R. Fujii

Electrodynamic parameters in the nighttime sector during auroral substorms

The characteristics of the large-scale electrodynamic parameters, field-aligned currents (FACs), electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulge-type auroral emission region has been deduced from auroral images taken by the Dynamics Explorer 1 (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with little net current in the surge horn and the west of bulge. The downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are associated with narrow, intense antisunward convection, corresponding to an equatorward directed spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high-latitude boundary of the upward region 1 FAC. In the surge, the convection reversal is near the low-latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region 1 and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents.

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.

Characteristics of the field‐aligned current system in the nighttime sector during auroral substorms

Fujii et al. (1994) obtained characteristics of the electrodynamic parameters, that is, field-aligned currents, electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, through a unique analysis that places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. In this paper we investigate in considerably more detail the characteristics of the field-aligned currents using data from the same set of passes as the previous study. We show for the first time that the net upward field-aligned currents throughout the surge and surge horn are sufficient to account for most if not all of the converging currents of the auroral electrojets. Current densities are largest in the surge and surge horn. Current region continuity does not appear to exist across the substorm bulge region. Much of the auroral substorm field-aligned current is composed of filamentary currents and finite current segments at large angles to each other. The westward electrojet may contain large gradients in intensity both in local time and latitude due to sets of localized field-aligned currents. The net downward current for several hours to the west of the surge is insufficient to account for the eastward electrojet, consistent with the concept that this electrojet originates primarily on the dayside. Our pattern of field-aligned currents associated with the surge has common features and also differs significantly from the patterns previously derived from data from radars and ground-based magnetometer arrays. Our pattern is considerably more complex, probably due to the much higher resolution in latitude of the satellite data. It is also larger in area, since our average substorm is much larger than those pertaining to the previous patterns, giving a substorm wedge considerably wider than that obtained from the radar and array data.

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.

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.

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.

Small‐scale field‐aligned currents observed by the Akebono (EXOS‐D) satellite

The EXOS-D fluxgate magnetometer data obtained at 3,000-10,000 km altitude have shown that small-scale field-aligned currents always exist in large-scale region 1, region 2, cusp and polar cap current systems. Assuming that these small-scale field-aligned currents have current sheet structure, the width of current sheet is estimated to be 5-20 km at ionospheric altitude. By comparing the magnetometer data with charged particle and high frequency plasma wave data simultaneously obtained from EXOS-D, it is found that small-scale currents have one-to-one correspondence with localized electron precipitation events characterized by flux enhancement over a wide energy range from 10 eV to several keV and broadband electrostatic bursts occasionally extending above local plasma frequencies or electron cyclotron frequencies.

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 ...

Generation of atmospheric gravity waves associated with auroral activity in the polar F region

Relations between auroral activities and the generation of neutral-wind oscillations in the polar F region (150–300 km) were investigated using data from the European Incoherent Scatter (EISCAT) radar, the all-sky auroral camera, and the IMAGE (International Monitor for Auroral Geomagnetic Effects) magnetograms. We dealt with two cases: observations on March 1, 1995 (case 1), and on March 29, 1995 (case 2). For both cases the field-aligned component of the neutral-wind velocity estimated from EISCAT radar data had dominant oscillation periods of 20–30 min, which are longer than the typical Brunt-Vaisala period in the polar F region (≃13 min). The observed oscillations showed the downward propagation of the phase with time. These properties on the oscillation period and the phase are general ones of atmospheric gravity waves (AGWs). For case 1 the all-sky auroral images obtained at Kilpisjarvi showed the auroral arc extending in an almost zonal direction near a distance estimated using wave parameters derived from the equation of the dispersion relation for AGWs applicable to the observed oscillations. This suggested that the auroral arc appeared to be the effective generator of the observed oscillations. The comparison of observed phase lines with predicted ones using models by Francis [1974] and Kato et al. [1977] showed agreements between the two for both cases. The comparison suggests that effective parameters of the wave source in characterizing neutral-wind oscillations would be the horizontal distribution of the wave source and the distance between the observing point and the source region. It was concluded that geomagnetic activities on March 1 and 29, 1995, in northern Scandinavia significantly related to the generation of the observed oscillations. The conclusion implies that geomagnetic activities at high latitudes are an important source to generate AGWs, as indicated by previous theoretical studies.

A comparison of thermospheric winds and temperatures from Fabry-Perot interferometer and EISCAT radar measurements with models

Abstract During the nights of 8–9 and 9–10 February 1997, Fabry-Perot interferometers were operated from the EISCAT radar site at Ramfjord (69.59° N, 19.23° E) and Skibotn (69.35° N, 20.36° E). From Ramfjord, horizontal neutral winds were measured in the lower and upper thermosphere using the auronal/airglow emissions at 557.7 and 630 nm, respectively. From Skibotn, thermospheric neutral temperatures were measured using the same wavelengths. The EISCAT radar measured ion temperatures up the local magnetic field line in the height range 90 – 580 km during the first night. Neutral winds are compared to the HWM-90 and CTIP-200 models with poor agreement. Neutral temperatures are compared to the MSISE-90 and CTIP-200 models as well as EISCAT ion temperatures with good agreement.