Satonori Nozawa

Simultaneous EISCAT Svalbard and VHF radar observations of ion upflows at different aspect angles

A simultaneous EISCAT Svalbard and VHF radar experiment has shown that field-aligned (FA) ion upflows observed at an altitude of 665 km in the dayside cusp are associated with significant anisotropy of ion temperature, isotropic increases of electron temperature and enhancements of electron density. There is no clear correspondence between the enhancements of the electric field strength and the occurrence of the ion upflows. This suggests that the upflow is driven primarily by precipitation. The data support that in addition to “direct” precipitation effects, namely enhanced ambipolar diffusion and heat flux, also wave-particle interaction, like wave-induced transverse ion heating, which causes a hydrodynamic mirror force, may play a role.

Studies of the E region neutral wind in the disturbed auroral ionosphere

Mean neutral wind velocity vectors observed at auroral E region altitudes by European incoherent scatter radar (EISCAT) for disturbed days are found to be rather similar to the mean wind velocity vectors observed on quiet days. It is demonstrated that the main difference in the E field between quiet and disturbed days is an enhancement in the diurnal component by a factor of 3. The semidiurnal, 6-, and 8-hour components display only minor differences. The amplitudes of the tidal components of the neutral wind increase in general above 109 km during ionospheric disturbances in particular for the diurnal tide of the zonal wind. The disturbance pattern of the neutral wind has a stronger equatorward component above ∼109 km than the corresponding quiet time pattern. In general, on disturbed days there is an additional eastward motion from the late evening until prenoon. During the daytime the northward wind is enhanced, while in the afternoon stronger westward velocities are seen above 109 km. In the afternoon sector until late evening during disturbed conditions there is also a southward component above ∼109 km. Below 109 km the semidiurnal tide appears to be enhanced during disturbed conditions. It is found that only part of the disturbance in the E region neutral wind can be explained by ion drag while Joule heating and particle precipitation may play an additional role. Satisfying agreement is found between the variations of the diurnal components of the E region neutral wind observed in the present work and the recent model predictions by Fesen et al. [1991].

Statistical characteristics of electromagnetic energy transfer between the magnetosphere, the ionosphere, and the thermosphere

We have determined, based on 28 days of European Incoherent Scatter Common Program 1 mode I data obtained between 1989 and 1991, statistical characteristics of the energy-coupling processes between the lower thermosphere, ionosphere, and magnetosphere through an analysis of the electromagnetic energy transfer rate J·E, the Joule heating rate J·E′, and the mechanical energy transfer rate U·(J×B) at altitudes of 125, 117, 109, and 101 km. At all altitudes the input electromagnetic energy is distributed to both Joule heating and mechanical energy. The energy distributed to Joule heating is larger than that to mechanical energy, but the latter is generally not negligible. All three rates respectively have two maxima, not in the midnight region but in the dawn and dusk. The enhancements of these rates have positive correlations with the increase of geomagnetic activity represented by the Kp index. The electromagnetic energy transfer rate is greatest at 117 km, becoming smaller with decreasing altitude. It is mostly positive but can be negative. At 117 km the mechanical energy transfer rate is considerably smaller than the electromagnetic energy transfer rate, suggesting that most of the electromagnetic energy at this altitude is converted to Joule heating and a small portion of the electromagnetic energy goes to mechanical energy. At 125 km the mechanical energy transfer rate is larger than that at 117 km. On average, 65% of the input electromagnetic energy is converted to Joule heating and 35% is converted to neutral mechanical energy. At 109 and 101 km altitude the mechanical energy transfer rate becomes negative, hence the Joule heating rate is greater than the electromagnetic energy transfer rate, suggesting that not only electromagnetic energy but also mechanical energy contribute to Joule heating.

Studies of the E region neutral wind in the quiet auroral ionosphere

Quiet time auroral E region neutral wind data obtained by European Incoherent Scatter Radar (EISCAT) have been analyzed in order to establish the background mean neutral velocities as well as tidal influence on the wind. There is a rather strong and persistent mean eastward wind present in all seasons in the E region below 120 km. It is strongest in summer (∼60 m s−1), weak in fall and spring, and weaker in winter (∼20 m s−1). The zonal wind is westward above 120 km, in general stronger in winter (∼70 m s−1) and weaker in the other seasons (∼30 m s−1). The northward mean wind is rather small (∼10 m s−1). It is shifted gradually from being mainly southward in winter to becoming northward in summer. The mean vertical velocity is of the order of 5 m s−1 in the auroral E region. The amplitude of the diurnal tides of the vertical component have broad minima in the height region between 90 and 120 km. The horizontal diurnal tide is dominant in the upper E region, while the semidiurnal tide has a maximum at 110 km, in particular in the eastward component. The 8- and 6-hour tides are rather constant in amplitude by height. The results obtained are in good agreement with other results published on the basis of incoherent scatter radar measurements. The comparison with theoretical models underlines the need for more modeling of the auroral E region neutral tides including the ionized plasma as well as electric fields and mean background winds. Theoretical studies of the vertical motion with respect to the influence of tides are especially lacking in this important region.

