Lower ionosphere response to external forcing: A brief review

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Advances in Space Research 43 (2009) 1–14 www.elsevier.com/locate/asr

Lower ionosphere response to external forcing: A brief review Jan Lasˇtovicˇka * Institute of Atmospheric Physics, AS CR, Bocni II, 14131 Prague, Czech Republic Received 18 August 2008; received in revised form 2 October 2008; accepted 3 October 2008

Abstract There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Lower ionosphere; Space weather forcing; Solar activity; Solar forcing; Atmospheric waves; Atmospheric forcing

1. Introduction The lower ionosphere at heights of about 50(60)–100 km is the lowermost part of the ionosphere. It includes the Dregion and bottom E region of the ionosphere. The lower ionosphere is a very minor component of the mesosphere and lower thermosphere, therefore typical plasma processes are not very important in this part of the ionosphere, which is controlled by ionization/recombination processes and atmospheric chemistry and dynamics. In spite of the fact that it is the closest region of the ionosphere to Earth’s surface, it belongs to less known and understood regions of

*

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the ionosphere. One reason is that it is not accessible to in situ measurements either by satellites, or by balloons; it is accessible only to rocket in situ measurements and to ground-based soundings using various radio-physical methods. The second reason is its complexity and quite dominant role of minor components in ion chemistry, including the dominant role of water cluster ions below about 85 km and important role of negative ions, particularly at lower altitudes at night. The lower boundary of the lower ionosphere is a height where free electrons no more exist as they all form negative ions through attachment processes. The third reason is the extreme variability of the lower ionosphere compared to higher levels, partly due to deposition of energy of extremely variable solar X-rays and high-energy particle fluxes, particularly electrons E > 10–20 keV. The fourth reason is that while above

0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.10.001

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the lower ionosphere the Sun and magnetosphere-driven processes dominate in the ionosphere, even though impact of the neutral atmosphere processes cannot be neglected, and below the lower ionosphere the atmosphere is driven by neutral atmosphere dynamics and chemistry, even though a small Sun-driven influence still exists, in the lower ionosphere both types of processes compete, sometimes solar-driven processes being dominant (e.g., solar flare or geomagnetic storm effects), whereas at other occasions dynamical atmospheric processes are dominant (e.g., winter anomaly). The lower ionosphere has extensively been studied in the 1960s and 1970s and to some extent in the 1980s, then interest fell down due to development of new techniques capable to provide good and broad data sets and information about higher levels of the ionosphere, due to development of models describing these parts of the ionosphere better, and due to more important role of higher levels of the ionosphere for commercial and military radio communications and for the Global Navigation Satellite Systems (GNSS) like GPS. Another reason was that some types of lower ionosphere measurements were terminated, like the A3 radio wave absorption measurements due to change of working regime of commercial radio transmitters used. However, the interest in the lower ionosphere has been partly re-appearing in recent years (e.g., Chevalier et al, 2007; Inan et al., 2007; Rodger et al., 2007; Todoroki et al., 2007; Osepian et al., 2008), among others in relation to lightning-induced phenomena like sprites. Investigations of the lower ionosphere are important both from the point of view of understanding our more distant environment and a part of the upper atmosphere, which plays a role in vertical coupling in the ionosphere– atmosphere system, and from the point of view of practical applications, as the lower ionosphere affects radio waves passing through or reflecting from it. The LF, MF and HF radio waves may fully be attenuated at high and higher middle latitudes during strong geomagnetic storms for a day or even more, and at low and middle latitudes during extreme solar flares for up to a few hours. One of the reasons of death of Roald Amudsen, the first man who reached the South Pole, during his last trip to Arctic and accident, was absence of radio connection for more than two days due to a geomagnetic storm. The external forcing of the lower ionosphere is of two different categories: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The solar origin forcing of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being the very phenomenon in the daytime not-high-latitude lower ionosphere due to the enormous increase of the solar X-ray flux. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particu-

