A nearly universal solar wind‐magnetosphere coupling function inferred from 10 magnetospheric state variables

Abstract

[1] We investigated whether one or a few coupling functions can represent best the interaction between the solar wind and the magnetosphere over a wide variety of magnetospheric activity. Ten variables which characterize the state of the magnetosphere were studied. Five indices from ground-based magnetometers were selected, namely Dst, Kp, AE, AU, and AL, and five from other sources, namely auroral power (Polar UVI), cusp latitude (sin(L c )), b2i (both DMSP), geosynchronous magnetic inclination angle (GOES), and polar cap size (SuperDARN). These indices were correlated with more than 20 candidate solar wind coupling functions. One function, representing the rate magnetic flux is opened at the magnetopause, correlated best with 9 out of 10 indices of magnetospheric activity. This is dF MP /dt = v 4/3 B T 2/3 sin 8/3 (q c /2), calculated from (rate IMF field lines approach the magnetopause, $v)(% of IMF lines which merge, sin 8/3 (q c /2))(interplanetary field magnitude, B T )(merging line length,). The merging line length is based on flux matching between the solar wind and a dipole field and agrees with a superposed IMF on a vacuum dipole. The IMF clock angle dependence matches the merging rate reported (albeit with limited statistics) at high altitude. The nonlinearities of the magnetospheric response to B T and v are evident when the mean values of indices are plotted, in scatterplots, and in the superior correlations from dF MP /dt. Our results show that a wide variety of magnetospheric phenomena can be predicted with reasonable accuracy (r > 0.80 in several cases) ab initio, that is without the time history of the target index, by a single function, estimating the dayside merging rate. Across all state variables studied (including AL, which is hard to predict, and polar cap size, which is hard to measure), dF MP /dt accounts for about 57.2% of the variance, compared to 50.9% for E KL and 48.8% for vBs. All data sets included at least thousands of points over many years, up to two solar cycles, with just two parameter fits, and the correlations are thus robust. The sole index which does not correlate best with dF MP /dt is Dst, which correlates best (r = 0.87) with p 1/2 dF MP /dt. If dF MP /dt were credited with this success, its average score would be even higher.Citation: Newell, P. T., T. Sotirelis, K. Liou, C.-I. Meng, and F. J. Rich (2007), A nearly universal solar wind-magnetosphere coupling function inferred from 10 magnetospheric state variables,

Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

“…8) when the E m values of past hours are also taken into account. This favourable result is consistent with the report of Newell et al (2007). In our Figs.…”

“…To test the relation we used different indices to quantify activity. The merging electric field, E m , also termed "Newell coupling function" (Newell et al, 2007), quantifies the solar wind input into the magnetosphere. We find a very good linear relation to the latitude of the boundary (see Fig.…”

“…The apex latitudes of the detections were first sorted into bins covering a certain range of the index, then the mean apex latitudes of the bins were used for further correlation coefficient analyses. The merging electric field is calculated according to the coupling function as defined by Newell et al (2007):…”

On the Usage of Geomagnetic Indices for Data Selection in Internal Field Modelling

“…In general terms, large M EF values are associated with southward magnetic field (IM F B z < 0) and/or high speed of solar wind. For the purposes of internal magnetic field modelling often a revised version of M EF is used (Newell et al 2007;Olsen et al 2014):…”

“…−2nT < B z < 6 nT and |B y | < 8 nT Maus et al (2010)). For MEF (or for its revised version by Newell et al (2007)) a typical condition for weak electrojet activity is M EF < 0.8 mV /m (Olsen et al 2014;Maus et al 2010;Sabaka et al 2015).…”