Substorm activity, due to either substorms or substorm‐associated disturbances, shows two preferred recurrent intervals for periodic events. While the longer recurrent interval of ∼3 hr has been ...extensively studied, albeit still without a conclusive answer on its source mechanism, the source of the shorter 1‐hr recurrent interval has not been addressed at all. Here, we provide statistics on 102 events, each event being a sequence of substorm‐like dipolarizations with recurrent intervals of between 30 min and 2 hr, using Kp, Dst, AE, and solar wind speed, and relate them to statistics of other response modes, such as steady magnetospheric convections, sawtooth events, and isolated substorms. It becomes apparent that the 1‐hr events are a nonstorm phenomenon, which clearly distinguishes them from sawtooth events, which occur during larger Dst and Kp values. While there are statistical similarities to isolated substorms in terms of solar wind driving and Kp, our events also encompass pseudo‐breakups and smaller substorm activations. Overall, the range of AE values for the 102 events is very broad, suggesting small to large auroral activities during the events. Furthermore, we provide a comparison to published global MHD simulations with kinetic corrections, which suggests that the source mechanism of these events could be internally driven, quasi‐periodic reconnection in the magnetotail. Finally, we advocate the treatment of this global phenomenon as a separate response mode of the magnetosphere‐ionosphere system.
Key Points
The short‐period substorm activity, due to either substorms or substorm‐associated disturbances, is a nonstorm phenomenon
The periodic events are distinct from other magnetospheric response modes; thus, it is plausible to treat them as a separate response mode
Internally driven, quasi‐periodic reconnection in the magnetotail is a viable source mechanism
A multipoint analysis of conjugate magnetospheric and ionospheric flow vortices during the formation of the substorm current wedge (SCW) on 19 February 2008 is presented. During the substorm, four ...Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft were located close to the neutral sheet in the premidnight region between 9 and 12 RE geocentric distance, of which three closely (∼1–2 RE) clustered at ∼23 MLT and one was farther west at ∼21 MLT. The closely clustered spacecraft were engulfed by a counterclockwise plasma flow vortex, while the single spacecraft recorded a clockwise plasma flow vortex. Simultaneously, a pair of conjugate flow vortices with clockwise and counterclockwise rotation appeared in the ionosphere, as inferred from equivalent ionospheric currents. The counterclockwise space vortex, which corresponded to a downward field‐aligned current, was at least 1–2 RE in diameter and had rotational flow speeds of up to 900 km/s. Current density estimates associated with the formation of the space vortex in the first 30 s yielded 2.8 nA/m2 (14 μA/m2 mapped to the ionosphere), or a total current of 1.1 × 105 A. Model calculations based on midlatitude ground magnetometer data show a gradual increase of the field‐aligned current, with 1–2 × 105 A within the first minute and a peak value of 7 × 105 A after 10 min, associated with the SCW, and a matching meridian of the downward current of the SCW and the downward current (counterclockwise) space vortex. The combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field‐aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.
Global Pi2 pulsations have mainly been associated with either low/middle latitudes or middle/high latitudes and, as a result, have been treated as two different types of Pi2 pulsations, either the ...plasmaspheric cavity resonance or the transient response of the substorm current wedge, respectively. However, in some reports, global Pi2 pulsations have a single period spanning low/middle/high latitudes. This “super” global type has not yet been satisfactorily explained. In particular, it has been a major challenge to identify the coupling between the source region and the ground. Here we report two consecutive super global Pi2 events which were observed over a wide latitudinal and longitudinal range. Using four spacecraft that were azimuthally spread out in the nightside and one spacecraft in the tail lobe, it was possible to follow the Pi2 signal along various paths with time delays from the magnetotail to the ground. Furthermore, it was found that the global pulsations were a combination of various modes including the transient Alfvén and fast modes, field line resonance, and possibly a forced cavity‐type resonance. As for the source of the Pi2 periodicity, oscillatory plasma flow inside the plasma sheet during flow braking (e.g., interchange oscillations) is a likely candidate. Such flow modulations, resembling the ground Pi2 pulsations, were recorded for both events.