larly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and midlatitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales and are important locally rather than globally. The paper is based on solicited presentation at the 37th COSPAR Assembly in Montreal in July 2008. Its purpose is to provide a brief review of external forcing of the lower ionosphere. I apologize to authors of papers which have not been referred to, since Advances in Space Research is not a journal for long comprehensive review papers and the scope of the paper is rather broad. Section 2 deals with forcing by variable solar electromagnetic radiation. Section 3 describes forcing by space weather, i.e. by geomagnetic storms and substorms, other solar-wind related phenomena, and high-energy particle effects. Section 4 provides information about impact of atmospheric waves from below on the lower ionosphere, whereas Section 5 deals with another forcing from below through lightninginduced phenomena. Section 6 contains concluding remarks. 2. Solar forcing of the lower ionosphere There are three basic and most important solar ionizing variability forcings of the lower ionosphere: (a) 11-year solar cycle, (b) 27-day solar rotation, (c) solar flares. The 11-year solar cycle is de facto double-peak variability; strong solar cycles are about 10 years long, weaker solar cycles are about 12 years long, while 11-year long cycles occur less often. The solar maximum-solar minimum difference in yearly average values of solar Lyman-alpha radiation ionizing NO is up to a factor of 2 and in hard X-rays ionizing the D-region (wavelengths below 0.5– 0.8 nm) it is more than an order of magnitude. Investigations of the solar cycle effect on the lower ionosphere are complicated by relative shortage of direct electron and ion density measurements. Nevertheless the available rocket measurements clearly support the existence of significant solar cycle effect in electron density. Various indirect monitoring-type measurements also confirm its existence. The LF indirect phase reflection height measurements in Ku¨hlunsborn, Germany since the late 1940s (e.g. Lauter et al., 1984; transmitter–receiver distance 1000 km) monitor at a fixed solar zenith angle a height of constant electron density located near 81 km. This height

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decreases from the solar cycle minimum to the solar cycle maximum by 0.7–1 km (von Cossart, 1984) as shown in Fig. 1, which means an increase of electron density at fixed heights near 81 km from the solar cycle minimum to its maximum. This decrease of reflection height is a net effect of two competing effects: the increase of the Lyman-alpha flux itself would result in a 3 km decrease of reflection height, but it is partly compensated by a 2 km thermal expansion of the mesosphere due to its heating in the solar cycle maximum versus the solar cycle minimum. The multifrequency radio wave absorption measurements in Europe also clearly displayed a solar cycle effect; these results served to improvement of the International Reference Ionosphere (Singer et al., 1984). The solar rotation variation has a period of 27 days, but it is quasi-period. Due to solar differential rotation the period slightly varies during solar cycle. Moreover, since activity of various active regions is changing and in a given rotation cycle the most important active region may differ from that in the previous rotation cycle, in terms of the F10.7 index the solar rotation variation period of solar ionizing ionization was observed to vary in extreme cases between 24 and 25 days up to 30–31 days. Sometimes the active regions on the Sun are grouped in two symmetric areas and then we observe also important (in solar UV sometimes even dominant) 13.5-day component, which is however, less reflected in the solar EUV flux and F10.7 and, thus, also in the lower ionosphere. There is one more problem with ionospheric effects of the solar rotation variation of ionizing flux. The 27-day period in the radio wave absorption in the lower ionosphere is of solar origin, when it is well expressed in solar ionizing flux (or indices), but in winter under low solar activity conditions, when it is sometimes also well pronounced, it appears to be caused by a 27-day planetary wave type oscillation in the neutral atmosphere, as it is observed in winds near 90–95 km (Pancheva et al.,

Fig. 1. Seasonal variation of the LF phase reflection height at a solar zenith angle of 78.5° for solar maximum and solar minimum conditions (after von Cossart, 1984).

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1991b). Generally the solar rotation variation is not so regular and so important in the lower ionosphere as the solar cycle effect, but it plays a role. Solar flares result in a very large increase of the solar Xray flux, for very strong flares by as much as three to four orders of magnitude, accompanied by some increase of the solar EUV flux by as much as several tens of percents. The duration of flare X-ray bursts is from a few minutes to a few hours. Solar flares occur much more often during high solar activity; they are very rare under solar cycle minimum conditions. The lower ionosphere is that part of the ionosphere, which is by far the most sensitive one to solar X-ray flares. Large enhancements of the solar X-ray flux may result in temporary blackout of radio wave propagation in the MF–HF range, particularly at low and middle latitudes, as a consequence of a huge increase of radio wave absorption due to a large increase of electron density. Flare effects in the lower ionosphere are called Sudden Ionospheric Disturbances (SIDs) and consist of several types of effects: SCNA (sudden cosmic noise absorption) is a significant increase of absorption of cosmic radio noise at high HF or low VHF frequencies (tens of MHz) as observed by riometers. SWF (short-wave fadeout) in the MF–HF radio wave range. SFA (sudden field anomaly) in the LF radio wave range. SPA (sudden phase anomaly) is observed in the VLF radio wave range on long-range sub-ionospheric VLF paths. It is the most sensitive method of the SID detection. The VLF reflection height was lowered from 70 to 53 km during a super-flare of 4 November 2003 (Thomson et al., 2005). SES (sudden enhancement of signal) is observed in the VLF radio wave range on long-range sub-ionospheric VLF paths, but it is less pronounced and detectable than SPA. SEA (sudden increase of atmospherics) peaking near 27 kHz. SDA (sudden decrease of atmospherics) peaking near 5 kHz. An interplay of improved reflection from the upper boundary of the Earth–ionosphere VLF waveguide due to steeper electron density gradient and a decrease of the upper boundary of waveguide determine if the flare effect is an increase or a decrease of a level of atmospherics (atmospheric noise) at a given VLF frequency (e.g., Sao et al., 1970; Trˇ´ıska and Lasˇtovicˇka, 1972). Fig. 2 shows examples of various SIDs for a strong flare of 11 June 1969 as observed at Panska Ves (50.5°N, 14.6°E). Top panel shows SEA as a substantial enhancement of the atmospheric noise level compared to the expected level. Middle panel presents SFA. Bottom panel shows SWF as a total blackout of signal lasting for about 1.2 h, which has practical impact on radio communications and broadcasting. While start time of all three types of SIDs is almost identical, times of their maxima somewhat

