International Journal of Geomagnetism and Aeronomy
Published by the American Geophysical Union
Vol. 1, No. 1, April 1998
The structure of the intensity of disturbances of the ion
temperature in the thermosphere
N. M. Gavrilov
Physics Research Institute of St. Petersburg State
University, St. Petersburg, Russia
Abstract
1. Introduction
2. Analysis of Experimental Data
3. Results of Data Processing
4. Discussion
5. Conclusions
Acknowledgements
References
The altitude-latitude distributions of the intensity of
variations of the ion temperature at altitudes of 100-400 km with
periods of less than 6 hours have been obtained from incoherent
scatter data from the following stations: Arecibo (18oN, 67oW),
Millstone Hill (43oN, 71oW), Saint Santin (45oN, 2oE),
Chatanika (65oN, 147oW), Sondrestrom (67oN, 51oW), and
EISCAT (70oN, 19oE). The seasonal and latitudinal variations of
the intensity of disturbances of the ion temperature have different
structure at altitudes above and below the region of 150-180 km.
Above this region the intensity maximizes at high and low
latitudes. The high-latitude maximum can be attributed to the
effect of variations of magnetospheric electric fields, to
energetic particle precipitation, and to the generation of internal
gravity waves (IGW) in the auroral region and to their propagation
from the lower atmosphere. One reason for the appearance of the
low-latitude maximum in the intensity of disturbances of the ion
temperature may be the enhanced generation of IGW by tides in the
equatorial upper atmosphere.
Many studies of internal gravity waves (IGW) in the middle and
upper atmosphere have been performed. However, systematic data on
IGW above 100-120 km are lacking. This paper examines the
altitude, seasonal, and latitude variations of the IGW intensity at
altitudes of 100-400 km using the long-term data on the intensity
of disturbances of the ion temperature observed in 1984-1990 by a
network of incoherent scatter stations: Arecibo (18oN, 67oW),
Millstone Hill (43oN, 71oW), Saint Santin (45oN, 2oE),
Chatanika (65oN, 147oW), Sondrestrom (67oN, 51oW), and
EISCAT (70oN, 19oE).
Both meteorological and auroral sources can contribute to the
intensity of variations of the night temperature. The effect of
these sources on seasonal and latitude variability of the intensity
of disturbances of the ion temperature at different altitudes is
discussed.
The present paper examines the data on the ion temperature
calculated by a standard method from parameters measured at the
incoherent scatter (IS) stations. A listing of the data studied
here, together with the initial processing methods, can be found in
Emery and Barnes [1994].
The periods of atmospheric IGW at middle latitudes range from 5 min
to 17 hours. The central problem with the IGW analysis of the data
obtained at IS stations is the fact that for each particular
altitude there are only a few measurements during the day. In
addition, tidal oscillations of large amplitude with periods
comparable to (or exceeding) the duration of continuous observation
are superimposed on the short-period waves. For measurements below
200 km the duration of continuous observation is limited to daytime
hours, and therefore the traditional methods of spectral analysis
can be used only in a few favorable cases.
The antenna beam of an IS radar typically scans the sky. For the
Millstone Hill and Saint Santin stations we used the data obtained
with the antennas directed to the zenith. The experimental data on
the ion temperature from the other stations were usually averaged
over the period of the beam run for every altitude in the latitude
range of 10o centered on the latitude of the given IS station.
Next, the averaged data was filtered. The values of the daily
average component and of the 24-, 12-, 8-, and 6-hour harmonics
were calculated for each day, using the method of least squares.
Figure 1
shows an example of a few low-frequency components
calculated for a set of values of the ion temperature.
The deviations of the measured values of the ion temperature from
the low-frequency component contain information on the variations
with periods of less than 6 hours. After elimination of this
low-frequency component, the squares of the deviations are averaged
daily and monthly over the period of measurement at each station.
