STUDY OF ACOUSTIC GRAVITY WAVES DURING GEOMAGNETIC STORM
OVER EQUATORIAL LATITUDE, INDIA
M. Lal
Equatorial Geophysical Research Laboratory
Indian Institute of Geomagnetism
Krishnapuram, Maharaganagar, Tirunelveli – 627011, India e-mail: mlal@iig.iigm.res.in
INTODUCTION :
Despite the large number of studies examining the possibility that geomagnetic activity influences the lower atmosphere, there is no definable ‘current understanding’ in this field. There are, however, a number of recurrent themes and suggestions that appear in the modern literature. One of the earliest published examples of the modern analysis of the geomagnetic forcing of the lower atmosphere is Macdonald and Woodbridge’s (1959) paper. Their study examined changes in ‘jet stream’ level (300 hpa) circulation patterns following geomagnetic disturbances. Using the superposed epoch analysis method, they found that, for the 1956/57 and 1957/58 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 (1960) gave an important conclusion that the time delay between a geomagnetic event and changes in atmospheric circulation is not constant, and subsequently the geomagnetic nature of the relationship varies as each individual trough maximizes at a different location. Further studies ( Woodbridge, 1971; Roberts and Olsen, 1973) confirmed the earlier results with the use of additional data, but did not elaborate much on potential mechanism.
Wilcox et al. (1973) also used the super posed epoch analysis method to examine the response of the 300 hpa northern hemisphere, winter atmospheric circulation to solar events. Taylor (1986) points out that subsequent work, including research by Wilcox et al. (1983), shows that the relationship between solar magnetic sector boundary crossing

and the vorticity area index is only evident between 1963 and 1973, and fails beyond
1973. Furthermore, Burns et al. (1980) found that the relationship is not evident in the southern hemisphere.
Recent research has followed along these similar lines. One very encouraging sign is that most researchers have taken to examining possible mechanisms. Although the field is still contentious, there are many important concepts and developments in the recent literature, and those are presented in the following section.
One aspect of geomagnetic / weather relationship that shows a certain degree of coherence between studies is the timing of the effects. In the vast majority of studies, the lower atmosphere responds to a geomagnetic disturbance with seven days. Vovk et al.
(2000) noted that the delay between Forbush decrease events and responses in Antaractic temperature, pressure, and wind, could be approximated by a quasi – exponential 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, 1994 and Tinsley and Deen, 1991, found significant response at larger time lag intervals.
For most studies, the relationship are either strongest in winter (Veretenenko and
Pudovkin, 1995), or evident only in winter (Wilcox et al., 1973). Danilov and Lastovicka
(2001) suggest that the reason that the troposphere response to solar activity is more developed in winter is because the winter atmosphere is less stable. This view is shared by Gabis and Troshichev (2000).
Krishnamurty, B V (1993, 1998) proposed the tropical troposphere in the south and southeast asian region exhibits a strong easterly jet just below the tropopause at ~ 10 o
N latitude in the south – west monsoon season and the source of gravity waves. Sasi et al.
(2000) reported Inertia Gravity wave generated associated with the easterly jet.
Krishnamurthy (1998) 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 has been studied by Bowman and Shrestha (1998). The detailed review of the sun, weather and climate of the recent studies has been done by Avdyushin and Danilov (2000). Long period fading in atmospherics during severe meteorological activity and associated solar geophysical phenomena at low latitude has been studied by Bhattacharya et al. (1998).
We have made an attempt to study the influence of geomagnetic storm on the tropospheric acoustic gravity waves at equatorial latitude station, Tirunelveli (8.7
o N,
77.8
o E), India. Section 2 describes in details about the methodology. Section 3 describes the results and discussion, and section 4 describes the conclusion of the study.
Experimental Set – Up :
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 proved 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 lines of pipes have been used in the present study. In this regard, we have fabricated linear and circular noise reducer. The circular noise reducer is noise reducing lines of pipes typically consisting of about 300 m of pipes of various diameters, tapering from 3 inch (inside diameter) pipe at center in steps to 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 center 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 center in steps to 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.
Results and Discussion :

