Global and frequent appearance of small spatial scale field-aligned currents possibly driven by the lower atmospheric phenomena as observed by the CHAMP satellite in middle and low latitudes
© Nakanishi et al.; licensee Springer. 2014
Received: 27 December 2013
Accepted: 8 May 2014
Published: 27 May 2014
Using magnetic field data obtained by the Challenging Minisatellite Payload (CHAMP), we show global and frequent appearance of small-amplitude (1 to 5 nT on the dayside) magnetic fluctuations with period around a few tens of seconds along the satellite orbit in middle and low latitudes. They are different from known phenomena, such as the Pc3 pulsations. The following characteristics are presented and discussed in this paper: (1) The magnetic fluctuations are perpendicular to the geomagnetic main field, and the amplitude of the zonal (east–west) component is larger than that of the meridional component in general. (2) As latitude becomes lower around the dip equator, the period tends to become longer. (3) The amplitudes have clear local time dependence, which is highly correlated to the ionospheric conductivities in local time (LT) 06–18. (4) The amplitude of the fluctuations shows magnetic conjugacy to a certain extent. (5) The amplitude shows no dependence on solar wind parameters nor geomagnetic activity. (6) A seasonal dependence is seen clearly. The amplitudes in the northern summer and winter are larger than those in the equinoxes. In the northern summer, the amplitudes above the Eurasian and South American continents and their conjugate areas are larger. In the northern winter, those above the eastern Pacific Ocean are larger. We suggest that the above characteristics, (1) to (6), can be attributed to the small spatial scale field-aligned currents having a lower atmospheric origin through the ionospheric dynamo process.
High-precision magnetic field data have been obtained by low-altitude satellites such as Ørsted, Challenging Minisatellite Payload (CHAMP), and SAC-C since Magsat was launched in 1979. In particular, the CHAMP satellite kept the most stable attitude, and the resolution of the magnetic data is so far the highest. The magnetic fluctuations with period around a few tens of seconds along the orbits are the objective of this research.
So far, three kinds of phenomena have been reported on the basis of the magnetic data obtained by the CHAMP satellite.
Firstly, as a temporal variation, the Pc3 micro-pulsation has been reported (e.g., Vellante et al. 2004; Heilig et al. 2007; Ndiitwani and Sutcliffe 2009). It is thought to be a shear Alfvén wave to which the field line resonance (FLR) converts fast magnetosonic wave generated by a solar wind upstream wave (e.g., Yumoto 1985 and references therein).
Secondly, as a spatial variation, the equatorial plasma bubble occurring after post-sunset has been identified (e.g., Stolle et al. 2006; Park et al. 2009a). Pressure inside the plasma is smaller than that outside. Balancing the total pressures on both sides, magnetic pressure inside increases compared to that outside, which means that the parallel component to the geomagnetic main field is not zero.
Lastly, as a spatial variation, the mid-latitude magnetic fluctuation (MMF) on the nightside has also been investigated recently (Park et al. 2009b). The authors claim that the MMFs are a result of the Perkins instability forming the medium-scale traveling ionospheric disturbances (MSTIDs), and they report the following characteristics: (1) the MMFs are perpendicular to the geomagnetic main field and thought to be the effect of field-aligned current (FAC), (2) their occurrence is rare above the southern Atlantic ocean, and (3) the occurrence rate bears little connection to the geomagnetic activity.
Contrary to the above three phenomena, our analysis of the magnetic field observed by the CHAMP satellite shows global and frequent appearance of small-amplitude magnetic fluctuations with period around a few tens of seconds along the satellite orbit nearly all the time and at any local time.
In the following, we shall show the characteristics of the magnetic fluctuations such as the dependence on local time, solar wind parameters, geomagnetic activity, and season. Considering all of the characteristics obtained, we suggest a simple model, i.e., small spatial scale FACs generated by the ionospheric dynamo having the origin in the lower atmosphere, and discuss whether or not the characteristics predicted by the model are consistent with those obtained by our data analyses.
