Challenging Ring‐Current Models of the Carrington Storm.

A detailed analysis is made of horizontal‐component geomagnetic‐disturbance data acquired at the Colaba observatory in India recording the Carrington magnetic storm of September 1859. Prior to attaining its maximum absolute value, disturbance at Colaba increased with an e‐folding timescale of 0.46 hr (28 min). Following its maximum, absolute disturbance at Colaba decreased as a trend having an e‐folding timescale of 0.31 hr (19 min). Both of these timescales are much shorter than those characterizing the drift period of ring‐current ions. Furthermore, over one 28‐min interval when absolute disturbance was increasing, the data indicate an absolute rate of change of ≥2,436 nT/hr. If this is representative of disturbance generated by a symmetric magnetospheric ring current, then, assuming a standard and widely used parameterization, an interplanetary electric field of ≥451 mV/m is indicated. An idealized and extreme solar‐wind dynamic pressure could, conceivably, reduce this bound on the interplanetary electric field to ≥202 mV/m. If the parameterization for electric‐field extrapolation is accurate, but the field strengths obtained are deemed implausible, then it can be concluded that the Colaba disturbance data were significantly affected by partial‐ring, field‐aligned, or ionospheric currents. The same conclusion is supported by the shortness of the e‐folding timescales characterizing the Colaba data. Several prominent studies of the Carrington event need to be reconsidered. Key Points: Before (after) maximum, the absolute value of disturbance at Colaba increased (decreased) with an exponential timescale of 0.46 hr (0.31 hr)If Colaba disturbance was due to a symmetric ring current, standard extrapolations require an interplanetary electric field of >451 mV/mResults suggest significant contributions to Colaba disturbance by partial‐ring, dayside field‐aligned, or dayside ionospheric currents [ABSTRACT FROM AUTHOR]