Magnetospheric Morphology

1. Magnetospheric Morphology Prepared by Prajwal Kulkarni and Naoshin Haque Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

2. What is the Magnetosphere? • Magnetosphere is region of space controlled by the Earth’s magnetic field. • Within 5 - 6 Earth radii (RE), the Earth’s magnetic field can be approximated as a dipole tilted ~11 degrees from the rotation axis • (RE = 6370 km) • The Earth’s magnetic field is confined by solar wind plasma blowing outward from the Sun.

3. The Solar Wind • Source of solar wind: hot plasma (106 K) from the solar corona • The coronal plasma is accelerated and flows radially outward from the sun, filling interplanetary space • Solar wind properties at Earth: • Speed ~400 km/s • Speed range ~200-700 km/s • Number density ~ 7 cm-3 • Magnetic field ~ 5 nT • Electron temperature ~ 105 K • Proton temperature ~3 x 104 K Picture of a total solar eclipse: the solar corona is visible

4. The Magnetosphere Solar Wind • Solar wind flows past Earth and is deflected around Earth’s magnetic field. • The solar wind compresses the magnetic field on the sun-side, creating a boundary termed the magnetopause at ~10 RE. • On the night side, the solar wind-dipole field interaction results in a tail up to~60 RE. • The magnetosphere is the region within the magnetopause, from ~10 RE on the sun side to ~60 RE on the night side. • Plasma within ~4 – 6 RE rotates with the Earth—a region called the plasmasphere.

5. The Plasmasphere plasmapause • The plasmasphere is a region of dense (101 – 104 electrons/cm3), ‘cold’ (energies < 1 eV) plasma located up to ~5 RE. • The electron density drops by ~2 – 3 orders of magnitude outside the plasmasphere. This density drop occurs at a sharp boundary termed the plasmapause.

6. L-shell • Specified distance from Earth in space and magnetospheric physics • Describes a particular set of planetary magnetic field lines. • Given L-value describes magnetic field line that crosses the magnetic equator at a number of Earth-radii equal to the L-value. • For example, "L = 2" describes the set of the Earth's magnetic field lines which cross the Earth's magnetic equator at 2 RE from the center of the Earth. • From this point, we will often specify geospace locations in terms of L-shell.

7. L-shell mapped to ground

8. Radiation Belts • Within the cold plasma of the plasmasphere, there is a population of energetic (from ~100 keV to several MeV) electrons and protons. • These energetic particles comprise the radiation belts. There are two belts—an inner radiation belt and an outer radiation belt. • The inner electron radiation belt exists from L ~1.3 to L ~ 2. The outer radiation belts extend beyond the plasmasphere. • Between the two belts, there exists a slot region of depleted radiation belt particles. • The density of these ‘hot’ energetic particles is much lower than the • cold plasmaspheric density.

9. The Slot Region • The slot region—a region of depleted radiation belt particles—exists from approximately L = 2—3. • Studying source and loss processes in electron radiation belts and formation of slot region are major research questions.

10. The Ring Current • The ring current is another population of trapped, charged particles, approximately located from 2 < L < 7. • The ring current is a toroidally shaped current that flows westward (ions and electrons move in opposite directions). • Carried mainly by kilovolt protons. • Particle motion is caused by gradient and curvature drift induced by the Earth’s magnetic field. • The ring current is associated with strong perturbations in the magnetic field measured at Earth.

11. Geomagnetic Storms H F Z • Sharp temporary change occurring in Earth’s magnetic field. • SC: caused by compression of Earth’s outer atmosphere by flare-enhanced solar wind • Initial phase: continued compression • Main phase starts after plasma cloud passes Earth • Reduction in surface field  increase in ring current particle fluxes Mins-hours 10s of hours-1 week 1/2 hour-several hours

12. Dst Index • Geomagnetic index: monitors world wide magnetic storm level • Based on the average value of H measured hourly at 4 near-equatorial geomagnetic observatories. • Negative values indicate geomagnetic storm in progress • Negative deflections caused by storm time ring current main phase recovery phase

13. Kp Index • Measures magnetic activity based on value of H • Combine data from 12 observatories to assign number to disturbances • Figure ranges from 0, 0+, 1-, 1, 1+, …, 9+ • Measured in 3-hour increments • Quasi-logarithmic February 1986

14. References Abel, B., and R. M. Thorne, Electron scattering loss in the Earth's inner magnetosphere: 1. Dominant physical processes, J. Geophys. Res., 103, 2385, 1998a. (Correction, J. Geophys. Res., 104, 4627, 1999.) Abel, B., and R. M. Thorne, Electron scattering loss in the Earth's inner magnetosphere: 2. Sensitivity to model parameters, J. Geophys. Res., 103, 2397, 1998b. (Correction, J. Geophys. Res., 104, 4627, 1999.) Carpenter, D., Whistler evidence of a ‘knee’ in the magnetospheric ionization density profile, J. Geophy. Res., 68, 1675, 1963. Li, X., and M. Temerin, The electron radiation belt, Space Science Reviews, 95, January 2001. Lyons, L. R., R. M. Thorne, and C. F. Kennel, Pitch angle diffusion of radiation belt electrons within the plasmasphere, J. Geophys. Res., 77, 3455, 1972.