Comparison of magnetotail magnetic flux estimates based on global auroral images and simultaneous solar wind—magnetotail measurements

We compared simultaneous magnetotail magnetic flux F estimates, (1) based on in situ spacecraft measurements in the tail and solar wind (FT) with (2) the polar cap magnetic flux, estimated from global auroral images (using proton-induced or electron-induced emissions, Fp or Fe, respectively). Simultaneous Fp and Fe estimates gave the correlation coefficient CC=0.74, indicating that these measures are not absolutely precise. Regression analysis of FT versus Fe and Fp gave CC values 0.73 and 0.50, correspondingly. FT values, containing closed magnetic flux, are systematically higher than Fp and Fe by 20–30%. Motivated by diverse results, published by different groups, we reanalyzed the F dependence on the dayside merging electric field Em for different dynamical states. The linear regression F(Em) for substorm onsets shows a large slope ∼0.07–0.12GWb/(mV/m) for all Fp, Fe and FT, confirming the loading–unloading substorm scheme. For SMC intervals this slope is only 0.03 GWb/(mV/m).

Research highlights
► ‘Optical’ (Fp, Fe) estimates are not absolutely precise (CC=0.74 in our data set).
► CC=0.73, CC=0.50 for (FT, Fe), (FT, Fp) correspondingly (worse for more disturbed Fp periods).
► FT exceeds Fp, Fe by ∼20-30 % due to plasma sheet neglecting in FT calculations.
► Strong F(Em) dependence at SO, supporting the loading-unloading substorm scheme.