In mid- and low-latitude ionospheric F-region on the dayside, magnetic field and electron density (Ne) fluctuations with amplitude smaller than a few nT and 1–2% of Ne, respectively, are commonly ...observed. Their spatial scale along satellite orbit is around 70–250 km. It is presumed that they are generated by the waves propagated from lower atmosphere. However, the mode of waves (acoustic wave or internal gravity wave) and their source are not yet clear. Among the possible sources, cumulus convection and/or associated rainfall are considered to be the strong candidates for the atmospheric wave generation. We use the rainfall estimated by the hourly Global Satellite Mapping of Precipitation (JAXA/GSMaP) as a proxy of lower atmospheric disturbance as the wave source, and compare the rainfall with the amplitude of magnetic fluctuations (magnetic ripples) and electron density fluctuations observed by the Swarm satellites. The data from April 2014 to July 2020 are used. The global distribution of rainfall estimated by the GSMaP and its seasonal variation have similarities with amplitude distribution of magnetic ripples and electron density fluctuations on the dayside. We calculate the ratio of their magnitude, i.e., amplitude of magnetic ripples or electron density fluctuations in rainfall cases to those in no-rainfall cases. Although the longitudinally averaged ratio is not very large but around 1.1–1.2 in ± 10– ± 50° Apex latitudes, it is clearly larger than 1.0. The ratio increases when the intensity of rainfall (mm/h) increases. These results indicate that a cumulous convection which causes rainfall is one of the main sources of atmospheric waves that produce magnetic ripples and electron density fluctuations commonly observed in the dayside ionosphere. Anticipating acoustic waves as the driver of magnetic ripples and electron density variations, a difference in the generation mechanism of electron density fluctuations from that of magnetic ripples is suggested even if their sources are common.
Graphical abstract
A strong volcanic eruption caused a clear vertical acoustic resonance between the sea surface and the thermosphere. Its effects are observed as geomagnetic and GPS-TEC oscillations near the volcano ...and its geomagnetic conjugate area. The geomagnetic oscillations are observed at Apia and Honolulu geomagnetic observatories with amplitude of about 2 nT and 0.2 nT, respectively. The volcanic eruption started around 04:14 UT on January 15, 2022. The oscillations appeared at 04:21UT at Apia, Samoa, only about 7 min after the start of eruption. Because the distance between the volcano and Apia is about 841 km, it takes about 40 min for a sound wave to propagate from the volcano to Apia. Therefore, it is more plausible to assume that the magnetic oscillation observed at Apia about 7 min after the eruption is caused by the sound waves propagated vertically upward to the ionosphere and generated an electric current. The coherent appearance of geomagnetic oscillation at Honolulu located near the geomagnetic conjugate point of the volcano strongly support the idea that the ionospheric current generated over the volcano diverted as a field-aligned current which flew to the opposite hemisphere and caused the geomagnetic oscillation at Honolulu. The earliest start of GPS-TEC oscillation was around 04:15UT near the volcanic eruption, and it was around 04:20 UT at KOKV station in Hawaii. The time-lag of the TEC variations between Samoa and Hawaii obtained by a cross-correlation analysis is 4.5 min or 8.5 min. These time differences are much smaller than the travel time of the seismic waves from the volcano to Hawaii islands. Therefore, it is suggested that the electric field transmitted along geomagnetic field caused the TEC variation observed over Hawaii Islands. A sawtooth waveform of geomagnetic oscillation observed at Apia and Honolulu is analyzed and a possible generation mechanism is discussed.
Graphical Abstract
Periodic variations of the ionosphere were detected by ground‐based GPS total electron content (TEC) measurements after the Sumatra‐Andaman earthquake in 2004. The observational data showed that the ...4‐min periodic TEC variations occurred 1 h after the earthquake and continued for longer than 4 h. At the PHKT station, about 30 cycles of the 4‐min periodic TEC variations were observed from 0230 to 0430 UT. The maximum peak‐to‐peak amplitude of the variations was about 0.6 total electron content unit (TECU; 1 TECU = 1016 el m−2) around 0320 UT. The frequency of these periodic variations was 3.9 mHz. They were detected by the SAMP station in the northern Sumatra and the PHKT and BNKK stations in Thailand with the signal from seven GPS satellites: PRN 8, 11, 13, 19, 23, 27, and 31. They were observed in a limited area from 4°N to 15°N in latitude and from 96°E to 101°E in longitude, although the western boundary was not certain because of the limit of the observational field of view. The amplitude of these TEC variations showed the dependence on the zenith angle of the path between the GPS receiver and satellite. The amplitude had a maximum when the zenith angle was the smallest. This could be caused by the vertical structure of the electron density variations. This also suggested that the electrons were oscillating along the geomagnetic field. The 4‐min periodic TEC variations were interpreted to be induced by the long‐lasting free oscillation of the atmosphere set up by the earthquake. Their long duration also indicates that they were generated by a nontransient process like resonance.
