P. T. Newell

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.

Seasonal effects on auroral particle acceleration and precipitation

Global auroral images acquired from the Polar ultraviolet imager in the Northern Hemisphere during the winter of 1996 and the summer of 1997 (4 weeks before and after solstice) are used to study seasonal effects on auroral acceleration and precipitation. The energy flux of precipitating electrons is inferred from auroral luminosity in the long-wavelength bands (1600-1800 A) of N 2 Lyman-Birge-Hopfield (LBH1) auroral emissions, and the mean energy of precipitating electrons is inferred from the intensity ratio of LBH1 to LBHs (1400-1600 A. the shorter wavelength of LBH bands) auroral emissions. Results indicate that dayside and nightside regions of aurora reveal different seasonal effects: nightside (∼1900-0300 MLT) auroral power is suppressed in summer, while dayside auroral power is enhanced in summer and forms the so-called postnoon auroral hot spots, all by a factor of ∼2. The average energy of precipitating electrons is higher in the dark than in the sunlit hemisphere, while the number flux is lower in the dark than in the sunlit hemisphere for all regions. These changes, up to a factor of ∼3, are local time and latitude dependent. The suppression of the nightside auroral power in summer is associated with a large decrease in the electron energy, whereas the enhancement of dayside aurora in summer is associated with a large increase in the electron number flux. The increase of dayside auroral power in summer may be associated with the large-scale upward field-aligned currents, which peak in summer. Results are also discussed in the context of a conductivity feedback instability and a cyclotron maser instability. The asymmetric seasonal effects on the dayside and nightside auroras suggest a voltage generator for the dayside magnetosphere and a current generator for the nightside magnetosphere.

Evaluation of low‐latitude Pi2 pulsations as indicators of substorm onset using Polar ultraviolet imagery

Impulsive Pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes. It is well documented that there is usually more than one Pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm. This observational fact poses obstacles to determining substorm onsets with Pi2 signals. Although the Pi2 have become one of the most popular indicators for substorm onsets, the reliability of using the Pi2 in this fashion has not been seriously investigated. In this paper we address this question with a statistical approach by using ∼650 auroral substorm onsets identified with Polar ultraviolet images for a time interval from April 1996 to May 1997. A comparison of the low-latitude Pi2 pulsation onsets from Kakioka (L = 1.07) with the auroral breakups indicates that identifying substorm onset with the Pi2 alone is often ambiguous. Of a total of 119 isolated (defined as ∼10 min of quiet time preceding the onset) Pi2 bursts seen within ∼10 min from a magnetic positive bay, there were 65 events (∼55%) taking place within 3 min from breakups and 34 events (29%) indicating no sign of an auroral breakup within 10 min of the Pi2 burst. This result suggests that Pi2 may not be as a good indicator of the substorm onset as it was thought to be. Interestingly, it is always possible to associate Pi2 pulsations with some forms of auroral intensification. When compared to auroral breakups, Pi2 onsets are subject to a small delay of 1 – 3 min, with a peak around l min. Delays of Pi2 onsets are revealed to be a function of location relative to auroral breakup. This dependence is found to be consistent with the time of flight for a fast-mode wave, in a plasmapause cavity mode model, propagating in the magnetosphere.

Relationship between Birkeland current regions, particle precipitation, and electric fields

Data from eight DMSP F7 satellite passes coincident with Sondrestrom radar observations have been examined to determine how the large-scale dayside Birkeland currents are related to the particle precipitation regions and to the convection pattern. The classification schemes recently developed from DMSP particle data were adopted. The observations were limited to the prenoon local time hours and led to the following conclusions: (1) The local time of the mantle currents (which were traditionally called cusp currents) is not limited to the longitude of the cusp proper, but covers a larger local time extent. (2) The mantle currents flow entirely on open field lines (where “open field lines” is defined as a region where the ion precipitation and electron precipitation have the characteristics of plasma mantle, cusp, or polar rain.) This confirms and extends to all local times similar results obtained from other observations. (3) About half of region 1 currents flow on open field lines. This is consistent with the assumption that the region 1 currents are generated by the solar wind dynamo and flow within the surface that separates open and closed field lines. (4) More than 80% of the Birkeland current boundaries do not correspond to particle precipitation boundaries. Region 2 currents extend beyond the plasma sheet poleward boundary; region 1 currents flow in part on open field lines; mantle currents and mantle particles are not coincident. (5) On most passes when a triple current sheet is observed (region 2, region 1, and mantle currents), the convection reversal is located on closed field lines. When only two current sheets are observed (either region 2/region 1, or region 1/mantle currents), the convection reversal is on open field lines. (6) The data appear to be more consistent with a topology such that mantle currents are an extension of region 1 currents, rather than a separate system located poleward of the region 1 current system.