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Fig. 2. SIDs (from top to bottom – SEA, SFA, SWF) observed at Panska Ves (50.5°N, 14.6°E) on 11 June 1969. Start time of SID is about 16:22 UT (slightly differs for different methods).

differ (impossible to determine for SWF), and end times are difficult to determine undoubtedly and may be to some extent questioned. It should be mentioned that the effects of cosmic gamma ray bursts (Maeda et al., 2005; Inan et al., 2007) have been observed in the lower ionosphere, even though rarely and at nighttime, when the direct solar ionizing radiation is screened. Effects of galactic X-ray flux enhancements were

also reported (Kaufmann et al., 1970), but their reliability was questioned (Poppoff et al., 1975). 3. Space weather forcing of the lower ionosphere The space weather forcing of the lower ionosphere is a complex set of various inter-related phenomena. It is caused by solar, solar-wind and high-energy particle phenomena

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and variations on time scales of terrestrial weather, i.e. on time scales less than several days. Solar-wind related phenomena are monitored at Earth through monitoring of geomagnetic activity. From the point of view of the lower ionosphere, the three most important phenomena are geomagnetic storms and substorms, solar proton events (SPEs), and relativistic electron precipitation (REP) events. These phenomena will be treated hereafter in the paper. There are some other phenomena like Forbush decreases of cosmic ray flux or changes of polarity of components of the interplanetary magnetic field (IMF) (e.g., Lasˇtovicˇka, 1979), which are either less important (but not quite negligible), or are included in geomagnetic activity variations. 3.1. Geomagnetic storms and substorms Geomagnetic storms are the most important and most powerful space weather feature as for impact on the lower ionosphere. They act in the high and middle latitudes; effects of strong storms may be observed down to geomagnetic latitudes of about 35° (e.g., Beynon and Williams, 1974) but majority of effects do not reach latitudes well below 50°. Main agent responsible for mid-latitude effects are precipitating electrons of energies of about 10– 300 keV. Substorms are observed in the lower ionosphere as a weaker version of the geomagnetic storm effect confined to auroral and subauroral latitudes. Geomagnetic storm effects on the lower ionosphere were reviewed, e.g., by Lastovicka (1996). Massive injections of energetic particles into the outer radiation belts and their later precipitation into the lower ionosphere cause large increase of electron density in the high latitude lower ionosphere and related substantial increase of radio wave absorption, sometimes blackout of MF and partly HF radio wave propagation. Joule heating plays a secondary role contrary to higher altitudes. An interesting phenomenon is observed in the higher mid-latitudes, in a region which corresponds to slot between the outer and inner radiation belts. During geomagnetic storms, the slot region is filled in energetic particle injections from the magnetosphere. Main source of precipitation of such particles from slot region is a wave-particle interaction with plasmaspheric hiss and other ELF–VLF emissions, which substantially increases the pitch angle diffusion and moves particles into the loss cone, which is followed by their precipitation into the lower ionosphere. Time-development of interacting emissions is such that the main precipitation takes place after the storm. This is illustrated by ionospheric radio wave absorption measurements shown in Fig. 3. Top panel of Fig. 3 shows development of geomagnetic activity. Next panel shows measurements from auroral observatory Kiruna (fmin is proxy of ionospheric radio wave absorption); only a strong direct effect is observed. Subauroral station Uppsala also reveals direct effect, even though less strong and shorter, and after a gap displays a well-developed post-storm effect (PSE), called sometimes aftereffect. Higher-middle latitude radio wave absorption

