Metamaterial research is an extremely important global activity that promises to change our lives in many different ways. These include making objects invisible and the dramatic impact of metamaterials upon the energy and medical sectors of society. Behind all of the applications, however, lies the business of creating metamaterials that are not going to be crippled by the kind of loss that is naturally heralded by use of resonant responses in their construction. This review sets out some solutions to the management of loss and gain, coupled to controlled and nonlinear behavior, and discusses some critical consequences concerning stability. Under the general heading of active and tunable metamaterials, an international spectrum of authors collaborates here to present a set of solutions that addresses these issues in several directions. As will be appreciated, the range of possible solutions is really fascinating, and it is hoped that these discussions will act as a further stimulus to the field.

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Active and tunable metamaterials

Seismic waves are the most dramatic and most intensely studied manifestations of earthquakes. However, we also know of non-seismic phenomena, which precede large earthquakes. Some of them have been reported for centuries, even millennia. The list is long and diverse: bulging of the Earth's surface, changing well water levels, ground-hugging fog, low frequency electromagnetic emission, earthquake lights from ridges and mountain tops, magnetic field anomalies up to 0.5% of the Earth 's dipole field, temperature anomalies by several degrees over wide areas as seen in satellite images, changes in the plasma density of the ionosphere, and strange animal behavior. Because it seems nearly impossible to imagine that such diverse phenomena could have a common physical cause, there is great confusion and even greater controversy. This explains why reports on non- seismic pre-earthquake phenomena are regarded with suspicion in the scientific community. This may change with the recent discovery that igneous and metamorphic rocks, which make up a major portion of the Earth 's crust, contain electric charge carriers, which have been overlooked in the past. These charge carriers are defect electrons in the valence band, i.e., positive holes. Under normal conditions they are dormant, but when they ''wake up'', the rocks begin to sparkle and glow. This paper describes the physical and chemical nature of these positive holes, how they are introduced into minerals and rocks, and how they become activated. Evidence will be presented that, once the positive holes are generated, currents propagate through the rocks leading to electromagnetic emission, to positive surface potentials, to corona discharges, to positive ion emission, and to mid-infrared radiation. These phenomena are expressions of the same fundamental process: the ''awakening'' of dormant positive hole charge carriers that turn rocks momentarily into p-type semiconductors.

Rocks That Crackle and Sparkle and Glow: Strange Pre-Earthquake Phenomena

. A superimposed epoch analysis has been undertaken, in order to find the correlation of the ionospheric perturbations with seismic activity. We take the wave path from the Japanese LF transmitter (frequency=40 kHz) and an observing station of Kochi (wave path length of 770 km), and a much longer period (of five years) than before, is considered. This subionospheric LF propagation can be called "an integrated measurement" in the sense that any earthquakes in the LF sensitive area just around the great-circle path can influence the observed LF signals, so that we define the "effective magnitude" (Meff) by integrating the total energy from different earthquakes in the sensitive area on a current day and by converting it back into magnitude. A superimposed epoch analysis for the effective magnitude greater than 6.0 has yielded that the ionosphere is definitely disturbed in terms of both amplitude and dispersion, and that these perturbations tend to take place prior to an earthquake. The statistical z-test has also been performed, which has indicated that the amplitude is definitely depleted 2–6 days before the earthquake day and also that the dispersion is very much enhanced during the same period. This statistical study has given strong support to the existence of seismo-ionospheric perturbations for high seismic activity.

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A statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric LF propagation data in Japan

In order to model seismo-electromagnetic phenomena, we focus on one specific defect as an alternative source of charge carriers in igneous rocks. These charge carriers are defect electrons in the O2− sublattice that are chemically equivalent to O− in a matrix of O2− and are known as positive holes (p-holes). They are activated from peroxy defects: O3X–OO–XO3 (X = Si, Al, etc.) that are known as positive hole pairs (PHPs). Stressed igneous rocks behave like p-type semiconductors. In order to examine the contributions of p-holes to seismo-electromagnetic phenomena, we conducted two series of uniaxial loading tests using air-dry tiles of several types of rocks (granite, anorthosite, gabbro, limestone, and marble) and glass. We observed that the igneous rock tiles under central loading generated (1) a positive current, carried by holes and flowing from the central stressed volume through the surrounding unstressed volume to the edges of the tiles and (2) a negative current, carried by electrons and flowing from the central stressed volume into the load pistons. In addition, a positive potential appeared on the surface of the unstressed portion of the igneous rock tiles. On the other hand, limestone containing small amounts of silicate grains generated lower currents. Marble containing no silicate grains and glass generated much lower and no currents, respectively. The current generated per 1 m3 of stressed volume at a rapid loading up to 50 MPa was on the order of 10−5 A for granite. A slow increase of the stress level by 10 MPa superimposed on the 50 MPa added a 10−6 A of the current. Anorthosite and gabbro showed 10–50 times higher currents, and limestone and marble showed 1/10–1/100 times. The vertical electric field during a stress change from 50 MPa to 60 MPa was on the order of 102 V/m. We propose that, when stresses and strain change at or around a fault zone before and during an earthquake, similar currents and electric fields will be generated in and on the ground, respectively. Such currents and fields will be accompanied by changes in the conductivity of the rocks and may lead to abnormal electromagnetic fields and emissions and to ionospheric perturbations.

