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Tropospheric events and possible related gravity wave activity effects on the ionosphere
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
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Tropospheric events and possible related gravity wave activity e*ects on the ionosphere ∗ , J. Bo+ + P. Sauli ska Institute of Atmospheric Physics, Czech Academy of Sciences, Bocni II, 1401, 141 31, Prague 4, Czech Republic Received 3 December 1999; received in revised form 16 August 2000; accepted 7 September 2000
Abstract Short-term changes of electron concentration in the ionospheric F-region have been studied during a campaign of rapid sequence vertical ionospheric sounding. The wave-like structures occurred under low solar and geomagnetic activity. The most probable source of the observed waves was below the 175 km level and probably not in the auroral zone. The vertical components of propagation characteristics of the observed waves correspond to the theory of acoustic gravity wave propagation in the ionosphere and con6rm the position of the source being below the studied heights. Another case studied showed the c 2001 Elsevier Science e*ects of passage of strong tropospheric cold front under high solar activity during February 1990. Ltd. All rights reserved. Keywords: Gravity waves; Ionosphere; Cold fronts
1. Introduction The motion of a large meteorological phenomenon causes wave-like structures that propagate from the troposphere to the thermospheric heights (Clark et al., 1971). Acoustic gravity waves (AGW) caused by this phenomenon a*ect the neutral atmosphere and consequently the ionosphere, and perturbations of the ionosphere due to AGW are observable in many ionospheric parameters such as electron and ion temperatures and densities (Kelley, 1997; Liu et al., 1998). Some of the e*ects are, for instance, changes of electron density pro6les (by gravity waves launched by weather fronts in the troposphere that propagate to the F2 region), increases of F-layer critical frequencies, and variations in a sporadic E-layer. Changes in the ionosphere, which originated in meteorological phenomena have been observed using di*erent techniques and described by various authors (for instance, Bertin et al., 1975).
∗
Corresponding author. + E-mail address: [email protected] (P. Sauli).
The present work has been concerned with a study of the e*ects of AGW in the ionosphere connected with passages of weather fronts based on the results of ionospheric vertical sounding.
2. Experimental data A campaign of 5-min rapid-sequence soundings was run on 27 October–3 November 1997, and regular 15-min vertical ionospheric soundings were collected in February 1990 at the midlatitude ionospheric observatory of PruB honice (49:9◦ N; 14:5◦ E). To avoid the contamination of the data, the November 1997 campaign was run under relatively low solar and geomagnetic activity (especially no SSC events have occurred). The geomagnetic activity during this period is shown in Fig. 1. The results of ionograms from the rapid run ionospheric soundings were converted into a sequence of a true height electron density pro6les by polynomial analyses method (POLAN) (Titheridge, 1985). These series of N (h) pro6les were transformed to time variations of the electron concentrations at 6xed heights with 5 km steps from 150 to 235 km. Fig. 2 presents the results for
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 2 0 5 - 4
946
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
Fig. 1. Geomagnetic activity during campaign in November–October 1997. Daily values of Ap index and Sum Kp index (left). 3-h values of Kp index (right).
Fig. 2. Row data of 5 min campaign of the electron density at 6xed altitude of 31 October 1997 (a) and 3 November 1997 (b).
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
947
days 31 October 1997 and 3 November 1997. Among the random and quasi-periodic oscillations, wave-like structures are observable, for example, in the time interval 7–10 UT on 31 October 1997 and during the afternoon of 3 November 1997. During these 2 days the geomagnetic activity was extremely low (Ap = 5 for 31 October and Ap = 4 for 3 November). 2.1. Results of 1997 campaign Time variations of the electron density at constant heights were analysed with the aim to 6nd possible quasi-periodic oscillations of AGW type, using the correloperiodogram method (Kopecki and Kuklin, 1971; Vitinsky et al., 1986; Bloom6eld, 1976). The method consists in computating the correlation coeIcients of the time sequences of electron concentration in given altitudes with the periodic functions. For each frequency, the con6dence level is computed. Only periods with the con6dence level higher than 0.9 are taken into account as physical real cases. The results of spectral analyses on 3 November 1997 for the time interval 9:05 UT to 17:15 UT at altitudes of 150 –235 km in 5 km steps, show two quasi-periodic oscillations with a con6dence level exceeding 0.9 with periods 86 and 106 min. Fig. 3. Vertical amplitude pro6les for two dominant oscillating periods.
Fig. 4. Vertical pro6les of phase velocity (top) and group velocity (bottom) — wave period 86 min — of 3 November 1997.
