Ch 8 Magnetosphere Dungey 1961 Open magnetosphere model

Ch 8 Magnetosphere • Dungey 1961 – Open magnetosphere model • Axford and Hines 1961 – Closed magnetosphere model 1

As the magnetized solar wind flows past the Earth, the plasma interacts with Earth’s magnetic field and confines the field to a cavity, the magnetosphere. 2

The Magnetopause The solar wind compresses the terrestrial magnetic field into a blunt-nosed shape on the dayside, forming the dayside magnetopause inside the bow shock. The location of the magnetopause is determined by a balance between the dynamic pressure of the solar wind and the magnetic field pressure: The night-side magnetosphere is drawn out into a long tail structure. The magnetotail consists of two lobes of oppositely-directed magnetic flux -- S. hemisphere flux is directed away from the Sun, N. hemisphere towards the 3 Sun.

The shock represents a discontinuity in the medium of the solar wind. The solar wind flows across the bow shock in front of the earth where it is slowed to subsonic velocities. -1 m 0 k s 20 Solar Wind 450 km s-1 The solar wind is deflected by the obstacle presented by the earth's field at the boundary designated the magnetopause IMAGE 20 0 k m s -1 In crossing the shock, the solar wind plasma is slowed down to ~200 km s-1 and the loss of K. E. is dissipated as thermal energy, increasing the temperature to 5 x 106 K. (about 510 times hotter than the solar wind). Increased T in magnetosheath, => Cs is higher, also driving 200 km s-1 to be subsonic. The magnetosheath, consisting of relatively hot sub-sonic turbulent plasma. 4

25 March 2000 Vandenberg AFB Delta II Rocket 5

NASA 6 Sw. RI

IMAGE/RPI Sounding in the Polar Cap Local plasma resonances X-mode echo trace Z trace S/C 7

Polar Cap Plasma Distribution Kp=6 Kp=0 8 Nsumei et al. , JGR, 2003

Empirical Magnetospheric Density Distribution Measured by IMAGE/RPI Average 2000 -2001 L=7 6 5 March 2001 1200 LT June 2001 0800 LT 9 Reinisch et al. , ASR 2003

The neutral points are the only points that connect the earth's surface to the magnetopause. The neutral points are regions of interest since this is where solar wind particles (from the magnetosheath) can enter the magnetosphere without having to cross field lines. There is experimental evidence that this does happen. Particles with energies typical of the sheath are observed over some 5° of latitude around 77°, and over 8 hours of local time around noon. These regions, being more extended than an idealized point, are called the polar cusps or polar clefts. 10

The Magnetotail & Plasma Sheet Field lines emerging from the polar regions are swept back, away from the Sun; some of these would have connected on the dayside in a dipole field. These field lines constitute the magnetotail. Magnetosheath Polar Cusp The main distinguishing feature of the plasma sheet is that it consists of hot (ke. V) particles. Bow Shock Radiation Belts The plasma contained between the lobes, whether on closed or open field lines, is called the plasma sheet. Magnetopause Plasma sheet is a mixture of particles originating in the solar wind (H+) and ionosphere (O+). 11

The boundary of the plasma sheet is determined by a balance between the magnetic pressure of the tail lobes and the kinetic pressure of the plasma sheet plasma: where BT = tail field intensity outside the plasma sheet. The tail region is highly dynamic, and is only reasonably represented by the above description during quiescent periods. 12

Magnetospheric Circulation Historically, the current system (as inferred from ground magnetometers) led to the first speculations about circulation patterns in the magnetosphere, and theories about solar wind magnetosphere interactions. Given that electrons are bound to field lines in the E-region (where ionospheric current flows) and ions are stationary by comparison, the following motions of field lines (electrons, opposite to conventional current flow) are inferred from the current patterns. Motion of “feet” of field lines 13

Digisonde drift mesurments at Qaanaaq/Greenland 14 Reinisch, 2002

Magnetospheric Circulation The generally accepted explanation involves magnetic connection or merging (between the terrestrial magnetic field and the IMF) on the dayside and reconnection on the night side. The following figure (polar plane view) shows the sequences of magnetic merging, convection and reconnection, where the numbers represent the time in hours after a field line has merged on the dayside with a southward-directed IMF (BZ < 0): 1 -2 IMF merges with TMF 2 -5 The connected field lines are swept back by the solar wind 6 Reconnection of the TMF occurs 7 -9 Field lines convect back to the dayside (this must happen or an accumulation of magnetic energy would be implied on the night side) 15

