O. Saka

Correlations between the very low frequency chorus and pulsating aurora observed by low-light-level television at

A good correlation was found between the very low frequency chorus and pulsating aurora observed at Park Site (L ~ 4.4), Saskatchewan, Canada, during the recovery phase of a substorm on January 28, 1980. The activity of the chorus was low during the periods when the pulsating aurora was active over the station. However, seven chorus events were identified during that period and six of them showed one-to-one correlation with the brightening of a pulsating patch. The brightening of the patch leads chorus events by ~0.1–0.2 s. The corresponding patch detected by a low-light-level TV camera has an oval shape 75 km in the north–south direction and 150 km in the east–west direction at the ionospheric altitudes. The energy of precipitated electrons was inferred to lie between 20 and 90 keV.

Ground‐satellite correlation of low‐latitude Pi 2 pulsations: A quasi‐periodic field line oscillation in the magnetosphere

Magnetometer data at dip-equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous spacecraft (1982-019, 1984-037, 1984-129) are studied to investigate field line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field disturbances that axe launched in the midnight sector of the magnetosphere in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi-periodic field line oscillations and particle injections are often observed in the nightside sector in association with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi-periodic field line oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pulsations. The oscillations in space are polarized primarily in the azimuthal direction with a dc offset that develops to form a field-aligned current signature, namely, eastward (westward) directed in the premidnight (postmidnight) sector. Simulation of forced field line oscillations has been carried out by adding the sudden increase in plasma pressure in the equatorial midnight sector region that is expected to come from particle injection at substorm expansion onset. We discuss the relevance of the quasi-periodic oscillations to plasma injections at geosynchronous altitudes with reference to the result of the model calculations.

A possible driving source for transient field line oscillations in the postmidnight sector at geosynchronous altitudes

Geosynchronous orbit data from charged particle analyzer (30- to 300-keV electron flux) and magnetometer instruments on board the 1984-129, GOES 6, GOES 5, and 1982-019 satellites distributed in the midnight-to-dawn sector are examined during a particle injection event that occurred at 0820 UT on May 21, 1986. The injection was detected by 1984-129 in the midnight sector as a dispersionless and short-lived (duration was 9 min) event. The injected electron cloud drifted eastward and was detected by 1982-019 in the dawn sector. Meanwhile, the GOES 6 and 5 satellites were in the postmidnight sector between the satellites which measured the electron flux. They observed transient field line oscillations during the periods when the azimuthal plasma pressure arising from the electron cloud was in the postmidnight sector. It is suggested that field-aligned currents are likely to be built up preferentially by a divergence of diamagnetic currents in the region where the azimuthal plasma pressure gradient dominates. It is argued that these currents excite an azimuthally polarized transient field line oscillation.

Further investigation on distributions of tangential discontinuity in the solar wind

Abstract The frequency of the occurrence of the tangential discontinuity (TD) was investigated at thirteen interacting regions (the leading edges) of the solar wind, where each averaged value of the total pressure reaches very much higher values, ranging from 3 × 10−10 to 12 × 10−10 erg/m3, than the non-interacting regions (the troughs or the high velocity regions, ⩽2 × 10−10 erg/cm3). The present result that the degree of the occurrence is considerably higher in the interacting region than in the non-interacting region was confirmed to be consistent with our previous result. It was discovered that the occurrence frequency at the interacting region increases when strong interaction has occurred.

A substorm onset signature at the auroral zone as observed with SuperDARN and equatorial magnetometers

Ionospheric convection enhancement with high westward velocities exceeding 1500 m s−1 was observed in association with the particle injection event by SuperDARN in the dusk sector of the auroral zone. Such a convection enhancement could be attributable to the enhanced sunward plasma flows in the dusk sector of the magnetosphere. At the dip equator a ground Pi2 onset was found to occur in association with these convection transients. The onset of the Pi2 lagged behind the convection enhancement by 4–11 min. It is argued that the convection transient could be the earliest indicator of the substorm onset among energetic particle injection, low-latitude Pi2 onset, and, perhaps, the auroral breakup.