Fig. 5. Vertical pro6les of phase velocity (top) and group velocity (bottom) — wave period 106 min — of 3 November 1997.
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' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
Fig. 6. The 1-h course of tropospheric temperature measured at Mile+sovka observatory during February 1990.
and group velocities were computed for two waves with periods 86 and 106 min. The vertical pro6les of phase and group velocities con6rm that the direction of wave propagation is consistent with the acoustic gravity wave propagation theory (Hines, 1960) for both the 86 min wave (Fig. 4) as well as the 106 min wave (Fig. 5). The positive group velocity, in both cases, indicates the upward energy propagation while the negative phase velocity shows the downward phase progression at all examined altitudes. These results con6rm that the two observed internal gravity waves propagate to ionospheric heights from below. 2.2. February 1990 event
Fig. 7. Acoustic gravity wave spectra computed from results of A3 absorption measurement on 270 kHz, 7–9 February 1990.
The vertical amplitude pro6les of these dominant periods for heights 175 –235 km are shown in Fig. 3. Using the method described by Liu et al. (1998) and Altadill et al. (1999), the vertical components of the phase
The event observed during 6rst decade of February 1990 allows comparing the e*ect of passage of the cold front with the period of quiet weather conditions. The possible strong e*ect due to the passage was observed on 8 February 1990. During 6rst 10 days of February, there was a considerable northward eddy heat Mux by the strong wave leading to a pronounced minor stratospheric warming. An enormous temperature change was observed from 1 to 10 February. The warming was accompanied by the typical cooling in the mesosphere. Tropospheric temperature changes, during this time interval measured hourly at Mile+sovka observatory (867 m), are presented in Fig. 6. The passage of the strong cold front started in the afternoon of 8 February 1990 at 13 UT, occured during an interval of low geomagnetic activity (Ap = 8 on 8 February and Ap = 9 on 9 February). The interesting and possible ionospheric responses to the changing tropospheric conditions were observed in lower ionosphere (D-region) and as well in the thermosphere. The lower ionosphere responded with a pronounced decrease of radio wave absorption over central Europe during this
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
949
Fig. 8. Acoustic gravity wave activity (amplitude spectra and corresponding level of signi6cance) computed from vertical ionospheric sounding data measured at PruB honice observatory.
period (Naujokat et al., 1993). The interesting e*ects were also observed in ionospheric vertical sounding data at PruB honice station. The digital A3 absorption measurements are being run at the PruB honice observatory for many years. The digital night-time absorption measurements at 270 kHz allow the use of absorption for monitoring AGW activity in the lower ionosphere (La+stovi+cka et al., 1993) in the period range 10 min–3 h. Fig. 7 shows the changes of the gravity wave activity during days 7–9 February. During the day of cold front passage of signi6cant increasing of AGW activity at this period range is observed. (Dotted horizontal lines show level of signi6cance 0.9 for each day.) During the day before (7 February 1990) and the day after (9 February 1990) the event, signi6cant acoustic gravity wave activity was not present in the period range of 50–100 min. The variations of the electron density on 8 and 9 February 1990 based on 15 min vertical ionospheric sounding were calculated using the same procedure as during the campaign 1997. The similar thermospheric responses to the tropospheric changes are presented in Fig. 8. Results of spectral analysis in the upper part (the day of event) shows signi6cant AGW activity in contrast to the lower part (the day
after) in the same period range (50 –100 min). During the event of passage of cold tropospheric front, similar responses are observed in the lower ionosphere (absorption measurements at 270 kHz are mainly a*ected at heights 85 –95 km) as well as in the thermospheric heights.
3. Conclusion During a campaign of rapid-run ionospheric soundings, two cases were observed on 3 November 1997. The results of analyses show that the vertical component of group velocity of the detected waves con6rms the acoustic gravity waves propagating upward. The source of detected waves lies below the 180 km altitude. The data used for analyses does not allow the determination of the sources of these waves. The vertical characteristics show that the detected waves rather than auroral (TID) could be of tropospheric origin. In the cases 1997 and also in the case 1990 a similar increase of AGW activity at the period range 50 –100 min was observed. The possible explanation is that AGWs are launched by the system of tropospheric fronts observed over Central Europe.
