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ANNALS  OF  GEOPHYSICS, SUPPLEMENT TO VOL.  47, N.  2/3, 2004

10
Planetary and gravity wave signatures
in the F-region ionosphere with impact 
on radio propagation predictions 
and variability 
DAVID ALTADILL (1), EMIL M. APOSTOLOV (2), JOSEF BOŠKA (3),
JAN LAŠTOVIČKA (3) and PETRA ŠAULI (3) 

(1) Observatorio del Ebro (URL-CSIC), Roquetes, Spain
(2) Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria 
(3) Institute of Atmospheric Physics, Academy of Sciences of Czech Republic, Prague, Czech Republic 

The aim of this work within the WP 3.1 of the COST 271 Action is the characterization of the vari-
ability introduced in the F-region ionosphere by ‘Planetary Wave Signatures’ (PWS) and ‘Gravity Wave
Signatures’ (GWS). Typical patterns of percentage of time occurrence and time duration of PWS, their
climatology and main drivers, as well as their vertical and longitudinal structure have been obtained.
Despite the above characterization, the spectral distribution of event duration is too broad to allow for
a reasonable prediction of PWS from ionospheric measurements themselves. GWS with a regular morn-
ing/evening wave bursts and specific GWS events whose arising can be predicted have been evaluated.
As above, their typical pattern of occurrence and time duration, and their vertical structure have been
obtained. The latter events remain in the ionospheric variability during disturbed days while additional
wave enhancements of auroral origin occur. However, both types of disturbances can be distinguished.

10.1.  INTRODUCTION

Interest in the ionospheric F-region variability has increased recently (Forbes et al., 2000; Rish-
beth and Mendillo, 2001; Mendillo et al., 2002), partly due to the role that the ionosphere plays in
the Earth’s environment via coupling processes from above and below, the former being a signifi-
cant part of the space weather. The variability of the ionospheric F-region ranges from time-scales
of minutes (e.g., TID’s) to long-term changes (solar cycle variations and even secular). Table 1 from
Rishbeth and Mendillo (2001) summarizes the possible causes of the ionospheric F-region variabil-
ity. It is widely accepted that the geomagnetic activity is the main cause of the ionospheric variabil-
ity, although meteorological causes transmitted from below may contribute comparatively. Many of
the «meteorological influences» on the ionosphere arise from upward propagating gravity, tidal and
planetary waves (Kazimirovsky et al., 2003). Here we discus the main results on ‘Planetary Wave
Signatures’ (PWS) and ‘Gravity Wave Signatures’ (GWS) as sources of the variability of the F-re-
gion ionosphere obtained within the COST 271 Action. It should be mentioned that we also studied
the effects of gravity and planetary waves on the lower ionosphere (Boskova and Laštovička, 2001;
Laštovička, 2001; Laštovička et al., 2003c). The PWS in the F-region with an emphasis on their pos-


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David Altadill, Emil M. Apostolov, Josef Boška, Jan Laštovička and Petra Šauli

sible sources, time duration and time occurrence patterns, their typical longitudinal size, and their
climatology are presented in Section 10.2. Section 10.3 deals with the GWS of non-auroral origin in
the F-region with an emphasis on their regular occurrence and on other specific events. The last sec-
tion highlights the most relevant conclusions of our work within the COST 271 Action. 

