the reflector. Suitable means are provided for moving the reflector about its two axes
The Waldmeier Effect in Sunspot Cycles
We discuss two aspects of the Waldmeier Effect, namely (1) the rise times of
sunspot cycles are anti-correlated to their strengths (WE1) and (2) the rates
of rise of the cycles are correlated to their strengths (WE2). From analysis of
four different data sets we conclude that both WE1 and WE2 exist in all the
data sets. We study these effects theoretically by introducing suitable
stochastic fluctuations in our regular solar dynamo model.Comment: Magnetic Coupling between the Interior and Atmosphere of the Sun;
Astrophysics and Space Science Proceeding
Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope
CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter at the
Swedish 1-m Solar Telescope, opens a new perspective in solar polarimetry. With
better spatial resolution (0.13") than Hinode in the Fe I 6302 A line and
similar polarimetric sensitivity reached through postprocessing, CRISP
complements the SP spectropolarimeter onboard Hinode. We present some of the
data which we obtained in our June 2008 campaign and preliminary results from
LTE inversions of a pore containing umbral dots.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
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