Pi2 Waves Research Articles

Plasmaspheric Pi2 Pulsation Enhancement in Response to Plasma Sheet Pi2 Wave Source: Statistical Study Using Van Allen Probes and THEMIS Conjunctions

AbstractTo understand the enhancement of Pi2 pulsations inside the plasmasphere in response to the plasma sheet Pi2 wave source, we conduct a statistical investigation of 208 conjunction events by using simultaneous two‐point measurements with one satellite located in the plasmasphere and the other one located in the plasma sheet. All the events had a Pi2 compressional wave source observed in the plasma sheet as indicated by their association with bursty bulk flows (BBFs), but for about 25% of the events there were no corresponding enhancements in plasmaspheric Pi2 waves. For events with plasmaspheric Pi2 wave enhancements, a cavity or virtual resonance was likely the dominant wave mode, while excitation of field line resonance was also observed. We select two groups of events: strong (weak) group with the plasmaspheric compressional wave enhancements above 75% percentile (below 25% percentile), and conduct a statistical‐significance evaluation of the differences between the two groups. The strong events were observed closer to midnight than the weak events. The plasma sheet wave source that has a larger wave amplification or larger dipolarization associated with BBFs is more likely to excite stronger plasmaspheric wave enhancements. The strong events occurred more often with a pre‐condition of lower Auroral Electrojet (AE)* levels than did weak events. We explain these dependencies as strong events being associated with more favorable conditions that allow the inward‐propagating compressional waves from the plasma sheet wave source to reach the plasmapause and excite the plasmaspheric waves.

On the Source of the Anomalous ULF Waves Detected at Both Ground and Space‐Borne Data on 23 June 2020

AbstractWe have analyzed a highly monochromatic (f = 1.67 mHz) and large‐amplitude Ultra Low Frequency (ULF) wave event observed at satellites and ground observatories on 23 June 2020 during super solar quiet geomagnetic conditions. The train wave was detected between 6:22 and 7:55 UT across a wide longitudinal range of ground stations from low to high latitudes. Using Deep Space Climate Observatory and THEMIS‐B spacecraft, which were in the interplanetary medium, we have identified the possible driver of such global ULF wave activity in the impact of a small pressure pulse accompanied by a discontinuity in the magnetic field. The prolonged duration (90 min) of the ULF waves as well as their latitude‐independent frequency and the small azimuthal wave number (m ∼ 0−2) can be explained in terms of a global magnetospheric waveguide mode. The amplitude and cross‐phase analysis of the wave activity at ground, together with the polarization pattern, suggest the waveguide mode coupling with field line resonance. Nevertheless, during the same time interval, indirect evidence exists of a rapid reconfiguration of the magnetotail in the form of Pi2 waves in the night‐side region. The analysis of energetic particle flux at ionospheric height (500 km) shows a direct connection between the ULF wave activity and particle precipitation.

Magnetic Field and Energetic Particle Flux Oscillations and High‐Frequency Waves Deep in the Inner Magnetosphere During Substorm Dipolarization: ERG Observations

AbstractUsing Exploration of energization and Radiation in Geospace (ERG or Arase) spacecraft data, we studied low‐frequency magnetic field and energetic particle flux oscillations and high‐frequency waves deep in the inner magnetosphere at a radial distance of ~4–5 during substorm dipolarization. The magnetic field oscillated alternately between dipole‐like and taillike configuration at a period of 1 min during dipolarization. When the magnetic field was dipole‐like, the parallel magnetic component of the Pi2 waves was at trough. Both energetic ion and electron fluxes with a few to tens of kiloelectronvolts enhanced out of phase, indicating that magnetosonic waves were in slow mode. Field‐aligned currents also oscillated. These observations are consistent with signatures of ballooning instability. In addition, we found that broadband waves from the Pi1 range to above the electron cyclotron frequency tended to appear intermittently in the central plasma sheet near dipole‐like configuration.

