[Report published in Astronomy & Geophysics 42, 3.35-3.36 (2001)]
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by Andy Breen (University of Wales, Aberystwyth)
Published in
Astronomy & Geophysics 42, 3.35-3.36 (2001)
The RAS discussion meeting on "Solar activity and terrestrial
consequences", organised by Andy Breen (Aberystwyth) and Mike Hapgood
(Rutherford-Appleton Laboratory) was held at Burlington House on 9th March
2001.
Staying with eruptive events in the solar atmosphere, but moving from large
to small scales, Robertus Erdelyi (Sheffield), compared the results of 2D
MHD simulations of micro-scale reconnection with measurements from
instruments on the SOHO and TRACE spacecraft and observations in hydrogen
alpha from Tenerife and La Palma. The results confirmed that micro-scale or
even nano-scale flaring resulting from small-scale magnetic reconnection
could be an important source of mass flux and momentum for the solar wind,
and that these processes are important in the evolution of the corona.
Richard Harrison (Rutherford-Appleton Laboratory) then discussed the
relationship between coronal dimming and coronal mass ejections, reporting
results from a long-running SOHO multi-instrument campaign observing
extreme ultra-violet signatures of CME onsets. Some 15 such events have
been identified in this way, and detailed analyses of 7 of these were
presented. All showed clear dimming of coronal emission lines, which served
to identify the source region of the eruption and to estimate the amount of
mass removed from the low corona. These mass estimates were very similar to
the mass of material ejected through the outer corona as a CME. An
intriguing fact is that in many cases the low coronal dimming appears to
start before the CME onset.
Coronal mass ejections also formed the subject of the talk given by Rainer
Schwenn (Max-Planck Institut für Aeronomie), who discussed their
initiation, development and interplanetary propagation. Mass ejections,
though notoriously variable in mass, velocity and morphology, are readily
recognisable in coronagraph measurements. Importantly, a CME is an ejection
of mass through the corona not an ejection of coronal mass.
When considering the terrestrial consequences of CMEs, it is important to
recognise that small, unspectacular events can be highly geoeffective. The
important point is the rotation of the magnetic field in the CME - a
southward turn in the z-component of the interplanetary field is needed for
a geoeffective event. The topology and orientation of filaments are the
same before and after the eruption and in any ejected interplanetary
magnetic cloud, suggesting that the likely geoeffectiveness of a CME can
be estimated by considering the polarity of the region of origin.
One of the outstanding problems in understanding interplanetary CMEs is how
the 3-part structure of CMEs in the corona (leading edge, void and core)
evolves into the 2-part structure (shock and ejecta) seen in interplanetary
space. In-situ observations have shown remnants of cold prominence material
in the ejecta and hot material near the shock, but no evidence of a shock
has been seen in coronal observations of even the fastest CMEs. It appears
likely that the shock develops and starts to slow at distances somewhere
between 30 and 60 R.
The next talk, given by Bob Bentley (Mullard Space Science Laboratory,
UCL), concerned signatures used for CME prediction. To do this it is
necessary to predict both the CME onset and the likely Bz of the event. A
continued programme of observation, giving continuous coverage with good
cadence in many wavelengths is required to reconcile the predictions of
different models of CME initiation and development. Prediction of
geoeffective CMEs is still some way off, but this should improve as new
instruments become available.
The last talk of the morning session was given by Peter Cargill (Imperial
College), who reviewed the status of large-scale modelling of CMEs in the
solar wind and their interaction with the magnetosphere. He showed that
existing MHD models already do a quite reasonable job in the interplanetary
medium, but that more physics needs to be included, particularly in the
ring current and the interface with the ionosphere, thermosphere and lower
layers of the neutral atmosphere. He further suggested that the ultimate
aim of the modelling community should be a coupled solar corona to neutral
earth atmosphere model, which could be achievable by the UK community in
about 10 years.
Next, Richard Balthazor (Sheffield) discussed recent work on thermospheric
heating and showed that factors such as fine-scale Joule heating, transonic
effects of viscosity as neutrals approach Mach 1 and shock heating if they
exceed this speed, as well as plasma wave heating can be important. Coupled
ionosphere-thermosphere model results showed that high-speed ion flows
could drive neutrals to supersonic velocities, producing around 1000 K of
thermospheric heating at F-region altitudes.
Nigel Fox (National Physical Laboratory), then spoke concerning measurement
of the total radiative output of the Sun, the currently accepted record of
which is based on an amalgamation of measurements from different
space-based radiometers. An accuracy of around 0.1% is claimed for the
individual instruments but the results may differ by as much as 0.5%, which
is much greater than the variation over a solar cycle. A solar variability
mission under development at the National Physical Laboratory uses new
technologies to reduce uncertainties in irradiance by a factor of nearly
100. This improvement would aid climate modelling and aid studies of the
physical processes occurring within the Sun.
