Download A startling new Sun

Solar physics
A startling new Sun
3.10
here is only one star that we can observe from 5 microparsec – our Sun,
which is of enormous scientific interest in its own right, and also continues to
be of great importance for Solar System science and astronomy as a whole. Many
basic questions about the Sun have not yet been answered and many
fundamental space and cosmic plasma processes can be studied there in detail.
The satellite SOHO is bringing an unprecedented wealth of data about the Sun,
making it possible for solar physicists to address these questions in depth. The
results confirm some ideas but may also spring some surprises.
T
1367.0
1996
1997
1998
J F M A M J J A S O N D J F M A M J J A S O N D Jaaa
1366.5
solar irradiance (Wm2)
S
OHO, the Solar and Heliospheric Observatory, is a collaborative ESA/NASA mission with ESA as the senior partner, and
was launched on 2 December 1995. On 14 February 1996 it reached the first Lagrangian point
(L1), which is four times as far from us as the
Moon and 1% of the distance to the Sun where
the gravitational pulls of the Earth and the Sun
balance. SOHO is observing the Sun continuously in unprecedented detail with a suite of
instruments. They are giving the first comprehensive view of the Sun from the deep interior
through the different layers of the atmosphere
and out into the solar wind.
The interior of the Sun consists of a core,
where the energy is being generated, and a convection zone, where the convectively unstable
plasma is highly turbulent. The atmosphere
comprises three layers: a cool surface layer, the
photosphere, at a temperature of about
6000 K; the warmer and rarer chromosphere at
about 104 K; and the very much hotter corona
at 2–5 MK that stretches to the Earth and
beyond. In the photosphere you see sunspots,
dark regions where strong magnetic flux tubes
poke through the solar surface. The photosphere rotates more rapidly near the equator
than near the poles. In the chromosphere, you
see bright active regions around sunspots and
also prominences, which are strange flux tubes
(actually located up in the corona but at chromospheric temperatures) full of cold, dense
plasma. In addition, you sometimes see huge
eruptions of mass and magnetic flux associated
with erupting prominences that may be either
in active regions (and associated with solar
flares) or outside active regions. The corona
consists of magnetically open coronal holes
(along which the fast solar wind escapes) and
also magnetically closed coronal loops (which
contain the plasma).
The key unanswered puzzles are:
● What is the nature of the solar interior and
the dynamo that generates the Sun’s magnetic
field in a cyclic manner?
● How is the corona heated to 2–5 MK?
● What is the origin of magnetic eruptions
(coronal mass ejections)?
● How is the solar wind accelerated?
SOHO was designed to try and answer these
major unsolved problems in solar physics
which are of great importance for astrophysics
as a whole. It has a suite of 11 instrument packages (A&G 39/2 26). The interior is studied
indirectly by means of global oscillations measured in velocity and intensity by the VIRGO,
1366.0
1365.5
1365.0
1364.5
0
200
400
Days since 0 January 1996
600
1 The solar irradiance from VIRGO as a function of time (courtesy C Frölich).
GOLF and MDI instruments; the atmosphere is
investigated by EUV spectrometers, an imager
and coronagraphs called EIT, SUMER, CDS,
UVCS and LASCO; and the solar wind is sampled by in situ measurements with CELIAS,
COSTEP, ERNE and SWAN. The UK is playing
a large and active part in the mission, having a
principal investigator (R Harrison) for one and
co-investigators for most of the other instruments. The preliminary results are quite breathtaking (Solar Physics 170 1–205; 175 207–799)
and will clearly revolutionize our understanding of many aspects of solar physics. It is
impossible to review them all here, so I shall
just give a few examples.
The solar interior
The VIRGO instrument (Variability of solar IR
radiance and Gravity Oscillation) led by Claus
Fröhlich (Davos) aims to determine the characteristics of low-l p-modes (i.e. long-wavelength
global sound waves) from irradiance variations. It has found also by measuring the solar
irradiance as a function of time (figure 1) that
the “solar constant” is far from constant from
day to day. Also the irradiance level is beginning to increase with the start of the new
sunspot cycle.
GOLF (Global Oscillations at Low Frequency) led by Alan Gabriel (Paris) aims to deduce
the deep internal structure of the Sun from the
velocity variation of low-l p-modes and to try
and detect g-modes (gravity oscillations) for
the first time. It is an instrument that measures
Doppler shifts of sodium-D lines and has produced exceptionally noise-free p-mode spectra;
June 1998 Vol 39
Solar physics
from SOHO
Eric Priest summarizes results from the
satellite SOHO that are changing our
basic understanding of the Sun.
the Sun’s magnetic field. Furthermore, they
have ruled out the possibility of a rapidly rotating core and any significant solar-cycle variation of the oblateness.