Distributions of tangential discontinuity in the corotating solar wind structure

Abstract Distributions of the tangential discontinuity (TD) in the solar wind sector structure are investigated on the basis of the magnetic field data and the ion plasma parameters from the Explorer 33 satellite from 23 January to 23 March 1968. The TD is separated from the observed field fluctuations by calculating the direction of the plasma flow and also the direction of the minimum field fluctuation with respect to the ambient magnetic field direction. It is found that the TD is formed by the thin layered field-aligned currents (the current sheets), and that the TD is predominantly built up in the leading edge of the solar wind where the compression of the plasma and the magnetic field takes place. It is suggested that the current sheets might be locally generated in the leading edge in the turbulent conditions arising from collisions between the fast- and the slow-stream of the solar sector structure.

A concurrent modulation of the auroral luminosity and ground Pc3 activities at dip-equator

Abstract A diffuse and faint aurora was recorded in the all-sky image of Syowa station near the southern boarder (most tailward part) of the Syowa field-of-view during the period from 2200-0000 UT, 1 August 1987. The Syowa station was at the midnight sector, 2200 MLT-0000 MLT. Meanwhile, geomagnetic pulsations in Pc3 band were observed in the morning sector (0700-0900 LT) by ground magnetometer at the dip-equator (MEL: Melekeok in Western Pacific region, −1.12°, 205.06° in geomagnetic coordinates). We demonstrate a possible correlation of the occurrences between these two events at midnight auroral zone and at dayside dip-equator.

Amplitude modulation of the equatorial electrojet (EEJ) during a magnetospheric storm

Abstract An encounter of the Earth with a high velocity solar wind stream triggered the magnetic storm of 29 January 1995. The storm lasted for the following seven days, keeping the level of the Dst index in the range of −25–−50 nT. At the dip-equator, a regular enhancement of the magnetic H component during daytime, referred to as the Equatorial Electrojet (EEJ), was seen to be suppressed and modified during this storm interval. We attempted to classify the type of the EEJ modification by analyzing magnetometer data from two stations at the dip-equator but located in the opposite hemisphere, and energetic particle data from two geosynchronous satellites located close to the ground magnetometer meridian. As a result, three different types of modulation of EEJ amplitudes, with time scales of 15 min to 1 day, were found to appear during the periods when the flux level of low energy charged particles (>30 keV) in the midnight magnetosphere was increased above the quiet level. These modulations were characterized by referring to the nightside particle signatures. We argue that, although the EEJ is a local enhancement of the ionospheric currents at the dayside dip-equator, the EEJ is definitely affected by changes of the magnetosphere, probably in various ways arising from the complexities of the storm effects.

Ionospheric equivalent current systems of low-latitude Pc3 pulsations

Abstract Ionospheric equivalent current systems of low-latitude Pc3 pulsations observed on 7 June 1980 were studied using data from the AFGL (Air Force Geophysics Laboratory) magnetometer network. The network, which covers four hours in longitude, consists of seven stations: five along 55°N and two along 40°N geomagnetic latitude. Pulsations studied had a period between 11 and 23 s and were observed simultaneously at all seven stations. A cross spectral analysis of the interstation signals showed a phase structure in longitude as well as in latitude. In longitude, the apparent phase propagated eastward or westward, but was not always consistent between the H - and D -components. In latitude, on the other hand, the apparent phase was always poleward for both components. Ionospheric equivalent current systems were drawn by combining the results of the cross spectral analysis of the interstation signals with the phase differences of the H - and D -components at each station. Two different types of the current systems, namely Type A and Type B were found. Type A indicates that either a clockwise (CW) or counterclockwise (CCW) ionospheric current cell appears at the North-East end of the activity Region, moves toward the West, and then disappears at the North-West end of the activity region. This type of event has a spectral peak between 12 and 16 s and is observed between 6:00 and 10:00 M.L.T. Type B consists of two sub-cells moving from North to South independently. This type has a spectral peak between 11 and 23 s, a range much broader than that of Type A and is commonly seen between 7:30 and 15:00 M.L.T. The structure and movement of the current system associated with low-latitude Pc3 pulsations is discussed in relation to the field-line resonance model.

Thin layered field aligned current in the solar wind

Abstract The distribution of thin layered field aligned currents (current layers) in the leading edge of the fast stream at 1 a.u. is studied by comparing solar wind plasma parameters (bulk velocity, density proton temperature and magnetic field intensity) and occurrence frequency of the current layer. Each leading edge studied is either a stream interface or an interplanetary shock. It is found that the occurrence frequency is related best to the proton temperature variations; the occurrence frequency increases with the rise of proton temperature. Possible mechanisms which cause high occurrence frequency of current layers in the leading edge are discussed.