Fig. 3. The effect of a geomagnetic storm (Ap, Dst) in March 1970 on the lower ionosphere (fmin, A3 radio wave absorption) at auroral and middle latitudes in Europe (after Lauter and Bremer, 1983). Each column represents one day.

measurements (A3-MF, A3-LF) in Germany reveal for all three measuring paths only strong delayed PSE and weak, if any, direct effect. It is necessary to mention that Fig. 3 displays an extreme case; usually the PSE dominance is less pronounced. The high latitude boundary of the PSE region roughly coincides with the low latitude boundary of auroral electron precipitation. Motion of this boundary was sometimes monitored by the radio wave absorption as shown in Fig. 4. The first pulse of geomagnetic activity in Fig. 4 suppressed the boundary down to almost L = 3, and recovery to higher L-shells was delayed by two weaker pulses of geomagnetic activity. Unfortunately, the radio wave absorption monitoring network no more exists. Magnitude of the effect of geomagnetic storm in the mid-latitude lower ionosphere depends on several factors. Lasˇtovicˇka and Rapoport (1979) confirmed the following conditions to be unfavorable for the development of effects of geomagnetic storms at middle latitudes:

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3.2. Effects of solar proton events on the lower ionosphere

Fig. 4. Storm-related increases of radio wave absorption depending on Lshell and storm time. Shaded area – post-storm effect; downward-dashed area – auroral particle injections; upward-dashed area – direct storm effect (after Wagner and Ranta, 1983).

(a) Longer period of geomagnetic calm before the storm (i.e. slot region was quite empty). (b) Very short duration of the main phase of the storm, which may be a consequence of crossing the IMF sector boundary (less time for sufficient particle injections into slot region). (c) High ‘‘auroral jet” (AE) activity compared to Dst in the storm maximum (i.e. storm was confined to rather high latitudes). (d) Northward IMF Bz several days after the storm (i.e. very little transfer of solar-wind energy into the magnetosphere–ionosphere system). The particle precipitation is enhanced in the South Atlantic magnetic anomaly region and, therefore, the effect of geomagnetic storms is there stronger than that at respective middle/moderate latitudes of the Northern Hemisphere. Geomagnetic storms affect considerably the lowermost ionosphere; the VLF/ELF sub-ionospheric radio wave propagation is improved or attenuated depending on radio frequency and changes of electron density and shape of electron density profile. Atmospheric electricity (Reiter, 1989) and mesospheric electric field (Zadorozhny and Tyutin, 1998) are also affected by geomagnetic storms, as they depend on atmospheric ionization.

Solar proton events (SPEs) are mainly confined to high latitudes, to polar cap region. Strong SPEs are mostly (but not always) associated with strong geomagnetic storms. Their energy is carried essentially by protons of energies 3 to 300 MeV, which are geomagnetically shielded from lower latitudes. SPEs cause the polar cap absorption (PCA) events in the lower ionosphere of high latitudes, which are monitored by a network of riometers (measurements of cosmic noise absorption). Effects of solar protons are both direct via enhanced ionization rate and indirect through production of excessive NO, the only atmospheric component ionized by the solar Lyman-alpha radiation. In principle it is possible to use ionospheric multi-frequency measurements to estimate magnitude and spectrum of penetrating high-energy protons. However, such estimates are rather rough, not very accurate. Effects of high-energy particles on the middle atmosphere and lower ionosphere were reviewed, e.g., by Lastovicka et al. (1988). Fig. 5 shows the results of model calculations (Ondra´sˇkova´, 2005) of lower ionospheric effects of the SPE of July 14, 2000 and partly (bottom panel) also of the October 1989 SPE. Electron density enhancements are confined to altitudes below about 95–100 km and peak on day 2, the day of maximum proton flux. The largest electron density enhancement is observed in the lowermost ionosphere; around 60 km it reaches three orders of magnitude, at 50 km almost four orders of magnitude. Vertical gradients of electron density in the lower ionosphere are significantly reduced. The ratio of negative ion density to electron density is substantially reduced and reaches one slightly below 70 km. SPEs also considerably increase electrical conductivity in the high latitude stratosphere and mesosphere as can be expected based on large increase of electron density. Balloon measurements in the Antarctic stratosphere revealed substantial increases of electric conductivity and decreases of both horizontal and vertical electric fields almost to zero (Kokorowski et al., 2006). 3.3. Effects of relativistic electron precipitation events on the lower ionosphere Another important phenomenon of space weather, which affects the lower ionosphere, is the relativistic electron precipitation (REP) events. REP events occur in auroral and subauroral latitudes. REP events do not occur in polar cap lower ionosphere (Shirochkov et al., 2004). The high latitude boundary of REP events is approximately boundary between open and closed geomagnetic field lines. Typical duration of REP spikes is 1–3 h, but typical duration of REP enhancements is about 5–7 days (Baker et al., 1986). Fig. 6 shows time history of riometer absorption A (dB) at different high latitude stations (k – invariant latitude)

