On the relaxation of magnetospheric convection when &amp;lt;I&amp;gt;B&amp;lt;/I&amp;gt;&amp;lt;sub&amp;gt;z&amp;lt;/sub&amp;gt; turns northward

M C Kelley

doi:10.5194/angeo-30-927-2012

Abstract

Abstract. The solar wind inputs considerable energy into the upper atmosphere, particularly when the interplanetary magnetic field (IMF) is southward. According to Poynting's theorem (Kelley, 2009), this energy becomes stored as magnetic fields and then is dissipated by Joule heat and by energizing the plasmasheet plasma. If the IMF turns suddenly northward, very little energy is transferred into the system while Joule dissipation continues. In this process, the polar cap potential (PCP) decreases. Experimentally, it was shown many years ago that the energy stored in the magnetosphere begins to decay with a time constant of two hours. Here we use Poynting's theorem to calculate this time constant and find a result that is consistent with the data.

Highlights

When Bz turns northward after a long time southward, the energy stored in the distorted magnetic field decays, with a two to three-hour time constant
We show this is consistent with the Joule dissipation term in Poynting’s theorem as applied to the magnetosphere. This calculation is made for Bz north, it has applications for Bz south, since a major balance to the input of energy in that case is the Joule heating term, for which we know the time constant
The fact that the potential does not go to zero is due to the Poynting flux term for Bz north, which is due to a viscous interaction, cusp reconnection, or a combination of both

Summary

Introduction

The decay time constant of the PCP is about two to three hours. Our goal is to predict this time constant using Poynting’s theorem. Poynting’s theorem (PT) is not usually thought to apply to slowly varying systems, but only to wave phenomena. This is not a requirement of the theory, . A few papers have been written using PT for low-frequency magnetospheric and ionospheric phenomena (Kelley et al, 1991; Richmond, 2010). An excellent discussion of auroral physics is presented in Paschmann et al (2003) and references therein

Calculations

Conclusions