Long term change
Long-term change in
the upper atmosphere
G
lobal change interests the public as
well as scientists. We are all familiar
with pronouncements on “global
warming”, with its effect on climate and
oceans, and its possible consequences for agriculture, animals and ourselves. Arguments rage
as to whether observed changes are natural or
man-made. What can solar–terrestrial physics
contribute to this debate?
Possible signatures of long-term change in
the upper atmosphere include:
● cooling in the middle and upper atmosphere
due to global surface warming;
● changes in tides due to ozone depletion and
consequent weakening of solar forcing;
● chemical contamination by man.
These signatures may be affected by influences,
external and internal to the Earth, that we need
to identify:
● long-term changes in solar and interplanetary activity;
● chemical contamination by natural causes,
e.g. volcanic;
● changes in the Earth’s internal magnetic field.
In this paper we review some possible signatures of long-term change.
Long-term data sets in STP
To study long-term change we need long-term
data sets, carefully acquired, accurately calibrated, and well archived. In solar–terrestrial
physics (STP), some data sets cover a
respectable time-span. Regular observations of
sunspots extend back for 250 years, and from
historical records the solar cycle can be traced
back for 10 times as long. Routine geomagnetic field measurements were started almost 150
years ago, ionospheric soundings 68 years ago,
solar wind monitoring 34 years ago. The muchused indices derived from these data go back
for similar lengths of time: the Wolf sunspot
number R since 1749, the aa geomagnetic index
since 1868, the Kp index since 1932, the ionospheric index IF2 since 1938, and the solar
“10.7 cm flux index” since 1947. Each index
has its own peculiarities and deficiencies.
Long-term trends may be “cyclical” or “secular” (non-cyclical) and nowadays we also
enquire whether they are “natural” or “manmade”. To make sure of a trend, let alone to
determine its cause, we need sequences that
3.26
What can solar–terrestrial physics
add to the global warming debate?
Henry Rishbeth and Mark Clilverd
assess the usefulness of data
collection in the long term.
he upper atmosphere may
contain signatures of long-term
global change in its physical and
chemical behaviour. Robust data
sets from the ionosphere,
mesosphere and stratosphere can
be compared with, for example, the
solar cycle and the geomagnetic
field. We discuss in particular the
shrinking of the ionospheric F-layer
and the cooling of the mesosphere,
both trends consistent with global
warming at lower levels. We also
assess the usefulness of long-term
records for observations of
unpredicted, short-lived events.
Carefully acquired, accurately
calibrated, and well archived data
sets spanning decades are used in
order to evaluate either slow trends
such as ionospheric cooling or
unexpected happenings such as
volcanic eruptions.
T
extend over many periods of any natural cycle
that may be present in the data. The bestknown natural periods are the 24-hour solar
day and the 24.8-hour lunar day, the 27-day
solar rotation period and the 29-day lunar period, the 365-day year, and the 11-year or 22year solar cycle. The Wolf sunspot number and
the geomagnetic aa index meet the requirement
of extending over many solar cycles, the ionospheric data barely do so. Another requirement
is that the data are acquired “regularly”, “frequently” and “consistently”, the exact meaning
of these terms may differ from one example to
another. The STP measurements that meet these
requirements are largely ground-based, with
important contributions from long-lived satellites and space probes. Intermittent observations, by rockets for example, are seldom useful
for long-term studies, though similar measurements taken many years apart might be useful
to detect trends. For studying long-term
change, the value of data sets increases more
than linearly with their length.
Most of our numerical information about
long-term trends in the upper atmosphere
comes from ionospheric soundings. This is
because the charged particles are easier to
detect and monitor than the ambient neutral
air, even though the latter is much denser. Regular ionospheric measurements began at
Slough in 1931 and became widespread by the
1940s. Global coverage reached a peak during
the period of the IGY (“Geophysical Year”,
1957–58) and the subsequent IQSY (“Quiet
Sun Year”) in 1964–65, and has since declined,
particularly in the southern hemisphere. In
contrast, regular information on neutral air
density at heights above 200 km, derived from
satellite orbits, goes back only to 1960. At
lower heights, around 80–100 km, we have
data from the late 1940s, derived from radar
tracking of meteors, and longer term but more
qualitative information based on sightings of
aurora and noctilucent clouds.
Shrinking of the ionospheric F-layer
The first suggestion that global warming might
influence the upper atmosphere came from
Roble and Dickinson (1989). They estimated
that a doubling of the CO2 and CH4 concentrations at 60 km, as expected to occur by the
mid-21st century, would cool the atmosphere
about 10 K at that height because of the
increased infrared emissivity. They showed
that above 200 km the cooling would be
greater, possibly as much as 50 K (figure 1).
Building on this suggestion, Rishbeth (1990)
estimated that the accompanying thermal contraction would lower the height of the F2 peak
(known as hmF2) by 15–20 km. Detailed computations using a general circulation model
(Rishbeth and Roble 1992) reinforced this conclusion. This so-called “falling sky” (Rodger
June 1999 Vol 40
Long-term change
300
∆T = – 40° in 800°
F-layer
250
200
tent data sets. Rocket data are too sparse, and
satellites do not give usable measurements of
hmF2. In time, the decrease in air density
above 200 km should become apparent in
observations of satellite orbits.
