The Stretching and Pinchoff of Loops
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In the previous chapter, we found that a linear on-ramp of electric current produced a growing dipole field sandwiched between sheaths of relatively strong, oppositely directed meridional field. As the structure moved outward, the negative field in the trailing sheath weakened and formed a neutral point where the legs of the stretched field lines could pinch together to separate the outgoing bubble from the static dipole field that was left behind. For a linear off-ramp, the pinchoff occurred in the leading sheath, whose negative field weakened the previously established dipole field and produced a neutral point where the dipole field was split into two parts. The reversed polarity bubble emerged between these parts sweeping them away as it moved inward and then outward from the sphere. In this chapter, we will use space-time maps to examine this process in more detail.