The CHAMP satellite was launched on 15 July 2000 into a circular, near-polar (inclination 87.3°) orbit. Its initial altitude was 456 km and decreased to 250 km in 2010, and it had atmospheric reentry on 9 September 2010. One of the mission objectives is the study of the geomagnetic field. Instruments supporting the magnetic field investigations are precision scalar and vector magnetometers, a dual-head star camera system, and a digital ion drift meter.
The satellite data used in this study are the preprocessed (level 2) fluxgate vector magnetometer data from the CHAMP satellite (product identifier CH-ME-2-FGM-FGM). This instrument samples the magnetic field at a rate of 50 Hz with a resolution of digitization, 0.065 nT, and noise level less than 0.1 nT, which is sufficient to study the 1- to 5-nT (or even less) magnetic fluctuations having period around 10 to 30 s. In the standard processing, the data are averaged to 1-Hz samples which we used.
In this study, we did not use the vector data transformed into the north-east-central (NEC) coordinates, i.e., the geographic coordinate system, since the attitude determination was not accurate enough for the investigation of small-amplitude fluctuations around 1 to 5 nT with period around 10 to 30 s. However, because of the excellent attitude stability of the CHAMP satellite, we can make direct use of the high-performance fluxgate readings in the sensor coordinates (Vellante et al. 2004). It is precise enough for our purpose, i.e., the study of fluctuations with short period along the satellite orbit, to assume that the axes of the sensor coordinates are nearly the same (deviations of the North in the sensor coordinates from the true geographical North less than 3°) as those of the NEC coordinates except for the sign as is explained below. That is, in the sensor coordinates, the north–south component is approximately aligned with the orbit of the satellite, i.e., positive in the direction of the CHAMP flight; the vertical component is pointing to the center of the Earth; and the east–west component is along a vector product of the vertical and north–south directions.
The duration of the data used is 3,155 days in total from 2001 to 2010, which covers almost the whole CHAMP period, except for the data from the commissioning phase in 2000 that are not provided.
The satellite took around 11 days to cover 1 h of local time (LT) and around 131 days to cover all the LT sectors. To cover both all the local times and seasons, it took around 5 years. In our analysis, the local time is divided into four sectors; the dawnside (LT 03–09), the dayside (LT 09–15), the duskside (LT 15–21), and the nightside (LT 21–03). The data of 3,155 days used here are enough to clarify seasonal and local time dependence.
As other parameters, the geomagnetic Kp index and the solar wind parameters observed are used in our statistical analysis, and an ionospheric conductivity model is used in the ‘Results and discussion’ section. The Kp index is provided by the GFZ in Potsdam through the World Data Center (WDC) for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/index.html).
The solar wind data are obtained from the OMNI2 database where all the data are corrected for convection delay (http://omniweb.gsfc.nasa.gov/form/omni_min.html). These data are used in the geocentric solar magnetospheric (GSM) coordinate system with a time resolution of 1 min. The parameters used in this study are the IMF strength (BT), the X-component of the IMF (B x ), and the solar wind speed (Vsw). The IMF cone angles are calculated by the following equation: cone angle = arccos(|Bx|/BT).
The ionospheric conductivity model is provided by the WDC for Geomagnetism, Kyoto. Detailed information about the model can be found at the WDC website (http://wdc.kugi.kyoto-u.ac.jp/ionocond/sightcal/index.html).
Method of data analysis and some case studies
In the auroral zone higher than 60° of the magnetic latitude, there are magnetic fluctuations with much larger amplitude, which are reported to be caused by small spatial scale field-aligned currents having the origin in the magnetosphere (e.g., Iyemori et al. 1985). To separate from these phenomena, the region treated in this paper is limited to the region less than 50° in the dipole latitude.
The amplitude of the parallel component is much smaller than those of the perpendicular components and nearly zero, that is, the magnetic fluctuations are perpendicular to the geomagnetic main field. The fluctuations are often localized within the latitude range from a few degrees to 10°, i.e., they show packet-like structures.
The amplitude of the zonal component is, in general, larger than that of the meridional component.
Approaching the dip equator, the period and the signal amplitude of the two perpendicular components become longer and smaller, respectively. These characteristics are also apparent in the dynamic spectra in Figure 3. They can be seen in other longitude sectors except for the Brazilian anomaly sector, where the above characteristics often cannot be seen, for example, on the seventh path from the right in Figure 2.
