Katsuhide Marubashi

Magnetic Field Configuration Models and Reconstruction Methods for Interplanetary Coronal Mass Ejections

This study aims to provide a reference for different magnetic field models and reconstruction methods for interplanetary coronal mass ejections (ICMEs). To understand the differences in the outputs of these models and codes, we analyzed 59 events from the Coordinated Data Analysis Workshop (CDAW) list, using four different magnetic field models and reconstruction techniques; force-free fitting, magnetostatic reconstruction using a numerical solution to the Grad–Shafranov equation, fitting to a self-similarly expanding cylindrical configuration and elliptical, non-force-free fitting. The resulting parameters of the reconstructions for the 59 events are compared statistically and in selected case studies. The ability of a method to fit or reconstruct an event is found to vary greatly; this depends on whether the event is a magnetic cloud or not. We find that the magnitude of the axial field is relatively consistent across models, but that the axis orientation of the ejecta is not. We also find that there are a few cases with different signs of the magnetic helicity for the same event when we leave the boundaries free to vary, which illustrates that this simplest of parameters is not necessarily always clearly constrained by fitting and reconstruction models. Finally, we examine three unique cases in depth to provide a comprehensive idea of the different aspects of how the fitting and reconstruction codes work.

New method for detecting interplanetary flux ropes

We identified candidate flux ropes by using interplanetary magnetic field data obtained by the Wind spacecraft and selecting the periods when the third derivative of each interplanetary magnetic field component with respect to time was continuously smaller than a certain value. If the data gathered during these periods fit a cylindrical force-free field model, the candidates were classified as flux ropes. There were too many flux ropes that had a large impact parameter value to be explained by the finite lengths of the flux ropes (or possibly the curvatures of the flux rope axes). We thus excluded flux ropes having a large impact parameter value from our analysis of fitted parameters. We took into account short-duration flux ropes (2–10 hours), and we found that the number of small flux ropes is much larger than that of large ones. We found that this method works well in identifying flux ropes provided that valid criteria have been established.

A Comparison of the Initial Speed of Coronal Mass Ejections with the Magnetic Flux and Magnetic Helicity of Magnetic Clouds

To investigate the relationship between the speed of a coronal mass ejection (CME) and the magnetic energy released during its eruption, we have compared the initial speed of CMEs (V CME) and the two parameters of their associated magnetic clouds (MC), magnetic flux (F MC), and magnetic helicity per unit length (|H MC|/L), for 34 pairs of CMEs and MCs. The values of these parameters in each MC have been determined by fitting the magnetic data of the MC to the linear force-free cylindrical model. As a result, we found that there are positive correlations between V 2 CME and F MC, and between V 2 CME and |H MC|/L. It is also found that the kinetic energy of CMEs (E CME) is correlated with F MC and |H MC|/L of the associated MC. In contrast, we found no significant correlation between V MC2 and F MC, nor between V MC2 and |H MC|/L. Our results support the notion that the eruption of a CME is related to the magnetic helicity of the source active region.

Soft X-ray Solar Activities Associated with Interplanetary Magnetic Flux Ropes

We studied the soft X-ray solar events that could be associated with the interplanetary magnetic flux ropes observed by the WIND satellite during 1995 through 1998. The timings of the launches of the magnetic flux ropes from the Sun were estimated using flux rope speeds derived by the fitting of a cylindrical model. In the reasonable time window, soft X-ray solar signatures were found in approximately 70% of the flux ropes. Parameters (e.g., axis direction, strength of magnetic field, radius, and helicity) of the magnetic flux ropes obtained by the model fitting were compared with the characteristics of the corresponding soft X-ray events observed by Yohkoh. According to the result of the comparison, the magnetic flux ropes with strong magnetic fields or high speeds were observed in association with higher soft X-ray solar activities.

Electron temperature probe onboard Japan’s Mars orbiter

Japan’ s first Mars spacecraft PLANET-B was successfully launched on 4th of July, 1998 and was named “NO-ZOMI” after the launch. One of the scientific instruments is a unique electron temperature probe which was developed in Japan and has been used for more than 20 years on sounding rockets as well as on scientific satellites (Oyama, 1991). The electron temperature probe dubbed PET (Probe for Electron Temperature measurements) consists of two planar electrodes, 150 mm in diameter, placed at the edges of the two solar cell panels of the “NOZOMI” spacecraft. Electron temperatures can be measured in plasmas with densities exceeding 1000 cm−3 with sufficient accuracy. The maximum sampling rate of 8 data points per satellite spin for each probe allows high resolution measurements (i.e., an angular resolution around the spin axis of 23 degrees). Additionally, the probe can measure the anisotropy of the electron temperature, if it exists. It is also possible to infer the existence of nonthermal electrons.