Inward Propagation of Flow‐Generated Pi2 Waves From the Plasma Sheet to the Inner Magnetosphere

AbstractTo understand the inward propagation of the Pi2 waves generated by bursty bulk flows (BBFs) from the plasma sheet to the inner magnetosphere, we statistically investigate 137 midnight conjunction events with one satellite of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) in the plasma sheet and one satellite of Geostationary Operational Environmental Satellite (GOES) in the inner magnetosphere. All events have Pi2 wave enhancements associated with BBFs at THEMIS. In response, larger Pi2 wave enhancements at GOES are correlated with larger dipolarization associated with each BBF at THEMIS but not with stronger waves and flows at THEMIS. The larger wave enhancements at GOES occur preferentially under stronger solar wind driving. We discuss an explanation considering that BBFs are an earthward moving plasma sheet bubble having relatively lower flux tube entropy compared to that of the background plasma and that Pi2 waves are generated by the slowing down of the bubble. The stronger dipolarization of BBFs suggests a bubble of smaller entropy, while the stronger solar wind driving leads to higher background entropy in the inner magnetosphere, both favorable for a bubble to penetrate deeper, and thus, Pi2 waves generated by the bubble are more likely to propagate to the GOES location with less damping to result in larger wave enhancements. In addition, our analyses using empirical density and magnetic field models suggest that the strong Pi2 wave enhancements at GOES are less likely to be a cavity resonance driven inside the plasmasphere.

Low-Latitude Pi2 Waves according to Observations on SWARM Satellites and Ground Stations

We considered wave disturbances of the Pi2 type geomagnetic field (periods of 1–2 min) recorded simultaneously by magnetometers at low-latitude stations in Africa and on low-orbit SWARM satellites both during the beginning of a substorm and in non-substorm periods. At night, Pi2 waves in the upper ionosphere and on Earth are almost identical in amplitude and in phase. These waves on the satellite mainly manifest themselves in longitudinal (along the geomagnetic field) and radial magnetic components. A comparison of the observational results with the model of the interaction of MHD waves with the ionosphere–atmosphere–Earth system shows that night low-latitude Pi2 signals are generated by magnetospheric fast magnetosonic waves propagating to the Earth through the opacity region. The results of analytical estimates and numerical simulations are consistent with the properties of Pi2 signals recorded in the upper ionosphere and on Earth.

Impulsively Excited Nightside Ultralow Frequency Waves Simultaneously Observed on and off the Magnetic Equator

AbstractThe Arase spacecraft is capable of observing ultralow frequency waves in the inner magnetosphere at intermediate magnetic latitudes, a region sparsely covered by previous spacecraft missions. We report a series of impulsively excited fundamental toroidal mode standing Alfvén waves in the midnight sector observed by Arase outside the plasmasphere at magnetic latitudes 13–24°. The wave onsets are concurrent with Pi2 onsets detected by the Van Allen Probe B spacecraft at the magnetic equator in the duskside plasmasphere and by ground magnetometers at low latitudes. The duration of each toroidal wave packet is ∼20 min, which is much longer than that of the corresponding Pi2 wave packet. The toroidal waves cannot be the source of high‐latitude Pi2 waves because they were not detected on the ground near the magnetic field footprint of Arase. Overall, the toroidal wave event lasted more than 2 hr and allowed us to use the wave frequency to estimate the plasma mass density at L = 6.1–8.3. The mass density (in amu/cm3) is higher than the electron density (cm−3) by a factor of ∼6, which implies that 17–33% of the ions were O+.

Time-spatial correspondence between Pi2 wave power and ultra-violet aurora bursts

Time-spatial correspondence between Pi2 wave power and ultra-violet aurora bursts

Nightside Pi2 Wave Properties During an Extended Period With Stable Plasmapause Location and Variable Geomagnetic Activity

