L. R. Lyons

Association between Geotail plasma flows and auroral poleward boundary intensifications observed by CANOPUS photometers

Poleward boundary intensifications are nightside geomagnetic disturbances that have an auroral signature that moves equatorward from the poleward boundary of the auroral zone. They occur repetitively, so that many individual disturbances can occur during time intervals of ∼1 hour, and they appear to be the most intense auroral disturbance at times other than the expansion phase of substorms. We have used data from three nightside conjunctions of the Geotail spacecraft in the magnetotail with the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array in central Canada to investigate the relation between the poleward boundary intensifications and bursty plasma sheet flows and to characterize the bursty flows associated with the disturbances. We have found a distinct difference in plasma sheet dynamics between periods with, and periods without, poleward boundary intensifications. During periods with identifiable poleward boundary intensifications, the plasma sheet has considerable structure and bursty flow activity. During periods without such poleward boundary intensifications, the plasma sheet was found to be far more stable with fewer and weaker bursty flows. This is consistent with the intensifications being the result of the mapping to the ionosphere of the electric fields that give rise to bursty flows within the plasma sheet. Two different types of plasma sheet disturbance have been found to be associated with the poleward boundary intensifications. The first consists of plasma sheet flows that appear to be the result of Speiser motion of particles in a localized region of thin current sheet. The second, seen primarily in our nearest-to-the-Earth example, consists of energy-dispersed ion structures that culminate in bursts of low-energy ions and isotropic low-energy electrons and are associated with minima in magnetic field and temperature and maxima in ion density and pressure. Both types of plasma sheet disturbance are associated with localized regions of enhanced dawn-to-dusk electric fields and appear to be associated with localized enhanced reconnection. Our analysis has shown that poleward boundary intensifications are an important aspect of geomagnetic activity that is distinct from substorms. In addition to their very distinct auroral signature, we have found them to be associated with a prolonged series of ground magnetic Pi 2 pulsations and ground X component perturbations, which peak at latitudes near the ionospheric mapping of the magnetic separatrix, and with a series of magnetic Bz oscillations near synchronous orbit. Like substorms, the tail dynamics associated with the poleward boundary intensifications can apparently extend throughout the entire radial extent of the plasma sheet. Color versions of figures are available at http://www.atmos.ucla.edu/∼larry/geotail.html.

Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations

[1] A critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all‐sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north‐south (N‐S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre‐existing open‐closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward‐transport of the new plasma leads to a near‐Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere‐ionosphere electrodynamic coupling, which may play an important role in the motion of pre‐onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre‐onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well‐developed region 2 current system within the duskside convection cell, and also a high probability of diffuse‐appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

Identifying the Driver of Pulsating Aurora

Auroral Chorus Energetic particles that arrive from near-Earth space produce photon emissions—the aurora—as they bombard the atmosphere in the polar regions. The pulsating aurora, which is characterized by temporal intensity variations, is thought to be caused by modulations in electron precipitation possibly produced by resonance with electromagnetic waves in Earths magnetosphere. Nishimura et al. (p. 81) present a detailed study of an event that showed a good correlation between the temporal changes in auroral luminosity and chorus emission—a type of electromagnetic wave occurring in Earths magnetosphere. The results points to chorus waves as the driver of the pulsating aurora. Correlations are found between aurora light intensity and a type of electromagnetic wave in Earth’s magnetosphere. Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

Electron pitch-angle diffusion driven by oblique whistler-mode turbulence

A general description of cyclotron harmonic resonant pitch-angle scattering is presented. Quasi-linear diffusion coefficients are prescribed in terms of the wave normal distribution of plasma wave energy. Numerical computations are performed for the specific case of relativistic electrons interacting with a band of low frequency whistler-mode turbulence. A parametric treatment of the wave energy distribution permits normalized diffusion coefficients to be presented graphically solely as a function of the electron pitch-angle. The diffusion coefficients generally decrease with increasing cyclotron harmonic number. Higher harmonic diffusion is insignificant at very small electron pitch-angles, but becomes increasingly important as the pitch-angle increases. One thus expected the rate of pitch-angle scattering to decrease with increasing electron energy, since the resonant value of the latter varies proportionately with harmonic number. This indicates that, in mirror-type magnet field geometrics, such as the earths radiation belts, the diffusion losses of high energy electrons are likely to be appreciably slower than those at low energy.

