Lower Hybrid Drive in Solar Magnetic Reconnection Regions: Implications for Electron Acceleration and Solar Heating

Abstract

Lower hybrid (LH) drive involves the resonant acceleration of electrons parallel to the magnetic field by lower hybrid waves, often driven by ions with ring or ring-beam distributions. Charge-exchange between hydrogen atoms and protons with relative motions perpendicular to the magnetic field leads to ring distributions of pickup ions, with concomitant perpedicular ion ‘heating’. This paper considers the combination of LH drive and charge-exchange in the outflow regions of magnetic reconnection sites in the solar chromosphere and lower corona, showing that the combined mechanism naturally predicts major perpendicular ion heating and parallel electron acceleration, and exploring the mechanism’s relevance to specific solar reconnection phenomena, heating of the solar atmosphere, and production of energetic electrons that generate solar radio emission. Although primarily qualitative, analysis shows that the mechanism has numerous attractive aspects, including perpendicular ion heating that increases linearly with ion mass, parallel electron acceleration, predicted ion and electron temperatures that span those of the chromosphere and lower corona, and parallel electron speeds spanning those for type III bursts. Applications to chromospheric explosive events and low-lying active regions, and to heating the chromosphere, appear particularly suitable. Sweeping of plasma frozen-in to chromospheric and coronal magnetic field lines across the neutral atmosphere due to motions of sub-photospheric fields represents an obvious and important generalisation of the mechanism away from reconnection sites. The requirements that the neutrals not be strongly collisionally coupled to the plasma and that sufficient neutrals are available for charge-exchange restricts the LH drive mechanism to above the photosphere but below where the corona is essentially fully ionised. LH drive may thus be important in heating the chromosphere and low corona while other heating mechanisms dominate at higher altitudes. Although attractive thus far, quantitative analyses of LH drive in these contexts are necessary before definitive conclusions are reached.

Simulations of coronal type III solar radio bursts: 3. Effects of beam and coronal parameters

“…The ion temperature T i in the corona may also change, and sometimes T i can be higher than T e (e.g., owing to heating by ion cyclotron waves [ Cranmer , 2000] or lower hybrid waves [ Cairns , 2001]). Figure 5 shows the dynamic spectrum of 2 f p emission for a higher T i = 4 MK, and variations with T i of the spectral characteristics at two frequencies, for otherwise identical parameters to Table 1.…”

Excitation of lower hybrid wave by an ion beam in magnetized plasma

“…Such a redistribution of energy occurs in many regions of the heliosphere, including in the outer heliosheath where pick-up ions form a ring beam distribution (Cairns & Zank, 2002), and near magnetic reconnection sites where bulk ions flow across B and electrons accelerated along B are observed (Cairns, 2001), such as occur in the Earth's magnetotail and possibly in the solar corona. Lower hybrid waves are electrostatic waves with wave vectors nearly perpendicular to the magnetic field, and involve oscillations of both the ions and electrons.…”

Properties of lower hybrid waves

“…Waves in heliospheric plasmas are involved in many universal heliophysical processes, including plasma heating, particle acceleration, and emission processes that are signatures of energy releases. Lower hybrid (LH) waves are of particular interest in contexts that involve both electrons and ions; these waves can transfer energy between parallel motions of electrons and perpendicular motions of ions (Omel'chenko et al 1989;Melrose 1986;Cairns 2001;Cairns & Zank 2002).…”

“…Such distributions occur in many regions of the heliosphere, including in the outer heliosheath where pick-up ions form a ring beam distribution (Cairns & Zank 2002), and near magnetic reconnection sites where bulk ions flows across B and electrons accelerated along B are observed (Cairns 2001), such as occur in the Earth's magnetotail and possibly in the solar corona. However, to determine where this mechanism is efficient and could be relevant we must calculate the wave growth and particle diffusion rates.…”

“…Lower hybrid (LH) waves are of particular interest in contexts that involve both electrons and ions; these waves can transfer energy between parallel motions of electrons and perpendicular motions of ions (Omel'chenko et al 1989;Melrose 1986;Cairns 2001;Cairns & Zank 2002).…”