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energies E > 2 MeV (energy determined according to the perturbation recovery time). Since relativistic electrons penetrate into the mesosphere and stratosphere, they affect via ionization the electrical conductivity and, thus, also electric field. Hale et al. (1990) measured by rocket a change of the vertical electric field in the stratosphere during a REP event. 4. Forcing of the lower ionosphere by atmospheric waves from below The lower ionosphere is that part of the ionosphere, where Sun-driven influences from above and mostly atmospheric meteorology-driven influences from below compete. The latter forcing is primarily by atmospheric waves coming from below. Here, we will deal with forcing by planetary and gravity waves, and partly by tidal and infrasonic waves. Forcing of the whole ionosphere by atmospheric waves from below has recently been reviewed, e.g., by Lasˇtovicˇka (2006). 4.1. Forcing by planetary waves

Fig. 5. Model calculations of development of electron density at 70°N during and after the SPE of 14 July 2000 (top panel) and of electron density increase (right) and negative ion/electron density ratio (bottom panel) during SPEs of July 2000 and October 1989 (after Ondra´sˇkova´, 2005). Day 2 – maximum of proton flux.

during the REP event of May 1992. Stations located at k > 69° (three top panels and bottom panel) displayed only a strong but short effect. Then the magnetosphere was compressed by geomagnetic storm, these stations felt into polar cap, and the effect was no more observed. On the other hand, stations of Finnish riometer chain (Kevo–Nurmija¨rvi) and one southern hemisphere station, located at lower invariant latitudes, remained in the auroral zone and, therefore, revealed a week long effect with somewhat delayed peak. Rodger et al. (2007) observed in Sodankyla¨ (L = 5.2) hundreds of short-lived local VLF sub-ionospheric radio wave propagation perturbations. They corresponded to a rainstorm type precipitation of relativistic electrons of

Long-term and continuous observations are needed for investigations of planetary wave effects on the lower ionosphere. Such data have been provided by the radio wave absorption measurements in the lower ionosphere. Data on the radio wave absorption obtained by the A3 method in Europe (oblique incidence on the ionosphere, continuous wave transmission) at low (LF), medium (MF) and high (HF) frequencies between about 100 kHz and 10 MHz have been used. Pancheva and Lasˇtovicˇka (1989) and Pancheva et al. (1989) demonstrated for the daytime HF and MF, and nighttime LF absorptions that the planetary wave-like oscillations in absorption are related to similar oscillations in the neutral atmosphere, namely in wind, not to oscillations in solar or geomagnetic activity. Lasˇtovicˇka et al. (1994) performed model calculations which showed that the planetary waves in the neutral atmosphere were adequately transformed into planetary waves in the absorption, i.e. in the ionized component. Dominant periods are centered at 2, 5, 10 and 16 days, all being quasi-periods (period bands). Even the 27-day variation in absorption is not always of direct solar origin; relatively often it seems to be related rather to a planetary wave (Pancheva et al., 1991b). Planetary waves occur as bursts of activity. Lasˇtovicˇka et al. (2003) analyzed absorptions along two radio paths in central Europe, which were representative for altitudes of about 85–100 km, by means of the wavelet technique. Only wave events of three or more wave cycles were considered. Waves with periods near 5 days reveal a typical persistence of wave events around 5 wave cycles. Waves with periods near 10 days are less persistent with a typical persistence of 3–4 cycles. The typical persistence of waves with periods around 16 days is no more than 3 cycles.

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Fig. 6. Time history of riometer absorption A (dB) at different high latitude stations (k – invariant latitude) during the REP event of May 1992 (after Shirochkov et al., 2004).

