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Ionosphere

Summary

The ionosphere is a conducting region in the upper atmosphere. During magnetic storms, the ionosphere is in a high voltage state caused by an excess input of electromagnetic energy from the magnetosphere.

The newly developed electric field in the ionosphere is believed to be mapped to the inner region of the magnetosphere, acting as a major driver for the development of the ring current. Thus, the ionospheric electric field plays both passive and active roles in the Sun-Earth system. The primary goal of the GEMSIS-Ionosphere project is to understand the Sun-Earth system through examining the ionospheric electric field as well as energy flow by means of developing models and analyzing data.

Models and Scientific Objectives

Inductive Model of Global Electric Potential

We have developed an inductive scheme for the purpose of reconstructing the storm-time electric potential using data from the SuperDARN radars, including data from the recently developed SuperDARN Hokkaido radar. The Hokkaido radar covers a wide range of area from mid to high latitudes, enabling us to observe the wide spread distribution of the storm-time electric potential.

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Deductive Model of Global Electric Potential

We have developed a deductive scheme for the purpose of reconstructing the storm-time electric potential. By applying Ohm’s law, we can calculate the electric potential for given electric currents flowing along a field line (field-aligned current) and the ionospheric conductance. A realistic model of the field-aligned current has been developed as described below. The deductive model will be used to understand (1) a response of the ionospheric electric potential to the field-aligned current and the ionospheric conductance, and (2) an extreme condition of the ionospheric electric potential during which the inductive model cannot be reconstructed due to insufficient data.

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Empirical Model of Large-Scale Field-Aligned Currents

The realistic modeling of field-aligned currents is one of the essential parts for conducting space weather research from a deductive point of view. For this purpose, we have been developing an empirical model of large-scale field-aligned currents (LSFACs) in collaboration with The Institute of Statistical Mathematics (ISM) and The Johns Hopkins University Applied Physics Laboratory (JHU/APL). This model is made by applying new statistical methods for as many as 190,000 data sets of field-aligned current observations obtained by the DMSP and DE2 satellites. The model is parameterized by the solar wind speed, the interplanetary magnetic field (IMF), the tilt angle of the Earth’s magnetic field, and the solar activity indices to give a realistic spatial profile of LSFACs. Modeling procedures from past studies tend to give much smaller intensities of LSFACs than those actually observed because they were constructed by averaging adjacent LSFAC sheets of opposite polarities. To avoid this weakness and give quantitatively appropriate intensities as well as spatially sharp boundaries of LSFAC sheets, the present model has adopted a new method in which positions of upper/lower latitude boundaries and intensities of LSFACs are averaged separately. The results were presented at several domestic scientific conferences/meetings.

A resultant LSFAC structure of our empirical model for the southward IMF case was deduced from the magnetic field data obtained by the DMSP and the DE2 satellites. One of the advantages of the present model is to the ability to show sharp boundaries of LSFAC sheets as actually seen along individual satellite passes.

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Quantitative Evaluation of Transmission of Electromagnetic Energy

We have quantitatively evaluated transmission of electromagnetic energy between the inner magnetosphere and low-latitude ionosphere during geomagnetic storms using global ground magnetometers, radars and satellite observations.

Simultaneity of electric field transmission:
We found that quasi-periodic magnetic field variations (DP 2), which appear simultaneously between the polar region and equator, are produced by two kinds of electric fields. One is the dawn-to-dusk convection electric field due to southward IMF. The other is the dusk-to-dawn shielding electric field generated by the inner magnetosphere. We confirmed this process based on the results of Hokkaido HF radar and ring current simulation. This result shows that the electromagnetic energy transmitted from the polar region to the equator is simultaneous within a few of seconds. This is an important result in understanding the propagation process of electromagnetic energy.

Response of the magnetosphere associated with solar wind dynamic pressure enhancements:
We conducted detailed statistical analysis of the long-term geomagnetic observation data from middle latitudes to the equator in order to clarify a global picture of three-dimensional current systems generated by sudden commencements (SCs) associated with solar wind dynamic pressure enhancements. As a result, we showed that the global magnetic field signature is produced by a pair of field-aligned currents (FACs) resembling region 1 FACs during SCs. The most important fact in our results is that the magnetic effect produced by the FACs reached the nighttime equatorial region.

Inhomogeneity of transmission of electromagnetic energy:
From a detail analysis of satellite (CRRES etc.) observation data, we found that convection electric fields in the near-Earth region respond quickly with one-to-one correspondence to the IMF polarity while the response of the electric field in the plasmasheet region tends to be delayed by several minutes. We also verified that electromagnetic energy flows from the ionosphere to the magnetosphere. These important results have been presented at international and domestic meetings, and published in journals of the American Geophysical Union.

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Auroral Breakup and Magnetotail Reconnection During Substorms

The longitudinal locations of reconnection in the near-Earth magnetotail at the time of isolated auroral breakup were studied. The near-Earth reconnection is identified by tailward plasma flows with a southward magnetic field. We first identified 66 breakups in the Polar satellite ultraviolet imager observations of the night-side polar ionosphere. We then studied tailward flows during breakups using Geotail satellite in situ observations of the plasma sheet between 25 and 31 Re down the tail. It was found that most tailward flows were observed near the breakup longitudes. Auroral breakup is therefore inferred to always be accompanied by near-Earth reconnection near breakup-MLT.

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Inner Magnetosphere-Ionosphere Coupling

During magnetic storms, energetic particles are accumulated in the inner magnetosphere, resulting in the development of the ring current. The ring current is a huge electric current surrounding the Earth. The remnants of the electric current are thought to flow into/away from the ionosphere. In order to cancel out the space charge deposited by the electric current, an additional electric field is developed. We have succeeded to explain the variation of the storm-time electric field observed by the SuperDARN Hokkaido radar by using simulation that couples the magnetosphere and the ionosphere. We have suggested that the subauroral ionosphere can be a manifestation of the storm-time ring current during magnetic storms.

This shows the simulated plasma pressure (ring current) in the magnetosphere. The multiple-layered structures seen in this figure generates the multiple-layered field-aligned current, resulting in the temporal variation of the rapid, westward flow of the plasma in the subauroral ionosphere.

This shows the simulated plasma pressure (ring current) in the magnetosphere. The multiple-layered structures seen in this figure generates the multiple-layered field-aligned current, resulting in the temporal variation of the rapid, westward flow of the plasma in the subauroral ionosphere.

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Magnetosphere-Ionosphere Coupling System

By performing a three dimensional MHD simulation, we analyzed the non-linear properties of the shear-Alfven wave through the magnetosphere-ionosphere coupling processes, especially, focusing on the dynamics of vortex and helicity.

Integrated Analysis Environments

We have developed useful software tools which, in conjunction with general analysis software (IDL), can easily visualize satellite, radar and ground magnetometer observation data, and the output data of GEMSIS models of field-aligned currents and electric potential. The satellite, radar and ground magnetometer data based on integrated analysis have been output as a common data format. These analysis tools will be made available to many domestic and international collaborators in the future.

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