In this paper, we test whether time periods with hot proton temperature anisotropy are associated with electromagnetic ion cyclotron (EMIC) waves and whether the plasma conditions during the observed ...waves satisfy the linear theory threshold condition. We identify 865 events observed by the Composition Distribution Function instrument onboard Cluster spacecraft 4 during 1 January 2001 to 1 January 2011 that exhibit a positive temperature anisotropy (Ahp = T⊥ h/T∥ h − 1) in the 10–40 keV protons. The events occur over an L range from 4 to 10 in all magnetic local times and at magnetic latitudes (MLATs) within ±50°. Of these hot proton temperature anisotropy (HPTA) events, only 68 events have electromagnetic ion cyclotron (EMIC) waves. In these 68 HPTA events, for those at 3.8<L ≤ 5 and |MLAT| ≤ 10°, the EMIC waves with powers >1.0 nT2/Hz mainly appear in the region with fEMIC/fH,eq < 0.8. Two stop bands are present, one near the region with fEMIC/fH,eq ≈ 0.33, the other in the region with 0.8 < fEMIC/fH,eq < 0.9. Most of the EMIC waves in the He, H, and >H bands satisfy Ahp/(Ahp + 1) > fEMIC/fH,lo, Ahp/(Ahp + 1) > 0.45 × fEMIC/fH,lo, and Ahp/(Ahp + 1) < 0.45 × fEMIC/fH,lo. fEMIC, fH,eq, and fH,lo are the EMIC wave frequency, the magnetic equatorial, and the local proton gyrofrequencies. We also find that the EMIC waves predominantly occur with Ahp > 0.25. By testing a threshold equation for the EMIC instability based on linear theory, we find that for EMIC waves with |MLAT| ≤ 10° in the He, H, and >H bands, the percentages that satisfy the predicted conditions for wave growth by the threshold equation are 15.2%, 24.6%, and 25.6%. For the EMIC waves with |MLAT| > 10° the percentages that satisfy the wave growth predicted conditions are only 2.8%, 2.6%, and 0.0%. Finally, possible reasons for the low forecast accuracies of EMIC waves are suggested.
Key Points
We do the statistical analysis of EMIC waves from a 10 year Cluster observation
We test the A_hp versus EMIC wave frequency formula of Kennel and Petschek (1966)
We test the linear theory derived by Blum et al. (2009)
The DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) spacecraft detects short bursts of lightning-induced electron precipitation (LEP) simultaneously with newly ...injected upgoing whistlers. The LEP occurs within < 1 s of the causative lightning discharge. First in situ observations of the size and location of the region affected by the LEP precipitation are presented on the basis of a statistical study made over Europe using the DEMETER energetic particle detector, wave electric field experiment, and networks of lightning detection (Météorage, the UK Met Office Arrival Time Difference network (ATDnet), and the World Wide Lightning Location Network (WWLLN)). The LEP is shown to occur significantly north of the initial lightning and extends over some 1000 km on each side of the longitude of the lightning. In agreement with models of electron interaction with obliquely propagating lightning-generated whistlers, the distance from the LEP to the lightning decreases as lightning proceed to higher latitudes.
Using the four‐spacecraft Cluster system, we analyze rapid neutral sheet crossings near the Cluster apogee at about −18RE. In case studies of multiple oscillations of the locally quiet plasma sheet ...as well as in a statistical study of oscillations in dawn and dusk near‐flank plasma sheet portions we typically obtain that locally these dynamical current sheets are very corrugated and that subsequent crossings basically show portions of large‐scale kink‐like waves propagating from the tail center toward flanks. Propagation velocities are in the range of several tens km/s for the locally quiet sheets, and up to 200 km/s during fast flows. These results suggest that the flapping motions are of internal origin and that kink‐like waves are emitted in the central part of the tail by some impulsive source and propagate toward the tail flanks. The wave properties do not match any local excitation mechanism previously discussed so far in the literature.