The amplitudes of the two perpendicular components on the dayside are much larger than those on the nightside by around three times or more, which is statistically shown in the next section.5. From Figure 3, there seems to be a magnetic conjugacy about the amplitude, which can be seen generally in Figure 2. It should be kept in mind that although it is not easy to examine the magnetic conjugacy by one satellite, the examination of the magnetic conjugacy may be useful for the discussion of the spatial and temporal structures.
These characteristics can be found clearly in most cases although not shown in this paper. The magnetic fluctuations having the characteristics discussed in the previous paragraphs occur almost all the time and at any local time as shown by statistical analysis.
In the next section, in addition to the confirmation of the above characteristics, we conduct a statistical analysis of the magnetic fluctuations in order to examine dependence on local time, latitude, solar wind parameters, geomagnetic activity, and season.
We use the data from the period between 2001 and 2010 for statistical analysis. First, we describe the method of analysis and confirm statistically that the magnetic fluctuations have the five characteristics described in the previous section, as well as we show that they exist almost all the time and at any local time. Next, we examine the dependence of the amplitude on solar wind parameters, geomagnetic activity, and season.
As to the coordinates, the dipole coordinates are used in the analysis of local time, the dipole latitude, solar wind parameters, and geomagnetic activity dependencies, while the geographic coordinate system is used in the statistical analysis of the seasonal dependence to see the geographical effect. Statistical analysis is carried out with the data sets of the amplitude, the period, and the PSD for the years 2001 to 2010. The amplitude is calculated by the following process. Firstly, the standard deviation of the magnetic fluctuations is calculated for each orbit in each bin of size 5° of latitude in both the dipole and the geographic coordinates. The latitudinal scale of 5° yields about 60 data points and covers a few wave lengths; therefore, it is reasonable to regard the standard deviation as a measure for the amplitude in each bin.
Secondly, we average the standard deviations calculated for each bin of size 5° in longitude. The number of orbits passing through each bin is enough to see the statistical characteristics as described in the ‘Observations’ section. For example, the number of the orbits along which the CHAMP satellite passed through each bin is divided into each local time for the total period, i.e., 3,155 days, and the correspondent number is around 50 to 60. In the later part of the section, there are some cases in which local time is divided into four sectors, that is, the dawnside (LT03-09), the dayside (LT09-15), the duskside (LT15-21), and the nightside (LT21-03). In these cases, each bin includes about 350 orbits.
From these figures, we confirm statistically the five characteristics presented by a few typical examples in the previous section, as described below in order.Firstly, although not shown in Figure 5, it should be noted that the amplitude of the parallel component is statistically nearly zero at any local time.Secondly, the global distribution of the amplitude of the two perpendicular components shows that at any local time, the amplitudes of the zonal component are larger than those of the meridional component as seen in Figure 5 except for those above the Brazilian anomaly sector.Thirdly, as shown in Figure 5, the amplitude of the two perpendicular components is, in general, smaller around the dip equator than those at other latitudes and nearly zero on the dip equator except for the Brazilian anomaly sector. The amplitude minimum on the dip equator is also seen in Figure 6.Figure 7 shows that as the dip angle becomes smaller, the averaged period of the spectral peak becomes longer. In particular, from around 15° of the dip angle which corresponds to around 8° of the dipole latitude, the period becomes longer more steeply.Fourthly, as shown in Figure 5, the amplitude on the dayside is much larger than that on the nightside. As to the dawnside and the duskside, the amplitude on the duskside is slightly larger than that on the dawnside. Figure 6 indicates that the amplitude on the dayside is about five times larger than that on the nightside. Lastly, the global distribution of the amplitude in Figure 5 shows that there exists the magnetic conjugacy to a certain extent, which is also seen in Figure 6.
Therefore, the five characteristics mentioned in the previous section are also confirmed by the statistical analysis.
Summarizing the results of our statistical analysis, we found three characteristics: (1) There is almost no dependence on the solar wind parameters. (2) There is almost no dependence on geomagnetic activity. (3) There exists the clear seasonal dependence with the topographical characteristics. These cannot be explained by the phenomena having the origin in the solar wind, the magnetosphere, and the ionosphere, while it may be attributed to the processes in the lower atmosphere.