Other MDI studies have revealed that the
surface of the Sun is covered with a “magnetic
carpet” of positive and negative magnetic fragments continually emerging, cancelling, merging and fragmenting in such a way that the
magnetic flux is replaced every 40 hours!
The transition region and low corona
2 A Moreton wave observed on a series of difference images from EIT, where light shows the addition
and dark the subtraction of plasma (courtesy D Moses).
the solar noise at low frequencies (where
g-modes are expected) is a factor of 10 smaller
than expected.
If we compare six months data for the spectrum from the ground with only two months
from VIRGO, the advantage in going to space
is clear: the noise is much lower and spurious
signals (sidebands) disappear because of the
uninterrupted viewing. A surprise from a
wavelet analysis is that, when you follow individual modes in time, you find that individual
p-modes wander in frequency.
The third helioseismology instrument on
SOHO is MDI (Michelson Doppler Imager) led
by Phil Scherrer (Stanford), which detects
modes of much higher l (smaller wavelength)
than VIRGO and GOLF. It does so by measuring the velocity at a million points on the solar
surface every minute.
Seismological inversion techniques have been
used to deduce the sound speed squared as a
function of radius throughout the interior. The
difference between the observations and a
standard model is extremely small, typically
June 1998 Vol 39
0.2%, but they show an unexpected peak at
the base of the convection zone. This is probably due to a deficit in helium there, produced
somehow by mixing.
MDI observations have also been used to
deduce the rotation rate inside the Sun. At the
surface the Sun rotates more rapidly near the
equator than near the poles. So what happens
inside the Sun? The expectation was that the
rotation would be constant on cylinders parallel to the rotation axis, so that in planes
through the axis the lines of constant period
would be straight lines parallel to the axis.
However, the observations were a great surprise – instead the rotation is constant on radial lines (i.e. cones) throughout the convection
zone, and uniformly rotating below the convection zone (A&G 39/2 26). There is, therefore, an intense shear layer at the base of the
convection zone, which is likely to drive instabilities and turbulence and to be the agent that
mixes up the helium, as was required for the
sound speed peak. Indeed, this shear layer is
probably the site of the dynamo that generates
The EIT (EUV Imaging Telescope) led by JeanPièrre de la Boudinière (Paris) aims to produce
global images of the transition region and
corona up to 1.5 solar radii at a spatial resolution of 5 arcsec. The instrument uses multilayered normal incidence EUV optics to register
EUV lines in HeII, FeIX, FeXII and FeXI and
so produces images at four different temperatures. At coronal temperatures you see many
X-ray bright points continually sparkling like
jewels, and coronal loops dynamically evolving
and occasionally erupting. As well as providing
global images that other instruments can use, it
has observed sources of the fast solar wind and
of coronal mass ejections. Over the poles you
see in detail for the first time the location of the
source of the fast solar wind as plumes or interplume regions streaming out from magnetic
sources, sometimes in a dynamic time-dependent manner. The plumes arise from small
unipolar magnetic sources on chromospheric
network boundaries and are denser and cooler
than the surrounding coronal hole. Also the
beginning of a coronal mass ejection shows up
as the formation of a coronal void and the outward propagation of a bright shell, namely a
Moreton wave (figure 2). In another study, the
initiation of a huge coronal mass ejection was
found to be associated with a small prominence activation before it erupted.
SUMER (Solar UV Measurements of Emitted
Radiation) led by Klaus Wilhelm (Lindau) aims
to use emission lines to deduce physical properties of the transition region and corona (temperature, density, abundance, velocities) at
high spatial, spectral and temporal resolution,
and so to provide new insights on coronal
heating and solar wind generation mechanisms. It has observed: explosive events with
speeds of about 150 km s–1, several thousand
of which are present on the Sun per minute; the
abundance of low FIP elements (i.e. with first
ionization potential lower than 10 eV)
enriched in the corona by factors of 1–20
3.11
Solar physics
c 150
a
–60 km/s
0 km/s
50 km/s
counts/px
100
50
b
0
628.5
629.0
629.5 630.0
wavelength (Å)
630.5
3 Flows in an active region loop system from CDS.
a EIT image, Fe XII 195A, 27 July 1997.
b Monochromatic image from CDS, 27 July 1996.
c CDS line profiles. (Courtesy P Brekke.)
