SCIENCE CHINA Technological Sciences • RESEARCH PAPER •
May 2012 Vol.55 No.5: 1264–1272 doi: 10.1007/s11431-012-4799-4
Ionospheric absorption and planetary wave activity in East Asia sector HAO YongQiang1* & ZHANG DongHe1, 2 1 2
Institute of Space Physics and Applied Technology, Peking University, Beijing 100871, China; State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, China Received December 10, 2011; accepted February 12, 2012; published online March 26, 2012
In this paper, we focus on ionospheric absorption in the East Asia sector, and look for manifestations of atmospheric influences in this area. First, a 4-year historical record of absorption measurement at Beijing is presented. This record was obtained by a sweep frequency technique, in which 27-days periodic variation of the absorption level was found to be dominant, appearing in most seasons except winters. Instead, unusual enhancements of the absorption level appeared in winters (winter anomaly), at the meantime the level varied with periods mainly in the range of 8–12 days. Comparing to 27-days period from the Sun, the shorter period oscillations should be related to planetary wave activities in lower atmosphere. Second, fmin data from 5 mid-latitude ionosondes in Japan were used as an indirect but long-term measurement. With the fmin data covering two solar cycles, disturbances with various periods were found to be active around solar maximum years, but the 8–12 days oscillations always existed in winter, showing seasonal dependence instead of connection to solar activity. These results given in this paper demonstrate seasonal and solar cycle-dependent features of the ionospheric absorption in East Asia sector, and confirm the existence of influence from atmosphere-ionosphere coupling in this area, as well as the relationship between ionospheric winter anomaly and planetary wave activity. ionospheric absorption, winter anomaly, atmosphere-ionosphere coupling Citation:
Hao Y Q, Zhang D H. Ionospheric absorption and planetary wave activity in East Asia sector. Sci China Tech Sci, 2012, 55: 12641272, doi: 10.1007/s11431-012-4799-4
1 Introduction The ionosphere, as the ionized part of the atmosphere, is mainly formed by the solar ionizing radiation, so direct control of solar origin is primarily responsible for the variation of the ionosphere. Meanwhile, it is also known that the thermosphere-ionosphere system responds rapidly to coupling from below, which is also called “meteorological influences”. Radio wave absorption in the lower ionosphere has been extensively studied to investigate the influences of coupling between the ionosphere and the atmosphere below
*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2012
it. From ground observations in Europe, wave-like oscillations of absorption level have been identified and correlated with planetary wave activities in lower atmosphere. Evidences and experimental results have confirmed that the variability of ionosphere should be connected with dynamic processes in the troposphere and stratosphere [1, 2]. However, among all the parts of the atmosphere, the upper part of the atmosphere (thermosphere) which overlaps with the ionosphere is relatively stable, whereas the lower part (troposphere) is the most disturbed and is the main source for dynamical processes in the atmosphere. Some mechanisms are needed to transfer energy and momentum upward so as to connect the variability of thermosphere/ionosphere to dynamics in lower atmosphere, even to meteorological tech.scichina.com
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activities near earth surface. Among many mechanisms that have been suggested, the upward propagation of internal atmospheric waves is an essential way to explain the vertical coupling between the lower and upper atmosphere. Charney and Drazin [3] first investigated the vertical propagation of large scale atmospheric disturbances, and found the theoretical condition of zonal wind for planetary waves to propagate vertically. Then Brown and Williams [4] showed coupling between the lower and upper atmospheres by finding good correlation between variations of pressure at stratosphere and electron density at mesosphere. The manifestations of influences from planetary waves have been widely observed in the upper atmosphere and different regions of the ionosphere. In most cases the influences register as quasi-periodic fluctuations, e.g., the planetary waves with a period of 5–30 days were observed to propagate vertically up to 110 km [5]. Not only were such kind of oscillations found in lower ionosphere, but also variations with period in range of 2–15 days were identified in F2 region of the ionosphere, suggesting that in an indirect way the planetary waves can penetrate upward and influence the highest part of the ionosphere [6, 7]. Furthermore, in geomagnetic data obtained by ground observatories, 10and 16-day