Natsuo Sato

Conjugacy of isolated auroral arcs and nonconjugate auroral breakups

Fine examples of both conjugate and nonconjugate isolated auroral arcs were observed at two geomagnetically conjugate stations near L = 6, Syowa Station in Antarctica and Husafell in Iceland on September 12, 1988. These events exhibited some interesting characteristics. An auroral loop structure that appeared in both hemispheres was ∼2.0 times larger in the north-south direction at Syowa than at Husafell. This scale difference is greater than expected from the difference in geographic and geomagnetic (IGRF) coordinates between the two points of observation. However, temporal and spatial variations in the loop structures were almost identical in both hemispheres. After the disappearance of the loop structure, closely conjugate auroras were formed. Nonconjugate auroral features appeared again at Syowa on the poleward side, while the equatorward auroras maintained conjugacy. The nonconjugate aurora at Syowa then began to break up, showing fast moving vortex-like structures (auroral spirals). At this time, all auroral features at Husafell seemed to have their conjugate counterparts in equatorward auroras at Syowa and none exhibited rapid motions. These conjugate auroras at Husafell were gradually extending poleward, while the corresponding features at Syowa were compressed toward the equator and shrinking in size. The onset of auroral breakup was about one minute earlier at Syowa than at Husafell. The nonconjugate auroral features were reflected in corresponding magnetic field variations on the ground. The events summarized above give interesting clues to the development and decay of auroral conjugacy and the question why the beginning of auroral breakup is not simultaneous at conjugate stations. The time lag and nonconjugacy of auroral breakup in conjugate areas suggests that the triggering source of auroral breakup was not located near the equatorial plane in the magnetosphere but most likely in a localized region near the ionosphere in the southern hemisphere. The nonconjugate auroral spirals also suggest the existence of asymmetrical field-aligned currents.

On the altitude dependence of the spectral characteristics of decametre-wavelength E region backscatter and the relationship with optical auroral forms

Abstract. Observations of E region backscatter by the Ice-land East SuperDARN HF radar from the 30 minute period 2330 to 2400 UT on 13 September 1999 are presented, along with simultaneous observations of auroral luminosity from two all-sky cameras. Interferometric techniques are employed to estimate the altitude of origin of each echo observed by the radar. Under investigation is a region of backscatter which is L-shell aligned and exists in a region of low auroral luminosity bounded to the north and the south by two auroral arcs. The spectral characteristics of the backscatter fall into three main populations: broad, low Doppler shift spectra; narrow, high Doppler shift spectra; and exceptionally narrow, low Doppler shift spectra. The first two populations are similar to type II and type I spectra observed with VHF radars, respectively. These populations scatter from near the peak of the E region. The high Doppler shift population appears to exist in a region of sub-critical electric field. The third population originates below the E region peak at altitudes between 80 and 100 km. We argue that a non-coherent scattering process is responsible for this backscatter. Key words. Ionosphere (auroral ionosphere; ionospheric irregularities)

Global evolution of a substorm-associated DP2 current system observed by superDARN and magnetometers

Abstract This paper deals with an evolution of the electric field in the dayside auroral and equatorial ionosphere during a substorm on July 16, 1995. A southward turning of the IMF detected by WIND (171 Re) caused enhancements in the auroral electrojet intensity in the 7–10 MLT and 15.5–18.5 MLT sectors as observed by the IMAGE (74-56 cgmlat) and CANOPUS (70-58 cgmlat) magnetometer chains. SuperDARN detected an equatorward motion of the radar scattering region at speeds of several −10 degs/hour in the dayside (05–17 MLT), suggesting an increase in the flux of the open magnetic field in the polar cap. Furthermore, coherent magnetic variations are observed at subauroral to equatorial latitudes simultaneously with the auroral magnetic variations within a temporal resolution of 10 s. This suggests that the electric field increase during the growth phase is established instantaneously around the convection reversal in the 15.5–18.5 MLT sector, and furthermore penetrates instantaneously to mid and low latitudes. SuperDARN detected a continuous equatorward motion of the auroral oval during the expansion phase around the cusp, which implies a continuous magnetic merging at the day-side magnetopause during the expansion phase. A rapid decrease in the electric field is inferred from coherent auroral and equatorial magnetic field decreases during the recovery phase, which may have been caused by northward turning of the IMF. This magnetic field decrease resembles the change in magnetic field of the counter-electrojet at the dip equator in the afternoon sector.

Superposed epoch analysis of the ionospheric convection evolution during substorms: IMF BY dependence

We present superposed epoch analyses of the average ionospheric convection response in the northern and southern hemispheres to magnetospheric substorms occurring under different orientations of the interplanetary magnetic field (IMF). Observations of the ionospheric convection were provided by the Super Dual Auroral Radar Network (SuperDARN) and substorms were identified using the Far Ultraviolet (FUV) instrument on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft. We find that during the substorm growth phase the expected IMF BY‐dependent dawn‐dusk asymmetry is observed over the entire convection pattern, but that during the expansion phase this asymmetry is retained only in the polar cap and dayside auroral zone. In the nightside auroral zone the convection is reordered according to the local substorm electrodynamics with any remaining dusk‐dawn asymmetry being more closely related to the magnetic local time of substorm onset, itself only weakly governed by IMF BY. Owing to the preponderance of substorms occurring just prior to magnetic midnight, the substorm‐asymmetry tends to be an azimuthal extension of the dusk convection cell across the midnight sector, a manifestation of the so‐called “Harang discontinuity.” This results in the northern (southern) hemisphere nightside auroral convection during substorms generally resembling the expected pattern for negative (positive) IMF BY. When the preexisting convection pattern in the northern (southern) hemisphere is driven by positive (negative) IMF BY, the nightside auroral convection changes markedly over the course of the substorm to establish this same “Harang” configuration.