AbstractThe frequencies and amplitudes of inner magnetosphere Pi2 waves are affected by the radial plasma density profile. Variable geomagnetic activity and external driving conditions can affect both wave properties and density profiles simultaneously. When interpreting observations, this can lead to ambiguity about whether changing wave properties are due to changing external conditions, density profiles, or a combination of factors. We present a case study using multipoint ground‐based and in situ measurements to examine Pi2 wave properties during a period of variable geomagnetic activity. Multiple satellite passes demonstrate the density profile and plasmapause location are stable for at least 2 h over a wide range of magnetic local time. This stability allows us to examine how factors besides the radial density profile affect Pi2 wave properties. We find evidence for Pi2 waves with a broadband frequency spectrum as well as a discrete frequency plasmaspheric virtual resonance (PVR) that is observed at low, middle, and high latitudes and both inside and outside the plasmapause. The PVR is excited in repeated bursts before, during, and after (1) the development of a substorm, (2) several auroral intensifications, (3) the development of Subauroral Polarization Stream flows/electric fields/conductivities, and (4) variable interplanetary magnetic field conditions. Through all these changes the PVR frequency remains remarkably stable (8.2 ± 0.53 mHz, based on low‐latitude ground magnetometer observations), suggesting that these variations have little effect on the frequency. This is consistent with PVR model predictions for a stationary plasmapause.

Ultralow Frequency Waves Deep Inside the Inner Magnetosphere Driven by Dipolarizing Flux Bundles

AbstractDipolarizing flux bundles (DFBs) are small flux tubes (typical cross‐tail scale of 1–3 RE) in the nightside magnetosphere that have magnetic field more dipolar than the background. They are generated at or beyond 20 RE downtail and then travel earthward. Although DFBs usually stop before reaching the geosynchronous orbit (GEO), they may still transfer some portion of their energy into the inner magnetosphere by radiating ULF waves. We show the clearest evidence of this process, to date, using in situ data from a fleet of spacecraft and ground stations. We illustrate that a typical DFB stopped before reaching GEO but excited Pi2 waves that filled a volume of space extending from the plasma sheet to deep inside the plasmasphere. This event provides the first in situ, direct link between stopped DFBs and ULF waves deep inside the inner magnetosphere (as deep as L = 3). The waves were attenuated when traveling away from the DFB, but even at L = 3 (XGSM = −2.2), they were still traveling with a significant earthward Poynting flux. In addition, we observed evidence of interaction between these waves and electrons at GEO.

Occurrence and characteristics of nighttime ULF waves at low latitude: The results of a comprehensive analysis

AbstractThe occurrence and characteristics of ULF events (f ≈ 10–100 mHz) detected during the night at low latitude (L'Aquila, Italy, λ ≈ 36.3°), during quiet and moderately perturbed magnetospheric conditions, have been examined by means of a long‐term analysis between 1996 and 2012. Clearly defined events (≈8000 on each component) are typically more energetic in H than in D and basically consist of penetrating upstream waves, resonances of local field lines, and Pi2 waves. The global event occurrence shows a strong asymmetry about midnight, with a much higher wave activity before dawn than after dusk: it mostly comes from the intense penetration of upstream waves through the dawn flank of the magnetopause. D events are more frequent in summer and H events more frequent in winter, suggesting a different influence of the ionospheric modification of the downgoing signals. Between f ≈ 30 and 45 mHz, the reversal of the dominant polarization across midnight reveals tailward propagation of penetrating waves. Below f ≈ 25 mHz, intermingled with continuous Pc3 and Pc4 waves, a large fraction of events exhibit Pi2 characteristics: the dominant left‐handed polarization and the switch of the tilt angle of the major axis of the polarization ellipses are consistent with the pattern expected for waves related to the substorm current wedge. A relevant percentage of the power spectra shows a second enhancement above f ≈ 55 mHz, revealing resonance of local field lines also during the night.

Latitudinal dependence on the frequency of Pi2 pulsations near the plasmapause using THEMIS satellites and Asian-Oceanian SuperDARN radars

