R. E. Horita

EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° invariant latitude, ILAT) above the collision-dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low-energy (a few eV) upward ion flows in the high-altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field-aligned and cold (Ti 80° ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s, respectively. Both the ion velocity and temperature distributions exhibited a day-to-night asymmetry, with higher average values on the dayside than on the nightside.

Ion depletion zones in the polar wind: EXOS D suprathermal ion mass spectrometer observations in the polar cap

In the high-altitude polar cap, the suprathermal ion mass spectrometer (SMS) on the EXOS D (Akebono) satellite frequently “observed” ion depletion zones (IDZ) in which the thermal-energy ion flux was below the detection limit of SMS, corresponding to thermal-energy ion densities less than 10−2 cm−3. These IDZ are located primarily in the nightside region of the magnetosphere at invariant latitudes above 70° and at altitudes preferentially near apogee and between 8000 and 10,000 km (EXOS D apogee) but extending down to 3000 km. In contrast, outside the IDZ, the SMS regularly observed 3/16/2009 2:01:59 PMoutflowing H+, He+, O+, and O++ polar wind ions with energies typically less than 10 eV in the polar cap. Also, at sufficiently low altitudes below the IDZ the SMS instrument always observed H+, He+, O+, and O++ ions that were stationary in the Earths corotating frame, i.e., ions observed in the spacecraft ram direction.

IPDP source regions and resonant proton energies

Abstract We report the first satellite observations of protons involved in the generation of IPDP (intervals of pulsations of diminishing periods). Fifteen IPDP events observed on the ground during the period December 1971-February 1972 were correlated with particle data obtained on the Explorer 45 satellite. The analyses suggest that the IPDP events were generated by the proton cyclotron instability with westward drifting protons with energies of 1–100 keV; the approximate locations of the instability regions at the onset (low-frequency) of IPDP generation were generally near the plasmapause and were at L values between 4.7 and 5.5 with the majority at values between 5.0 and 5.3 in the afternoon/evening sector between 1700 and 2300 LT.

Proton cyclotron frequency phenomena in the topside ionosphere

Abstract Proton cyclotron echoes and spurs are phenomena related to the proton cyclotron frequency discovered on topside sounder ionograms from Canadian Alouette satellites. The echoes and spurs appears on the ionograms at apparent ranges which lead to a frequency close to the proton cyclotron frequency; the frequency is obtained by taking the reciprocal of the time elapsed between the transmission of the sounder pulse and the reception of the signal at the satellite. Aloutte II and ISIS and II ionograms for about sixty satellite passes were scaled to study the charateristics of these phenomena. Generally, proton cyclotron echoes and spurs occured on the ionograms at frequencies below the electron plasma frequency f N , the echoes predominantly slightly above the electron cyclotron frequency f H and the spurs just below f N . They appeared most often when a harmonic of the electron cyclotron frequency nf H (n = 1, 2, 3, 4) was approximately equal to one of the other characteristic frequencies, that is: (1) nf H ≈ f N , (2) nf H ≈ f z S , the frequency of the Z wave at the heght of the satellite, and (3) nf H ≈ f T , the upper hybrid resonance frequency. Proton cyclotron echoes, spurs and protein cyclotron wave patterns have many features in common in addition to their fundamental relationship with the proton cyclotron frequency. The echoes and spurs are observed most often when when nf H overlaps one of the other characteristic frequencies, that is: nf H ≈ f N , nf H ≈ f z S , and nf H ≈ f T . The proton cyclotron wave pattern is observed under the first of the three conditions. It appears that the occurence of the phenomena is related to the plama conditions, the geographic location not being important in itself except that reflects different plasma conditions. Although proton cyclotron echoes and spurs were observed more often near the geomagnetic equator, consistent with the results of Matuura and Nishizaki,(8) they still observed at high latitudes even near the north geomagnetic pole. The echoes and spurs occur at frequencies below f N , the echoes predominantly slightly above f H and the spurs just below f N . Generally it is easy to distinguish between the two since usually they appear separately or, if together, often an echo would terminate and a spur begin at a slightly different apparent range. But it is not always easy since sometimes it appeared that a proton cyclotron echo and a spur formed a continuous trace, suggesting that perhaps they may be different manifestations of the same phenomenon. Work is continuing in an attemp to understand the origin of proton cyclotron echoes, spurs, and proton cyclotron wave patterns.