Although the peak F2-layer electron density
(NmF2) does not directly depend on temperature, as does the F2-layer height, Bremer
(1998) and Danilov (1998) have reported longterm decreases in NmF2 of order 5% in 30
years, varying from station to station. These
changes are at the margin of detectability, and
might be due to a long-term increase in geomagnetic activity.
altitude (km)
Cooling of the mesosphere
150
thermosphere
E-layer
100
∆T=–5° in 20°
D-layer
mesosphere
550
0
stratosphere
∆T=+1° in 50°
0
0
200
troposphere
400
600
temperature (K)
800
1: Sketch of the atmospheric regions, with height profiles of temperature and its anticipated changes.
1999) may serve as a proxy for global temperature change at the Earth’s surface.
At present, ionosondes are the only usable
tool for detecting long-term changes of hmF2.
Despite the simplicity of the underlying radar
principle, the accurate determination of hmF2
is not easy. The problems are both computational and instrumental. So care is needed
when taking bucketfuls of data from CDROMs or the Web! Even with consistent data
sets from well-calibrated ionosondes, detecting
trends is not simple, because hmF2 varies considerably with local time and season, and with
solar and geomagnetic activity. But since these
systematic patterns are known from decades of
observation, the relatively small long-term
trends can be teased out.
The first success in detecting a long-term
change was claimed by Bremer (1992), who
derived an average drop of 0.25 km/year from
33 years of data from Juliusruh (52°N) in GerJune 1999 Vol 40
many. Ulich and Turunen (1997) found similar
results at Sodankylä (67°N) in Finland. Using
data for a 38 year period, 1958–96, Jarvis et al.
(1998) found a drop in hmF2 of 0.5 km/year at
Port Stanley (52°S), Falkland Islands, and
0.2 km/year at Argentine Islands (65°S),
Antarctica, though the trends for some individual months depart from the mean behaviour
(figure 2). Further analysis of the diurnal and
seasonal variability showed that it is either altitude dependent or is accompanied by a
decrease in thermospheric wind. Analysing
data for 31 European stations, Bremer (1998)
found an average downward trend of hmF2,
despite upward trends at some individual stations. This lack of complete consistency seems
to be a characteristic of the subject.
Could the drop in ionospheric height be measured in other ways? In principle, hmF2 can be
measured by incoherent scatter radars, but
they do not have sufficiently long and consis-
The summer mesopause temperature is
remarkably stable (Lubken and von Zahn
1991) with a variability of 20 K. As the predicted drop in temperature as a result of
increased CO2 in the biosphere is 10 K, the
mesopause should be a sensitive indicator of
changing global temperatures. Evidence of
long-term change in the mesosphere has been
reported by Clancy and Rusch (1989) using
SME limb scattering, Hauchecorne et al.
(1991) using lidar, and Aikin et al. (1991) using
lidar and TIROS satellite data. However, their
data series are shorter than a solar cycle, making long-term trend resolution difficult. Contrary to these results, however, Lubken et al.
(1996) reported the absence of a trend in summer temperature from falling-sphere measurements in Andøya, Norway. Taubenheim et al.
(1997) used low-frequency radio reflection
heights to deduce a decrease of 0.2 K/year over
32 years (1963–95) at 80 km and Clemesha et
al. (1992) observed a downward trend of
45 m/year in the height of sodium layers occurring at around 93 km height, equivalent to a
temperature drop of 0.3 K/year. Serafimov and
Serafimova (1992) detected changes in the
ionospheric absorption of medium-frequency
radio waves, consistent with this trend.
Further evidence for long-term change near
the mesopause is provided by three possibly
related phenomena seen at 80–95 km: noctilucent clouds, the strong radar reflections
known as PMSE (polar mesosphere summer
echoes), and polar mesospheric clouds. In the
northern hemisphere, noctilucent cloud occurrence doubled between 1970 and 1990, and
the trend continues (Gadsden 1990, 1997).
This may well be related to decreasing temperature, though Thomas (1996) suggested that it
is more likely to result from the rapid increase
in methane in the biosphere, which increases
the concentration of water at the mesopause
from which the clouds form. Long-term
changes might also be caused by chemicals
deposited in the upper atmosphere by rockets
used to launch spacecraft. For example, there
might be changes in the occurrence of iono3.27
Long-term change
spheric sporadic E layers, which are known to
contain metallic ions.
Argentine Islands
Geomagnetic changes and their effects
Link with solar activity
The upper atmosphere, in particular the ionosphere, is strongly influenced by the Sun and
by the geomagnetic field, both the main (internal) field and the external fields. Variations of
the external field largely stem from solar–interplanetary causes, basically originating at the
Sun and therefore dominated by the 11-year
and longer solar cycles. Over the 131-year history of the geomagnetic aa index, there has
been an increase in activity starting around
1915 (Clilverd et al. 1998).
