Earth s Magnetosphere

Transcription

1 Earth s Magnetosphere General Description of the Magnetosphere Shape Pressure Balance The Earth s Magnetic Field The Geodynamo, Magnetic Reversals, Discovery Current Systems Chapman Ferraro Cross Tail Currents Ring Current Partial Ring Current Birkeland Currents Boundary Regions Lobes Plasmasphere Radiation Belts

2 Magnetosphere The solar wind generally moves faster than its own sound speed. So in the frame of the solar wind, the magnetosphere is moving at supersonic speeds. This generates a (fluid dynamics) shock wave in front of the magnetosphere called the bow shock.

3 Earth s Magnetic Field Magnetosphere Solar Wind + = Similar to a bar magnet, the Earth s intrinsic field is roughly dipolar. The solar wind deforms the magnetic field, and creates both a magnetopause and bow shock.

4 The Earth s Magnetic Field Changes in the Earth s magnetic field Drift of the magnetic pole Reversal of the field direction recorded in the sea floor

5 Dynamo Theory & Earth s Magnetic Field The Geodynamo and Magnetic Reversals Freezing of liquid iron onto the solid inner core combined with buoyancy of lighter alloys provides free energy to set up convection. The Coriolis force causes helical fluid flows. This prevents fields from canceling each other out. Non-uniform heat transfer through Earth s mantle results in possibility of field reversals. The reversal rate seems to be controlled by the solid inner core. From Glatzmaier and Roberts

6 Earth s Magnetic Field Using essentially MHD models and treating the outer core as a turbulent conductive fluid, Glatzmeier & Roberts were the first to create a reversing dynamo model From Glatzmeier and Roberts

7 Magnetosphere Earth s magnetic cavity The frozen-in condition requires that magnetized solar wind plasma and Earth s magnetic field cannot mix. Therefore, Earth creates a magnetic cavity in the solar wind called the magnetosphere. The solar wind flow is deflected around the magnetosphere obstacle, and the magnetosphere is deformed to reach a pressure equilibrium with the solar wind.

8 Earth s magnetic cavity Magnetosphere The magnetopause is defined as the boundary between the influence of the solar wind and the influence of the Earth s magnetic field. From hydrostatic equilibrium, we can define the location of this boundary: P SW = ρ SW v SW 2 P Earth = B2 2µ o ρ SW v SW 2 = B2 2µ o

9 Magnetosphere Earth s magnetic cavity In order to deform Earth s dipole field, we need additional electric currents. The magnetosphere is the result of superimposing several large scale external current systems. All of these current systems are dynamic.

10 Magnetosphere Magnetospheric current systems Earth s magnetosphere is filled with a variety of current systems. These currents: shape the magnetic field connect different regions They are the result of particle motion in already existing fields. Magnetospheric currents may or may not demark magnetospheric boundaries.

11 Magnetosphere Earth s magnetic field has three sources: * Internal Magnetic field Produced by currents in Earth s outer core * External Magnetic Field Produced by currents in the magnetosphere and ionosphere * Anomalous, induced magnetic field Remnant magnetization in the crust Around 1832, Carl Friedrich Gauss concluded that 95% of Earth's magnetic field is internal, and 5% external. Earth s magnetic field is (practically) entirely due to macroscopic currents Earth s field is relatively strong and variable

12 Magnetosphere Chapman-Ferraro Current The Chapman-Ferraro current shapes the magnetosphere in light of the impinging solar wind. It limits the size of the magnetosphere and (at least for northward IMF) creates a closed magnetosphere. Solar Wind B = 0 ρ = n(m i + m e ) Magnetopause r ge j Magnetosphere B = B z ρ = 0 Here we show a simplified derivation of the Chapman-Ferraro current for a planar magnetopause separating the field-free solar wind (left) from the plasma-free magnetosphere (right). v r gi Current Density: j = 2r gi vne = 2nm i v 2 /B z

13 Chapman-Ferraro Current Magnetosphere The Chapman-Ferraro current must provide the j x B force integrated across the magnetopause in order to balance the solar wind dynamic pressure. This explains the current pattern over the whole magnetopause. Courtesy Encyclopedia Britannica & University of Sydney

14 Magnetosphere Closed Magnetosphere Setting up the Chapman-Ferraro currents and disallowing reconnection between field lines will cause a closed magnetosphere. There is no connection between solar wind and magnetosphere. A closed magnetosphere is best realized under northward interplanetary magnetic field (IMF) conditions.