Substorm and convection bay compared: Auroral and magnetotail dynamics during convection bay

Using observations from eight spacecraft and a ground network, we study two subsequent bay-like disturbances on December 10, 1996, initiated by southward interplanetary magnetic field intervals, one being a classic substorm and another one a convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2 pulsations in the auroral zone. Contrasting to the well-defined substorm signatures of the first event (poleward auroral expansion, substorm current wedge, strong particle injection to 6.6 R E ) resulting from energy loading/unloading and near-Earth reconnection in the tail, these signatures were virtually absent during the convection bay (CB). Distinctive features of the CB event were the same as those during the Steady Magnetospheric Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed lobe field, and enhanced magnetic flux closure (large B z ) and multiple bursty earthward flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 R E ; (4) auroral streamers associated with both BBFs and narrow injections. We emphasize the development of multiple and sporadic auroral streamers which start at the poleward oval boundary, propagate equatorward (in 3-8 min) and end with a long-duration bright spot in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during the convection bay was formed by sporadic narrow (a few R E wide) plasma streams (plasma bubbles) which transported the plasma sheet material from the distant magnetotail reconnection regions to the inner magnetosphere and may significantly contribute to the magnetospheric circulation on the nightside. We modeled the nightside tail configuration using magnetotail magnetic observations and low-altitude particle boundaries to show that at the beginning of the convection bay the increase of magnetic flux tube volume with distance was small in the midtail. Therefore the pressure crisis in the tail was significantly reduced during the convection bay, and the efficient earthward transport by sporadic narrow plasma streams was probably able to balance the magnetospheric circulation to avoid the large-scale instability of the magnetotail.

The quantitative relationship between auroral brightness and solar EUV Pedersen conductance

A quantitative relationship between auroral brightness and solar EUV produced Pedersen conductance is established by using Lyman-Birge-Hopfleld long and short bands of auroral emissions from Polar Ultraviolet Imager (UVI). We used a conductance model characterized by solar zenith angle, solar F10.7 radio flux, and the local magnetic field to estimate solar EUV Pedersen conductance for each pixel of the images. The hourly average IMP 8 solar wind data set is used to derive corresponding solar wind conditions. Pixels of the auroral brightness in Polar UVI images have been binned in terms of magnetic latitude and local time. We relate the auroral brightness to the conductance and find that a positive correlation occurs in the early morning sector, indicating increasing brightness with increasing conductance. An anticorrelation appears in the premidnight region, supporting the previously reported suppression of the discrete aurora occurrence in sunlight. The correlation becomes stronger for the southward interplanetary magnetic field (IMF) condition than for the all Bz condition in the early morning and premidnight sectors, indicating that the conductance effect on the auroral brightness becomes significant when the IMF is southward. The afternoon auroral bright spot is not eminent when the ionospheric conductance is low.

Characteristics of ionospheric convection and field-aligned current in the dayside cusp region

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high-latitude ionospheric convection and field-aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field-aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field-aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field-aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field-aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field-aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi-steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

Four large-scale field-aligned current systems in the dayside high-latitude region

A system of four current sheets of large-scale field-aligned currents (FACs) was discovered in the data set of simultaneous Viking and DMSP-F7 crossings of the dayside high-latitude region. This paper reports four examples of this system that were observed in the prenoon sector. The flow polarities of FACs are upward, downward, upward, and downward, from equatorward to poleward. The lowest-latitude upward current is flowing mostly in the CPS precipitation region, often overlapping with the BPS at its poleward edge, and is interpreted as a region 2 current. The pair of downward and upward FACs in the middle of the structure are collocated with structured electron precipitation. The precipitation of high-energy (>1 keV) electrons is more intense in the lower-latitude downward current sheet. The highest-latitude downward flowing current sheet is located in a weak, low-energy particle precipitation region, suggesting that this current is flowing on open field lines. Simultaneous observations in the postnoon local time sector reveal the standard three-sheet structure of FACs, sometimes described as region 2, region 1, and mantle (referred to the midday region 0) currents. A high correlation was found between the occurrence of the four FAC sheet structure and negative interplanetary magnetic field (IMF) By. We discuss the FAC structure in terms of three types of convection cells: the merging, viscous, and lobe cells. During strongly negative IMF By, two convection reversals exist in the prenoon sector; one is inside the viscous cell, and the other is between the viscous cell and the lobe cell. This structure of convection flow is supported by the Viking electric field and auroral UV image data. Based on the convection pattern, the four FAC sheet structure is interpreted as the latitudinal overlap of midday and morning FAC systems. We suggest that the four-current sheet structure is common in a certain prenoon local time sector during strongly negative IMF By.