Fine-Scale Cavitation of Ionospheric Plasma Caused by Inertial Alfvén Wave Ponderomotive Force

Abstract

Deep, very narrow magnetic-field-aligned density depletions were observed by the Freja spacecraft during auroral oval crossings. These cavities have perpendicular width of the order of the electron skin depth c͞v pe and are associated with low-frequency electromagnetic perturbations and with discrete auroral structures. We demonstrate here that these cavities are likely produced by the ponderomotive force of inertial Alfvén waves. [S0031-9007(98) [7].The purpose of this Letter is to show that these density cavities result from the ponderomotive force created by the large field-aligned oscillating current [7] of an inertial Alfvén shear mode. Unlike previous models which tended to ignore dynamics parallel to the equilibrium magnetic field, we find that parallel electric fields and the resulting parallel electron motion (i.e., electron inertia effects) are of fundamental importance. We demonstrate in this Letter that very weak oscillating parallel electric fields associated with the inertial Alfvén shear mode can lead to substantial density depletions in magnetized plasma; this behavior is analogous in some ways to the density depletion associated with large amplitude lower hybrid resonance cones [8] and has important implications for the creation of striated cavities and other nonlinear phenomena in low b space plasmas.The inertial Alfvén shear mode is a cold-plasma wave which exists when the plasma b is smaller than the mass ratio m e ͞m i (or, equivalently, the Alfvén velocity y A is larger than the electron thermal velocity y Te ). This mode involves finite parallel electric field and finite electron mass (in contrast to ideal MHD where both these quantities are assumed to be vanishingly small). The parallel acceleration of finite-mass electrons explicitly determines the parallel current (this physics is neglected in ideal MHD). Superposition of the k spectrum generated by a localized source excites resonance cones [9,10], and it has recently been shown that small-scale auroral shear Alfvén signals are consistent with resonance cone excitation by a distant, pulsed, localized source [11].We begin by presenting in Fig. 1(a) typical Freja measurements of the transverse magnetic field B Ќ ; these perturbations have been interpreted as being due to inertial Alfvén resonance cones [7]. Figure 1(a) shows that the magnetic structure is irregular and occasionally has sharp gradients. Each time increment of 1 sec in Fig. 1 corresponds to Freja traveling a distance of 6.7 km perpendicular to the equilibrium magnetic field. Figure 1(b) shows measurements of the corresponding electron density. The sharp density cavities evident in Fig. 1(b) are correlated with the magnetic field gradients in Fig. 1(a). These measurements were made in auroral regions at an altitude of 1500 km where T e 0.5 1 eV, B 0 2.6 3 10 4 nT, and the oxygen content of the plasma was typically 50%-75% with the balance being mainly hydrogen. Thus, b ø m e ͞m i so that the shear Alfvén modes are

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