Magnetospheric reconnection driven by solar wind pressure fronts

Abstract. Recent work has shown that solar wind dynamic pressure changes can have a dramatic effect on the particle precipitation in the high-latitude ionosphere. It has also been noted that the preexisting interplanetary magnetic field (IMF) orientation can significantly affect the resulting changes in the size, location, and intensity of the auroral oval. Here we focus on the effect of pressure pulses on the size of the auroral oval. We use particle precipitation data from up to four Defense Meteorological Satellite Program (DMSP) spacecraft and simultaneous POLAR Ultra-Violet Imager (UVI) images to examine three events of solar wind pressure fronts impacting the magnetosphere under two IMF orientations, IMF strongly southward and IMF Bz nearly zero before the pressure jump. We show that the amount of change in the oval and polar cap sizes and the local time extent of the change depends strongly on IMF conditions prior to the pressure enhancement. Under steady southward IMF, a remarkable poleward widening of the oval at all magnetic local times and shrinking of the polar cap are observed after the increase in solar wind pressure. When the IMF Bz is nearly zero before the pressure pulse, a poleward widening of the oval is observed mostly on the nightside while the dayside remains unchanged. We interpret these differences in terms of enhanced magnetospheric reconnection and convection induced by the pressure change. When the IMF is southward for a long time before the pressure jump, open magnetic flux is accumulated in the tail and strong convection exists in the magnetosphere. The compression results in a great enhancement of reconnection across the tail which, coupled with an increase of magnetospheric convection, leads to a dramatic poleward expansion of the oval at all MLTs (dayside and nightside). For near-zero IMF Bz before the pulse the open flux in the tail, available for closing through reconnection, is smaller. This, in combination with the weaker magnetospheric convection, leads to a more limited poleward expansion of the oval, mostly on the nightside. Key words. Magnetospheric physics (solar windmagnetosphere interactions; magnetospheric configuration and dynamics; auroral phenomena)

Substorm triggering by new plasma intrusion: Incoherent‐scatter radar observations

Received 4 December 2009; revised 16 March 2010; accepted 30 March 2010; published 27 July 2010. [1] In the companion paper, we identified a repeatable sequence of events leading to substorm onset in THEMIS all‐sky imager observations: enhanced flows bring new plasma into the plasma sheet. The new plasma then moves earthward as a flow channel, bringing it to the near‐Earth plasma sheet and where it produces onset instability. New plasma entering the dusk (dawn) convection cell drifts equatorward and eastward and then around the Harang reversal, leading to pre‐midnight (near‐ and post‐midnight) onset. Here we present evidence supporting this sequence using incoherent scatter radar (ISR) ionospheric observations. Using the Sondrestrom ISR, we find that enhanced flows of new plasma commonly enter the plasma sheet from the polar cap ∼8 min prior to onset. These flows are related to poleward boundary intensification signatures, consistent with the inferences from the imagers. Using the Poker Flat ISR (PFISR), we find that shortly before onset, enhanced westward flows reach the subauroral polarization streams (SAPS) region equatorward of the Harang reversal (dusk‐cell onsets) or enhanced eastward flows enter the onset region from the poleward direction (dawn‐cell onset). PFISR proton precipitation signatures are consistent with the possibility that the enhanced flows consist of reduced‐entropy plasma sheet plasma, and that onset occurs poleward of much of the enhanced SAPS flow (dusk‐cell onsets) or equatorward of the enhanced eastward flows (dawn‐cell onsets). Consistency with reduced entropy plasma is seen only within the enhanced flows, leading us to suggest that intrusion of low‐entropy plasma may alter the radial gradient of entropy toward onset instability.

