The SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 373

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                      The SOCIETY for POPULAR ASTRONOMY
              Electronic News Bulletin No. 373  2014 April 6

    Here is the latest round-up of news from the Society for Popular
    Astronomy.  The SPA is Britain's liveliest astronomical society, with
    members all over the world.  We accept subscription payments online
    at our secure site and can take credit and debit cards.  You can join
    or renew via a secure server or just see how much we have to offer by
    visiting  http://www.popastro.com/

    HUBBLE SEES MARS-BOUND COMET SPROUT MULTIPLE JETS
    Space Telescope Science Institute
    Comet C/2013 A1 Siding Spring is plunging towards the Sun along a
    roughly 1-million-year orbit.  The comet, discovered in 2013, was
    within the radius of Jupiter's orbit when the Hubble Space Telescope
    photographed it on 2014 March 11.  Hubble resolves two jets of dust
    coming from the solid icy nucleus.  They seem to be persistent jets,
    as they were first seen in Hubble pictures taken on 2013 October 29.
    They should allow astronomers to determine the direction of the
    nucleus's pole, and hence its rotation axis.  The comet will come to
    perihelion on October 25, at a distance of 130 million miles, well
    outside the Earth's orbit.
    On its inbound passage, Comet Siding Spring will pass within 84,000
    miles of Mars on October 19, which is less than half the Moon's
    distance from the Earth.  The comet is not expected to become bright
    enough to be seen with the naked eye.  A Hubble observation made on
    January 21 this year caught the comet as the Earth was crossing its
    orbital plane.  That special geometry facilitates the determination of
    the speed of the dust coming off the nucleus — information that helps
    astronomers to determine how likely and how much the dust grains in
    the coma will impact Mars and Mars spacecraft.

    FIRST RING SYSTEM AROUND ASTEROID
    ESO
    Observations at many sites in South America have made the surprising
    discovery that the remote asteroid Chariklo is surrounded by two dense
    and narrow rings.  It is by far the smallest object found to have
    rings, and only the fifth body in the Solar System, after Jupiter,
    Saturn, Uranus and Neptune.  The rings may be the result of a
    collision that created a disc of debris.  The rings of Saturn are
    among the most spectacular sights in the sky, and less prominent rings
    have also been found around the other giant planets.  Despite many
    careful searches, no rings had been found around smaller objects in
    the Solar System until now, when observations of the distant minor
    planet (10199) Chariklo as it passed in front of a star have shown
    that it has two fine rings.
    Chariklo is the largest member of a class known as the Centaurs and it
    orbits between Saturn and Uranus.  Predictions had shown that it would
    pass in front of the star UCAC4 248-108672 on 2013 June 3, as seen
    from South America.  Astronomers using telescopes at seven different
    locations, including the 1.54-m Danish and TRAPPIST telescopes at La
    Silla, and saw the star apparently vanish for a few seconds as its
    light was occulted by Chariklo.  But they found much more than they
    were expecting.  A few seconds before, and again a few seconds after,
    the main occultation there were two further very short dips in the
    star's apparent brightness.  By comparing what was seen from different
    sites the team could reconstruct not only the shape and size of the
    object itself but also the shape, width, orientation and other
    properties of the newly discovered rings.  The team found that the
    ring system consists of two sharply confined rings only seven and
    three kilometres wide, separated by a clear gap of nine kilometres;
    Chariklo itself has a diameter of about 250 km.  Astronomers think
    that such rings are likely to be formed from debris left over after a
    collision.  It must be confined into the two narrow rings by small
    unseen satellites.  So, as well as the rings, it is likely that
    Chariklo has at least one small moon still waiting to be discovered.

    SURFACE OF TITAN SEA IS MIRROR SMOOTH
    Stanford University
    New radar measurements of a large sea on Titan offer insights into
    the weather patterns and landscape composition of the Saturnian moon.
    The measurements, made in 2013 by the Cassini spacecraft, reveal that
    the surface of Ligeia Mare, Titan's second-largest sea, possesses a
    mirror-like smoothness, possibly owing to a lack of wind.  Titan has
    a dense, planet-like atmosphere and large seas made of methane and
    ethane.  Measuring roughly 420 by 350 km, Ligeia Mare is larger than
    Lake Superior.  Titan is the best analogue that we have in the Solar
    System to a body like the Earth, because it is the only other body
    that we know of that has a complex cycle of solid, liquid, and gas
    constituents.  Titan's thick cloud cover makes it difficult for
    Cassini to obtain clear optical images of its surface, so scientists
    must rely on radar, which can see through the clouds, instead of a
    camera.
    To paint a radar picture of Ligeia Mare, Cassini bounced radio waves
    off the sea's surface and then analyzed the echo.  The strength of the
    reflected signal indicated how much wave action was happening on the
    sea.  The surface of Ligeia Mare seemed eerily still.  One possible
    explanation for the sea's calmness is that no winds happened to be
    blowing across that region of the moon when Cassini made its fly-by.
    Another possibility is that a thin layer of some material is
    suppressing wave action.  For example, on Earth, a layer of oil on top
    of a sea suppresses small waves.  Cassini also measured microwave
    radiation emitted by the materials that make up Titan's surface.
    Those measurements confirmed previous findings that the land around
    Ligeia Mare is composed of solid organic material, probably the same
    methane and ethane that make up the sea.  Like water on the Earth,
    methane on Titan can exist as a solid, a liquid, and a gas all at
    once.