Diffusion (merging) region diffusion region Note southward IMF 16

Magnetic Connection In the correct geometry, magnetic field lines can interconnect. In general, the field lines must be anti-parallel and there must be an electric field as shown. The electric field causes plasma and field lines to drift into the merging region (black arrows pointing along Z) After merging, the B field lines have a component directed along Z, causing a drift perpendicular to the new B direction and to the E field. The new flow is along X as shown. Magnetic connection occurs in the magnetotail and at the dayside magnetopause. 17

Recall from earlier notes that in a magnetoplasma an applied E-field results in a plasma drift E and Vp are equivalent in highly conducting plasmas such as the magnetosphere and solar wind. 18

Conversely, if a magnetoplasma moves at a velocity VP with respect to a stationary observer, the observer will measure an electric field The above is just a restatement of the Lorenz force on the particle. This provides a useful way to understand the concept of a field line; that is, the motion of a field line is such that an observer moving with it detects zero electric field. From a stationary observer's viewpoint, the dynamics of the magnetosphere may be understood in terms of electric fields instead of moving field lines. From this point of view, then, the depicted antisunward flow over the polar cap in previous figures can be interpreted in terms of a dawn-to-dusk electric field (for an earth-fixed observer). Typically, the magnitude is about 6 m. V m-1 which corresponds to a cross-cap potential difference of 60 k. V. 19

Magnetospheric Circulation - Equatorial Plane View Dawn E Dusk The cross-cap potential is often used as a measure of intensity of solarwind/magnetosphere interaction, and can reach values of order 200 k. V during intense 20 magnetic disturbances.

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Polar Cap Electric Potential - Southward IMF (e. g. , Cannon and Reinisch, JGR, 1999) 22

Modern Ionosonde and Transmit Antenna The UMASS Lowell Digisonde Transmit antenna 23

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Polar Cap Electric Potential - Northward IMF (e. g. , Cannon and Reinisch, JGR, 2001) 26

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Principal Plasma Populations in Earth’s Magnetosphere 29

Main Zones of Magnetospheric Plasma Precipitation Boundary between Open field lines at High latitudes and Closed field lines at Low latitudes 30

The Plasmapause The outer boundary of the plasmasphere, at about 4 RE, is where the plasma density undergoes a sudden drop. This is the plasmapause. Ring Current However, the plasmapause boundary is very dynamic, and varies between about 3 to 6 RE, sometimes getting as low as 2 RE. Although not depicted as such in the previous figure, note that the plasmasphere overlaps with a considerable part of the radiation belt region. However, these represent two entirely different particle (energy) populations. 31

Plasmasphere Depleting and Refilling DST full depleted Lpp, storm full 32 Reinisch et al. , JGR, 2003

Plasmapause Boundary Now, the co-rotating plasmasphere sets up a "co-rotation" electric field: Outside the plasmapause the plasma is not co-rotating, and the circulation there is determined by the cross-tail potential. Essentially, the plasmapause represents the boundary where these two electric fields are of the same order: where BE = equatorial magnetic flux density at the surface, L = distance in RE, and RE = radius of earth. 33

Putting in numbers, m. Vm-1 ~ 1 m. Vm-1 at 4 RE Put another way, the plasmapause represents the boundary between the "inner magnetosphere" and "outer magnetosphere" plasma circulation patterns. The former is co-rotating, and the latter is strongly influenced by the solar wind interaction (see following figure): Viewed this way, one expects intensification of the outer magnetospheric circulation to lead to a contraction of the plasmasphere (inward movement of the plasmapause). This indeed happens (see subsequent figures). In fact, it is thought that the intensified outer circulation leads to a peeling off of outer layers of the plasmasphere, which are then lost as detached plasma chunks in the magnetotail and solar wind. 34

Satellite observations of ion density, showing the plasmapause at several Kp levels L 35

