Atmospheric Gravity Wave Activity Research Articles

Observation of Ultra-Low-Frequency Wave Effects in Possible Association with the Fukushima Earthquake on 21 November 2016, and Lithosphere–Atmosphere–Ionosphere Coupling

The study presents seismogenic ULF (ultra-low-frequency) wave effects, as observed at our own new magnetic observatory at Asahi (geographic coordinates: 35.770° N, 140.695° E) in Chiba Prefecture. Our target earthquake (EQ) is a huge one offshore of Fukushima prefecture (37.353° N, 141.603° E) with a magnitude (M) of 7.4, which occurred at 20.59 h on November 21 UT, 2016. As a sampling frequency of 1 Hz was chosen for our induction magnetometer, we could detect both ULF wave effects: ULF radiation from the lithosphere, and the ULF depression effect, indicative of lower ionospheric perturbations. Observing the results of polarization analyses, we detected clear enhancements in ULF (frequency = 0.01–0.03 Hz) lithospheric radiation 14 days, 5 days, and 1 day before the EQ, and also observed a very obvious phenomenon of ULF (0.01–0.03 Hz) depression just 1 day prior to the EQ, which is regarded as the signature of lower ionospheric perturbations. These findings suggest that pre-EQ seismic activity must be present in the lithosphere, and also that the lower ionosphere was very much perturbed by the precursory effects of the Fukushima EQ. These new observational effects from our station have been compared with our previous investigations on different seismogenic topics for the same EQ, including the ULF observations at another magnetic observatory at Kakioka, belonging to the Japan Meteorological Agency (JMA), about 50 km north of our Asahi station, subionospheric VLF/LF propagation data (Japanese and Russian data), AGW (Atmospheric gravity wave) activity in the stratosphere, and satellite observation of particle precipitations. We have found that seismogenic anomalies of different parameters tend to happen just around the EQ day, but mainly before the EQ, and have found the chain-like tendency of the effects of the lithosphere, which seem to propagate upwards the lower ionosphere. Finally, we will try to gain a better understanding of the physical phenomena or mechanisms of the lithosphere–atmosphere–ionosphere coupling (LAIC) process during the EQ preparation phase.

Multi-Parameter Observations of Seismogenic Phenomena Related to the Tokyo Earthquake (M = 5.9) on 7 October 2021

Multi-parameter observations, powerful for the study of lithosphere–atmosphere–ionosphere coupling (LAIC), have been performed for a recent Tokyo earthquake (EQ) with a moderate magnitude (M = 5.9) and rather larger depth (~70 km) on 7 October 2021, in the hope of predicting the next Kanto (Tokyo) huge EQ, such as the 1923 Great Kanto EQ (with a magnitude greater than 7). Various possible precursors have been searched during the two-month period of 1 September to 31 October 2021, based on different kinds of data sets: (i) ULF (ultra-low frequency) magnetic data from Kakioka, Japan, (ii) ULF/ELF (extremely low frequency) magnetic field data from the Chubu University network, (iii) meteorological data (temperature and humidity) from the Japan Meteorological Agency (JMA), (iv) AGW (atmospheric gravity wave) ERA5 data provided by the European Centre for Medium-Range Weather Forecast (ECMWF), (v) subionospheric VLF/LF (very low frequency/low frequency) data from Russia and Japan, (vi) ionosonde Japanese data, and (vii) GIM (global ionosphere map) TEC (total electron content) data. After extensive analyses of all of the above data, we have found that there are a few obvious precursors: (i) ULF/ELF electromagnetic radiation in the atmosphere, and (ii) lower ionospheric perturbations (with two independent tools from the ULF depression and subionospheric VLF anomaly) which took place just two days before the EQ. Further, ULF/ELF atmospheric electromagnetic radiation has been observed from approximately one week before the EQ until a few days after the EQ, which seems to be approximately synchronous in time to the anomalous variation in meteorological parameters (a combination of temperature and humidity, atmospheric chemical potential). On the other hand, there have been no clear anomalies detected in the stratospheric AGW activity, and in the NmF2 and TEC data for the upper F region ionosphere. So, it seems that the lithospheric origin is not strong enough to perturb the upper F region. Finally, we discuss the possible hypothesis for the LAIC process, and we can conclude that the AGW hypothesis might be ruled out, but other possible channels such as the chemical channel (radon emanation) and the associated effects might be in operation, at least, for this Tokyo EQ.

