arXiv subjects

Motoharu Nowada

Publications and source records attributed to Motoharu Nowada.

5 recordsLinked to original sources

arXiv2024

Dependence of the Solar Wind Plasma Density on Moderate- and Extremely High-Geomagnetic Activity Elucidated by Potential Learning

The relationship between moderate and extremely high levels of geomagnetic activity, represented by the Kp index (2- to 5+ and 6- to 9), and solar wind conditions during southward IMF intervals was revealed utilizing a newly developed machine learning technique. Potential learning (PL) is a neural network algorithm that emphasizes input parameters with the highest variance during training and identifies the most significant ones influencing the outputs based on a computed metric called "potentiality". We focus on the dependence of solar wind plasma density on moderate-geomagnetic conditions. It has been unclear from what stage of geomagnetic activity the solar wind density begins to control the Kp level. Previously, PL extracted solar wind velocity as the predominant parameter at extremely low (0 to 1+) and high-Kp ranges under southward IMF. In this study, the IMF three components, solar wind speed, and plasma density from the OMNI database (1998-2019), covering solar cycle 23 to early 25, were used as inputs. Again, PL selected solar wind velocity as the most significant parameter for moderate and extremely high Kp. The potentiality of solar wind density for these ranges was, however, 3.5 times higher than in the previous study, suggesting its impact on geomagnetic activity cannot be ignored. We statistically investigated the relation between solar wind speed and plasma density used as PL inputs under all Kp levels. Above moderate Kp, geomagnetic conditions become high even under slow solar wind if density is large, suggesting that not only velocity but also density contributes significantly. These PL and statistical investigations show that solar wind density begins to regulate Kp above moderate geomagnetic activity under southward IMF. They also help understand the relationship between solar wind and geomagnetic activity and forecast geomagnetic activity under various IMF conditions.

physics.space-ph↗

arXiv2024

Contemporaneous Appearances of Auroral Spiral and Transpolar Arc: Polar UVI Observations and Global MHD Simulations

A local vortex-structured aurora and a large-scale transpolar arc (TPA) were contemporaneously observed by the Polar ultraviolet imager (UVI) during the late recovery phase of a substorm, and the interplanetary magnetic field (IMF) BY and BZ were negative and negative-to-positive. The TPA grew along the dawnside auroral oval from the nightside to the dayside, and an auroral spiral and several spots were located azimuthally near the poleward edge of the nightside auroral oval. Both auroras had tailward elongated source regions with scales of ~30 RE (spiral) and more than ~45 RE (TPA). To examine their magnetospheric/ionospheric field-aligned current (FAC) profiles, we performed global magnetohydrodynamic (MHD) simulations, using two different types of code: Block-Adaptive-Tree Solar-wind Roe Upwind Scheme (BATS-R-US) and improved REProduce Plasma Universe (REPPU). Both MHD simulations reproduced the tailward elongated TPA-associated FAC structures. The spiral-associated FAC intensity was, however, approximately three orders of magnitude weaker than the TPA-associated FAC intensity. Only improved REPPU simulations replicated faint but continuous poleward extending streak-like structures without evident FACs, instead of the auroral spiral. Geomagnetic field measurements showed that the spiral had upward (from the ionosphere to the magnetosphere) FACs, and its appearance might be accompanied by ultra-low-frequency Pc5 waves. Our results suggest that 1) a local-scale spiral might be formed with much weaker magnetotail FACs than global-scale TPA-associated FACs, although the spiral source region is elongated tailward, and 2) a solar wind-magnetosphere-ionosphere coupling system with minimal or no significant substorm effects is required to form the spiral with the weak magnetotail FACs.

physics.space-ph↗

arXiv2023

Auroral Morphological Changes to the Formation of Auroral Spiral during the Late Substorm Recovery Phase: Polar UVI and Ground All-Sky Camera Observations