period planetary wave activities were detected as well [8]. These periods seem to be of planetary waves, however, in most time the planetary wave links processes at all altitudes, instead of directly impacting the ionosphere by themselves. So the relationship between planetary wave activity and corresponding ionospheric disturbance is not a simple one. Recent numerical modeling study has found that when there is quasi-stationary planetary wave in the winter stratosphere, large ionospheric variability is found at low to middle latitude, and it was suggested that the planetary wave modulates tides, which impact the ionosphere through the E-region wind dynamo [9]. In previous studies, researchers usually focused on the lowest part of the ionosphere (i.e. D-region) to investigate the relationships between ionosphere behavior and planetary waves. The D-region is weakly ionized and the ions frequently collide with high neutral particles density at this altitude, which leads to significant effects on the absorption of high frequency radio waves. The coupling between the atmosphere and ionosphere can change the interactions between neutral and ionized species, which is directly reflected in the variations of D-region absorption. Actually, by penetration of ionizing radiations, the D-region is mainly produced in daytime by the energy from the Sun. As a consequence, solar radiation burst can largely enhance the D-region absorption, and to first order the D-region shows clear periodic variations which are directly related to the Sun (e.g., 27-day solar rotation, and seasonal variations caused by the changes of solar zenith angle). The connection between the ionosphere and solar radiation is well known, even the transient response of ionosphere to sudden burst of solar flare has been carefully studied recently (e.g.,
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refs. [10, 11]). But, on the other hand the ionosphere often exhibits more complex behaviors under the influence from the atmosphere, significant anomalies of D-region absorption in its normal diurnal and seasonal variations for example. This is the effect of internal atmospheric waves, which is below the ionosphere and generated by energy sources in lower atmosphere. The energy sources are season-dependent or meteorological event related, and all kinds of waves superimpose oscillations of various periods on the ionosphere. The effects from above and below coexist in the ionosphere, which makes it difficult to investigate individually the coupling between the atmosphere and ionosphere. Nevertheless, for the ionospheric observation data, some methods can be employed to separate the influence of the atmosphere origin from those of the solar origin, then the features and mechanism of the coupling can be discussed. In the past decades, the ionosphere over Europe has been extensively studied for its absorption property (see the references cited above), and has been taken as the representative of mid-latitude ionosphere. However, in sectors with different longitudes, some energy sources or dynamic processes in lower atmosphere are localized, which could be coupled into the ionosphere, leading to different longitudinal features. With riometer data obtained at Zhongshan station, Chinese scientists have carried out research on ionospheric absorption over Antarctica [12, 13]. In this paper we focus on the regional features of ionospheric absorption in East Asia sector, and the related planetary wave activity in this area. First, measurement data of ionospheric absorption in Beijing are presented, and they are analyzed to investigate winter anomaly and periodic variations related to atmospheric waves. Then the ionosonde fmin data from 5 stations in Japan are also considered, from which the features of D-region absorption variability in a solar cycle are found and demonstrated. These data give direct or indirect measurements of ionospheric absorption, in which oscillation component with period of about 27 days is clear, but by separating components of different periods, we pay attention to shorter period waves related to planetary waves, and their season-dependent and solar cycle-dependent behaviors. The data and analysis results presented here give an overview of the absorption property of the ionosphere in the East Asia sector, and could be a basis on which the regional features of coupling processes are investigated.
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Absorption measurements at Beijing
For many years the radio wave absorption in the lower ionosphere has been measured by three methods. In each method radio waves are transmitted through the ionosphere, and the attenuation is obtained at the receiving end: A1, receiving waves reflected in the vertical direction; A2, receiving extraterrestrial radio noise (riometer); A3, receiving waves reflected from the ionosphere in oblique incidence.