15 Closed Magnetosphere Magnetosphere Open Magnetosphere Reconnection

16 Magnetosphere Lobes require tail current Earth has an extended tail region. Within most of the non-equatorial tail, field lines are essentially parallel to the equatorial plane and point towards (northern hemisphere) or away (southern hemisphere) from Earth. In order to achieve the observed field geometry in these tail lobes, an additional current is required to increase the tail magnetic field. Using Ampere s Law it is easy to see that we need a horizontal current sheet with a duskward flowing current.

17 Magnetosphere Lobes require tail current Earth has an extended tail region. Within most of the non-equatorial tail, field lines are essentially parallel to the equatorial plane and point towards (northern hemisphere) or away (southern hemisphere) from Earth. In order to achieve the observed field geometry in these tail lobes, an additional current is required to increase the tail magnetic field. Using Ampere s Law it is easy to see that we need a horizontal current sheet with a duskward flowing current. B Current Layer B

18 Magnetosphere Particle Motion in the Inner Magnetosphere In the inner magnetosphere, charged particles perform three distinct motions: gyration around a field line bounce motion along a field line drift to B towards west (protons) or east (electrons)

19 Magnetosphere Particle Drift Causing a Current Exerting an external force perpendicular to the local magnetic field will result in a drift of charged particles across magnetic field lines. The combination of all force terms describes in principle the macroscopic drift of plasma in the inner magnetosphere. Positive particles will move west, negative particles east, causing the ring current. E x B Drift (no current) Drift caused by external forces Gradient B drift Curvature drift

20 Partial Ring Current Magnetosphere In addition to the ring current completely encircling Earth there is also a partial ring current on the night side. It closes via Field-aligned Currents (FAC). Create Region 2 currents. Field aligned currents close in the ionosphere.

21 Field-aligned Currents Region 1 Currents: Come from the distal tail, essentially shortcircuits the tail current. Courtesy of Encyclopedia Britannica

22 Birkeland Currents: Magnetosphere Connect the magnetosphere with the ionosphere, causing a coupling of dynamical behavior. Region 1 currents connect to the (fartail) plasma sheet and boundary layers. They are responsible for the convection pattern found in the ionosphere, and they relate to discrete electron aurora. Region 2 currents connect closer to Earth (partial ring current). Approx 75% of R1 currents are returned as R2 currents. Into Ionosphere Out of Ionosphere Region 1 Region 2

23 Birkeland Currents Magnetosphere Region 1 currents connect to the (far-tail) plasma sheet and boundary layers, therefore they map to higher latitudes. Region 2 currents connect closer to Earth (partial ring current).

24 Current

25 Boundary Layers/Regions We have learned, that a collection of currents determines the principal shape of the magnetosphere. We will next move to visit various magnetospheric regions and describe their characteristics. Magnetosphere

26 Boundary Layers/Regions Magnetosphere The magnetospheric boundary layer is the region of the magnetosphere just inside the magnetopause. It consists of (at least): The Mantle/HLBL: covers the high-latitude magnetosphere poleward of the cusps de-energized magnetosheath plasma (0.01/cc - 1/cc; 100 ev; km/s) flowing tailward gradual transition from magnetosheath to magnetosphere Low Latitude Boundary Layer (LLBL): dayside boundary region equatorward of the cusp contains mix of sheath and magnetosphere plasmas contains open and closed field lines The Cusps: funnel-shaped regions of minimum B-field that provide direct entry of magnetosheath plasma into the magnetosphere the Cusps are responsible for dayside auroral precipitation

27 Magnetosheath Boundary Layers/Regions Magnetosphere Low Latitude Boundary Layer (LLBL), High Latitude Boundary Layer (HLBL), Cusps Bow Shock Cusp HLBL Magnetotail Lobes LLBL Magnetopause

28 Boundary Layers/Regions The magnetotail lobes are a region of low density plasma which directly connects the solar wind through the mantle with the polar cap region of the ionosphere. The polar cap is the region inside (i.e. poleward) of the auroral oval. Lobe field lines carry polar rain (few 100 ev solar wind electrons) and ionospheric outflow (ev s ev ionospheric ions). Magnetosphere

