Mitsue Den

Main-phase creation of "seed" electrons in the outer radiation belt

During a geomagnetic storm in early November 1993, NOAA satellite observations revealed that a population of energetic electrons appeared in the center of the outer radiation belt during the main phase of the storm. At the beginning of the main phase of the magnetic storm, the number of electrons with energies from 30 keV to 100 keV increased rapidly and contributed to build up of the ring current. At the end of the main phase the flux of electrons with energies greater than 300 keV increased significantly. Akebono satellite observations showed that the flux of electrons with energies ranging from 300 keV to 950 keV increased late of the storm main phase and that the flux of electrons with energies from 950 keV to 2.5 MeV increased during the storm recovery phase. The electron flux increase observed by both NOAA and Akebono took place first in the central part of the outer radiation belt (L~4) and propagated to higher L shells with a significant time delay. We think that the ring current electrons that appeared first and near L~4 during the main phase seeded the subsequent increase in the flux of MeV electrons in the entire outer radiation belt.

Energetic electron variation in the outer radiation zone during early May 1998 magnetic storm

Abstract Using NOAA and Akebono observations, we examined variations of the energetic electron flux in the outer radiation zone during the May 2 and 4, 1998 magnetic storm, which was a “two-step” storm. Both a flux dropout and an inward shift of the outer belt MeV electrons were recorded during the main phase of the May 2 magnetic storm. A very big injection of the intermediate energy (30– 100 keV ) electrons to the heart of the outer radiation zone took place during the main phase of the storm. During the recovery phase of the storm an increase in the MeV electron flux was seen, which surpassed the pre-storm level in one day. Comparison of NOAA and Akebono observations yields that the injected electrons with the energy of ∼100 keV seeded a subsequent enhancement of the MeV electrons in the outer radiation zone. A more inward shift of the peak position as well as a flux dropout occurred during the main phase of the May 4 magnetic storm. No significant injection of the intermediate energy (30– 100 keV ) electrons was, however, seen during the main phase of the May 4 magnetic storm. A remarkable increase of the MeV electron flux was seen during the recovery phase of the storm. The pre-existing intermediate electrons seeded the increase of the MeV electrons near the new peak portion. The increase propagated to higher L values with a significant time delay, suggesting an enhanced radial diffusion.

Effects of the IMF and substorms on the rapid enhancement of relativistic electrons in the outer radiation belt during storm recovery phase

Abstract It is often the case that the flux of the relativistic electrons in the outer radiation belt decreases substantially once the major magnetic storm takes place. After the disappearance of the relativistic electrons which can last as much as one day, an increase in the flux occurs, often to levels that exceed the pre-storm level. We have investigated a correlation of the increment of the relativistic electron flux at geosynchronous orbit with the interplanetary magnetic field (IMF) properties as well as the magnetic disturbance signature during the magnetic storms. Results demonstrate that a large flux enhancement occurs when the IMF is southward during the storm recovery phase.

Solar Flare Prediction Model with Three Machine-learning Algorithms using Ultraviolet Brightening and Vector Magnetograms

We developed a flare prediction model using machine learning, which is optimized to predict the maximum class of flares occurring in the following 24 h. Machine learning is used to devise algorithms that can learn from and make decisions on a huge amount of data. We used solar observation data during the period 2010-2015, such as vector magnetogram, ultraviolet (UV) emission, and soft X-ray emission taken by the Solar Dynamics Observatory and the Geostationary Operational Environmental Satellite. We detected active regions from the full-disk magnetogram, from which 60 features were extracted with their time differentials, including magnetic neutral lines, the current helicity, the UV brightening, and the flare history. After standardizing the feature database, we fully shuffled and randomly separated it into two for training and testing. To investigate which algorithm is best for flare prediction, we compared three machine learning algorithms: the support vector machine (SVM), k-nearest neighbors (k-NN), and extremely randomized trees (ERT). The prediction score, the true skill statistic (TSS), was higher than 0.9 with a fully shuffled dataset, which is higher than that for human forecasts. It was found that k-NN has the highest performance among the three algorithms. The ranking of the feature importance showed that the previous flare activity is most effective, followed by the length of magnetic neutral lines, the unsigned magnetic flux, the area of UV brightening, and the time differentials of features over 24 h, all of which are strongly correlated with the flux emergence dynamics in an active region.

Global MHD simulation of magnetospheric response of preliminary impulse to large and sudden enhancement of the solar wind dynamic pressure

A sudden increase in the dynamic pressure of solar wind generates a prominent and transient change in ground-based magnetometer records worldwide, which is called a sudden commencement (SC). The magnetic field variation due to an SC at high latitudes shows a bipolar change, which consists of a preliminary impulse (PI) and main impulse (MI). The largest recorded SC had an amplitude of more than 200 nT with a spiky waveform at low latitudes, and the mechanism causing this super SC is unknown. Here, we investigate the cause of the super SC using a newly developed magnetosphere-ionosphere coupling simulation, which enables us to investigate the magnetospheric response to a large increase in the solar wind dynamic pressure. To simulate SCs, the dynamic pressure of the solar wind is increased to 2, 5, 10, and 16 larger than that under the stationary condition, and two different types of dynamic pressure increase are adopted by changing the solar wind density only or the solar wind speed only. It was found that the magnetic field variations of the PI and MI are several times larger and faster for a jump in the speed than for a jump in the density. It is inferred that a solar wind velocity of more than 2500 km/s in the downstream shock, which cannot be directly simulated in this study, would be consistent with the super SC.