Results and discussion
The characteristics of the magnetic fluctuations treated in this paper cannot be explained by the instrumental effect nor the phenomena reported before.
At first, the instrumental effect is described. We consider whether or not the instrumental effect is caused by density perturbation at the satellite altitude, i.e., gravity waves. Firstly, the satellite attitude is expected to be stable enough to the density perturbation because the CHAMP satellite is a heavy satellite (500 kg) with a large moment of inertia. Secondly, the objective magnetic fluctuation has the three observational characteristics, i.e., the geomagnetic conjugacy, no altitude dependence, which implies that they do not have atmospheric density dependence, and the smaller amplitude on the dip equator. They cannot be explained by the attitude fluctuation. Therefore, from the above reasons, the observed magnetic fluctuation cannot be attributed to the instrumental effect. However, it should be noted that because the amplitude on the nightside is small, the artificial noises, for example, the digitization step size (0.065 nT), may not be ignorable on the nightside.
Next, the phenomena reported previously are described. Here, we mainly refer to the MMFs reported by Park et al. (2009b) and later the Pc3 micro-pulsations.
Secondly, most of the objective magnetic fluctuations are different from the magnetic fluctuation accompanying the equatorial plasma bubble (EPB) because our objective variations exist almost all the time and globally and appreciable variation is not seen in the parallel component. But for the case of the statistical analysis on the nightside, the EPBs should be considered because the magnetic fluctuations related to the EPBs are extracted by the similar high-pass filter as reported by Park et al. (2009a) and the amplitude of our objective variations is small on the nightside. Actually, from LT 19 to LT 00, the amplitude of the parallel component gets larger although not shown here. In addition, the effect of the artificial noise may not be ignorable. Therefore, local time range for consideration of local time dependence is limited from LT 06 to LT 18 in this paper.
We interpret that all of the characteristics mentioned so far can be attributed to spatial structure of the field-aligned currents (FACs). The phenomenon may be related to the ionospheric dynamo driven by the atmospheric gravity waves propagating from the lower atmosphere to the ionosphere.First, the spatial structure of the FACs is described. The characteristic that the small-amplitude magnetic fluctuations are perpendicular to the geomagnetic main field implies whether the fluctuations are due to the effect of the FACs or the shear Alfvén waves. As shown in Figure 7, at the dip angles lower than 15°, which is around 8° of the dipole latitude, the averaged period and PSD of the zonal component tend to become longer and smaller, respectively. The same characteristics are seen in the meridional component and at other local time sectors except for the region between 10° and 60° of the dipole longitude, i.e., in the Brazilian anomaly sector. We interpret these characteristics of the PSD in middle and low magnetic latitudes not as temporal variations of Alfvén wave but as spatial structures of the small spatial scale FACs with both ends in the ionosphere.
The above considerations, with the characteristics of the seasonal dependence including the topographical dependence, strongly suggest that the magnetic fluctuations are caused by the E-layer dynamo. That is, the FACs having the lower atmospheric origin are generated through the E-layer dynamo process. It should be kept in mind that the two-dimensional ionospheric dynamo, within a finite region, causes a three-dimensional electric current closure which is made up of currents in the ionosphere and FACs, generating the two perpendicular components, i.e., the meridional and zonal magnetic components at the satellite altitude.
The reason why the strongest amplitude is seen around 8° in the dipole latitudes may be interpreted as follows. In our model, the value of the FACs is approximately proportional to the conductivities. The conductivity becomes larger as latitude decreases to the dip equator. On the other hand, in less than around 8° in the dipole latitude, because the apparent period gets longer, the amplitude gets attenuated by the high-pass filter. In the region, the effect of the attenuation is steeper than that of the amplification due to the conductivity enhancement toward the dip equator. Therefore, the strongest amplitude of the objective magnetic fluctuation is located at about 8° in latitude from the dip equator.
It is important to clarify what drives the ionospheric dynamo from the lower atmosphere and why the seasonal dependence of the amplitude shows the topographical characteristics as shown in Figure 10. We consider the effect of the upward propagating atmospheric gravity waves (AGWs), i.e., the acoustic gravity waves and the internal gravity waves, from the lower atmosphere to the ionosphere. Some reports confirm the propagation of the atmospheric gravity waves from the troposphere to the ionosphere by computer simulation (e.g., Horinouchi et al. 2002; Walterscheid et al. 2003). As to a possibility that magnetic fluctuations are caused by the AGWs, after the Sumatra earthquake on 26 December 2004, Iyemori et al. (2005) observed a peculiar geomagnetic pulsation near the conjugate point of the epicenter of the earthquake. The period of a sharp spectral peak was 3.6 min, near one of the vertical resonance periods of the acoustic gravity wave between the ground and the ionosphere predicted by several papers (e.g., Tahira 1995). Iyemori et al. (2005) suggested the following process to generate the magnetic pulsation. First, the ocean floor suddenly moves vertically up (or down) and is accompanied by neutral atmosphere movement there. The acoustic wave which propagates vertically to the lower ionosphere is reflected back at an altitude where the wave frequency is equal to the cutoff frequency. If the condition matches, the vertical acoustic resonance occurs. At last, the dynamo generates the long-period magnetic pulsations. Iyemori et al. (2013) analyzed the barometric data as well as the observed magnetic field on the ground after the 2008 Iwate-Miyagi Nairiku earthquake, 2007 Mieken-Chubu earthquake, and 2010 Chile earthquake and found the sharp spectral peaks at the acoustic resonance periods in each event, which strongly supports the above suggestion.
There are some quantitative studies about the effects of the acoustic gravity waves generated by big earthquakes on ionospheric disturbances. For example, just after the 2011 Off the Pacific coast of Tohoku earthquake, Saito et al. (2011) observed the oscillation of the GPS total electron content (TEC) having the acoustic resonance frequency near the tsunami source and the propagation of the GPS-TEC variation having the wave-like structure centering the tsunami source region. About the event, Matsumura et al. (2011) showed that there is a remarkable agreement with numerically simulated atmospheric perturbations at an altitude of 300 km with the GPS-TEC observation. It shows the atmospheric perturbations causing the ionospheric disturbance were the acoustic gravity wave generated by a sudden ocean lift up at the tsunami source region, and the acoustic gravity wave was converted to the internal gravity wave which propagated in the horizontal direction.
Here, we suggest the possibility that atmospheric gravity waves generated by the lower atmospheric disturbances, which are expected to exist globally and frequently, propagate upward and cause the ionospheric dynamo. The ionospheric dynamo currents must be accompanied by the FACs, because the dynamo region caused by the gravity wave is limited in the horizontal scale and the dynamo currents need to divert along a highly conductive geomagnetic field, forming the FAC system. That is, as an Alfvén wave with the polarized electric field propagates along the geomagnetic field line being accompanied by the FAC, resulting in a three-dimensional closure circuit of the ionospheric dynamo current. It should be noted that in our model, the generator of the ionospheric current is the acoustic wave or the internal gravity wave generated by the disturbance in the lower atmosphere. The temporal variation is assumed to be slow enough compared to the spatial variation divided by the CHAMP velocity, so that the temporal changes can be ignorable. If the AGW has a period longer than a few minutes as the temporal variation, the above assumption is reasonable.
With the possibility of a tropospheric origin shown by computer simulation (Zettergren and Snively 2013), the field-aligned currents are caused by the propagation of acoustic gravity waves generated by tropospheric sources through the E-layer dynamo. The period of the acoustic wave is around 3.5 min, i.e., near the vertical resonance period. The order of the speed of the neutral wind at an altitude of 100 km, i.e., the E-layer, caused by the acoustic wave is 10 m/s, and the spatial scale is around 100 km. As a result, the order of the density of the field-aligned current at an altitude of 100 and 400 km is 0.1 and 0.01 μ Am−2, respectively. On the other hand, with use of Ampere's law, the density can be estimated from the observed magnetic fluctuations. The order of the amplitude of the typical magnetic fluctuations, main magnetic field, and the latitudinal spatial scale estimated from the CHAMP observation are 1 nT, 10,000 nT, and 100 km, respectively. Then, the order of density at the CHAMP altitude is 0.01 μ Am−2, which is consistent with that predicted by Zettergren and Snively (2013).