We herein describe a harmonic Pi2 wave that started at 09:12 UT on August 19, 2010, with data that were obtained simultaneously at 19:00–20:00 MLT by three mid-latitude Asian-Oceanian Super Dual Auroral Radar Network (SuperDARN) radars (Unwin, Tiger, and Hokkaido radars), three Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites (THEMIS A, THEMIS D, and THEMIS E), and ground-based magnetometers at low and high latitudes. All THEMIS satellites, which were located in the plasmasphere, observed Pi2 pulsations dominantly in the magnetic compressional (B //) and electric azimuthal (E A) components, i.e., the fast-mode component. The spectrum of Pi2 pulsations in the B // and E A components contained two spectral peaks at approximately 12 to 14 mHz (f 1, fundamental) and 23 to 25 mHz (f 2, second harmonic). The Poynting flux derived from the electric and magnetic fields indicated that these pulsations were waves propagating earthward and duskward. Doppler variations (V) from the 6-s or 8-s resolution camping beams of the Tiger and Unwin SuperDARN radars, which are associated with Pi2 pulsations in the eastward electric field component in the ionosphere, observed Pi2 pulsations within and near the footprint of the plasmapause, whose location was estimated by the THEMIS satellites. The latitudinal profile of f 2 power normalized by f 1 power for Doppler velocities indicated that the enhancement of the normalized f 2 power was the largest near the plasmapause at an altitude-adjusted corrected geomagnetic (AACGM) latitude of 60° to 65°. Based on these features, we suggest that compressional waves propagate duskward away from the midnight sector, where the harmonic cavity mode is generated.

Propagation of Pi2 pulsations in a dipole model of the magnetosphere

AbstractLocalized fast flows that impinge on the inner magnetosphere from the plasma sheet are observed to oscillate on time scales of minutes. The compression ahead of these flows will launch fast mode waves, while the velocity shears at the edges of these flows directly excite shear Alfvén waves. These waves, which are coupled by gradients in the Alfvén speed, have been suggested as a source for the Pi1 and Pi2 waves that are observed at both high and low latitudes in the ionosphere. A new three‐dimensional simulation of the propagation of ULF waves in the dipolar region of the magnetosphere has been developed to study these coupled wave modes. This model includes a height‐resolved ionospheric conductivity so that ionospheric fields can be more realistically determined, as well as a direct calculation of ground magnetic fields to compare with ground magnetometers using an inductive ionosphere model. Results from this model show that a plasmaspheric resonance can be set up by waves with periods about 1 min and that field line resonances can be excited both inside and outside the plasmasphere. The use of the inductive ionosphere model leads to the conclusion that even a uniform Hall conductivity can break the dawn‐dusk symmetry of the convection pattern. Waves from a source at 10 Earth radii reach the ionosphere with time delays between high and low latitudes of tens of seconds, with implications for the timing of substorm phenomena observed by spacecraft and by ground magnetometers and radars.

Near-equatorial Pi2 and Pc3 waves observed by CHAMP and on SAMBA/MAGDAS stations

We have examined simultaneous ULF activity in the Pi2 and Pc3 bands at the near-equatorial magnetic stations in South America from SAMBA and MAGDAS arrays and low-orbiting CHAMP satellite during its passage over this meridional network. At the nighttime, both Pi2 and Pc3 waves in the upper ionosphere and on the ground are nearly of the same magnitude and in-phase. At the same time, the daytime Pc3 pulsations on the ground and in space are nearly out-of-phase. Comparison of observational results with the theoretical notions on the MHD wave interaction with the system ionosphere–atmosphere–ground suggests that nighttime low-latitude Pi2 and Pc3 wave signatures are produced by magnetospheric fast compressional mode. The daytime near-equatorial Pc3 waves still resist a quantative interpretation. These waves may be produced by a combination of two mechanisms: compressional mode leakage through the ionosphere, and by oscillatory ionospheric current spreading towards equatorial latitudes.

Pi2 pulsation simultaneously observed in the E and F region ionosphere with the SuperDARN Hokkaido radar

