David versus Goliath 1

Transcription

1 David versus Goliath 1 or A Comparison of the Magnetospheres between Jupiter and Earth 1 David and Goliath is a story from the Bible that is about a normal man (David) who meets a giant (Goliath) Tomas Sverin 1(10) tosv0007@student.umu.se

2 Introduction A magnetosphere is a sphere of influence around a planet in which the forces linked with the magnetic field of the planet win through over all other forces. 2 Thomas Gold proposed the name magnetosphere in 1959 when he wrote: "The region above the ionosphere in which the magnetic field of the earth has a dominant control over the motions of gas and fast charged particles is known to extend out to a distance of the order of 10 earth radii; it may appropriately be called the magnetosphere 3 The solar wind is diverted away from the planet by the magnetic field of the planet and carving a cavity, which contains hot plasma that has low density, this plasma is derived from the solar wind or the planet. The magnetosphere and the plasma of the solar wind are kept apart by a thin layer. This layer is called the magnetopause, where strong surface current circulate. There are three factors that are needed to form and maintain a magnetosphere: 1. A strong enough planetary magnetic field that halts the solar wind. 2. A source of plasma internal or external to the magnetopause to populate it. 3. A source of energy to power it. Jupiter and Earth differ when it is about how the magnetosphere is driven: Earth: Solar wind-driven magnetosphere; plasma and energy are derived from the solar wind. Jupiter: In rotationally-driven magnetosphere; the rotation of the planet derives the bulk of the energy, whereas the plasma is derived from the planet or a satellite of the planet. 4 The geomagnetic field is associated with the magnetic field of Earth. 5 The Interplanetary Magnetic Field (IMF) is another name for the Sun s magnetic field. 6 Figure 1: The IMF is represented by blue arcs and the picture shows the spiral nature of the IMF. 7 2 The Configuration of Jupiter s Magnetosphere 3 Gold, Journal of Geophysical Research, volume 64, page 1219, The Configuration of Jupiter s Magnetosphere Tomas Sverin 2(10) tosv0007@student.umu.se

3 Tellus (Earth) Introduction The magnetosphere of Earth can be divided into to two regions: Region 1: The Outer Magnetosphere The Bow Shock and the Magnetosteath The Magnetopause The Magnetotail Region 2: The Inner Magnetosphere The Plasma sheet The Plasmasphere The Ring current 8 Figure 2: The magnetosphere of Earth 9 For a magnetic dipole in vacuum the field lines extend out to infinite distances. In the region called the magnetopause, which is the boundary of the magnetosphere, the field lines are almost completely confined, because they are further out. The geomagnetic field is separated from the interplanetary magnetic field because of bend field lines, which is a cause of currents in the magnetosphere. The magnetopause is also a boundary between the magnetosperic plasma and the solar wind, because the magnetic fields are frozen-in to the plasma. The Outer Magnetosphere Bow shock and Magnetosheath The bow shock is in the outer layer of the magnetopause, which means it is the first thing the plasma of the solar wind will approach. The speed of sound is approximately 340 m/s and the speed of the 8 Lecture Notes on 9 Tomas Sverin 3(10) tosv0007@student.umu.se

4 solar wind at Earth s orbit is much higher, around 450 km/s. The bow shock is a shock wave that is formed in the boundary between then supersonic solar wind and the subsonic flow of compressed plasma. At the bow shock, the flow velocity goes from U Solar Wind in the solar wind to U Magnetosheath in the magnetosheath and the same occurs with the density when it goes from ρ Solar Wind to ρ Magnetosheath. In the stationary case: ρ Solar Wind U Solar Wind = ρ Magnetosheath U Magnetosheath, The magnetosheath is the region between the bow shock and the magnetopause, and it is characterized by irregular flows and enhanced densities, temperatures and magnetic field, when compared to the solar wind. Figure 3: Cross section of the simplest model of the magnetosphere in the noon-midnight meridian. The solar wind flows is deflected at the Bow Shock and flows around the magnetosphere, forming the magnetosteath. The current sheet at the magnetopause separates the interplanetary magnetic field from the magnetosphere. 10 Magnetopause In the magnetopause the flow of solar wind is very limited, and the field lines on the magnetospheric side are mainly connected to the geomagnetic field. A neutral line can be created by magnetic reconnection when the interplanetary magnetic field has a southward component. The magnetic field is weak in the vicinity of a neutral line, which leads to large deviations from ideal MHD. Southward interplanetary magnetic field leads to an enhanced influx of solar wind into the magnetosphere, which is a result of the magnetic reconnection Tomas Sverin 4(10) tosv0007@student.umu.se

