Study of geomagnetic storm induced acoustic gravity waves over equatorial latitude

Transcription

1 Indian Journal of Radio & Space Physics Vol. 35, June 2006, pp Study of geomagnetic storm induced acoustic gravity waves over equatorial latitude M Lal Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism, Krishnapuram, Maharaganagar, Tirunelveli , India mlal@iigs.iigm.res.in Received 23 March 2005; revised 3 January 2006; accepted 17 February 2006 An attempt has been made to study the influence of geomagnetic storm on the tropospheric acoustic gravity waves at equatorial station, Tirunelveli (8.7 N, 77.8 E), India. The daily average of the amplitude and phase of acoustic gravity waves has been studied for the period Severe geomagnetic storms occurred during April and September 2000, April 2001, October and November 2003 and November It is found that geomagnetic storm influences the tropospheric acoustic gravity waves. There is a delay between the occurrence of geomagnetic storm and tropospheric acoustic gravity waves. The delay is a function of strength of the geomagnetic storm. Severe geomagnetic storm such as that of November 2004 shows a short-period delay of ~ 5 days, and strong geomagnetic storm like the one of July 2004 shows the long-period delay of about 15 days. Keywords : Geomagnetic storm, Acoustic gravity waves, Gravity waves, Equatorial latitude PACS Nos : Lr ; Dj IPC Code : G08C23/02; G01W1/02 1 Intoduction It is known that the upper atmosphere shows a clearly defined response to geomagnetic storms. A response to geomagnetic disturbances is also observed in the lower stratosphere and troposphere. One of the earliest work of the geomagnetic forcing of the lower atmosphere is by MacDonald and Woodbridge 1. They examined changes in jet stream level (300 hpa) circulation patterns following geomagnetic disturbances. Using the superposed epoch analysis method, they 1 found that, for the and winters, troughs in atmospheric waves appearing in the Alaska-Aleutian area, roughly three days after a geomagnetic disturbance, tended to develop larger amplitudes. Macdonald and Roberts 2 concluded that the time delay between a geomagnetic event and changes in atmospheric circulation is not constant. The geomagnetic nature of the relationship varies as each individual trough maximizes at a different location. Further studies 3,4 agreed with these results. Wilcox et al. 5 also used the superposed epoch analysis method to examine the response of the 300 hpa northern hemisphere, winter atmospheric circulation to solar events. As pointed out by Taylor 6, the subsequent work, including research by Wilcox et al. 7, showed that the relationship between solar magnetic sector boundary crossing and the vorticity area index was only evident between 1963 and 1973, and failed beyond Further, Burns et al. 8 found that the relationship is not evident in the southern hemisphere. One aspect of geomagnetic-weather relationships is the timing of the effects. In majority of studies, it is shown that the lower atmosphere responds to a geomagnetic disturbance within seven days. Vovk et al. 9 noted that the delay between Forbush decrease events and responses in Antarctic temperature, pressure and wind, could be approximated by a quasiexponential curve. Therefore, it is possible that the actual delay between geomagnetic activity and the associated atmospheric response is a function of the magnitude of the geomagnetic event. However, with some exceptions, Stening 10 and Tinsley and Deen 11, found significant response at larger time lag intervals. For most of the studies, the relationship is either strongest in winter 12 or evident only in winter 5. Krishnamurthy 13,14 proposed that the tropical troposphere in the south and south-east Asian region exhibits a strong easterly jet just below the tropopause at ~10 N latitude during the south-west monsoon season and is the source of gravity waves. Sasi et al. 15 reported about the inertia gravity wave associated with the easterly jet. Krishnamurthy 14

2 LAL: GEOMAGNETIC STORM INDUCED AGW OVER EQUATORIAL STATION 175 observed that the solstice season is more favourable for gravity waves propagation than the equinoxes at Indian equatorial zone. Tropospheric gravity waves and their relationship with geomagnetic activity at the Australian station have been studied by Bowman and Shrestha 16. Long period fading in atmospherics during severe meteorological activity and associated solar geophysical phenomena at low latitude have been studied by Bhattacharya et al 17. In this paper an attempt has been made to study the influence of geomagnetic storm on the tropospheric acoustic gravity waves at equatorial latitude station, Tirunelveli (8.7 N, 77.8 E), India. 2 Experimental set-up 2.1 Microbarograph Atmospheric waves are characterized by variations in the wind speed, the atmospheric density and the atmospheric pressure. Therefore, it is possible to determine the existence of the waves by measuring the small changes in the atmospheric pressure produced during the passage of a wave. This is done by using microbarograph that is capable of measuring pressure changes of the order of a few microbars. The microbarograph unit, basically a condenser microphone, has been designed to measure the infrasonic pressure variation of the order of hundreds of microbars in the range of ± 1000 μbar in full scale division. These variations are generated when the infrasonic wave ranges from to 2 Hz (periods ranging from tens of minutes to a fraction of a second). The capacitor sensing element consists of a diaphragm and two stator plates acting as a balanced capacitor. The sensing element has a controlled air leak from the front cavity by means of a leak tube. The leak tube allows the pressure to the compartment behind the diaphragm to become equal to atmospheric pressure without allowing the acoustic pressure of interest to reach the back of the diaphragm. The front cavity has a leak hole in the lid which determines the high frequency response of the capacitor element. The acoustic filters and sensing elements of the microbarograph is shown in Fig. 1(a). The dual balanced capacitor element is a component of the chopper which generates a square wave of approximately 25 khz. This chopper is a free running collector-coupled multivibrator. This free running multivibrator is, in turn, controlled by the sensing element capacitance. The entire system presents an excellent linear transducer over very wide acoustic pressure range. The symmetry of the square wave is varied by the oppositely charging capacitance of the sensing element resulting from acoustic signals. This variation in symmetry constitutes a variation in time average voltage output of the chopper. This signal, when passed through the balanced filter section to eliminate DC offsets and chopper frequency, is a varying voltage, the amplitude and frequency of which correspond to the acoustic signals incident upon the microbarograph element. The signal is then passed on to an operational amplifier which supplies the necessary amplification and low output impedance to drive long lines for field application. The relative sensitivity of the microbarograph is changed by altering the setting of the feedback resistor of the operational amplifier. A 470 Ω resistor is connected between the operationalamplifier output plug and the microbarograph output plug. Thus, an indefinite short circuited output can be tolerated with no damage to the instrument. The microbarograph functional block diagram is shown in Fig. 1(b). The spectrum of periods measured by microbarograph ranges from 10 1 to 10 5 seconds. The high frequency, generally non-periodic noise, is due to turbulence, whereas the low-frequency portion of the pressure spectrum is characterized, principally, by the semi-diurnal and diurnal tides. Within the narrow band of periods, uniform sinusoidal oscillations in the pressure variation appear frequently. These are the results of tropospheric gravity waves. Fig 1 (a) Acoustic filters and sensing elements of microbarograph and (b) Microbarograph functional block diagram

3 176 INDIAN J RADIO & SPACE PHYS, JUNE Mechanical noise reducer The present study is aimed at detecting global variation of surface pressure by minimizing the effect of the local meteorological effects, such as cloud variation and wind variability. The array spacing has been proved to be well-suited for recording most classes of subsonic pressure disturbances. To minimize the effect of pressure fluctuations caused by local turbulent wind eddies, noise reducing pipe lines have been used in the present study. In this regard, linear and circular noise reducers have been fabricated. The circular noise reducer is noise reducing pipe lines typically consisting of about 300 m of pipes of various diameters, tapering from 3 inch (inside diameter) pipe at the centre in steps of 0.5 inch (inside diameter) pipe at the ends. The pipe line is equipped with capillary ports to the atmosphere usually set at 10 ft intervals, and the input to the microphone is connected to the centre of this spatial filter. Similar configuration has been used for the linear noise reducer, which is consisting of about 150 ft pipe of various diameters, tapering from 3 inch (inside diameter) pipe at the centre in steps of 0.5 inch (inside diameter) pipe at the ends. The pipe line is equipped with capillary ports to the atmosphere usually set at 5 ft intervals, and the input to the microphone is connected to the noise reducing system. data have been recorded at 1s interval. The pressure variation has been obtained by using the circular noise reducer system, which enhances the S/N ratio by a factor of about 6 as compared to the linear noise reducer system. The diurnal variation of the surface pressure shows a maxima before sunrise and another maxima during afternoon period. The minimum value of the surface pressure is found to be after sunrise period. The influence of wind variation has been minimized by using mechanical noise reducer system. The pressure is found to be varying between 200 and 400 μbar. Figure 2(b) shows the spectra for the surface pressure variations obtained on 1 Apr (1024 points). It may be noted that the prominent gravity wave activity in surface pressure is found to be near the periodicity of 40, 70 and 90 min. The summation of the spectral power has been obtained between 15 and 120 min. The summation is the averaged power of the gravity wave for one day period. 2.3 Method The diurnal variation of the pressure measured by the microbarograph has been converted into digital form by using data logger system. The pressure variation is recorded at 1s interval. Then one-minute average pressure change has been used as a single point and the total number of points in a single day (24 h period) becomes Each set of data contains time in minute against pressure changes (in microbar). The diurnal variation of pressure is converted into frequency against amplitude by using fast Fourier transformation. The summation of amplitude has been taken between 15 and 150 min period for each set of data. The superpose epoch analysis for severe geomagnetic storm has been performed for the period between 2000 and The minimum value of the D st index (in nt) has been chosen as a zero day, and the corresponding gravity wave amplitude for ± 30 days has been averaged. 3 Results Figure 2(a) shows the average surface pressure variation obtained between 1 and 10 Apr The Fig. 2 (a) Diurnal variation of surface pressure over Tirunelveli and (b) Power derived from the diurnal variation of pressure

4 LAL: GEOMAGNETIC STORM INDUCED AGW OVER EQUATORIAL STATION 177 Figure 3(a) shows the power (in microbar 2 ) of acoustic gravity waves (AGW) for November- December 2004 at equatorial station, Tirunelveli (8.7 N, 77.8 E), India. The lower panel shows the D st index variation. The minimum value of D st index has reached 390 nt on 7 Nov. 2004, and the corresponding amplitude of AGW has been shown in Fig. 3(a). The maximum in the amplitude of AGW has been obtained on 12 Nov There is a time lag of 5 days between the D st index minimum and maximum of gravity wave amplitude. Figure 4 shows the power variation of AGW for July-September 2004 at Tirunelveli. The power (in microbar 2 ) along y-axis is taken as the sum of power between 15 and 120 min, obtained from the diurnal Fig 3 (a) Amplitude of acoustic gravity waves obtained during November-December 2004 and (b) Variation of D st index for November-December 2004 pressure variation. The upper panel shows the power of AGW and the lower panel shows the D st index variation. There was a strong geomagnetic storm of D st index ~ 190 nt on 27 July 2004 (day number 209). After short period, one more event of severe geomagnetic storm was found on 1 Sep (day number 245), with the minimum D st index of 150 nt. The influence of both the severe geomagnetic storms is clearly seen in the amplitude of AGW variation. The first minimum of the D st index was found on 27 July and the corresponding maximum in the amplitude of the acoustic gravity wave was found on 12 Aug (day number 225). Thus, a time lag of about 15 days is seen between the maxima of D st index variation and amplitude of AGW. In the second case, the maximum of D st index was found on 1 Sep and the corresponding maximum of the power of acoustic gravity wave was found on 15 Sep Hence a time lag between the minimum of D st index