Ensang Lee

Plasma blob events observed by KOMPSAT-1 and DMSP F15 in the low latitude nighttime upper ionosphere

[1] In this paper we report plasma blob events (plasma density enhancements) that were observed from KOMPSAT-1 (685-km altitude, 2250 LT) and from DMSP F15 (840-km altitude, 2130 LT) in the low-latitude F region. The blobs were observed mostly along the ±15° magnetic latitudes. Their global distribution showed a seasonal-longitudinal dependence similar to the distribution of the equatorial plasma bubbles. The blobs drifted upward relative to the ambient plasma, and the electron temperatures and H + proportions were lower within the blobs compared to those in the background. The characteristics of the plasma blobs were similar to those of the equatorial plasma bubbles. Therefore, it is suggested that the blobs originated from the lower altitudes by a mechanism that drives an upward drift of the plasma bubbles. The blob events did not occur in a correlated way with the magnetic activity or daily variation of solar activity.

Substorms associated with azimuthal turnings of the interplanetary magnetic field

Abstract Whether the magnetospheric substorms can be triggered by the interplanetary magnetic field (IMF) variations is an important issue in the substorm research. In this work we investigate observationally the relationship between substorm activities and IMF By variations, i.e., azimuthal turnings. We have searched for the IMFs azimuthal turning events for a period of one year using the data from multispacecraft monitoring the solar wind, WIND, IMP 8 and Geotail. Based on specific selection criteria, we have found 11 such events that exhibit pure azimuthal turnings while the IMF Bz remains quasi-steady. These events are found to be mostly in reasonable temporal associations with the substorm activities which were identified by multipoint measurements using the geomagnetic bays, auroral images, geosynchronous energetic particle injections and magnetic dipolarizations. We find an average response time of ∼8– 9 min between the substorm onsets and the dayside magnetopause contact times of the IMF By turnings. The results suggest that in addition to the more popular trigger by northward turnings of the IMF, its azimuthal (both positive and negative) turnings be also regarded as another possible external trigger of substorms.

Effect of By in three-dimensional reconnection at the dayside magnetopause

Asymmetric magnetic reconnection with a non-zero By component is studied using a three-dimensional magnetohydrodynamic simulation and the results are interpreted in terms of the dayside reconnection at the magnetopause. The By component is implemented by rotating the magnetosheath field. It is seen that a magnetic bulge is formed in the magnetosheath region but displaced in the y direction due to the tilted magnetic field line geometry of reconnection. The bulge moves away from the reconnection region along the magnetopause with a speed significantly slower than that of the symmetric case. In fact, the reconnection rate is seen much slower than the symmetric case. It is also seen that the reconnection rate decreases as the angle of rotation increases. The discontinuity associated with magnetic reconnection geometry is identified as a slow shock with Alfven waves attached to it on the magnetospheric side, while on the magnetosheath side an intermediate shock is seen, which evolves into a slow shock and a r...

Effect of nonzero dawn–dusk magnetic field component on particle acceleration in three-dimensional geomagnetic reconnection

The acceleration of protons is investigated by tracing their trajectories in the electric and magnetic fields obtained from a three-dimensional magnetohydrodynamic simulation of local magnetic reconnection. The magnetic reconnection is induced by imposing a localized anomalous resistivity in two different initial configurations, the first one without a By component and the second with a positive By superimposed in the two-dimensional Harris field lying in the xz plane. Applying the trajectory calculations to the geotail condition, the previously published results have been mostly recovered for the By=0 case. For the nonzero By case, remarkable asymmetry about the z=0 plane appears in the trajectories, and the pitch angle and spatial dependencies of energy gain. For the particles found above the z=0 plane at the earthward edge from the X-line, results are similar to those in the By=0 case, while they are quite different for the particles found below the z=0 plane. The trajectories of the particles exiting ...

Computer studies of the three‐dimensional magnetic reconnection with the superimposed B y component

Three-dimensional magnetic reconnection is studied using magnetohydrodynamic simulations. The initial configuration is based on the two-dimensional Harris neutral sheet model that lies in the xz plane and is extended in the y direction. Localized anomalous resistivity is applied to the central region, and the subsequent evolution of spontaneous magnetic reconnection is observed. Special attention is given to the results with a finite By superimposed on the Harris model. Significant changes are seen in the reconnection morphology, as the By component causes asymmetries. The reconnected field lines are skewed, and the plasma flows, shock structures, and current flows show the corresponding peculiar asymmetries. The plasma sheet is also seen twisted. A broader region is affected by magnetic reconnection as By increases, and it is seen that energy conversion over the whole simulation domain is more significant when By is larger, unless By is the dominant component of the magnetic field. The field-aligned component of the current, which initially exists because of the finite By component, is enhanced off the central plane when reconnection develops, while it is reduced on the central plane. The spatial scale of resistivity affects the reconnection rate as in previous studies of By = 0, yielding a small energy conversion for a very localized model resistivity.

Effect of electron collisions on the plasma–sheath formation

Abstract The effects of electron collisions in the plasma–sheath are numerically investigated. We employ a one-dimensional electrostatic particle-in-cell code, with collisions incorporated using the Monte Carlo method. While it has been reported previously that the presheath potential drop increases with the ion collision frequency, the effects of electron collisions were not explored. The present study shows the inclusion of electron collisions reduces the potential drop, especially in the presheath region. The result is more or less the same when a weak magnetic field is imposed. We note that the reduced electron flux, caused by electron collisions, makes the presheath potential drop shallower as the required ion drift motions to balance the electron current are smaller.