Solar atmosphere. Solar activity and solar wind. Reading for this week: Chap. 6.2, 6.3, 6.5, 6.7 Homework #2 (posted on website) due Oct.

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1 Solar activity and solar wind Solar atmosphere Reading for this week: Chap. 6.2, 6.3, 6.5, 6.7 Homework #2 (posted on website) due Oct. 17 Photosphere - visible surface of sun. Only ~100 km thick. Features include sunspots Chromosphere -temperature rises from 6000 C to about 20,000 C.; get H-alpha emission- reddish color ; Ca-K purple Carrington (1859) reported bright flare of light from sunspot. Sunspot looked different. Followed by magnetic storm on earth. Corona - extremely hot, low density >10 6 C Emission lines Temperature/Density Profile of Solar Atmosphere Seeing different parts of solar atmosphere sohowww.nascom.nasa.gov

2 Temperature and emission lines Huge magnetic loops extending tens of thousands of km into space trapping hot gases inside them. The images show CDS' unique ability to map such regions. The coolest gases (20,000 degrees) are shown in the top left image, and the hottest (2 million degrees) in the bottom right. The edge of the Sun is seen on the left of each image and different gases show different loops giving a spectacular display above the surface of the Sun. sohowww.nascom.nasa.gov Yang et al., A&A 501, (2009) DOI: / / Sunspot to flare to cme sunspot 1302 has already produced two X-flares (X1.4 on Sept. 22nd and X1.9 on Sept. 24th). Each of the dark cores in this image from SDO is larger than Earth, and the entire active region stretches more than 100,000 km from end to end CMEs By Jack Newton in British Columbia FLARES

3 Sunspots to flares and cmes Sunspots to flares (Left) magnetic field model of the solar corona, (middle) magnetohydrodynamic simulation of ejective solar flare and (right) particle-in-cell simulation of magnetic reconnection, which causes solar flare explosions Selected model cartoons from Two-types: simple loop vs. ribbon flares A single loop flare is a single flux tube that twists and reconnects. In the ribbon model, multiple reconnection sites lead to extended region of acceleration due to sheared magnetic footpoints Flare models Evidence for tether-cutting reconnection in the powerful flare of October 29, The upper left panel shows a GOES SXI image of the flare, with RHESSI hard X-ray contours superposed; this would correspond to the tether-cutting stage of the flare as shown at lower left. In later development (a TRACE EUV image in the background) the event has simplified as shown at lower right.

4 Electromagnetic Emissions from Flares Energy up to J in a few minutes Radio White light flares rare; brightness in flares usually ~1% of photospheric Visible and UV X-rays and!-rays AR9077: Solar Magnetic Arcade Credit: TRACE, Stanford-Lockheed ISR, NASA X-rays and gamma rays from flare Flare Classification Scheme Flares classified according to output in 1-8 Å range: B: I < 10-6 W/m 2 C: 10-6 < I < 10-5 M: 10-5 < I < 10-4 X: I > 10-4 W/m 2 Lin, R.P., et al. 2003, ApJ, 595, L69 High energy emissions were detected from the loop top. This great discovery indicates that energy release is occurring above the loops. YOKOH olar-b/3/index_e.html A numeral is appended to the letter class to give the abscissa, e.g.., M5 is an intensity of 5!10-5 W/m 2. Only X-flares can have a two-digit numeral: Largest ever recorded during Halloween, 2003, storm (11/4/2003): X28, or 2.8!10-3 W/m 2 (later claimed to be X45!)