10.2.  PLANETARY WAVE SIGNATURES ON THE F-REGION

The Planetary Waves (PW) in the Mesosphere/Lower Thermosphere (MLT) (Salby, 1984) may in-
directly reach the F-region ionosphere and drive PW effects there. The PW modulation of upward
propagating tides or gravity waves, the interaction of PW with the strong diurnal/semidiurnal de-
pendence of the ionosphere, and the transmission of PW activity via vertical plasma drift or mean
vertical mass transport may explain the arising of PW effects in the F-region from the PW activity in
the MLT (Laštovička and Šauli, 1999; Pancheva et al., 2002; Altadill and Apostolov, 2003; and ref-
erences therein). Experimental results show also the vertical propagation structure, from above and
below, of such oscillation activity in the F-region (Altadill and Apostolov, 2001; Altadill et al.,
2001b). The latter case studies assess that PW effects in the F-region originated from the PW activi-
ty in the MLT propagate upwards. However, these investigations stated that a «quasi-periodic» se-
quence of geomagnetic storms and sub storms and/or large storms can generate wave-like oscillations
in the ionosphere as well. Hereafter we call ‘Planetary Wave Signatures’ (PWS) both effects, the os-
cillation activity of the ionosphere driven by the PW activity in the MLT and geomagnetic activity.
The PWS discussed here have periods of about 2, 5, 10, 13.5, and 16 days, these PWS being an im-
portant source of variability of the ionosphere – they may contribute up to 50-60% of the variability
of the f 0F2 in the periodic range 2-35 days (Apostolov et al., 1998). 

10.2.1.  Time duration and time occurrence patterns

An important input to COST 271 is the evaluation of the persistence of PWS in f 0F2 over Europe
(Laštovička et al., 2003a). They found a typical duration of 4 cycles for the 5-day events. The typi-
cal duration being of about 3.5 and rather 3 cycles for the 10-day and 16-day events, respectively. So,
in terms of numbers of cycles PWS with longer period of oscillation have shorter duration, but they
have longer duration in terms of time. However, the spectral distribution of event duration is too broad
to allow for a reasonable prediction of event duration (fig. 10.1a), which means that PWS-type os-
cillations remain to be unpredictable noise in ionospheric predictions until a reasonably reliable com-
bination of external predictors will be found. Laštovička et al. (2003b) found quite similar results for
stations at middle latitudes in Japan and U.S.A.

By taking into account the average values of event duration and numbers of events found by
Laštovička et al. (2003a), one can conclude that the 5-day, 10-day and 16-day events manifest them-
selves during about 16%, 26% and 34% of the time respectively. Altadill and Apostolov (2003) con-
firmed the latter results at midlatitude global scale and that 2-day and 13.5-day events manifest them-
selves during about 12% and 22% of the time, respectively (fig. 10.1b). 

10.2.2.  Longitudinal size pattern

Altadill et al. (2001a, 2003) evaluated the zonal structure of PWS in some case studies, and they
showed that PWS are large-scale phenomena in the F-region. The typical longitudinal size of such
events in the mid-latitude F-region of the Northern Hemisphere has been obtained recently (Altadill
and Apostolov, 2003). They did not distinguish between traveling and standing waves and did not


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Planetary and gravity wave signatures in the F-region ionosphere with impact on radio propagation predictions and variability

Fig. 10.1a,b. The top plot shows an example of the typical duration of PWS in terms of number of cycles for
the 5-day events (after Laštovička et al., 2003a). The bottom plot depicts the typical percentage of occurrence
of PWS events according to their dominant oscillating periods (after Altadill and Apostolov, 2003). 

Fig. 10.2. The plots shows the percentage of occurrence of PWS events (y axes) referred to the number events
found to exist in sector 1 as function of their longitudinal size (x axis). The horizontal error bars mean the lon-
gitudinal size of the labeled midlatitude sector. The horizontal lines of each plot indicate the assumed threshold
for the typical longitudinal size (after Altadill and Apostolov, 2003). 

10.2

consider the possibility that standing oscillations would not necessarily be seen at a given single sec-
tor if the minima in the wave were present in that sector. They compared the coherent PWS at con-
secutive midlatitude longitudinal sectors and their result may be summarized as follows (fig. 10.2).
The PWS with periods of 2-3 and 5-6 days have a typical longitudinal size of 80°. Although they can
be observed on a larger zonal scale, it is quite unlikely. PWS with periods of 10 and 13.5 days have
a typical longitudinal size of 100°, they being likely observed on a larger zonal scale. PWS with a
period of 16 days have a typical longitudinal size of 180°, they being likely observed at global scale. 