HI Lyman-α
O VI
600
V1/e (km s–1)
V1/e (km s –1)
250
200
150
400
200
V1/e (TH = Te )
V1/e ( TO = Te )
100
0
1.5
2.0
2.5 3.0
ρ/R!
3.5
4.0
1.5
2.0
2.5
ρ/R!
3.0
3.5
4 Line-of-sight line widths in a polar coronal hole from UVCS (courtesy J Kohl).
relative to the photosphere; non-thermal velocities that are associated with coronal heating
by MHD turbulence in quiet regions.
The CDS (Coronal Diagnostics Spectrometer) led by Richard Harrison (Rutherford
Appleton Lab) has the same aim as SUMER,
but it uses normal and grazing incidence spectrometers in the EUV range 150–785 Å (Harrison 1995). This instrument is able to deduce
the temperature, density and velocity in many
structures. It can take the atmosphere to pieces
and inspect the different temperature regimes,
deducing the density from line ratios and the
velocity from Doppler shift and broadenings. It
finds that the transition region and corona are
highly dynamic with numerous small regions
having velocities of 50 km s–1 or up to
3.12
300–400 km s–1 in explosive events. In addition, low-energy microflares called blinkers
have been discovered by Harrison at transition
region temperatures with intensity increases by
a factor of 3 for typically 13 minutes located at
network junctions. An example of the power
of the instrument can be seen in figure 3, which
analyses the flows in an active-region loop system observed at the limb (global Sun top left,
close up bottom left). Line profiles (right) at
different points reveal that plasma at point A is
moving towards you at 60 km s–1, whereas
plasma at C is moving away at 50 km s–1.
The outer corona
The UVCS (Ultra-Violet Coronagraph Spectrometer) team, led by John Kohl (Harvard)
aims to measure the properties of the solar wind
and identify the mechanisms that accelerate it.
A great surprise is that in a polar coronal
hole, between 1.5 and 3.5 R while the proton
outflow velocity increases from 50 to
200 km s–1, the line-widths of Lyman-α
increase from 190 km s–1 up to 250 km s–1, but
those of OVI increase much more dramatically
from 100 km s–1 to 600 km s–1 (figure 4), which
suggests an ion cyclotron resonance by MHD
waves that creates an anisotropic velocity distribution, with v⊥ larger than v. In addition,
the oxygen outflow velocity becomes about
twice as large as the proton velocity. The line
widths in plumes are lower than in interplume
regions by 10–30 km s–1.
One study suggests that the slow solar wind
originates in so-called stalks near the axes of
coronal streamers. Another surprise is the low
emission of O VI in a long-lived coronal
streamer, where gravitational settling is producing a lowering of the abundance of oxygen
by an order of magnitude.
LASCO (Large-Angle Spectroscopic Coronagraph) led by Guenther Brueckner (Washington) aims to image and analyse the corona for
the first time out to 30 R from the Sun. It has
three overlapping coronagraphs called C1, C2
and C3 and has discovered global halo events.
In addition, it has explored the detailed properties of great eruptions of mass and magnetic
flux called coronal mass ejections (CMEs).
Helmet streamers are often found to bend
towards the equatorial plane. Further out, they
merge into one large equatorial “streamer
sheet” clearly discernible out to 30 R. With
LASCO occasionally you also see Kreutz sungrazing comets and it has observed the disconnection of magnetic structures during CMEs.
Perhaps the most surprising discovery from
LASCO is that many of these huge mass ejections (with ten times as much mass than was
thought before) are global or halo events (figure 5), in which the ejection is not just in one
direction but goes out on both sides of the Sun
and presumably also towards us. Occasionally
they reach the Earth and have far-reaching
effects: for example, a coronal mass ejection
that started on 6 January 1997 reached the
Earth on 10 January and caused a communications black-out, together with the permanent
disabling of a new $200 million communications satellite – this was especially annoying
because Star Trek was being broadcast on one
of the TV channels at the time!
What happened was that charge from the
radiation belts built up on the satellite and then
fried the circuits when it discharged. We are
now beginning to be able to forecast such
events, but clearly some care is needed in public relations, as evidenced by the reaction to a
NASA person who went on TV after watching
one CME start its two- or three-day journey to
June 1998 Vol 39
Solar physics
the Earth last April and reportedly said: “At
this very moment a coronal mass ejection is
hurtling towards the Earth like a missile!”
5 Coronal mass ejection
seen in the C3 coronagraph of
LASCO (courtesy G Simnett).