Gordon Wrenn (T.S. Space Systems) presented results of a study of the
effects on spacecraft of electrons accelerated to relativistic velocities
by solar activity. "Phantom commands" occurring in spacecraft systems are
often caused by changes in the space environment, with electrons in the MeV
energy range penetrating the spacecraft skin. Results presented showed a
remarkable degree of correlation between phantom commands and GOES
observations of electrons with energies of more than 2 MeV. There is a
clear association between high energy electrons and persistent coronal
holes as well as with solar proton events and CMEs. This rare demonstration
of environmental cause with engineering effect clearly establishes and
important new aspect of "space weather".
Recent studies suggest that the proportion of flux emerging in the corona
which contributes to the solar wind is governed by the relative areas of
faculae and sunspots on the Sun. The faculae/sunspot area ratio also
governs solar irradiance, and models of solar total irradiance all show a
small increase over the 20th century. If this change in solar irradiance is
used as an input to the Hadley centre climate model then an amplifying
factor for solar effects of 2.5 is required to match the observed climate
record. Increased shielding of the Earth from cosmic rays by the enhanced
coronal flux, resulting in fewer ionisation nuclei in the atmosphere and a
lower rate of cloud formation could provide this factor. It is important to
note that including the solar-driven component of climate change increases
the importance of the anthropogenic component over the second half of the
century.
The meeting closed with thanks to the speakers and poster viewing.
Programme:
Summary:
Ground and space-based observations have shown that the Sun is far from
being a stable, unchanging object. Rather, we live within the outer atmosphere
of a variable star. Taking place just after solar maximum, this meeting draws
together the results of recent research into the Sun-Earth energy flow and
the terrestrial consequences of solar variability.
Other comments: Recent discoveries from space- and ground-based
instruments, from sophisticated computer modelling and from painstaking
analysis of long-period datasets have led to new insights into the physics
of the magnetically coupled Sun-Earth system. These, in turn, have led to
a new appreciation of the effects of solar activity on the Earth and
on man-made systems. This meeting, taking place just after the peak in the
11-year solar activity cycle, draws together the results of recent studies
of the active Sun, both on short time-scales (solar flares, coronal mass
ejections and their effects on geospace) and over longer periods (11-year
and longer solar variations and their terrestrial consequences) in a
series of invited and contributed talks and posters. The speakers include
many of the leading figures in the field, of solar-terrestrial physics as
well as Ph.D. students and younger researchers, presenting results that
trace the effects of solar activity from their origins on the Sun, through
interplanetary space and their interactions with the Earth and man-made
systems.
The highlights of the meeting include reviews of the most recent
investigations into the link between solar flares and coronal mass
ejections (Fletcher, Harrison), the propagation of these events
though interplanetary space (Schwenn) and their interaction with the
magnetic field and upper atmosphere of the Earth (Cargill, Lanchester,
Aylward and Millward), as well as the effects of solar activity on
man-made systems (Thompson, Wrenn) and climate (Lockwood). The
results are placed in their historical context by the talk by Henry
Rishbeth.
The last talk of the meeting is by Mike Lockwood and reviews the most
recent developments in the very important investigation of the effects of
long-period solar changes on climate and the implications for interpreting
climate change. This talk is therefore of interest to a wide
audience.
09:30 Registration, coffee and poster viewing
Morning Session
Chair: Dr M. Hapgood (Rutherford-Appleton Laboratory)
09:55 Andy Breen (University of Wales, Aberystwyth):
Introduction and announcements.
10:00 Lyndsey Fletcher (Glasgow University):
"Solar flares and their relationship to coronal mass ejections"
11:10 Bob Bentley (Mullard Space Science Laboratory,
University College London):
"Signatures used for CME prediction"
10:20 Robert von Fay-Siebenburgen (Sheffield University):
"Nanoscale eruptive events"
10:30 Richard Harrison (Space Science Division, Rutherford-Appleton
Laboratory):
"Coronal dimming and coronal mass ejection onsets"
10:40 Rainer Schwenn (Max-Planck Institut für Aeronomie,
Katlenburg-Lindau, Germany):
"Coronal mass ejection initiation, development and interplanetary propagation"
11:10 Alexi Glover (Mullard Space Science Laboratory, University
College London):
"Sigmoidal Solar Features and their Implications for coronal mass
ejection prediction"
11:20 Henry Rishbeth (Southampton University):
"The centenary of solar-terrestrial physics?"