Impacts of torus model on studies of geometrical relationships between interplanetary magnetic clouds and their solar origins

Our recent analysis of interplanetary magnetic clouds (MCs) showed that the orientations of MC axes determined by a model fitting with curvature of MCs taken into account (referred to as a torus model, hereafter) can be significantly different from those obtained from fittings with a straight cylinder model. Motivated by this finding, we re-examined geometrical relationships between magnetic field structures of MCs and their solar origins. This paper describes the results of the re-examination with special attention paid to two MC events, for which different orientations of MC axes were obtained from a torus model and a cylinder model. For both cases, it is shown that the torus models give the MC geometries of magnetic field structures in good agreement with those of coronal arcade structures which were formed in association with the launch of MCs along the magnetic field inversion lines. Summarizing the analysis results for 12 MCs investigated here, we conclude that: (1) the formation of coronal arcade structure is a good indication of MC formation; (2) MC geometries can be obtained that are consistent with the coronal arcades with respect to the axis orientation and the magnetic field structure including chirality, indicating that no significant direction changes occurred during the propagation of MCs through the interplanetary medium.

Torsional Alfvén Waves As Pseudo‐Magnetic Flux Ropes

We examined two classes of the solar wind magnetic field variations which show well‐ordered smooth rotations with time scales of 2–7 hours. In one class, the solar wind velocity changes in good correlation with the rotating magnetic field. It is shown that the torsional Alfven wave model can explain the observed variations in this class. In another class, no significant correlation is seen between the magnetic field and the velocity. This class is interpreted by a flux rope model and taken as a small‐scale magnetic flux rope. It is also shown that the observed magnetic field variations in the first class could be easily mistaken as small‐scale magnetic flux ropes, if a careful analysis was not undertaken. The present study provides a baseline to distinguish the above two classes of rotational variations in the solar wind magnetic field.

Geomagnetic effects of high-density plasma with southward magnetic field in the interplanetary coronal mass ejection observed on May 2-3, 1998

This paper aims to clarify the effect of high-density plasma in interplanetary coronal mass ejection (ICME) observed during the May 2–3, 1998 geomagnetic storm. The examination is performed based on the estimation of Dst index, which is calculated with the observed solar wind parameters of the ICME. The estimated Dst index variation is compared with Dst index variation provided by the World Data Center for Geomagnetism, Kyoto (WDC, Kyoto). From this examination, we find that the trend of the estimated Dst is in good agreement with that of the provided Dst when the thresholds are taken into account for both the solar wind plasma density and the dawn-to-dusk solar wind electric field, as 30 #/cc and 0.49 mV/m, respectively. From the result, we can conclude that the effect of high-density plasma is important on the enhancement of geomagnetic storm as well as the effect of the other solar wind parameters, such as the interplanetary magnetic field (IMF) Bz and solar wind velocity. On the other hand, the solar source of the magnetic field of this ICME is examined. The magnetic field structure of the ICME is examined by fitting the flux rope model to the observed magnetic field and solar wind speed. The results are compared with the magnetic structure of the bases of coronal helmet streamers. From this comparison we can find that the magnetic structure of the interplanetary flux rope is in good agreement with that of the neutral line of the base of coronal helmet streamers. The result suggests that if we look for the causes of geomagnetic storm we should take into account both the plasma structure and the magnetic structure of the base of coronal helmet streamers.

Solar activities associated with interplanetary magnetic flux ropes

Abstract A smoothly rotating interplanetary magnetic field with duration on the order of a day has been detected in solar wind observations. This structure is called an “interplanetary magnetic flux rope.” Here, we studied the relationship between interplanetary magnetic flux ropes observed by Ulysses when it was near the solar limb and coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) spacecraft. We found that interplanetary flux ropes with large inclination to the equatorial plane of the Sun were associated with CMEs with large angular widths. The angular widths were measured using the SOHO/LASCO observations. The flux ropes with small inclination were associated with CMEs with small angular widths. These observations suggest that the projection of the erupted flux ropes are observed as CMEs. Interplanetary flux ropes have the same orientation as their solar sources, CMEs.