5 Figure 4: Dungey open magnetosphere, in which there is an essential breakdown of frozen-in flow at the dayside magnetopause and in the tail leading to the occurrence of reconnection. The arrowed solid lines indicate magnetic field lines, the arrowed dashed lines plasma streamlines, and the heavy long-dashed lines the principal boundaries (the bow shock and magnetopause). The circled dots marked E indicate the electric field associated with the flow, which is perpendicular to the flow and the field. 11 Magnetotail The magnetosphere is compressed on the dayside and an extended magnetotail is formed on the night side, which is a result of the interaction with the solar wind. Through magnetic reconnection the solar wind will cause the plasma to circulate within the magnetosphere, the circulation is called magnetospheric convection. The Inner Magnetosphere In the outer magnetosphere the magnetic field lines are weak and the plasma high, the plasma can then shape the magnetic field lines. Closer to the Earth the magnetic field contains its dipole shape, and take control over the plasma flows. The atmosphere and the ionosphere are carried along with the rotation of the Earth, this creates an electric field in a non-rotating frame of reference. 12 Plasma sheet The plasma sheet contains of a sheet of plasma that extends down to the magnetotail and dividing the magnetic field of Earth into two lobes. Another name for the plasma sheet is the current sheet, because of the main current which circles cuts the magnetosphere through the center of the magnetotail. 13 Plasmasphere The plasmasphere is a torus-shaped region in the magnetosphere of the Earth, the edge is called the plasmapause and is very sharp. In the plasmapause the densities drop by a factor of in a relatively short distance. The plasmasphere is basically an extension of the ionosphere to high altitudes. The magnetic field lines rotate with the Earth inside the plasmapause and plasma that is flowing up from the ionosphere is trapped by the corotaing field lines, the plasma builds up to higher densities. Magnetic field lines are unable to corotate at the outside of the plasmapause, the inability is because of the magnetic field lines are influenced strongly by electric field of solar wind origin Lecture Notes on 13 Tomas Sverin 5(10) tosv0007@student.umu.se

6 They convect to the magnetosphere boundary on the dayside of the Earth, sweep the ionospheric plasma out of the magnetosphere and form the plasmasphere boundary. 14 Figure 5: Summary of the main features of the plasmasphere. 15 Ring current One of the major current systems in the Earth s magnetosphere is the ring current, which circles the Earth by drift of energetic charged particles that are trapped on field lines. 16 Figure 6: The figure shows the directions of the drifts and the resultant westwards direction of the current, which opposes the Earth s field in the region interior to the current but adds to the Earth s field in the exterior region Tomas Sverin 6(10) tosv0007@student.umu.se

7 Jupiter Introduction Jupiter is the planet of a lot of superlatives: Most massive planet in the solar system. Rotates the fastest. The strongest magnetic field. The most massive satellite system of any planet. 18 The strong magnetic field of Jupiter and its fast rotation makes a unique magnetosphere that is known for its enormous size. The magnetosphere of Jupiter differs from other magnetosphere because it derived much of its plasma internally from the satellite Io. The plasma from the satellite Io is heavy, it contains of various charge states of S and O, which inflates the magnetosphere from the combined actions of centrifugal force and thermal pressure, responsible for generating an azimuthal current in the equatorial region. The magnetosphere of Jupiter can be divided into three regions: Region 1: The Inner Magnetosphere (<R J ) Region 2: The Middle Magnetosphere (10-40 R J ) Region 3: The Outer Magnetosphere (>40 R J ) R J is the radii of the Jupiter. The Inner magnetosphere The inner magnetosphere is the seat of plasma production for the magnetosphere and there are four major plasma sources: The satellite Io The satellite Io is the main source of plasma in Jupiter s magnetosphere and it is estimated that upward 1 tons per second of plasma mass is added to the magnetosphere. 19 The source of Io is volcanoes with low-charge state oxygen and sulphur ions from dissociation of sulphur dioxide. Inelastic collisions of tours ions with Io s atmosphere heat the atmospheric gases and causing a significant population of neutral molecules and atoms to gain speeds above Io s 2.6 km per second gravitational escape speed. These neutral form an extensive corona encircling most of the way around Jupiter. Io loses 1-3 tons of neutral atoms per second. Compared with the local plasma that is corotating with Jupiter at 74 km per second, the neutral atoms are moving slowly, close to Io s orbital speed of 17 km per second. When a neutral atom becomes ionized via electron impact, it experiences an electric field resulting in a gyro motion of approximately 57 km per second. Between a third and half of the neutral atoms is ionized to produce additional fresh plasma, while the rest are lost via reactions in which a neutral atom exchanges an electron with a torus ion. When the particle has become neutralized it is no longer confined by the magnetic field and flies off as an energetic neutral atom. This charge exchange process adds gyro-energy to the ions, and extracts momentum from the surrounding plasma but does not add more plasma to the system. 20 The torus can be divided into an inner cold region with temperature of 2-10 ev, where ions are tightly confined to the centrifugal equator, and an outer warm torus with temperature of 60 ev. The high density and the cold temperature of the inner torus reflect the long residence time of ions in the cold torus where they are subject to significant cooling The Magnetosphere of Jupiter: Coupling the equator to the poles 19 The Configuration of Jupiter s Magnetosphere 20 The Magnetosphere of Jupiter: Coupling the equator to the poles 21 The Configuration of Jupiter s Magnetosphere Tomas Sverin 7(10) tosv0007@student.umu.se

