Substorm effects on magnetospheric VLF hiss observed by ISIS satellites

Narrow-band VLF electric field data at 300 Hz, 1.5kHz, 5kHz, 8kHz, 16kHz, and 20kHz received from ISIS satellites at Syowa station, Antarctica are compared with AE and AL indices between February, 1982 and January, 1983 for investigating substorm effects on magnetospheric VLF hiss. The telemetry coverage for ISIS satellites from Syowa Station is from about 80° to about 50° in geomagnetic invariant latitude. In geomagnetic quiet and weak substorm recovery phase periods, a narrow-band mid-latitude hiss and a broad-band polar hiss appear, respectively, at invariant latitudes from 50° to 63° and from 66° to 78° from the afternoon to the midnight in MLT. In substorm expansion phase, the polar hiss region moves to lower latitudes and approaches the mid-latitude hiss region, and also, a LHR hiss whose intensity peak latitude increases with decreasing frequency (from 20kHz to 1.5kHz) appears in the mid-latitude hiss region near the plasmapause. Statistical study of VLF hiss shows that the mid-latitude hiss occurs most often in the quiet period and is independent of the substorm activity. While, the occurrence rate of polar hiss is 61 % in the substorm period (200nT AL ≧ -583nT) and 29% in the quiet period (30nT AL ≧ - lOOnT). Hence, the generation of polar hiss is clearly influenced by the substorm

Anomalous sporadic-E layers observed before M7.2 Hyogo-ken Nanbu earthquake; Terrestrial gas emanation model

Before the M7.2 Hyogo-ken Nanbu earthquake of January 17, 1995, anomalous foEs increases above 7MHz were observed at Shigaraki about 120km northeast of the epicenter and at Kokubunji about 500km east of it in the daytime on January 15, 1995. Since there were very quiet geomagnetic conditions and no solar event on January 15 and the foEs is normally below 5MHz in the Japanese winter, the anomalous foEs increases above 7MHz observed on January 15 may be presumably an ionospheric seismic precursor. An upward tornado-type seismic cloud with rapid air currents occurred between altitudes of 680m and 2000m over the epicenter region, and anomalous radon ion density increases of about 100 times the normal radon ion density were also observed at epicentral distance of about 200km before this earthquake. The epicentral area of the anomalous foEs increase occurrence before this earthquake is about the same as that of the terrestrial gas emanation from active faults before great earthquakes(C.-Y. King; J. Geophys. Res., 91B, 12269, 1986). The radon ions carried up to cold high altitudes produce positively charged ice crystals in the topside cloud and the bottom-side cloud has negative charges. An electrostatic field is set up in the lower ionosphere by positively charged cloud-to-ground lightning discharges. A temporal electrostatic field above the air breakdown electric field causes ambient electron heating and ionization of neutrals in the lower ionosphere. Times for producing the anomalous foEs increases computed by the quasi-electrostatic heating and ionization process at altitude of 100km are comparable with seismic flash duration before great earthquakes

Generation region of polar cusp hiss observed by ISIS satellites

Polar cusp VLF hiss of irregular spectra is observed mainly at geomagnetic invariant latitudes from 74° to 84°, and at geomagnetic local times from 10 to 14 hours. This occurrence region corresponds well to a region of cusp precipitations of lowenergy electrons below 1 keV. Changes of lower cutoff frequency of polar cusp hiss are related with plasma irregularities in the polar cusp ionosphere. The whistler-mode Cerenkov radiation from electrons below I keV is discussed for generating the polar cusp hiss, but this mechanism can not work in the polar cusp ionosphere for VLF frequencies much lower than a local electron gyrofrequency. The whistler-mode VLF Cerenkov radiation is generated from the electrons below 1 keV at high altitude(～6 R_E) cusp magnetosphere, where a local electron gyrofrequency is in VLF band. The whistler-mode cusp hiss generated at high altitudes will propagate along field-aligned cusp plasma down to a strong geomagnetic field region where the cusp structure fades out. Below this region, the whistler-mode hiss spreads over the polar cusp ionosphere where low energy electron precipitations produce ionospheric irregularities

Plasma particles drifting in the equatorial plane of quantitative magnetospheric model and related magnetospheric phenomena