The mean values of the intensity of the ion temperature
disturbances thus obtained include contribution from both the
short-period atmospheric processes similar to IGW and the
measurement errors. It is difficult to distinguish these two
factors in the observed intensity of ion temperature disturbances
because different radar scanning programs are used by the different
IS stations. The dispersion of measurement errors for an individual
station can be estimated from the experimental data obtained when
the atmospheric component intensity is close to minimum
[Gavrilov et al., 1994].
Subtraction of these minimal values eliminates the
contribution from the measurement errors. Obviously, the
intensities of irregular variations thus obtained are lower bounds
since the minimal values subtracted also contain contributions from
the "atmospheric" component. However, in most cases the minimal
values of the intensity do not exceed 10 K, so one can assume that
the error elimination procedure only weakly affects the results
obtained.
The data processing method described above has been applied to
measurements of the ion temperature at the network of IS stations.
To obtain the altitude, latitude, and seasonal variations of the
intensity of ion temperature disturbances, the experimental data
are averaged for the given altitude and month over the entire
observational period at each station.
Figure 2 shows the altitude dependence of the intensity of the ion
temperature disturbances observed at the Arecibo station. A
pronounced maximum is seen at altitudes of 150-180 km. The maximum
relative increase in value is about 15-17% and decrease with
increasing latitude. For instance, the increase measured at the
Saint Santin station [Gavrilov et al., 1994],
at latitude 42oN,
at altitudes 150-180 km is 7-11%. At higher latitudes (Chatanika,
Sondrestrom, and EISCAT stations) this maximum further decreases
and even vanishes.
Figure 3 presents an example of the seasonal variation of the ion
temperature at fixed altitudes for the Arecibo station. It is seen
that the seasonal variation differs in structure in the lower and
upper thermosphere. Above 180-200 km altitude the amplitude of the
variation is much less than that at lower altitudes. Similar
seasonal variations are also observed at other IS stations. The
seasonal variations include annual, semiannual, and sometimes
shorter-period harmonics. According to the data from the
high-latitude IS stations, at an altitude of 125 km the maximum
intensity of variation of the ion temperature is about 100-170 K
(or 16-25%) in January-February and October-November. At low
latitudes the semiannual and higher harmonics with maximum
intensity of 100-120 K (or 10-15%) dominate in June-July and
December-January.
Figures 4 and
5 show the latitude
distribution of the intensity of
the ion temperature disturbances obtained from the data of the
network of IS stations during the solstice and equinox periods,
respectively. The data for each latitude are taken for 2 months
separated by a half-year interval (for instance, January and July).
The differences in the latitude dependencies appear above and below
the region of 180-200 km. In the lower thermosphere in January and
in June the disturbances of the ion temperature maximize near the
winter pole and minimize near the summer pole. In the equinox
period the latitude distribution of the intensity is more
symmetrical. At altitudes higher than 180-200 km the latitude
distribution of the intensity has maximums near the poles and at
the equator and changes only weakly during the year (see
Figures 4 and
5).
According to theory, the ion temperature is equal to the
temperature of the neutral atmosphere plus the temperature due to
the ion and electron components. This addition depends on the
density and composition of the atmosphere as well as on solar
activity and the electric fields in the upper atmosphere. The
theory also predicts that under quiescent conditions and at middle
and low latitudes the ion temperature differs little from the
temperature of the neutral component at altitudes of the E and
low-F regions of the ionosphere.
The disturbances of the ion temperature observed in the upper
atmosphere can be attributed to dynamical processes in the
atmosphere and/or to the variation of solar emission and
corpuscular fluxes absorbed in the thermosphere. At high latitudes,
Joule heating and rapid variations of the magnetospheric electric
fields are possible
[Alekseeva and Tashkinova, 1978; Hunsucker, 1977;
Millword et al., 1993; Rees et al., 1984;
Richmond, 1978; Williams et al., 1993].
The generation of waves by dynamical
processes in the lower atmosphere [Bertin et al., 1975;
Gavrilov, 1992; Gavrilov and Yudin, 1986;
Vincent, 1984] is the most probable
source of dynamical variations of the neutral atmosphere with
periods from several minutes to several hours in the thermosphere.
