ESS 200CESS 200CLecture 14Lecture 14The Solar Wind Interaction with The Solar Wind Interaction with UnmagnetizedUnmagnetizedBodiesBodies• To be an obstacle to the solar wind a body must be conducting. • Imagine a planet with an atmosphere. – In sunlight some of the neutral atoms and molecules can be ionized.– If the solar wind is magnetized currents can be generated in theionosphere that will keep the magnetic field from penetrating the planet. – This condition will persist as long at the magnetic field keeps changing (otherwise it will eventually diffuse into the planet).• Venus– The main constituent of the Venus’ atmosphere is carbon dioxide. – Venus’ lower atmosphere is warmer than Earth’s because of the greenhouse effect, the upper atmosphere is much colder because of the absence of heating by the magnetosphere.– Scale height of Venus’ atmosphere is small being only a few kilometers on the night (cold) side. Atmospheric density falls off with height according to the equation of hydrostatic equilibrium that balances the upward pressure gradient with the downward force of gravity. where n is the number density of molecules, m is their mass, g is the force of gravity, k is Boltzman’s constant, T is the temperature of the gas and h is the height. For an isothermal atmosphere the density decreases as where . This isothermal description fails at the highest altitudes. For instance atomic oxygen has a cold component with a scale height of km and a hot component with a scale height of 100’s of km.()dhnkTdnmg−=)exp(0 nHhnn−=mgkTHn=– The upper atmosphere is partially ionized by solar ultraviolet radiation. The rate of ionization decreases rapidly with decreasing altitude at low altitudes where ionizing radiation is absorbed.  The rate of ionization decreases with increasing altitude at high altitudes where the number of neutral particles decreases. There is a maximum ion production rate at some altitude hm. The rate of ionization, Q, varies with altitude for a simple one-component isothermal ionosphere aswhere Qmis the peak production rate and with Hnis the neutral scale height and hmthe height of peak production. The rate of recombination of electrons and ions is proportional to the product of the electron and ion number densities (provided there is no vertical transport). where is the solar zenith angle and the density have been referenced to the maximum density at the subsolar point, Nm0, z to its altitude, hm0, and the scale height, Hn.()[]yyQQm−−−=exp1exp()nmHhhy−=()[]{}2/expsec10zzNNme−−−=χχ– The Chapman layer ionosphere of Venus. Top – UV radiation drops as it is absorbed in the photoionizationprocess. Middle – The rate of electron production versus altitude. Bottom - The electron density profile.– The high altitude electron temperature is about 5000K.–– The peak of the density is at about ~140 km and the density is ~106 cm-3. At 400 km the density is ~20000 cm-3under solar maximum conditions.– In general the solar wind plasma doesn’t penetrate below about 400 km.– At Venus’ the solar wind is supermagnetosonic.– A bow shock forms upstream of Venus.– At solar maximum the shock front is about 2000 km above the subsolar point. – At solar minimum the typical ionosphere doesn’t completely hold off the solar wind. – Downstream of the shock the velocity of the solar wind drops drastically.– The IMF is compressed near the stagnation point and the field drapes around the obstacle.– In the compressed region the thermal velocity of the plasma empties the field lines and a magnetic barrier results.– The magnetic barrier has the effect of confining the ionosphere to regions close to Venus.– The boundary is called the ionopause. Ionospheric plasma is not detected above the barrier because the ions produced there are immediately removed by the interplanetary magnetic field. The ionopause nominally forms where solar wind dynamic pressure ( ) equals the ionospheric thermal pressure (nkT).– Examples of observed altitude profiles for ionospheric electron densities (points) and magnetic fields (solid line). The ionopause is where the magnetosheath field decreases and the plasma density increases. 2vρ–When the solar wind pressure increases the ionopause moves closer to Venus.–When the boundary layer is close to Venus it thickens and a large-scale magnetic field appears inside of the ionosphere. –These penetrating magnetic fields have roughly the same orientation as the IMF and can be thought of as IMF field that is not completely shielded by the ionospheric currents.– The currents at the ionopauseare very good at excluding the magnetic field from the ionosphere but some gets through.– The spikes in the magnetic field are “magnetic flux ropes”. Flux ropes are untwisted at high altitudes. They become twisted as they sink toward the planet. It is thought to arise as the structure sinks through a vertical shear in the horizontal flow.– When the solar wind pressure increases the ionopause moves toward Venus. At low altitudes some field diffuses into the ionosphere. The inward diffusion on the dayside is matched by outward diffusion at night. In plasma on the field lines recombines leaving a flux tube.– The “flux ropes” tend to become “force free” structures. The currents in the flux rope tend be along the magnetic field so that This means that pressure gradient forces are not important.0=× BJrr0=×+−∇= BJPFrrr– The downward magnetic flux on the dayside is balanced by upward magnetic flux at night.  Since magnetic field lines in the ionosphere have their ends in the solar wind those on the night side are far from Venus and the field is mainly radial.  These regions appear as “holes” to a plasma detector.– When the dayside “magnetosphere” becomes completely magnetized the plasma to the nightside is cut off and the “ionosphere vanishes”. A similar phenomenon occurs at solar minimum when the dayside solar wind pressure is higher than the typical solar wind pressure.• Comets– Gas sublimed from comets is ionized in the solar wind. Conservation of momentum causes the solar wind to slow down.– This also happens at Venus but the effect is less dramatic. Venus’ large size keeps the atmospheric gas from expanding rapidly. Venus