    SPA SOLAR SECTION  2014 February
    By Geoff Elston, SPA Solar Section Director
    Solar Cycle 24 continues to fascinate and confound us.  It was thought
    that we had reached sunspot maximum in late 2011/early 2012.  The
    expectation was that we would see a second slightly lower peak in
    sunspots in 2013-14 (double peaks were seen in Cycles 22 and 23),
    before we entered the gradual decline of sunspot activity thereafter.
    That has not happened.  While the intensity of Cycle 24 is not as high
    as that of the previous two, the length of sustained activity has been
    longer.  The increase in sunspot activity in late 2013 and early 2014
    (mostly in the southern hemisphere of the Sun) seems to have presented
    us with a higher second peak in sunspot activity than the first peak
    two years ago.  We may only just have reached the maximum of Cycle 24,
    some five years after sunspot minimum in 2008.
    Rotation Nos. 2146/2147: Despite the varied and sometimes intense
    level of sunspot activity in early- to mid-February, overall there was
    a decrease in sunspot activity this month.  The Mean Daily Frequency
    went down to 4.42 and the Relative Sunspot Number to 75.25.
    WHITE-LIGHT ACTIVITY: February began with a bang in the form of Active
    Region 1967 (AR 1967) which was the second return of the sunspot group
    designated AR 1944 on the previous appearance.  AR 1967 was visually
    stunning through a telescope with a full-aperture solar filter and was
    immediately apparent to the (protected) naked eye.  On the 1st AR 1967
    was seen midway between the E limb and the Central Meridian (CM) with
    AR 1968 (another very complex sunspot group) just to the north.
    AR 1967 is said to have spanned more than Jupiter's diameter at its
    maximum extent.  Both 1967 and 1968 were intensely flare-active, so
    much so that they produced several Coronal Mass Ejections (CMEs),
    leading to the appearance of aurora on the 7th.  Both groups were
    nearing the W limb by about the 8th to 10th, showing many bright
    faculae because of limb-darkening.
    From the 10th onwards three active groups remained visible — AR 1975,
    nearing the W limb, and 1974 and 1976, moving away from the E limb.
    1974 developed as it passed over the CM and headed westwards, showing
    a highly fractured structure of numerous spots and pores following a
    developing larger pair of leader sunspots.  As 1974 neared the W limb
    there was still much to see in 1976, and in AR 1977 and 1980 as they
    lay stretched out just west of the CM.  In the last week of February,
    another complex group appeared over the E limb, designated AR 1981,
    1982 and 1984.  That trail of spots was almost over the CM on the
    22nd.  On the 25th the former AR 1967 reappeared on the E limb and was
    re-designated AR 1990 for this appearance.  It produced a massively
    strong flare (one of the strongest in the current cycle) leading to a
    large and a bright CME at the solar limb.  The result was an extensive
    auroral display that was seen over many parts of the UK and Europe as
    night fell on the 28th.
    H-ALPHA ACTIVITY
    In the first few days of February, both AR 1967 and 1968 showed a
    significant number of plages around them.  1967 also displayed some
    small filaments, and a dark long convoluted filament was seen on the
    SW quarter of the solar disc with some fine prominences along the E
    limb.  As AR 1971 came over the E limb on or around the 5th, filaments
    were also visible, noticeably a long snake-like one extending away
    from the limb to the south of 1971.  Many prominences were seen
    around the limb.  On the 6th the whole disc was busy with filaments
    and plages, and the NE limb was quite heavily populated with
    prominences.  By the 7th AR 1967 and 1968 were nearing the W limb and
    still putting on a fine display of filaments and plages.  Elsewhere
    many filaments were on view, but notably there was a long dark
    convoluted one across the SE quarter.  On the S limb a fine example
    of a 'smoking chimney' prominence was visible, as were two typical
    'hedgerow'-type prominences on the W and SW limbs.
    Filaments were still very evident on the 9th.  On the 10th, with
    AR 1973, 1974, 1975 and 1976 visible right across the disc, many
    plages and filaments were also apparent.  The N, S and SW limbs showed
    the brightest prominences.  On the 13th the area surrounding AR 1974
    and towards the SW limb was busy with plages and filaments.  The same
    was true of 1976 and 1977 which showed much filament and plage
    activity.  The SE and SW limbs displayed some fine low-lying and
    arch-type prominences.  A long line of sunspots (comprising AR 1974,
    1976, 1980 and 1977) with plages and filaments was on view on the 16th.
    In the last week of the month, AR 1981 and 1982 were accompanied by
    several filaments spread across the southern half of the disc.
    Meanwhile, AR 1987 and 1989 brought their own plage and filament
    activity with them as they emerged from the E limb.  A magnificent
    large prominence was visible on the E limb on the 25th, just south
    from the returned sunspot group AR 1990.  The SW limb was also active
    with prominences too.  A broad dark filament was visible to the south
    of AR 1987, which was by then over the CM, on the 27th. The filament
    seemed to have broadened and become less well-defined by the following
    day.  The disc was still populated with plages and smaller filaments
    associated with the numerous sunspot groups visible on that day.
    MDF (P): 8.29
    I am very grateful to all our members who, despite often cloudy
    weather, keep their solar images and drawings coming in to me on an
    almost daily basis.  That really helps me to know what is happening on
    the Sun when I cannot observe because of the weather or other
    commitments.  The images are also highly valuable for these monthly
    reports.

    Bulletin compiled by Clive Down
    (c) 2014 the Society for Popular Astronomy
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