Key Points
Identification of several propagation paths for super global Pi2
Proposal of generation mechanism for super global Pi2
We present ground‐based and in situ observations from March 13, 2007. The THEMIS satellites were in the evening sector conjugate to THEMIS ground‐based imagers. At ∼0507 UT there was an optical onset ...on inner CPS field lines. This involved near‐simultaneous brightening of 1 MLT hour longitudinal segment of the onset arc. The part of the arc that brightened was that closest to the equatorward boundary of the diffuse (proton) aurora. Within one minute, a dipolarization front moved across four THEMIS satellites. Based on their locations, the order in which they detected the dipolarization front, and the auroral evolution, we assert that the expansion phase began earthward of the four satellites and evolved radially outwards. We conclude that this onset occurred in an azimuthally localized region of highly stretched field lines.
We present the first simultaneous observations of Alfvén waves at Polar and FAST altitudes, ∼7 RE geocentric and ∼3500 km, respectively, at ∼23 MLT in the main phase of a major geomagnetic storm on ...22 October 1999. We compare the Poynting flux for these waves and the electron energy flux at the two spacecraft. We also present a new method of Alfvén wave analysis, examining Poynting flux magnitude and directionality along with the perturbation electric to magnetic field ratio of these waves as a function of wave temporal scale (frequency). The results of this analysis are compared with those expected from kinetic Alfvén wave models. There is a mean net loss of ∼2.1 ergs cm−2 s−1 (mW m−2) in earthward Poynting flux over the altitude region between Polar and FAST, a mean net increase in earthward electron energy flux of up to ∼1.2 ergs cm−2 s−1 over the same region, frequency characteristics consistent with a mixture of Alfvén waves obeying the kinetic Alfvén wave dispersion relation mixed with some coupling to the ionosphere, and high‐frequency kinetic Alfvén wave generation between Polar and FAST. Current models are found to be generally consistent with the study results but are not yet sufficiently well formulated to account for the details, including evidence for temporal and/or spatial modulation of reflectivity.
During the encounter of a substorm on 23 March 2007, the THEMIS constellation observed energetic particle injections and dipolarizations in the premidnight sector. Clear injection and dipolarization ...signatures were observed during the main intensification by three probes (A, B, and D) in the region around 11 RE and 2100 local time (LT). THEMIS C, which was leading in the constellation at 8.3 RE, also observed a clear injection signature, but the dipolarization was not so clear. From the timing based on these observations, a fast westward expanding ion injection and dipolarization front was identified. In combination with the energetic particle observations from Los Alamos National Laboratory (LANL) geosynchronous satellites, the particle injection seemed to initiate between 2100 and 0100 LT. This event provides an excellent opportunity to examine the dipolarization and particle injection processes beyond geosynchronous orbit and over a wide LT range. We model this injection event by means of test particle simulation, setting up an initial particle distribution and sending an earthward dipolarization‐like pulse from the tail that also expands azimuthally, then recording the ions and electrons at the various satellite locations. Most features of the injected particles are reproduced by the test particle simulation. These include not only the earthward injections but also the fast westward expansion of the injection, as well as the timing of the injections as observed among different satellites that made the observations. On the basis of the observations and the simulation results, we suggest that this substorm injection was initiated around 2300 LT, farther down the tail, and propagated radially inward and expanded azimuthally.
