Mitsunori Ozaki

Displays of paternal mouse pup retrieval following communicative interaction with maternal mates

Compared with the knowledge of maternal care, much less is known about the factors required for paternal parental care. Here we report that new sires of laboratory mice, though not spontaneously parental, can be induced to show maternal-like parental care (pup retrieval) using signals from dams separated from their pups. During this interaction, the maternal mates emit 38-kHz ultrasonic vocalizations to their male partners, which are equivalent to vocalizations that occur following pheromone stimulation. Without these signals or in the absence of maternal mates, the sires do not retrieve their pups within 5 min. These results show that, in mice, the maternal parent communicates to the paternal parent to encourage pup care. This new paradigm may be useful in the analysis of the parental brain during paternal care induced by interactive communication.

Ground‐based ELF/VLF chorus observations at subauroral latitudes—VLF‐CHAIN Campaign

We report observations of very low frequency (VLF) and extremely low frequency (ELF) chorus waves taken during the ELF/VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) of 17–25 February 2012 at subauroral latitudes at Athabasca (L=4.3), Canada. ELF/VLF waves were measured continuously with a sampling rate of 100 kHz to monitor daily variations in ELF/VLF emissions and derive their detailed structures. We found quasiperiodic (QP) emissions whose repetition period changes rapidly within a period of 1 h without corresponding magnetic pulsations. QP emissions showed positive correlation between amplitude and frequency sweep rate, similarly to rising-tone elements. We found an event of nearly simultaneous enhancements of QP emissions and Pc1/electromagnetic ion cyclotron wave intensities, suggesting that the temperature anisotropy of electrons and ions developed simultaneously at the equatorial plane of the magnetosphere. We also found QP emissions whose intensity suddenly increased in association with storm sudden commencement without changing their frequency. Falling-tone ELF/VLF emissions were observed with their rate of frequency change varying from 0.7 to 0.05 kHz/s over 10 min. Bursty-patch emissions in the lower and upper frequency bands are often observed during magnetically disturbed periods. Clear systematic correlation between these various ELF/VLF emissions and cosmic noise absorption was not obtained throughout the campaign period. These observations indicate several previously unknown features of ELF/VLF emissions in subauroral latitudes and demonstrate the importance of continuous measurements for monitoring temporal variations in these emissions.

Polarization analysis of VLF/ELF waves observed at subauroral latitudes during the VLF-CHAIN campaign

Chorus wave emissions are one of the most intense naturally occurring phenomena in the very low (VLF) and extremely low frequency (ELF) ranges. They are believed to be one of the major contributors to acceleration and loss of electrons in the radiation belts. During the VLF Campaign observation with High-resolution Aurora Imaging Network (VLF-CHAIN) from 17 to 25 February 2012, several types of VLF/ELF emissions, including chorus, were observed at subauroral latitudes in Athabasca, Canada. To our knowledge, there has not been any comprehensive study of the physical properties of such emissions at these latitudes. In this study, we calculate spectral and polarization parameters of VLF/ELF waves with high temporal resolution. We found that the polarization angle of several emissions depended on both frequency and time. We suggest that the frequency-dependent events, which usually last several tens of minutes, might be the consequence of the broadening of the ray path that the waves follow from their generation region to the ground. Furthermore, time-dependent events, also lasting tens of minutes, have a polarization angle that changes from negative to positive values (or vice versa) every few minutes. We suggest that this could be due to variations of the wave duct, either near the generation region or along the wave propagation path. Using another ground station in Fort Vermillion, Canada, about 450 km northwest of Athabasca, we tracked the movements of the ionospheric exit point of three chorus emissions observed simultaneously at both stations. Although we found that movement of the ionospheric exit point does not follow a general direction, it is subject to hovering motion, suggesting that the exit point can be affected by small-scale plasma processes.

Current-Sensitive CMOS Preamplifier for Investigating Space Plasma Waves by Magnetic Search Coils

In order to considerably reduce circuit resources (mass, volume, and power) for the analog front ends of plasma wave measurement systems, a current-sensitive preamplifier for magnetic search coils (MSCs) is designed with standard 0.25- μm complementary metal-oxide-semiconductor (CMOS) technology. Since the input noise current determines the output noise levels around the resonant frequency of an MSC, a CMOS preamplifier operating with low noise current is suitable when combined with an MSC instead of using bipolar junction transistors. The current-sensitive CMOS preamplifier consists of a current amplifier for the detection of current flowing through the MSC and an equalizer for adjusting the transfer function. A prototype of the circuit structure was fabricated on a 1.9 × 3.3- mm2-silicon chip. The noise equivalent magnetic induction of the CMOS preamplifier combined with a 100-mm-long MSC is 3.5 pT/Hz1/2 at 10 Hz and 30 fT/Hz1/2 at 2 kHz with a power consumption of 4.6 mW for a 3.3-V supply. This paper is the first to examine a current-sensitive CMOS preamplifier for probing plasma waves.