11:40 Mark Lester (University of Leicester) :
"On the Impact in the Ionosphere of Solar Variability:
SuperDARN observations"
12:00 Peter Cargill (Imperial College of Science, Technology and
Medicine, London):
"Solar wind - magnetosphere coupling associated with
Interplanetary CMEs: Lessons from global modelling"
12:30 LUNCH AND POSTER VIEWING
Afternoon Session
Chair: Dr W.P. Wilkinson (University of Brighton)
13:20 Betty Lanchester (Southampton University):
"Magnetosphere-Ionosphere coupling under disturbed conditions:
interpreting auroral signatures from above and below"
13:50 R. Balthazor (Sheffield University):
"Magnetosphere-Ionosphere-Thermosphere coupling:
understanding the building blocks in thermospheric reservoir heating"
14:00 Alan Aylward & George Millward (University College London):
"Stirring up the thermosphere: ionosphere thermosphere coupling"
14:30 Nigel Fox (National Physical Laboratory):
"Quantifying solar irradiance variability"
14:40 Alan Thomson (British Geological Survey):
"Geomagnetic Induced Currents in Power Grids of Northern Europe:
Terrestrial Consequences of Magnetic Storms in 2000"
14:50 Gordon Wrenn (T.S. Space Systems):
"Relativistic Electrons, 1989-2000: a changing threat to
spacecraft in the magnetosphere"
15:00 Mike Lockwood (Rutherford-Appleton Laboratory):
"Long term variations in the coronal magnetic field and its
implications for mankind's environment and systems"
15:30 Tea At Savile Row followed by the A&G (Ordinary) Meeting
9:30 Registration: No charge. Coffee will be provided in the Lower
library of the Geological Society
A simple lunch will be available for purchase in the Lower Library
of the Geological Society
15:30 Tea will be provided in the Scientific Societies Lecture Theatre,
Savile Row for those attending the A&G (Ordinary) Meeting at 16.00
A Drinks Party will be held in the Coffee Room of the Scientific
Society's Lecture Theatre 18.00 to 19.00 cost just £1.00 per head.
Posters:
"Solar energetic particles and radiation levels experienced by commercial
flights" (provisional title)
Bob Bentley (Mullard Space Science Laboratory, University College London)
"Sigmoidal Solar Features and their Implications for coronal mass ejection
prediction"
Alex Glover (Mullard Space Science Laboratory, University College London)
"Interplanetary propagation of coronal mass ejections: case studies"
(provisional title)
Alison Canals (University of Wales, Aberystwyth)
"How to avoid effects of space weather on satellites - a Tsunami initiative"
Norma B. Crosby, Andrew J. Coates (University College London), Richard B. Horne
and Mervyn P. Freeman (British Antarctic Survey, Cambridge)
"Scintillation intensity signatures of coronal mass ejections"
(provisional title)
Richard Fallows (University of Wales, Aberystwyth)
"Trajectories of prominence eruptions and their association with the coronal
activities"
K. Hori and L. Culhane (Mullard Space Science Laboratory, University College
London)
"The European Science Foundation's Scientific Network SPECIAL, Space
Processes and Electrical Changes Influencing Atmospheric Layers"
Michael Rycroft (ISU Strasbourg)
"Upgrading EISCAT - proposals for improving the effectiveness of EISCAT
as a solar-terrestrial observatory"
G. Wannberg (EISCAT Scientific Association, Kiruna) and Andy Breen
(University of Wales, Aberystwyth)
RAS G/MIST discussion meeting, 9 March 2001
Living with an Active Star: Solar cycle changes, eruptive events
and terrestrial consequencesMeeting Report
Flares and mass ejections
Mike Hapgood chaired the morning session, which opened with a review of
solar flares and their relationship to coronal mass ejections (CMEs) by
Lyndsay Fletcher (Glasgow). The phenomena both appear to have their origin
in restructuring of the coronal magnetic field, but their relationship is
far from clear. Furthermore, the flare volume is much smaller than that of
the CME, CMEs and flares can originate at very different latitudes, there
is no clear relationship in time between flares and CMEs and the standard
2-dimensional model of flares suggests spatial and timing relationships
which are not seen in data. It is therefore not useful to think of a flare
causing a CME or vice versa, but rather both types of event are
consequences of changes in the configuration of the magnetic field in the
corona. The magnetic field configuration also determines the relationship
between the two phenomena - a flare that involves flux transfer rather than
large-scale field opening may not be associated with a CME.
