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Ordinary Meeting, 2009 January 28
held
at the Royal Astronomical Society, Burlington House, Piccadilly, London
SE1
Roger
Pickard, President
Ron
Johnson, Hazel Collett and Nick James,
Secretaries
The President opened the fourth meeting of the
119th Session and invited Mr Tom Boles to read the minutes of the
previous meeting on the behalf of the Meetings Secretary. These were
approved by the audience and duly signed. It was announced that 25
new members were proposed for election, and those 8 new members who
had been proposed at the previous meeting were approved by the
audience and declared duly elected. Mr Nick James, Papers Secretary,
reported that two papers had been approved for publication in the
Journal:
[check these]
CCD UVB-Photometry of Three Southern Galactic
Open Clusters.... , by David Boyd et al.
The Comets of 1999, by Jonathan Shanklin
The President announced that the next Ordinary
Meeting would be held on Wednesday March 31 at the present venue.
Before then, the Association would be holding a Back to Basics
workshop in Canterbury on January 31. The President then introduced
the evening's first speaker, Dr Nick Achilleos of the Atmospheric
Physics research group at Imperial College, London.
Rotation, Magnetism and Mass Loss
at Saturn
Dr Achilleos opened his talk with an overview of
Saturn's magnetosphere, which he explained was the region of space
around the planet where its magnetic field was strong enough to
disrupt the flow of the solar wind; he added that in practice, this
region extended out to around 20 times the planet's radius. He
described the morphology of this region, explaining that it had a
sharp boundary on Saturn's sunward side, termed a bow shock,
where the outward streaming flow of the solar wind impacted upon it.
He explained that the exact position of this shock front changed over
time: at times of intense solar activity, Saturn's magnetosphere
tended to become a little compressed and so the shock front moved
inwards, but at times when the solar wind was weaker, its
magnetosphere tended to expand outwards. By contrast, he explained
that its boundary on the side of the planet facing away from the Sun
was rather more weakly defined, because here there was much less
pressure being exerted on it from the solar wind. Consequently, the
magnetosphere stretched out into a long and variable comet-like tail
called a magnetotail, which gradually tapered away as it moved
away from the planet. As the magnetotail grew narrower and narrower,
the solar wind particles surrounding it gradually reformed into a
steady flow having passed around the planet.
The speaker added that although it was not
generally possible for solar wind particles to penetrate into
Saturn's magnetosphere – the combination of the electrical charge
of the particles and the magnetic field of the planet made this
impossible – there were two points on the magnetosphere's
boundary immediately above the planet's north and south poles where
the planet's magnetic field was relatively weak, and where charged
solar wind particles could penetrate it; these were termed cusps.
The speaker explained that it was the flow of particles through these
'holes' which gave rise to aurorae on Saturn as on the Earth, but
that the detailed behaviour of the magnetospheres of the two planets
differed considerably. In particular, the high density of ionised
particles in Saturn's magnetosphere suggested that it must have its
own internal source of such particles, meanwhile almost all of the
charged particles in the Earth's magnetosphere seemed to have
leaked in from the solar wind.
The speaker explained that recent data from the
Cassini spacecraft had identified eruptions of water from
vents on Saturn's icy moon Enceladus as the source of both the ice
particles in Saturn's E ring, and also of the ionised particles
which seemed to populate the plane of the ring and moon system.
Current models suggested that at present Enceladus was ejecting a
steady flow of around 10 to 100 kg of such material per second, but
that the total mass of plasma in the ring plane was remaining roughly
constant as a result of a corresponding leakage of material down
Saturn's magnetotail, whence it was lost into the solar wind.
The speaker then turned to describe Jupiter, which
he explained provided a good benchmark against which to compare the
other gas giant planets. He explained that the Jovian magnetosphere
also had its own source of plasma in the form of the planet's
innermost moon, Io. This moon was well known for its extremely active
volcanism, and, as in the case of Enceladus, it seemed to be venting
some material into space. Perhaps unsurprisingly on account of its
vigorous volcanism, Io was in fact very much more active as a source
of plasma than Enceladus, venting around 1000 kg of sulphurous
compounds to space per second. The speaker added that one of the most
compelling demonstrations of this leakage of material were
ultraviolet images of Jovian aurora taken by the Hubble Space
Telescope (HST), which showed that, in addition to the
solar-triggered auroral ovals around the poles, there was also a
bright auroral spot on Jupiter's surface at a significantly lower
latitude, which followed Io's orbit around the planet. It was
apparent from this that electrical currents were flowing between
Jupiter and Io through some conductive medium – undoubtedly a sea
of plasma.