An obvious question about the ionospheric
changes is whether they are just caused by
changes in solar and geomagnetic activity,
which must be presumed to be of natural origin. Extending the work of Friis-Christensen
and Lassen (1991) on the relation between the
length of the solar cycles and lower atmosphere temperatures, Ortiz de Adler et al.
(1997) found correlations between solar cycle
length, solar EUV flux measured outside the
atmosphere, F2-layer electron density and
stratospheric temperature. The long-term
change in geomagnetic activity, mentioned pre3.28
altitude trend (km/yr)
The geomagnetic effects in the ionosphere and
magnetosphere embrace timescales ranging
from seconds to solar cycles. Superimposed on
these are longer-term trends associated with
the secular geomagnetic drift and weakening of
the geomagnetic dipole which, though cyclical,
have such long time constants that they appear
secular. We may study the changing location of
the auroral oval; we may detect changes in the
equatorial ionosphere due to the migration of
the geomagnetic dip equator; and ultimately
we can speculate on the ionospheric and magnetospheric consequences of the next geomagnetic polarity reversal.
On shorter time scales (days and hours), magnetometers record the effects of currents flowing
in the ionospheric E-region. They provide the
only quantitative measurement of conditions in
the upper atmosphere during the 19th and early
20th centuries. They can detect the modulation
of E-region tides by waves originating in the
stratosphere, such as planetary waves and the
quasi-biennial oscillation of the semidiurnal tide
(Jarvis 1996). This indirect monitoring of
stratospheric winds using magnetometer data is
possible at least as far back as 1900, 50 years
further than balloon measurements. Magnetometers also provide a way of measuring the
effect of solar activity on the geomagnetic field;
for example, variations of high-latitude dayside
magnetic pulsations recorded on magnetometers have been tentatively associated with solar
wind conditions (Vennerstrøm 1997).
0.0
–0.5
–1.0
Jan Mar May Jul Sep Nov Jan
month
2: Long-term trends in the daily mean of the height of
the ionospheric F2 peak (hmF2) at Argentine Islands
(65°S), from Jarvis et al. (1998). With acknowledgement to the American Geophysical Union.
viously, is probably of solar origin. The long
timescales of solar activity will always present
problems for interpreting terrestrial changes.
Through the resulting modulation of cloud
cover, the Earth’s surface temperature follows
variations in galactic cosmic-ray flux, which is
strongly modulated by solar wind conditions
(Svensmark 1998). This research might possibly be extended back over 100 years through
the analysis of magnetometer data. Though
many questions remain, this might provide the
long sought-after link between heliospheric
activity and the Earth’s climate and weather.
Conclusions to date
We have described long-term changes in the
solar–terrestrial environment that have been
detected, or might become apparent in future.
The “falling sky” in the ionosphere (the drop
in F2-layer height by up to 0.5 km/year) and
the detected cooling of the mesosphere by
0.2–0.3 K/year both indicate that the predicted
“global cooling” in the upper atmosphere is
indeed taking place. We can say that, if there is
“global warming” in the lower atmosphere
(whether or not due to human activity), the
existing ionospheric observations are consistent with the expected consequences. Making
measurements in the mesosphere has historically been difficult. However, a new era has
begun with concentrated studies of the region
using improved instruments and powerful analytical techniques. Efforts are under way to
improve the modelling of the mesospheric
region, both by extending upwards the stratospheric models and lowering the boundaries of
ionospheric models. But, particularly because
of the complication of the 11-year solar cycle,
perhaps another 30 years is needed to make
sure that the long-term effect is real. This does
not meet media desires for instant drama!
The subject clearly has wide ramifications.
With contributions from many countries
including Argentina, Brazil, Finland, Germany,
India, Russia, UK, USA and others, it attracts
worldwide attention in the solar–terrestrial
community. It carries an important lesson for
scientists and their masters: long-term observations really matter! Sequences of good data are
valuable resources; the longer they are, the
greater their potential for detecting and monitoring long-term change.
Past data are also used to investigate sudden
and unexpected events, but only if the data
have been preserved. Examples are:
● the magnetic signature of the passage of
Comet Halley through the magnetosphere in
1910;
● geophysical effects of the Siberian meteorite
in 1908;
● magnetic and ionospheric signatures of catastrophic explosions, e.g. Krakatoa 1883,
Flixborough 1974.
Once observations are stopped and sequences
of data are broken, their value quickly
declines. Unfortunately, the worth of an observatory is sometimes judged on the immediate
usage of its data. We cannot expect every longterm data-gathering operation to be kept going
for ever; but the “global change” issue highlights the need for setting priorities, taking
account of the length and consistency of data
sets and the scientific importance of the
observing site (Willis et al. 1994). It is clear too
that “old data” must be kept available for full
and open access by the scientific community. ●
Henry Rishbeth, Dept of Physics and Astronomy,
University of Southampton, Southampton SO17
1BJ, and Mark Clilverd, British Antarctic Survey,
High Cross, Madingley Road, Cambridge CB3 0ET.
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