1. introduction
DESCRIPTION
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. Newell-Meng Particle Precipitation Map (1992). 1. Introduction. Cusp region (Newell and Meng,1988): - PowerPoint PPT PresentationTRANSCRIPT
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 ResearchChinese Academy of Sciences
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)
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The two narrow funnel-shaped cusps are generally recognized as being key regions for the Solar Wind-Magnetosphere-Ionosphere Coupling.
cusp
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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
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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.
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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
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★ 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.
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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
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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
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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.
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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
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★ 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].
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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.
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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
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