The speaker closed his talk with a discussion of
his current research into the interaction of the magnetospheres of
Jupiter and Saturn with the plasma that they contained. He explained
that whilst the Cassini probe was returning a wealth of
valuable information, the complex feedback mechanisms and internal
regulation which seemed to be at work required some very detailed
mathematical modelling.
Following the applause, the President introduced
the evening's second speaker, Dr John Rogers, Director of the
Association's Jupiter Section.
Jupiter in 2007 and 2008
Dr Rogers explained that what he had to say would
neatly follow on from the previous talk: he would be providing an
overview of the visual appearance of Jupiter over the past couple of
years, as observed by amateurs. He began with an overview of the
circumstances of Jupiter's 2008 apparition, as seen by the casual
observer. He commented that since Jupiter had lain in Sagittarius
throughout the whole year, at a declination a little south of –20°,
this apparition had not been at all well placed for observation from
the UK. At opposition, Jupiter had reached a peak altitude of only
16° in the UK sky. However, naked-eye observers with a good southern
horizon had nonetheless been able to enjoy two conjunctions of
Jupiter with Venus which had neatly bracketed the apparition: the
first had occurred close to its beginning and the second had occurred
close to its end. On the occasion of the first, around 2008 February
1, the pair had passed within 35' of each other, forming a
brilliant conjunction. On the occasion of the second, around 2008
December 1, the pair had made a more distant conjunction at a
separation of around 2°, but the addition of a three-day-old waxing
crescent Moon to the pair's west had led to the event's being
dubbed the 'smiley face' conjunction. The speaker noted that
later on the evening of December 1, the Moon had gone on to occult
Venus.
Dr Rogers then turned to describe the fine detail
which amateur planetary imagers had been able to resolve with the use
of modern webcams, some good seeing, and the stacking of large
numbers of short exposure images. Among these imagers, the speaker
reserved especial mention for local expert Damian Peach: whilst it
had to be conceded that his images from Jupiter's 2008 apparition
were far from his best, and certainly could not compete with those
taken by the southern-hemisphere's amateurs, they were nonetheless
highly impressive given that he had been working exclusively from the
UK, against the problems of the planet's southerly declination at
this apparition. Dr Rogers explained that the remainder of his talk
would be about what could be learnt from such amateur images, with a
bias towards images taken by those southerly-based amateurs who had
had the best observing conditions.
The speaker identified three key milestones which
amateur imagers of Jupiter had passed for the first time in 2008, and
which he believed summarised their achievements rather well. The
first of these was that substantially more fine detail had been
recorded in the alternating latitudinal wind patterns across the
planet's disk than had been recorded at previous apparitions. He
explained that for many years it had been possible to observe and
measure the speeds of Jupiter's well-known currents and jet
streams. It had long been known that each of these had its own
individual speed which could be measured by observing the movements
of conspicuous spots in them. The resulting speeds were typically
found to be around 1° of longitude per month relative to System I.
The speaker went on to explain that there were much faster currents
called jet streams on the edges of the belts at the equator
which moved at characteristic speeds of several degrees of longitude
per day. These too had been observable by amateurs in some detail for
many years, but spacecraft observations had revealed, by looking at
the movements of subtle textures in the clouds, that there was a
smooth transition in rotation speed with latitude between these jet
streams and the neighbouring belts, which amateur observations had
not had sufficient angular or time resolution to record prior to
2008.
Dr Rogers explained that it was interesting that
the first amateur observations of this zonal wind profile, made by a
wide range of imagers and then collated and analysed by the JUPOS
project over the course of 2008, agreed well with older data taken in
situ by Voyager and Cassini. He explained that this
might not have been expected, given that the planet had undergone a
global upheaval in 2007, and that substantial changes had been
observed in the visual appearances of many of the belts at that time.
Specifically, he remarked that during the global upheaval, the
generation of many new vortices had been observed in several of the
belts. The South Equatorial Band (SEB) had been observed to become
very faint before undergoing a vigorous outburst of spots which
appeared to have revived it. The Equitorial Zone (EZ) had darkened
substantially from its previous pale colour, but had since cleared
again. And the North Temperate Belt (NTB), which had been absent for
several years prior to 2007, had undergone a vigorous outbreak of
spots and revived. In view of these planet-wide changes, the speaker
explained that the agreement of the jet speeds measured by amateurs
in 2008 with previous data – with the single exception of the NTB
whose speed did appear to have changed – was surprising, and
appeared to reveal something significant about the nature of the
global upheaval. It appeared to have been a purely meteorological
phenomenon which had not affected the underlying pattern of the
planet's jets and winds.
