Space Weather: Solar Wind Interactions with the Earth’s Magnetic field 24 October 2011 William J. Burke Air Force Research Laboratory/Space Vehicles Directorate Boston College Institute for Scientific Research DMSP C/NOFS CRESS

2 Space Weather Course Overview Lecture 1:Overview and Beginnings Lecture 2:The Aurorae Lecture 3:Basic Physics (painlessly administered) Lecture 4:The Main Players Lecture 5:Solar Wind Interactions with the Earth’s Magnetic Field Lecture 6:Magnetosphere – Ionosphere Interactions Lecture 7:Solar Induced Disruptions Lecture 8:Magnetic Storms and Substorms Lecture 9The Satellite Drag Problem Lecture 10:Verbindung (to help make up for your rash decision not to take Wollen Sie Deutch Sprechen?)

3 Space Weather Solar Wind Interactions This talk discusses the main sources of space weather on the Sun In this context the Sun atmosphere has three layers, the photosphere, chromosphere and corona – The Photosphere: visible surface layer of the Sun and has a temperature of ~ 5000  K. – The Chromosphere: up to 2000 km from surface with temperatures up to ~ 20,000  K. – The Corona; uppermost layer with temperatures of ~1,000,000  K. From a space weather perspective, the most important driver is the corona which is the source of the solar wind, solar flares, coronal mass ejections (CME) and coronal holes. Flares and CMEs are the most geo-effective sources of disturbances – Ionospheric blackouts – Magnetic storms Overview

4 Space Weather Solar Wind Interactions The Photosphere Photosphere is the 100 km thick solar layer that is visible to our eyes. How do we know its temperature is ~5,000  K? In 1893 Wilhelm Wien showed that: max (cm)  0.29 /T (  K) In 1610 when Galileo turned his telescope towards the Sun he discovered that its surface was pocked marked by dark blemishes or sun spots. Low temperatures ~ 2200  K and strong magnetic fields Wien’s Law

5 Space Weather Solar Wind Interactions The Chromosphere Samuel Heinrich Schwabe (1789 – 1875) The Solar Cycle Edward Walter Maunder (1851 – 1928) Between 1645 and 1715 few sunspots observed. Little ice age in Europe and North America. Layer above photosphere in which T rises from ~ 6000  to 22,000  Lyman alpha emissions Trace metallic ions

6 Space Weather Solar Wind Interactions The Solar Corona The uppermost layer of solar atmosphere in which T rises to ~ 1,000,000  K First observed from the ground during total eclipses of the Sun Source region for the solar wind Magnetic structures rooted in photosphere poke through corona. As seen through x-ray telescopes the coronal is relatively cool near poles

7 Space Weather Solar Wind Interactions The Solar Wind First direct evidence for existence of solar wind found in images of comet tails (Biermann, 1948). Neutral dust follow trajectories along comet’s path Ionized gas directed radially away from the Sun Early Explorer and IMP missions monitored characteristics of solar wind’ The Ulysses mission, launched in 1990, decommissioned in 2008 was placed into polar solar orbit to determine where parcels of solar wind that reach Earth originate. Demonstrated high speed streams come from polar latitudes and low speed streams come from equatorial latitudes.

8 Space Weather Solar Wind Interactions Solar Wind Interactions with the Earth’s Magnetic Field By 1960 NASA missions showed that there was always a solar wind reached the vicinity of Earth. Typical densities are ~ 5  3 /cc. Typical speeds are ~ 400  100 km/s Hydrodynamic models of the flow by the Earth’s magnetic field were put assembled: Predicted a cavity in solar wind dominated by the Earth’s magnetic field called the magnetosphere. Predicted that supersonic flow demanded a bow shock across which flow becomes subsonic. Intermediate layer called the magnetosheath. Boundary called the magnetosphere and magnetosheath called the magnetopause. How does Solar wind alter internal magnetospheric dynamics?

9 Space Weather Solar Wind Interactions Interactions with the Earth’s Magnetic field In 1958 Eugene Parker of the University of Chicago first proposed that the pressure gradients in the would drive solar wind should drive a supersonic solar wind that flows radially outward from the Sun in all directions. Axford and Hines (1960) proposed that a viscous interaction along the magnetopause was responsible for the for driving auroral current systems. James Dungey of the Imperial College London suggested that if the solar wind carried a weak magnetic field it was possible to explain auroral and magneto- spheric dynamics in terms of magnetic merging. Eugene N Parker Current layer

10 Space Weather Solar Wind Interactions The Interplanetary Magnetic Field NASA scientists began to search for a weak Interplanetary magnetic field in the solar wind Technology challenges Bingo!! After lots of great arguments a picture emerged that was close to Dungey ‘s suggestion. Lots of unexpected benefits went with the discovery of the IMF Sources Heliospheric current sheet Magnetic clouds

11 Space Weather Solar Wind Interactions Scott Furbush The Interplanetary Magnetic Field The Furbush decrease in galactic cosmic ray fluxes during solar maximum years Simulation of Earth’s interaction with southward IMF

12 Space Weather Solar Wind Interactions It took half the semester, but we are now have all the elements in hand to understand how space weather develops We can now turn our attention to the various manifestation of space weather in the near-Earth environment