Geosynchronous magnetic field response to solar wind dynamic pressure pulse

The present study examines the morning-afternoon asymmetry of the geosynchronous magnetic field strength on the dayside (magnetic local time [MLT] = 06:00~18:00) using observations by the Geostationary Operational Environmental Satellites (GOES) over a period of 9 years from February 1998 to January 2007. During geomagnetically quiet time (Kp < 3), we observed that a peak of the magnetic field strength is skewed toward the earlier local times (11:07~11:37 MLT) with respect to local noon and that the geosynchronous field strength is larger in the morning sector than in the afternoon sector. That is, there is the morning-afternoon asymmetry of the geosynchronous magnetic field strength. Using solar wind data, it is confirmed that the morning-afternoon asymmetry is not associated with the aberration effect due to the orbital motion of the Earth about the Sun. We found that the peak location of the magnetic field strength is shifted to ward the earlier local times as the ratio of the magnetic field strength at MLT = 18 (B-dusk) to the magnetic field strength at MLT = 06 (B-dawn) is decreasing. It is also found that the dawn-dusk magnetic field median ratio, B-dusk/B-dawn, is decreasing as the solar wind dynamic pressure is increasing. The morning-afternoon asymmetry of the magnetic field strength appears in Tsyganenko geomagnetic field model (TS-04 model) when the partial ring current is included in TS04 model. Unlike our observations, however, TS-04 model shows that the peak location of the magnetic field strength is shifted toward local noon as the solar wind dynamic pressure grows in magnitude. This may be due to that the symmetric magnetic field associated with the magnetopause current, strongly affected by the solar wind dynamic pressure, increases. However, the partial ring current is not affected as much as the magnetopause current by the solar wind dynamic pressure in TS-04 model. Thus, our observations suggest that the contribution of the partial ring current at geosynchronous orbit is much larger than that expected from TS-04 model as the solar wind dynamic pressure increases.

Accuracy of using 6300 Å auroral emission to identify the magnetic separatrix on the nightside of Earth

Ground observations of 6300 A auroral emission at the polar cap boundary are studied to determine the accuracy with which the latitudinal profile of emission intensity can be used to identify the separatrix. Meridian scanning photometers at Rankin Inlet and Gillam provide the observations of 6300 A emission, and the separatrix determinations obtained from the photometer data are compared with those determined from DMSP F9 precipitating particle data obtained within 1.5 hours in MLT of the ground stations. We assume that the separatrix lies at the poleward edge of boundary plasma sheet precipitation. We find that the average intensity of 6300 A in the polar cap is fairly uniform at 60 R. In the auroral zone the average emission is fairly uniform at 170 R. On the basis of the efficiency and accuracy of separatrix identification, choosing a threshold of 110 R provides the best identification of the polar cap boundary. The rms error in this identification is 1.2°, and the boundary is located in 54% of the cases. The latitudinal gradient of the emission intensity is also investigated as a possible identifier of the polar cap boundary. However, using the intensity gradient to identify the polar cap boundary is less accurate, with a minimum uncertainty of 2.4°. Finally, fitting the measurements of the 6300 A emission to a latitudinal step function, which represents an idealized emission profile, reduces the rms error in the identification of the separatrix to 1.0° while still identifying the boundary in 54% of the cases.