Gravity Wave Activity in the Stratosphere before the 2011 Tohoku Earthquake as the Mechanism of Lithosphere-atmosphere-ionosphere Coupling.

The precursory atmospheric gravity wave (AGW) activity in the stratosphere has been investigated in our previous paper by studying an inland Kumamoto earthquake (EQ). We are interested in whether the same phenomenon occurs or not before another major EQ, especially an oceanic EQ. In this study, we have examined the stratospheric AGW activity before the oceanic 2011 Tohoku EQ (Mw 9.0), while using the temperature profiles that were retrieved from ERA5. The potential energy (EP) of AGW has enhanced from 3 to 7 March, 4–8 days before the EQ. The active region of the precursory AGW first appeared around the EQ epicenter, and then expanded omnidirectionally, but mainly toward the east, covering a wide area of 2500 km (in longitude) by 1500 km (in latitude). We also found the influence of the present AGW activity on some stratospheric parameters. The stratopause was heated and descended; the ozone concentration was also reduced and the zonal wind was reversed at the stratopause altitude before the EQ. These abnormalities of the stratospheric AGW and physical/chemical parameters are most significant on 5–6 March, which are found to be consistent in time and spatial distribution with the lower ionospheric perturbation, as detected by our VLF network observations. We have excluded the other probabilities by the processes of elimination and finally concluded that the abnormal phenomena observed in the present study are EQ precursors, although several potential sources can generate AGW activities and chemical variations in the stratosphere. The present paper shows that the abnormal stratospheric AGW activity has also been detected even before an oceanic EQ, and the AGW activity has obliquely propagated upward and further disturbed the lower ionosphere. This case study has provided further support to the AGW hypothesis of the lithosphere-atmosphere-ionosphere coupling process.

A LOFAR observation of ionospheric scintillation from two simultaneous travelling ionospheric disturbances

This paper presents the results from one of the first observations of ionospheric scintillation taken using the Low-Frequency Array (LOFAR). The observation was of the strong natural radio source Cassiopeia A, taken overnight on 18–19 August 2013, and exhibited moderately strong scattering effects in dynamic spectra of intensity received across an observing bandwidth of 10–80 MHz. Delay-Doppler spectra (the 2-D FFT of the dynamic spectrum) from the first hour of observation showed two discrete parabolic arcs, one with a steep curvature and the other shallow, which can be used to provide estimates of the distance to, and velocity of, the scattering plasma. A cross-correlation analysis of data received by the dense array of stations in the LOFAR “core” reveals two different velocities in the scintillation pattern: a primary velocity of ~20–40 ms−1 with a north-west to south-east direction, associated with the steep parabolic arc and a scattering altitude in the F-region or higher, and a secondary velocity of ~110 ms−1 with a north-east to south-west direction, associated with the shallow arc and a scattering altitude in the D-region. Geomagnetic activity was low in the mid-latitudes at the time, but a weak sub-storm at high latitudes reached its peak at the start of the observation. An analysis of Global Navigation Satellite Systems (GNSS) and ionosonde data from the time reveals a larger-scale travelling ionospheric disturbance (TID), possibly the result of the high-latitude activity, travelling in the north-west to south-east direction, and, simultaneously, a smaller-scale TID travelling in a north-east to south-west direction, which could be associated with atmospheric gravity wave activity. The LOFAR observation shows scattering from both TIDs, at different altitudes and propagating in different directions. To the best of our knowledge this is the first time that such a phenomenon has been reported.

Abnormal Gravity Wave Activity in the Stratosphere Prior to the 2016 Kumamoto Earthquakes

