Dipole tilt effects in plasma sheet <I>B_y</I>: statistical model and extreme values

• Main Author: A. A. Petrukovich
• Format: Article
• Language: English
• Published: Copernicus Publications 2009-03-01
• Series: Annales Geophysicae
• Online Access: https://www.ann-geophys.net/27/1343/2009/angeo-27-1343-2009.pdf
• ISSN: 0992-7689; 1432-0576

With 11 years of Geotail measurements we construct a model of plasma sheet <I>B_y</I>, depending on IMF <I>B_y</I>, coordinates <I>X,Y</I> and geodipole tilt angle. At midnight and pre-midnight local times <I>B_y</I> is positively correlated with tilt (positive in summer). Thus in summer <I>B_y</I> is shifted towards positive values and in winter towards negative values, so that up to several nT could be added to the IMF influence. The dawn side plasma sheet <I>B</sub>y</sub></I> generally does not exhibit any tilt dependence, but within 15 <I>R_E</I> the weaker negative correlation with tilt was revealed. The tilt dependence is just a useful parametrization and several mechanisms actually affecting plasma sheet <I>B_y</I> were previously suggested. In particular, similar coupling between tilt and IMF <I>B_y</I> was earlier found in the ionospheric convection patterns. Besides this average response, extreme <I>B_y</I> (|<I>B_y</I>|&gt;5 nT, <I>B_y</I>&gt;IMF <I>B_y</I>) were often observed (up to 20–25% of cases during solar maximum and in the pre-midnight sector within 20 <I>R_E</I>). They can not be explained by our statistical model and are preliminary interpreted as an "over-reaction" of the magnetosphere in some individual events. Large <I>B_y</I> field radically changes dynamics of the current sheet and has to be taken into account during substorm-related studies.