ASEN 5335 Aerospace Environments -- Magnetospheres 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 2 The Magnetopause The location of the magnetopause is determined by a balance between the dynamic pressure of the solar wind and the magnetic field pressure: 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 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 Sun.

ASEN 5335 Aerospace Environments -- Magnetospheres 3 Solar Wind 450 km s km s -1 The solar wind flows across the bow shock in front of the earth where it is slowed to subsonic velocities. 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 10 6 K. (about 5-10 times hotter than the solar wind). Increased T in magnetosheath, => C s is higher, also driving 200 km s -1 to be subsonic. The magnetosheath, consisting of relatively hot sub-sonic turbulent plasma. The shock represents a discontinuity in the medium of the solar wind. The solar wind is deflected by the obstacle presented by the earth's field at the boundary designated the magnetopause

ASEN 5335 Aerospace Environments -- Magnetospheres 4 The neutral points are the only points that connect the earth's surface to the magnetopause. These regions, being more extended than an idealized point, are called the polar cusps or polar clefts. 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. 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 5 The plasma contained between the lobes, whether on closed or open field lines, is called the plasma sheet. 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 Magnetopause Radiation Belts Bow Shock Polar Cusp The main distinguishing feature of the plasma sheet is that it consists of hot (keV) particles. Plasma sheet is a mixture of particles originating in the solar wind (H + ) and ionosphere (O + ).

ASEN 5335 Aerospace Environments -- Magnetospheres 6 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 B T = 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 7 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. Magnetospheric Circulation Given that e- 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

ASEN 5335 Aerospace Environments -- Magnetospheres 8 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 (B Z < 0): 1-2IMF merges with TMF 2-5The connected field lines are swept back by the solar wind 6Reconnection of the TMF occurs 7-9Field lines convect back to the dayside (this must happen or an accumulation of magnetic energy would be implied on the nightside) Magnetospheric Circulation

ASEN 5335 Aerospace Environments -- Magnetospheres 9 Note southward IMF

ASEN 5335 Aerospace Environments -- Magnetospheres 10 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. Magnetic connection occurs in the magnetotail and at the dayside magnetopause. Magnetic Connection E x B 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. The electric field causes plasma and field lines to drift into the merging region (black arrows pointing along Z)

ASEN 5335 Aerospace Environments -- Magnetospheres 11 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 12 Conversely, if a magnetoplasma moves at a velocity V P with respect to a stationary observer, the observer will measure an electric field 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.3 mV m -1 which corresponds to a cross-cap potential difference of 60 kV. 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 13 Dusk E Magnetospheric Circulation - Equatorial Plane View Dawn The cross-cap potential is often used as a measure of intensity of solar- wind/magnetosphere interaction, and can reach values of order 200 kV during intense magnetic disturbances.

ASEN 5335 Aerospace Environments -- Magnetospheres 14 Some of the sunward-convecting particles precipitate into the upper atmosphere and produce the aurora. video

ASEN 5335 Aerospace Environments -- Magnetospheres 15

ASEN 5335 Aerospace Environments -- Magnetospheres 16

ASEN 5335 Aerospace Environments -- Magnetospheres 17 The outer planets -- the "gas giants" (Jupiter, Saturn, Uranus, Neptune) provide interesting comparisons with the earth's magnetosphere.

ASEN 5335 Aerospace Environments -- Magnetospheres 18 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).

ASEN 5335 Aerospace Environments -- Magnetospheres 19 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.

ASEN 5335 Aerospace Environments -- Magnetospheres 20 MAGNETOSPHERIC SIZE The size of a magnetosphere is determined by the relative importance of the planetary magnetic field strength and the solar 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:

ASEN 5335 Aerospace Environments -- Magnetospheres 21 Relative magnetospheric sizes based on pressure balance