T. Sotirelis

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

[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.

The role of small‐scale ion injections in the buildup of Earth's ring current pressure: Van Allen Probes observations of the 17 March 2013 storm

Energetic particle transport into the inner magnetosphere during geomagnetic storms is responsible for significant plasma pressure enhancement, which is the driver of large-scale currents that control the global electrodynamics within the magnetosphere-ionosphere system. Therefore, understanding the transport of plasma from the tail deep into the near-Earth magnetosphere, as well as the energization processes associated with this transport, is essential for a comprehensive knowledge of the near-Earth space environment. During the main phase of a geomagnetic storm on 17 March 2013 (minimum Dst ~ −137 nT), the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on the Van Allen Probes observed frequent, small-scale proton injections deep into the inner nightside magnetosphere in the region L ~ 4 – 6. Although isolated injections have been previously reported inside geosynchronous orbit, the large number of small-scale injections observed in this event suggests that, during geomagnetic storms injections provide a robust mechanism for transporting energetic ions deep into the inner magnetosphere. In order to understand the role that these injections play in the ring current dynamics, we determine the following properties for each injection: (i) associated pressure enhancement, (ii) the time duration of this enhancement, and (iii) the lowest and highest energy channels exhibiting a sharp increase in their intensities. Based on these properties, we estimate the effect of these small-scale injections on the pressure buildup during the storm. We find that this mode of transport could make a substantial contribution to the total energy gain in the storm time inner magnetosphere.

Boundary-oriented electron precipitation model

A boundary-oriented model of the global configuration of electrons precipitating into the polar ionosphere is presented. It provides the differential energy flux of precipitating electrons from 32 eV to 30 keV for five different activity levels. Data from 12 years and eight DMSP spacecraft were incorporated into the model. The defining characteristic of this model is that only observations similarly located relative to auroral boundaries (e.g., observations just equatorward of the open-closed boundary) are averaged together. The model resulting from this approach more closely resembles instantaneous observations than previous efforts. A distinct polar cap surrounds a narrow auroral zone, transitions between different regions are appropriately sharp, and model spectra are more realistic. This increased fidelity with observation is a significant advantage for the model, broadening its applicability. Also new is the calculation of both mean and median model spectra. The mean is dominated by sporadic flux enhancements, where present, while the median resembles more commonly observed background fluxes, permitting both of these aspects to be addressed. Parameterization for activity is based on the degree of magnetotail stretching, as indicated by the latitude of the ion isotropy boundary. A variety of features can be discerned in the model. There is a large difference between the mean and median energy flux in regions where upward region 1 Birkeland currents are commonly observed. The smooth ∼1 to 10 keV precipitation seen at most local times, in the equatorward portion of the oval, is nearly absent in much of the afternoon sector. Enhanced number fluxes are seen at the poleward edge of the oval near midnight, likely due to the frequent presence of field-aligned bursts. Structured precipitation dominates the energy flux at all local times except between dawn and noon, where the contribution from unstructured precipitation dominates. The total hemispheric energy flux due to mean spectra varies with activity from 6 to 38 GW and exceeds the energy flux due to median spectra by a factor of approximately 4, regardless of activity.

Ground‐based optical determination of the b2i boundary: A basis for an optical MT‐index

[1] The equatorward boundary of the proton aurora corresponds to a transition from strong pitch angle scattering to bounce trapped particles. This transition has been identified as the b2i boundary in Defense Meteorological Satellite Program (DMSP) ion data [Newell et al., 1996]. We use ion data from 29 DMSP overflights of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Meridian Scanning Photometer (MSP) located at Gillam, Canada, to develop a simple algorithm to identify the b2i boundary in latitude profiles of proton auroral (486 nm) brightness. Applying this algorithm to a ten year set of Gillam MSP data, we obtain ∼250,000 identifications of the optical b2i, the magnetic latitude of which we refer to as b2i Λ . We intercompare ∼1600 near-simultaneous optical and in situ b2i Λ , concluding that the optical b2i Λ is a reasonable basis for an optical equivalent to the MT-index put forward by Sergeev and Gvozdevsky [1995]. Using ∼17,000 simultaneous measurements, we demonstrate a strong correlation between the optical b2i Λ and the inclination of the magnetic field as measured at GOES 8. We develop an empirical model for predicting the GOES 8 inclination, given theuniversal time, dipole tilt, and the optical b2i Λ , as determined at Gillam. We also show that in terms of information content, the b2i boundary is an optimal boundary upon which to base such an empirical model.