To some extent the above results contradict the estimation of the scale size of the ‘meteoro-
logical influences’ reported by Mendillo et al. (2002), who find that these variations are coher-
ent some 2500 km apart only. By considering PWS as a part of the ‘meteorological influences’
on the ionospheric variability, the above results indicate that PWS in the F-region ionosphere are
large scale phenomena (note that a longitudinal scale of 80° at latitude of 50° means a distance
of 6000 km), having larger zonal scale for PWS with longer oscillating periods. 

10.2.3. Climatology and sources 

The typical climatology of PWS in the f 0F2 at midlatitudes has been also evaluated within COST
271 Action (Altadill and Apostolov, 2003). From their results one senses that the occurrence of PWS

a

b

10.1


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David Altadill, Emil M. Apostolov, Josef Boška, Jan Laštovička and Petra Šauli

in the f 0F2 is better expressed during the summer half of the year (fig. 10.3). The latter is better ex-
pressed for PWS with shorter period. This is not surprising because the contribution of the PWS to
the variability of the f 0F2 is higher also at that time (Apostolov et al., 1998). However a quite sin-
gular pattern is obtained when the seasonal occurrence of PWS in the f 0F2 is considered in relation
to their dominant ‘driver’. According to Altadill and Apostolov (2003) these drivers may be the PW
activity in the MLT, the geomagnetic activity, and PWS events ‘independent’ from the above two,
there being a very small contribution of solar flux variations to the ionospheric variability at periods
of 2-30 days, see also Forbes et al. (2000). These results are summarized in table 10.I. The PWS re-
lated with the PW activity in the MLT tend to occur according to the climatology of the latter, PWS
with shorter period occur during summer half year and those with longer period occur during winter
half year. An exception to that is the PWS with a period of 13.5 days, which are related to the geo-
magnetic activity variations. The PWS related with the geomagnetic activity tend to occur during
summer half year. The so-called independent PWS events tend to occur during summer half year al-
so. The evaluation of the possible ‘main driver’ of the PWS in the ionosphere suggests that geomag-
netic activity variations play the most important role (Altadill and Apostolov, 2003). This is espe-
cially clear for the 13.5-day events. The geomagnetic activity variations practically drive 100% of the
latter events in the ionosphere. The geomagnetic activity variations can drive at least 20% of the 2-3-
day events, at least 30% of the 5-6-day events, 20-70% of the 10-day events, and 25-65% of the 16-
day events. A likely candidate for the ‘independent’ events may be the non-linear interaction or the
amplitude modulation between different PWS (e.g. Altadill et al., 2001b; Pancheva et al., 2002). The
PW activity in the MLT can drive about 20-30% of PWS in the ionosphere with periods near 2-3, 5-
6, 10, and 16 days. Note that the above percentages agree with those reported by Forbes et al. (2000),
which refer to the ‘meteorological influences’ on the ionospheric variability. 

10.3.  GRAVITY WAVE SIGNATURES ON THE F-REGION

The Atmospheric Gravity Waves (AGW) can generate disturbances in the ionosphere, the lat-
ter being mainly driven by heating produced by electric currents, energetic particle precipitation
and mechanical forces in the auroral ionosphere. Other sources of AGW disturbances in the ion-
osphere are from lower- and middle-atmosphere associated with regions of turbulence and wind
shear (Hunsucker, 1987; Hocke and Schlegel, 1996), and strong tropospheric events (Šauli and
Boška, 2001). There are a variety of studies that show the moving Solar Terminator as a source
of such type of disturbances (Somsikov, 1995; Galushko et al., 1998). Moreover, several studies
showed the arising of AGW in the ionosphere as a consequence of solar eclipses. The latter can
contribute to a gravity wave field and build up a wave front that may produce ionospheric distur-
bances (Chimonas and Hines, 1970; Liu et al., 1998). Solar eclipses can generate temperature dis-
turbances in situ in the thermosphere and electron density changes in situ in the ionosphere and
such disturbances will propagate as a wave (Müller-Wodarg et al., 1998). Hereafter we call Grav-
ity Wave Signatures (GWS) to the above discussed disturbances in the F-region with characteris-
tics of AGW. Our results were obtained mainly during quiet periods of geomagnetic activity.
However, some important results were obtained during campaign «HIRAC/SolarMax» (Feltens et
al., 2001). As an input to the COST 271 Action we will emphasize mainly on regular occurrence
of GWS and on specific events whose arising can be predicted. 