5 Flare models: which is correct? We don t know! Area of active research. Where does energy (and for CMEs, mass) come from? Coronal mass ejections 3 part structure: a bright frontal loop; a dark cavity; and a bright core Two types : fast and slow. Slow can see acceleration in images. Final speeds up to 1000s km/s Total mass usually kg Believed to originate on originally closed field lines. Some associated with flares; some are not Lots of ongoing work on how to predict RHESSI spacecraft images of gamma-rays (blue) and X-rays (red) thrown off by the hottest part of the flare are shown with UV images from the TRACE spacecraft. The gamma rays are made by energetic protons at the Sun. Scientists were surprised that the gamma rays matched the energy spectrum of protons at Earth: the proton storm may have come directly from the Sun and not from the CME as anticipated. Coronal mass ejections Example CME models Mass-loading model of Low et al From Zhang and Low, AAAA, 2005 Slow vs fast explained by different initial structures

6 MHD CME model Coronal mass ejections Alternative models of CME origin The left figure shows an SAIC model simulation of field line behavior during a CME. In this case the simulated CME was initiated by reversing the polarity of the photospheric magnetic field under an idealized sheared coronal arcade structure. The right shows an alternative model from NRL where a CME was initiated by emerging flux in a quadrupolar coronal field configuration.

7 STEREO observations of solar energetic particles Intro to solar wind Early idea of solar corpuscular radiation: Stormer - aurora due to electron beams from sun Chapman-Ferraro - origin of geomagnetic disturbances Forbush - modulation of cosmic rays Hoffmeister (1943) - comet tails not radially out Biermann (1951) - 2 tails, need particle wind Chapman (1955) - hot corona must extend far out since some fraction above escape velocity Parker (1958)- supersonic expansion of solar wind Direct observations Lunik (1959) and Explorer 10 (1961) - possible observation Mariner II (1962)- definitive; observed fast and slow streams with recurrent 27 day period Hydrostatic model Chapman: Assume also hydrostatic equilibrium. Thermal energy ~.1 escape energy dp /dr =!GM S Aside: our atmosphere and scale heights dp /dr =!GM E difference is gravity doesn't change much over relevant height so dp /dr =!g dp = nmgdr! = nm! = nm Use p = nkt and assume T = constant dp = ktdn so ktdn = nmgdr dn/n = mgdr/kt n = n 0 exp(mg(r! r 0 ) /kt) where n o is density at r 0 H = scale height n = n 0 exp((r! r 0 ) /H) For sun, dp /dr =!GM S Again assume isothermal, n = n 0 exp(mgm S /kt(1/r S "1/r)) dp/dr =!GM S Hydrostatic model Chapman realized that energy transport via thermal conduction larger than radiative in hot outer solar atmosphere. Chapman added energy conservation and got scale height Concluded that n at Earth would be ~ 10 2 /cm 3 at T ~ 10 6 K (~ 100 ev) Parker noted that results in very large pressure far from Sun Not static, equilibrium Need to include momentum equation

8 Parker s model for solar wind Back to original HD equations (ignore Lorentz force)!dv/dt = -"p +F g no explicit time dependence and only radial variations!vdv /dr = #dp/dr #!GM s Include continuity (spherical geometry!) (1 )d(!vr 2 ) /dr = 0 Assume isothermal dp/dr = 2kTdn/dr 1 v dv ( dr v 2! 2kT /m) = 4kT mr! GM s r 2 Following Lysak(2009) and Parker(1965) What do solutions look like? 1 v dv ( dr v 2! 2kT /m) = 4kT mr! GM s r 2 Look at limiting cases. RHS = 0 for coronal temperatures T C! GM s m /4R c with R c the radius of coronal base T C ~ 6x10 6 K for T < T C, RHS negative for R C < r < r crit (r crit /R C = T C /T) RHS postive for r crit < r <! at r crit LHS = 0 either dv/dr = 0 1 v or v 2 = 2kT /m = c s 2 dv dr v 2! 2kT /m ( ) = 0 so for isothermal 1. dv /dr = 0; v is maximum or minimum. dv/dr changes sign across r crit, but v 2! 2kT /m = v 2! c s 2 does not change sign. flow is either supersonic or subsonic everywhere 2. v 2 = c s 2 at r crit then v either increases or decreases monotonically