The solar wind
The CELIAS (Charge Element and Isotope
Analysis) instrument is led by Peter Bochsler
(Berne) and Dietrich Hovestadt (Garching) and
has detected many elements and isotopes for
the first time. COSTEP (Comprehensive SupraThermal and Energetic Particle Analyser) led by
H Kunow (Kiel) and ERNE (Energetic and Relativistic Nuclei and Electrons) led by J Torsti
(Turku) together make up the CEPAC consortium, which is measuring energetic particle
events, both from co-rotating interaction
regions in the solar wind and from active
regions on the Sun as well as an anomalous cosmic ray component. Furthermore, SWAN
(Solar Wind ANisotropies) led by J Bertaux
(Verrière) has measured the global solar wind
mass flux from the Sun’s UV radiation illuminating the cavity in the interstellar hydrogen
cloud. Figure 6 is a map of interstellar hydrogen glowing in Lyman-α from SWAN in which
hot stars show up as white points and the asymmetry in intensity is caused by the motion of the
Sun at 26 km s–1 through the interstellar cloud.
Conclusions
SOHO is clearly a brilliant success and is producing major breakthroughs in our understanding of the Sun. Progress on the large questions is as follows:
● For the solar interior, the structure and properties of the convection zone have been determined and the probable location of the dynamo
identified, but the structure of the core is uncertain and g-modes have not yet been detected.
● Part of the coronal heating problem has been
solved. There are several mechanisms at work:
magnetic reconnection has been shown to heat
X-ray bright points; large-scale diffuse loops
are probably heated in an MHD turbulent manner (Priest et al. 1998a); but how small loops
and coronal holes are heated is not known.
● The solar wind consists of two parts: the fast
solar wind in coronal holes may well be accelerated by ion-cyclotron resonance, but the role
of plumes is not yet known; the slow solar
wind is a mystery, but is highly dynamic and
has a higher contribution from coronal mass
ejections than thought before. ●
Prof. E R Priest, Dept of Mathematical and Computational Sciences, St Andrews University, St Andrews
KY16 9SS. The figures are courtesy of the SOHO
VIRGO, MDI, EIT, SUMER, CDS, UVCS, LASCO
and SWAN consortia. SOHO is a project of international co-operation between ESA and NASA.
References
Antonucci E et al. 1997 in The Corona and Solar Wind near
June 1998 Vol 39
90
upwind: direction from
which the interstellar
breeze comes
no data
(safety)
Sun: source
of solar wind
0
downwind: void in
the interplanetary
hydrogen
stars of the Milky Way
90
100
200
300
400
500
600
17 March 1996 ecliptic co-ordinates
700
800
6 Map of interstellar hydrogen from SWAN (courtesy J Bertaux).
Minimum Activity ESA SP-404, Noordwijk 175–182.
Appourchaux T et al. 1997 Solar Phys. 170 27–41.
Bertaux J L et al. 1997 Solar Phys. 175 737–770.
Bochsler P et al. 1997 Proc. 5th SOHO Workshop Oslo 37–43.
Brekke P et al. 1997 Solar Phys. 170 143–161.
DeForest C E et al. 1997 Solar Phys. 175 393–410.
Fröhlich C et al. 1997 Solar Phys. 170 1–25.
Gabriel A H et al. 1997 Solar Phys. 175 207–226.
Habbal S R et al. 1997 Astro-phys. J. 489 L103–L106.
Harrison R A et al. 1995 Solar Phys. 162 233–290.
Harrison R A et al. 1997 Solar Phys. 170 123–141.
Kohl J L et al. 1997 Solar Phys. 175 613–644.
Kosovichev A G et al. 1997 Solar Phys. 170 43–61.
Kuhn J R et al. 1997 Proc. of IAU Symposium 181 103–110.
Lemaire P et al. 1997 Solar Phys. 170 105–122.
Mason H E et al. 1997 Solar Phys. 170 143–161.
Moses D et al. 1997 Solar Phys. 175 571–599.
Priest E R et al. 1998 Nature in press.
Raymond J C et al. 1997 Solar Phys. 175 645–665.
Schrijver C J et al. 1997 Solar Phys. 175 329–340.
Schwenn R et al. 1997 Solar Phys. 175 667–684.
Simnett G et al. 1997 Solar Phys. 175 685–698.
Torsti J et al. 1997 Solar Phys. 175 179–191.
Wilhelm K et al. 1997 Solar Phys. 170 75–104.
3.13