Satoko Nakamura

Fine structure of plasmaspheric hiss

Plasmaspheric hiss has been widely regarded as a broadband, structureless, incoherent emission. In this study, by examining burst-mode vector waveform data from the Electric and Magnetic Field Instrument Suite and Integrated Science instrument on the Van Allen Probes mission, we show that plasmaspheric hiss is a coherent emission with complex fine structure. Specifically, plasmaspheric hiss appears as discrete rising tone and falling tone elements. Our study comprises the analysis of two 1 h samples within which a total of eight 1 s samples were analyzed. By means of waveform analysis on two samples, we identify typical amplitudes, phase profiles, and sweep rates of the rising and falling tone elements. The exciting new observations reported here can be expected to fuel a reexamination of the properties of plasmaspheric hiss, including a further reanalysis of the generation mechanism for hiss.

Electromagnetic Ion Cyclotron Rising Tone Emissions Observed by THEMIS Probes Outside the Plasmapause

We report observations of electromagnetic ion cyclotron (EMIC) triggered emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes outside the plasmasphere. Although these phenomena have recently received much attention because of the possibility of strong interaction with particles, only a few events of EMIC triggered emissions have been reported near the equatorial plasmapause. We performed a survey of the THEMIS probe data and found various types of emissions mainly on the dayside at radial distances of 6–10 RE. We study three distinctive events in detail. The first is a typical event with an obvious rising tone emission in the afternoon sector. The emissions in the second event are simultaneously excited in different frequency bands separated by the cyclotron frequency of helium ions. In the third event, which occurred near local noon, rising tone emissions were excited in an extended region near the equator where the field-aligned B gradient was much reduced because of compression of the magnetosphere by the solar wind. We compare these events with the nonlinear wave growth theory developed by Omura et al. (2010). In all events, it is found that the observed relationship between the amplitudes and frequencies of the emissions are in good agreement with the theory.

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

We 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.

Nonlinear wave growth theory of coherent hiss emissions in the plasmasphere

Recent observations of plasmaspheric hiss emissions by the Van Allen Probes show that broadband hiss emissions in the plasmasphere comprise short-time coherent elements with rising and falling tone frequencies. Based on nonlinear wave growth theory of whistler mode chorus emissions, we have examined the applicability of the nonlinear theory to the coherent hiss emissions. We have generalized the derivation of the optimum wave amplitude for triggering rising tone chorus emissions to the cases of both rising and falling tone hiss elements. The amplitude profiles of the hiss emissions are well approximated by the optimum wave amplitudes for triggering rising or falling tones. Through the formation of electron holes for rising tones and electron hills for falling tones, the coherent waves evolve to attain the optimum amplitudes. An electromagnetic particle simulation confirms the nonlinear wave growth mechanism as the initial phase of the hiss generation process. We find very good agreement between the theoretical optimum amplitudes and the observed amplitudes as a function of instantaneous frequency. We calculate nonlinear growth rates at the equator and find that nonlinear growth rates for rising tone emissions are much larger than the linear growth rates. The time scales of observed hiss emissions also agree with those predicted by the nonlinear theory. Based on the theory, we can infer properties of energetic electrons generating hiss emissions in the equatorial region of the plasmasphere.

A statistical study of EMIC rising and falling tone emissions observed by THEMIS

Electromagnetic ion cyclotron (EMIC) waves with rising or falling frequency variations have been studied intensively because of their effects on energetic particles in the Earths magnetosphere. We develop an automated classification method of EMIC events based on the characteristics of frequency variations. We report some basic statistical properties of frequency variations in EMIC waves observed over 5–10 RE by three Time History of Events and Macroscale Interactions during Substorms probes from January 2012 to December 2014. We clarify whether rising tones or falling tones are observed in each chosen 20 min time segment. In the present analysis, we find that the occurrence rate of EMIC rising or falling tone events is more than 30% of the total EMIC wave events. The dayside magnetosphere is a preferential region for the EMIC frequency variations. The occurrence rate of rising tone events is slightly greater than that of falling tone events. We examine the relation between the frequency characteristics and the magnetospheric conditions. The solar wind pressure strongly controls the occurrence rates of frequency variations. We also calculate ranges of frequency variations. Large-amplitude EMIC waves tend to have wider frequency variations, and the range of frequency variation is largest around the prenoon region. In addition, rapid variations in wave amplitudes called “subpacket structures” are found in 70% of the EMIC rising or falling tone events in the dayside region. Subpacket structures appear mainly in large-amplitude EMIC rising or falling tones. These features are consistent with nonlinear wave growth theory.

Ion hole formation and nonlinear generation of electromagnetic ion cyclotron waves: THEMIS observations

Electromagnetic plasma waves are thought to be responsible for energy exchange between charged particles in space plasmas. Such an energy exchange process is evidenced by phase space holes identified in the ion distribution function and measurements of the dot product of the plasma wave electric field and the ion velocity. We develop a method to identify ion hole formation, taking into consideration the phase differences between the gyromotion of ions and the electromagnetic ion cyclotron (EMIC) waves. Using this method, we identify ion holes in the distribution function and the resulting nonlinear EMIC wave evolution from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. These ion holes are key to wave growth and frequency drift by the ion currents through nonlinear wave-particle interactions, which are identified by a computer simulation in this study.

Direct measurements of two-way wave-particle energy transfer in a collisionless space plasma

Two-step energy transfer in space plasma Plasmas are ionized gases that contain negative electrons, positive ions, and electromagnetic fields. These constituents can oscillate in position over time, carrying energy as plasma waves. In principle, such waves could transfer energy between two different ion populations. Kitamura et al. analyzed data from the Magnetospheric Multiscale mission, a group of four spacecraft that are flying in tight formation through Earths magnetosphere. They discovered an event in which energy was transferred from hydrogen ions to plasma waves and then from the waves to helium ions. This energy transfer process is likely to occur in many other plasma environments. Science, this issue p. 1000 Energy transfer between H+ ions, plasma waves, and He+ ions is observed in a space plasma. Particle acceleration by plasma waves and spontaneous wave generation are fundamental energy and momentum exchange processes in collisionless plasmas. Such wave-particle interactions occur ubiquitously in space. We present ultrafast measurements in Earth’s magnetosphere by the Magnetospheric Multiscale spacecraft that enabled quantitative evaluation of energy transfer in interactions associated with electromagnetic ion cyclotron waves. The observed ion distributions are not symmetric around the magnetic field direction but are in phase with the plasma wave fields. The wave-ion phase relations demonstrate that a cyclotron resonance transferred energy from hot protons to waves, which in turn nonresonantly accelerated cold He+ to energies up to ~2 kilo–electron volts. These observations provide direct quantitative evidence for collisionless energy transfer in plasmas between distinct particle populations via wave-particle interactions.