AbstractWe investigated Pi2 pulsations in the nightside ionosphere that began at 14:15 UT (2315 LT) on 11 July 2010, and they were observed with high‐temporal (8 s) resolution by beam 4 of the Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar. These pulsations were simultaneously observed in both the ground/sea scatter echoes reflected from the F region height and in ionospheric echoes from field‐aligned irregularities in the sporadic Es region. They had the same period of 110 s and approximately no phase lag. From the radar observations and the International Geomagnetic Reference Field model, the amplitude of the eastward (EEW) component of the electric field of the Pi2 pulsations in the ionosphere was estimated ~8.0 mV/m in the F region and ~2.0 mV/m in the E region. Corresponding Pi2 pulsations appeared dominantly in the horizontal northward magnetic field component (H) at nearby ground stations, Moshiri (MSR), St. Paratunka (PTK), and Stecolny (STC), with amplitudes ranging from 6 nT (MSR) to 10 nT (STC). At the dominant frequency of 8.8 mHz, the coherences between H and EEW were high (>0.9), the cross phases of EEW relative to H were −56° and −45°, and the amplitude ratios were 2.7 × 105 m/s and 8.4 × 105 m/s, in the E and F regions, respectively. Based on a comparison of these results with theoretical predictions, we suggest that the concept of a pure cavity mode is not sufficient to explain the combined observations for midlatitude Pi2 waves and that the contribution of an Alfvén waves must be taken in account.

Interrelationship between preonset auroral and magnetic signatures at a geomagnetically conjugate Iceland‐Syowa pair

AbstractWe present the initiation and development of Pi1‐Pi2 band pulsations in concert with spatiotemporal evolution of preonset auroral arc during an isolated auroral substorm reported by Motoba et al. (2012), and their interhemispheric similarities/dissimilarities, based on the colocated optical and magnetic field measurements at a geomagnetically conjugate Iceland‐Syowa pair. At least ~7 min prior to the auroral expansion onset, the first identifiable signature of preonset arc began with a small‐scale (~30–50 km) azimuthal beading. Interestingly, the early development of the visible arc beading for ~3–4 min was not accompanied by any ground magnetic perturbation. Then the bead‐like forms in the arc evolved into brighter, larger undulations, eventually developing into the poleward expansion. Such a preonset optical sequence was almost identical at both stations. In the transition from the beads into undulations, Pi2 pulsations were first identified clearly, followed by Pi1 pulsations a few minutes later. Whereas the Pi2 onset was coincident with the initiation of progressive increase of the preonset arc luminosity, the Pi1 onset was associated with a subsequent steep increase. The Pi2 wave cycles superimposed on the magnetic negative bay were well correlated with the preonset arc luminosity time series. These results imply the potential linkage between the ground Pi1‐Pi2 signatures in the onset region and the preonset auroral arc processes. Although the Pi1‐Pi2 features were generally similar between the two hemispheres, there was also some dissimilarity in their temporal behaviors, which could reflect a small but noteworthy interhemispheric asymmetry in the overhead conjugate preonset aurora.

On the influence of open magnetic flux on substorm intensity: Ground‐ and space‐based observations

AbstractUsing the location of maximum region 1 current determined by the Active Magnetosphere and Planetary Electrodynamics Response Experiment as a proxy for the open/closed field line boundary, we monitor the evolution of the amount of open magnetic flux inside the magnetosphere during 772 substorms. We then divide all substorms into three classes, depending on the amount of open flux at expansion phase onset. Studying the temporal variations during the substorms of each class for a number of related geophysical parameters, we find that substorms occurring while the amount of open flux is large are generally more intense. By intense we mean that the auroral electrojet, region 1 current, auroral brightness, tail dipolarization and flow speed, ground magnetic signatures, Pi2 wave power, as well as the intensity and extent of the substorm current wedge (SCW) are all larger than during substorms that occur on a contracted polar cap. The SCW manifests itself as an intensification of the nightside R1 and R2 current system after onset. Our analysis shows that to dispose of large amounts of accumulated open magnetic flux, large substorms are triggered in the terrestrial magnetosphere.

Transition of Pi2 ULF wave polarization structure from the ionosphere to the ground

Trains of ultra low frequency (ULF) waves in the Pi2 range (40–150 s) are regularly detected in near‐Earth space. While some Pi2 occur just before substorm onset, others do not show any casual relation to substorm activity so that the Pi2 generation and propagation mechanisms are still unclear. The bulk of information on Pi2 morphology is provided by ground‐based magnetometers, subject to distortions due to transition through the highly conducting lower ionosphere. While space‐borne magnetometers provide important in situ information, they cannot match the spatio‐temporal continuity of ground‐based instruments. In this paper, we devise a novel approach using co‐located high‐frequency ionospheric radars and ground magnetometers for the first direct observation of the Pi2 wave polarization transition between the ionosphere and the ground. Furthermore, application of the Hilbert transform enables us to reconstruct the transition dynamics, which resolves an apparent disagreement between the Pi2 spectra measured by the two instruments.