AbstractEarthquake (EQ) prediction is an essential topic in recent geophysics and disaster management. In order to explain EQ precursory phenomena, especially the ionospheric perturbations, a few hypotheses have been proposed as the mechanism of lithosphere‐atmosphere‐ionosphere coupling. The most reliable seems to be the atmospheric gravity wave (AGW) hypothesis, because a lot of subionospheric very low frequency (VLF) observations and also ground surface measurements support this hypothesis. However, no direct signature of AGW activity in the middle atmosphere has been yet obtained. So in this study, we have used the ERA5 reanalysis data to investigate the tempo‐spatial evolution of stratospheric AGW activity before a recent huge seismic event of the 2016 Kumamoto EQ on 15 April (Mw7.2). It was found that the AGW potential energy is enhanced significantly during about 1 week before the EQ. The enhancement was mainly distributed around the EQ epicenter and expanded toward eastern Japan. The AGW activity was highest around 11 April, then it calmed down and returned to the background level right before the EQ. These results were compared with the subionospheric VLF observations during the same period, and then the tempo‐spatial evolutions of the lower ionospheric perturbation are found to be very consistent with the stratospheric behavior. The present paper reports on the first finding that the abnormal AGW activity in the stratosphere is detected before a major EQ, and the coincidence of stratospheric AGW activity with VLF subionospheric perturbations provides further support to the AGW hypothesis of the lithosphere‐atmosphere‐ionosphere coupling process.

Statistical characteristics of atmospheric gravity wave in the mesopause region observed with a sodium lidar at Beijing, China

Nightly and seasonal variations of atmospheric gravity wave (AGW) activity in the sodium layer are investigated by analyzing 253 nights of sodium lidar observations. These observations were made from April 2010 to September 2011 at Yanqing, Beijing (40.3°N,116.2°N). Vertical wavelengths, periods and phase velocities of 162 quasi-monochromatic AGWs are extracted from the lidar data. An average parameter relationship λz=0.212Tob0.544 is found. Nightly statistical results show that the activity of AGW has maximum from 22 to 24 LST in night. Monthly statistical results reveal that AGW activity maximizes in summer and minimizes in winter. Seasonal variation characteristic of AGW activity is interpreted based on a qualitative analysis of probable main sources in Northern China. Influences of seasonally varying temperature and background winds on AGW activity are discussed.

Analysis on the Global Morphology of Middle Atmospheric Gravity Waves

AbstractUsing temperature profiles obtained by the SABER/TIMED experiment from January 2002 to December 2009, we have extracted mesoscale temperature perturbations with vertical wavelengths ranging from 2 to 10 km. Global distribution of middle atmospheric gravity wave activity is revealed by observing the temperature perturbations. Comparison shows that gravity wave fluctuations in summer and winter are stronger than those in spring and autumn, below 70 km the disturbance intensity in summer is weaker with respect to winter, and contrary above 70 km. The larger gravity waves appear in winter hemisphere and in the tropics between 25°N and 25°S. Meanwhile, the perturbation peak points in the tropics move northward with increasing altitude. Furthermore, relatively larger values of gravity wave activity are present at the edge of the Antarctic polar vortex. They also appear to vary with longitudes at equatorial latitudes, which may be resulted from wind filtering, topography, planetary wave modulations and other factors. Moreover, gravity wave disturbance intensity changes with height. It indicates a decrease at 25~30 km and an increase above 42 km. Comparing the average distribution of gravity waves of eight years at different heights, we can see that the strength of gravity waves is related to topography obviously at lower altitudes, but the relationship is not significant at higher altitudes. It indicates that the formation of gravity waves is closely correlated with topography, but in the process of propagation the distribution significantly changes with altitudes.

Characteristics of atmospheric gravity wave activity in the polar regions revealed by GPS radio occultation data with CHAMP

Using GPS radio occultation data during 2001–2005, we studied the climatological behavior of atmospheric gravity waves in the polar stratosphere. We calculated temperature fluctuations with vertical wavelengths shorter than 7 km and then determined the wave potential energy, Ep, every month in a longitude‐latitude cell of 20° × 10° between 12 km and 33 km. In the Arctic region (50–90°N), Ep shows an annual variation with maximum in winter, consistent with the zonal mean horizontal wind, V, and the Eliassen‐Palm (E‐P) flux, Fz. The large Fz values indicate higher planetary wave activity, resulting in distortion of the polar vortex. The unbalanced flow can then excite gravity waves through geostrophic adjustment. In the Antarctic region (50–90°S), Ep gradually increases during winter and reaches its maximum in spring before decreasing rapidly. The time derivative of V coincides with the Ep peak and the horizontal distribution of Ep has a similar structure to V, suggesting that the Ep enhancement is closely related to the decay of the polar vortex. During major warming events over the Arctic, the divergence of E‐P flux, ΔF, was enhanced, coinciding with large Ep. In the Antarctic, ΔF strongly correlates with Ep in spring. Gravity waves seem to be effectively generated through planetary wave transience and/or breaking. Orographic generation of gravity waves seems to be important in limited areas only, such as Scandinavia and the Antarctic Peninsula, showing that it is less important than the polar night jet in determining the climatological behavior of gravity waves.