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The data obtained by these methods have greatly improved our knowledge and understanding of the Earths ionosphere. For example, the periods of planetary waves were found in ionosphere variations over Europe, through the analysis on observations by A3 method [14, 15]. In addition to the above three methods, Ilias and Gupta [16] suggested a sweep frequency technique which receives various man-made radio transmissions for broadcasting. These radio waves are reflected by the ionosphere and trapped in the Earth-ionosphere cavity. Monitoring of the variation of the total radio power in a particular frequency band can give a way to study the mean level of ionospheric absorption. In 1982, we established a similar instrument at Peking University in Beijing (40N, 116E). This device sweeps the frequency range of 2–4 MHz, and integrates the power strength of the radio signals in this band. The total power strength is recorded and considered as a relative measurement of the signal level after attenuation by the ionosphere, thus it is inversely proportional to the mean level of ionospheric absorption. To study the daytime absorption, the power strength data at noon (11–13 LT) were selected and averaged for every single day, as shown in Figure 1(a) for time range from May 1982 to April 1986. As the conditions of solar flux are critical to the absorption strength of lower part ionosphere, the panel (c)–(e) of Figure 1 also shows data of sunspot number (SSN), F10.7 index and Lyman- (1216 Å) flux. The sun-
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spot number and F10.7 index were from SPIDR website (http://spidr.ngdc.noaa.gov/), and Lyman- data were downloaded from NGDC (http://www.ngdc.noaa.gov/stp/ solar/solaruv.html), which were obtained by space borne measurements of Solar Mesosphere Explorer [17]. 2.1
Annual variation
From year 1982 to 1986, the solar activity was in the descending phase of solar cycle 21, when the irradiation of the Sun decreased securely. The decrease can be seen clearly from the sunspot number, F10.7 index and Lyman- flux shown in Figure 1. Also, Figure 1(b) shows the daytime (noon) electron density at 80 km altitude (D-region) calculated by IRI model. Though the electron density shows strong yearly oscillations, with the Sun being inactive the mean level also decreases year after year. Comparing Figures 1(a) and (b) it is clear that both the shortwave power strength and D-region electron density show reasonable yearly oscillations, which are the result of solar zenith angle variation in a year; also they are well anti-correlated with each other, that is, with high electron density and low shortwave strength (high ionospheric absorption) in summers of the northern hemisphere. A special feature of solar cycle 21 should be mentioned, that is, periodic variations can be found in SSN, F10.7 and Lyman- data in the range of time from 1982 to 1984. As
Figure 1 Ionospheric absorption measurements at Beijing and the background ionosphere electron density, as well as solar irradiation data from May 1982 to April 1986. (a) Relative power strength of radio waves in 2–4 MHz measured by sweep frequency technique, (b) Electron density at 80 km calculated by IRI model, (c) Sun spot number, (d) Solar F10.7 index, and (e) Lyman-α flux measured by SME.
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can be seen in Figure 1, the minimum levels of SSN, F10.7 and Lyman-α flux all appeared around Feb. 1983, Jan. 1984 and Jan. 1985, which showed clearly oscillation of 1-year period. This is not in accordance with our knowledge of the Sun: it is well known that solar irradiation varies with period of 27-days and 11-years, and corresponding oscillations are dominant in the ionosphere and atmosphere; however, no evidence has been found to indicate that the Sun varies by season consistently. Although the observation data show that this kind of annual variation exists simultaneously in solar irradiation and D-region electron density (as well as in the ionospheric absorption), they are not dependent on each other. Instead, the changing of solar zenith angle in a year is considered as a primary driving force, which is responsible for the annual variations in the ionosphere. Actually, the yearly variation of solar irradiation was just a special case in year 1982–1984, which was not persistent throughout the solar cycle. The enhancement of irradiation comes from solar active regions, which are not distributed randomly, but are clustered in groups. A long-lived active region group may endure for six months, as has been observed during the ascending phase of the 21st solar cycle [18]. If great groups of active regions emerge and disappear approximately one time a year, then the solar irradiation may seem to have annual variation. This is the case for solar activity in the range of time 1982–1984. 2.2
Winter anomaly and planetary waves
In Figure 2(a), the shortwave strength curve is shown again, and superimposed on it is a sine curve with period of 1 year. The sine curve roughly presents a yearly variation pattern of the shortwave strength. Comparing the two curves, unusual low values of the shortwave strength can be found in winters of the successive 4 years (the winter time is marked by vertical dotted lines, that is, the months of Dec. and Jan.). Low shortwave strength means an enhancement of ionospheric absorption. But from the Lyman-α and F10.7 data shown in Figure 1, in these winters no solar irradiation flux burst occurred. So the absorption enhancement in winters (so-called winter anomaly) cannot be explained by variation of solar irradiation, the intense ionization of the lower ionosphere was due to non-solar factors. Because of the season-dependence (always occurs in winters), the winter anomaly was possibly connected to special winter conditions of the atmosphere/ionosphere of the Earth. To distinguish between the influences of solar and atmospheric origin, we used band-pass filter technique to isolate oscillation components of different periods in the shortwave absorption data. The components of period around 27-days and shorter than 27-days were taken as the signals of solar and atmosphere origin influences, respectively. Also the band-pass filter was applied to the solar Lyman-α flux data as a contrast. Signal of solar control was first tested by isolating the
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component with period around 27-days. A 25–30 day period band-pass filter was applied and the results were demonstrated in Figures 2(b) and (c). The signals of periodic oscillations around 27-days were strong in both of the two datasets. They are certainly related to the solar irradiation modulated by the self-rotation of the Sun. Despite the dominant 27 days signals, oscillations of other frequencies were identified by selecting band-pass range as 8–12 days: the result of shortwave strength data were shown in Figure 3(b), in which the oscillation component of 8–12 days was significant in winters (Feb. and Jan.) and suppressed in other seasons; the isolated component of Lyman- data was shown in Figure 3(c), which was stable all the time without unusual activities in winters. Comparing the two results, the 8–12 days oscillations were not evident in the Sun, nor their seasonal dependence; but they were pronounced in winter ionospheric absorption, and coincident in time with the winter abnormal enhancements of absorption level shown in Figure 3(a). The comparison denies the influence of solar origin with period shorter than 27 days, and implies a relationship between winter anomaly of absorption and 8–12 day oscillations in the atmosphere.