29 Boundary Layers/Regions Magnetosphere The (central) plasma sheet (PS) is a region of closed magnetic field lines occupying the equatorial region of Earth s magnetotail. It is separated from the lobes by the plasma sheet boundary layer (PSBL). The Plasma Sheet (PS): is hot (few kev), has weak magnetic fields and an average density of cm-3 fed by ionospheric outflow and solar wind plasma (via reconnection) flows are earthward inside approx. 30 RE, tailward outside; fast flow bursts exist at times. The Plasma Sheet boundary Layer (PSBL): an active region with field aligned ion beams, field-aligned currents, likely on closed field

30 Magnetosphere Inner Magnetosphere - The Ring Current Tsyganenko, 2005 ENA Image of Ring Current

31 Magnetosphere Inner Magnetosphere - The Partial Ring Current In addition to the ring current encircling Earth there is also a partial ring current on the night side. It closes via field-aligned currents. ENA image & extracted model Courtesy P. Brandt

32 Magnetosphere Inner Boundary - Ionosphere The ionosphere is created by interaction between ionizing solar UV radiation and Earth s atmosphere. It creates a layer of plasma from km altitude. The ionosphere is coupled both to the magnetosphere and the neutral atmosphere. The ionosphere consists of layers, and can be roughly separated into three regions according to latitude (or better: magnetic field inclination). Ionosphere features a variety of plasma physics effects, including convection, electric currents, waves, etc.

33 Magnetosphere - Ionosphere structure

34 Magnetosphere Inner Boundary - Ionosphere The ionosphere is created by interaction between ionizing solar UV radiation and Earth s atmosphere. It creates a layer of plasma from km altitude. The ionosphere represents less than 0.1% of the total mass of the Earth's atmosphere, and is an extension of the thermosphere.

35 Magnetosphere Inner Boundary - Ionosphere The ionosphere is also the region where precipitating electrons and ions generate the aurora. Just as with atomic line emissions, the color of the emitted light is a function of the energy of the precipitating charged particle (which may collide multiple times) and the electron energy structure of the particle is collides with. Courtesy of JAMSTEC Yellow-green color for Atomic ~100km Crimson red from Molecular <100km Blue-ish light from Ionic > 150km

36 Magnetosphere Inner Boundary - Ionosphere Phots from:

37 Magnetosphere The plasmasphere can be thought of as an extension of the mid-latitude ionosphere. Its cold (few ev) and dense (10 s to 100 s cm-3) plasma exists on closed field lines and is co-rotating with Earth. Its outer edge is the plasmapause, located between 3 and 6 RE. The boundary between co-rotation and open convection is also called Alfven Layer.

38 The radiation belts (van Allen belts) were discovered 1958 by van Allen. Inner Belt: RE, MeV p+ - caused by cosmic rays Outer Belt: RE, 1-10 MeV e- - created internally, through injection and energization during storms. Magnetosphere The radiation belts are harmful to space-based assets, causing component degradation, electronic errors, instrument noise, charge-up in insulators, and biological damage.

39 Magnetosphere The radiation belts (van Allen belts) were discovered 1958 by van Allen by placing a Geiger counter onboard the Explorer 1 & 3 satellite to detect charged particles (launched to answer the Soviet s first satellite, Sputnik, launched 16 months earlier). Credit above:

40 Magnetosphere -Summary " The magnetosphere is a result of the interaction between the magnetized solar wind and Earth s magnetic field. " The dipole magnetic field is shaped by a series of current systems (Chapman-Ferraro, tail current, ring currents, field aligned currents). " The magnetosphere is filled by a variable mixture of ionospheric and solar wind plasma. Several distinct, nonoverlapping regions can be identified. Each region shows a typical and distinct plasma content. Those regions are often separated by boundary layers. Plasma moves between different regions and gets energized (or de-energized). The magnetosphere is not static!

41 Outlook Homework 4 is short and covers recent topics in magnetospheric configuration, currents and pitch angle. Due Monday 2/27 Midterm will cover through todays notes: in class 3/3 No Class on Monday (2/20), consider attending the research symposium 9-12 or Dr. Jim Green s lecture at 3pm (see for details). Also, Dr. Linda Spilker s talk on 2/22 at 7pm! Upcoming Next topics: Magnetic Reconnection in detail as a lead up to: The Earth s dynamic magnetosphere!

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