Monday, 15 January 2007
Estimation of solar wind pressure and its impact on geomagnetic field during energetic solar events
Exhibit Hall C (Henry B. Gonzalez Convention Center)
The solar wind is now know to be a mixture of materials found in the solar plasma, composed of ionized hydrogen (electrons and protons) with an 8% component of helium and trace amounts of heavy ions. The solar wind has been detected inward toward the sun to the orbit of Mercury, and outward past the orbits of Uranus and Neptune. The flux of particles is modulated by the pressure (both magnetic and thermal) at the base of the wind in the solar corona, and to a certain extent, some of the features of the solar wind, particularlly in the case of high speed streams, can be identified with specific large scale coronal features, the coronal hole structures and solar energetic events. The solar wind, in fact, is the medium which connects the magnetic variation of the Sun out through the heliosphere, the volume of interplanetary space which is influenced by solar magnetic fields, to various bodies found in the solar system. The scientific interest in the solar wind stems from this fact, and has held the attention of researchers for two reasons: (1) To the extent that the physics of the solar wind is known, it is possible to establish causality, and establish predictive capability for solar-geophysical events. (2) The wide range of physical conditions found in the solar wind and its interactions with magnetized and non-magnetized planets, dust, cosmic rays, comets, and spacecraft allow the investigation of physical mechanisms not easily duplicated in the terrestrial laboratory. Many contributions to the basic understanding of plasma processes have been identified and understood in the context of the solar wind. Numerous problems await further investigation and resolution. There is a significant pressure in the interstellar medium, the expansion must eventually stop. The resulting quasi-static bubble is then of the order R AU where R is determined by equating the ram pressure of the solar wind to the total interstellar pressure, P (internal + dynamic + magnetic + cosmic ray pressure):
P = (m * n * V * V)/(R * R)
Here n is the ion number density at 1 AU, m the average ion mass, and V the speed of the solar wind. It is implicit in this relationship, essentially the same as used currently, that the solar wind is highly supersonic and therefore the left hand side of equation (1) need contain only the solar wind ram pressure. Davis, using what little information was available 40 years ago, suggested R = 200 - 2000 AU. Taking n = 8 protons per cubic centimeter, V = 400 km/s and (allowing for the presence of helium), we find that R ~ 85 AU for The solar flares, coronal holes and the coronal mass ejections are the main source of geomagnetic activity. The main role is attributed mainly to coronal mass ejections —CMEs. There is no simple association between CMEs and flares: there are large CMEs without flares and big flares without CMEs. Flares and CMEs seem to be part of a single phenomenon, a solar eruption, and neither one is the cause of the other. There have been a number of detailed analyses of geomagnetic contributions due to different solar wind sources, and also on the classification of geomagnetic activity according to each solar wind structure. Observations from Skylab, Helios, Ulysses, ACE and SOHO have been very useful for the understanding of the Sun – Earth system. A set of recent solar events were selected and using the ACE data and geomagnetic observation in the form of Ap, Kp and Dst we performed a detailed study of the control of IMF and solar wind parameters on the enhancements of geomagnetic activity. It is found that there are three types of events for the enhancement in geomagnetic activity associated with (1) negative or southward Bz, (2) oscillatory variation in Bz and (3) positive or northward Bz. Here these observations and their implications will be discussed. The solar wind RAM pressure on logarithmic scale show almost linear relationship with Kp and not so with other geomagnetic indices. The slope of the line is found to be different from event to event. Kp enhancement varies from ~ 2 to 6 for a tenfold increase in the solar wind RAM pressure during different solar events.