3
fmin data and solar cycle variation
The ionosphere is variable and displays oscillations over a wide range of timescales. To investigate long-term variations of the ionospheric absorption, datasets covering at least one solar cycle are necessary. Here, fmin parameter measured by standard vertical incidence ionosonde is used, because fmin represents the minimum frequency below which the radio signal transmitted by ionosonde is strongly absorbed by the ionosphere, so it is expected to be a rough measure of radio wave absorption in the ionosphere (e.g. refs. [19, 20]). Also, thanks to decades of continuous operation of the global ionosonde network, the published data cover several solar cycles. The fmin parameter data were obtained from the 1994 NGDC/WDCA Ionospheric Digital Database (CD-ROM) for the following analysis, with the fmin values at noon (12 LT) extracted for every single day. Because our study focuses on the East Asia sector, we choose fmin data from 5 mid-latitude ionosonde stations in Japan, whose latitudes are similar to Beijing as shown in Table 1. To show some straightforward features of the fmin data, they are displayed together with shortwave absorption data which have been introduced previously, and with solar Lyman-α flux as well. We choose the first and the last year in this period from 1982 to 1986, and demonstrate the data in Figures 4 and 5. The first year-long dataset is from Apr. 1982 to Mar. 1983 and the second one is from Apr. 1984 to Mar. 1985. Being in the descending phase of the cycle, the solar activity was much lower for the later dataset. In Figures 4 and 5, in all the three kinds of measurement data pe-
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Figure 2 25–30 day band-pass filtering results for 4-year absorption and solar irradiation data. (a) Shortwave strength data, (b) Result of band-pass filtering on shortwave strength, (c) Band-pass filtering result of solar Lyman-α flux.
Figure 3 Table 1
Similar to Figure 2, but the period range of the filter is 8–12 days.
Names and locations of 5 ionosonde stations
Code OK426 YG431 TO535 AK539 WK545
Station OKINAWA YAMAGAWA KOKUBUNJI AKITA WAKKANAI
Geo. Lat. 26.3 31.2 35.7 39.7 45.4
Geo. Long. 127.8 130.6 139.5 140.1 141.7
riodic variations are found to be pronounced around summer (May to Aug.). With assistance of the time markers
(27-day spacing vertical dotted lines), the period of fmin data is clearly identified to be about 27 days in summer, but much shorted in winter (Dec. and Jan.). This is consistent with the band-pass filter results in the last section that 8–12 day oscillations emerged in the ionosphere during winter. Furthermore, comparing the fmin values among the 5 ionosondes shown in Figures 4 and 5, in summer lower latitude ionosondes (OK426 and YG431) had higher fmin levels, due to intense ionization of low ionosphere close to the equator; however, in winter the fmin values of higher latitude iono-
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sondes (AK539 and WK545) were greatly enhanced and even exceed the levels of lower latitude stations. This is a typical manifestation of winter anomaly which occurs in mid-latitude ionosphere, and in this study it is found in the observations at Beijing (39.9N), AK539 (39.7N) and WK545 (45.4N). This also indicates that in the East Asia sector the winter anomaly is detectable at latitudes around
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40° (AACGM Lat. 34N) and above, but not significant at lower latitudes. Long-term evolution of ionospheric absorption in East Asia region is also a key to understand the solar cycle dependence of planetary wave influence in the area. For long-term analysis, the continuous fmin data of AK539 covering two solar cycles (from year 1969 to 1987) were taken
Figure 4 Absorption and solar irradiation data from Apr. 1982 to Mar. 1983. (a) Shortwave power strength at Beijing; (b) Solar Lyman-α flux; (c) fmin values of 5 ionosondes.
Figure 5
Similar to Figure 4, but the data were of time period from Apr. 1982 to Mar. 1983.
