Proton Cyclotron Instability Research Articles

Proton-cyclotron instabilities in non-uniform loss-cone magnetospheric plasma

The waves, propagating nearly transverse to the ambient magnetic field, with frequencies near the harmonics of the proton-cyclotron frequency are studied in an inhomogeneous plasma with protons having loss-cone distributions. Three types of drift cyclotron instabilities have been studied: (i) non-flute instability; (ii) ∇B-resonant instability; and (iii) non-resonant instability. Increases of loss-cone and density gradient increase the growth rates of all three instabilities. Increases in the positive temperature gradient and βt (ratio of thermal pressure of trapped protons to magnetic field pressure) have a stabilizing effect on the non-flute and non-resonant instabilities and a destabilizing effect on the ∇B-resonant instability. The non-resonant instability has an interesting feature: a particular harmonic can be excited in two separate bands of unstable wave numbers. These instabilities can play an important role in the dynamics of the ring current and the inner edge of the plasma sheet region of the magnetosphere. The discrete turbulence generated by them would give rise to precipitation of protons on the auroral field lines, which may contribute to the excitation of diffuse aurora. These instabilities may be relevant to the observation of harmonic waves at 6RE by Perrautet al. (1978).

IPDP source regions and resonant proton energies

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.

Drift instabilities associated with the particles in ring current and inner edge of plasma sheet

Electromagnetic waves propagating transverse to the magnetic field, containing inhomogenous and loss cone plasma, may become unstable due to the excitation of resonant proton, resonant electron and drift cyclotron instabilities. Resonant proton instability gets excited in inhomogenous plasma, irrespective of the presence of temperature anisotropy, loss cone or temperature gradient. However, the growth rate of this instability is much smaller than the other two instabilities. The maximum growth rates of resonant electron instability are enhanced with the increase of loss cone index, gradients in transverse temperature and magnetic field, and with the decrease of temperature anisotropy and gradients in density and parallel temperature. The drift cyclotron instability exists in a bounded range of wave numbers and its growth rate increases with the increase of electron temperature, density and magnetic field gradient, and with the decrease of proton temperature and temperature anisotropy. In the region of ring current for beyond plasmapause the resonant proton and resonant electron instabilities have the characterstic frequencies around 0.1Ω p and growth rates ~10 −6Ω p and 10 −3Ω p , respectively. In the ring current region the drift cyclotron instability is not excited whereas in the plasma sheet region the frequency and growth rate of this instability are around Ω p and 10 −2Ω p , respectively. These instabilities can accelerate the ring current particles along the magnetic field lines and dump them into the auroral region.

Plasma fluctuations in the solar wind

Ogo 5 plasma and magnetic field data are used to compute power spectra of solar wind fluctuations over the frequency interval 10−3‐10−1 Hz. We confirm the validity of the assumption made in earlier papers that the power spectra calculated from total flux measurements are approximately equal to the power spectra of density fluctuations times the square of the average solar wind speed. The relative density power spectrum Pn/no² is usually of the same order of magnitude as the power spectrum of speed fluctuations relative to the Alfvén speed, Pv/vA². All cases studied show evidence of the presence of Alfvén waves in this frequency range. In some data sets the density and field fluctuations are consistent with magnetosonic waves. In other sets the ratio of the power in field magnitude fluctuations to that in density fluctuations is inconsistent with magnetosonic waves; for these cases we postulate static inhomogeneities with a balance between electron thermal and magnetic pressures. Finally, we suggest that the power enhancements near 1 Hz reported in earlier papers may be caused by a resonant proton cyclotron instability driven by the proton thermal anisotropy in the solar wind.

Modifications in the maximum convective growth rate of the electromagnetic proton cyclotron instability due to the presence of thermal ions

The effect of thermal ions on the convective properties of the electromagnetic proton cyclotron instability is investigated. It is shown that when Ap < 1, where Ap is the proton anisotropy, cold ions, with relative specific charge mj/Zj mp ≥ 1, stabilize the convective growth of the instability. When Ap >1, the effect of thermal ions is to enhance the convective growth of the instabifity in the range where the real part of the frequency, ωr, is smaller than the ion gyrofrequency, Ωj. The maximum enhancement, corresponding to an optimum concentration of heavy ions, is shown to be a constant independent of the species. However, for each species, even for Ap > 1 (except for protons), there is always a range of values of Ap such that the species will stabilize the system. Heavier ions are shown to be more effective in enhancing the convective growth of the instability, in the sense that, the heavier they are, smaller relative concentrations are required to reach the constant value mentioned above. For sufficiently heavy species, the required concentration to optimize the convective growth, is independent of the species.

Requirements on singly ionized lithium concentrations for magnetospheric seeding experiments

Optimum concentrations of singly ionized Li relative to warm protons, η ≡ nLi/np,w, for magnetospheric seeding experiments aimed at enhancing the electromagnetic proton cyclotron instability have been obtained analytically and numerically. This has been done by calculating the absolute maximum growth rates γmaxabs and enhancement F ≡ γmaxabs/γmax(η = 0) for the proton instability branch upon addition of cold lithium ions to an infinite anisotropic proton plasma. A necessary condition for enhancement is P ≡ βpAp²(Ap + 1) ≪ 1, where βp ≡ 8πnpKT∥,p/B0² and Ap ≡ (T⊥/T∥)p − 1. The shape of the linearly unstable spectrum strongly depends on the parameter [Ap/(Ap + 1)]/(mLi/ZLimp)−1. When the condition stated above is satisfied, the growth rate and the extension of the unstable spectrum in k space may increase with η. An optimum η value is found to exist.

Type IPDP Magnetospheric Plasma Wave Events

It is stated that recently there has been an interest in a type of micropulsation event referred to as IPDP (intervals of pulsations diminishing in period). This interest arises partly owing to the possibility that IPDP characteristics might be used to determine the values of certain magnetospheric parameters, assuming that the waves are produced through electronmagnetic proton cyclotron instability. The interest also arises owing to the occurrence of IPDP events on the afternoon-evening side during the expansive phase of substorms, which is thought to be related to the westward drift of protons injected near midnight. Investigation of this question must include observations of IPDP activity at stations well separated in longitude, and some observations are presented from a pair of sites separated by 3 hours in longitude. Based on these observations some revision to the westward drift mechanism for IPDP production are suggested. Some of the observations also appear to imply that westward drift is not the only mechanism involved in the production of IPDP-like events. (UK)

Evening micropulsation events with a rising midfrequency characteristic

Many examples of evening micropulsation events with rising midfrequencies were found on the several years of continuous College micropulsation records. The midfrequency typically rises from 0.1 to 0.5 cps over a ∼1-hour event duration. There is a pronounced diurnal variation in occurrence, with maximum near 2000 local time, and there is a seasonal variation with most occurrences in summer. Three of the possible source mechanisms are briefly discussed: (1) systematic variations in the characteristics of the lower exosphere resonance cavity, (2) excitation via a growing wave mechanism with the additional assumption of a time-decreasing midenergy in the electron or proton beam, and (3) excitation via proton cyclotron instabilities near the equatorial plane with inward diffusion of the protons.