950
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
References Altadill, D., Boska, J., SolNe, J.G., Alberca, L.F., 1999. Short-time Fluctuations in the F region Electron Density. Paper presented at COST 251 meeting, Madeira 1999. Bertin, F., Testud, J., Kersley, L., 1975. Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances. Planetary Space Science 23, 493–507. Bloom6eld, P., 1976. Fourier Analysis of Time Series: An Introduction, John Wiley & Sons, New York. Clark, R.M., Yeh, K.C., Liu, C.H., 1971. Interaction of internal gravity waves with the ionospheric F2-layer. Journal of Atmospheric and Terrestrial Physics 33, 1567–1576. Hines, C.O., 1960. Internal atmospheric gravity waves at ionospheric heights. Canadian Journal of Physics 38, 1441–1481. Kelley, M.C., 1997. In situ ionospheric observations of severe weather-related gravity waves and associated small-scale plasma structure. Journal of Geophysical Research 102, 329–335. Kopecki, M., Kuklin, G.V., 1971. K voprosu ob 11-letneiQ variacii sredneiQ prodol2itelbnosti 2izni grupp
solneqnyh p%ten. Issledovanie po geomagnetizmu, a&ronomii i fizyke Solnca, vyp. 2, str. 167–179. La+stovi+cka, J., Bo+ska, J., Bure+sovNa, D., 1993. Digital measurements of LF radio wave absorption in the lower ionosphere and inferred gravity wave activity. Annals of Geophysicae 11, 937–946. Liu, J.Y., Shiao, C.C., Tsai, L.C., Liu, C.H., Kuo, F.S., Lue, H.Y., Huang, C.M., 1998. Vertical phase and group velocities of internal gravity waves derived from ionograms during the solar eclipse of 24 October 1995. Journal of Atmospheric and Solar-Terrestrial Physics 60, 1679–1686. Naujokat, B., Labitzke, K., Lenschour, R., Petzdolt, K., Wohlfast, R.-C., 1993. Report on the winter 1989/90: very cold with a pronounced minor warming and a late 6nal warming. STEP-Collection of reports on the stratospheric circulation during the winters 1974=75 –1991=92, pp. 237–257. Titheridge, J.E., 1985. Ionogram analysis with the generalised program POLAN. Report UAG-93. Vitinsky, Y.I., Kopecki, M., Kuklin, G., 1986. Statistics of Sunspot-Creating Activity (in Russian), Nauka, Moscow.
www.elsevier.nl/locate/jastp
Tropospheric events and possible related gravity wave activity e*ects on the ionosphere ∗ , J. Bo+ + P. Sauli ska Institute of Atmospheric Physics, Czech Academy of Sciences, Bocni II, 1401, 141 31, Prague 4, Czech Republic Received 3 December 1999; received in revised form 16 August 2000; accepted 7 September 2000
Abstract Short-term changes of electron concentration in the ionospheric F-region have been studied during a campaign of rapid sequence vertical ionospheric sounding. The wave-like structures occurred under low solar and geomagnetic activity. The most probable source of the observed waves was below the 175 km level and probably not in the auroral zone. The vertical components of propagation characteristics of the observed waves correspond to the theory of acoustic gravity wave propagation in the ionosphere and con6rm the position of the source being below the studied heights. Another case studied showed the c 2001 Elsevier Science e*ects of passage of strong tropospheric cold front under high solar activity during February 1990. Ltd. All rights reserved. Keywords: Gravity waves; Ionosphere; Cold fronts
1. Introduction The motion of a large meteorological phenomenon causes wave-like structures that propagate from the troposphere to the thermospheric heights (Clark et al., 1971). Acoustic gravity waves (AGW) caused by this phenomenon a*ect the neutral atmosphere and consequently the ionosphere, and perturbations of the ionosphere due to AGW are observable in many ionospheric parameters such as electron and ion temperatures and densities (Kelley, 1997; Liu et al., 1998). Some of the e*ects are, for instance, changes of electron density pro6les (by gravity waves launched by weather fronts in the troposphere that propagate to the F2 region), increases of F-layer critical frequencies, and variations in a sporadic E-layer. Changes in the ionosphere, which originated in meteorological phenomena have been observed using di*erent techniques and described by various authors (for instance, Bertin et al., 1975).
∗
Corresponding author. + E-mail address: [email protected] (P. Sauli).
The present work has been concerned with a study of the e*ects of AGW in the ionosphere connected with passages of weather fronts based on the results of ionospheric vertical sounding.