Fig. 1 shows the updated schema of possible solar – terrestrial mechanism linking geomagnetic activity to recent climate change (Daniel Palamara, Ph.D. thesis, 2003).
The most likely pathways through which geomagnetic activity influences climate has been shown in right panel. Unfeasible mechanism has been shaded and shown in left panel. It has been demonstrated in details, Daniel thesis, that geomagnetic activity influences tropospheric circulation, and subsequently climate, via the stratosphere. This is especially evident in the northern hemisphere, but no so obvious from the results for the southern hemisphere. There is a strong evidence that the source of the stratospheric circulation variations is upper atmospheric circulation changes and the interaction of the planetary waves (see, for example, Arnold and Robinson, 1998, 2001). This figure also demonstrated that geomagnetic activity is not a proxy for solar irradiance variations and the figure has been marked accordingly.
The alternative source of the stratospheric circulation changes, that geomagnetic activity is directly or indirectly influencing ozone concentrations within the polar vortex, has been left open as a possibility because of the vague and contradictory nature of geomagnetic – ozone studies. If it is assumed that geomagnetically – induced ozone changes are responsible for stratospheric zonal – mean zonal – wind and temperature changes associated with geomagnetic activity then the stratospheric aerosol ‘twilight effect’ can explain the temporal pattern. In this scenario, the seasonal aspect of the northern hemisphere results in partly due to the solar zenith angle required, the twilight effect being suitable only in winter. Overall, however, this mechanism is much less refined than the previously described one and while it warrants further investigation and modeling, it is not the preferred explanation for the solar – terrestrial relationship.
Fig.2 shows the amplitude of acoustic gravity waves obtained during Nov-Dec 2004 at equatorial latitude station, Tirunelveli (8.7
o N, 77.8
o E), India. The lower panel shows the
–Dst index variation which is the minimum value of each day. The minimum value of the
Dst index has reached up to -390 nT on 7 th
Nov 2004. Corresponding amplitude of gravity wave has been obtained and is shown in upper panel of Fig. 2. There is a maxima

in the amplitude of gravity wave has been obtained on 14 th
Nov 2004. There is a time lag of 7 days has been obtained between the –Dst index maxima and maxima of gravity wave amplitude. Thus, this figure shows that the influence of geomagnetic storm on the tropospheric acoustic gravity waves at equatorial latitude station has been obtained at the time lag of 7 days.
Fig. 3 shows the amplitude variation of acoustic gravity wave obtained during July – Spt
2004 at equatorial latitude station, Tirunelveli (8.7
o N, 77.8
o E), India. The upper panel shows the amplitude of acoustic gravity wave variation and the lower panel shows the –
Dst index variation for July – Spt 2004. There was an strong geomagnetic storm of Dst index ~ -190 nT on 27 th
July 2004. During the same period of observation, one more event of severe geomagnetic storm has been found and it is found to be be minimum Dst index was on 1 st
Spt 2004, and the minimum Dst index is found to be -150 nT. Hence, we got the opportunity to study the two severe geomagnetic storm within the period of observation. The influence of both the severe geomagnetic storm is clearly found to be present in the amplitude of acoustic gravity wave variation. The first minima of the Dst index has been obtained on 27 July and corresponding maxima in the amplitude of the acoustic gravity wave has been obtained on 14 Aug 2004. Thus there is a time lag of about 18 days has been obtained between the maxima of –Dst index variation and amplitude of acoustic gravity wave. In the second case, the maxima of - Dst index has been obtained on 1 st
September 2004 and the corresponding maxima of the amplitude of acoustic gravity wave has been obtained on 18 th
Spt 2004. This shows that there is a time lag of 17 days has been obtained between the maxima of –Dst index variation and amplitude of acoustic gravity waves. Though the amplitude of AGW obtained in second event is less compared to the first event. It appears that there is a dependence between the strength of the Dst index and the time lag of effect reflection on the equatorial tropospheric altitude.
Fig. 4 shows the –Dst index variation and amplitude of the acoustic gravity waves obtained between 20 Oct and 30 Dec 2003. The upper panel shows the –Dst index variation and the lower panel shows the amplitude of the tropospheric acoustic gravity