Auroral forms and the field‐aligned current structure associated with field line resonances

We show evidence to suggest that some auroral forms may be related to the field-aligned current structure associated with magnetohydrodynamic wave activity, specifically field line resonances. These auroras appear as a series of azimuthally spaced east-west elongated arcs, inclined slightly from north to south. Modeling shows that this “braided” appearance reflects the locations of upward field-aligned current driven by the wave. Moreover, more complicated auroral structures can be produced if several harmonically related waves occur together. We model a situation in which two harmonic components are present at the latitude of resonance and demonstrate the striking similarity between the resulting field-aligned current structures and actual auroral observations.

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.

The ionospheric response to interplanetary magnetic field variations: Evidence for rapid global change and the role of preconditioning in the magnetosphere

We have found observational evidence for a rapid communication of interplanetary magnetic field (IMF) changes to the global ionosphere and evidence for the state of the magnetosphere in the previous hour conditioning this response. These conclusions are drawn from a case study of sunward flow bursts on the nightside polar cap boundary observed by geomagnetically conjugate HF radars. The flow burst excitation consists of two factors: (1) At the time of the flow burst, the magnetosphere still held a memory of the stable and northward IMF period that had persisted up until 1 hour before the flow burst (internal condition). During the northward IMF period a theta aurora associated with a sunward flow channel was formed in the polar cap. After that the IMF turned southward, and the transpolar arc decayed antisunward. However, by the time of the flow burst (i.e., 1 hour after the IMF southward turning), the Sun-aligned arc had not yet completely vanished, and in the poleward expanded portion of the northern plasma sheet, there was still a remnant of the sunward flow channel susceptive to an external forcing. (2) One hour after the southward turning of the IMF a sharp IMF transition from southward to northward Bz impinged on the dayside magnetopause (external condition). On arriving at the dayside cusp ionosphere the Bz transition signal pervaded the entire polar cap ionosphere instantaneously (<1 min) and reached the nightside plasma sheet. There, the remnant of the sunward flow channel was reactivated by the Bz transition, and a sunward flow burst was observed first in the northern ionosphere and then in the southern ionosphere with a 7-min time delay. Thus the sunward flow burst represents a rapid global response of the ionosphere starting 2–3 min after the IMF change at the subsolar magnetopause.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Japanese research project on Arctic and Antarctic observations of the middle atmosphere

Abstract An all-sky optical imager is in routine observation at the South Pole. Monochromatic images of aurora and air glow at N 2 + 427.8nm, OI 557.7nm, OI 630nm and OH 730nm are supplying significant information on the magnetospheric process in the polar cap and cusp/cleft region along with atmospheric wave signature at this particular point. SuperDARN radars in Antarctica make observations over the South Pole. At Syowa Station, Antarctica, a multi-instrumental observation project is now being implemented for the study of the polar upper atmosphere from the mesosphere to the thermosphere, where complex physical and chemical processes take place making the region very attractive for scientific research. Two HF radars, which are part of SuperDARN radars, have been already installed and started observations. By the end of 1999, all-sky imagers, photo meters, a Na temperature Lidar, an MF radar and a Fabry-Perot interferometer will be introduced and start collecting various physical parameters on a routine basis. In the Arctic region, we are planning to deploy coordinated ground-based observations with optical, radio and radar sensing of the polar middle and upper atmosphere in conjunction with EISCAT radars. Scientific goals are versatile to shed light on the tangled coupling processes in response to magnetospheric disturbances from above and bi-lateral interactions with high-density lower atmospheric layers. These are outlined in this paper.

Acceleration mechanism of high-speed neutral wind observed in the polar lower thermosphere

[1] We analyzed data obtained with the European incoherent scatter (EISCAT) Svalbard Radar (ESR) at Longyearbyen (78.2°N, 16.0°E in geographic coordinates, 75.2°A in invariant latitude) to advance our understanding of the acceleration mechanism of the lower thermospheric neutral wind when the ionospheric convection becomes enhanced. The Advanced Composition Explorer satellite observed a southward turning in the interplanetary magnetic field at 0843 UT on 16 June 2005. At 0900 UT, the F region ion velocity and the lower thermospheric neutral wind (at 118-km altitude) observed with the ESR began to accelerate significantly in the westward and northwestward directions, respectively. The neutral wind was remarkably accelerated within I hour from 0900 UT, and its speed became a value of ∼500 m s ―1 at 1000 UT. The wind speed was significantly higher than a typical wind speed at 118 km. We evaluated the ion drag contribution on the generation of the high-speed neutral wind. Our evaluation verifies that the ion drag force could not generate the high-speed neutral wind within such a short time (∼1 hour). This result implies that the major driving force was the horizontal pressure gradient force induced by the Joule heating. We deduced the contribution of the pressure gradient force based on a quantitative estimation of the Joule-heating-induced pressure gradient from the ESR and the Super Dual Auroral Radar Network data. We concluded that the pressure gradient force was the most probable force in this event.