Sun-Earth connections
Henry Rishbeth (Southampton), provided an historical context for
discussions of the Sun-Earth system by presenting a review of the
beginnings of solar-terrestrial physics The subject may be said to
originate in 1900, with letters published by Lodge and Fitzgerald,
suggesting that electron streams from the Sun were the cause of magnetic
storms and aurora. Fitzgerald also speculated that the Earth has a
"magnetic tail". Marconi's transatlantic radio transmission in 1901 was
followed by Lodge's suggestion of an "ionosphere" produced by solar
radiation. From this fascinating discussion of the beginnings of our field,
the meeting came up to date with a talk by Mark Lester (Leicester) on the
impact on the ionosphere of solar variability, as seen in SuperDARN
observations. The Super Dual Auroral Radar Network (SuperDARN) can be used
to monitor the high-latitude ionospheric convection pattern on a routine
basis. These data can be used to understand the coupling between the solar
wind, magnetosphere and ionosphere and as inputs for models of this coupled
system. The propagation characteristics on the radar beams can also be used
to investigate the effects of large solar events on the ionosphere, e.g.
the black-out of all northern hemisphere SuperDARN radars caused by the
Bastille Day storm in July 2000.
Interpreting the aurora
The afternoon session, chaired by William Wilkinson (Brighton), was opened
with a talk by Betty Lanchester (Southampton), on interpreting the aurora
from above and below. Detectors on spacecraft now provide superb views of
the aurora from space in wavelengths that separate the emissions resulting
from electron and ion precipitation. The unique signature of proton
precipitation is the Doppler-shifted emission from hydrogen atoms, formed
by charge exchange with atmospheric molecules. Observations made with the
Southampton/UCL imaging spectrograph on Svalbard have provided higher
spectral resolution of the hydrogen Balmer beta line than any previous
measurements. These measurements, combined with Lyman-alpha data from the
IMAGE satellite and particle data from polar orbiting satellites, make it
possible to establish the source and energy of the precipitating protons
and their relation to the electron population.
Modelling the upper atmosphere
The theme of ionosphere-thermosphere coupling continued in the next talk,
given by Alan Aylward. One framework that is being increasingly used to
interpret observations of the "receiving end" of the solar-terrestrial
system are 3-dimensional numerical models, represented in the UK by CTIP
(the Coupled Thermosphere Ionosphere Plasmasphere). This model is already
capable of accurately simulating long-term variability such as solar-cycle
or seasonal effects, and in most circumstances the large-scale
thermospheric and ionospheric consequences of geomagnetic activity. The
development is now more towards understanding the coupling to regions above
and below. The model has been extended to include the middle atmosphere. If
the ultimate goal is to model solar-terrestrial interactions from the
surface of the Sun to the surface of the Earth, this type of model will be
the crucial "anchor" at the terrestrial end, though there is still a lot to
be done to couple it to models of the lower stratosphere and troposphere
and to models of the magnetosphere.
Magnetic storms
The meeting then moved on to consider the effects of solar variability on
technological systems, with a talk given by Alan Thompson (British
Geological Survey). Magnetic disturbances can have severe consequences for
power transmission systems, and the talk reviewed the impact of two
magnetic storms in 2000 on power grids in Scotland and Finland. An example
of monitoring equipment installed in the Scottish grid was discussed, and
animations of the changes in measured induced current and the estimated
geoelectric current were shown for northern Europe during the storms of 6th
April and 15th July. The rapid time variations and spatial complexity of
the disturbances were very apparent in these results.
Space climate change
The final talk of the meeting was given by Mike Lockwood
(Rutherford-Appleton Laboratory) and concerned the recent discovery of long
term variations in the coronal magnetic field the implications of this
variation for mankind's environment and systems. The aa geomagnetic index
provides a continuous record back to 1868, while sunspot number records and
the number of nights when aurora were seen in southern Finland cover an
even longer period, and all of these measures of magnetic activity have
increased by a factor of about 2 over the last 100 years. Calculated values
of aa derived from solar wind velocities over the whole period that
interplanetary measurements are available showed a very high order of
agreement with the actual aa index, with a correlation coefficient of
95-97%. There is a similarly high order of agreement between coronal source
fluxes calculated from solar wind measurements and aa. This good agreement
is maintained out of the ecliptic when source fluxes estimated from aa and
determined from Ulysses measurements of the polar solar wind are compared,
showing that aa provides a reliable long-term measure of the coronal source
flux. The increase in aa seen over the last 100 years thus indicates that
the coronal source flux has doubled since 1900, producing a decrease in
cosmic ray flux, seen both in direct cosmic ray measurements and Beryllium
10 levels in ice cores. The ice core record shows that the coronal source
flux was about 25% of its present value during the 17th century "little ice
age", when sunspot numbers were very low, suggesting a link between coronal
flux and climate.
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