[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.

Plasmaspheric hiss is known to play an important role in controlling the overall structure and dynamics of radiation belt electrons inside the plasmasphere. Using newly available Van Allen Probes wave data, which provide excellent coverage in the entire inner magnetosphere, we evaluate the global distribution of the hiss wave frequency spectrum and wave intensity for different levels of substorm activity. Our statistical results show that observed hiss peak frequencies are generally lower than the commonly adopted value (~550 Hz), which was in frequent use, and that the hiss wave power frequently extends below 100 Hz, particularly at larger L shells (> ~3) on the dayside during enhanced levels of substorm activity. We also compare electron pitch angle scattering rates caused by hiss using the new statistical frequency spectrum and the previously adopted Gaussian spectrum and find that the differences are up to a factor of ~5 and are dependent on energy and L shell. Moreover, the new statistical hiss wave frequency spectrum including wave power below 100 Hz leads to increased pitch angle scattering rates by a factor of ~1.5 for electrons above ~100 keV at L~5, although their effect is negligible at L ≤ 3. Consequently, we suggest that the new realistic hiss wave frequency spectrum should be incorporated into future modeling of radiation belt electron dynamics.

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.

International audienceA statistical survey of plasma densities and electron distributions (0.5-100 keV) is performed using data obtained from the Time History of Events and Macroscale Interactions During Substorms spacecraft in near-equatorial orbits from 1 July 2007 to 1 May 2009 in order to investigate optimum conditions for whistler mode chorus excitation. The plasma density calculated from the spacecraft potential, together with in situ magnetic field, is used to construct global maps of cyclotron and Landau resonant energies under quiet, moderate, and active geomagnetic conditions. Statistical results show that chorus intensity increases at higher AE index, with the strongest waves confined to regions where the ratio between the plasma frequency and gyrofrequency, f_pe/f_ce, is less than 5. On the nightside, large electron anisotropies and intense chorus emissions indicate remarkable consistency with the confinement to 8 R_E. Furthermore, as injected plasma sheet electrons drift from midnight through dawn toward the noon sector, their anisotropy increases and peaks on the dayside at 7 < L < 9, which is well correlated with intense chorus emissions observed in the prenoon sector. These statistical results are generally consistent with the generation of both lower-band and upper-band chorus through cyclotron resonant interactions with suprathermal electrons (1-100 keV). Two typical events on the nightside and dayside are studied in greater detail and additional interesting features are identified. Pancake distributions of electrons with energy below ˜2 keV, which could be responsible for the excitation of upper-band chorus, are observed at lower L shells (<7) on the nightside and at larger L shells (>6) on the dayside. In addition, very isotropic distributions at a few keV, which may be produced by Landau resonance and contribute to the formation of the typical gap in the chorus spectrum near 0.5 f_ce, are commonly observed on the dayside

Citizen scientists, along with satellite and ground-based sensors, have revealed a new arc boundary at subauroral latitudes.

[1] Statistical results on the global distribution of suprathermal electron (0.1-10 keV) fluxes are shown both outside and inside the plasmasphere separately, using electron data from THEMIS. Significant electron fluxes are found within the plasmasphere, although they are nevertheless smaller than the populations outside the plasmasphere. Electron fluxes outside of the plasmapause increase with stronger magnetic activity on the nightside and decrease as a function of increasing magnetic local time (MLT). Inside the plasmasphere, electron fluxes increase just inside of the plasmapause, particularly from the midnight to the dawn sector during active times, while electron distributions are less MLT-dependent during quiet times. Inside the plasmasphere, electron fluxes are larger and more stable at smaller L shells at higher energy (a few to 10 keV), while electron fluxes decrease at smaller L shells at lower energy (less than a few keV). Our new statistical results on the suprathermal electron distribution both inside and outside the plasmasphere provide essential information for the evaluation of wave propagation characteristics. Case analyses have been performed in order to understand potential mechanisms responsible for electron access into the plasmasphere. The first case analysis shows that during a relatively quiet time following a disturbed interval, deeply injected suprathermal electrons remain trapped at low L shells during the refilling of the plasmasphere and eventually form the plasmaspheric population. The second case analysis suggests that a combination of locally enhanced electric field and subsequent energy-dependent azimuthal magnetic drift may be able to trap the suprathermal electrons inside the plasmasphere during a geomagnetically active period.

[1] Using observations from the THEMIS spacecraft, we investigate the modulation of whistler mode chorus waves in the magnetosphere by compressional Pc4-5 pulsations (i.e., pulsations with periods from tens of seconds to a few minutes) with an anticorrelation between the total electron density and the background magnetic field intensity. We find that such compressional pulsations are associated with modulations of resonant electron fluxes and chorus intensity. Changes in the total electron density, background magnetic field, and the flux and anisotropy of resonant electrons could all be responsible for triggering the excitation of chorus waves. To quantitatively investigate which parameters dominantly contribute to chorus generation, we evaluate the changes in linear growth rates of whistler mode waves due to variations in either the ratio of resonant electrons to the total electrons R(V R ) or the electron anisotropy A(V R ). In the majority of cases, the modulation of whistler mode wave intensity is dominated by R(V R ) variations caused by compressional Pc4-5 pulsations and tends to occur at large L shells of 8-12 in the dawn sector. Only a small fraction of events are associated with A(V R ) modulations and these typically occur at lower L shells (<∼8).

[1] The characteristics of the Poynting flux and wave normal vectors of whistler-mode waves outside the plasmapause are investigated for the lower (0.1-0.5 f ce ) and upper bands (0.5-0.8 f ce ), where f ce is the equatorial electron cyclotron frequency. To analyze the wave properties, we utilized high-resolution waveform data from multiple THEMIS spacecraft in the near-equatorial magnetosphere from June 2008 to November 2012. Full measurements of the wave electric and magnetic fields are used to calculate the Poynting fluxes and construct the wave normal vectors, which are then used to calculate the polar and azimuthal angles with respect to the background magnetic field. Statistical results show that the majority of whistler-mode waves propagate away from the magnetic equator, suggesting that the major source region is very close to the equator. The lower band wave normal angle distribution shows a major peak close to the field line direction and a secondary peak near the resonance cone. In contrast, the wave normal distribution of upper band waves exhibits a broad distribution between 0 and 60 with the largest probability at~0 . The azimuthal component of the wave normal vector predominantly points radially outward for both lower and upper band waves, but a tendency for azimuthal propagation is observed for lower band waves in the day and dusk sectors probably due to pronounced azimuthal density gradients in the afternoon sector. Our statistical results provide crucial information on the Poynting fluxes and wave normal vectors of whistler-mode waves, which play a significant role in radiation belt electron dynamics.