2. Experimental data A campaign of 5-min rapid-sequence soundings was run on 27 October–3 November 1997, and regular 15-min vertical ionospheric soundings were collected in February 1990 at the midlatitude ionospheric observatory of PruB honice (49:9◦ N; 14:5◦ E). To avoid the contamination of the data, the November 1997 campaign was run under relatively low solar and geomagnetic activity (especially no SSC events have occurred). The geomagnetic activity during this period is shown in Fig. 1. The results of ionograms from the rapid run ionospheric soundings were converted into a sequence of a true height electron density pro6les by polynomial analyses method (POLAN) (Titheridge, 1985). These series of N (h) pro6les were transformed to time variations of the electron concentrations at 6xed heights with 5 km steps from 150 to 235 km. Fig. 2 presents the results for
c 2001 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 2 0 5 - 4
946
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
Fig. 1. Geomagnetic activity during campaign in November–October 1997. Daily values of Ap index and Sum Kp index (left). 3-h values of Kp index (right).
Fig. 2. Row data of 5 min campaign of the electron density at 6xed altitude of 31 October 1997 (a) and 3 November 1997 (b).
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
947
days 31 October 1997 and 3 November 1997. Among the random and quasi-periodic oscillations, wave-like structures are observable, for example, in the time interval 7–10 UT on 31 October 1997 and during the afternoon of 3 November 1997. During these 2 days the geomagnetic activity was extremely low (Ap = 5 for 31 October and Ap = 4 for 3 November). 2.1. Results of 1997 campaign Time variations of the electron density at constant heights were analysed with the aim to 6nd possible quasi-periodic oscillations of AGW type, using the correloperiodogram method (Kopecki and Kuklin, 1971; Vitinsky et al., 1986; Bloom6eld, 1976). The method consists in computating the correlation coeIcients of the time sequences of electron concentration in given altitudes with the periodic functions. For each frequency, the con6dence level is computed. Only periods with the con6dence level higher than 0.9 are taken into account as physical real cases. The results of spectral analyses on 3 November 1997 for the time interval 9:05 UT to 17:15 UT at altitudes of 150 –235 km in 5 km steps, show two quasi-periodic oscillations with a con6dence level exceeding 0.9 with periods 86 and 106 min. Fig. 3. Vertical amplitude pro6les for two dominant oscillating periods.
Fig. 4. Vertical pro6les of phase velocity (top) and group velocity (bottom) — wave period 86 min — of 3 November 1997.
Fig. 5. Vertical pro6les of phase velocity (top) and group velocity (bottom) — wave period 106 min — of 3 November 1997.
948
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
Fig. 6. The 1-h course of tropospheric temperature measured at Mile+sovka observatory during February 1990.
and group velocities were computed for two waves with periods 86 and 106 min. The vertical pro6les of phase and group velocities con6rm that the direction of wave propagation is consistent with the acoustic gravity wave propagation theory (Hines, 1960) for both the 86 min wave (Fig. 4) as well as the 106 min wave (Fig. 5). The positive group velocity, in both cases, indicates the upward energy propagation while the negative phase velocity shows the downward phase progression at all examined altitudes. These results con6rm that the two observed internal gravity waves propagate to ionospheric heights from below. 2.2. February 1990 event
Fig. 7. Acoustic gravity wave spectra computed from results of A3 absorption measurement on 270 kHz, 7–9 February 1990.
The vertical amplitude pro6les of these dominant periods for heights 175 –235 km are shown in Fig. 3. Using the method described by Liu et al. (1998) and Altadill et al. (1999), the vertical components of the phase
The event observed during 6rst decade of February 1990 allows comparing the e*ect of passage of the cold front with the period of quiet weather conditions. The possible strong e*ect due to the passage was observed on 8 February 1990. During 6rst 10 days of February, there was a considerable northward eddy heat Mux by the strong wave leading to a pronounced minor stratospheric warming. An enormous temperature change was observed from 1 to 10 February. The warming was accompanied by the typical cooling in the mesosphere. Tropospheric temperature changes, during this time interval measured hourly at Mile+sovka observatory (867 m), are presented in Fig. 6. The passage of the strong cold front started in the afternoon of 8 February 1990 at 13 UT, occured during an interval of low geomagnetic activity (Ap = 8 on 8 February and Ap = 9 on 9 February). The interesting and possible ionospheric responses to the changing tropospheric conditions were observed in lower ionosphere (D-region) and as well in the thermosphere. The lower ionosphere responded with a pronounced decrease of radio wave absorption over central Europe during this
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
949
Fig. 8. Acoustic gravity wave activity (amplitude spectra and corresponding level of signi6cance) computed from vertical ionospheric sounding data measured at PruB honice observatory.
