The Korean heliospheric community, led by the Korea Astronomy and Space Science Institute (KASI), is currently assessing the viability of deploying a spacecraft at the Sun-Earth Lagrange Point L4 in collaboration with National Aeronautics and Space Administration (NASA). The aim of this mission is to utilize a combination of remote sensing and in situ instruments for comprehensive observations, complementing the capabilities of the L1 and L5 observatories. The paper outlines long-term scientific objectives, underscoring the significance of multi-point in-situ observations to better understand critical heliospheric phenomena. These include coronal mass ejections, magnetic flux ropes, heliospheric current sheets, kinetic waves and instabilities, suprathermal electrons and solar energetic particle events, as well as remote detection of solar radiation phenomena. Furthermore, the mission’s significance in advancing space weather prediction and space radiation exposure assessment models through the integration of L4 observations is discussed. This article is concluded with an emphasis on the potential of L4 observations to propel advancements in heliospheric science.

The Ionospheric Anomaly Monitoring by Magnetometer And Plasma-probe (IAMMAP) is one of the scientific instruments for the Compact Advanced Satellite 500-3 (CAS 500-3) which is planned to be launched by Korean Space Launch Vehicle in 2024. The main scientific objective of IAMMAP is to understand the complicated correlation between the equatorial electro-jet (EEJ) and the equatorial ionization anomaly (EIA) which play important roles in the dynamics of the ionospheric plasma in the dayside equator region. IAMMAP consists of an impedance probe (IP) for precise plasma measurement and magnetometers for EEJ current estimation. The designated sun-synchronous orbit along the quasi-meridional plane makes the instrument suitable for studying the EIA and EEJ. The newly-devised IP is expected to obtain the electron density of the ionosphere with unprecedented precision by measuring the upper-hybrid frequency (fUHR) of the ionospheric plasma, which is not affected by the satellite geometry, the spacecraft potential, or contamination unlike conventional Langmuir probes. A set of temperaturetolerant precision fluxgate magnetometers, called Adaptive In-phase MAGnetometer, is employed also for studying the complicated current system in the ionosphere and magnetosphere, which is particularly related with the EEJ caused by the potential difference along the zonal direction.

Ionospheric Anomaly Monitoring by Magnetometer And Plasma-probe (IAMMAP) is a suite of scientific instruments used in the Compact Advanced Satellite 500-3 (CAS 500-3), which is planned to be launched by the KSLV (Korean Space Launch Vehicle) in 2025. The main scientific objective of IAMMAP is to understand the complicated correlation between the equatorial electrojet (EEJ) and the equatorial ionization anomaly (EIA), both of which play important roles in the dynamics of ionospheric plasma in the dayside equator region. The magnetic field generated by the EEJ is tiny (~several nT) compared to the background geomagnetic field (~30,000 nT); thus, a high-resolution magnetometer with a wide dynamic range is required to investigate its correlation with the EIA. To achieve this required resolution, IAMMAP includes a set of improved fluxgate magnetometers named the AIMAG (Adaptive In-phase MAGnetometer), which was developed by adopting new technologies. Here, we report the analysis results of the manufactured AIMAG performance based on trade-off studies for optimizing the circuit and sensor configurations. The results support that the AIMAG sensor and electronic circuits with new concepts and technologies will lead to successful observations of EEJ signatures in the ionosphere.

In this study, we identified 188 heliospheric current sheet (HCS) crossing events observed by Voyager 2 between 1977 and 1990, spanning heliocentric distances from 1 to 33.6 au. The identified HCS crossings revealed well-defined trends including a >30% decrease in magnetic field intensity and >20% increases in proton density and temperature. The Harris sheet model fitting to all 188 HCS crossing events indicates that the estimated HCS thickness spans over 2 orders of magnitude, ranging from 7 × 10 3 to 2.5 × 10 6 km, with a median value of 3.3 × 10 5 km. For 84.6% of the events, the HCS planes lie within − 30 ° to 30 ° in latitude from the equatorial plane, whereas the others indicate highly tilted HCS planes. Similarly, for 85.6% of the events, the HCS planes align with the Parker spiral field from the Parker field direction (within 30°), whereas the others exhibit significant longitudinal tilt. Additionally, a substantial fraction of HCS crossings indicate a significant normal magnetic field component (e.g., B normal / B > 0.3 for 38.8% of the total events), implying rotational discontinuities. Comparing our results with previous studies on HCS thickness from near the Sun to the heliosheath, we find a general increase in thickness with distance. Notably, at 1–33.6 au, HCS thickness remains significantly larger than the ion inertial length, implying conditions generally unfavorable for magnetic reconnection. This contrasts with recent reports of frequent reconnection at HCSs closer to the Sun. Our findings provide a comprehensive insight into the HCS structures across a wide range of heliospheric distances.

Context. Magnetic flux ropes are frequently observed in association with magnetic reconnection in space plasmas, yet their formation, evolution, and observational visibility are not fully understood, particularly with respect to the near-Sun heliosphere. Aims. We investigate the properties and origin of magnetic flux ropes embedded within reconnection exhausts during heliospheric current sheet (HCS) crossings close to the Sun. We assess the conditions under which such structures are most readily identifiable in in situ measurements. Methods. We analyzed high-resolution magnetic field and plasma observations from Parker Solar Probe (PSP) during two consecutive HCS crossings, separated by ∼10.5 hours, at a heliocentric distance of ∼12 R _⊙ . Flux ropes were identified, with particular attention paid to intervals when PSP traversed the HCS central regions. Results. For each crossing, we identified a series of flux ropes embedded within reconnection exhausts. Their passage durations are below 20 seconds, corresponding to spatial scales of a few thousands kilometers. This result is still greater by three orders of magnitude than the ion inertial length. This identification was possible particularly during intervals when PSP was closest to the central region of HCS. These flux ropes can be distinguished from the background exhausts by enhancements in magnetic field strength, which are shown to be particularly significant in the guide field component. In addition, they travel at speeds that diverge slightly (typically by < 10 km/s) from the speeds of the surrounding outflows, along with the possibility of increased density and reduced temperatures. Conclusions. We attributed the origin of magnetic flux ropes to a secondary reconnection within the exhausts and subsequent merging of smaller flux ropes into larger structures, consistent with predictions by various simulations. We stress that such flux ropes are most readily identifiable at the HCS center, where the background magnetic field is weakest, so that the relative enhancement in flux rope field becomes most prominent. This observational advantage is particularly notable closer to the Sun, where the high ambient magnetic field strength can otherwise obscure such structures, unless the spacecraft trajectory remains within the HCS central region for a sufficient duration.

This paper presents ground calibration and orthogonality correction methods for the tri-axis fluxgate magnetometer (FGM), named as adaptive in-phase magnetometer (AIMAG), aboard the CAS500-3 satellite. The orthogonality errors of the FGM among the axes can lead to significant inaccuracies in magnetic field measurements. In this study, we employed Helmholtz coils and an autocollimator to apply controlled magnetic fields and adjust the magnetometer’s alignment. By deriving the correction matrix, we could transfer the sensor axes to the ideal orthogonal coordinate system. We validated the correction method by analyzing the sensor’s output under various magnetic field conditions. This correction method is expected to enhance the in-flight magnetic field measurements of the CAS500-3 satellite.