Magnetosheath Entry Layer sma a l P le t Man Details of the Cusp Region Low-Latitude Boundary Layer 36

Note: since ~1028 -1029 particles/s impact the dayside magnetopause, and ~ 1026 particles/s are estimated to enter the plasma sheet, only 1% efficiency of this process is required. 37

Particle Flow in the Merging - Reconnection - Convection Process “Dipolarization” of the B-field During the return flow, the particles are energized in their attempt to satisfy the first adiabatic invariant, = const As particles convect towards the earth, B increases, therefore the particle energies increase. The energy comes from the E-field. 38

Producing the Auroras are produced by electrons and protons striking Earth’s atmosphere. When oxygen and nitrogen atoms are hit by these energetic particles, they become excited. As they relax to their original state, they emit light of a characteristic color Green = Oxygen Red = Oxygen (lower energy electrons) Blue = Nitrogen Also emitted in UV and X-ray 39

Electron Precipitation & the Aurora 40

Early Auroral Studies 41

The Phases of the Aurora Quiet Expansion Growth Maximum Area Onset 42 Recovery

MAGNETOSPHERIC CURRENTS The combination of plasma and electric fields in the magnetosphere allows electric current to flow. Several current systems have been identified: • magnetopause current (A) • Birkeland (field-aligned) currents • tail current (B) • ring current (C) The strong deviations of the magnetosphere from a dipole shape are in fact due to the first three (A, B, C) of the above current systems. These are now discussed in turn. 43

Plasma Populations and Current systems of the Magnetosphere 44

MAGNETOPAUSE CURRENT If we ignore any magnetic or electric field in the solar wind, the origin of the magnetopause current and the corresponding modifications of the magnetic field can be grossly understood as follows: Consider a small section of the dayside magnetopause with the solar wind normal to it (see following figure). The ions are deflected one way and the electrons the other, causing a current to flow (consisting mostly of ions due to their greater penetration depth). The current flow at the magnetopause is such that its magnetic field cancels the geomagnetic field outside the boundary. Similarly, earthward of the boundary the field strength is doubled -- this is essentially the "compression" of the dayside magnetosphere that we have alluded to before. 45

Currents and Fields at the Magnetopause Boundary 46

It turns out that the force produced by this current (the so-called Lorentz or J X B force) balances the momentum force of the solar wind, which is another way of stating the "dynamic pressure" vs. "magnetic pressure" balance we discussed before. When the solar wind intensifies, the magnetopause current is increased: -- This further "compresses" the dayside magnetosphere; -- The ground magnetic signature of this sudden current increase associated with the compression is called a "sudden impulse" (SI), or if it is connected with the beginning of a storm, it is called a "sudden storm commencement" (SSC). 47

TAIL CURRENT The down-wind extension of the magnetosphere into a tail indicates the presence of a current system as follows: View from Earth 48

This is what keeps the dark-side magnetic field from assuming its dipolar form. The energy to do this comes from the solar wind. The magnetic flux in a current loop is Since BT ~ 20 n. T, then i. T ~. 016 A/m. Assuming something reasonable for the tail length, i. Tail ~ 108 A. For a cross-tail potential of 60 k. V, the power extracted from the solar wind ~ 6, 000 GW !! The neutral sheet separates the northern lobe from the southern lobe. 2 1 • j 3 • • Current out of page • • "neutral sheet" here refers to the magnetic field, and the region where currents flow so that reconnection is inhibited, similar to the heliospheric current sheet. 49

Currents and Magnetic Fields in the Geotail 50

Energy is Imparted to the Plasma During Reconnection 51

RING CURRENT Under magnetic storm conditions the magnetic field of the earth at low latitudes may be depressed ~ 1 -2% for a day or two (main phase). This is due to a westward ring current which we have already discussed in relation to particle drift (gradient-curvature drift) on curved field lines with the magnitude of B increasing towards the earth. Recall that the gradient drift depends on the particle "magnetic dipole moment m" -- hence gradient drift is not important for "cold" particles like those populating the ionosphere and plasmasphere; these particles co-rotate. 52