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into consideration. For the minimization and compensation of the instrumental errors, we used another parameter dfmin which is defined as the difference between the daily noon value and monthly median noon value of fmin. In this analysis, the dfmin data were first divided into sections, with each section coving 3 months, i.e. the data were grouped into seasons (Dec.–Feb., Mar–May, Jun.–Aug., Sep.–Nov.); then for every section of the data, its frequency spectrum was calculated by maximum entropy method (MEM); finally, the resulting spectra were displayed in Figure 6 for different seasons and sorted by year. From Figure 6, we can summarize the behavior of ionospheric absorption in a solar cycle as follows. 1) In every season, the 27-day (0.03–0.04 cycles/day) os-
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cillations are dominant, but they are more pronounced around solar high active years (e.g., around years 1970 and 1980). 2) Planetary waves related oscillations of period 8–12 days (0.08–0.12 cycles/day) always appear in winters. However, around solar maximum years this kind of short period variations is also evident in the other seasons, which may be the result of frequent solar irradiation bursts. 3) In solar inactive years (e.g., around years 1976 and 1986), the solar control is minimum. At this time, both longer and shorter period oscillations are suppressed in all seasons except winters. Disturbances of various periods are enhanced in winters, implying a non-solar origin influence on the ionospheric absorption which is localized or related
Figure 6 The MEM spectra of fmin data at AK539 for every season from 1969 to 1987. (a) Winter (Dec.–Feb.); (b) Spring (Mar.–May); (c) Summer (Jun.–Aug.); (d) Autumn (Sep.–Nov.).
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to the atmospheric activities underneath.
4
Summary and discussion
In this paper we focus on the radio wave absorption property of the ionosphere over the East Asia sector, which was measured by two types of technique, at Beijing and 5 ionosonde stations in Japan, respectively. The observation data show that periodic oscillations of 27-days are dominant almost all the time, especially in solar active years; meanwhile, in the years near solar maximum, solar bursts also drive shorter period variations which emerge in every season. Both of the two kinds of disturbances are controlled by sudden or continuous variations of solar irradiation. However, coupling from dynamics processes in the atmosphere affects the ionospheric behavior as well. With energy and momentum transferred by planetary waves, the ionosphere can be largely disturbed. The observational evidence shows that in the East Asia sector the impact from atmosphere is significant in regional ionosphere with latitude around 40° (AACGM Lat. 34N) and above; the ionospheric absorption is always unusual enhanced in winters no matter what level the solar activity is, and the absorption level varies with period between 8 and 12 days. These results indicate that the coupling between the atmosphere and ionosphere is season-dependent, instead of solar cycle dependent. Through the data and analysis presented above, control from the atmosphere is clearly distinguished from that origin in the Sun, and some special regional characteristics are found for the atmosphere-ionosphere interaction above the East Asia sector. Unusual disturbance in the ionosphere has been found to be related to at least two types of events in low atmosphere, one is the sudden stratospheric warming (SSW), and the other is the typhoon (or hurricane, tropical cyclone). The occurrence of SSWs is related to the growth of quasi-stationary planetary waves, which interact nonlinearly with the tides at lower latitudes, and amplify the amplitude of tidal modes in the mesosphere-lower thermosphere region, hence the ionosphere is indirectly modulated by the planetary wave [9, 21]. A typhoon, however, is surely one of the important ground sources of the wave-like disturbances in troposphere. Observational facts have shown close relation of TIDs or spread F phenomena in the ionosphere with typhoons landing on or near the east coast of China [22, 23]. Their studies emphasize the impact on the ionosphere by cyclone exciting acoustic-gravity waves, and imply that the response of ionosphere is limited and localized in the region approximately above the cyclone location in troposphere. Actually, statistical comparison has been made on the occurrence of spread F between two ionosonde stations in China. The two stations are separated by 38° in longitude and with very different ground meteorological conditions.
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The results showed that the occurrence rate at one station (near coast) is always much larger than at the other one (in the very center of the continent) [24]. It is obvious that the lower atmosphere forcing mentioned above are usually longitudinally dependent, and even localized in a small area, hence predicting the ionosphere is even more difficult. In general, the forcings from both above and below forms a broad spectrum, which makes the lower ionosphere very variable. Further understanding of the mechanism and processes in atmosphere-ionosphere coupling is needed in order to improve models and methods, then numerical simulation and forecasting of regional ionosphere will be interesting and applicable subjects in the future. This work was supported by the National Natural Science Foundation of China (Grant No. 40904036), the Public Science and Technology Research Funds Projects of Ocean, State Oceanic Administration of China (Grant No. 201005017), the National Basic Research Program of China (“973” Project) (Grant No. 2011CB811405) and the Specialized Research Fund for State Key Laboratories. 1
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