variation and maximum power of AGW is found. Figure 5 shows the variation of D st index and power of AGW obtained between 20 Oct (day number 293) and 30 Dec (day number 364). The lower panel shows the variation of D st index and the upper panel shows the power of AGW. There were severe geomagnetic storms on 31 Oct. and 21 Nov The minimum value of D st index obtained on 31 Oct (day number 304) was 401 nt and that on 21 Nov (day number 325) was 472 nt. The maximum in the power of AGW was obtained at a time lag of 15 days after the severe geomagnetic storm took place on 31 Oct The change in AGWs power during the second event, i.e., 21 Nov. Fig. 4 (a) Amplitude of acoustic gravity waves obtained during July-September 2004 and (b) Variation of D st index for July- September 2004 Fig. 5 (a) Amplitude of the acoustic gravity waves obtained between 20 October and 30 December 2003 and (b) Variation of D st index for 20 October-30 December 2003

5 178 INDIAN J RADIO & SPACE PHYS, JUNE does not show a pronounced increase, but a secondary maximum has been seen after 9 days of the storm day. It may be possible that there was interference between the two events which might have caused significant changes in the power of AGW from one event to the other. Further, an increase in the power of AGW has been obtained on the last week of December 2003, which is about 60 days after the severe geomagnetic storm took place on 31 Oct Bhattacharya et al. 17 have studied the solarmeteorological-geomagnetic relationship at low latitude station, Calcutta and noticed that the occurrence of depression is followed by the severe geomagnetic storm condition. The increase in amplitude of gravity waves obtained during the last week of December 2003 may be due to the occurrence of depression in Bay of Bengal which took place between east coast of Tamil Nadu and Sri Lanka. Similar phenomenon has also been observed after the severe geomagnetic storm during September Figure 6 shows the power (in microbar 2 ) obtained in September The upper panel shows the variation of D st index. A minimum value of 201 nt was seen on 17 Sep The effect of the strong geomagnetic storm has been found in the power of AGW, and the maximum power of AGW is found on 22 Sep Thus, a lag of 5 days has been obtained between the minimum value of the D st index and the power of AGW. The weather record shows that there was cyclone in the last week of November Similar to the event of October-November 2003, September 2000 geomagnetic storm also shows that there was occurrence of depression in Bay of Bengal, near the east coast of observatory (ocean is 30 km east of the observatory), after about 60 days of the onset of magnetic storm. The superposed epoch analysis has been performed for the entire period and for severe and strong geomagnetic storm condition. Figure 7 shows the resultant power of AGW obtained during severe geomagnetic storm condition. The minimum value of the D st index has been considered as zero day number, and the corresponding power of AGW has been obtained for ± 30 days. Figure 7 shows that there are maxima in the power of AGW on 5, 15, and 25 days. Planetary waves of period 10 days appear to be associated with the maxima of AGW power. 4 Discussion Bowman and Mortimer 18 studied the enhanced gravity wave activity over four sunspot minimum periods. They have found both short and long period delays. The short delay hypothesis as suggested briefly by Bowman 19 involves changes to weather system, influenced by atmospheric gravity waves generated at the time of delayed ionospheric D-region absorption. Another hypothesis considers the delays associated with the movement of weather patterns from polar regions following enhancement in geomagnetic activity 20 (EGA). The short period delays of up to five days or more, following the occurrence of EGA has been reported by Bowman and Shrestha 16. The long period delays are around 15 days. The EGA associated with these delays is a part of sequences of recurrent geomagnetic activity involving periodicities of around 27 days and 13.5 days. Pap et al. 21 suggested that the solar 13.5-day periodicity comes from two new magnetically active solar regions, about 180 apart in longitude. The Fig. 6 (a) Variation of D st index for September 2000 and (b) Power spectral density of acoustic gravity waves obtained during September 2000 Fig. 7 Superpose epoch analysis of AGW amplitude obtained for severe geomagnetic storm condition between 2000 and 2004.