The speaker then turned to discuss in more detail
the revival of the NTB which he had earlier alluded to; he explained
that this was normally the fastest jet stream on the planet. He
explained that, in its normal state, a series of conspicuous vortices
ran along it, from whose motion the speed of the jet could easily be
estimated. These had been monitored extensively between the mid-1990s
and 2002, revealing that the jet had maintained a roughly constant
speed of 120-130 m/s relative to System III over that time period. In
2002, however, the vortices had disappeared, leaving only a few
flecks with which the jet speed could be measured. Observations since
that time had suggested that the jet stream had been steadily
increasing in speed. The speaker explained that in 1990, following a
similar disappearance of the vortices from the NTB, there had been an
outbreak of very fast-moving spots in the jet, moving at speeds
greatly in excess of those normally observed. There had in the past
few years been some anticipation that a similar outbreak might be
seen again.
He could now confirm that such an outbreak had
indeed been seen during the global upheaval of 2007: great white
plumes had been seen at an altitude much above the planet's normal
visible surface, becoming the brightest features on its disk at any
wavelength accessible to amateur observers, and moving at speeds of
up to 13°/day relative to System II. Dr Rogers identified the
observation of these plumes as the second milestone which amateur
imagers had passed in Jupiter's 2008 apparition, and reported that
recent professional attempts to model them suggested that they were
clouds of water vapour which had emerged from a water layer deep
beneath the planet's cloud level. These models went on to argue
that the high speed of these plumes could only be explained by the
presence of a permanent fast jet at this depth1, and that
in the coming months and years these plumes would disperse and the
vortices which normally populated the NTB would reform. To date,
amateur observations seemed to be in agreement with this analysis;
they had recently revealed the formation of a deep orange belt, the
formation of some subtle lozenges within it, and a slowing of the jet
speed. If the belt did indeed revert to its pre-2002 behaviour as was
now expected, this would represent the first continuous amateur
observations of the increase and subsequent decrease in the jet's
speed accompanying a complete cycle in its behaviour.
Finally, the speaker turned to discuss the merger
of two red anticyclonic ovals in the southern hemisphere – the
Great Red Spot (GRS) and a smaller red spot which had appeared at a
similar latitude in 2008 and which had been dubbed the Baby Red Spot
(BRS). The speaker added that the appearance of the BRS had been a
highly exceptional event: the appearance of new red spots on Jupiter
was very rare and that only a handful had ever been observed. Whilst
it was well established that such spots were anticyclonic
circulations, the exact reason for their red colour remained unclear.
Empirically, though, it appeared that red spots were those which were
unusually vigorous and deep-rooted as compared to other circulations.
The speaker added that the precise origin of the BRS was somewhat
unclear, since it had formed during Jupiter's solar conjunction of
2007-8 and its birth had been largely unobserved.
Returning to the story of the end of the BRS's
life, the speaker explained that on 2008 June 30 the spot had
approached the following edge of the GRS, and had subsequently been
squeezed through a vanishingly narrow current between the southern
edge of the GRS and the northern edge of Oval BA, which had happened
to lie due south of the GRS at this time. At the time, there had been
much speculation as to whether the BRS would be torn apart during
this passage, or whether its deep rooting would be sufficient to
ensure its survival. The speaker explained that in the event, it had
reappeared a few days later on the leading edge of the GRS, appearing
coherent but highly deformed. A white track, however, had also become
apparent, connecting at one end to the emergent BRS, and which could
be followed around a full 360° path around the rim of the GRS. The
speaker favoured the interpretation that this track represented
material which had been torn from the BRS by the GRS. Within a few
days, the main part of the BRS's circulation had apparently been
drawn back into the GRS and entirely swallowed, and so it appeared
that it had been entirely torn apart over the course of two
encounters with the GRS. The speaker noted that amateurs had never
before seen the merger of two red spots on Jupiter, and so these
observations were the third milestone which he identified in amateur
observations of Jupiter's 2008 apparition.
The speaker closed his talk by showing video
compilations of the amateur observations discussed earlier in his
talk. Following the applause, the President invited Mr David Arditti
to present Sky Notes.
The January Sky
Mr Arditti opened with a review of the
circumstances of the solar system bodies. He reported that the Sun
remained quiet and continued to show very little sunspot activity,
but that some prominences could still be observed on the solar limb
in Hα. Of the planets, he reported that Venus and Saturn would be
well placed for observation in the coming month, but that Mercury,
Mars and Jupiter were not currently observable. He added that whilst
it might still be possible to catch Uranus in evening twilight, its
present apparition was near its end.
Turning first to Venus, Mr Arditti explained that
it presently appeared as a brilliant object in the evening sky and
continued to brighten; it would attain a maximum brightness of mag.