Development of multi-hierarchy simulation model with non-uniform space grids for collisionless driven reconnection

A multi-hierarchy simulation model aimed at magnetic reconnection studies has been developed, in which macroscopic and microscopic physics are solved self-consistently and simultaneously. In this work, the previous multi-hierarchy model by these authors is extended to a more realistic one with non-uniform space grids. Based on the domain decomposition method, the multi-hierarchy model consists of three parts: a magnetohydrodynamics algorithm to express the macroscopic global dynamics, a particle-in-cell algorithm to describe the microscopic kinetic physics, and an interface algorithm to interlock macro and micro hierarchies. For its verification, plasma flow injection is simulated in this multi-hierarchy model and it is confirmed that the interlocking method can describe the correct physics. Furthermore, this model is applied to collisionless driven reconnection in an open system. Magnetic reconnection is found to occur in a micro hierarchy by injecting plasma from a macro hierarchy.

Global simulation study for the time sequence of events leading to the substorm onset

We have developed a global simulation code which gives numerical solutions having an extremely high resolution. The substorm solution obtained from this simulation code reproduces the precise features of the substorm onset in the ionosphere. It can reproduce the onset that starts from the equatorward side of the quiet arc, two step development of the onset, and the westward traveling surge (WTS) that starts two minutes after the initial brightening. Then, we investigated the counter structures in the magnetosphere that correspond to each event in the ionosphere. The structure in the magnetosphere promoting the onset is the near-earth dynamo in the inner magnetospheric region away from the equatorial plane. The near-earth dynamo is driven by the field-aligned pressure increase due to the parallel flow associated with the squeezing, combined with equatorward field-perpendicular flow induced by the near-earth neutral line (NENL). The dipolarization front is launched from the NENL associated with the convection transient from the growth phase to the expansion phase, but neither the launch nor the arrival of the dipolarization front coincides with the onset timing. The arrival of flow to the equatorial plane of the inner magnetosphere occurs two minutes after the onset, when the WTS starts to develop toward the west. The expansion phase is further developed by this flow. Looking at the present result that the onset sequence induced by the near-earth dynamo reproduces the details of observation quite well, we cannot avoid to conclude that the current wedge (CW) is a misleading concept.

Multi-Hierarchy Simulation of Collisionless Driven Reconnection by Real-Space Decomposition

The first results on analysis of collisionless driven reconnection with a multihierarchy simulation model are reported. In the multi-hierarchy simulation model, real space in a simulation consists of three parts: a magnetohydrodynamics (MHD) domain to deal with macroscopic dynamics, a particle-in-cell (PIC) domain to solve microscopic kinetic physics from the first principle, and an interface domain to interlock the two domains. By means of multi-hierarchy simulations, the influence of macroscopic dynamics on microscopic physics of magnetic reconnection is investigated. Dynamical behaviors of collisionless reconnection in the PIC domain depend strongly on plasma inflows from the MHD domain. It is found that if the width of an MHD inflow increases as vw vA0, where vA0 is the Alfven speed at the upstream boundary, magnetic reconnection has only a single X-point., while in cases of vw2.0vA0, reconnection with multiple X-points takes place.

Polar cap potential saturation during the Bastille Day storm event using global MHD simulation

We investigated the temporal variations and saturation of the cross polar cap potential (CPCP) in the Bastille Day storm event (2000/7/15) by global magnetohydrodynamics (MHD) simulation. The CPCP is considered to depend on the electric field and dynamic pressure of the solar wind as well as on the ionospheric conductivity. Previous studies considered only the ionospheric conductivity due to solar extreme ultraviolet (EUV) variations. In this paper, we dealt with the changes in the CPCP attributable to auroral conductivity variations caused by pressure enhancement in the inner magnetosphere owing to energy injection from the magnetosphere because the energy injection is considerably enhanced in a severe magnetic storm event. Our simulation reveals that the auroral conductivity enhancement is significant for the CPCP variation in a severe magnetic storm event. The numerical results concerning the Bastille Day event show that the ionospheric conductivity averaged over the auroral oval is enhanced up to 18 mho in the case of Bz of less than -59 nT. On the other hand, the average conductivity without the auroral effect is almost 6 mho throughout the entire period. Resultantly, the saturated CPCP is about 240 kV in the former and 704 kV in the latter when Bz is -59 nT. This result indicates that the CPCP variations could be correctly reproduced when the time variation of auroral conductivity caused by pressure enhancement due to the energy injection from the magnetosphere is correctly considered in a severe magnetic storm event.

Macro- and microphysics of magnetic reconnection in a multi-hierarchy open system

Based on particle-in-cell (PIC) simulation results of collisionless driven reconnection in a steady state, an effective resistivity model is developed for a magnetohydrodynamic (MHD) simulation in order to bridge the huge gap between macro- and microphysics of magnetic reconnection. The PIC simulation reveals that the reconnection electric field sustained by microscopic physics is found to evolve so as to balance the flux inflow rate, which is determined by global dynamics in a macroscopic system. This effective resistivity model is applied to MHD phenomena controlled by magnetic reconnection in the Earths magnetosphere. Although this model does not include any adjustable parameters related to kinetic dissipation processes, some global phenomena such as the onset of magnetic substorm, dipolarization and propagation of flux rope, detailed processes of which are longstanding questions, are reproduced well in the MHD simulation and are consistent with the observations.