Nozomu Nishitani

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.

Dynamical property of storm time subauroral rapid flows as a manifestation of complex structures of the plasma pressure in the inner magnetosphere

[1] During the intense magnetic storm of 15 December 2006, the midlatitude Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar observed a dynamical character of rapid, westward flows at 50-56 magnetic latitude. The simulation that couples the inner magnetosphere and the subauroral ionosphere was performed using a realistic boundary condition of the hot ion distribution determined from four Los Alamos National Laboratory satellites at 6.6 R E . The following results are obtained using the simulation: (1) In general, morphology of the azimuthal component of the simulated ionospheric plasma flow is consistent with that known as the subauroral polarization stream (SAPS), (2) an increase in the hot ion density in the plasma sheet results in the temporal reduction and subsequent intensification of the rapid flow at certain subauroral latitudes with a delay of ~40 min, and (3) influence of the plasma sheet temperature on the rapid flow is not evident. The simulated line-of-sight velocity is compared with that obtained by the SuperDARN Hokkaido radar. Agreement between them is found in terms of the temporal and spatial variations of the rapid flows as well as the flow velocity. It is suggested that the dynamical character of the subauroral plasma flow is a direct manifestation of the plasma pressure distribution in the inner magnetosphere (the ring current) especially during the magnetic storm.

Giant ionospheric disturbances observed with the SuperDARN Hokkaido HF radar and GPS network after the 2011 Tohoku earthquake

Giant ionospheric disturbances induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) on 11 March 2011 are studied by using data from the SuperDARN Hokkaido HF radar and GPS receiver network (GEONET) in Japan. The HF radar observations revealed strong disturbances to the north of Hokkaido that propagated northward at velocities of 6.7–1.8 km/s triggered by northward-propagating seismic surface waves. An induction magnetometer in Hokkaido recorded part of the seismic wave propagation from the epicenter. After the passage of the 6.7–1.8 km/s waves the radar observed northward-propagating disturbances (343–136 m/s) due to atmospheric gravity waves (AGW) perhaps excited near the epicenter. Interestingly, the radar first detected peculiar disturbances with periods of about 2–4 min caused by the acoustic resonance. GEONET, which covers the area on the south of the radar field of view, provided total electron content (TEC) data. Comparisons between radar and TEC observations indicate the following: (1) 6.7–1.8 km/s waves observed with the radar do not always have counterparts in TEC. (2) Acoustic waves of 1.3–0.7 km/s identified in TEC are not observed with the radar. (3) Disturbances caused by both AGW and acoustic resonance are simultaneously discernible in both TEC and radar data.

Azimuthal flow bursts in the inner plasma sheet and possible connection with SAPS and plasma sheet earthward flow bursts

We have combined radar observations and auroral images obtained during the Poker Flat Incoherent Scatter Radar Ion Neutral Observations in the Thermosphere campaign to show the common occurrence of westward moving, localized auroral brightenings near the auroral equatorward boundary and to show their association with azimuthally moving flow bursts near or within the subauroral polarization stream (SAPS) region. These results indicate that the SAPS region, rather than consisting of relatively stable proton precipitation and westward flows, can have rapidly varying flows, with speeds varying from ~100 m/s to ~1 km/s in just a few minutes. The auroral brightenings are associated with bursts of weak electron precipitation that move westward with the westward flow bursts and extend into the SAPS region. Additionally, our observations show evidence that the azimuthally moving flow bursts often connect to earthward (equatorward in the ionosphere) plasma sheet flow bursts. This indicates that rather than stopping or bouncing, some flow bursts turn azimuthally after reaching the inner plasma sheet and lead to the bursts of strong azimuthal flow. Evidence is also seen for a general guiding of the flow bursts by the large-scale convection pattern, flow bursts within the duskside convection being azimuthally turned to the west, and those within the dawn cell being turned toward the east. The possibility that the SAPS region flow structures considered here may be connected to localized flow enhancements from the polar cap that cross the nightside auroral poleward boundary and lead to flow bursts within the plasma sheet warrants further consideration.

