Yoshihiro Kakinami

Latitudinal distribution of anomalous ion density as a precursor of a large earthquake

[1] Data obtained by the U.S. satellite DE‐2 are used to investigate possible precursor features in the ionosphere associated with a large earthquake (latitude �33.13°, longitude 73.07°, M = 7.5), which occurred during a moderate geomagnetic disturbance. Atomic oxygen ion and molecular ion distributions show characteristic latitudinal features similar to the well‐known equatorial ionization anomaly (EIA) feature but centered around the earthquake epicenter. We name this the precursor ionization anomaly (PIA). The density minima of both the atomic oxygen and molecular ions are in two latitude zones, depending on the distance from the epicenter. One of the PIA minima aligns with the geomagnetic latitude crossing the epicenter. Another minimum is found along the geographic latitude of the epicenter. These minima are located in an area spanning about 40° in latitude and about 140° in longitude. It is noted that the molecular ion minimum is more clearly defined even when the atomic ion density minimum is not indicated clearly. The ion density reduction seems to be caused by a superposition of natural/quiet time ionospheric eastward electric field and an electric field associated with the earthquake. Although we studied one single event, our careful examination of results suggests that the location and day of occurrence of the PIA can be predicted for some large earthquakes even during moderate geomagnetic disturbance if the satellite orbit is properly chosen.

Ionospheric electron content anomalies detected by a FORMOSAT-3/COSMIC empirical model before and after the Wenchuan Earthquake

An empirical model of ionospheric electron content (IEC), based on FORMOSAT3/COSMIC (F3/C) data, is constructed in order to detect pre-earthquake anomalies. The empirical model provides IEC with four parameters of local time, season, longitude and latitude. For the first time we try to detect anomalies in the F3/C IEC by comparing the model values with observations during the 12 May 2008 Wenchuan Earthquake period. It is found that around the epicentre an IEC enhancement appears on day 3 (9 May) and sequential IEC reductions occur on day 6 to day 1 (6 to 11 May) before the earthquake.

A new index to monitor temporal and long-term variations of the equatorial electrojet by MAGDAS/CPMN real-time data: EE -Index

A new index, EE-index (EDst, EU, and EL), is proposed to monitor temporal and long-term variations of the equatorial electrojet by using theMAGDAS/CPMN real-time data. The mean value of the H component magnetic variations observed at the nightside (LT = 18–06) MAGDAS/CPMN stations along the magnetic equatorial region is found to show variations similar to those of Dst; we defined this quantity as EDst. The EDst can be used as a proxy of Dst for the real-time and long-term geospace monitoring. By subtracting EDst from the H component data of each equatorial station, it is possible to extract the Equatorial Electrojet and Counter Electrojet components, which are defined as EU and EL, respectively.

Seismo-ionospheric anomalies of the GPS-TEC appear before the 12 May 2008 magnitude 8.0 Wenchuan Earthquake

In this paper, the total electron content (TEC) of Global Ionosphere Maps (GIMs) is normalized and employed to study the seismo-ionospheric anomalies at the time of the 12 May 2008 M 8.0 Wenchuan Earthquake. The space weather conditions are taken into account. It is found that remarkable reductions appear locally around the epicentre and their conjugate points during the daytime of 29 April and 6–10 May 2008. A global study and a strict criterion are applied to detect anomalies. Results show that the anomalies on 29 April and 6 and 7 May 2008, which are respectively days 13, 6 and 5 before, are possibly related to the earthquake. The conjugate signature implies the seismo-generated electric field is essential.

A comparison of a model using the FORMOSAT-3/COSMIC data with the IRI model

In this study, an empirical model constructed using data of FORMOSAT3/COSMIC (F3/C) from 29 June, 2006, to 17 October, 2009, retrieves altitude profiles of electron density (Ne). The model derives global Ne profiles from 150 to 590 km altitude as functions of the solar EUV flux, day of year, local time and location under geomagnetically quiet conditions (Kp < 4). Ne profiles derived by the model are further compared with those of the International Reference Ionosphere (IRI). Results show that the F2 peak altitude hmF2 and the electron density NmF2, as well as the electron density above, derived by the model are lower than those of the IRI model. The F3/C model reproduces observations of F3/C well at 410-km altitude while the IRI model overestimates them. The overestimation of the IRI model becomes large with decrease of EUV flux. It is found that the topside vertical scale height of the F3/C model shows high values not only magnetic dip equator but also middle latitude. The results differ significantly from those of IRI, but agree with those observed by topside sounders, Alouette and ISIS satellites.

