Yasunobu Ogawa

Global imaging of polar cap patches with dual airglow imagers

During a 2 h interval from 2240 to 2440 UT on 12 November 2012, regions of increased 630.0 nm airglow emissions were simultaneously detected by dual all-sky imagers in the polar cap, one at Longyearbyen, Norway (78.1°N, 15.5°E) and the other at Resolute Bay, Canada (74.7°N, 265.1°E). The Resolute Bay incoherent scatter radar observed clear enhancements of the F region electron density up to 1012 m−3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap. During this interval of simultaneous airglow imaging, the nightside/dawnside (dayside/duskside) half of the patches was captured by the imager at Longyearbyen (Resolute Bay). This unique situation enabled us to estimate the dawn-dusk extent of the patches to be around 1500 km, which was at least 60–70% of the width of the antisunward plasma stream seen in the Super Dual Auroral Radar Network convection maps. In contrast to the large extent in the dawn-dusk direction, the noon-midnight thickness of each patch was less than 500 km. These observations demonstrate that there exists a class of patches showing cigar-shaped structures. Such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation.

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.

A negative phase shift of the winter AO/NAO due to the recent Arctic sea-ice reduction in late autumn

This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.

The stratospheric pathway for Arctic impacts on midlatitude climate

Recent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic-midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary-scale waves. Those results and consistency with observation-based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase.

Near-Infrared Polarimetric Study of Monoceros R2 IRS

We present H- and Ks-band imaging polarimetry for the star-forming region Mon R2 IRS, and spectropolarimetry from 1.2 to 4.2 μm for the infrared sources IRS 2 and IRS 3. The nebular complex in Mon R2 IRS is revealed to be composed of four domains dominated by the infrared sources IRS 1, IRS 2, IRS 3, and IRS 6. IRS 2 is the predominant illuminating source in the region and dominates the emission of the IR ring and most of the western part of the nebulosity. IRS 1 is responsible for the enhanced unpolarized intensity in the southeast part of the IR ring and inside the IR ring. IRS 3 illuminates the bright eastern nebula, and together with IRS 2 illuminates the eastern region inside the IR ring. IRS 6 illuminates a small nebula to the south of the IR ring. An arc structure of polarized intensity is observed abutting the northwest IR ring, consistent with the outflow cavity structure inferred from millimeter-wave observations. IRS 3 is associated with a disklike condensation oriented southeast-northwest, perpendicular to the elongated eastern nebula. The magnetic field in the core region exhibits an hourglass structure oriented north-south.

Observations of the lower thermospheric neutral temperature and density in the DELTA campaign

The rotational temperature and number density of molecular nitrogen (N2) in the lower thermosphere were measured by the N2 temperature instrument onboard the S-310-35 sounding rocket, which was launched from Andøya at 0:33 UT on 13 December 2004, during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. The rotational temperature measured at altitudes between 95 and 140 km, which is expected to be equal to neutral temperature, is much higher than neutral temperature from the Mass Spectrometer Incoherent Scatter (MSIS) model. Neutral temperatures in the lower thermosphere were observed using the auroral green line at 557.7 nm by two Fabry-Perot Interferometers (FPIs) at Skibotn and the Kiruna Esrange Optical Platform System site. The neutral temperatures derived from the look directions closest to the rocket correspond to the rotational temperature measured at an altitude of 120 km. In addition, a combination of the all-sky camera images at 557.7 nm observed at two stations, Kilpisjärvi and Muonio, suggests that the effective altitude of the auroral arcs at the time of the launch is about 120 km. The FPI temperature observations are consistent with the in situ rocket observations rather than the MSIS model.

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.

Solar activity dependence of ion upflow in the polar ionosphere observed with the European Incoherent Scatter (EISCAT) Tromsø UHF radar

The influence of solar activity upon ion upflow in the polar ionosphere was investigated using data obtained by the European Incoherent Scatter (EISCAT) Tromso UHF radar between 1984 and 2008. In a ...

Auroral fragmentation into patches

Auroral patches in diffuse auroras are very common features in the postmidnight local time. However, the processes that produce auroral patches are not yet well understood. In this paper we present two examples of auroral fragmentation which is the process by which uniform aurora is broken into several fragments to form auroral patches. These examples were observed at Athabasca, Canada (geomagnetic latitude: 61.7°N), and Tromso, Norway (67.1°N). Captured in sequences of images, the auroral fragmentation occurs as finger-like structures developing latitudinally with horizontal-scale sizes of 40–100 km at ionospheric altitudes. The structures tend to develop in a north-south direction with speeds of 150–420 m/s without any shearing motion, suggesting that pressure-driven instability in the balance between the earthward magnetic-tension force and the tailward pressure gradient force in the magnetosphere is the main driving force of the auroral fragmentation. Therefore, these observations indicate that auroral fragmentation associated with pressure-driven instability is a process that creates auroral patches. The observed slow eastward drift of aurora during the auroral fragmentation suggests that fragmentation occurs in low-energy ambient plasma.

Ionospheric electron density profiles inverted from a spectral riometer measurement

The first implementation of the so-called spectral riometer technique for the ionospheric electron density profile estimation is presented. In contrast to the traditional riometer operating at a single frequency, this experiment monitors the cosmic radio noise at 244 frequencies, ranging between 10 and 80 MHz, by using the new Kilpisja rvi Atmospheric Imaging Receiver Array radio telescope. The received power at each time and frequency is compared to the corresponding quiet-day value, resulting in the cosmic radio noise absorption spectrum as a measurement of ionization in the ionosphere. In this study, the observed absorption spectrum is used to invert the corresponding electron density profile by applying a simple parameterized electron precipitation model. By comparing the inverted electron density profiles to a simultaneous and nearly colocated European Incoherent Scatter VHF radar measurement on 13–14 November 2012, we show that the spectral riometry approach is capable of producing realistic electron density profiles under conditions of substorm-related electron precipitation.