The passage of the Cluster satellites in a polar orbit through Earth's magnetotail has provided numerous observations of harmonically related Pc 1–2 ULF wave events, with the fundamental near the ...local proton cyclotron frequency Ωcp. Broughton et al. (2008) reported observations by Cluster of three such events in the plasma sheet boundary layer, and used the wave telescope technique to determine that their wave vectors k were nearly perpendicular to B. This paper reports the results of a search for such waves throughout the 2003 Cluster tail passage. During the 4 month period of July–October 2003, 35 multiple‐harmonic wave events were observed, all in the plasma sheet boundary layer (PSBL). From the first observed event (22 July) to the last (28 October), 13 of Cluster's 42 tail passes had at least one event. The wave events were rather evenly distributed from XGSE = −7 RE out to the Cluster apogee distance of −18 RE, with one event observed at −4 RE. ZGSE for these events ranged from −10 to −3 RE and +3 to +7 RE (i.e., there were no events for ∣Z∣ < 3 RE). The wave events, with durations from ∼1 to 50 min, were consistently associated with signatures of the PSBL: elevated fluxes of counterstreaming ions with energies ranging from ∼3 to 30 keV, and elevated fluxes of electrons with energies ranging from 0.25 to ∼5 keV. Analysis of plasma parameters suggests that although waves occurred only when the ion beta exceeded 0.1 (somewhat larger than typical for the PSBL), ion particle pressure may be of more physical importance in controlling wave occurrence. Electron distributions were more isotropic in pitch angles than the ion distributions, but some evidence of counterstreaming electrons was detected in 83% of the events. The ions also showed clear signatures of shell‐like or ring‐like distributions; i.e., with reduced fluxes below the energy of maximum flux. The suprathermal ion fluxes were asymmetric in all events studied, with more ions streaming earthward (for events both north and south of the central plasma sheet). Good agreement between the observed frequency of the fundamental harmonic and the local Ωcp suggests that the waves were observed near the region of their origin and did not propagate along B, consistent with the wave telescope analysis.
Large-scale, electric currents flowing along magnetic field lines into the polar regions of Earth are thought to be the main contributors of the energy that powers the ionospheric aurora. However, we ...have found evidence for global contributions from electromagnetic waves (Alfvén waves). Data that were collected from the Polar spacecraft over the course of 1 year show that the flow of wave electromagnetic energy at altitudes of 25,000 to 38,000 kilometers delineates the statistical auroral oval. The Poynting flux of individual events distributed along the auroral oval was larger than 5 ergs per square centimeter per second, which is sufficient to power auroral acceleration processes. This evidence suggests that in addition to magnetic field-aligned currents, the dayside and nightside aurora is globally powered by the energy flow of these high-altitude Alfvén waves.
We present evidence based on measurements from the Polar spacecraft for the existence of small‐scale, large‐amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at ...altitudes of 4–6 RE. These structures coincide with larger‐scale Alfvénic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small‐scale spikes have electric field amplitudes up to 300 mV m−1 and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger‐scale Alfvén waves have periods of ∼20–60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller‐scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is ∼4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20–120 km (with an electron inertial length c/ωpe∼10 km and an ion gyroradius ∼20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of ∼1 keV, consistent with estimates of the parallel ∫ Edl associated with small‐scale kinetic Alfvén wave structures. The energy flux in the electron “beams” is ∼0.7 ergs cm−2 s−1. These observations suggest that the small‐scale kinetic Alfvén waves are generated from the larger‐scale Alfvén waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large‐amplitude Alfvén waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2 RE, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.
We present evidence based on measurements from the Polar spacecraft for the existence of small‐scale, large‐amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at ...altitudes of 4–6
R
E
. These structures coincide with larger‐scale Alfvénic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small‐scale spikes have electric field amplitudes up to 300 mV m
−1
and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger‐scale Alfvén waves have periods of ∼20–60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller‐scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is ∼4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20–120 km (with an electron inertial length
c
/ω
pe
∼10 km and an ion gyroradius ∼20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of ∼1 keV, consistent with estimates of the parallel ∫
Edl
associated with small‐scale kinetic Alfvén wave structures. The energy flux in the electron “beams” is ∼0.7 ergs cm
−2
s
−1
. These observations suggest that the small‐scale kinetic Alfvén waves are generated from the larger‐scale Alfvén waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large‐amplitude Alfvén waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2
R
E
, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.
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