Time History Of Events And Macroscale Interactions During Substorms Probes Research Articles

On the Origin of Ionospheric Hiss: A Conjugate Observation

AbstractWe present a conjugate observation on whistler mode electromagnetic hiss from the low Earth orbit satellite Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) and the high‐altitude elliptical orbit spacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS). The conjugate observation was performed at 14:51:10 to 15:12:00 UT on 15 June 2010, when DEMETER was flying across the L shell region from ~1.39 to 2.80 at an altitude of ~660 km; meanwhile, THEMIS probes were passing through the L shell region from ~1.64 to 1.91 at altitudes from ~1.6 to 2.0 RE. The conjugated observations demonstrate similar time‐frequency structures between the ionospheric hiss (~350 to 800 Hz) captured by DEMETER and the plasmaspheric hiss (~350 to 900 Hz) recorded by THEMIS probes, including similar peak frequencies (~500 to 600 Hz), similar lower cutoff frequencies (~350 to 400 Hz), and upper cutoff frequencies (~730 to 800 Hz). The wave vector analyses show that the ionospheric hiss propagates obliquely downward to the Earth and slightly equatorward with right‐handed polarization, suggesting that its source comes from higher altitudes. Ray tracing simulations with the constraint of observations verify that the connection between ionospheric and plasmaspheric hiss is physically possible through wave propagation. This study provides direct observational evidence to support the mechanism that high‐altitude plasmaspheric hiss is responsible for the generation of low‐altitude ionospheric hiss.

Subpacket structures in EMIC rising tone emissions observed by the THEMIS probes

AbstractWe report subpacket structures found in electromagnetic ion cyclotron (EMIC) rising tone emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. We investigate three typical cases in detail. The first case shows a continuous single rising tone with four obvious subpackets, and the second case is characterized by a patchy emission with multiple subpackets triggered in a broadband frequency. The third case looks like a smooth rising tone without any obvious subpacket in the fast Fourier transform spectrum, while its amplitude contains small peaks with increasing frequencies. The degree of polarization of each subpacket is generally higher than 0.8 with a left‐handed polarization, and the wave direction of the subpackets is typically field aligned. We show that the time evolution of the observed frequency and amplitude can be reproduced consistently by nonlinear growth theory. We also compare the observed time span of each subpacket structure with the theoretical trapping time for second‐order cyclotron resonance. They are consistent, indicating that an individual subpacket is generated through a nonlinear wave growth process which excites an element in accordance with the theoretically predicted optimum amplitude.

Formation and evolution of high‐plasma‐pressure region in the near‐Earth plasma sheet: Precursor and postcursor of substorm expansion onset

AbstractCause of substorm expansion onset is one of the major problems in the magnetospheric study. On the basis of a global magnetohydrodynamic (MHD) simulation, Tanaka et al. (2010) suggested that formation and evolution of a high‐pressure region (HPR) in the near‐Earth plasma sheet could result in sudden intensification of the Region 1 field‐aligned current and the westward auroral electrojet. In this sense, the formation and evolution of the HPR are a key in understanding the cause of the onset. On 5 April 2009, three probes of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) were located at XGSM~−11 Re around the equator, which provide unique opportunity to investigate the spatial‐temporal evolution of the HPR near the substorm expansion onset. Just before the onset, a positive excursion of the plasma pressure appeared at the outermost probe first, followed by the inner ones. Just after the onset, the opposite sequence took place. A positive excursion of the Y component of the current density was observed near the onset by the THEMIS probes and followed by a decrease trend. A similar variation was also found in the MHD simulation. All these features are consistent with the simulation result that a squeeze of the plasma from the plasma sheet results in the formation of the HPR before the onset and that the accumulated plasma spreads outward after the onset. The HPR is shown to be important for the dynamics of the magnetosphere during a substorm.

Association of consecutive Pi2‐Ps6 band pulsations with earthward fast flows in the plasma sheet in response to IMF variations