However, it is also true that it is not the energetic Van Allen particles that are the main contributors to the ring current. The fluxes of these particles are too small. The main ring current particles are protons of 20 - 100 ke. V (see following figure). The ring current is located between 4 and 6 RE, close to the inner edge of the plasma sheet and outer edge of the trapping zone. Where do the ring current particles come from? -- magnetospheric convection, after reconnection, accelerates particles inward, but they eventually mirror in the stronger field near the earth, and this is where the inner edge of the plasma sheet forms. 53

BIRKELAND (FIELD-ALIGNED) CURRENTS Magnetometers carried on board satellites have detected persistent perturbations of the earth's magnetic fields over the auroral zones which can only be interpreted as resulting from currents flowing into and out of the ionosphere. The following figure shows the average locations of these currents for two levels of magnetic disturbance as determined from measurements on the TRIAD satellite. The solar wind/magnetosphere interaction provides energy and momentum to the magnetosphere system; the magnetospheric circulation is determined by redistributing its plasma and fields in a way that allows for dissipation of this energy. This dissipation occurs in the form of: • energizing particles which give up their • dispelling blobs of plasma out the energy to the neutral atmosphere; magnetotail; • developing a current system capable of dissipating energy through ohmic losses; • transferring momentum to the neutral atmosphere. 54

Region 1 and Region 2 Current Systems R 2 R 2 R 1 R 2 Quiet Active Current flow is also consistent with the requirement for dissipation of the energy deposited into the magnetosphere by the solar wind; ohmic dissipation of currents in the ionosphere is one way of doing this. 55

Region 1 Current System Sun B Dusk Dawn 56

• Concurrent with the flow of Region-1 currents into the ionosphere at the onset of a substorm, currents and electric fields spread throughout the conducting ionosphere; even at equatorial latitudes the signatures of “penetration” electric fields are seen. • After ~1 hour, the Region-2 currents close in the ionosphere and set up a counter-potential field, shielding the lower latitudes from electro-dynamic coupling from high latitudes. • The magnetosphere can be viewed as a voltage generator imparting the same potential across N. and S. polar regions; the currents that flow depend upon the conductivity of the ionosphere. 57

Auroral Potential Structure 58

Major Magnetic Storms A major magnetic storm generally follows an extended period of southward IMF. However, although severe and nearly continuous substorms always accompany major magnetic storms, a magnetic storm is not just a collection of magnetic substorms. The main distinguishing feature is the build up of an enhanced radiation environment in the inner magnetosphere. Consequences of Major Magnetic Storms 59

Consequences of Major Magnetic Storms 60

Consequences of Major Magnetic Storms 61

Consequences of Major Magnetic Storms 62

COMPARATIVE MAGNETOSPHERES 63 The outer planets -- the "gas giants" (Jupiter, Saturn, Uranus, Neptune) provide interesting comparisons with the earth's magnetosphere.

The magnetospheres of the outer planets have been explored by the Pioneer, Voyager, and Voyager 2 missions. V 2 64

Some of the factors accounting for differences between the magnetospheres of the outer planets and that of earth include: • Properties of the solar wind change as we move outward, affecting the coupling of energy flux from the solar wind to the magnetospheres. • Magnetic fields of the outer planets are generally much larger than earth's. • Rapid rotation of the outer planets provide centrifugal forces large compared to those of earth. • Jupiter's moon Io represents a dominant source of plasma to the Jovian magnetosphere. • The outer planets have rings capable of absorbing trapped radiation (especially in the case of Saturn). 65

VARIATION IN SOLAR WIND PROPERTIES The solar wind number density and the radial component of the IMF decrease (inverse square) as distance from the sun increases. (electron and ion temperatures of the solar wind plasma also decrease with distance). The solar wind velocity, on the other hand, increases (slightly) with distance. The increased Mach numbers consistent with the above imply stronger shock fronts at the outer planets. The weaker dynamic pressure suggests larger magnetospheres for comparable B-values. Weaker IMF suggests merging and reconnection not so important. 66

MAGNETOSPHERIC SIZEsolar wind dynamic pressure. The following table provides data on the relative sizes of the magnetospheres (in terms of subsolar magnetopause distances) for earth and the four outer planets: 67