Shoya Matsuda

Electromagnetic ion cyclotron waves suggesting minor ion existence in the inner magnetosphere observed by the Akebono satellite

It is well known that electromagnetic ion cyclotron (EMIC) waves exhibit characteristic frequencies on the basis of dispersion relations in multiple-component plasma. We present a case of EMIC waves exhibiting a sudden decrease in intensity (characteristic lower cutoff) to just above half of the proton cyclotron frequency observed in the vicinity of the geomagnetic equator by the Akebono satellite along its trajectory during a magnetic storm in April 1989. It was found that the waves propagate with a large wave normal angle with respect to the geomagnetic field line and that they had a crossover frequency above the characteristic lower cutoff. Because of stormy conditions, ion constituents were expected to fluctuate, suggesting that the characteristic frequencies of EMIC waves should have been fluctuating as well. However, the characteristic frequencies of each event did not vary despite disturbances in the inner magnetosphere, represented by a sudden decrease in the Dst index and electron density fluctuation. In addition, the waves were repeatedly observed within a half day after sudden decreases in Dst; however, they disappeared when the recovery of the Dst index became moderate. Wave generation appears to be closely correlated to fresh energetic particle injection. We study the dispersion relations of EMIC waves under the condition of multiple ion species and suggest the existence of a few percent of alpha particles (He++) or deuterons (D+), which can explain the lower cutoff of EMIC waves in the inner magnetosphere.

Pulsating aurora from electron scattering by chorus waves

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

The ERG Science Center

The Exploration of energization and Radiation in Geospace (ERG) Science Center serves as a hub of the ERG project, providing data files in a common format and developing the space physics environment data analysis software and plug-ins for data analysis. The Science Center also develops observation plans for the ERG (Arase) satellite according to the science strategy of the project. Conjugate observations with other satellites and ground-based observations are also planned. These tasks contribute to the ERG project by achieving quick analysis and well-organized conjugate ERG satellite and ground-based observations.

High‐altitude M/Q=2 ion cyclotron whistlers in the inner magnetosphere observed by the Akebono satellite

An ion cyclotron whistler is a left-handed polarized electromagnetic ion cyclotron mode wave converted from a lightning electron whistler. Some ion cyclotron whistlers have been identified as deuteron whistlers. This paper reports M/Q = 2 ion cyclotron whistlers observed by the Akebono satellite in the altitude region around 3200–10000 km (L = 1.5–3.4), which is a considerably higher altitude than those previously reported. This evidence indicates that M/Q = 2 ions are present not only in the low-altitude region but also in the inner magnetosphere around L = 3.4. We estimated the ion concentration from the crossover frequency of the M/Q = 2 ion cyclotron whistler wave observed in the altitude region around 4200 km (L = 1.7) and concluded that the concentration of M/Q = 2 ions was plausibly up to 12.6% during this event. Our observation results give important clues to study unknown minor ion profiles such as circulation mechanism of deuterons or injection mechanism of alpha particles in the solar wind.

M/Q = 2 ion distribution in the inner magnetosphere estimated from ion cyclotron whistler waves observed by the Akebono satellite

In this study, we examine the spatial occurrence distributions of H+, He+, and M/Q = 2 ion band ion cyclotron whistler waves observed by the Akebono satellite below an altitude of 10,500 km. These ion cyclotron whistler waves are categorized as electromagnetic ion cyclotron mode waves with characteristic spectrum properties that depend on the ion composition in the plasma. Statistical research is particularly important for clarifying the variation of ion composition in a plasmaspheric ion environment. In this study, essential differences are noted among the observed regions of each ion cyclotron whistler wave band. Our statistical analysis showed that the generation of H+ band ion cyclotron whistlers in the equatorial region is difficult, while M/Q = 2 ion band ion cyclotron whistlers are frequently observed near this region. Thus, a certain amount of M/Q = 2 ions is evident. To explain these statistical results, we propose a model for generation of several bands of ion cyclotron whistlers along a propagation path. In addition, we examine the magnetic local time dependence of the observed ion cyclotron whistlers. The spatial occurrence distribution of the M/Q = 2 ion cyclotron whistler waves is greatest inward of L ∼ 2.4 in the local dayside; however, they extend to L ∼ 3.0 in the local nightside. Our results suggest a density enhancement process of M/Q = 2 ions in the nightside plasmasphere, which is consistent with previous satellite observations. Thus, this study presents important knowledge on the effects of minor ion population on wave propagation and generation.

Ion cyclotron whistlers related to heavy minor ions observed by the akebono satellite and their distribution in the inner magnetosphere

It is well known that lightning whistler waves are caused by lightning discharge; these waves propagate along geomagnetic field lines as R-mode plasma waves of less than several tens kHz. Ion cyclotron whistler waves, which are electromagnetic ion cyclotron (EMIC) mode waves, have a close relation to lightning whistlers. Propagation characteristics of ion cyclotron whistler strongly depend on ion concentrations in plasma as well as nature of general EMIC waves. Gurnett et al.(1965) proposed a generation mechanism for ion cyclotron whistlers along their propagation characteristics. One of the most important features is the lowest frequency of an ion cyclotron whistler that denotes the local crossover frequency (ωcr) of the EMIC mode wave. The asymptotic frequency of a typical ion cyclotron whistler is close to each local ion cyclotron frequency (Ωi). These facts suggest that we can estimate the ion species and concentrations at a local point or in the propagation path of the waves by analyzing these characteristics of ion cyclotron whistlers.