Lidar observations of middle atmospheric gravity wave activity over a low-latitude site (Gadanki, 13.5° N, 79.2° E)

Abstract. The low-latitude middle atmospheric gravity wave characteristics are presented using 310 nights of Rayleigh lidar observations made at Gadanki (13.5° N, 79.2° E) over the period from March 1998 to December 2002. The gravity wave characteristics are presented in terms of vertical wave number and frequency spectra, along with the estimated potential energy for the four seasons, namely, spring, summer, autumn and winter. The computed wave number spectra for both the stratosphere and the mesosphere are found to differ significantly from a saturated model predicted spectrum. The spectra were found to be shallower at lower wave numbers and steeper at higher wave numbers with transition at ~8.85×10-4 cy/m. The computed frequency spectra seem to follow the model plot with a power law index of -5/3 above a frequency of ~2×10-4 Hz. The estimated potential energy per unit mass increases gradually up to ~60 km and then rather rapidly above this height to reach values of the order of 200J/kg at ~70 km.

Studies on atmospheric gravity wave activity in the troposphere and lower stratosphere over a tropical station at Gadanki

Abstract. MST radars are powerful tools to study the mesosphere, stratosphere and troposphere and have made considerable contributions to the studies of the dynamics of the upper, middle and lower atmosphere. Atmospheric gravity waves play a significant role in controlling middle and upper atmospheric dynamics. To date, frontal systems, convection, wind shear and topography have been thought to be the sources of gravity waves in the troposphere. All these studies pointed out that it is very essential to understand the generation, propagation and climatology of gravity waves. In this regard, several campaigns using Indian MST Radar observations have been carried out to explore the gravity wave activity over Gadanki in the troposphere and the lower stratosphere. The signatures of the gravity waves in the wind fields have been studied in four seasons viz., summer, monsoon, post-monsoon and winter. The large wind fluctuations were more prominent above 10 km during the summer and monsoon seasons. The wave periods are ranging from 10 min-175 min. The power spectral densities of gravity waves are found to be maximum in the stratospheric region. The vertical wavelength and the propagation direction of gravity waves were determined using hodograph analysis. The results show both down ward and upward propagating waves with a maximum vertical wave length of 3.3 km. The gravity wave associated momentum fluxes show that long period gravity waves carry more momentum flux than the short period waves and this is presented.

DC electric field measurement in the SEEK Campaign

DC electric fields were measured by the Electric Field Detector ‐ Beam experiment (EFD‐B) onboard the sounding rocket S‐310‐26 above the E‐region ionosphere during the Sporadic‐E Experiment over Kyushu (SEEK) campaign. The sounding rocket was launched into the ionosphere when a strong quasi‐periodic (QP) echo structure appeared. The EFD‐B measured both the electric field and the magnetic field from X+169 s until X+232 s (altitude of 167 km to 173 km). Accuracy of the measurements were 0.19 mV/m for the electric field and 1.9 nT for the magnetic field. The magnetic field measurements were in very good agreement with a model field (IGRF 95). The electric fields, which were projections of the ionospheric electric fields to the altitude of the observation point along the magnetic field line, reached to 11.5 mV/m at maximum. The observed electric field is much larger than expected, and is associated with the 4 km periodic structure of the electric field which may be related to the atmospheric gravity wave activity in the E region.

On the lack of southern hemisphere polar mesosphere summer echoes

We report VHF radar observations of the southern high‐latitude mesopause region using wind profilers that were installed recently on King George Island, Antarctica, and Ushuaia, Argentina. Briefly, our observations, which were made during January and February 1993, show almost no evidence of so‐called polar mesosphere summer echoes, or PMSE. Since these echoes are a predominant feature of the northern high‐latitude mesosphere in summer, their absence in the southern hemisphere is both surprising and intriguing. In this paper we present evidence demonstrating the virtual absence of the echoes and demonstrate that our systems were capable of detecting them had they been present. We also outline some of the consequences of this intriguing result, which are supported by observed hemispheric differences in polar mesospheric clouds, mesospheric temperatures, upper atmospheric gravity wave activity, and mean circulation patterns.