Kazuo Makita

Transpolar auroras, their particle precipitation, and IMF By component

Transpolar auroras, their associated particle precipitation, and their occurrence with respect to the IMF By polarity are examined on the basis of DMSP F6 auroral images and the corresponding particle data. It is found that the transpolar arcs are located in the poleward edge of the soft particle precipitation region extending from either the dawn or dusk part of the auroral oval precipitation; they are not embedded in the polar rain region. This finding suggests that the transpolar arcs are located along the poleward boundary of the closed field line region (or the equatorward boundary of the open region) as suggested by Meng (1981). Further, the appearance of the extended precipitation region from the oval depends on the polarity of the IMF By, in the northern hemisphere morning sector for IMF By 0. In general, the precipitating particle flux in the extended precipitation region is not high enough to produce appreciable luminosity. Thus only the transpolar arcs (associated with relatively intense precipitation) near the poleward boundary tend to become much more luminous, forming the so-called “theta aurora.”

Convective generation of “giant” undulations on the evening diffuse auroral boundary

Convective generation of “giant” undulations on the equatorward boundary of an evening diffuse aurora is numerically simulated. A giant undulation is defined as a waveform having the crest-to-trough amplitude comparable to the wavelength. The two-dimensional electrostatic particle code is used for studying the motion of magnetospheric plasma perpendicular to the geomagnetic field. According to the simulation results by Yamamoto et al. [1993], the giant undulation is a manifestation of the Kelvin-Helmholtz (K-H) waves arising from the polarization of “an arc sheet” (dense plasma population of ionospheric origin) in the magnetosphere, which is assumed to be located just equatorward of the region of proton diffuse aurora. In this previous simulation, initially, irregularities are given evenly over the entire azimuthal length of the arc sheet so that the resulting undulations are periodic. The present simulation deals with a different situation, that a K-H wave starts growing from local irregularities on the polarized arc sheet. The simulation results show that the disturbance propagates both westward and eastward (relative to the background flow), forming a series of K-H waves along the arc sheet. As a consequence, giant undulations with spatially varying amplitudes are developed on the equatorward boundary of a diffuse aurora, which is located just poleward of the arc sheet. A series of giant undulations convectively produced in the simulation is remarkably similar to some auroral images photographed from the Defense Meteorological Satellite Program (DMSP) satellites. In addition, we show the direct evidence for the presence of an arc sheet, probably associated with the giant undulations, which is provided by the measurements of precipitating particles from the DMSP F7 satellite crossing the equatorward boundary of the diffuse aurora; the high-density (∼10 cm−3) ions with energy of a few hundred electron volts are detected near the edge of the region of energetic (∼10 keV) ion precipitation. The primary cause of the formation of that arc sheet is thought to be escape of the oxygen and hydrogen ions from the topside ionosphere, due to the transverse acceleration by the ion-cyclotron waves.

Biological Sensors for Solar Ultraviolet Radiation

Solar ultraviolet (UV) radiation is widely known as a genotoxic environmental agent that affects Earth ecosystems and the human population. As a primary consequence of the stratospheric ozone layer depletion observed over the last decades, the increasing UV incidence levels have heightened the concern regarding deleterious consequences affecting both the biosphere and humans, thereby leading to an increase in scientific efforts to understand the role of sunlight in the induction of DNA damage, mutagenesis, and cell death. In fact, the various UV-wavelengths evoke characteristic biological impacts that greatly depend on light absorption of biomolecules, especially DNA, in living organisms, thereby justifying the increasing importance of developing biological sensors for monitoring the harmful impact of solar UV radiation under various environmental conditions. In this review, several types of biosensors proposed for laboratory and field application, that measure the biological effects of the UV component of sunlight, are described. Basically, the applicability of sensors based on DNA, bacteria or even mammalian cells are presented and compared. Data are also presented showing that on using DNA-based sensors, the various types of damage produced differ when this molecule is exposed in either an aqueous buffer or a dry solution. Apart from the data thus generated, the development of novel biosensors could help in evaluating the biological effects of sunlight on the environment. They also emerge as alternative tools for using live animals in the search for protective sunscreen products.