10.3.1.  Specific GWS events 

Specific GWS events arising in the F-region have been evaluated by preparing ionospheric campaigns
of rapid sequence sounding in order to study the source-response relationships between production and


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Planetary and gravity wave signatures in the F-region ionosphere with impact on radio propagation predictions and variability

Fig. 10.3. Percentage of occurrence of PWS events ( y axes) as function of month during the time interval 1983-
2001 (adapted from Altadill and Apostolov, 2003).

Table  10.I. Percentages of seasonal occurrence of the different PWS in the f 0F2 according to their possible
drivers. WIN means from November to February, EQU means March, April, September and October, and SUM
means from May to August. MLT means PWS driven by PW activity in the MLT, GEO by the geomagnetic ac-
tivity and IND means PWS events ‘independent’ from the above two (after Altadill and Apostolov, 2003). 

PWS 2-3 5-6 10 13.5 16

WIN 5 4 2 0 3
IND EQU 4 11 0 0 3

SUM 15 13 5 0 3

WIN 5 8 11 0 10
MLT EQU 14 6 5 0 11

SUM 13 7 4 0 5

WIN 9 3 10 25 11
GEO EQU 12 21 30 38 21

SUM 23 27 33 37 33

loss mechanisms, and dynamics. These events are related to solar eclipses and the passage of cold tro-
pospheric fronts (e.g., Altadill et al., 1999, 2001c; Boška et al, 2000; Boška and Šauli, 2001; Šauli,
2001a,b). The main results on these GWS events are related to their vertical structure, time life, degree of
variability and their dominant oscillating periods (fig. 10.4). Although the source of disturbances are quite
different, the ionosphere reacts at very similar periods (between 60 to 80 min), their amplitudes in the
plasma frequency range from 0.2 to 1.2 MHz (showing a high degree of variability), their life time is of
about 4-6 cycles of oscillation (up to 6 h), and their energy propagates vertically at an average speed of


Fig. 10.4. Vertical structure of a GWS event caused by a cold front passage during 2 November 1997 as ob-
served above Ebro station (40.8°N, 0.5°E). The left plot shows the vertical phase and group velocities of the
wave packet with dominant period of 75 min as function of the altitude. The right plot shows the disturbance of
this event caused in the plasma frequency as function of time and altitude, where the black line simulates the en-
ergy progression of the wave with an averaged velocity of 8 ms-1 and the white lines simulate their phase pro-
gression with an averaged velocity of − 47 ms-1 (adapted from Altadill et al., 1999). 

Fig. 10.5. Vertical structure of a GWS event observed during solar eclipse 11 August 1999 above Pr °uhonice obser-
vatory (49.9°N, 14.5°E). The plot shows the vertical phase and group velocities, time occurrence of central period of
the wave packet with dominant period of 85 min as function of the altitude (adapted from Šauli, 2001b).

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David Altadill, Emil M. Apostolov, Josef Boška, Jan Laštovička and Petra Šauli

about 5-10 ms-1. However, their vertical structure is quite different. Whereas the GWS caused from cold
fronts propagates upwards in the F-region (fig. 10.4), the GWS caused by solar eclipse originates at tran-
sition region between the F1- and F2-layers and it propagates upwards and downwards simultaneously
from that level (fig. 10.5) – see Altadill et al. (2001c), Šauli (2001a). Thus the vertical structure of GWS
displays different patterns according to their sources of origin. 