AbstractOn 11 March 2009, the H component had four consecutive bay‐like variations accompanied by positive and negative deflections in the D component across the Atlantic like those affected by the substorm current wedge formation. A train of pulsations with a frequency range 2–10 mHz (referred to as Pi2‐Ps6 band), sensed by Time History of Events and Macroscale Interactions during Substorms (THEMIS)/Canadian Array for Real‐time Investigations of Magnetic Activity (CARISMA) magnetometers, had clearly three consecutive Pi2s followed by a Ps6 at low latitudes, but first Pi2 and then Ps6 at high latitudes mixed with large‐amplitude Ps6 at midlatitudes. The geostationary orbit magnetometers sensed similar magnetic perturbations. THEMIS probes first observed earthward fast flows, magnetic dipolarizations, and modulated energetic particle fluxes at ~ XGSM −9.2 RE, then at ~ XGSM −7.5 RE for Pi2 and at ~ XGSM −18.0 RE only for Ps6. They appeared during a very quiet period for northward interplanetary magnetic field (IMF) with a clock angle variation of low to high and then low. The H spectrum shows two harmonic frequencies ~2–4 mHz and ~8–10 mHz but the D spectrum one dominant frequency ~2–4 mHz. Pi2 can result from a combination of fast magnetospheric and plasmaspheric cavity resonances and Ps6 from a fast magnetospheric cavity resonance. The surface waves at the interface separating braking earthward fast flows from the ambient plasma convection region could lead to large‐amplitude Ps6 at midlatitudes. Hence, consecutive Pi2‐Ps6 band pulsations can be associated with earthward fast flows in the plasma sheet, expectedly driven by magnetotail reconnection, respectively, in the near‐Earth region and the distant Earth one in response to IMF variations as in the two‐neutral‐point model.

Dipolarization fronts and associated auroral activities: 1. Conjugate observations and perspectives from global MHD simulations

Earthward‐propagating dipolarization fronts (DFs) are often found to be associated with magnetic reconnection and bursty bulk flows (BBFs) in the magnetotail. Recent THEMIS (Time History of Events and Macroscale Interactions During Substorms) probe observations have shown a DF propagating over 10REfrom the mid‐tail region to the near‐Earth tail region, and THEMIS All‐Sky Imager data show a north‐south auroral form and intensification of westward auroral zone currents. In this study, we examine THEMIS in situ observations of DFs in the magnetotail and simultaneous observations of the proton aurora from ground‐based CANOPUS (the Canadian Auroral Network for the OPEN Program Unified Study) Meridian Scanning Photometers (MSPs). We find that earthward‐moving DFs are often associated with intensification of proton aurora when the THEMIS probes are conjugate to the meridian of the MSP. The proton auroral intensifications are transient and in some cases detached from the background proton precipitation. Just before the DFs, the ion distribution is anisotropic in the field‐aligned direction (mostly earthward) and the ion energy increases. These observations suggest that plasma sheet protons can be reflected and energized by earthward‐moving DFs as they propagate through the magnetotail. We postulate that this population of ions is the source of the proton auroral intensification observed on the ground. This conjecture is tested using our global MHD simulation results, where the proton precipitation is calculated with the field‐line curvature (FLC) model. The MHD simulation results show that proton precipitation enhancement can be caused by compression of plasma by approaching DFs/BBFs, which is consistent with ion reflection at DFs. Thus, using the conjugate observations from THEMIS spacecraft and MSP in this study, we are able to directly link the magnetotail dynamics, i.e., dipolarization fronts, with ground auroral activities. However, understanding of DF‐associated ion energization requires detailed test‐particle simulations with an analytical magnetotail model, such as those in our companion paper.

Local time‐dependent Pi2 frequencies confirmed by simultaneous observations from THEMIS probes in the inner magnetosphere and at low‐latitude ground stations

Using electric and magnetic field data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) fast mode waves away from midnight. probes (TH‐A, TH‐D, and TH‐E) acquired in the inner magnetosphere (L < 4) and low‐latitude ground magnetic field data from Bohyun (BOH, L = 1.35) and Hermanus (HER, L = 1.83) stations, we studied longitudinal variations of Pi2‐associated fast mode waves away from midnight. We selected 48 nightside Pi2 events at BOH identified when THEMIS probes were in the inner magnetosphere for 1 month (February 2008). During that period the probes were located between 4.0 and 20.0 magnetic local time (MLT), i.e., at and on either side of the dayside sector. This choice was motivated by our interest in studying loss of Pi2 wave energy away from its nominal source at the nightside, all the way to the dayside. Between 4.0 and 8.0 MLT the probes often observed poloidal oscillations in space during BOH Pi2 events. The poloidal oscillations had high coherence (>0.6) with BOH Pi2s and radially standing fast mode structures. Thus, these fast mode waves are explained by plasmaspheric resonance. On the duskside, however, few events at THEMIS probes had high coherence with BOH Pi2s. Furthermore, the THEMIS probe data showed no evidence of Pi2 signals at 9.0–18.0 MLT, which is consistent with previous studies. Most of the high‐coherence events were detected when the local time separation between the THEMIS probes and BOH was less than 3 h. These observations suggest that Pi2 wave energy is lost as it propagates azimuthally from a source region localized in longitude. From longitudinally separated simultaneous multipoint observations at THEMIS probes and BOH and HER stations, we found that the Pi2 frequency varies with longitude both in space and on the ground. This implies that although plasmaspheric fast mode waves establish a standing wave structure on a given meridional plane, their frequency changes with longitude if the plasmasphere is not axisymmetric. Finally, we show that a low‐latitude daytime Pi2 is not a fast mode wave propagating to the dayside through the magnetosphere.