6 LAL: GEOMAGNETIC STORM INDUCED AGW OVER EQUATORIAL STATION 179 observation by Mursula and Zieger 22 that these periodicities last for a few (about 4) solar rotations is found to be consistent with the observation by Bowman and Mortimer 18. The presence of long period of waves after EGA for about few solar rotations could be responsible for the formation of cyclone like conditions. It has been demonstrated that geomagnetic activity influences tropospheric circulation, and subsequently climate, via the stratosphere 23. There is a strong evidence that the source of variations in stratospheric circulation is the changes in upper atmospheric circulation and the interaction of the planetary waves 24,25. Danilov and Lastovicka 26 suggest that the tropospheric response to solar activity is more developed in winter because the winter atmosphere is less stable. This view is shared by Gabis and Troshichev 27. A detailed review of the studies on sun, weather and climate has been done by Avdyushin and Danilov 28. Roberts and Olson 4 found a relation between the pressure drop in winter at a level of 300 hpa and geomagnetic storms in the North American and North Atlantic zones. A change in the near-earth pressure in the European and Siberian sectors after strong sporadic magnetic storm, was found in the series of works by Mustel et al. 29. Bucha 30 found nearearth pressure in the North Atlantic region to be decreased as a result of magnetic storms, deepened Icelandic depression and enhanced zonal circulation at a level of 500 hpa, and corresponding temperature variations in the North Atlantic region and Europe were observed. Here, it was established that the corresponding effects are most pronounced in winter than in summer. Clearly defined effects of magnetic storms on a change in the vorticity index (which largely reflects the degree of troposphere disturbance) at a level of 500 hpa were revealed by Padgaonkar and Aurora 31. Lastovicka et al. 32 formulated three specific features of the tropospheric response to geomagnetic storm : (i) Tropospheric responses have a microregional character, possibly due to changes in circulation and orography. (ii) The tropospheric response to magnetic storm is much more pronounced in winter than in summer, possibly, because the direct solar radiation input to the troposphere is lower, and the atmosphere is less stable in winter. (iii) The winter response of the troposphere substantially depends on the phase of QBO. 5 Conclusions An attempt has been made to study the influence of geomagnetic storm on the tropospheric AGW at equatorial latitude station, Tirunelveli (8.7 N, 77.8 E), India for different seasons. The effect of geomagnetic storm on the AGW is found to be sensitive to the season and more pronounced in winter condition. The present study says that there are both short and long period delays in the effect of geomagnetic storm on the tropospheric AGW. The severe geomagnetic storms such as of October- November 2003 and November 2004 show the influence of geomagnetic storm on AGW at a time delay of about 5 days. On the other hand, the strong geomagnetic storm shows the influence on tropospheric AGW at a time delay of about 15 days. Acknowledgements The author would like to thank Mr M V Subramanian and Mem Nazarath Begam (Project Student) for helping in the data analysis, and also the World Data Center-C2 for Geomagnetism, Kyoto, Japan for making available the geomagnetic indices. The author is thankful to his colleagues for providing necessary help to complete the work. References 1 MackDonald N J & Woodbridge D D, Relation of geomagnetic disturbances to circulation changes at 30,000 ft level, Science (USA), 129 (1959) Mack Donald N J & Roberts W O, Further evidence of a solar corpuscular influence on large-scale circulation at 300 mbar, J Geophys Res (USA), 65 (1960) Woodbridge D D, Comparison of geomagnetic storms and trough development at solar activity maximum and minimum, Planet & Space Sci (UK), 19 (1971) Roberts W O & Olson R H, Geomagnetic storms and wintertime 300 mbar trough development in the North Pacific-North America Area, J Atmos Sci (USA), 30 (1973) Wilcox J M, Scherrer P H & Hoeksema J T, In Weather and climate responses to solar variations, edited by B M McCormac (Colorado Associated University Press, Boulder), 1983, p Taylor (Jr.) H A, Selective factors in sun-weather research, Rev Geophys (USA), 24 (1986) Wilcox J M, Scherrer P H, Svalgaard L, Roberts W O & Olsen R H, Solar magnetic sector structure: Relation to circulation of the earth s atmosphere, Science (USA), 180 (1973) Burns G B, Bond F R & Cole K D, An investigation of the southern hemisphere vorticity response to solar sector boundary crossing, J Atmos & Terr Phys (UK), 42 (1980) Vovk V Y, Egorova L V & Moskvin I V, Effects of GCR Forbush decreases on atmospheric parameters in the Antarctic, Geomagn & Aeron (Russia), 40 (2000) 792.

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