Sky & Telescope - May 2024 - 28

Aurora Hunting
is the Kp-index. This is a scale used to characterize the magnitude
of a geomagnetic disturbance. Unfortunately, it isn't very
useful for aurora chasers because the number is determined by
averaging the previous three hours of data. This means that the
Kp-index won't register activity until it is well under way, often
after the best part of the display has already occurred! Instead,
scroll down to the listings below, which utilize real-time data.
The next entry is Solar Wind, which displays the speed of
the incoming solar wind. The higher the speed, the better
the chance of a northern lights display. Ambient solar wind
averages between 200 and 400 kilometers per second. This
fluctuates between solar minimum and maximum. Anything
over 500 km/s can trigger faint aurorae, and over 600 km/s
may produce a strong display.
The next listing is Density - the concentration of ionized
plasma impacting the magnetosphere. This measurement can
signal the initial shock from a CME or a stream interaction
region or corotating interaction region - the boundary between
the fast and slow solar wind.
Another important value to watch is the Interplanetary
Magnetic Field. Think of this as a " potential " indicator. The
higher this number, the more potential energy there is in the
field to interact with an incoming CME.
Finally, there's the Bz value. This displays the solar wind's
magnetic-field direction as it reaches Earth and is a critical
value to monitor when determining if it's worth heading out
on a chase. It acts like an on/off switch for auroral activity.
Why? Because Earth's magnetic field naturally points north.
Since opposites attract, we want an incoming stream of solar
plasma with a south-pointing magnetic field so it will couple
with Earth's magnetic field and allow for an auroral display.
A north-north coupling will repel and deflect the solar wind
out away from Earth. So, the higher the southern value is, the
better chance of a good show.
Of course, these are all in-the-moment observations. What
if you need to travel to a distant location to see a display? This
is where the difficulty lies. Several satellites monitor the Sun,
and when a CME or coronal hole is Earthward-facing, we can
infer approximate times of arrival. However, we don't have
any way to monitor these events during their 149-millionkilometer
(93-million-mile) transit. So, the predicted timing
of a CME can be off by hours or even days. Also, we can't
measure the magnetic polarity of an event until it hits one
of our upstream monitors, like NASA's DSCOVR satellite. At
most, we have a few days' lead time to get to a good location
and then wait it out.
Location, Location, Location!
As mentioned earlier, aurorae tend to be visible at highlatitude
locations, so you'll likely have to travel to a location
where aurorae are more commonly seen. They typically
appear in the Arctic and Antarctic regions. Places
like Alaska, Iceland, northern Canada, the Scandinavian
Peninsula, and Siberia offer the best places to spot them. Sky
PERFECT PAGEANT Photographing a good display of the aurora low towards the horizon requires an unobstructed view. But these low shows can
offer some better photographic opportunities, such as this picturesque scene along the southern shores of Lake Superior in Eagle Harbor, Michigan.
28 MAY 2024 * SKY & TELESCOPE

Sky & Telescope - May 2024

Table of Contents for the Digital Edition of Sky & Telescope - May 2024