Another known source is the generation of IGW in the E layer of the
auroral ionosphere by Joule heating
[Alekseeva and Tashkinova, 1978; Hunsucker, 1977;
Rees et al., 1984; Richmond, 1978]. Also
possible is the generation of IGW in the thermosphere caused by
pulsing magnetospheric electric fields.
The IGW generated in the auroral region grow in amplitude with
altitude [Richmond, 1978];
therefore the increase in intensity
(seen in
Figures 4 and
5) of the ion temperature variations
observed above 150 km at high latitudes, can be explained by the
action of auroral sources of IGW and magnetospheric electric
fields. The wave amplitudes increase with altitude because the
density decreases; therefore the auroral source can cause growth of
the IGW amplitude throughout the thermosphere above the auroral
region.
As was mentioned in section 3, the latitude structure of the
intensity of the ion temperature variations is different above and
below the 150-180 km region. The solar activity and electric
fields affect the ion temperature at high latitudes at altitudes
above 110-120 km
[Richmond, 1978; Williams et al., 1993];
therefore one can expect that in the lower thermosphere and at
middle and low latitudes the IGW, propagating from lower
atmospheric layers, are the major contributors to the formation of
the ion temperature variations.
The IGW intensity at an altitude of 125 km depends
on season and latitude (see Figures 2-5). The maximum and minimum
intensity of IGW is observed at high latitudes in January and July,
respectively. As is seen from Figure 4, the experimental
points for January at high latitudes are higher than those for July at all
altitudes. Hence the enhanced flux of the wave energy from lower
layers can cause an increase of disturbances in the entire
near-polar winter thermosphere.
The maximum in intensity of the ion temperature variations,
observed in the thermosphere at low latitudes, requires a more
complicated explanation. Direct satellite measurements of fluxes of
solar UV radiation
[London, 1993] demonstrate that these variations
do not exceed several percent at a moderate level of solar
activity; therefore the disturbances of the ion temperature at low
latitudes are, apparently, of dynamical origin and can be
associated with the IGW.
The generation of IGW by atmospheric tides whose amplitudes peak
near the equator can be the reason for the high intensity of the
ion temperature disturbances at low latitudes. The transfer of
energy from tides to the IGW has been confirmed in a number of
publications, for example, by Gavrilov et al. [1981].
According to
these papers, diurnal and semi-diurnal tides become unstable and
generate the IGW at altitudes of about 100 km and higher. Then
these IGW can propagate upward and initiate a growth of
disturbances of the ion temperature in the upper thermosphere. At
middle and high latitudes the amplitudes of the tides are less than
those near the equator; therefore the generation there turns out to
be weaker.
The seasonal and altitude-latitude variations of the intensity of
the ion temperature disturbances in the altitude region from 100 to
400 km are considered based on data of long-term measurements
undertaken by a network of incoherent scatter stations.
The intensity of the ion temperature disturbances is maximizes in
the 150-180 km region at low latitudes and decreases with
increasing latitude. The seasonal variation of the intensity
includes the annual, semiannual and higher-frequency harmonics. The
seasonal and latitude structure of the intensity of the ion
temperature disturbances is different below and above 150-180 km.
Above this region, the intensity of variation of the ion
temperature maximizes in both high and low latitudes. The
high-latitude maxima are related to the corpuscular fluxes,
magnetospheric electric fields and generation of IGW in the auroral
region. One reason for the low-latitude maximum is the enhanced
generation of IGW by tides in the equatorial upper atmosphere.
The author thanks A. Richmond, M. Buonsanto,
O. Biudjardie, K. Jou, P. Collis, K. Mazaudie,
and G. Foster for
permission to use their experimental data and for useful
suggestions. This work was supported by the National Foundation for
Basic Research (project 93058211). We also used the incoherent
scatter radar database, collected under the CEDAR Program of the
U.S. National Center for Atmospheric Research.
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