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The hope is that by the time of the next maximum, solar weather forecasting will have some of the predictive capability that meteorologists have achieved for terrestrial weather at Earth’s surface. However, the most difficult events to predict are the largest and most damaging storms—hurricanes on Earth and extreme, rare storm events on the Sun. Thus, it is inevitable that the Sun will continue to surprise us.

The timing of solar events

A basic equation is useful in figuring out when events on the Sun will impact Earth:

distance = velocity × time, or D = v × t

Dividing both sides by v , we get

T = D / v

Suppose you observe a major solar flare while astronauts are orbiting Earth. If the average speed of solar wind is 400 km/s and the distance to the Sun as 1.496 × 10 8 km, how long it will before the charged particles ejected from the Sun during the flare reach the space station?

Solution

The time required for solar wind particles to reach Earth is T = D / v .

1.496 × 10 8 km 400 km/s = 3.74 × 10 5 s , or 3.74 × 10 5 s 60 s/min × 60 min/h × 24 h/d = 4.3 d

Check your learning

How many days would it take for the particles to reach Earth if the solar wind speed increased to
500 km/s?

Answer:

1.496 × 10 8 km 500 km/s = 2.99 × 10 5 s , or 2.99 × 10 5 s 60 s/min × 60 min/h × 24 h/d = 3.46 d

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Earth’s climate and the sunspot cycle: is there a connection?

While the Sun rises faithfully every day at a time that can be calculated precisely, scientists have determined that the Sun’s energy output is not truly constant but varies over the centuries by a small amount—probably less than 1%. We’ve seen that the number of sunspots varies, with the time between sunspot maxima of about 11 years, and that the number of sunspots at maximum is not always the same. Considerable evidence shows that between the years 1645 and 1715, the number of sunspots, even at sunspot maximum, was much lower than it is now. This interval of significantly low sunspot numbers was first noted by Gustav Spӧrer in 1887 and then by E. W. Maunder in 1890; it is now called the Maunder Minimum    . The variation in the number of sunspots over the past three centuries is shown in [link] . Besides the Maunder Minimum in the seventeenth century, sunspot numbers were somewhat lower during the first part of the nineteenth century than they are now; this period is called the Little Maunder Minimum.

Numbers of sunspots over time.

A graph titled “Monthly Average Sunspot Numbers”. The graph shows the number of sunspots on the y-axis (0 to 400) and the year on the x-axis (1750 to 2000). A scalloped line shows the rise and fall of sunspot numbers throughout the solar cycle.
This diagram shows how the number of sunspots has changed with time since counts of the numbers of spots began to be recorded on a consistent scale. Note the low number of spots during the early years of the nineteenth century, the Little Maunder Minimum. (credit: modification of work by NASA/ARC)

When the number of sunspots is high, the Sun is active in various other ways as well, and, as we will see in several sections below, some of this activity affects Earth directly. For example, there are more aurora    l displays when the sunspot number is high. Auroras are caused when energetically charged particles from the Sun interact with Earth’s magnetosphere    , and the Sun is more likely to spew out particles when it is active and the sunspot number is high. Historical accounts also indicate that auroral activity was abnormally low throughout the several decades of the Maunder Minimum.

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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