Fig. 7 shows an example of long-term development of the planetary wave activity inferred from the A3 MF absorption in southeastern Europe. Each data point represents average 2-month planetary wave activity. A strong seasonal variation with wintertime maxima and summertime minima, particularly pronounced in the 1980 s, is evident. Such a variation is expected. Much stronger filtering by stratospheric winds in summer is responsible for much

smaller penetration of the tropospheric planetary waves to the lower ionosphere heights in summer. Typical values of absorption for the given measuring path are 35–40 dB, thus the amplitudes of planetary waves in absorption are up to a bit more than 10%. Fig. 7 displays also a long-term trend; an increase of planetary wave activity is observed for the 1980s (since the very late 1970s), whereas no trend, only perhaps a

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Fig. 7. Long-term development of planetary wave activity in the lower ionosphere at periods near 5 and 10 days as inferred from the A3 absorption measurements at 1412 kHz in southeastern Europe (after Lasˇtovicˇka, 2002).

wave-like variation is observed in the 1970s. Altogether nine radio wave absorption data sets (all available longterm absorption data sets) have been studied. They all revealed either positive (often statistically insignificant due to the large seasonal variation) or no trend, none of them revealed a negative trend. In general it may be said that there was no trend in the 1960s, early 1970s and 1990s, while there was a tendency to a positive trend (typical increase of the planetary wave type oscillation amplitude by 20–40%) in a part of the 1970s and in the 1980s, a bit earlier in the northern than southern Europe (Lasˇtovicˇka, 2002). Lasˇtovicˇka (1993) investigated possible effects of the quasi-biennial oscillation (QBO) on the planetary wave activity inferred from radio wave absorption. No significant effect of the QBO was found in the planetary wave activity. No significant effect of the 11-year solar cycle on the planetary wave activity was established, either. Pancheva et al. (1991a) found that the planetary wave activity in the lower ionosphere was enhanced at shorter periods during the periods of enhanced planetary wave two in the high latitude stratosphere at the 30 hPa level, while longer periods were enhanced during the periods of enhanced planetary wave one in the high latitude stratosphere. The strongest stratospheric disturbance, the major sudden winter stratospheric warming, is triggered by enhancement of planetary wave activity. Such a strong phenomenon should reflect in the lower ionosphere. Its effect has been observed as a temporary breakdown of the winter anomaly lasting for several days up to more than a week (e.g., Lasˇtovicˇka, 1984a,b).

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sphere in the MLT region. Gravity waves in the ionosphere are either of ‘‘meteorological” origin coming to the ionosphere from below, or are of auroral origin coming quasi-horizontally from the auroral zone, or are excited in situ for instance by solar terminator or solar eclipse. Here, we deal only with the gravity waves of ‘‘meteorological” origin, which means excited by processes in the neutral atmosphere. Gravity waves of ‘‘meteorological” origin are of primary importance to the momentum and energy budget of the MLT region and for its wind system compared to the role of gravity waves of other two origins (e.g., Fritts and Alexander, 2003). The electron density profiles measured by rockets since the 1960s displayed usually well-developed gravity wave type vertical structures, and profiles measured by other techniques with sufficient vertical resolution also displayed such structures, as illustrated by Fig. 8. Gravity wave type oscillations are expressed at all heights, shorter vertical wavelengths in the bottom part, longer vertical wavelengths at higher altitudes. They are expressed best in rocket profile, but two other methods also display some gravity wave type oscillations. The rocket profiles make the determination of characteristics of individual gravity waves possible, but their small number and irregular geographic distribution do not allow for the determination of the gravity wave statistical characteristics and long-term changes. Studies of the latter type were made with several years of the 272 kHz A3 continuous nighttime digital radio wave absorption measurements in the Central Europe described by Lasˇtovicˇka et al. (1993). The results on nighttime gravity wave activity near 95 km over Central Europe derived from these measurements for periods 10–180 min were summarized by Lasˇtovicˇka (2001). The maximum average nighttime amplitude of gravity waves varied between 2–8%. The gravity wave

4.2. Forcing by gravity waves Gravity waves are very important in the momentum and energy budget of the MLT region and for its wind system, as shown, e.g., in a review by Fritts and Alexander (2003), and they have been broadly studied in the neutral atmo-

Fig. 8. Comparison of electron density profiles on 31 January 1972 derived from slightly spaced rocket measurements, partial reflection measurements and incoherent scatter radar measurements (after Sechrist, 1974). Gravity wave type oscillations are expressed at all heights, shorter vertical wavelengths in the bottom part, longer vertical wavelengths at higher altitudes.

