The Super Dual Auroral Radar Network (SuperDARN) has been operating as an international co-operative organization for over 10 years. The network has now grown so that the fields of view of its 18 radars cover the majority of the northern and southern hemisphere polar ionospheres. SuperDARN has been successful in addressing a wide range of scientific questions concerning processes in the magnetosphere, ionosphere, thermosphere, and mesosphere, as well as general plasma physics questions. We commence this paper with a historical introduction to SuperDARN. Following this, we review the science performed by SuperDARN over the last 10 years covering the areas of ionospheric convection, field-aligned currents, magnetic reconnection, substorms, MHD waves, the neutral atmosphere, and E-region ionospheric irregularities. In addition, we provide an up-to-date description of the current network, as well as the analysis techniques available for use with the data from the radars. We conclude the paper with a discussion of the future of SuperDARN, its expansion, and new science opportunities.

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A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

[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(A 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 dΦ Mp / dt = v 4/3 B T 2/3 sin 8/3 (θ c /2), calculated from (rate IMF field lines approach the magnetopause, ∼v)(% of IMF lines which merge, sin 8/3 (θ c /2))(interplanetary field magnitude, B T )(merging line length, ∼(B MP /B T ) 1/3 ). 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 dΦ 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), dΦ 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 d ΦMP /dt is Dst, which correlates best (r = 0.87) with p 1/2 dΦ MP /dt. If dΦ MP /dt were credited with this success, its average score would be even higher.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JA012015

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

A model for the formation of the low-boundary layer in which a magnetosheath flux tube reconnects in the north and south beyond the cusp when the IMF is strongly northward is presented. For northward IMF the geomagnetic field captures solar wind flux tubes through intermittent reconnection at the cusp region. These newly captured flux tubes shorten and sink into the magnetosphere while the flux tubes reorient themselves as they become assimilated into the magnetosphere. There is no significant acceleration for the particles within the flux tubes. In the magnetosphere the interchange instability disperses the flux tube azimuthally along the magnetopause to form a boundary layer. Subsequent reconnection forms sublayers of the boundary layer and different sublayers represent different ages after reconnection. The interchange instability is stable radially, which keeps sharp boundaries between sublayers and between the boundary layer and magnetosphere.

Model of the formation of the low-latitude boundary layer for strongly northward interplanetary magnetic field

We examine 65 ionospheric convection changes associated with changes in the Y and Z components of the interplanetary magnetic field (IMF). We measure the IMF reorientations (for all but six of the events) at the Wind satellite. For 22 of the events the IMF reorientation is clearly observed by both Wind and IMP 8. Various methods are used to estimate the propagation time of the IMF between the two satellites. We find that using the magnetic field before the IMF orientation change gives the smallest error in the expected propagation time. The IMF is then propagated to the magnetopause. The communication time between when the IMF encounters the magnetopause and the start of the convection change is estimated to be 8.4 (±8.2) min. The resulting change in the ionospheric potential is examined by subtracting a base potential pattern from the changing potential patterns. From these residual patterns, a number of conclusions are made: (1) the location of the change in convection is stationary, implying that the change in convection is broadcast from the cusp region to the rest of the ionosphere in a matter of seconds and that the elctric field mapped down the cusp controls the entire dayside ionospheric convection pattern; (2) the shape of the change in the ionospheric convection is dependent on the IMF component that changes, which is indicative of the change in the merging rate on the dayside magnetopause; (3) 62% of the events change linearly form one state to another, while 11% of the events change asymptotically; (4) the change in the ionospheric potential is linearly related to the magnitude of the IMF orientation, with Bz changes having a larger proportionality constant than By changes; (5) the ionospheric convection takes, on average, 13 min to completely reconfigure; and (6) some of the ionospheric convection changes occur on a timescale shorter than that of the corresponding IMF reorientation, possibly as a result of thresholding in the dayside merging region.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JA03328

A statistical study of the ionospheric convection response to changing interplanetary magnetic field conditions using the assimilative mapping of ionospheric electrodynamics technique

This study employs observations from several sources to determine the location of the polar cap bound- ary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral im- ages from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the mag- netosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, al- lowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and de- creases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to de- crease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap bound- ary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times sup- ports this finding.

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Variations in the polar cap area during two substorm cycles

Dayside and nightside merging rates usually differ; first one dominates and then the other to maintain long-term flux balance. The polar cap, defined as the area in the ionosphere penetrated by open field lines, expands when closed field lines open during dayside merging and contracts when open field lines close during tail merging. The patterns of convection, electric field and current for polar cap inflation and deflation differ from those of steady state convection. This report models the flow and electrical parameters of polar cap inflation and deflation. We treat the idealized case of uniform conductivity, a planar ionosphere, and “pure Bz” merging (i.e., no By effect, although we note how the approach used here is readily modified to account for the By effect). For this idealized case, the potential associated with polar cap inflation and deflation is seen to be described by the same equations that govern the two-dimensional hydrodynamic flow into and out of an expanding and contracting cylinder with a gap in its side. The gap corresponds to the breach in the polar cap boundary through which magnetic flux passes as a result of merging. The calculated potential pattern displays the usual two cell convection configuration, but the foci of the two cells are the edges of the flux gaps. Thus the cells are displaced sunward during dayside merging and tailward during tail merging.

T. S. Huang

Papers

Polar cap inflation and deflation