Study on neutral wind contribution to the electrodynamics in the polar ionosphere using EISCAT CP‐1 data

Energy coupling between the thermosphere, ionosphere and magnetosphere is studied quantitatively through an analysis using the European Incoherent Scatter (EISCAT) Common Program (CP) −1 version H data obtained on May 3, 1988. A negative excursion of the H component in the Tromso magnetogram occurred during the experiment period, which involved the following two features: (1) the electric potential across the polar cap was expected to be reduced abruptly in association with a sudden change of the interplanetary magnetic field (IMF) Bz polarity from southward to around null and (2) the negative excursion had a relatively long duration of development (about 4 hours), which may drive neutrals to move significantly through ion drag. In order to investigate the energy coupling between the thermosphere and ionosphere, we evaluate quantitatively the electromagnetic energy flux J·E, the Joule heating rate J·E′ (E′ = E + U × B), and the mechanical energy transfer rate U·(J × B), where U is the neutral wind velocity. The CP-I-H experiment provides directly or indirectly all quantities above at altitudes of 101 km, 109 km, 119 km, and 132 km. The results are summarized as follows. (1) The amplitude of the neutral wind related electric field U × B varied greatly with altitude, i.e., at altitudes above 119 km it often became larger than 50% of the amplitude of the observed electric field; (2) during the late recovery phase of the negative excursion of the H component of the Tromso magnetic field, the neutral wind related electric field tended to be canceled with the observed electric field; (3) in the E region the neutral wind mechanical energy transfer rate U·(J × B) is not negligible but is comparable to the Joule heating rate J·E′; and (4) in particular, at higher altitudes (132 km high) the conversion from the neutral wind mechanical energy to the electromagnetic energy occasionally may occur.

Studies of the auroral E region neutral wind through a solar cycle: Quiet days

On the basis of the data obtained by the European Incoherent Scatter radar located in northern Scandinavia, we have studied E region neutral wind characteristics between 95 and 119 km under geomagnetic quiet conditions (Ap < 16) over a solar cycle between November 1986 and October 1996. Data of 56 days were analyzed and grouped in order to investigate variations of the wind in different seasons and solar cycle conditions. Seasonal and solar cycle dependences of mean winds as well as diurnal and semidiurnal amplitudes are found. Phase differences of the diurnal and semidiurnal components are less than 2 hours between different season and solar activity conditions. No prominent seasonal nor solar cycle dependence is found for the vertical component. Comparisons are made with model predictions and radar observations as well as UARS observations in terms of seasonal and solar cycle dependences of the wind. Possible roles of gravity waves are discussed in terms of the seasonal variation of zonal mean wind.

Dynamics of Neutral Wind in the polar region observed with two Fabry-Perot Interferometers

Optical observations were made at Ramfjord, Norway from January 10 to February 14,1997. Two types of Fabry-Perot interferometers (FPIs), Doppler-imaging and scanning, were installed at the EISCAT radar site and were used to acquire data simultaneously with radio instruments. Both FPIs can observe emissions of two different wavelengths simultaneously. We can estimate the horizontal and vertical wind in different emission layers simultaneously with high time-resolution (∼1 min). The observations on February 8 and 9, 1997, show some notable characteristics: (1) large-scale perturbations (≈ ±150 m/s) are observed in the upper thermospheric wind. They seem to begin 30 min after the onset of a magnetic substorm and to stop when the next substorm begins. (2) Clear wave-like structures are found in the horizontal wind variations. Some of them can be seen over the entire sky, and one of them is found in a restricted regions. (3) A clear wave-like structure is also found in the vertical wind in the upper thermosphere. A similar structure can be seen in the lower thermosphere, but these structures are not always in phase. This phases difference starts at the same time that horizontal winds between the two layer has their phase difference. (4) The relation between the vertical wind and the divergence of horizontal wind seems to change with time. The correlation coefficient between them changes one-hours before and on-time of a substorm on-set. This sign of the coefficient is negative in most of the time, with considering about time-lag. It means the vertical morion is caused by divergent flow of horizontal wind.

Development of low-cost sky-scanning Fabry-Perot interferometers for airglow and auroral studies

We have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromsø site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010–2011 (FP02–FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 × 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to −80°C. The large incident angle (maximum: 1.3°–1.4°) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromsø by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s−1 and from 4 to 27 m s−1 for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02–FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s−1 to more than 50 m s−1 depending on airglow intensity.

Observations of mesospheric neutral wind 12-hour wave in the Northern Polar Cap

Abstract Combined three-station neutral wind observations from 70° to 80°N are used to study the 12-h oscillation from 87 to 130 km altitude. A strong 12-h wave with a 37 km vertical wavelength was observed at Troms o (69.6°N). The observed phases and vertical wavelength are consistent with the predictions of the Global Scale Wave Model-98 (GSWM-98) for the westward zonal wavenumber two semi-diurnal migrating tide (SDW2). However, the observed amplitudes are much greater than the model prediction at Troms o . At Resolute (74.9°N), the observed 12-h oscillation in neutral winds appears to be have large contribution from the SDW2, based on the zonal phase shift from Troms o to Resolute. At Eureka (81.1°N), the 12-h oscillation (not the strongest wave) does not have the predicted phase shift from Troms o based on the zonal wavenumber of the SDW2. The amplitudes of the 12-h oscillation at Resolute and Eureka are much smaller than those predicted by the GSWM-98 for the SDW2. We believe that the contribution from non-migrating semi-diurnal tide (perhaps, the zonal wavenumber one SDW1) is the likely cause of the inconsistency between the observed and predicted SDW2 in phases and amplitudes at high latitudes.

Upper atmosphere cooling over the past 33 years

Theoretical models and observations have suggested that the increasing greenhouse gas concentration in the troposphere causes the upper atmosphere to cool and contract. However, our understanding of the long-term trends in the upper atmosphere is still quite incomplete, due to a limited amount of available and well-calibrated data. The European Incoherent Scatter radar has gathered data in the polar ionosphere above Tromso for over 33 years. Using this long-term data set, we have estimated the first significant trends of ion temperature at altitudes between 200 and 450 km. The estimated trends indicate a cooling of 10–15 K/decade near the F region peak (220–380 km altitude), whereas above 400 km the trend is nearly zero or even warming. The height profiles of the observed trends are close to those predicted by recent atmospheric general circulation models. Our results are the first quantitative confirmation of the simulations and of the qualitative expectations.