Wave-particle interaction around the lower hybrid resonance

Abstract Wave-particle interaction in the ionosphere is studied theoretically for wave frequencies around the lower hybrid resonance (LHR) frequency. An expression is derived for the growth rate of whistler-mode waves propagating in a magneto-active plasma penetrated by a tenuous beam of nonthermal particles. This expression is an extension of that derived in a previous paper by employing the electrostatic dispersion equation; here, the full-wave dispersion equation is used which reduces to the electrostatic one for large values of refractive index. It is shown that around the LHR frequency there are two regions in the propagation angle θ (the ‘electrostatic’ and ‘electromagnetic’ regions) where growth by the Landau instability process may occur. The maximum growth rate has an abrupt discontinuity just above the LHR frequency and increases rapidly to values several orders of magnitude higher than those below the LHR frequency. Also examined is the influence on the growth rate of the following parameters: the ratio of the temperatures perpendicular and parallel to the background magnetic field T ⊥ T ‖ , the streaming velocity V s of the nonthermal electrons, and the ratio φ 2 of the kinetic energy in the streaming motion to the thermal energy of the streaming electrons. The theory is discussed in relation to LHR noise bands discovered by the Canadian Alouette I satellite and to other VLF signals such as auroral hiss.

Ion gyrofrequency phenomena observed on Whistlers, auroral hiss and ELF hiss

Abstract Modulation of auroral hiss at harmonics of the proton gyrofrequency has been observed by the VLF receiver on the Canadian ISIS 2 satellite; the observed value of the proton gyrofrequency is less than the local proton gyrofrequency in the immediate vicinity of the satellite. In addition, modulation at harmonics of the helium gyrofrequency has been observed on ELF hiss inside and outside the plasmasphere. These observations and a number of other related VLF phenomena observed by others (electrostatic noise seen on the Javelin 8.46 rocket, V-shaped VLF noise and saucers) are discussed in terms of the Landau instability process excited by non-thermal electrons streaming along the magnetic field of the Earth. Whistler modulation is also discussed.

The source regions of fine structure in whistlers, auroral hiss and ELF hiss

Abstract Fine structure modulation at harmonics of the proton and/or helium gyrofrequency of whistlers, auroral hiss and ELF hiss observed on the ISIS I and II satellites was analyzed to determine the source regions where the modulation took place. The proton and helium modulated whistlers have been found to have source regions with apparent altitude range around 0–1800 km. Proton modulated auroral hiss was found to have source regions in the altitude range 1000km to 2500km. Helium modulated ELF hiss observed on the ISIS II satellite was found to have source regions in the altitude range around 1400 km to 2000 km for geomagnetic latitudes above around 30° and below the satellite for geomagnetic latitudes below around 30°. Some ELF hiss observed on the ISIS I satellite has modulation source regions below the satellite for slightly larger geomagnetic latitudes. For the ELF hiss near the Equator the location of the modulation source region occurs along a magnetic field line related to the equatorial anomaly.

ELF emissions observed near the plasmapause and plasma sheet

Abstract Alouette 2 VLF data often show broad-band ELF emissions in the invariant latitude range 50°–80° N. These emissions have maximum intensity over a region several degrees in width. The occurrence of these emissions is shown to be related to the period of Pi2 micropulsations, to the plasma-sheet inner boundary and also, during increased magnetic activity, to the plasmapause. This result is consistent with a prediction of Saito and Sakurai (1970) that the plasma-sheet inner boundary comes close to the plasmapause when K p ≥ 2 0 .