Next, the relation between the seasonal dependence and the lower atmospheric origin is discussed. Sato et al. (2009) sum up the characteristics of the small-scale gravity wave from the lower atmosphere with horizontal wavelength of tens to hundreds of kilometers and the seasonal dependence of the activity of the lower atmospheric disturbance. According to their paper, in the northern summer, the activities above the Andes and the east coast of Australia are high due to topographically forced gravity waves, the so-called mountain waves, and those above the Indian and African continents are high due to gravity waves generated by strong convection in the monsoon regions. These disturbances could be related to the seasonal dependence shown in Figure 10. However, there is no appropriate reference about the lower atmospheric disturbances during the northern winter; therefore, at the present, we cannot refer to the characteristic of the larger amplitudes above the eastern Pacific Ocean.
For investigating the lower atmospheric origin in detail, the correlation between the amplitude of our objective magnetic fluctuations and meteorological conditions should be examined for each pass of the CHAMP satellite, which is not done in this paper.
Our analysis of the CHAMP magnetic data shows existence of small-amplitude (1 to 5 nT) magnetic fluctuations with period around a few tens of seconds along the orbits of the satellite at middle and low latitudes, i.e., at the geomagnetic dipole latitudes within 50°, nearly all the time and at any local time.
The following characteristics can be seen: (1) The signal is perpendicular to the geomagnetic main field, and the amplitude of the zonal component is generally larger than that of the meridional component. (2) Around the dip equator, as the latitude becomes lower, the period and the amplitude of the two components perpendicular to the main field respectively tend to become longer and smaller (to nearly zero on the dip equator). (3) The amplitudes of the magnetic fluctuations have the clear local time dependence, which highly correlates to the electric conductivity of the ionospheric dynamo layer. (4) The amplitude shows symmetry about the magnetic dip equator which indicates a magnetic conjugacy to a certain extent. (5) The amplitude shows almost no dependence on the solar wind parameters such as the IMF cone angle nor the solar wind speed, which strongly suggests no association with the Pc3 micro-pulsation. (6) The amplitude also shows almost no dependence on the geomagnetic activity. (7) There is a clear seasonal dependence with topographical characteristics. That is, the amplitudes are larger in the northern summer and winter than those during the equinoxes. In the northern summer, the amplitudes above the Eurasian and South American continents and their conjugate areas are larger, and in the northern winter, the amplitudes above the eastern Pacific Ocean are larger than those above other areas. These characteristics cannot be explained by the mechanisms reported so far, i.e., Pc3 micro-pulsations and equatorial plasma bubbles. In addition, the characteristics of the occurrence frequency on the nightside of our objective magnetic fluctuations have the same characteristics with the mid-latitude magnetic fluctuations (MMFs) on the nightside reported by Park et al. (2009b), and they may be a part of our objective magnetic fluctuations.
We interpret that all of the above characteristics can be attributed to the spatial structures of field-aligned currents (FACs) having the origin in the lower atmosphere. The expected spatial structure of the small spatial scale FACs having their origin in the lower atmosphere is consistent with the above observed characteristics. In addition, the amplitudes of the perpendicular components are highly correlated to the ionospheric conductivities, implying that the dependence of the amplitudes on local time may be attributed to the efficiency of the ionospheric E-layer dynamo.
In this paper, many aspects are not discussed and left unsolved, for example, the characteristics that the amplitude of the zonal component is larger than that of the meridional component, and the relationship between the zonal and the meridional components in the region between 20° and 45° of the dipole longitude, i.e., in the Brazilian anomaly sector, differs from that in other longitudinal zones. They will be discussed in separate papers.
The CHAMP magnetic data are provided by the Information Systems and Data Center in German Research Centre for Geosciences, Potsdam. This study was partly supported by the 2011 Kyoto University ‘Core Stage Backup’ program and by grant 25287128 under Japan Society for Promotion of Science (JSPS).
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