Correlation between rainfall and short period GPS-TEC (total electron content) variations are investigated by using the precipitation data obtained on the ground and estimated from satellite ...observations (JAXA/GSMaP) as a proxy of lower atmospheric wave activity. The GPS-TEC data obtained at a tropical station, PHIM, in Phimai, Thailand, for 2014–2020, and the data obtained at a mid-latitude station, NAKG, in Tokara Nakanoshima Island, Japan, for 2017–2019, are examined. A statistical analysis of MEM (maximum entropy method) power spectral density (PSD) in the period range from 50 to 1200 s over PHIM clearly shows an enhancement in the cases of rainfall from that in no-rainfall cases, in particular, on the dusk side. The enhancement is observed both acoustic wave periods less than 5–6 min and internal gravity wave periods more than 10 min. The enhancement after sunset could be an effect of strong rainfall more frequent on the dusk side than that in other local time, or it could suggest the importance of ionospheric electron density profile change for the TEC variation. On the other hand, the PSD does not show such clear enhancement over NAKG on the dusk side, although it shows a small enhancement on both dayside and night-side. A clear PSD bulge near the main vertical acoustic resonance periods, i.e., around 275 s, appears in the average PSD profile of the TEC at PHIM, which suggests that the resonance effect contribute to some extent the PSD enhancement under rainy condition. An event analysis also suggests the contribution of acoustic resonance to the enhancement of the short period TEC variation. A complicated spatial distribution of TEC oscillation over a rainfall area around PHIM, where the TEC oscillations with various periods co-exist, is presented.
Graphical Abstract
Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. ...Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.
The Swarm satellites observed small-amplitude (0.1–5 nT) magnetic fluctuations perpendicular to the geomagnetic field. These so-called magnetic ripples (MRs) have a period of around a few tens of ...seconds along the satellite orbit in the topside ionosphere at middle and low latitudes. They are spatial structures from small-scale field-aligned currents. We investigated the following three characteristics of the MRs. First, we used Swarm observations to confirm their basic characteristics obtained from the Challenging Minisatellite Payload satellite. That is, the global distribution of the average MR amplitudes has clear geographic, seasonal, and local time dependence that is highly correlated with ionospheric conductivities. Second, we found that the average amplitudes of the MRs derived from the Swarm-B satellite, which flies at a ~50 km higher altitude, are slightly smaller than those of the Swarm-A and Swarm-C. This difference suggests that the location of the origin of MRs is below ~460 km altitude, i.e., not in the magnetosphere. Last, to provide evidence of correlation between the MRs and meteorological phenomena, we performed statistical and event analyses with typhoon track data, which are a source of acoustic and gravity waves. The data from 54 typhoons during the period from November 26, 2013, to July 31, 2016, were used for statistical analysis. The results show that the average amplitudes of the MRs during typhoon activity on the dawn, dusk, and night sides are larger than those during non-typhoon conditions. Event analyses indicate amplitude enhancements of the MRs around typhoons, and the latitude of the enhancement migrated with the typhoon. These analyses indicate that typhoon activity is correlated with MR activity and that cumulus convection activity other than typhoons may also affect MR amplitudes.
Graphical abstract
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The Calbuco volcano in southern Chile erupted on April 22, 2015. About 2 h after the first eruption, a Swarm satellite passed above the volcano and observed enhancement of small-amplitude (~0.5 nT) ...magnetic fluctuations with wave-packet structure which extends 15° in latitude. Similar wave packet is seen at the geomagnetic conjugate point of the volcano. Just after the eruption, geomagnetic fluctuations with the spectral peaks around the vertical acoustic resonance periods, 215 and 260 s, were also observed at Huancayo Geomagnetic Observatory located on the magnetic equator. Besides these observations, around 4-min, i.e., 175, 205 and 260 s, oscillations of total electron content (TEC) were observed at global positioning system stations near the volcano. The horizontal propagation velocity and the spatial scale of the TEC oscillation are estimated to be 720 m/s and 1600 km, respectively. These observations strongly suggest that the atmospheric waves induced by explosive volcanic eruption generate TEC variation and electric currents. The Swarm observation may be explained as a manifestation of their magnetic effects observed in the topside ionosphere.
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.
A time-dependent, two-dimensional, nonlinear, non-hydrostatic, compressible, numerical model is used to investigate the horizontal extension of acoustic waves trapped in a thermal duct between the ...lower thermosphere and the ground or the stratosphere, and evanescent and gravity waves around them. The simulation shows that an impulsive point source on the ground excites acoustic waves ducted in the mesosphere. The ducted acoustic waves have horizontal wavelengths longer than 130km and frequencies of 3.7–3.8mHz. This ducted acoustic waves are identified as acoustic resonance modes expected in previous studies. They extend to 300km distance from the source after 2h. In the case of the source with a finite width, the acoustic resonance would extend to 300km out of the edge of the source. The temporal variation of two resonance modes is also declared. In the vicinity of the source, two acoustic resonance modes are formed which have frequencies of 3.7 and 4.4mHz. The mode of 4.4mHz is dominant above the mesopause for about 50min after the impulsive perturbation of the source since the mode is leaky. On the other hand, the mode of 3.7mHz is well trapped in the thermal duct and persist over 2h. In addition to the acoustic waves, evanescent gravity waves with horizontal wavelengths shorter than 100km and with frequencies between 2.4 and 3.6mHz are excited in the stratosphere. Gravity waves freely propagating are also excited with frequencies lower than 2mHz and with horizontal wavelengths shorter than 130km.
► Acoustic resonance extends to 300km from a point source after 2h. ► The resonance mode of 4.4mHz is dominant above the mesopause at first. ► Evanescent gravity waves are excited together with resonant acoustic waves.
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