Comment on “A turning point in auroral physics”] An apparent controversy in auroral physics

Abstract

In his article “A turning point in auroral physics,” Bryant [2006] argued against what he called the ‘standard’ theory of auroral acceleration, according to which the electrons “gain their energy from static electric fields,” and offered wave acceleration as an alternative. Because of the importance of the process, not only for the aurora borealis but also for other cosmic plasmas, a clarification of this apparent controversy seems to b e in place.Indeed, theoretical descriptions of the acceleration process often use an electrostatic approach. One postulates or derives a quasi‐Ohm's Law for the relation between current density and potential drop along the magnetic field and attributes the energy gain of the electrons to the ‘fall’ through this potential drop. Strong confirmation comes from the findings that a ‘potential,’ derived from integrating the transverse electric field measured by a satellite crossing above an auroral arc, agrees well with the energy gain of ions accelerated below the spacecraft [e.g., McFadden et al., 1999]. The solution of the problem lies in the word ‘nearly’ In the height range of several thousand kilometers, where auroral acceleration is concentrated, the electric field is nearly electrostatic. In reality, it is applied by an electromagnetic Alfvén wave launched in the outskirts of the magnetosphere. When it reaches the stiff magnetic field near Earth, where its phase speed is close to the velocity of light, it traverses the acceleration region so fast that a nearelectrostatic description of the field is permissible. However, above the low magnetosphere the field continues as an induction field. In other words, auroral field lines are not equipotentials. In conclusion, Bryant is right in principle but wrong in discarding auroral acceleration by nearly electrostatic fields.