Magnetopause from pressure balance

The shape of the magnetopause and the field due to magnetopause currents are calculated from the requirement that the pressure in the magnetosheath be balanced by magnetic pressure inside the magnetosphere. The field due to magnetopause currents is calculated to be consistent with the iteratively adjusted magnetopause shape. The field due to current systems inside the magnetosphere is taken from the T96 model [Tsyganenko, 1996], which carries information from ∼47,000 magnetic field observations. Many different magnetospheric configurations were found for a variety of conditions. Changes in the shape of the magnetopause with varying dipole tilt angle stood out. The magnetotail and the nose (the point closest to the Sun) were found to shift vertically, in opposite directions, for nonzero dipole tilt. The vertical offset of the nose from the Earth-Sun line varied linearly with dipole tilt angle, reaching ∼3 RE for maximal tilt and having a weak dependence on solar wind dynamic pressure. The formation of a secondary stagnation point just above the sunward cusp was indicated for absolute dipole tilts in excess of 15°. The magnitude of the field strength at its local maximum just behind the cusp was determined as a function of dipole tilt angle and the subsolar field strength. Calculated magnetopause shapes and observed magnetopause crossings were found to be consistent when the tilt angle was taken into account. Variations in the latitude of the magnetic cusps with dynamic pressure, interplanetary magnetic field Bz, and dipole tilt were reasonably consistent with observed variations in the latitude of the particle cusp.

Shape of the open–closed boundary of the polar cap as determined from observations of precipitating particles by up to four DMSP satellites

The shape of the open–closed boundary is studied using auroral oval crossings from up to four DMSP satellites, providing a maximum eight-point determination per hemisphere. The spectra of both precipitating ions and electrons are examined, and boundary crossings are determined by visual inspection. A subset of crossings with between six- and eight-point determinations observed during intervals spanning 15 to 58 min are used to form a cubic spline approximation to the open–closed boundary for each interval. The variability in the size, shape, and location of the boundary is characterized. Approximately half the time the points can be well fit by a circle, but for the remaining intervals the shape is more complex. The speed at which the boundary moves is estimated, and the accuracy of determining the amount of open flux from n observed boundary crossings is tested. When only one point is used to determine the open flux (by assuming an offset circle), the standard deviation of the relative error is 33%, dropping to 16% for a four-point measurement. The variation of the open flux with measures of solar wind coupling is tested and found to be roughly proportional. These findings have implications for space weather applications which require the construction of a data set of open flux.

Auroral precipitation power during substorms: A Polar UV Imager‐based superposed epoch analysis

The Polar UV Imager (UVI) is useful both for determining substorm onset times with precision on the order of 1 min and for making quantitative estimates of global power associated with auroral precipitation. Combining these capabilities, we studied 390 substorms, not necessarily isolated, using the superposed epoch analysis technique. The results quantify the phenomenology of auroral power during substorms. Precipitating auroral power rises with three distinct timescales based on distance from the average onset position. The most dramatic results are seen in the location of the auroral bulge, 2100-0000 magnetic local time (MLT). Integrated over this premidnight sector, auroral power decreases by ∼10% in the few minutes before onset. After onset, premidnight auroral power increases by a factor of 3.4 during the first 9 min. The largest increase occurs within 3 min after onset. After peaking at an average of ∼9 GW, premidnight auroral power declines at a steady 0.045 GW min -1 over ∼100 min. Less dramatic increases with longer risetimes are seen elsewhere on the nightside. Outside the LT of the bulge origin, but close enough that the bulge is likely to eventually reach it, the delay from onset to peak auroral power is ∼15-18 min, and the increase is typically in the range of a factor of 2 or 3. Finally, several hours away from the bulge, auroral power increases by less than a factor of 2, with a time delay of ∼36-45 min. Predawn, this represents the travel time of hot eastward drifting electrons in the diffuse aurora. Postdusk (1800-2100 MLT), a fairly weak (between 1.5 and 2.0 fold) increase is seen for unclear reasons. Altogether, a substorm averages 59% greater auroral power dissipation over the entire nightside in the 12() min following onset than in the preceding 120 min (152 and 96 TJ, respectively). By contrast, dayside auroral power is virtually invariant during substorms. Uncertainty in defining onset time using Polar UVI images was less than the 3-min bin period adopted. Auroral onset is thus demonstrated to be well specified.