period (Naujokat et al., 1993). The interesting e*ects were also observed in ionospheric vertical sounding data at PruB honice station. The digital A3 absorption measurements are being run at the PruB honice observatory for many years. The digital night-time absorption measurements at 270 kHz allow the use of absorption for monitoring AGW activity in the lower ionosphere (La+stovi+cka et al., 1993) in the period range 10 min–3 h. Fig. 7 shows the changes of the gravity wave activity during days 7–9 February. During the day of cold front passage of signi6cant increasing of AGW activity at this period range is observed. (Dotted horizontal lines show level of signi6cance 0.9 for each day.) During the day before (7 February 1990) and the day after (9 February 1990) the event, signi6cant acoustic gravity wave activity was not present in the period range of 50–100 min. The variations of the electron density on 8 and 9 February 1990 based on 15 min vertical ionospheric sounding were calculated using the same procedure as during the campaign 1997. The similar thermospheric responses to the tropospheric changes are presented in Fig. 8. Results of spectral analysis in the upper part (the day of event) shows signi6cant AGW activity in contrast to the lower part (the day
after) in the same period range (50 –100 min). During the event of passage of cold tropospheric front, similar responses are observed in the lower ionosphere (absorption measurements at 270 kHz are mainly a*ected at heights 85 –95 km) as well as in the thermospheric heights.
3. Conclusion During a campaign of rapid-run ionospheric soundings, two cases were observed on 3 November 1997. The results of analyses show that the vertical component of group velocity of the detected waves con6rms the acoustic gravity waves propagating upward. The source of detected waves lies below the 180 km altitude. The data used for analyses does not allow the determination of the sources of these waves. The vertical characteristics show that the detected waves rather than auroral (TID) could be of tropospheric origin. In the cases 1997 and also in the case 1990 a similar increase of AGW activity at the period range 50 –100 min was observed. The possible explanation is that AGWs are launched by the system of tropospheric fronts observed over Central Europe.
950
' P. Sauli, J. Bo'ska / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 945–950
References Altadill, D., Boska, J., SolNe, J.G., Alberca, L.F., 1999. Short-time Fluctuations in the F region Electron Density. Paper presented at COST 251 meeting, Madeira 1999. Bertin, F., Testud, J., Kersley, L., 1975. Medium scale gravity waves in the ionospheric F-region and their possible origin in weather disturbances. Planetary Space Science 23, 493–507. Bloom6eld, P., 1976. Fourier Analysis of Time Series: An Introduction, John Wiley & Sons, New York. Clark, R.M., Yeh, K.C., Liu, C.H., 1971. Interaction of internal gravity waves with the ionospheric F2-layer. Journal of Atmospheric and Terrestrial Physics 33, 1567–1576. Hines, C.O., 1960. Internal atmospheric gravity waves at ionospheric heights. Canadian Journal of Physics 38, 1441–1481. Kelley, M.C., 1997. In situ ionospheric observations of severe weather-related gravity waves and associated small-scale plasma structure. Journal of Geophysical Research 102, 329–335. Kopecki, M., Kuklin, G.V., 1971. K voprosu ob 11-letneiQ variacii sredneiQ prodol2itelbnosti 2izni grupp
solneqnyh p%ten. Issledovanie po geomagnetizmu, a&ronomii i fizyke Solnca, vyp. 2, str. 167–179. La+stovi+cka, J., Bo+ska, J., Bure+sovNa, D., 1993. Digital measurements of LF radio wave absorption in the lower ionosphere and inferred gravity wave activity. Annals of Geophysicae 11, 937–946. Liu, J.Y., Shiao, C.C., Tsai, L.C., Liu, C.H., Kuo, F.S., Lue, H.Y., Huang, C.M., 1998. Vertical phase and group velocities of internal gravity waves derived from ionograms during the solar eclipse of 24 October 1995. Journal of Atmospheric and Solar-Terrestrial Physics 60, 1679–1686. Naujokat, B., Labitzke, K., Lenschour, R., Petzdolt, K., Wohlfast, R.-C., 1993. Report on the winter 1989/90: very cold with a pronounced minor warming and a late 6nal warming. STEP-Collection of reports on the stratospheric circulation during the winters 1974=75 –1991=92, pp. 237–257. Titheridge, J.E., 1985. Ionogram analysis with the generalised program POLAN. Report UAG-93. Vitinsky, Y.I., Kopecki, M., Kuklin, G., 1986. Statistics of Sunspot-Creating Activity (in Russian), Nauka, Moscow.