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Solar Radius
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Title: Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits
Authors: Marcelo Emilio, Jeff R. Kuhn, Rock I. Bush, Isabelle F. Scholl

The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8. Contact times between Mercury and the solar limb have been used since the 17th century to derive the Sun's size but this is the first time that high-quality imagery from space, above the Earth's atmosphere, has been available. Unlike other measurements this technique is largely independent of optical distortion. The true solar radius is still a matter of debate in the literature as measured differences of several tenths of an arcsecond (i.e., about 500 km) are apparent. This is due mainly to systematic errors from different instruments and observers since the claimed uncertainties for a single instrument are typically an order of magnitude smaller. From the MDI transit data we find the solar radius to be 960".12 0".09 (696,342 65 km). This value is consistent between the transits and consistent between different MDI focus settings after accounting for systematic effects.

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RE: The Sun
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Title: Direct Measurements of Magnetic Twist in the Solar Corona
Authors: A. Malanushenko, M. H. Yusuf, D. W. Longcope

In the present work we study evolution of magnetic helicity in the solar corona. We compare the rate of change of a quantity related to the magnetic helicity in the corona to the flux of magnetic helicity through the photosphere and find that the two rates are similar. This gives observational evidence that helicity flux across the photosphere is indeed what drives helicity changes in solar corona during emergence.
For the purposes of estimating coronal helicity we neither assume a strictly linear force-free field, nor attempt to construct a non-linear force-free field. For each coronal loop evident in Extreme Ultraviolet (EUV) we find a best-matching line of a linear force-free field and allow the twist parameter alpha to be different for each line. This method was introduced and its applicability was discussed in Malanushenko et. al. (2009).
The object of the study is emerging and rapidly rotating AR 9004 over about 80 hours. As a proxy for coronal helicity we use the quantity averaged over many reconstructed lines of magnetic field. We argue that it is approximately proportional to "flux-normalised" helicity H/Phi^2, where H is helicity and Phi is total enclosed magnetic flux of the active region. The time rate of change of such quantity in the corona is found to be about 0.021 rad/hr, which is compatible with the estimates for the same region obtained using other methods Longcope et. al. (2007), who estimated the flux of normalized helicity of about 0.016 rad/hr.

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Faint young sun paradox
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Our sun may have been much bigger and brighter long ago

Standard models predict that our sun was much dimmer in its youth, but devising a way to keep the early Earth from freezing over has not been easy for climate modellers. An alternative solution - currently being re-examined by a group of researchers - is to assume our sun started out a bit heftier (and therefore brighter) than expected.
Most stars tend to increase in luminosity as they get older. This is due to their cores becoming denser and thus hotter over time. Assuming our sun has followed this same trend, one can estimate that it was 30 percent fainter 4.5 billion years ago.

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RE: The Sun
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SDO Helps Measure Magnetic Fields on the Sun's Surface

A subset of data that helps map out the sun's magnetic fields was recently released from the Solar Dynamics Observatory (SDO). Observations that measure the strength and direction of magnetic fields on the solar surface -- known as vector magnetograms -- play a crucial role in understanding how those fields change over time and trigger giant eruptions off the surface of the sun such as solar flares and coronal mass ejections (CMEs).
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Title: Small-scale, Dynamic Bright Blobs in Solar Filaments and Active Regions
Authors: Y. Lin, O. Engvold, L.H.M. Rouppe van der Voort

High cadence high spatial resolution observations in H-alpha with the Swedish 1-m Solar Telescope on La Palma have revealed the existence of small-scale highly dynamic bright blobs. A fast wavelength tuning spectro-polarimeter provides spectral information of these structures. The blobs slide along thin magnetic threads at speeds in the range from 45 km/s to 111 km/s. The blobs have a slight elongated shape and their lengths increase by a factor of 3 from close to arcsec when they first appear till they disappear 1-2 min later. The brightest blobs show the highest speed. The widths of the H-alpha line emission of the blobs correspond to non-thermal velocities in the plasma less than 10 km/s which imply that they are not the result of shock driven heating. The dynamic character of the bright blobs is similar to what can be expected from an MHD fast mode pulse.

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Title: Interaction and Eruption of Two Filaments Observed by Hinode, SOHO, and STEREO
Authors: Y. Li, M. D. Ding

We investigate the interaction between two filaments and the subsequent filament eruption event observed from different view angles by Hinode, the Solar and Heliospheric Observatory (SOHO), and the Solar Terrestrial Relations Observatory (STEREO). In the event, the two filaments rose high, interacted with each other, and finally were ejected along two different paths. We measure the bulk-flow velocity using spectroscopic data. We find significant outflows at the speed of a few hundreds of km/s during the filament eruption, and also some downflows at a few tens of km/s at the edge of the eruption region in the late stage of the eruption. The erupting material was composed of plasmas with a wide temperature range of 10^4-10^6 K. These results shed light on the filament nature and the coronal dynamics.

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Title: On cyclic activity of the Sun and solar-type stars
Authors: E. A. Bruevich, G. S. Ivanov-Kholodnyj

The cyclicity of 33 solar-type stars that are similar to 11-year and to the quasi-biennial variations of solar radiation have studied. Our calculation were based on new simultaneous observations of the flux variations of the photospheric and chromospheric emissions of 33 solar-type stars and the Sun during the HK project that were conducted over the last 20 years. The method of Fast Fourier Transform was applied to these observed data. In addition to the known cyclic chromospheric emission variations of stars at the 11-year time scale, which were discovered at the Mount Wilson Observatory, we found a recurrences on the quasi-biennial time scale. The results of calculations of periods of the star's fluxes variations at the 11-year and quasi-biennial time scales are presented

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Sun storms 'to become more disruptive within decades'

Within decades, solar storms are likely to become more disruptive to planes and spacecraft, say researchers at Reading University.
The work, published in Geophysical Research Letters, predicts that once the Sun shifts towards an era of lower solar activity, more hazardous radiation will reach Earth.
The team says the Sun is currently at a grand solar maximum.

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Monster Waves Behind Sun's Coronal Heating Mystery?

Why the sun's atmosphere is so hot has foxed solar physicists for decades, and while it may not appear to have the glamour of the mysteries of dark matter, dark energy and the hunt for the Higgs boson, this solar mystery is at the very core of space weather prediction, impacting our everyday lives.
And today, it has been announced that we may be one step closer to understanding why the sun's atmosphere -- a.k.a. the corona -- is over 20 times hotter than the solar surface and how the solar wind is accelerated to hundreds of kilometres per second.

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The Sun lets loose an M9 flare



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Scientists Prove Existence of 'Magnetic Ropes' that Cause Solar Storms

George Mason University scientists discovered recently that a phenomenon called a giant magnetic rope is the cause of solar storms. Confirming the existence of this formation is a key first step in helping to mitigate the adverse effects that solar storm eruptions can have on satellite communications on Earth.
The discovery was made by associate professor Jie Zhang and his graduate student Xin Cheng using images from the NASA Solar Dynamic Observatory (SDO) spacecraft.
Though the magnetic rope was believed to be the cause of these giant eruptions on the Sun, scientists had previously not been able to prove this phenomenon existed because of how quickly the rope moves.
However, through close examination of images taken by the Atmospheric Imaging Assembly (AIA) telescope on board the SDO, Zhang was able to pinpoint an area of the sun where a magnetic rope was forming. The AIA telescope suite is able to capture images of the Sun every 10 seconds, 24 hours a day. This unprecedented cadence in time helped the discovery.

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