Topics 2018.01.17

Mechanism for theta aurora formation

GP_Web_Topics_1801_PPARC_Fig1.jpg

Fig.1, Theta aurora taken form the satellite over the norther polar region (Obara et al. GRL, 1988)

From the space, we see aurora very clearly.ツ黴€ During north ward IMF condition, we sometimes see theta aurora. Theta aurora consists of main oval and theta bar, which connects dayside and night side ovals (see Fig.1). Obara et al. (GRL, 1988) identified simultaneous appearance of theta aurora in both hemispheres. With a help of super computer, Tanaka, Obara and Kinitake (JGR, 2004) produced theta aurora structure in the polar cap during the northward IMF condition. They assumed the polarity change of IMF By (east west component). Plasmasheet penetrated into the high latitude region forming the theta bar. Our recent study could describe magnetic field lines not only over the theta bar but also over main oval (JGR, Tanaka, Obara et al., 2017).

Fig.2 shows the high latitude region in the northern hemisphere, in which we see theta aurora together with several field lines. Field lines labeled THETA are field lines, connecting to theta bar. LLLC/ELC shows field lines connecting to the open close boundary in the evening side auroral oval. LLLC/MLC indicates field lined which originated from the open close boundary in the morning side oval in the southern hemisphere. In Fig.2, LLLC/MLS got together forming cusp like structure at the end of theta bar in the day side.

Fog.3 shows the high latitude region in the southern hemisphere. THETA shows field lines over the theta bar. LLLC/MLS shows field lines connecting to the open close boundary in the morning side auroral oval. LLLC/ELC indicates field lined which originated from the open close boundary in the evening side oval in the northern hemisphere. In Fig.3, LLLC/MLS got together forming cusp like structure at the end of theta bar in the day side, too.
ツ黴€ Fig.4 shows the field lines vied from the night side in the magnetotail. Field lines labeled THETA connect theta bars in both hemispheres. LLLC/ELC and LLLC/MLC show the similar feature, forming cusp like structure in both hemisphere.

In this article, we did not explain in detail, but theta bar actuary moves dawnward/ duskward depending on the situation of IMF By polarity change; i.e. negative to positive or positive to negative. Our current challenge will be to realize theta aurora by using real data of IMF.

ツ黴€ツ黴€ツ黴€ツ黴€ ( Prof. Dr. Tahahiro Obara, Planetary Plasma and Atmospheric Research Center)

GP_Web_Topics_1801_PPARC_Fig2.jpg

Fig.2, High latitude region in the northern hemisphere. Field lines labeled THETA are field lines, connecting to theta bar. LLLC/ELC shows field lines connecting to the open close boundary in the evening side auroral oval. LLLC/MLC indicates field lined which originated from the open close boundary in the morning side oval in the southern hemisphere.

GP_Web_Topics_1801_PPARC_Fig3.jpg

Fig.3, High latitude region in the southern hemisphere. Field lines labeled THETA are field line, connecting to theta bar. LLLC/MLC shows field lines connecting to the open close boundary in the morning side auroral oval. LLLC/ELC indicates field lined which originated from the open close boundary in the evening side oval in the northern hemisphere.

GP_Web_Topics_1801_PPARC_Fig4.jpg

Fig.4, Field lines vied from the night side. Field lines labeled THETA connect theta bars in both hemispheres. LLLC/ELC and LLLC/MLC show the similar feature, forming cusp like structure in both hemisphere.

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