The Wave-Like Auroral Structure around Auroral Expansion Onset

We present the direct connection between the wave-like auroral structure around the time of auroral expansion onset and the ballooning mode waves in the near-Earth magnetotail. Based on the NASA mission time history of events and macroscale interactions during substorms (THEMIS) ground-based all-sky imagers, we show that around the time of auroral expansion onset, a wave-like auroral structure first has four luminosity peaks separated by 2–3° magnetic longitude (MLON). Subsequently, the wave-like structure propagates in the azimuthal direction and an overall bright arc spans approximately 1 h magnetic local time. The wavelength is estimated to be 120–180 km. Finally, a noticeable poleward auroral expansion is observed. The ballooning mode waves are identified by two THEMIS probes in the near-Earth magnetotail. The observed wavelength of the ballooning mode waves is approximately equal to the order of the ion Larmor radius. The wavelength of 1500–3000 km in the near-Earth magnetotail is comparable with the wave-like auroral structure estimate. This study suggests that the ballooning mode waves might play a crucial role in auroral expansion onset, corresponding to the wave-like auroral structure in this study.

Analysis of radiation belt energetic electron phase space density using THEMIS SST measurements: Cross-satellite calibration and a case study

[1] In this study we perform an energy channel dependent cross-satellite calibration of Time History of Events and Macroscale Interactions during Substorms (THEMIS) solid state telescope (SST) flux measurements based on electron phase space density (PSD) conjunctions at fixed phase space coordinates. By comparing PSD around L* = 6 between THEMIS SST and Los Alamos National Laboratory (LANL) satellite LANL-01A synchronous orbit particle analyzer (SOPA) for a half year period starting from 1 July 2007, we evaluate systematic errors in the THEMIS SST measurements and quantify the cross-instrument calibration factors for the 11 SST energy channels from 40 to 2159 keV. Good consistency in electron PSD conjunctions between the five THEMIS probes indicates that the SST instrument aboard each spacecraft responds quite similarly to the ambient electron radiation environment. Compared to the LANL-01A SOPA instrument, the THEMIS SST underestimates the electron fluxes within a factor of 2 for the 40–140 keV energy channels and overestimates the electron fluxes within a factor of 3 for the 204–2159 keV energy channels. Using the cross-satellite calibrated SST flux data for the five THEMIS spacecraft and the SOPA measurements from the LANL-01A and 1989–048 satellites, we analyze the response of radiation belt electrons to a sudden solar wind pressure enhancement event. The electron PSD conjunctions between the THEMIS probes and the two geostationary satellites show good agreement, suggesting a reasonable cross-satellite calibration of the SST measurements. Our results also indicate a clear correlation between electron PSD dropouts and the solar wind pressure pulse, which is likely due to the combination of magnetopause shadowing and outward radial diffusion. The gradual buildup of electron PSD after the abrupt pressure enhancement most likely results from a combined effect of local acceleration and inward and outward radial diffusion that refills the main phase PSD dropout when the magnetopause moves outward. A longer-term quantitative analysis of the temporal evolution and radial profile of electron PSD based on multiple satellite measurements, including the cross-satellite calibrated THEMIS SST data, will be required to improve our understanding of the dynamic responses of radiation belt electrons to solar activity.

Multipoint in situ and ground‐based observations during auroral intensifications

Analysis of in situ and ground‐based observations is performed to establish the precise timing of observed signatures during two successive auroral activations. The first, minor activation was interpreted as a pseudobreakup, while the second, major one was classified as a substorm. Timing of observations aboard four of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes situated in the plasma sheet in the tail‐aligned conjunction indicates initial activity at XGSM ≈ −15 RE. Magnetic field variations, tailward fast flows, and signatures in particle behavior observed by two THEMIS probes in the midtail plasma sheet suggest magnetic reconnection as the source of the first activation. The substorm onset, detected 6 min after the pseudobreakup, was found to be associated with the rapid decrease of the magnetic field strength, dipolarization, and increase of plasma density and pressure, i.e., signatures of the cross‐tail current reduction (disruption), observed in the near‐Earth plasma sheet at XGSM > −10 RE. Thus, in this case, reconnection in the midtail preceded the near‐Earth current reduction. A scenario based on a model of the near‐Earth breakup triggered by the fast earthward flow, generated by preceding reconnection, is proposed.