No abstract

A complete set of ISEE plasma wave, plasma, and field data are used to identify the plasma instability responsible for the generation of extremely low frequency (ELF) electromagnetic lion roars. Lion roars detected close to the magnetopause are generated by the cyclotron instability of anisotropic (Tz-/Tll-= 1.2) thermal electrons When the local plasma critical energy, EM = B2/8zrN, falls to values (EM --10-30 eV) close to or below the electron thermal energy, 25 eV, as a result of decreases in B. A companion theoretical paper, Thorne and Tsurutani (1981), demonstrates that the convective growth rates of lion roars under these conditions is greater than 100 dB RE -•, The lion roars are terminated by increases in the ambient magnetic field magnitude and consequential increases in E• to values greater than 100 eV. Because there are few resonant particles at these high energies, the growth rate decreases by 3 orders of magnitude and measurable growth ceases. The value of the absolute upper limit of the frequency of unstable waves predicted by theory, 60ma x --A-f•-/(A-+ 1), is compared with observations. The predictions and observations are found to be in general, but not exact, agreement. Several possible explanations are explored. The quasi-periodic, --•20-s magnetic and plasma oscillations which cause the variations in E• and hence alternately drive the cyclotron waves unstable and then stable are also investigated. The plasma and field pressures are shown to be out of phase, while the total pressure (electron + ion + field) remains relatively constant. Most of the pressure is associated with the particle thermal motion. The large 2:1 variations in field strength cause large oscillations in/3 (8•rP/B2), from/3 = 1-2 at field maximum to/3 = 10-25 at field minimum. Analysis of the high-resolution magnetic fields at the two closely separated spacecraft, ISEE 1 and 2, rule out the possibility that these field and plasma oscillations could be due to magnetopause motion. Crosscorrelation analyses of the magnetic fields at the two spacecraft and the time delays for maximum correlation are shown to be consistent with the magnetic structures being quasi-static in nature. The temporal variations of the plasma and fields are due to spatial structures convecting past the spacecraft at the magnetosheath flow speed. The quasi-periodic structures are -20 proton gyroradii in scale in the plasma rest frame. Magnetic structures with similar scale lengths are also shown to exist in the magnetosheaths of Jupiter and Saturn (Pioneer 11 data). The results are consistent with the interpretation that these magnetohydrodynamic structures are nonoscillatory 'waves' generated by the drift mirror instability. The condition for instability,/3_d/311 > 1 + (1//3•), is met for the cases studied in this paper. The electron and ion instabilities are intimately coupled. The generation of high/3 (> 10), low critical energy (E• = 10-30 eV) regions by the drift mirror instability leads to the electrons becoming cyclotron unstable. The consequential w...

We present a study of the variation of the relative abundance of helium to hydrogen in the solar wind as a function of solar wind speed and heliographic latitude over the previous solar cycle. The average values of A He , the ratio of helium to hydrogen number densities, are calculated in 25 speed intervals over ($27 day) Carrington rotations using Faraday cup observations from the Wind spacecraft between 1995 and 2005. We find that for solar wind speeds between 350 and 415 km s À1 , A He varies with a clear 6 month periodicity, with a minimum value at the heliographic equatorial plane and a typical gradient of 1% per degree in latitude. Once the gradient is subtracted, we find that A He is a remarkably linear function of solar wind speed. We identify the implied speed at which A He is zero as 259 AE 12 km s À1 and note that this speed corresponds to the minimum solar wind speed observed at 1 AU. The vanishing speed may be related to previous theoretical work in which enhancements of coronal helium lead to stagnation of the escaping proton flux. During solar maximum the A He dependences on speed and latitude disappear, and we interpret this as evidence of two source regions for slow solar wind in the ecliptic plane, one being the solar minimum streamer belt and the other likely being active regions.

[1] We examine the implications of the widely used, force-free, constant-a flux rope model of interplanetary magnetic clouds for the evolution of these mesoscale (fraction 1 AU) structures in the heliosphere, with special emphasis on the inner ( 1 AU) heliosphere. We employ primarily events observed by the Helios 1 and 2 probes between 0.3 and 1 AU in the ascending and maximum phases of solar cycle 21 and by Wind at 1 AU in a similar phase of solar activity cycle. We supplement these data by observations from other spacecraft (e.g., Voyagers 1 and 2, Pioneers 10 and 11, and others). Our data set consists of 130 events. We explore three different approaches. In the first, we work with ensemble averages, binning the results into radial segments of width 0.1 AU in the range 0.3 r h 1 AU. Doing this, we find that in the inner heliosphere the modeled average central axial field strength, hB 0 i, varies with heliospheric distance r h as hB 0 i [nT] = 18.1 Á r h À1.64 [AU], and the average diameter increases quasilinearly as hDi [AU] = 0.23 r h 1.14 . The orientation of the axis of the underlying magnetic flux tube in our data set is generally found to lie along the east-west direction and in the ecliptic plane at all values of r h , but there is considerable scatter about these average directions. In the second, we monitor the evolution of magnetic clouds in snapshot fashion, using seven spacecraft alignments. The results are in broad agreement with the statistics reported under step 1. In the final approach, we obtain the functional dependence of B 0 and D predicted by an analytic expression for a freely expanding Lundquist flux tube. We find D to vary linearly with r h , broadly similar to that obtained under approach 1. The maximum field strength scales as r h À2 compared to a r h À1.3dependence obtained from statistics. We compare our findings with those of Bothmer and Schwenn (1998), who used a different methodology. The results obtained form a good background to the forthcoming Solar Terrestrial Relations Observatory (STEREO) and Sentinels missions and to multispacecraft studies of magnetic clouds.

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5‐10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.