Current and surface potential induced by stress-activated positive holes in igneous rocks

Formation mechanism of great positive TEC disturbances prior to Wenchuan earthquake on May 12, 2008

Penetration of an electrostatic field from the lithosphere into the ionosphere and its effect on the D-region before earthquakes

A mechanism of electric-photochemistry channel of seismo-ionospheric coupling is investigated. In particular, the penetration of electric field from the lithospheric source into the ionosphere and effect of this field on the photochemistry coefficient and ionospheric parameters in the altitude range in the lower D region are modeled numerically. It is shown that observable effects can be expected when lithospheric electric source strength is of the order of 1.5 kV/m. In this case, variations of electron temperature and electron concentration will be of the order of (40–60)% and (25–40)% respectively at the range of altitudes 60–70 km. An increase of near-ground conductivity (caused by increasing humidity and/or radon emanation) by ∼2.3 times can cause increase of electric field intensity by ∼2 times in altitude ranges of 60–70 km. Corresponding relative change of T e increases up to ∼50%, as compared with the case of lower near-ground temperature. Spatial shapes of relative distribution of electron temperature and the ratio of negative ion–electron concentration map the spatial shape of the lithospheric electric field distribution. Spatial shapes of electron concentration distribution and electric field strength distribution of the lithospheric source are “opposite” to each other.

Change of ionospheric plasma parameters under the influence of electric field which has lithospheric origin and due to radon emanation

A new model is developed to describe the response of the unstable near-equatorial F region plasma to the spatial packet of atmospheric gravity waves (AGWs) of the lithospheric origin. Presence of both propagating and non-propagating (upward) modes in the spectrum of exited AGW packet is accounted for. It is shown that peculiarities of spectral and spatial distributions of relative perturbation of ion concentration in the ionosphere are determined by spectrum peculiarities of both AGW and increment of plasma instability. It is shown, in particular, that both periodical structures with spatial period of order of 500–800 km (for AGW periods of order of 25–40 min) and localized structures with characteristic dimension of the order of 1000–2000 km are excited in plasma, and localized plasma structures are shifted to the west over the distance of a few thousand kilometers relatively to the maximum of the neutral atmosphere response in the presence of eastward wind.

Stable and unstable plasma perturbations in the ionospheric F region, caused by spatial packet of atmospheric gravity waves

In this paper we propose a new 3D model for the generation of electrostatic field in the lithosphere–atmosphere–ionosphere (LAI) system by two external electric sources; namely lithospheric electric field or external current placed near the ground and (simultaneously) in the mesosphere/lower ionosphere. Then we present the results of numerical calculations. We discuss an influence of the generated electric field on the photochemical and ionospheric characteristics in the altitude range of the lower ionosphere. Appearance of strong electric field and strong change of the ionospheric characteristics (such as electron temperature and concentration) in the lower ionosphere is predicted in the presence of external electric sources in the lower atmosphere and mesosphere. An influence of the excited electric field on the losses of VLF electromagnetic waves propagating in the “Earth–ionosphere” waveguide is considered qualitatively, and rough estimation of this phenomenon is carried out. We prove a possibility of “blooming” of the ionosphere for VLF waves, if in the presence of mesospheric external current, an additional external (seismogenic) electric source appears in the lower atmosphere.

V.N. Ivchenko

Papers

Penetration of an electrostatic field from the lithosphere into the ionosphere and its effect on the D-region before earthquakes

Change of ionospheric plasma parameters under the influence of electric field which has lithospheric origin and due to radon emanation

Stable and unstable plasma perturbations in the ionospheric F region, caused by spatial packet of atmospheric gravity waves

Modeling electrostatic-photochemistry seismoionospheric coupling in the presence of external currents

Model of the internal gravity waves excited by lithospheric greenhouse effect gases