The ultraviolet imager (UVI) of the Polar spacecraft and an all-sky camera at Longyearbyen contemporaneously detected an auroral vortex structure (so-called "auroral spiral") on 10 January 1997. From space, the auroral spiral was observed as a "small spot" (one of an azimuthally-aligned chain of similar spots) in the poleward region of the main auroral oval from 18 h to 24 h magnetic local time. These auroral spots were formed while the substorm-associated auroral bulge was subsiding and several poleward-elongated auroral streak-like structures appeared during the late substorm recovery phase. During the spiral interval, the geomagnetically north-south and east-west components of the geomagnetic field, which were observed at several ground magnetic stations around Svalbard island, showed significant negative and positive bays caused by the field-aligned currents related with the aurora spiral appearance. The negative bays were reflected in the variations of local geomagnetic activity index (SML) which was provided from the SuperMAG magnetometer network at high latitudes. To pursue the spiral source region in the magnetotail, we trace each UVI image along field lines to the magnetic equatorial plane of the nightside magnetosphere using an empirical magnetic field model. Interestingly, the magnetotail region corresponding to the auroral spiral covered a broad region from Xgsm ~ -40 to -70 RE at Ygsm ~ 8 to 12 RE. The appearance of this auroral spiral suggests that extensive areas of the magnetotail (but local regions in the ionosphere) remain active even when the substorm almost ceases, and geomagnetic conditions are almost stable.

physics.space-ph↗

arXiv2022

Ionospheric Plasma Flows Associated with the Formation of the Distorted Nightside End of A Transpolar Arc

We investigate ionospheric flow patterns occurring on 28 January 2002 associated with the development of the nightside distorted end of a J-shaped transpolar arc (nightside distorted TPA). Based on the nightside ionospheric flows near to the TPA, detected by the SuperDARN (Super Dual Auroral Radar Network) radars, we discuss how the distortion of the nightside end toward the pre-midnight sector is produced. The J-shaped TPA was seen under southward Interplanetary Magnetic Field (IMF) conditions, in the presence of a dominant dawnward IMF-By component. At the onset time of the nightside distorted TPA, particular equatorward plasma flows at the TPA growth point were observed in the post-midnight sector, flowing out of the polar cap and then turning toward the pre-midnight sector of the main auroral oval along the distorted nightside part of the TPA. We suggest that these plasma flows play a key role in causing the nightside distortion of the TPA. SuperDARN also found ionospheric flows typically associated with Tail Reconnection during IMF Northward Non-substorm Intervals (TRINNIs) on the nightside main auroral oval, before and during the TPA interval, indicating that nightside magnetic reconnection is an integral process to the formation of the nightside distorted TPA. During the TPA growth, SuperDARN also detected anti-sunward flows across the open-closed field line boundary on the dayside that indicate the occurrence of low-latitude dayside reconnection and ongoing Dungey cycle driving. This suggests that nightside distorted TPA can grow even in Dungey-cycle-driven plasma flow patterns.

physics.space-ph↗

arXiv2022

Investigation of the Relationship between Geomagnetic Activity and Solar Wind Parameters Based on A Novel Neural Network (Potential Learning)

Predicting geomagnetic conditions based on in-situ solar wind observations allows us to evade disasters caused by large electromagnetic disturbances originating from the Sun to save lives and protect economic activity. In this study, we aimed to examine the relationship between the Kp index, representing global magnetospheric activity level, and solar wind conditions using an interpretable neural network known as potential learning (PL). Data analyses based on neural networks are difficult to interpret; however, PL learns by focusing on the "potentiality of input neurons" and can identify which inputs are significantly utilized by the network. Using the full advantage of PL, we extracted the influential solar wind parameters that disturb the magnetosphere under southward Interplanetary magnetic field (IMF) conditions. The input parameters of PL were the three components of the IMF (Bx, By, -Bz(Bs)), solar wind flow speed (Vx), and proton number density (Np) in geocentric solar ecliptic (GSE) coordinates obtained from the OMNI solar wind database between 1998 and 2019. Furthermore, we classified these input parameters into two groups (targets), depending on the Kp level: Kp = 6- to 9 (positive target) and Kp = 0 to 1+ (negative target). Negative target samples were randomly selected to ensure that numbers of positive and negative targets were equal. The PL results revealed that solar wind flow speed is an influential parameter for increasing Kp under southward IMF conditions, which was in good agreement with previous reports on the statistical relationship between the Kp index and solar wind velocity, and the Kp formulation based on the IMF and solar wind plasma parameters. Based on this new neural network, we aim to construct a more correct and parameter-dependent space weather forecasting model.

physics.space-ph↗