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activity revealed a remarkable delayed increase of duration of at least two years after the Mt. Pinatubo volcanic eruption for longer periods (T > 60 min) but no effect for short periods (T < 30 min); the change at longer periods was at least partly caused by changes in tropospheric sources. No detectable effect of QBO and of solar 27-day variation was found in the gravity wave activity. On the other hand, the magnitude of the gravity wave activity and its seasonal variation were evidently affected by the 11-year solar cycle. The gravity wave activity weakened from high to low solar activity level, at least partly due to shift of storm tracks during solar cycle. Fig. 9 shows seasonal variation of gravity wave activity inferred from the nighttime A3 radio wave absorption at 270 kHz measured at Pruhonice (50°N, 15°E). Whereas under high solar activity conditions (1989–1991) there is no seasonal variation, when solar activity decreases (1992, 1993), summertime peak appears. A shift of storm tracks (important source of gravity waves) with solar cycle contributes to the solar cycle variation of seasonal variation (Bosˇkova´ and Lasˇtovicˇka, 2001). The transformation of gravity waves in the neutral atmosphere into gravity waves in the ionized component at lower ionosphere heights is not simple. Fritts and Thrane (1990) found a frequency-dependent phase shift of gravity waves in the ionized component with respect to waves in the neutral atmosphere, mainly to chemical relaxation effects. Sugiyama (1988) pointed out other problems caused by temperature effects of gravity waves on chemical reaction rates. Xu (1999) found some influence of diabatic processes due to photochemical heating on long-period gravity

waves. Fortunately, for the 270 kHz measuring paths, nighttime conditions and heights where majority of absorption was formed (around 90 km), all these effects are negligible. 4.3. Forcing by tides As far as I know, no comprehensive investigations of tidal oscillations have been made in the lower ionosphere until now. The diurnal variation in the lower ionosphere is basically controlled by the solar zenith angle, thus the diurnal tide is to a large extent masked. On the other hand, the ionized component in the lower ionosphere is used for wind measurements via drift measurements (D1 method), meteor radar and partial reflection radar measurements. The ionized component serves as a tracer in tidal investigations in the upper middle atmosphere winds, where tides are very important, tidal amplitudes being sometimes larger than the prevailing wind. 4.4. Forcing by infrasonic waves The infrasonic waves have been studied in the higher levels in the ionosphere using Doppler type measurements, but not in the lower ionosphere. Model calculations show (Krasnov and Drobzheva, 2005) that for point-type blast sources including underground nuclear explosions, most of the infrasonic energy is deposited into the atmosphere or reflected at heights around 100 km. Sinusoidal acoustic waves emitted by a point source near or at the earth surface behave partly different. Due to non-linear processes they lose and deposit most of energy in the so called transformation region, as shown by model calculations (Krasnov et al., 2007). Fig. 10 shows the evolution of the sinusoidal acoustic signal during its propagation from the earth surface to 95 km. One can see that the sinusoidal signal of f = 10 Hz was destroyed at heights below 95 km. Only the initial and final impulses remained at 95 km. Energy was deposited to the atmosphere (and ionosphere) between heights of about 80– 90 km. Height of the transition zone rapidly increases with decreasing frequency. Thus sinusoidal point source-emitted infrasound will deposit large part of energy at lower ionosphere heights for periods from a few seconds to tenths of seconds and less. Experimental investigations in the area of lower ionosphere infrasound are very desirable. Infrasound is not a very important source of energy for the lower ionosphere but it can play quite non-negligible role. 5. Lower ionosphere forcing by lightning-induced phenomena

Fig. 9. Seasonal variation of nighttime gravity wave activity (in terms of maximum GW amplitudes) inferred from radio wave absorption measurements in central Europe for 1989–1993 in the period band 31–60 min. Each plot is offset from the preceding one to avoid overlap. The gravity wave amplitudes vary between 1.03 and 2.07 dB. After Bosˇkova´ and Lasˇtovicˇka (2001).

Lightning-induced red sprites, blue jets and elves are spectacular but very short living optical events. They are associated with electromagnetic perturbations, which cause disturbances in the lower ionosphere (typically around and below 90 km and down to the bottom boundary of the ionosphere), which are observed by high-accuracy sub-iono-

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Fig. 10. The evolution of the sinusoidal acoustic signal during its propagation from the earth surface to 95 km, f = 10 Hz, emitted by a point source (after Krasnov et al., 2007).

spheric VLF propagation measuring/monitoring systems. Increase of the MF radio wave absorption caused by lightning-induced heating of the ionosphere around 90 km has also been reported (Farges et al., 2007). A review of ULF signatures of lightning discharges has quite recently been published (Bo¨singer and Shalimov, 2008). The first connection between sprites and early/fast VLF events having short onset duration <20–50 ms was reported by Inan et al. (1995). More recently also early/ slow VLF events have been discovered (e.g., Haldoupis et al., 2006). Their onset time duration is 0.5–2 s and they are associated with sprites (e.g., Mika et al., 2005). Fig. 11 shows an example of records of early/fast and early/slow VLF events, which are evidently different. The EuroSprite-2003 campaign measurements did not reveal any early