The GWS caused by tropospheric fronts are due to dynamics from lower atmospheric levels, that
is why GWS propagate upwards. The GWS caused solar eclipses are due to the cooling/heating
processes linked with the decreasing/increasing solar radiation that leads to a reduction/augment of
the scale height for both plasma and neutrals. This fact causes a downward/upward motion of plas-
ma and neutrals as well as of F1 production peak and of the transition region between F1- and F2-
layers, that is why the source of disturbance is located in that region from where its energy propa-
gates upwards and downwards. 

10.3.2.  Regular occurrence of GWS 

It was already mentioned that solar terminator acts as a source of GWS in the F-region, and this is
a regular phenomenon occurring at sunrise/set times. That is why the diurnal variation of the activity
of GWS related with that has been evaluated (Boška et al., 2003). They found systematic occurrence
of these events during sunrise time and lasting several hours (fig. 10.6). Diurnal variations of AGW ac-
tivity during periods of low solar and geomagnetic activity (1996, 1997 years) show a clear enhance-
ment during and several hours after sunrise. The observed internal AGW (with main periods between
60 and 75 min) propagates with significant vertical component through ionosphere from a source lo-

10.4 10.5


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Planetary and gravity wave signatures in the F-region ionosphere with impact on radio propagation predictions and variability

10.6 10.7

Fig. 10.6. Time variation of the probability of existence of a GWS event with dominant period of 61 min ob-
served above Pr °uhonice station (49.9°N, 14.5°E) during 31 October 1997 (adopted from Boš ka et al., 2003).

Fig. 10.7. Altitude variation of the amplitude (left), phase velocity (middle) and group velocity (right) obtained
for the wave event with dominant period of 72-min observed during October 12, 1996 in the Pr °uhonice data
(adopted from Boš ka et al., 2003).

Fig. 10.8. Wavelet power spectra of electron concentration variation on 24 April 2001 at 230 km with sub-
stantial intensification of wave-like activity in dawn and dusk hours at period range 70-100 min. 

cated at altitude of 180-220 km. Vertical components (amplitude, phase and group velocity) of the typ-
ical structure with dominant period 72 min are shown in fig. 10.7. Similar effects of GWS were ob-
served in a period range 50-100 min during the period of high solar activity (2000-2001). Particular
wave enhancements corresponding to solar terminator movement on 24 April 2001 are shown on the
wavelet power spectra of the electron concentration at altitude 230 km (fig. 10.8). Amplitudes of the
observed oscillations reach the maximum usually above 200 km and below F-layer peak, with typical
value in electron concentration of 0.3-0.5 [1011m-3], in the plasma frequency range from 0.2 to 1.2 MHz,
their time life is of about 4-6 cycles of oscillation, and their energy propagates vertically at an average


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David Altadill, Emil M. Apostolov, Josef Boška, Jan Laštovička and Petra Šauli

speed of a few ms-1. Their vertical structure (fig. 10.7) recalls that described for GWS caused by solar
eclipses (fig.10.5). One senses that GWS originates at altitudes of about 180-200 km, i.e. near the tran-
sition region between the F1- and F2-layers and it propagates upwards and downwards simultaneous-
ly. As a consequence of the above discussion, the heating processes linked with the rapid increase of
the solar radiation during sunrise/set can act as a source of turbulence in the ionospheric F-region and
generate GWS in the electron density (see also Somsikov, 1995). 

Effects of the morning and evening wave bursts were observed during all campaigns and seem to be
a regular wave enhancement in the F-region ionosphere. Results show that GWS that could be related to
the solar terminator remains in the spectra during day of storm and they show similar vertical structures
as during previous days while additional strong wave enhancements of auroral origin occur in the elec-
tron concentration. During the periods of high solar and geomagnetic activity (1991, 2001), a substantial
intensification of AGW activity, associated with the response of magnetosphere to a sharp increase of so-
lar wind, was observed. The amplitudes of the GWS during the latter periods increase twice to three times
compared to the GWS detected during geomagnetic quiet time. Campaign HIRAC/SolarMax (23 April-
29 April 2001) started during a period of low geomagnetic activity and finished during a magnetic storm.
The clear difference between GWS and disturbances of auroral origin was studied and presented (Šauli,
2001a,b; Šauli and Abry, 2003; Šauli et al., 2003a,b). Wave structures occurring during a geomagnetic
storm do not have a vertical component of phase and group velocities. Onset time of the particular wave
structure at distant observatories confirm the horizontal propagation of the structure. 