We provide evidence of the simultaneous occurrence of large‐amplitude, quasi‐parallel whistler mode waves and ion‐scale magnetic structures, which have been observed by the Cluster spacecraft in the ...plasma sheet at 17 Earth radii, during a substorm event. It is shown that the magnetic structures are characterized by both a magnetic field strength minimum and a density hump and that they propagate in a direction quasi‐perpendicular to the average magnetic field. The observed whistler mode waves are efficiently ducted by the inhomogeneity associated with such ion‐scale magnetic structures. The large amplitude of the confined whistler waves suggests that electron precipitations could be enhanced locally via strong pitch angle scattering. Furthermore, electron distribution functions indicate that a strong parallel heating of electrons occurs within these ion‐scale structures. This study provides new insights on the possible multiscale coupling of plasma dynamics during the substorm expansion, on the basis of the whistler mode wave trapping by coherent ion‐scale structures.
Key Points
Observation of ion‐scale magnetic structures coupled with whistler mode waves
Ion‐scale structures favor the whistler wave instability onset
Trapped whistlers may enhance electron precipitations in the ionosphere
The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the ...Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both.
We report on the existence of a large‐scale ion flow vortex, a curled tailward flow of solar wind H+ (SW H+), and ionospheric O+ in the Venus plasma tail. The vortex commences at dusk (−Y), driven by ...a transverse (to the solar wind) aberration flow component. Dusk magnetosheath and ionospheric ions move westward across the nightside into the dawn sector, from where the tailward and lateral flow merges into a tailward‐moving vortex. A fluid analysis of the SW H+ energy and momentum (E&M) transfer to O+ at the terminator, shows that E&M balance (efficiency ≈1) is achieved in the altitude range of 1200–600 km. Below 600 km a westward O+ flow, moving along the direction of the atmospheric superrotation, dominates. Conversely, SW H+ dominates the high‐altitude vortex. The Venus large‐scale tail vortex is hardly unique. Other gaseous celestial objects (comets) orbiting the Sun may develop similar tail vortices.
Key Points
A large‐scale ion flow vortex is formed in the Venus plasma tailA vortex driven by the aberrated + radially expanded solar wind flow over VenusAn efficient transfer of solar wind energy and momentum to dayside/dusk O+ ions
We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007–2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following ...identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfvén Mach number, and (d) Te/Tp higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvénic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, ∼81% of which lie in the slow solar wind (≤ 450 km s−1). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is ∼4.3 h, 75%<6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that Tp in STs is not significantly less than the expected Tp. Thus, whereas a low Tp is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma β∼1, force‐free modeling of STs having a magnetic flux rope geometry may be inappropriate.
Key Points
Tp in small transients is not much less thanthe expected proton temperature
Low Tp is not a robust signature of small transients
Force‐free modeling of flux rope small transients may be inappropriate
High energy resolution DEMETER satellite observations from the Instrument for the Detection of Particle (IDP) are analyzed during an electromagnetic ion cyclotron (EMIC)‐induced electron ...precipitation event. Analysis of an Interval Pulsation with Diminishing Periods (IPDP)‐type EMIC wave event, using combined satellite observations to correct for incident proton contamination, detected an energy precipitation spectrum ranging from ∼150 keV to ∼1.5 MeV. While inconsistent with many theoretical predictions of >1 MeV EMIC‐induced electron precipitation, the finding is consistent with an increasing number of experimentally observed events detected using lower resolution integral channel measurements on the POES, FIREBIRD, and ELFIN satellites. Revised and improved DEMETER differential energy fluxes, after correction for incident proton contamination shows that they agree to within 40% in peak flux magnitude, and 85 keV (within 40%) for the energy at which the peak occurred as calculated from POES integral channel electron precipitation measurements. This work shows that a subset of EMIC waves found close to the plasmapause, that is, IPDP‐type rising tone events, can produce electron precipitation with peak energies substantially below 1 MeV. The rising tone features of IPDP EMIC waves, along with the association with the high cold plasma density regime, and the rapidly varying electron density gradients of the plasmapause may be an important factor in the generation of such low energy precipitation, co‐incident with a high energy tail. Our work highlights the importance of undertaking proton contamination correction when using the high‐resolution DEMETER particle measurements to investigate EMIC‐driven electron precipitation.