–4.6 on February 19. He added that as it was presently getting
nearer to the Earth, its disk was growing in size – this explained
why it was getting brighter – but that the phase of the
illumination of its disk was also waning to a crescent. He remarked
that whilst the illuminated portion of Venus' disk showed
notoriously little detail to the visual observer on account of the
thick layers of cloud that enshrouded the planet's surface, some
planetary imagers had recently been experimenting in imaging the
night side of its disk. By using CCD cameras with near-infrared
filters operating at around 1000 nm, some had reported being able to
see some clear morphological features on this unilluminated portion
of its disk. These were often found to correlate well with surface
maps compiled from observations made by the Venus Express
spacecraft, suggesting that these observers had, for the first time,
successfully made amateur observations of the planet's surface. It
seemed likely that this infrared band happened to coincide with a
wavelength window where the Venusian clouds were at least partially
transparent, and that the surface of the planet was hot enough to
glow thermally at these wavelengths, though it had to be conceded
that the correspondence of the morphological features in the amateur
images with those present in Venus Express observations was
not perfect. It consequently seemed likely that at least some of
features apparent in the amateur images were actually of atmospheric
origin.
The speaker also commented upon the 200-year-old
speculation that the planet's night side could be observed to
faintly glow – a phenomenon termed the Ashen Light. This
effect was exceptionally difficult to observe due to the glare of the
nearby illuminated portion of the disk, to the extent that its
existence remained controversial to this day, but it had nonetheless
been reported very widely by visual observers. To date, no
photographic observations had ever been made of it, and the speaker
posed this as a challenge to the audience.
The speaker then turned to the Moon, which he
explained had passed New Moon two days previously on January 26,
would reach Full Moon on February 9, and would reach New Moon again
on February 25. On February 4 it would occult the Pleiades between
2h15 and 4h30 UT, though the circumstances were not favourable for
observation from the UK since the Moon would be at an altitude of
only 7° at the start of the occultation, and would set before its
conclusion. The brightest star to be occulted by the Moon in the
coming month would be ε-Gem (mag. 3), which would disappear behind
the Moon at 19h25 UT on February 6 and reappear at 20h30 UT; at this
time, the Moon would be 89%-sunlit and at an altitude of 50° in the
UK sky. Finally, the speaker briefly mentioned that in the early
morning sky of January 30, the Moon would pass within 1.5° of Venus,
forming a photogenic pair.
Mr Arditti reported that Saturn's rings
continued to present themselves in an edge-on orientation, as had
been discussed in previous Sky Notes talks. He added that they had
opened out a little in the past few months, from 1° inclination in
December to 2° presently, and that it was now just about possible to
resolve the Cassini division. They would, however, narrow again later
in its apparition and eventually close completely in August, though
this ring-plane crossing would itself not be observable owing to
Saturn's proximity to the Sun at the time; it would be only a few
weeks away from reaching conjunction on September 17.
Several minor planets were well placed for
observation from the UK at present, including 2 Pallas, at mag. 8.2
in Eridanus, and 27 Euterpe, at mag. 9.0 in Cancer, close to the
Praesepe Cluster (M44). Early in the evening, 4 Vesta could be found
at mag. 8.0 in Cetus, and later in the night, 1 Ceres, could be found
at mag 7.4 in Leo.
The principal comet of interest over the next few
weeks would be Comet C/2007 N3 (Lulin), which was currently a mag.
6-7 object in Libra. Though it did not rise until the pre-dawn hours
at present, it was rapidly moving westward and brightening; it was
forecast to pass close by Spica on February 16, and then pass M44 on
March 5-6. If it continued to brighten as expected, it was predicted
to reach around mag. 4 in late February, becoming an easy binocular
target in the UK sky.
The speaker identified ζ-Aur as a variable star
to watch over coming months; this was an eclipsing binary whose next
eclipse was forecast to begin early in March. The ingress and egress
of this eclipse would each take 1.5 days, during which time the
star's magnitude would change modestly but detectably from 4.0 to
3.7 in the V-band. Totality was expected to last for 37 days. The
speaker also went on to urge members to observe the Variable Star
Section's Variable Star of the Year, IP Pegasi, about which more
details could be found in the Association's Handbook for
2009.
Turning lastly to the deep sky, Mr Arditti
extended warm congratulations to Mr Ron Arbour upon the discovery of
his 20th supernova, 2009M, on January 20 in NGC 1028 in Cetus. He
then gave a brief review of the open clusters which were accessible
in the winter sky. At present, the three famous clusters in Auriga,
M36, M37 and M38 were all at high latitudes and relatively easy
targets in the UK evening sky. However, he added that M46, M47, M48
and M50 were more challenging and not often observed from the UK on
account of their southerly declinations; he closed his talk by
showing some sketches of these clusters by Dale Holt.
Following the applause, the President adjourned
the meeting until Wednesday March 31.
-----
Dominic Ford
References
1 Sénchez-Lavega et al.,
Nature, 451, 437 (2008)
©
2009 Dominic Ford / The British Astronomical Association.
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