Plasma density suppression process around the cusp revealed by simultaneous CUTLASS and EISCAT Svalbard radar observations

Simultaneous CUTLASS and EISCAT Svalbard radar (ESR) observations on February 1, 1998, are used to study the generation of plasma density suppression, which may ultimately result in polar cap patch formation, occurring around the cusp region under IMF Bz negative and By positive conditions. The CUTLASS HF radars in Iceland and Finland observed F region plasma drifts in a wide area including the ESR field of view while the ESR monitored electron density, electron temperature, ion temperature, and ion motion along the geomagnetic field. We focus on two events for which the density suppressions (30–60%) formed in harmony with strong plasma drifts (> 1500 m s−1) lasting for 5–12 min. In one event, the suppression is mainly caused by enhanced chemical reactions in the F region due to the intensified convection flows which also raise ion temperature by 1500–2000 K through frictional heating. This process can chop preexisting high-density region produced by energetic particle precipitation, maybe giving rise to polar patches. In the other event, the density suppression is related to an appearance of eastward directed high-speed plasma jets in a limited region with a latitudinal width of > 100 km and a longitudinal extension of > 500 km. The ESR data, however, show no ion temperature increase, suggesting that the suppression may not be caused by enhanced chemical reactions. We tentatively propose that the eastward plasma jets transported less dense plasma from earlier local times over the ESR. Rapid change of IMF By polarity is another candidate for producing the density suppression. A role of HF radar wave refraction in explaining this event is discussed.

A theory for generation of the paired region 1 and region 2 field‐aligned currents

We present a new theoretical model for generation of a pair of region 1 and region 2 field-aligned currents (FACs) under the condition of a southward interplanetary magnetic field. On the basis of the satellite observations it is assumed that the hot (≳1 keV) plasma particles are distributed in a magnetic shell connected to two ovals of diffuse auroras on the northern and southern polar ionospheres. The hot plasma population contained in this magnetic shell having several degrees of latitude in width is called the hot plasma torus (HPT). It is proposed that the region 1/region 2 FACs can be generated as a result of natural distortion of the HPT due to the solar wind convection. When the interplanetary magnetic field has a southward component, i.e., the IMF Bz is negative, the solar wind flow across open geomagnetic field lines gives rise to electric field convection patterns over the polar caps, which are modeled as twin vortex cells with antisunward flows in the center of the polar caps. The convection thus driven by the solar wind is referred to as the solar wind convection. If it were not for an E × B convection flow, the HPT would be shaped such that the HPT particles are contained in the “magnetic drift shells,” which are tangent to the averaged total magnetic drift velocity. In the presence of the solar wind convection, the configuration of the HPT will be deformed from the magnetic drift shells. Because of the distortion of the HPT, the pressure gradient in the HPT gains a component parallel to the magnetic drift. Therefore the HPT can be polarized because of oppositely directed magnetic drifts of the HPT electrons and protons: the high-latitude and low-latitude sides of the HPT on the eveningside are negative and positive, respectively, and the polarity is reversed on the morningside. The resulting pattern of large-scale field-aligned currents due to the polarization of the HPT is consistent with the observations of region 1 and region 2 FACs. Moreover, provided that the solar wind acts as a voltage generator in the interaction with the open field lines, as a long-term characteristic of the paired region 1 and region 2 FACs we can obtain the relationship between the FAC intensity and the ionospheric conductivity: both the region 1 and region 2 intensities increase linearly with the Pedersen conductivity, while the proportionality constant for the region 2 FAC is smaller than that for the region 1 FAC. Our predicted relation for geomagnetic quiet conditions quantitatively agrees with the regression lines between the current intensities and the Pedersen conductivities obtained on the basis of Magsat satellite observations by Fujii and Iijima [1987].

