K. Liou

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.

Earthward flow bursts, auroral streamers, and small expansions

Earthward flow bursts associated with small auroral expansions, including pseudobreakups, and auroral streamers are studied by using Geotail plasma and magnetic field data and Polar ultraviolet imager data. These flow bursts are accompanied by dipolarization and decrease in the plasma pressure, which are consistent with the characteristics of so-called bubbles, and have a timescale of 2.5 min on average. Based on a statistical study of the flow bursts, it is shown that the location of the flows are centered about 0.4 hour magnetic local time east of the center of auroral expansion and are localized with a width of 3 – 5 RE. This relationship supports the idea that a dawn-to-dusk polarization electric field is created in the bubble to enhance the flows. The flow bursts associated with the small expansions, which are mainly observed in the region earthward of 15 RE, show more distinct signatures of compression at the front side of the flow, which possibly leads to the stopping of these flows. Flow bursts related to auroral streamers, which are observed mainly tailward of 15 RE, take place during relatively thick plasma sheet configurations, and are accompanied by stronger flow shear.

Multiple-spacecraft observation of a narrow transient plasma jet in the Earth's plasma sheet

We use observations from five magnetospheric spacecraft in a fortuitous constellation to show that narrow transient plasma flow jets of considerable length formed in the tail can intrude into the inner magnetosphere and provide considerable contribution to the total plasma transport. A specific auroral structure, the auroral streamer, accompanied the development of this narrow plasma jet. These observations support the ‘boiling’ plasma sheet model consisting of localized underpopulated plasma tubes (bubbles) moving Earthward at high speeds as a realistic way to resolve the ‘convection crisis’ and to close the global magnetospheric circulation pattern.

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Estimation of global field aligned currents using the iridium® System magnetometer data

The Iridium® System satellite constellation consists of 66 satellites in circular, polar, 780 km altitude orbits in six equally spaced planes. Each satellite carries an engineering magnetometer which has sufficient resolution to sense the Birkeland currents. This paper presents a spherical harmonic fitting (SHF) technique for estimating field aligned currents (FACs) using the cross track component of the magnetic field measurements from the Iridium satellites. The SHF magnetic field perturbations along with Amperes law are used to derive the global FACs from 40° MLAT to the pole in either hemisphere. Data for 10–11 UT, 23 August, 1999 were obtained from Iridium, Defense Meteorology Satellite Program (DMSP) Fl3 and the Ultraviolet Imager (UVI) onboard the POLAR satellite. The SHF data were evaluated along the DMSP F13 track. The range of east-west component, SHF magnetic perturbations from Iridium was (−530,465) nT compared with ( −630,634) nT from DMSP F13. The derived upward FACs ranged from −0.8 to 0.9 µAm−2. Upward FACs were co-located with bright dayside UVI emissions.

Development of auroral streamers in association with localized impulsive injections to the inner magnetotail

During continuous magnetospheric activity it is not uncommon to observe narrow (in MLT) transient particle injections (duration about 1–2 minute at E=100 keV and local time extent ≤ 1 hour MLT) in the nightside part of geosynchronous orbit. Using global UV images from POLAR spacecraft we analyze the development of auroral activity on December 22, 1996 during a sequence of such injections observed by two LANL spacecraft. We found that narrow transient injections are associated with specific localized auroral form, the auroral streamer, which develops in this local time sector. The streamer first appear as a bright spot in the poleward part of the double oval ≈2–5 minutes before the geosynchronous plasma injection, and then develops equatorward, reaching in many cases the equatorward boundary of the UV aurora. We interprete the observations as evidence that some high speed flow bursts (BBFs) of small cross-tail extent (less than 1 h MLT), formed in the distant tail or midtail, can intrude as close to the Earth as the geosynchronous distance before being stopped.

Flow bursts and auroral activations: Onset timing and foot point location

Flow burst events with a flux transfer rate exceeding 2 mV/m and with a duration of less than 10 min observed by Geotail are compared with auroral signatures obtained from the Polar ultraviolet imager. It is shown that all the flow bursts correspond either to localized auroral intensifications associated with small poleward expansions and pseudobreakups or to an activation starting at the poleward edge of the expanded auroral oval that develop equatorward toward the foot point of the satellite, including auroral streamers. Earthward flow bursts related to pseudobreakups and small expansions are observed mainly in the region earthward of 15 RE, more inward than those flows related to high-latitude auroral activations and auroral streamers. Although most of these auroral activations precede the observations of the flow bursts by a few minutes, the activations that break up near the foot point of the satellite start typically within ±1 min of the onset of flow burst observation.