Global auroral responses to abrupt solar wind changes : Dynamic pressure, substorm, and null events

[1] Global auroral images are used to investigate how specific types of solar wind change relate to the resulting type of auroral-region disturbance, with the goal of determining fundamental response types. For not strongly southward IMF conditions (B z ≥ -5 nT), we find that IMF changes that are expected to reduce the convection electric field after ≥30 min of negative IMF B z cause typical substorms, where expansion phase auroral activity initiates within the expected location of the Harang electric field reversal and expands in ∼10 min to cover ∼5 hours ofMLT. For not strongly southward IMF conditions, solar wind dynamic pressure (P dyn ) enhancements compress the entire magnetosphere, leading to a global auroral enhancement with no evidence for substorm bulge-region aurora or current wedge formation. Following prolonged strongly southward IMF (B z ≤ -8 nT), an IMF change leading to convection electric field reduction gives a substorm disturbance that is not much different from substorms for less strongly southward IMF conditions, other than the expansion phase auroral bulge region seems to expand somewhat more in azimuth. However, under steady, strongly southward IMF conditions, a P dyn enhancement is found to cause both compressive auroral brightening away from the bulge region and a Harang-region substorm auroral brightening. These two auroral enhancements merge together, leading to a very broad auroral enhancement covering ∼10-15 hours of MLT. Both current wedge formation and compressive energization in the inner plasma sheet apparently occur for these events. We also find that interplay of effects from a simultaneous IMF and P dyn change can prevent the occurrence of a substorm, leading to what we refer to as null events. Finally, we apply the plasma sheet continuity equation to the IMF and pressure driven substorm responses and the null events. This application suggests that solar wind changes cause substorm onset only if the changes lead to a reduction in the strength of convection within the inner plasma sheet.

Geomagnetic disturbances: characteristics of, distinction between types, and relations to interplanetary conditions

Abstract This tutorial emphasis disturbances in auroral emissions and ionospheric currents and their relation to interplanetary conditions and the overall level of geomagnetic activity. Auroral zone disturbances are divided into three fundamentally different types: poleward boundary intensifications (PBIs), substorms, and effects of solar wind dynamic pressure enhancements. The most common type of auroral-zone disturbance is the PBI, which occurs during all levels of geomagnetic activity. PBIs have an auroral signature that often can be seen to move equatorward from the magnetic separatrix. They are typically associated with ground magnetic perturbations of few tens of nT, but perturbations can be as high as ∼500 nT . Individual PBIs are longitudinally localized, associated with the longitudinally localized flow bursts in the tail plasma sheet, and occasionally traverse essentially the entire latitudinal extent of the plasma sheet. PBIs appear to generally be the dominant type of auroral-zone disturbance during periods of enhanced magnetospheric convection, including the growth phase of substorms, convection bays, and the main phase of magnetic storms. Substorms are a far more dramatic and large scale, but far less common, disturbance than PBIs. They occur after a ≳30 min growth-phase period of enhanced convection. It is now known that at least ∼50% of substorms are associated with IMF changes that lead to a reduction in the strength of convection. However, it has not yet been shown whether or not all are most substorm onsets are caused by these types of IMF changes. Auroral activity during substorms typically initiates within a ∼1–2 h MLT sector near the equatorward boundary of the auroral oval and then expands both poleward and azimuthally. Substorms are associated with ground magnetic disturbances that range from ∼50 to ∼2000 nT , a reduction in strength of the cross-tail current, a poleward displacement of the inner edge of the plasma sheet, and a large release of plasma and magnetic field energy from the region earthward of the new inner edge of the plasma sheet. The reduction of cross-tail current is also believed to often be associated with a severance, and loss from the magnetotail, of the outer portion of the plasma sheet (r≳25RE). Recent studies have shown that solar wind dynamic pressure increases caused large auroral-zone disturbances during a stormtime period of strongly enhanced convection, affecting the poleward boundary, latitudinal width, and intensity of the auroral oval. Dynamic pressure increases also appear to also enhance the entire magnetospheric current system, including the magnetopause, cross-tail, region 1 field-aligned, and global ionospheric currents. Thus, in addition to PBIs, significant variations in solar wind dynamic pressure should be considered as a possibly important source of geomagnetic disturbances during periods of enhanced magnetospheric convection.