The onset of microinstability and its consequences in the solar wind

Alfvén waves propagating outward from the coronal base set the solar wind into oscillatory motion. The effect of this wave motion on the broadening and shape of the spectral lines of the resonantly scattered Ly α radiation emitted by a low‐density coronal hole is investigated at hights above 1.5 Rs from Sun center. Usually it has been assumed that the broadening due to waves can be characterized by a kinetic temperature proportional to the wave velocity amplitude squared, δυ². It is shown that the accuracy of this approximation depends on the wave velocity amplitude and the assumed wave spectrum.

Section: Non-maxwellianmentioning

confidence: 99%

Broadening of the resonantly scattered H I Lyman alpha line caused by Alfvén waves

Esser¹

1990

“…Unfortunately, there are no direct observations bearing on this question. But if the conclusion that there must be substantial high-frequency left-hand wave power at the sun does hold up under closer scrutiny, then it is Iikely that these waves must originate at the sun itself, since internal instabilities, such as heat-conduction-driven modes, can only be expected to become operable at least several solar radii above Ss, ergs nps, cm -s cm -'" s-• a the solar surface [Singer and Roxburgh, 1977]. [Hirshberg, 1973;Hirshberg et al, 1972aHirshberg et al, , 1974Hundhausen, 1970].…”

Section: The Case a • 15 Is Particularly Interesting Because It Is Tmentioning

confidence: 99%

Acceleration of solar wind He⁺⁺3. Effects of resonant and nonresonant interactions with transverse waves

Hollweg¹,

Turner²

1978

Simple models are described which investigate the combined effects on solar wind He++ of resonant and nonresonant acceleration by left‐hand transverse waves. The principal points are the following. (1) For a wide range of parameters, (υα – υp) at 1 AU is close to the effective phase speed of the left‐hand waves. (2) The most important factor in determining υα/υp at 1 AU is whether the high‐frequency left‐hand waves are predominantly outward propagating, inward propagating, or a mix of both. The resonant acceleration may be more important than the effects of heating or stream‐stream interactions. (3) Reasonable values of υα/υp at the sun (and of nα/np at 1 AU) are obtained for a power law index α ≅ 1.5 in the wave power spectrum if the effective phase speed of the resonant waves near the sun is not small in comparison to the Alfvén speed there. This requires a substantial level of high‐frequency power in outward going waves at the sun, which cannot come from heat‐conduction‐driven instabilities. (4) The present models do not allow one to decide whether the coronal He++ abundance is greater or less than that at 1 AU. (5) Some of the models show a positive correlation between nα/np and υp at 1 AU, roughly as has been observed. (6) The models suggest that variations in nα/np at 1 AU can result from variations in the wave properties near the sun and not necessarily from variations in the coronal abundance. (7) Some models indicate that (υα/υp) may decrease with increasing r in the vicinity of 1 AU. (8) The resonant acceleration is more efficient than Coulomb friction in the sense that it does not exhibit a runaway effect. (9) Observations of minor species may be used to deduce wave properties and plasma processes in the solar wind.

“…The resulting microturbulence may seriously alter the physics of the solar wind. For example, microturbulence, originating either from the sun or from instabilities in the wind, has been cited as a possible source of proton and helium thermal energy and acceleration [Singer and Roxburgh, 1977;Hollweg, 1973Hollweg, , 1974aMcKenzie et al, 1978]. These plasma properties will be explored in this and subsequent work.…”

Section: Introductionmentioning

confidence: 97%

Microturbulence and the solar wind 1. Analytical results for fast mode instability growth rates

Schwartz¹

1978

Analytical expressions for the growth rates of unstable low‐frequency ‘fast’ modes are derived here in the low‐frequency, low‐βi limit for a plasma obeying classical linear transport theory. The results indicate that at the onset of instability (∼10 Rs from the sun) the first growing mode in the solar wind has a frequency of ∼0.1 times the proton gyrofrequency and propagates at an appreciable angle to the background magnetic field. Substantial inhomogeneity is required for the growth to overcome competing processes. These modes may have consequences for thermal conduction, proton and helium heating, the observable wave spectrum, and other facets of solar wind theory, though considerable additional theory will be required.