Topside ionospheric electron temperature and density along the Weddell Sea latitude

It has been well known that the ionospheric electron density Ne is greater in the summer nighttime than daytime around the Weddell Sea region, which is named Weddell Sea Anomaly (WSA). This paper for the first time reports unusual increases (decreases) of the daytime (nighttime) electron temperature Te at about 830 km altitude over the WSA latitudes probed by Tatiana-2 during December 2009 to January 2010. Concurrent measurements at 660–830 km altitude observed by Tatiana-2, Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER), and Formosa Satellite 3/Constellation Observing System for Meteorology, Ionosphere and Climate (F3/C) reveal the anticorrelation between Te and Ne along the WSA latitudes in the daytime and nighttime. Based on F3/C Ne along the WSA latitudes observed at various local times, the associated Te values are computed. The Tatiana-2 and DEMETER observations as well as the computed results show that Te yield the maximum values over the WSA region during daytime and over the Indian and Atlantic Ocean area during nighttime. The maxima or minima in F3/C Ne and the computed Te reveal eastward phase shifts.

Correlations between ion density and temperature in the topside ionosphere measured by ROCSAT‐1

From the ROCSAT-1 satellite plasma data at an altitude of 600 km, the correlation between ion temperature (Ti) and density (Ni) was investigated. The data were obtained in a magnetic dip latitude (MLAT) of less than ±40° in 2000–2004. Positive and negative correlations between Ni and Ti were observed around the magnetic dip equator, while weak positive correlations were observed in |MLAT| > 25° during daytime (10:00–16:00 local time). These variations were found in all longitudes, seasons, solar flux (F10.7) levels, and magnetic disturbance levels, although the minimum value of Ti clearly increased with increasing solar flux levels. The results suggest that the solar flux dependence of Ti arises from the solar flux dependence on neutral temperature (Tn). Since Ti is determined by heating through Coulomb collision with electrons and cooling through elastic collision with neutral species, the ratio of ion density to neutral density is an important factor. The ratio reaches its maximum value around the magnetic dip equator and decreases with increasing MLAT. The correlation between Ni and Ti in the topside ionosphere can be explained by electron temperature (Te) and Tn as well as the ratio because Ti follows Te variation when the ratio is high, while it follows Tn when the ratio is low.

Using the IRI, the MAGIC model, and the co-located ground-based GPS receivers to study ionospheric solar eclipse and storm signatures on July 22, 2009

The longest total solar eclipse in the 21st century occurred in Southeast Asia on 22 July 2009 from 00:55 to 04:15 UT, and was accompanied by a moderate magnetic storm starting at 03:00 UT with a Dst reduction of −78 nT at 07:00 UT. In this study, we use the ionospheric reference model IRI, the data assimilation model MAGIC, and ground-based GPS receivers to simulate and examine the ionospheric solar eclipse and geomagnetic storm signatures in Taiwan and Japan. Cross-comparisons between the two model results and observations show that IRI fails to simulate the two signatures while MAGIC partially reproduces the storm features. It is essential to include ground-based GPS measurements to improve the IRI performance.

Observation and simulation of the ionosphere disturbance waves triggered by rocket exhausts

Observations and theoretical modeling of the ionospheric disturbance waves generated by rocket launches are investigated. During the rocket passage, time rate change of total electron content (rTEC) enhancement with the V-shape shock wave signature is commonly observed, followed by acoustic wave disturbances and region of negative rTEC centered along the trajectory. Ten to fifteen minutes after the rocket passage, delayed disturbance waves appeared and propagated along direction normal to the V-shape wave fronts. These observation features appeared most prominently in the 2016 North Korea rocket launch showing a very distinct V-shape rTEC enhancement over enormous areas along the southeast flight trajectory despite that it was also appeared in the 2009 North Korea rocket launch with the eastward flight trajectory. Numerical simulations using the physical-based nonlinear and non-hydrostatic coupled model of neutral atmosphere and ionosphere reproduce promised results in qualitative agreement with the characteristics of ionospheric disturbance waves observed in the 2009 event by considering the released energy of the rocket exhaust as the disturbance source. Simulations reproduce the shock wave signature of electron density enhancement, acoustic wave disturbances, the electron density depletion due to the rocket induced pressure bulge and the delayed disturbance waves. The pressure bulge results in outward neutral wind flows carrying neutrals and plasma away from it and leading to electron density depletions. Simulations further show, for the first time, that the delayed disturbance waves are produced by the surface reflection of the earlier arrival acoustic wave disturbances

A possible space-based tsunami early warning system using observations of the tsunami ionospheric hole

Ionospheric plasma disturbances after a large tsunami can be detected by measurement of the total electron content (TEC) between a Global Positioning System (GPS) satellite and its ground-based receivers. TEC depression lasting for a few minutes to tens of minutes termed as tsunami ionospheric hole (TIH) is formed above the tsunami source area. Here we describe the quantitative relationship between initial tsunami height and the TEC depression rate caused by a TIH from seven tsunamigenic earthquakes in Japan and Chile. We found that the percentage of TEC depression and initial tsunami height are correlated and the largest TEC depressions appear 10 to 20 minutes after the main shocks. Our findings imply that Ionospheric TEC measurement using the existing ground receiver networks could be used in an early warning system for near-field tsunamis that take more than 20 minutes to arrive in coastal areas.