8 Figure 7: The interaction of magnetospheric plasma with Io s atmosphere (a). A 3-D view showing the fieldaligned currents that couple Io to Jupiter and the region of plasma stripped directly from Io s ionosphere. (b) View looking down on Io with the Sun to the bottom of the page and Jupiter to the top. 22 Solar wind The next most important source of plasma in Jupiter s magnetosphere is the solar wind whose source strength can be estimated by a consideration of the solar wind mass flux incident on Jupiter s magnetopause and the fractional amount that makes it into the magnetosphere. This plasma source is estimated to less than 100 kg per second. Escape of ions from the ionosphere of Jupiter The escape from ions, mainly and from the ionosphere of Jupiter provides a significant source of plasma in Jupiter s magnetosphere. The ionospheric plasma escapes along field lines when the gravity of Jupiter is not able to contain the hot plasma. The escape is not uniform and depends on the local photoelectron density, the temperature variations of the ionosphere with the solar zenith angle, other factors such as the auroral precipitation of ions and electrons and the ionospheric heating from Pedersen currents. Surface sputtering The surface sputtering of the three icy satellites by Jovian plasma provides the last significant source of plasma in Jupiter s magnetosphere. Because the icy satellites lack extended atmospheres and the fluxes of the incident plasma are low at the locations of these satellites, the total pickup of plasma from these satellites is estimated to be less than 20 kg per second based on the plasma spattering rates The Magnetosphere of Jupiter: Coupling the equator to the poles 23 The Configuration of Jupiter s Magnetosphere Tomas Sverin 8(10) tosv0007@student.umu.se

9 The Middle Magnetosphere The structure of the middle magnetosphere is very complex and a simple picture of rotating-out flowing plasma is not valid for the whole of Jupiter s magnetosphere. The strong magnetic field of Jupiter provides an ideal trapping geometry for this plasma and only a small fraction of this plasma is able to precipitate on to Jupiter. The only escape route for this plasma is outflow through the magnetotail and magnetopause. The plasma outflow in the magnetopause is driven by diffusion, interchange motion and reconnection with the interplanetary magnetic field. 24 The Outer Magnetosphere Magnetotail A magnetotail forms downstream if a magnetized planet as a direct consequence of momentum transfers between the magnetosphere and the solar wind. In the dusk magnetosphere the field lines are closed, which leads to that the momentum transfer occurs between the solar wind and the magnetospheric plasma at the boundaries of the magnetosphere. The magnototail of a closed magnetosphere is relatively short in the subsolar direction, so that the magnetosphere has a teardrop shape. The dawn magnetosphere contains of open field lines connecting to the magnetotail, which means that direct entry of solar wind density and momentum flux can occurs. The magnetotail then contains two lobe regions above and below the central current sheet which house open magnetotail flux connected on one end to the planet and at the other and to the interplanetary magnetic field. The magnetotail of an open magnetosphere tends to be very long. 25 Figure 8: Major current systems of the magnetosphere of Earth and Jupiter. 26 The pictures in figure 8 compare the current systems in the magnetosphere of Earth and Jupiter. Note that the opposite directions of the planetary dipoles results in currents of opposite directions. 24 The Configuration of Jupiter s Magnetosphere 25 The Configuration of Jupiter s Magnetosphere 26 The Magnetosphere of Jupiter: Coupling the equator to the poles Tomas Sverin 9(10) tosv0007@student.umu.se

10 List of References Information The Magnetosphere of Jupiter: Coupling the equator to the poles Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) The Configuration of Jupiter s Magnetosphere Journal of Geophysical Research Gold, Journal of Geophysical Research, volume 64, page 1219, 1959 Encyclopedia The Free Dictionary Windows to the Universe Lecture Notes on Lecture Notes on : From the Sun to The Aurora Windows to the Universe Windows to the Universe Southwest Research Institute Earth's Magnetosheath, Magnetopause and Magnetosphere Pictures Earth's Magnetosheath, Magnetopause and Magnetosphere Windows to the Universe Cosmicopia (NASA) Earth in Space Vol. 8, No. 7, March 1996, p American Geophysical Union Windows to the Universe Tomas Sverin 10(10) tosv0007@student.umu.se