Observation of IMF and seasonal effects in the location of auroral substorm onset

We use Polar ultraviolet imager (UVI) and Wind observations to study the location of 648 well-defined Northern Hemisphere auroral breakups (substorm onsets) in response to interplanetary magnetic field (IMF) orientation and season. The most likely onset location is at 2230 MLT and 67° Λm with half-maximum widths of 3 hours of MLT and 2° Λm, respectively. The onset latitude depends primarily on IMF Bz, but also Bx: the onset latitude decreases for Bx > 0 or Bz 0. The onset longitude depends on season and IMF By. In summer, substorms tend to occur in the early evening at ∼2200 MLT, whereas in winter they tend to occur near midnight at ∼2300 MLT. The average summer-winter difference in the onset location is ∼1 hour of MLT. Large By effects on the onset longitude occur only when Bx and By are small. Onset locations shift toward earlier local times for By > 0 and toward midnight for By 0 in summer and latest (2330 MLT) for By 0 the onset location shifts toward dusk when By > 0 but toward dawn when By < 0; the sense of this shift reverses for Bx < 0. An implication of the results is that auroral breakup is not conjugate.

Synoptic auroral distribution : A survey using Polar ultraviolet imagery

The global distribution of the ultraviolet auroral emission was investigated for the period between April and July 1996 using over 17,000 imagery acquired by the ultraviolet imager (UVI) on board the Polar satellite. Average brightness of the N2 Lyman-Birge-Hopfield (LBH) auroral emissions at 1700 A, which is approximately proportional to the total energy flux of precipitating electrons, was calculated with dayglow subtracted. The results of this investigation indicate that there exist two distinctive auroral emission regions, one in the premidnight sector of the auroral oval and one in the postnoon sector of the auroral oval. The maximum occurrence of nightside aurorae is found to be centered at 2230 magnetic local time (MLT) and 68° magnetic latitude (MLAT) while the dayside aurorae maximize at both 1500 MLT and 75° MLT and 1000 MLT and 75° MLAT, with the later one much weaker. This statistical auroral distribution is quite similar to previously reported distribution of discrete aurorae, suggesting that at this wavelength and at the sensitivity of the UVI detector, discrete aurorae contribute a major portion of the total emissions. The seasonal distribution of the nightside LBH auroral emissions is found to be consistent with previously reported particle result, namely nightside discrete auroral activities are more common in the dark hemisphere (winter) than in the sunlit hemisphere (summer). However, on the dayside part of auroral oval, auroral emissions are brighter in summer than in spring. The dayside auroral emissions, in particular the 1500 MLT bright spots, are also found to be correlated with the maximum region 1 upward field-aligned currents which are most intense in summer because of a higher ionospheric conductivity produced by photoionization in the dayside region. These results point up the controlling role played by ionospheric conductivity and further illustrate how dayside and nightside aurorae behave in fundamentally different ways.

Characteristics of the solar wind controlled auroral emissions

We performed a high-time resolution (5 min) correlative study of the energy deposition rate in the northern auroral zone with the concurrent solar wind plasma and interplanetary magnetic field (IMF) observations for a 4 month period from March 30 to July 29, 1996. Auroral power, inferred by auroral emissions, was derived from images acquired by the ultraviolet imager (UVI) on board the Polar satellite, and the interplanetary parameters were based on Wind observations. It is found that dayside aurorae in the afternoon sector (65°–80° magnetic latitude (MLAT) and 1300–1800 magnetic local time (MLT)) are more active for large IMF cone angles and large solar wind electric fields. This result can be attributed to the manifestation of the antiparallel magnetic field merging in different locations and the partial “penetration” of the IMF on the dayside magnetopause. The integrated nightside (60°–75° MLAT and 2000–0100 MLT) auroral brightness is moderately correlated with the north–south component of the IMF and the solar wind speed with correlation coefficients of 0.49 and 0.35, respectively. The mean nightside auroral power is found to be approximately linearly proportional to the IMF Bz with a constant slope of 2 GW/nT. The solar wind speed, however, affects the nightside auroral power for both polarities of IMF Bz. Interestingly, the solar wind dynamic pressure shows no effect on the nightside auroral brightness. All these findings indicate that both reconnection and viscous-like interaction mechanisms play an important role in producing auroral emissions in the night sector. It is also found that the nightside auroral brightness responds to the southward turning of the IMF with a peak delay time of ∼60 min. This result favors the model of loading-unloading magnetosphere. We also found that a negative IMF By condition favors the nightside auroral activity, and we attributed this effect to the partial penetration of the IMF By. Finally, the response function for nightside aurora is given as ∼VB4Tsin4θc2) with a median correlation coefficient of 0.63, indicating that there may be other factors other than the solar wind and IMF responsible for lightening up the northern–southern hemispheric sky.