Localized polar cap flow enhancement tracing using airglow patches: Statistical properties, IMF dependence, and contribution to polar cap convection

Recent radar observations have suggested that polar cap flows are highly structured and that localized flow enhancements can lead to nightside auroral disturbances. However, knowledge of these flows is limited to available echo regions. Utilizing wide spatial coverage by an all-sky imager at Resolute Bay and simultaneous Super Dual Auroral Radar Network measurements, we statistically determined properties of such flows and their interplanetary magnetic field (IMF) dependence. We found that narrow flow enhancements are well collocated with airglow patches with substantially larger velocities (≥200 m/s) than the weak large-scale background flows. The flow azimuthal widths are similar to the patch widths. During the evolution across the polar cap, the flow directions and speeds are consistent with the patch propagation directions and speeds. These correspondences indicate that patches can optically trace localized flow enhancements reflecting the flow width, speed, and direction. Such associations were found common (~67%) in statistics, and the typical flow speed, propagation time, and width within our observation areas are 600 m/s, tens of minutes, and 200–300 km, respectively. By examining IMF dependence of the occurrence and properties of these flows, we found that they tend to be observed under By-dominated IMF. Flow speeds are large under oscillating IMF clock angles. Localized flow enhancements are usually observed as a channel elongated in the noon-midnight meridian and directed toward premidnight (postmidnight) for +By (−By). The potential drops across localized flow enhancements account for ~10–40% of the cross polar cap potential, indicating that they significantly contribute to polar cap plasma transport.

Spatial and temporal characteristics of giant undulations

A giant undulation event observed at SANAE, Antarctica (L = 4) is reported. During the event the geomagnetic activity was high, as was the case in previously reported observations of giant undulations. Data were recorded by all-sky and small field-of-view TV cameras, from 2346 UT on September 10, to 0007 UT on September 11 (MLT ∼ 2200 h) 1987. The fine temporal and spatial resolution of the TV data made a detailed study of the undulation possible. The average wavelength was 170 km and the amplitude of the wave increased from 70 km to 140 km in 10 minutes. Wave propagation, measured at different positions on the undulation, was duskward with a phase speed of 540 to 650 m/s. An inner structure, propagating at a speed of 2.5 to 4.0 km/s, about 3 times as fast as the undulation form, was observed just poleward of the undulation boundary, indicating the presence of strong velocity shear. The linear theory of the velocity shear instability is examined as a possible generation mechanism on the basis of these observations.

A very large scale flow burst observed by the SuperDARN radars

We examined the dynamics of the ionospheric plasma in the dayside sector by using the HF radar data at Iceland West and at Finland from 1100 to 1230 UT on September 5, 1995. During that period, the solar wind density was high and the IMF was strongly southward. The dayside magnetopause was highly compressed nearly to the geosynchronous orbit. The two radars simultaneously detected a poleward flow burst in the noon sector which, assuming uniformity of flow in the region of the data gap (1.5 MLT) between the two radars, showed a magnetic local time extent of 5 hours. This local time extent is 2 to 3 hours wider than previous results. The maximum poleward plasma velocity of the flow burst is ∼750 m/s, and the latitudinal size of the flow burst region is ∼100 to 200 km. This flow burst region initially expanded in longitude up to 5 hours, and then shifted poleward with a phase speed of 400 to 670 m/s. The flow burst has a duration of ∼20 min. This large-scale poleward flow burst is likely to be due to large-scale reconnection occurring at the dayside magnetopause and subsequent convection as the magnetic field lines are transported across the polar cap.

Antarctic HF radar observations of irregularities associated with polar patches and auroral blobs: A case study

We report a case study of decameter-scale electron density irregularities associated with polar cap patches and auroral (boundary) blobs in the southern high-latitude F region ionosphere. The observations were carried out on July 14, 1995, with the Antarctic Super Dual Auroral Radar Network HF radars located at Syowa Station and Halley. On that day, 17 irregularity events associated with the patches were identified in the polar cap. The time distribution of these events is consistent with previous model calculations of patch formation and transportation in the northern hemisphere for southward interplanetary magnetic field (IMF) conditions (Bz 0). These patches seem to have been transported into the polar cap from the dayside cusp where the patches had been generated under negative Bz conditions. The striated radar echo patterns due to a series of auroral blobs, clearly observed at Halley in the evening auroral zone, are well explained by previous simulations that calculated the time evolution and transportation of a patch initially located in the polar cap.