[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,

[1] We have developed an auroral precipitation model which separately categorizes the discrete aurora and both the electron and ion diffuse aurora. The discrete aurora includes acceleration by two distinct physical mechanisms, namely, quasi-static electric fields, producing monoenergetic peaks, and dispersive Alfvén waves, producing broadband electron acceleration. The new model is not merely finer in magnetic latitude (MLAT) and magnetic local time (MLT) resolution than previous models but is parameterized by solar wind driving instead of Kp and is based on functional fits to the solar wind coupling function which best predicts auroral power. Each of the four auroral types in each MLAT and MLT bin is separately fitted, a departure from the traditional compilation of a handful of discrete models, each assigned to represent a Kp (or other activity index) range. The variation of any of these four types of aurora at any local time can be predicted on the basis of the specific solar wind history of an epoch. This approach permits perhaps the first comprehensive comparison of the hemispheric contribution of each type of aurora. It turns out that the diffuse aurora is surprisingly dominant, constituting 84% of the energy flux into the ionosphere during conditions of low solar wind driving (63% electrons, 21% ions). The diffuse aurora is far from quiescent, tripling in power dissipation from our low to high solar wind-driving conditions. Even under the latter condition, the diffuse aurora contains 71% of the hemispheric energy flux (57% electrons, 14% ions). The monoenergetic aurora contributes more energy flux (10% quiet, 15% active) than does broadband acceleration signatures (6% quiet, 13% active). However, the broadband aurora rises fastest with activity, increasing by a factor of 8.0 from low to high driving. Moreover, this most dynamic auroral type contributes very high number fluxes, even exceeding monoenergetic aurora under active conditions (28% of hemispheric precipitation versus 21%). Thus, dynamic ionospheric heating and ion outflow is likely heavily affected by the wave aurora. Although energy flux peaks on the nightside, number flux peaks on the dayside. The cusp, as previously reported, is much better defined by ions than electrons. Hence, the ion number flux peak is confined, corresponding to the cusp, while the region with high electron number flux is broad (a cleft, corresponding to the boundary layers, including the closed low-latitude boundary layer).

[1] The only previously established seasonal auroral variation is that of intense monoenergetic aurora, corresponding to quasi-static acceleration by geomagnetic-fieldaligned electric fields. Here we investigate the separate seasonal dependence of both types of electron accelerated aurora (broadband, or wave, in addition to monoenergetic) and both ion and electron diffuse aurora. Dayside and nightside variations are separately considered, as are conditions of low and high solar wind driving. Across these many combinations, several clear patterns emerge. One is that the dayside tends to maximize precipitation in the summer and much more so for low solar wind driving. Nightside precipitation is higher in the winter and much more so for high solar wind driving. The dayside effects are strongest in number flux and stronger in diffuse aurora than accelerated aurora. The ease of ion entry through the summer cusp, along with the constraints of charge quasi-neutrality, and the rise in dayside currents in the summer hemisphere adequately explain much (perhaps all) of the dayside behavior. Nightside effects are more apparent in energy flux, with the winter/summer ratio of monoenergetic aurora being the largest: 1.70 for high solar wind driving. However, both other types of electron aurora, diffuse (1.30) and broadband (1.26), also have winter/summer energy fluxes well above unity for high solar wind driving. The nightside seasonal variation of ions is much smaller, with slightly more energy flux postmidnight in the winter but with slightly higher energy fluxes premidnight in the summer. Since the increased nightside fluxes into the winter hemisphere occur primarily under strong solar wind driving, and are much more prominent in energy flux than number flux, they likely reflect increased energization in the winter ionosphere when stronger currents are being driven into the ionosphere from the magnetosphere. Equinoctial behavior tends to lie between the summer and winter hemisphere values, but typically closer to the latter. As a result, nightside electron energy flux summed over the hemispheres is higher around equinox, simply because there is no summer hemisphere.

[1] The brightest energetic neutral atom (ENA) intensity viewed by spacecraft in highinclination Earth orbit is from low-altitude emission (LAE). It is a prominent feature in the stereo ENA images obtained by cameras on the NASA TWINS 1/2 Mission of Opportunity. This emission is produced by energetic magnetospheric ions precipitating into the atomic oxygen exosphere at latitudes near the auroral zones at altitudes ∼300 km. The ions undergo multiple atomic collisions including charge exchange of ions and stripping of the resulting neutrals. Consequently, this is a "thick target" process. We introduce a "thick-target" approximation that allows us to extract the shape of the spatially averaged spectra of the precipitating ions from the ENA spectra in the TWINS 1/2 pixels viewing the LAE from near their orbital apogees. These ENA-extracted ion spectra are compared with in situ precipitating ion spectra measured concurrently by DMSP satellites (F15 and F16) at ∼825 km altitude, while they are passing directly over the LAE regions in the TWINS 1/2 images. We obtain good agreement between the shape of the ENAextracted and in situ ion spectra from three distinct precipitation regions over energies from 2 to 32 keV (assuming precipitating protons). The absolute normalization of the ENA-extracted and in situ spectra depends upon the TWINS viewing geometry because the ENA LAE source is not resolved by the imager. None of the spectral shapes obtained is consistent with a simple thermal intensity spectrum with kT = 5 keV.

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