waves obtained between 20 Oct and 31 Dec 2003. There was a severe geomagnetic storm on 30 Oct and 21 Nov 2003. The Dst index minimum value obtained on 31 Oct
2003 was – 401 nT. On the other hand, the Dst index obtained on 21 Nov 2003 was –
472 nT. Both the events were of severe geomagnetic storm condition. The maximum in the amplitude of acoustic gravity waves has been obtained at the time lag of 12 days after the severe geomagnetic storm took place on 31 Oct 2003. the changes in the acoustic gravity waves amplitude due to the second event, i.e., 21 Nov 2003 ; Dst ~ -472 nT, does not show a pronounced increase in the amplitude of AGW, but there is a secondary maxima has been obtained after 9 days of the storm day. It may be possible that there could be interaction between the two events which might have caused significant changes in the amplitude of AGW from one event to the other. Further there is an increase in the amplitude 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 2003. Bhattacharya et al. (1998) has studied the solar – meteorological – geomagnetic relationship at low latitude station, Calcutta and noticed that the occurrence of depression is followed by the severe geomagnetic storm condition. This figure shows that 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 Bangal which took place between East coast of Tamil
Nadu and Sri. Lanka. Similar phenomena has also been observed after the severe geomagnetic storm of September 2000.
Fig. 5 shows the Power Spectral Density (PSD) obtained in September 2000. the lower panel of the graph shows the –Dst index variation which found to be having maximum value on 17 September 2000 and its maximum value was – 201 nT. The effect of the strong geomagnetic storm has been found in the variation of amplitude of acoustic gravity waves, and the maximum value of the amplitude of AGW is found on 25 Spt
2000. Thus, there is a lag of 8 days has been obtained between the maximum value of the
–Dst index and the Power Spectral Density of the acoustic gravity waves. The weather record shows that there was cyclone in last week of Nov 2000. Similar to the event of Oct
– Nov 2003, Spt 2000 geomagnetic storm also shows that there is occurrence of depression in Bay of Bangal, near to the east coast of Observatory ( Ocean is 30 Km east

from the Observatory), has occurred. This could be triggered by the geomagnetic storm events.
DISCUSSION :
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. However, this response is not so pronounced as e.g., in the ionospheric F2 region and depends on quite different processes (Avdyushin & Danilov,
2000). Roberts and Olson (1973) found a relation between the winter pressure drop 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. (1977).
Bucha (1991) found near – Earth pressure in the North Atlantic region decreased as a result of magnetic storms, deepened Icelandic depression, enhanced zonal circulation at a level of 500 hPa, and corresponding temperature variations in the North Atlantic region and Europe were found. 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 in Padgaonkar and Aurora (1981).
Lastovicka et al. (1992) formulated three specific features of the tropospheric response to geomagnetic storm :
(1) Tropospheric responses have a micro regional character, possibly due to changes in circulation and orography.
(2) 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.
(3) The winter response of the troposphere substantially depends on the phase of
QBO.

Bowman and Moritmer (2001) found weather associations at reasonably high levels of significance with enhanced geomagnetic activity (EGA). Statistically this EGA involved either short delays of several days or long delays of about 20 days. The long delays has been explained due to the active regions on the sun to be displaced by 180 degrees of solar longitude. Bowman (2000) suggested that the changes to weather systems, influenced by atmospheric gravity waves generated at times of delayed ionospheric Dregion absorption. Another hypothesis considers the delays associated with the movement of weather pattern from polar regions following EGA.
CONCLUSION :
We have made an attempt to study the influence of geomagnetic storm on the tropospheric gravity waves at equatorial latitude station, Tirunelveli (8.7
o
N, 77.8
o
E),
India. The diurnal average of gravity wave amplitude has been derived for 2002, 2003, and 2004. Some of the severe geomagnetic storm studied in the present case are Oct –
Nov 2003, Nov 2004, and September 2000. Our studies says that there is a short period delay as well as long period delay present in the effect of geomagnetic storm on the tropospheric acoustic gravity waves (AGW). The severe geomagnetic storm such as Oct
– Nov 2003, and Nov 2004 shows the influence of geomagnetic storm at the time delay of about 7 days. On the other hand, the strong geomagnetic storm shows the influence on tropospheric AGW at the time delay of about 20 days. Some of the strong geomagnetic storm studied in the present case are July – Aug 2004, Jan – Feb 2004, July
– Aug 2003, and July 2000. Also the presence of jet stream of period about 25 min has been observed in some storm which found to be continue for longer period and lead to the onset of cyclone. Some of the cases are Oct – Nov 2003, and Jan – Feb 2004. Our study also shows that the severe geomagnetic storm occurrence leads to the formation of cyclone after about 60 days. Some of the mechanism responsible for the influence of geomagnetic storm on the tropospheric AGW has been explained by Bowman &
Mortimer (2001). The long delay of influence could be the two new magnetically active

solar regions, about 180 o
apart in longitude, and these periodicities last for a few (about
4) solar rotations (Mursula and Zieger (1996)). The short delay hypothesis suggested and mentioned briefly by Bowman (2000) involves changes to weather system, influenced by atmospheric gravity waves generated at times of delayed ionospheric D-region absorption. Another hypothesis considers the delays associated with the movements of weather patterns from polar regions following EGA (Bucha and Bucha, 1998).
ACKNOWLEDGEMENTS:
The author would like to thank Mr. M.V. Subramanian and Mem. Nazarath Begam for helping in the data analysis, and also the World Data Center – C2 for Geomagnetism,
Kyoto, Japan for making available geomagnetic indices. The author is thankful to his colleague for providing necessary help to complete the work.
REFERENCES :