The mechanism of mid-latitude Pi2 waves in the upper ionosphere as revealed by combined Doppler and magnetometer observations

Abstract. The interpretation of simultaneous ionospheric Doppler sounding and ground magnetometer observations of low-latitude Pi2 waves is revised. We compare the theoretical estimates of the ionospheric Doppler velocity for the same amplitude of the ground magnetic disturbances produced by a large-scale compressional mode and an Alfvén mode. The plasma vertical displacement caused by the wave electric field is shown to be the dominating effect. Taking into account the correction of the previous paper, the observations of low-latitude Pi2 in the F layer ionosphere by Doppler sounding and SuperDARN (Super Dual Auroral Radar Network) radars give consistent results. We suggest that the Doppler response to Pi2 waves is produced by the Alfvén wave component, but not the fast-mode component, whereas the ground magnetic signal is composed from both Alfvén and fast magnetosonic modes.

Wp index: A new substorm index derived from high‐resolution geomagnetic field data at low latitude

Geomagnetic field data with high time resolution (typically 1 s) have recently become more commonly acquired by ground stations. Such high time resolution data enable identifying Pi2 pulsations which have periods of 40–150 s and irregular (damped) waveforms. It is well‐known that pulsations of this type are clearly observed at mid‐ and low‐latitude ground stations on the nightside at substorm onset. Therefore, with 1‐s data from multiple stations distributed in longitude around the Earth's circumference, substorm onset can be regularly monitored. In the present study we propose a new substorm index, the Wp index (Wave and planetary), which reflects Pi2 wave power at low‐latitude, using geomagnetic field data from 11 ground stations. We compare the Wp index with the AE and ASY indices as well as the electron flux and magnetic field data at geosynchronous altitudes for 11 March 2010. We find that significant enhancements of the Wp index mostly coincide with those of the other data. Thus the Wp index can be considered a good indicator of substorm onset. The Wp index, other geomagnetic indices, and geosynchronous satellite data are plotted in a stack for quick and easy search of substorm onset. The stack plots and digital data of the Wp index are available at the Web site (http://s‐cubed.info) for public use. These products would be useful to investigate and understand space weather events, because substorms cause injection of intense fluxes of energetic electrons into the inner magnetosphere and potentially have deleterious impacts on satellites by inducing surface charging.

Behavior of substorm auroral arcs and Pi2 waves: implication for the kinetic ballooning instability

Abstract. We present synoptic observations of the 21 December 2006 substorm event by the THEMIS ground-based All-Sky-Imagers, the ISUAL CCD Imager aboard the FORMOSAT-2 satellite, the geosynchronous satellites and the ground-based magnetometers, and discuss the implication of the observations. There are three subsequent arc breakups with time separation of <1 min during the substorm expansion phase. In particular, we investigated the mode number of the substorm arc bead-like structure and the concurrent behavior of the arc intensity, the westward electroject intensity, and the ground Pi2 pulsation amplitude. Prior to each arc breakup there was a clear azimuthally-spaced bright spot structure along the arc with high mode number (~140–180) and the arc intensity increased together with the westward electrojet and the ground Pi2 pulsation amplitude under the arc. The Pi1 perturbations observed under the arc appeared at or after the arc breakup started. This suggests that the Pi2 pulsation is related to the arc formation. The Pi2 pulsation may be caused by the kinetic ballooning instability (KBI) that is excited in the strong cross-tail current region. The longitudinal extent of the earthward expansion front of the substorm dipolarization region at the geosynchronous orbit is estimated from timings of the energetic proton and electron injections and is roughly located between ~19.50 MLT and ~23.00 MLT, which is consistent with the corresponding longitudinal extent of the auroral substorm activity.