Long before scientists had even thought about the Earth's dependence on
the Sun, various ancient cultures - including the Egyptians, Greeks and
Romans - revered the Sun as a god, a benign provider of light and life.
Apart from this instinctive urge to worship, there also grew a desire to
study and comprehend. In the sixth century [b.c]{.smallcaps}., the Greek
mathematician Pythagoras (C 540 [b.c.]{.smallcaps}) concluded that the
Earth revolved around a "central fire" (not the Sun), with an orbital
partner, the counter-Earth, which periodically shielded the Earth from
it. The Sun was supposed to shine by light reflected from this central
fire. In 434 [b.c]{.smallcaps}., the Greek philosopher Anaxagoras
postulated that the Sun was actually this ball of central fire which was
"as large as Pelopennesus floating 4000 miles above the Earth's (flat)
surface".

Despite the very early philosophizing and theorizing on the Sun, the
solar wind emanating from it received little scientific attention until
the 1950's. Before then a few astronomers had speculated that some force
might originate from the Sun, but none defined it. In 1900, for example,
Sir Oliver Lodge suggested that the Sun was a source of a "torrent or
flying cloud of charged atoms or ions" which propagated through
interplanetary space and gave rise to magnetic storms. Three decades
later in 1932, Julius Bartels noted that there was a strong tendency for
moderate magnetic storms to recur at intervals of 27 days, the same
period as the rotation of the Sun. Bartels assumed that active solar
regions existed which were responsible for the magnetic disturbances on
Earth. He could not link these disturbances with flares or sunspots, so
he named the regions, "M-regions" (where the "M" stood for magnetic).
These M‑regions were later identified as coronal holes.

A scientist named Biermann made the first suggestion that the solar wind
was a continuous outflow of solar material in 1951. He interpreted the
fact that comet tails pointed in a direction radially away from the Sun
as evidence that interplanetary space must be filled with continuously
radially streaming material, the outer layer of the corona. This hot
coronal material, called plasma, carries with it the solar magnetic
field into the interplanetary region. This weak field embedded in the
solar wind is called the Interplanetary Magnetic Field (IMF). Eugene
Parker considered how the solar wind flow and solar rotation influence
the direction of this interplanetary magnetic field. The Sun rotates
with a period of 27 days and this causes the radially extending magnetic
field lines to become 'wound up' into a configuration known as the
Parker spiral. The IMF tends to adopt a "garden hose" orientation such
that the angle between the radial vector and the IMF is about 45^o^ to
the Earth's orbit.

+-----------------------------------+-----------------------------------+
| ![](media/image1.                 | The Parker Spiral -- "Plasma"     |
| jpeg){width="2.923611111111111in" | moves radially outwards from the  |
| height="2.786111111111111in"}     | Sun while the magnetic field      |
|                                   | become wound into a spiral. A     |
|                                   | garden hose effectively           |
|                                   | demonstrates the way that the     |
|                                   | Parker spiral is formed           |
|                                   |                                   |
|                                   | ![](media/image2                  |
|                                   | .png){width="2.998611111111111in" |
|                                   | height="1.5944444444444446in"}    |
+-----------------------------------+-----------------------------------+

The solar wind consists of charged particles, mainly protons and
electrons, emanating from the Sun in all directions at speeds of several
hundred kilometers per second. The solar wind also contains, in much
smaller amounts, positive ions stripped of many of their electrons by
the extremely high temperatures of the Sun.

The very active, very hot corona continually expands outward moving at
velocities of hundreds of kilometers per second, a million miles per
hour! This allows these protons and electrons to escape the gravity of
the sun and pour out into space well beyond the solar system. The
Earth\'s magnetic field (magnetosphere) protects us from these
particles, acting like a rock in a stream, diverting the solar wind
around us.

Solar Wind Velocity Activity

The table contains the solar wind velocity recorded by the WIND
spacecraft from 00:00-12:00 on January 10, 1997.

1\. Using the values in the table, plot the solar wind velocity (y-axis)
against time (x-axis) -- use a y-axis scale from 350 to 500.

2\. Using the values in the table, calculate the average solar wind
velocity during this period

3\. Using the average velocity and location of the WIND spacecraft,
calculate how long ago the solar wind left the sun.

150,000,000 km (distance from sun to Earth)

550,000 km (distance from WIND spacecraft to Earth)

HINT: Distance = Velocity x Time

4\. Now calculate how long it will take the solar wind detected by the
WIND spacecraft to reach the Earth.

5\. What is the total transit time for the solar wind from the Sun to
the Earth?

6\. Convert your answer from seconds into hours and days:

3600 seconds = 1 hour

24 hours = 1 day

7\. Calculate the length of time it would take the solar wind to reach
the Earth given velocities of 200 km/sec to 700 km/sec (in intervals of
50 km/sec). You can write your answers in the table provided.

  ------------------------ -------------- --------------- ----------------
  Solar Wind Velocity in   Transit time   Time in hours   Time in days
  km/sec                   in sec                         

  200                                                     

  250                                                     

  300                                                     

  350                                                     

  400                                                     

  450                                                     

  500                                                     

  550                                                     

  600                                                     

  650                                                     

  700                                                     
  ------------------------ -------------- --------------- ----------------

8\. Plot these results -- solar wind velocity (y-axis) versus days for
the solar wind to reach the earth (x-axis)

9\. From your graph, predict times for 100 and 800 km/sec.

  ---------------------------------- ------------------------------------
  **TIME**                           **Solar Wind Velocity**

  0:00                               374

  0:15                               377

  0:30                               378

  0:45                               381

  1:00                               408

  1:15                               423

  1:30                               423

  1:45                               419

  2:00                               426

  2:15                               444

  2:30                               442

  2:45                               441

  3:00                               450

  3:15                               434

  3:30                               435

  3:45                               457

  4:00                               469

  4:15                               457

  4:30                               441

  4:45                               472

  5:00                               477

  5:15                               458

  5:30                               470

  5:45                               465

  6:00                               431

  6:15                               424

  6:30                               424

  6:45                               420

  7:00                               399

  7:15                               392

  7:30                               393

  7:45                               394

  8:00                               395

  8:15                               395

  8:30                               390

  8:45                               388

  9:00                               404

  9:15                               401

  9:30                               400

  9:45                               390

  10:00                              375

  10:15                              386

  10:30                              397

  10:45                              389

  11:00                              380

  11:15                              378

  11:30                              375

  11:45                              373

  12:00                              372
  ---------------------------------- ------------------------------------