No abstract

We extend previous theories of stochastic ion heating to account for the motion of ions along the magnetic field B. We derive an analytic expression for the temperature ratio T ⊥i /T ⊥p in the solar wind assuming that stochastic heating is the dominant ion heating mechanism, where T ⊥i is the perpendicular temperature of species i and T ⊥p is the perpendicular proton temperature. This expression describes how T ⊥i /T ⊥p depends upon U i and β p , where U i is the average velocity along B of species i in the proton frame and β p is the ratio of the parallel proton pressure to the magnetic pressure, which we take to be 1. We compare our model with previously published measurements of alpha particles and protons from the Wind spacecraft. We find that stochastic heating offers a promising explanation for the dependence of T ⊥α /T ⊥p on U α and β p when the fractional cross helicity and Alfvén ratio at the proton-gyroradius scale have values that are broadly consistent with solar-wind measurements. We also predict how the temperatures of other ion species depend on their drift speeds.

Spacecraft measurements show that protons undergo substantial perpendicular heating during their transit from the Sun to the outer heliosphere. In this paper, we use Helios 2 measurements to investigate whether stochastic heating by low-frequency turbulence is capable of explaining this perpendicular heating. We analyze Helios 2 magnetic-field measurements in low-β fast-solar-wind streams between heliocentric distances r = 0.29 AU and r = 0.64 AU to determine the rms amplitude of the fluctuating magnetic field, δB p , near the proton gyroradius scale ρ p . We then evaluate the stochastic heating rate Q ⊥stoch using the measured value of δB p and a previously published analytical formula for Q ⊥stoch . Using Helios measurements we estimate the 'empirical' perpendicular heating rate Q ⊥emp = (k B /m p )BV (d/dr)(T ⊥p /B) that is needed to explain the T ⊥p profile. We find that Q ⊥stoch ∼ Q ⊥emp , but only if a key dimensionless constant appearing in the formula for Q ⊥stoch lies within a certain range of values. This range is approximately the same throughout the radial interval that we analyze and is consistent with the results of numerical simulations of the stochastic heating of test particles in reduced magnetohydrodynamic turbulence. These results support the hypothesis that stochastic heating accounts for much of the perpendicular proton heating occurring in low-β fast-wind streams.

We investigate the conditions under which parallel-propagating Alfvén/ion-cyclotron (A/IC) waves and fastmagnetosonic/whistler (FM/W) waves are driven unstable by the differential flow and temperature anisotropy of alpha particles in the solar wind. We focus on the limit in which w α 0.25v A , where w α is the parallel alphaparticle thermal speed and v A is the Alfvén speed. We derive analytic expressions for the instability thresholds of these waves, which show, e.g., how the minimum unstable alpha-particle beam speed depends upon w α /v A , the degree of alpha-particle temperature anisotropy, and the alpha-to-proton temperature ratio. We validate our analytical results using numerical solutions to the full hot-plasma dispersion relation. Consistent with previous work, we find that temperature anisotropy allows A/IC waves and FM/W waves to become unstable at significantly lower values of the alpha-particle beam speed U α than in the isotropic-temperature case. Likewise, differential flow lowers the minimum temperature anisotropy needed to excite A/IC or FM/W waves relative to the case in which U α = 0. We discuss the relevance of our results to alpha particles in the solar wind near 1 AU.

We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.