Dual-Resonant Search Coil for Natural Electromagnetic Waves in the Near-Earth Environment

A dual-resonant search coil (DRSC) is designed as a wideband magnetometer for natural electromagnetic waves in the near-Earth environment. Since the frequency response of a search coil is determined by its resonant frequency, its bandwidth can be increased by using multiple resonances. Two resonances are obtained by capacitive coupling with negative mutual inductance between two coils. The resulting measurement band of the DRSC is from 10 Hz to 1 MHz for a sensor 100 mm in length, 25 mm in diameter, and 0.11 kg in mass. A theoretical analysis of the resonant frequencies and gain profile of the DRSC agree with measurements. The noise equivalent magnetic induction of the DRSC is 40 fT/Hz1/2 at 3 kHz and 14 fT/Hz1/2 at 140 kHz. This magnetometer is the first to realize two resonances using a single sensor and preamplifier.

Equivalent Circuit Model for the Electric Field Sensitivity of a Magnetic Search Coil of Space Plasma

Magnetic search coils (MSCs) are sensitive to both magnetic and electric fields, but detecting electric fields is unnecessary for magnetic observations of plasma waves. However, it is important to evaluate both sensitivities for different geometries and electrostatic shields to avoid electric field pickup. An equivalent circuit model for the electric field sensitivity of an MSC in a collisionless isotropic cold plasma is developed here using electrical coupling through a sheath capacitance. That sensitivity is defined by a relationship between the MSC impedance and the sheath capacitance. To confirm the validity of the circuit model, the sensitivity to an electric field is measured by imposing an external electric field using charged parallel metallic plates in laboratory experiments. The coupling capacitance between the MSC and charged plates is equivalent to the sheath capacitance in a space plasma. The measured results show good agreement with an approximate expression deduced from the equivalent circuit model.

Numerical analysis of spherical wave absorption by a thin metamaterial absorber

Absorption and reflection characteristics of a spherical wave incident on a metamaterial absorber are numerically calculated on the basis of the plane-wave expansion technique. The absorbed and reflected field and power distributions on the absorber surface can be rigorously evaluated for a spherical wave radiated from a nearby RF source.

Radio-frequency power distribution measurement system using thin metamaterial absorber

A radio-frequency (RF) power distribution measurement system is developed. The 2-d distribution of an incident RF power is measured on the surface of a finite size metamaterial absorbers, which is designed to suppress undesired edge scattering. The system is evaluated by the measurement of an RF wave transmitted from a dipole antenna. The measured distributions of incident RF power are consistent with those obtained by EM simulations, demonstrating practical effectiveness of the technique to measure RF power distributions.

Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2>+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensity showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. Furthermore, these results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.

Simultaneous observations of magnetospheric ELF/VLF emissions in Canada, Finland, and Antarctica†

To investigate longitudinal extent of electromagnetic wave activity, we report the first simultaneous ground-based observations of magnetospheric ELF/VLF emissions at the following three longitudinally-separated stations at auroral and subauroral latitudes: Athabasca, Canada (ATH; magnetic latitude: 61.3∘N); Kannuslehto, Finland (KAN; 64.4∘N); and Syowa Station, Antarctica (SYO; 70.5∘S). The magnetic local time (MLT) separations of SYO-KAN, ATH-SYO, and ATH-KAN, are 3, 8, and 11 h, respectively. Simultaneous observation data at these stations are available for a total of 48 days in 2012-2014. The simultaneous occurrence rates of ELF/VLF emissions are 9.8%, 2.5%, and 3.6% for SYO-KAN, ATH-SYO, and ATH-KAN, respectively. We found that the simultaneous wave occurrence rate between two stations is higher in the morning-dayside sector, indicating that the longitudinal extent of the emissions exhibits MLT dependence. When emissions are simultaneously observed at two stations, the average AE and |Dst| indices tend to be higher. Similarly, if the two stations are more separated in MLT, the average |Dst| index increases. These results suggest that the longitudinal extent of ELF/VLF emissions increases with increasing geomagnetic activity.