Plain Language Summary
Energetic electrons are lost rapidly from the outer radiation belt. Several processes are thought to drive the electron losses. One process is through interactions with electromagnetic ion cyclotron (EMIC) waves. Theoretical studies suggest that electrons primarily with energy >1 MeV are lost through this process, however, previous experimental satellite observations indicate that precipitation bursts with much lower electron energies are more common. One issue is that the previous satellite observations were made with poor energy resolution and are challenging to interpret due to coincident proton precipitation, which contaminate the electron measurements. Here we use observations from the DEMETER satellite which we have corrected for proton contamination. The measurements, made with higher energy resolution than before, confirm that indeed, low energy electron precipitation can happen when EMIC waves drive electron losses. The study finds that this lower energy characteristic is likely to be driven by a small subset of rising tone EMIC waves, known as Interval Pulsation with Diminishing Periods (IPDP), typically confined to the magnetic local time evening sector.
Key Points
An electromagnetic ion cyclotron wave event (an interval of pulsations with diminishing period, IPDP) was studied from Low Earth Orbit
Co‐incident satellite observations detected IPDP‐induced energetic electron precipitation, starting at 150 keV, peaking at 215 keV
High‐resolution measurements from the DEMETER satellite show enhanced fluxes from 215 keV to 1.5 MeV exhibiting a “hard” power‐law spectrum
We present a case study of eight successive plasma sheet (PS) activations (usually referred to as bursty bulk flows or dipolarization fronts), associated with small individual BZGSM increases on 31 ...March 2009 (0200–0900 UT), observed by the Time History of Events and Macroscale Interactions During Substorms mission. This series of events happens during very quiet solar wind conditions, over a period of 7 h preceding a substorm onset at 1230 UT. The amplitude of the dipolarizations increases with time. The low‐amplitude dipolarization fronts are associated with few (1 or 2) rapid flux transport events (RFT, Eh>2 mV/m), whereas the large‐amplitude ones encompass many more RFT events. All PS activations are associated with small and localized substorm current wedge (SCW)‐like current system signatures, which seems to be the consequence of RFT arrival in the near tail. The associated ground magnetic perturbations affect a larger part of the contracted auroral oval when, in the magnetotail, more RFT are embedded in PS activations (>5). Dipolarization fronts with very low amplitude, a type usually not included in statistical studies, are of particular interest because we found even those to be associated with clear small SCW‐like current system and particle injections at geosynchronous orbit. This exceptional data set highlights the role of flow bursts in the magnetotail and leads to the conclusion that we may be observing the smallest form of a substorm or rather its smallest element. This study also highlights the gradual evolution of the ionospheric current disturbance as the plasma sheet is observed to heat up.
Key Points
Small dipolarization fronts not used in statistical studies also create wedgelet
The first rapid flux transport event in a sequence creates the wedgelet
The size or amplitude of a substorm seems to depend on how much it comprises RFT
A small separation between Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes allows us to analyze a sudden activation in the near‐Earth current sheet (CS) at ...microscales. The start of the activation coincides with the appearance of an earthward plasma flow and dipolarization front (DF) at THEMIS location. The time sequence of observations of the fast plasma flow and the associated DF by three THEMIS probes denotes their dawnward displacement and the localization of the flow channel in the dawn‐dusk direction. The onset of kink perturbations of the CS was generated on the dawn side of the flow. These fluctuations also propagated dawnward and were followed by the CS thinning (L ~ ρi) and by the development of tearing instability with transient appearance of a magnetic null point. The region of the unstable CS with a magnetic null point was localized in the X and, possibly, in the Y directions. The CS perturbations were most likely triggered by the intrusion of the fast flow into the ambient plasma in the course of the global dawnward displacement of the flow structure. Although no substorm onset was observed during the CS activation, a ground signature of a pseudobreakup was detected just after the excitement of the tearing mode in the near‐Earth tail. Probably the pseudobreakup was caused by a localized diversion of the current, which could result from the disruption of the cross‐tail current in a localized region of the near‐Earth CS.
Key Points
The intrusion of fast flow triggered kink and tearing modes in the CS
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