Using the quantitative magnetospheric model derived by MEAD and FAIRFIELD (J. Geophys. Res., 80,523,1975) from satellite observations of the magnetic field, we have computed equatorial profiles of the total geomagnetic field, electric equipotential lines and drift paths of plasma particles with pitch angle of 90° in the magnetosphere under the superquiet (SQ) and superdisturbed (SD) conditions in case of the magnetospheric tilt angle of 0°. The uniform dawn to dusk electric field used is 0.1mV/m for a quiet time and 0.4mV/m for a disturbed time. All particles start from geocentric circles of 17 earth\u27s radii (R_E) in the equatorial plane of the tail region. Electric equipotential lines in the equatorial plane of the Mead-Fairfield (MF) model are concave on the dawn and dusk sides since the geomagnetic field lines of the MF model in the equatorial plane are curved greatly tailwards on the dawn and dusk sides compared with the radial field lines of a dipole model. The Alfven layer or the boundary of the forbidden region for the zero-energy particles computed in the MF magnetospheric model is compared with the average location of the plasmapause in the equatorial plane. The computed drift paths of zero-energy particles for the uniform dawn to dusk electric field of 0.1mV/m show a stagnation region in the late evening sector which agrees well with the plasmapause bulge observed by ground whistlers. The trapped particle region for zero-energy particles and energetic electrons in the late-evening outer magnetosphere seems to be produced by the particle drift motion in the late-evening outer magnetosphere, where the geomagnetic field lines are greatly curved tailwards under a weak dawn to dusk electric field of 0.1mV/m. The dayside extent of equatorial drift paths for electrons with 0.5keV/nT in the MF-SQ and MF-SD geomagnetic fields for the dawn to dusk electric field of 0.4mV/m corresponds well with the hard electron precipitation region associated with the active mantle aurora

Relationship between solar wind electric field and geomagnetic activity in increasing and decreasing phases of solar wind velocity

Solar wind velocity normally increases to a maximum above 600km/s from a quiet value below 400km/s in a few days and returns to the quiet one in a week. We select 445 time profiles of solar wind velocity data with simple variation in the interval from January 1964 to April 1985. Averaged ΣK_p values at the solar wind velocity of 500km/s are 28 and 19 in the increasing (leading) and decreasing (trailing) phases of solar wind velocity for the above time-profiles, respectively. The maximum ΣK_p was found to occur around the middle in the leading phase, to statistically coincide with a large southward IMF-Bz (≦-4 nT) with duration longer than 5 hours. The K_p rapidly increased with increasing the southward IMF in first 10 hours of the geomagnetic storm on July 22-25,1974. A maximum K_p=7 occurred together with a maximum westward solar wind electric field (VBz) of -6.9mV/m. Correlative variations of AE with VBz continued from 22 UT, July 22 to 11 UT, July 23 when a maximum southward IMF-Bz of -12.4 nT occurred. After 11 UT, July 23,the correlation between VBz and AE became irregular. It is concluded that, for the initial storm phase, the solar wind electric field directly penetrates into the polar magnetosphere along interconnected field lines and produces higher AE indices. After the maximum southward IMF, the magnetic merging could no longer occur at the dayside magnetopause. Rapid AE increases associated with Dst decreases are discussed in terms of the ring current and the cross-tail current. The AE activities after the maximum southward IMF seem to be related with magnetospheric processes which are independent of the solar wind electric field

Occurrence map of broad-band auroral VLF hiss observed by ISIS satellites

A polar map of occurrence rate of broad-band auroral VLF hiss is drawn by using narrow-band VLF intensity data of 6 frequencies which were processed from ISIS electric field (50Hz-30kHz) tapes of 347 passes received at Syowa Station, Antarctica from June 1976 to January 1983.The high-latitude contour of occurrence rate of 0.3 lies at invariant latitude of about 82° for all local times. The low-latitude contour of 0.3 lies at 74° around 10h geomagnetic local time (MLT) and it extends down to 67° around 22h MLT, showing a symmetricity with respect to the 10-22h MLT meridian. Two regions of high occurrence rate above 0.5 lie between 76° and 78° in the afternoon sector and also between 78° and 81° in the late night-morning sector. The result is not consistent with the occurrence map of auroral hiss at 9.6kHz with a pronounced dawn-dusk asymmetry obtained from Ariel magnetic field data by HUGHES et al. (Space Research XI, ed. by K. Ya. KONDRATYEV et al. Berlin, Akademie, 1323-1330,1971). This discrepancy between the ISIS and Ariel results is discussed from view points of auroral particle precipitation and difference in VLF data used