VLF event without association with a sprite; on the other hand, basically no VLF events occurred in relation to cloud-to-ground discharges that did not lead to sprites (Mika et al., 2005). Cotts and Inan (2007) found a new class of early/fast VLF events of extremely long recovery times up to 20 min, much longer recovery than for typical early/fast VLF events. At least some of these events are consistent with possibility of persistent ionization at heights below 60 km. The importance of lightning-induced events for the lower ionosphere is a matter of debate. Mende et al. (2005) came to conclusion that thunderstorms could be an important source of ionization in the low and middle latitude nighttime D-region (80–90 km). However, their role is probably rather local, not global. Rycroft et al.

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Fig. 11. Examples of early/fast (top panel) and early slow (bottom panel) VLF amplitude perturbations observed on the 18.3 kHz path HWW-Crete (after Mika et al., 2005).

(2007) estimated the role of lightnings and sprites in ionospheric potential and found their role to be negligible, particularly as for sprites.

6. Concluding remarks Main external forcings of the lower ionosphere coming from above (solar origin) and/or below (atmospheric origin) and their ionospheric effects have briefly been reviewed. There are basically four categories of external forcing: (1) Variability of solar ionizing radiation. (2) Space weather. (3) Atmospheric waves coming from below. (4) Lightning-induced phenomena like sprites and their ionospheric counterparts. We discussed the most important phenomena but there are also other, less important but not quite negligible (at least locally/regionally) effects. Under (1) the solar cycle, solar rotation and solar flare effects have been briefly described. There are also other, less important variations, like the 13.5-day period sometimes appearing in solar ionizing radiation, the solar QBO

(quasi-biennial variation) with period somewhat shorter than the atmospheric QBO, etc. In the space weather area (2), effects of geomagnetic storms and substorms, solar proton events and highly relativistic electron events have been discussed. There are other high-energy particle effects including variability of galactic cosmic rays affecting the lowermost ionosphere, but they are less important. Various IMF polarity effects do exist, as well, but they are partly acting through changes of geomagnetic activity (e.g., changes of Bz polarity). Atmospheric waves coming from below (3) consist of planetary, gravity, tidal and infrasonic (acoustic) waves. They significantly affect the lower ionosphere, both directly and indirectly (e.g. through major stratospheric warmings). The lower ionosphere is affected also by secondary waves excited in situ in the MLT region by non-linear interactions between planetary, tidal and gravity waves (e.g., Pancheva and Mitchell, 2004). Gravity waves excited by solar terminator and gravity waves excited by geomagnetic activity in the auroral region and propagating quasi-horizontally may also to some extent influence the lower ionosphere. Lightning-induced phenomena associated with phenomena like sprites are probably unimportant globally but might be important locally in the lower ionosphere. There is another category of lightning-induced phenomena, whistler-excited precipitation of energetic electrons from magnetosphere/plasmasphere, which has also been observed in the lower ionosphere as perturbations of sub-ionospheric VLF propagation (e.g., Inan et al., 1990). The lower ionosphere is a very variable region, controlled by a broad spectrum of external forcings from above and from below. Further investigations are needed in order to determine better the relative role and importance of various external forcing and to try to develop reasonably reliable prediction methods of the state of the lower ionosphere. Acknowledgements This work was supported by the Grant Agency of the Czech Republic through Grants 205/07/1367 and 205/08/ 1356. References Baker, D.N., Blake, J.B., Klebesadel, R.W., Higbie, P.R. Highly relativistic electrons in the Earth’s outer magnetosphere, 1, Lifetimes and temporal history 1979–1984. J. Geophys. Res. 91, 4265–4273, 1986. Beynon, W.J.G., Williams, E.R. Magnetic activity and ionospheric absorption. J. Atmos. Terr. Phys. 36, 699–702, 1974. Bo¨singer, T., Shalimov, S.L. On ULF signatures of lightning discharges. Space Sci. Revs., doi:10.1007/s11214-008-9333-4, 2008. Bosˇkova´, J., Lasˇtovicˇka, J. Seasonal variation of gravity wave activity in the upper middle atmosphere in Central Europe. Studia Geophys. Geod. 45 (1), 85–92, 2001. Chevalier, M.W., Peter, W.B., Inan, U.S., Bell, F.T., Spasojevic, M. Remote sensing of ionospheric disturbances associated with energetic particle precipitation using the South Pole VLF beacon. J. Geophys. Res. 112, A11306, doi:10.1029/2007JA012425, 2007.

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