Polar Ultraviolet Imager observations of global auroral power as a function of polar cap size and magnetotail stretching

The Polar Ultraviolet Imager (UVI) instrument can quantitatively determine important magnetospheric descriptors, notably including substorm onset time and global auroral output. Previous research has related the input variables of the magnetospheric system, namely solar wind parameters, to various output variables. However, a complex system such as the magnetosphere includes, in addition to inputs and outputs, state variables. Polar cap flux and magnetotail stretching are two such that can be estimated from the Defense Meteorological Satellite Program (DMSP) series satellites. We herein determine that both polar cap flux, Φc, and the magnetotail stretching index, b2i, do correlate well with 40-min averages of nightside auroral power observed by UVI. There were a total of n = 638 distinct 40-min intervals within which b2i, Φc, and nightside auroral power could be determined. The correlations with premidnight auroral power were r = 0.72 for Φc and r = −0.76 for b2i. The multiple-correlation coefficient of these two variables with nightside auroral power was 0.81. These sample correlations are far better than the sample correlations of solar wind input variables to nightside auroral power. Thus accurate space weather forecasting can demonstrably benefit greatly by monitoring current magnetospheric state variables (nowcasting), rather than attempting to reproduce output variables solely from solar wind inputs. Attempts to predict substorm onsets were less successful. Although the average polar cap flux prior to onset is larger than normal, the difference is not large enough to have significant predictive capability. Specifically, polar cap flux averaged 404 ± 133 and 422 ± 148 MWb for the entire years of 1996 and 1997, respectively, while the polar cap flux at the time of substorm onset averaged 455 ± 143 MWb.

Source region of 1500 MLT auroral bright spots : Simultaneous Polar UV-images and DMSP particle data

We compare auroral images from the Polar ultraviolet imager (UVI) and simultaneous particle observations from the Defense Meteorological Satellite Program (DMSP) in the afternoon (1300 – 1600 MLT) sector along the oval in the northern hemisphere to determine the magnetospheric source region of postnoon auroral bright spots. Auroral bright spots are determined with Polar UVI, while their magnetospheric source regions are determined from DMSP F13 particle data. A total of 65 events of good temporal and spatial coincidence were identified after searching through over 1 year of data, from April 1996 to June 1997. Instances occur of auroral arcs mapping to each of several different regions, including the plasma sheet, the low-latitude boundary layer, and the plasma mantle. However, our results indicate that ∼2/3 of the time the most prominent auroral arcs are associated with plasma sheet electron precipitation and slightly less than 1/3 of the time they are found to occur near (less than 1° in magnetic latitudes) the boundary between the plasma sheet and other regions.

Statistical relationship between large-scale upward field-aligned currents and electron precipitation

Simultaneous observations of Birkeland currents by the constellation of Iridium satellites and N2 Lyman-Birge-Hopfield (LBH) auroral emissions measured by the Global Ultraviolet Imager (GUVI) onboard the Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics (TIMED) satellite are used to establish relationships between large-scale upward field-aligned currents and electron precipitation during stable current configurations. The electron precipitation was inferred from GUVI data using a statistical relationship between LBH intensity and electron energy flux. LBH emissions with >5% contribution from protons, identified by Lyman-alpha intensity, were excluded from the analysis. The Birkeland currents were derived with a spatial resolution of 3° in latitude and 2 h in local time. For southward interplanetary magnetic field (IMF), the electron precipitation occurred primarily within and near large-scale upward currents. The correspondence was less evident for northward IMF, presumably because the spatial variability is large compared to the areas of interest so that the number of events identified is smaller and the derived statistical distributions are less reliable. At dusk, the correlation between upward current and precipitation was especially high, where a larger fraction of the electron precipitation is accelerated downward by a field-aligned potential difference. Unaccelerated electron precipitation dominated in the morning sector, presumably induced by scattering of eastward-drifting energetic electrons into the loss cone through interaction with whistler-mode waves (diffuse precipitation) rather than by field-aligned acceleration. In the upward Region 1 on the dayside, where the electron precipitation is almost exclusively due to field-aligned acceleration, a quadratic relationship between current density and electron energy flux was observed, implying a linear current-voltage relationship in this region. Current density and electron energy flux in the regions of the large-scale upward currents from pre-midnight through dawn to noon are essentially uncorrelated, consistent with diffuse electron precipitation dominating the incident energy flux.