1. State Key Laboratory of Space Weather An inter-hemisphere asymmetry of the cusp region against the geomagnetic dipole tilt Jiankui Shi Center for Space Science and Applied Research Chinese Academy of Sciences

2. State Key Laboratory of Space Weather 1. Introduction  Cusp region (Newell and Meng,1988): is the dayside region in which the entry of magnetosheath plasma to low altitude is most direct. Entry into a region is considered more direct if more particles make it in (the number flux is higher) and if such particles maintain more of their original energy spectral characteristics. Newell-Meng Particle Precipitation Map (1992)

3. State Key Laboratory of Space Weather  The two narrow funnel-shaped cusps are generally recognized as being key regions for the Solar Wind-Magnetosphere-Ionosphere Coupling. cusp

4. State Key Laboratory of Space Weather  The cusp location is always variation because of: -- Variation of the solar wind dynamic pleasure -- The IMF Bz’s direction, and IMF By -- The reconnection in the magnetopause -- Geomagnetic Dipole Tilt Effect -- and so, on

5. State Key Laboratory of Space Weather  In this research: -- the Cluster data are used to study the variation of the cusp location in the middle altitude. -- the Latitudinal Displacement of the cusp region are studied by statistics. -- some results are obtained -- especially, it seems that there is an inter-hemisphere asymmetry of the cusp region against the Geomagnetic dipole tilt effect.

6. State Key Laboratory of Space Weather 2. Data Presentation ★ Data Set 2001- 2005: July – October Altitude: 4 -- 7Re 307 Cluster cusp crossings Northern : 173 Southern : 134 Satellite: C1, C3 and C4 FGM: B PEACE: e CIS: H +, He + + IMF: NASA’s Space Physics Data Facility (SPDF) ★ The Cluster satellite cross the cusp region at middle altitude from July to October each year

7. State Key Laboratory of Space Weather ★ Identification Criteria (zhou,1999) -- a sudden increase in the low-energy ion and electron density (greater than 5 cm-3) -- an electron thermal energy less than 100 eV -- the presence of significant He++ ( greater than 0.5 cm-3) which signifies a solar wind origin -- a decrease of the magnetic field from background greater than 1 nT.

8. State Key Laboratory of Space Weather 3. Statistical Analysis The MLT-ILAT distribution of all the cusp crossings The mean location of northern cusps MLT:12.05 ILAT:76.60 The mean location of southern cusps MLT:11.68 ILAT:75.70 3.1 Polar Cusp Location

9. State Key Laboratory of Space Weather 1) Bz<0 The cusp moves equatorward with more negative Bz 2) Bz>0 The cusp is almost stable 3.2 Latitudinal Displacement: IMF Bz

10. State Key Laboratory of Space Weather 3.2 Latitudinal Displacement: Dipole Tilt The dipole tilt angle is defined as the angle between the Earth’s north dipole axis and the GSM Z-axis The dominant effect on cusp latitude is IMF Bz, but for northward IMF, the cusp is almost immune to the IMF variation.

11. State Key Laboratory of Space Weather Cusp moves poleward when the dipole tilts more toward the Sun. Fitted dipole tilt angle dependence of the cusp latitudinal location. Northern Cusp : ILAT = 77.79 + 0.065Φ Southern Cusp : ILAT = 77.97 + 0.048Φ 3.2 Latitudinal Displacement: Dipole Tilt

12. State Key Laboratory of Space Weather ★ The polar cusps have been explored extensively at low-, mid-, and high-altitude by many spacecrafts: -- Low-altitude (DMSP): The cusp is found to move by ±2°MLAT from the mean position, in response to the changing dipole tilt angle [Newell et al., 1989]. -- Mid-altitude (Polar): The position of the cusp is significantly dependent on the title angle with roughly 1°for every 14°of tilt [Zhou et al.,1999]. -- High-altitude (MAGION-4): The footprint of the cusp-like plasma, made in the vicinity of the magnetopause, have a substantial latitudinal dependence on the dipole tilt with a slope of 0.16°MLAT /1°tilt [Nĕmeček et al., 2000].

13. State Key Laboratory of Space Weather  From the former author’s studies with other satellite data, the slope nearly linearly increases with the increasing altitude.  Our result is contrary to the trend in other studies. 3.2 Latitudinal Displacement: Dipole Tilt  Implies an inter-hemispheric asymmetry of the dipole tilt angle effects.

14. State Key Laboratory of Space Weather 4 summary We perform a statistical survey of the cusp on both hemispheres from the location, the displacement, the dipole tilt effect. An interhemispheric difference in the dependence of the cusp latitudinal location on the dipole tilt angle. It may imply an interhemispheric asymmetry

15. State Key Laboratory of Space Weather