Radar studies of plasma parameters in the polar cap and the auroral zone

Incoherent scatter radar measurements are an important source for studies of ionospheric plasma parameters. Data from the EISCAT Svalbard radar (ESR), which covers the polar cap and cusp, and from the EISCAT Tromsø radars, which covers the auroral zone, can be used to obtain information about the electron density, electron- and ion temperature, and line-of-sight plasma velocity. As the ESR started operations in 1996, and the Tromsø UHF radar in 1981, the accumulated database covers several solar cycles, giving a unique overview of the polar ionosphere. In this dissertation, the accumulated EISCAT database is used to study variations in the polar ionosphere on several time scales. The dependence of ionospheric parameters on season, time of day, solar cycle and geomagnetic activity has been investigated. A comparison between the ESR data and the International Reference Ionosphere (IRI) model was conducted, in order to understand how well the IRI model reproduces the polar cap ionosphere during different ionospheric conditions. The comparison showed that the IRI model is biased towards an underestimation of the F-region polar cap electron density. Furthermore, we derived the Hall conductivity from the Tromsø UHF data and used this to search for trends in the peak height of the Hall conductivity and in the E-region ion temperature. Such trends are expected to occur due to the anthropogenic emissions of greenhouse gases. However, as these trends are expected to be very small, no conclusive trend could be found with the present instrumentation. Lastly, we studied high latitude depletion regions, and observed an early morning depletion region in the polar cap ionosphere. This region expands with increasing geomagnetic activity. ESR ion temperature measurements show a heating at approximately the same time as the depletion region, suggesting that this depletion region might be connected to ion frictional heating

An evaluation of International Reference Ionosphere electron density in the polar cap and cusp using EISCAT Svalbard radar measurements

Publisher's version, source: http://doi.org/10.5194/angeo-34-751-2016.Incoherent scatter radar measurements are an important source for studies of ionospheric plasma parameters. In this paper the EISCAT Svalbard radar (ESR) long-term database is used to evaluate the International Reference Ionosphere (IRI) model. The ESR started operations in 1996, and the accumulated database up to 2012 thus covers 16 years, giving an overview of the ionosphere in the polar cap and cusp during more than one solar cycle. Data from ESR can be used to obtain information about primary plasma parameters: electron density, electron and ion temperature, and line-of-sight plasma velocity from an altitude of about 50 and up to 1600 km. Monthly averages of electron density and temperature and ion temperature and composition are also provided by the IRI model from an altitude of 50 to 2000 km. We have compared electron density data obtained from the ESR with the predicted electron density from the IRI-2016 model. Our results show that the IRI model in general fits the ESR data well around the F2 peak height. However, the model seems to underestimate the electron density at lower altitudes, particularly during winter months. During solar minimum the model is also less accurate at higher altitudes. The purpose of this study is to validate the IRI model at polar latitudes

An evaluation of International Reference Ionosphere electron density in the polar cap and cusp using EISCAT Svalbard radar measurements

Incoherent scatter radar measurements are an important source for studies of ionospheric plasma parameters. In this paper the EISCAT Svalbard radar (ESR) long-term database is used to evaluate the International Reference Ionosphere (IRI) model. The ESR started operations in 1996, and the accumulated database up to 2012 thus covers 16 years, giving an overview of the ionosphere in the polar cap and cusp during more than one solar cycle. Data from ESR can be used to obtain information about primary plasma parameters: electron density, electron and ion temperature, and line-of-sight plasma velocity from an altitude of about 50 and up to 1600 km. Monthly averages of electron density and temperature and ion temperature and composition are also provided by the IRI model from an altitude of 50 to 2000 km. We have compared electron density data obtained from the ESR with the predicted electron density from the IRI-2016 model. Our results show that the IRI model in general fits the ESR data well around the F2 peak height. However, the model seems to underestimate the electron density at lower altitudes, particularly during winter months. During solar minimum the model is also less accurate at higher altitudes. The purpose of this study is to validate the IRI model at polar latitudes

Long-term variations in electric conductivities measured by the EISCAT Tromsoe UHF radar

第6回極域科学シンポジウム[OS] 宙空圏11月16日（月）　国立極地研究所 2階 大会議

Seasonal and hemispheric asymmetries of F‐region polar cap plasma density: Swarm and CHAMP observations

One of the primary mechanisms of loss of Earth's atmosphere is the persistent “cold” ( urn:x-wiley:jgra:media:jgra56068:jgra56068-math-0001 20 eV) ion outflow that has been observed in the magnetospheric lobes over large volumes with dimensions of order several Earth radii. As the main source of this cold ion outflow, the polar cap F region ionosphere and conditions within it have a disproportionate influence on these magnetospheric regions. Using 15 years of measurements of plasma density Ne made by the Swarm spacecraft constellation and the Challenging Mini Satellite Payload (CHAMP) spacecraft within the F region of the polar cap above 80° Apex magnetic latitude, we report evidence of several types of seasonal asymmetries in polar cap Ne. Among these, the transition between “winter‐like” and “summer‐like” median polar cap Ne occurs 1 week prior to local spring equinox in the Northern Hemisphere (NH) and 1 week after local spring equinox in the Southern Hemisphere (SH). Thus, the median SH polar cap Ne lags the median NH polar cap Ne by approximately 2 weeks with respect to hemispherically local spring and fall equinox. From interhemispheric comparison of statistical distributions of polar cap plasma density around each equinox and solstice, we find that distributions in the SH are often flatter (i.e., less skewed and kurtotic) than those in the NH. Perhaps of most significance to cold ion outflow, we find no evidence of an F region plasma density counterpart to a previously reported hemispheric asymmetry whereby cold plasma density is higher in the NH magnetospheric lobe than in the SH lobe. Plain Language Summary The Earth's magnetic poles are not perfectly aligned with the Earth's geographic poles, and the degree of misalignment is greater in the Southern Hemisphere. Furthermore, as a result of the Earth's elliptical orbit around the Sun, summer and fall in the Northern Hemisphere together are approximately 1 week longer than summer and fall in the Southern Hemisphere, because the Earth is very slightly closer to the Sun around December solstice (summer in the Southern Hemisphere). These seasonal asymmetries, together with the asymmetric displacement of the Earth's magnetic poles relative to the geographic poles, suggest that the plasma density in the topside ionosphere's geomagnetic polar regions may also be subject to seasonal and hemispheric asymmetries. The polar regions are the primary site of loss of the Earth's atmosphere via so‐called ion outflow processes that, over geological time scales, are believed to lead to loss of the Earth's atmosphere. Using 15 years of plasma density measurements made by four different satellites to statistically study the plasma density of each hemisphere's geomagnetic polar cap ionosphere in the altitude range 350–520 km, we find that the polar cap ionosphere at these altitudes exhibits a variety of seasonal and hemispheric asymmetries