Acceleration of the Solar Wind

Transcription

1 From Sun to Mud: Solar and Space Physics for the UG Classroom Acceleration of the Andrew Jordan All images from SOHO spacecraft This presentation helps introductory physics students apply their skills in calculating acceleration to an environment that s unlike the everyday world. At the same time, it will also help them relate their discoveries back to their world. Discussion question: What is the extent of the Sun s atmosphere? Answer: The Sun s atmosphere, or solar wind, extends all the way out to 100 times the distance of the Earth from the Sun, or three times further out than Pluto. That means that Earth is actually inside the Sun s atmosphere, although it is more tenuous than the best vacuums achievable in labs. Eventually the Sun s atmosphere encounters the interstellar medium.

2 Continuous stream of charged particles emanating from the Sun in every direction - All images are from NASA and ESA s Solar and Heliospheric Observatory (SOHO) and are in false color. - The inner image is from the Extreme ultraviolet Imaging Telescope (EIT), and shows a prominence at the lower right - The other two images are from SOHO s coronagraphs (LASCO), telescopes with central disk to cover the Sun. This enables the much fainter corona of the Sun to be seen, just as the Moon does during a total solar eclipse. The wispy structures are variations in the solar wind. Many of the dots are stars. Some of the dots and all the small streaks are energetic charged particles hitting the camera. At lower right is a coronal mass ejection that has erupted from the Sun.

3 Fast wind Slow wind Slow wind Fast wind - Typically fast solar wind comes from higher latitudes, while slow solar wind comes from low latitudes - Reality can be much more complicated, however!

4 Fast wind Slow wind Slow wind Fast wind Airliner would take 17 years to fly from the Sun to Earth Solar wind takes about 4 days!

5 rfast = 5x106 km rslow = 1.5x107 km - 6 When the fast wind reaches 5x10 km from the Sun s surface, it has a speed of 750 km/s 7 - When the slow wind reaches 1.5x10 km, it has a speed of 400 km/s

6 Near the Sun (r = 0), both fast and slow solar wind have speeds of 10 km/s At the distances shown (rfast and rslow), the fast and slow winds have the typical speeds shown rfast = 5x106 km rslow = 1.5x107 km

7 Near the Sun (r = 0), both fast and slow solar wind have speeds of 10 km/s At the distances shown (rfast and rslow), the fast and slow winds have the typical speeds shown rfast = 5x106 km rslow = 1.5x107 km Questions What are the accelerations of the fast and slow solar wind? Do you think a human could survive these accelerations?

8 rfast = 5x106 km rslow = 1.5x107 km

9 - 2 The last couple of steps answer the question about whether human body can survive such acceleration. It s easier to grasp this if acceleration is written in terms of the gravitational acceleration at Earth s surface (~10 m/s ). - While the acceleration may not seem like much, remind the students that the acceleration occurs over many hours.

10 What accelerates the solar wind to these speeds? No one knows! Perhaps it has something to do with waves in the magnetic fields near the Sun. - This is an active area of research in the solar physics community. Homework problem: The Sun sporadically erupts and sends a burst of plasma and strong magnetic field into the solar system. Such events are called coronal mass ejections, or CMEs (one is going to the lower right in the background 6 image). At r = 0, one CME had a speed of about 40 km/s. When the CME reached r = 2x10 km, it had a speed of 2000 km/s. Assuming constant acceleration, calculate the acceleration of the CME. How does it compare to the fast and slow solar winds? 2 2 Answer: The acceleration would be about 1 km/s (1000 m/s ), which is about 100 G! (CME information taken from Figure 3 of P. T. Gallagher, G. R. Lawrence, and B. R. Dennis (2003), Rapid acceleration of a coronal mass ejection in the low corona and implications for propagation, The Astrophysical Journal, Vol. 588, L53-L56.)

11 From Sun to Mud: Solar and Space Physics for the UG Classroom Links and References: Basic intruduction to the Sun: D. Alexander, The Sun, Greenwood Press, 2009 Advanced textbook about the Sun: M. Stix, The Sun: An Introduction, Springer, 2004 NASA Marshall Space Flight Center s resources about solar wind and the Sun: NASA s Solar and Heliospheric Observatory (SOHO) Acceleration of the Andrew Jordan, University of New Hampshire