10.4.  SUMMARY AND CONCLUSIONS

The research work carried out under COST 271 Action as refers to the PWS in the periodic range
2-16 days and GWS leads to the following conclusions. 

PWS in the midlatitude F-region ionosphere may contribute up to 50-60% of the total variability
in the period range 2-35 days. The latter is better expressed during summer half year. The typical time
of occurrence of PWS with periods from 2 to 16 days is from 12 to 35% respectively, and their typ-
ical duration ranges from 4 to 3 cycles of oscillation, respectively. However, the spectral distribution
of event duration is too broad to allow for a reasonable prediction of event duration, which means that
PWS remain to be unpredictable at present from ionospheric measurements themselves. 

PWS in the midlatitude F-region ionosphere are large-scale phenomena. Their typical longitudi-
nal size ranges from 80° for PWS with periods of 2-3 and 5-6 days to 180° for PWS with periods of
16 days, those being observed on a larger zonal scale and even global. 

One senses that geomagnetic activity variations are the main drivers of PWS in the midlatitude F-
region ionosphere. However, PW activity in the MLT may contribute comparatively in some cases,
there being a very small contribution of solar flux variations to the ionospheric variability at periods
of 2-30 days. The PWS related with the geomagnetic activity tend to occur during summer half year.
The so-called independent PWS events tend to occur during summer half year also. The PWS relat-
ed with the PW activity in the MLT tend to occur during summer half year for PWS with shorter pe-
riod, and those with longer period tend to occur during winter half year. The latter events display a
distinct vertical structure with upwards energy propagation. 

GWS in the midlatitude F-region ionosphere with a regular occurrence and specific GWS events
whose arising can be predicted have been evaluated. The results were obtained mainly during quiet
periods of geomagnetic activity. However, some important results were obtained during disturbed pe-
riods as well. 

The specific GWS events are those caused by meteorological cold fronts passages and by solar
eclipses. Despite the different source of disturbance, the ionosphere reacts at very similar periods, their
amplitudes in the plasma variations show a high degree of variability, their time life is up to 6 h, and
their energy propagates vertically at an average speed of a few ms-1. Their vertical structure however is


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Planetary and gravity wave signatures in the F-region ionosphere with impact on radio propagation predictions and variability

quite different, the GWS caused cold fronts propagates from below in the F-region and the GWS caused
by solar eclipse originates at transition region between the F1- and F2-layers and it propagates upwards
and downwards simultaneously from that level. 

Morning and evening wave bursts are regularly observed in the F-region ionosphere during all
ionospheric sounding campaigns, showing that these GWS could be related to the solar terminator.
Their typical duration is about 4-6 h, their amplitudes in the plasma variations show a high degree of
variability, their vertical structure is similar to the GWS caused by solar eclipse, it originates at transi-
tion region between the F1- and F2-layers and it propagates upwards and downwards simultaneously
from that level. The GWS related to the solar terminator remain in the spectra during day of storm and
shows similar vertical structures as during quiet days while additional strong wave enhancements of au-
roral origin occur. The latter however do not have vertical energy propagation. So that sunrise termina-
tor-related GWS events are a good candidate for being included into ionospheric prediction algorithms. 

ACKNOWLEDGEMENTS

The work of J.L., J.B. and P.Š. has been supported by the Ministry of Education, Sports and Youth
of the Czech Republic via grant OC.271.10, and by the Grant Agency of the Czech Republic via
grants 205/01/1071 and 205/02/077P. The work of D.A. and E.M.A. has been carried out under the
International Cooperation Project between Spain and Bulgaria 2001BG0012 and under project
BTE2000-0825 (MCYT, Spain). The authors would like to acknowledge the contribution of L.F. Al-
berca and J.G. Solé to the WP 3.1 of COST 271 Action. 

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