Keisuke Hosokawa on World Wide Web

IMAGING STRUCTURES IN THE POLAR CAP IONOSPHERE

Prior to the early 70s, the polar cap had been thought as a very quiet region or blank. However, subsequent ground-based optical observations since the late 70s demonstrated that the polar cap ionosphere is far from quiet. It is known that there exist at least two outstanding structures there, which are polar cap patches and sun-aligned arcs. I and co-workers are operating all-sky airglow imager (Optical Mesosphere Thermosphere Imager: OMTI) at Resolute Bay in the northern part of Canada to see what's going on in the polar cap. 2D Imaging capability of the all-sky camera enables us to visualize dynamical nature of the polar cap patches and the sun-aligned arcs.

Polar Cap Patches

Polar cap patches are defined as regions of enhanced electron density generated near the dayside cusp region and drifting anti-sunward across the polar cap. I have been trying to answer the following unresolved issues regarding the fundamental characteristics of the polar cap patches.

What physical process creates polar cap patches?
("temporal variation of large-scale convection" or "nonlinear process through plasma instabilities"?)
How do they get structured? How fast do they become fully structured?
How long do they survive? How do they decay and die away?
How significant is their impact on the trans-ionospheric (satellite) communication?

Sun-Aligned Arcs (Polar Cap Aurora)

Sun-aligned arcs are structure much thiner and brighter than the polar cap patches. They are mostly aligned with the sun-earth line and prominent in the dawn and dusk sectors. They often move dynamically in the dawn-dusk direction, possibly in association with changes of the upstream IMF orientation. Here is a list of remaining questions which now I am tackling.

Where is the source of the sun-aligned arcs in the magnetosphere?
What controls the dawn-dusk motion of the sun-aligned arcs? High-latitude reconnection?
How is the ionospheric plasma convection (= electric field) in the vicinity of sun-aligned arcs?
What is the difference between sun-aligned arcs in the dawn and dusk sectors? Different source?

DYNAMICS OF THE AURORAL IONOSPHERE

Aurora (Northern Lights) are one of the most famous geophysical phenomena. They have been attracting us very much. Not only that, they are known as a very important physical process in terms of diagnostics of the space plasma environment surrounding magnetized planets. I am trying to reveal dynamical nature of aurora by combining ground-based radio/optical observations and spacecraft in-situ plasma/field measurements. The followings are the two major research topics on aurora which now I am interested in.

Generation of Pulsating Aurora: Role of Ionosphere

Pulsating aurora are popular phenomena that are almost always observed during the recovery phase of auroral substorms. Their morphological characteristics such as the pulsating period and motion have been studied for long time. However, the generation mechanism of pulsating aurora is still unclear. I am working with a hypothesis that the ionosphere plays an important role in characterizing pulsating aurora. To evaluate this working hypothesis, modulations of ionospheric electron density and/or electric field are currently under investigation using ground-based optical (all-sky TV camera) and radar (SuperDARN, EISCAT) observations.

Physics of Auroral Breakup in the Magnetosphere-Ionosphere Coupling System

Physics of aurora breakup (=explosive evolution of aurora) are studied by combining various types of observational techniques such as ground-based optical/radio measurements (all-sky TV camera and SuperDARN radars) and in-situ plasma/field observations with spacecrafts (REIMEI, THEMIS). Research topics regarding this subject are as follows: