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 or in the evening 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 l,,minous, forming the so-called "theta aurora."Paper number 90JA02323. 0148-0227/91/90IA-02323 $05.00 sector; and (p4) high-latitude nu'dm'ght auroras shifting from the oval. She showed that these very high-latitude arcs occur under high solar wind speed and positive Bz conditions and that pl and p2 auroras are mostly observed during negative By periods. However, the dependence of By for 1)4 aurora was not necessarily clear in her analysis. From electron precipitation observation, Meng [1981] suggested that high-latitude, Sun-aligned arcs result from the poleward expansion of the morning or evening auroral oval during quiet time.Most recently, using DE 1 global images, Frank et al. [1986] showed that a Sun-aligned transpolar aurora is connected to both the midday auroral oval and the midnight sector of the oval. They called such an auroral configuration the "theta aurora." Other DE insmunent data were used to correlate the theta aurora with cur-

Solar ultraviolet (UV) radiation is widely known as an environmental genotoxic agent that affects ecosystems and the human population, generating concerns and motivating worldwide scientific efforts to better understand the role of sunlight in the induction of DNA damage, cell death, mutagenesis, and ultimately, carcinogenesis. In this review, general aspects of UV radiation at the Earth's surface are reported, considering measurements by physical and biological sensors that monitor solar UV radiation under different environmental conditions. The formation of DNA photoproducts and other types of DNA damage by different UV wavelengths are compared with the present information on their roles in inducing biological effects. Moreover, the use of DNA-based biological dosimeters is presented as a feasible molecular and cellular tool that is focused on the evaluation of DNA lesions induced by natural sunlight. Clearly, direct environmental measurements demonstrate the biological impact of sunlight in different locations worldwide and reveal how this affects the DNA damage profile at different latitudes. These tools are also valuable for the quantification of photoprotection provided by commercial sunscreens against the induction of DNA damage and cell death, employing DNA repair-deficient cells that are hypersensitive to sunlight. Collectively, the data demonstrate the applicability of DNA-based biosensors as alternative, complementary, and reliable methods for registering variations in the genotoxic impact of solar UV radiation and for determining the level of photoprotection sunscreens provided at the level of DNA damage and cell death.

On the basis of the auroral precipitating electron data along the dawn‐dusk meridian from a Defense Meteorological Satellite Program satellite (DMSP‐F2), we show that the electron precipitation region extends poleward, often to the geomagnetic latitudes ≳85° from the average oval location, during quiet periods (namely, during periods of a large positive Bz component). This result may be interpreted as an indication that only a small amount of geomagnetic flux interconnects with the northward IMF, resulting in a contracted “open” region (the polar cap). The control of the location of the poleward boundary by the north–south component of the IMF is also statistically examined. The rate of latitudinal movement is about 0.4° to 0.8° per 1 nT for positive IMF Bz values and about 0.7° to 1.1° per 1 nT for negative IMF Bz values. The shift of the equatorward boundary is not obvious, however, for positive IMF Bz values. As geomagnetic activity increases, both the poleward and equatorward boundaries shift toward lower latitudes. The equatorward shift of the poleward boundary is greater than that of the equatorward boundary, resulting in a thinner electron precipitation region during disturbed periods than during quiet periods.

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.

Abstract.A geomagnetic negative sudden impulse (SI') occurred on 3 August 1997 in association with a sudden decrease of solar wind dynamic pressure. The discrete auroras observed by an all-sky TV imager at Zhongshan in Antarctica (magnetic latitude is-•74.5øS) suddenly enhanced their luminosity and moved poleward at the time of the SI'. Following the primary enhancement, three cycles of quasi-periodic luminosity pulsations were observed with periods of about 10-14 min and their average position drifted equatorward. The luminosity pulsations showed one-to-one correspondence with the magnetic pulsations, with the characteristic features of a field-line resonance. The low-altitude DMSP-F13 satellite was almost directly over Zhongshan and observed accelerated electron precipitation with an inverted 'V' structure and an upward field-aligned current. These signatures suggest that the solar wind negative pressure impulse triggered a field-line resonance of the geomagnetic field. The resonance caused the upward and downward field-aligned current sheets, and the field-aligned electron acceleration.