Synoptic observations of auroras along the postnoon oval: a survey with all-sky TV observations at Zhongshan, Antarctica

Abstract All-sky TV data obtained at Zhongshan, Antarctica, have been used to survey auroral displays along the postnoon auroral oval. The auroral occurrence peak around 15 MLT, which was previously shown by satellite observations, is confirmed to exist in ground observations as well. The so called ‘midday gap’ of discrete aurora, however, is not confirmed by ground observations. This survey reveals that the noon region appears to involve another auroral occurrence peak. The noon auroral peak observed from the ground is dominated by an aurora termed as dayside corona in the present study. A dayside corona is usually weak and changing rapidly in its appearance, luminosity and locale. The electron precipitation causing the dayside corona might be too soft, have reduced flux and/or have too rapid a motion of its rayed structures which would result in less luminosity, all of which could account for the ‘midday gap’ in satellite observations. We thus argue for a new synoptic picture of auroral displays along the postnoon oval, in which beside the 15 MLT peak, the noon region is filled with the dayside corona rather than a ‘gap’ in discrete aurora.

A particle simulation of large-amplitude undulations on the evening diffuse auroral boundary

The pattern of undulation on the equatorward boundary of the diffuse aurora, occasionally observed in the afternoon-evening sector, is studied by two-dimensional particle simulations for the motion of magnetospheric plasma perpendicular to the geomagnetic field. In the simulation model, the poleward electric field is assumed to be initially enhanced locally in latitude and independent of longitude, as suggested by the observations near the undulations of the diffuse auroral boundary. Such an electric field is assumed to be maintained by space charges carried by a cold plasma. The diffuse aurora is initially distributed over a longitudinally extended zone. Since this type of diffuse aurora is probably caused by precipitation of the energetic protons with energies in excess of a few keV, the temporal evolution of the diffuse auroral pattern, as displayed in photographs from the satellites, can be visualized by following the cluster of the energetic protons. Sheared plasma flow arising from the electric field distribution drives the Kelvin-Helmholtz instability and causes undulations on the equatorward boundary of the diffuse aurora with wavelengths and amplitudes comparable to the typical observed values. Notably, the particle simulation can reproduce the outstanding indentations characteristic of large-amplitude undulations which have occasionally been observed. The good agreement between the simulation results and the observed undulations suggests that the Kelvin-Helmholtz shear-driven process is responsible for undulations on the equatorward boundary of the diffuse aurora in the afternoon-evening sector. It is also shown that the magnetic drift of the energetic protons producing the diffuse aurora is important in creating the characteristic pattern of the diffuse auroral boundary.

Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic Magnetic Anomaly

An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (∼20 h LT) near the maximum depression (Dst ∼ −164 nT) and the following positive excursion during the strong geomagnetic storm on September 22–23, 1999. The unusual absorption that has short time-duration of 30–40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100–150 km) appearing from the zenith to the low-latitude part of the IRIS field-of-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (∼250 m/s) of the sheet absorption, precipitating electrons are estimated to be ∼20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.

Changes of the size of the open field line region during substo rms

Abstract The dimensions of both the near noon-midnight and near dawn-dusk meridians of the polar cap are examined during a number of substorms on the basis of particle data taken from the DMSP satellites. There is an annular belt of precipitation of soft electrons (∼300 eV) which extends poleward from the auroral oval. In general, the energy flux of these precipitating soft electrons is rather low and does not always cause sufficient luminosity to be detected by the present satellite-borne aurora imagers. This makes it very difficult to determine, conclusively, the open field line region on the basis of the present auroral imagers. In the present study, the open field line region is determined accurately in terms of the area of void of the soft electron precipitation. It is in this area where the so-called “polar rain” is observed. Our results show clearly that, at least for both medium and weak substorms, the dimension of the open field line region varies roughly in harmony with the AE index. The open region reaches the largest dimension at about the maximum epoch of substorms and becomes smallest during a quiet period, after slowly contracting during the recovery phase. The expansion and contraction of the open region result primarily from a decrease and increase, respectively, of the width of the soft electron precipitation belt, although the hard electron precipitation belt also expands and contracts.

DNA damage profiles induced by sunlight at different latitudes

Despite growing knowledge on the biological effects of ultraviolet (UV) radiation on human health and ecosystems, it is still difficult to predict the negative impacts of the increasing incidence of solar UV radiation in a scenario of global warming and climate changes. Hence, the development and application of DNA‐based biological sensors to monitor the solar UV radiation under different environmental conditions is of increasing importance. With a mind to rendering a molecular view‐point of the genotoxic impact of sunlight, field experiments were undertaken with a DNA‐dosimeter system in parallel with physical photometry of solar UVB/UVA radiation, at various latitudes in South America. Onapplying biochemical and immunological approaches based on specific DNA‐repair enzymes and antibodies, for evaluating sunlight‐induced DNA damage profiles, it became clear that the genotoxic potential of sunlight does indeed vary according to latitude. Notwithstanding, while induction of oxidized DNA bases is directly dependent on an increase in latitude, the generation of 6‐4PPs is inversely so, whereby the latter can be regarded as a biomolecular marker of UVB incidence. This molecular DNA lesion‐pattern largely reflects the relative incidence of UVA and UVB energy at any specific latitude. Hereby is demonstrated the applicability of this DNA‐based biosensor for additional, continuous field experiments, as a means of registering variations in the genotoxic impact of solar UV radiation. Environ. Mol. Mutagen. 2012.

Spore dosimetry of solar UV radiation: applications to monitoring of daily irradiance and personal exposure.

Environmental UV radiation can be quantified using spore dosimetry, which measures the inactivation of repair-deficient Bacillus subtilis spores dried on a membrane filter. The system exhibits highly selective sensitivity to UV radiation, not being affected by various environmental adversities, such as high and low temperature and humidity. Biologically-effective dose rate and cumulative dose of ambient radiation are measurable under various conditions at various places on the earth, including tropical, temperate, and polar sites. Applications to monitor the exposure at the surface of organisms including humans and plants have also been advanced.

Energetic particle precipitation in the Brazilian geomagnetic anomaly during the “Bastille Day storm” of July 2000

Ionospheric absorption associated with a great geomagnetic storm on July 15–16, 2000 (the “Bastille Day storm”) was observed in the Brazilian geomagnetic anomaly using a two-dimensional 4 × 4 imaging riometer (IRIS). In the afternoon of July 15, weak absorption (≈0.2 dB) was observed during the initial phase of the storm; large spatial-scale absorption exceeded the IRIS field of view (330×330 km). During the sharp magnetic decrease in the main phase of the storm, absorption was intensified (<0.5 dB) in the evening, showing a sheet structure with ≈150 km latitudinal width and >330 km longitudinal elongation. Subsequently, absorption was intensified (≈1 dB), having a small spatial-scale (≈150 km) in the background sheet structure and a pronounced westward drift (≈570 m s-1). In association with large magnetic fluctuations in the Bz component of the interplanetary magnetic field (IMF), the ground magnetic variation in the night sector showed large positive swings during the initial to main phases of the storm. With the subsequent southward turning of the IMF Bz, the ground magnetic variation in the evening sector showed rapid storm development. Particle fluxes measured by a geosynchronous satellite (L =≈6.6) demonstrated large enhancements of low-energy protons (50–400 keV) and probably electrons (50–225 keV) during the storm’s initial phase. Particle fluxes from the low-altitude NOAA satellite (≈870 km) indicated the invasion of low-energy particles into the region of L < 2 during the main phase of the storm. These results indicate that low-energy particles injected into the outer radiation belt in association with frequent and strong substorm occurrences, were transported into the inner radiation belt through direct convective access by the storm-induced electric fields during the storm’s development. These particles then precipitated into the ionosphere over the Brazilian geomagnetic anomaly. Notably, the most intense absorption could be dominantly caused by proton precipitation with energies of ≈40 keV. Key words: Bastille Day storm, Brazilian geomagnetic anomaly, energetic particle precipitation, imaging riometer.