Re: SUMMARY: Geomagnetic Sudden Impulse



Shawn Curry wrote:
Seriously, what is a "Geomagnetic sudden impulse"?

A sudden geomagnetic pulse means that a series of stations around the
world have registered a rapid change in the overall planetary magnetic
field.

Sudden changes in the geomagnetic field of the Earth are often caused
by the solar wind - either the Earth crossing the ambient solar wind in
the ecliptic plane or by coronal mass ejections from sunspots sending
bullet shaped clouds of charge gas particles into the ecliptic plane.

Note that the ambient solar wind is not a flat disk in the ecliptic.
It follows the magnetic field lines that eminate from the Sun in the
shape of a Parker spiral.
http://helios.gsfc.nasa.gov/gloss_op.html#parker

The bullet or tear-drop shaped CMEs are many times the size of any
planetary body. Here's a shock propogation movie that simulates a CME.
Note the Earth is the small blue dot at 1 A.U. on the plus Y axis and
that it takes about 38 hours for the CME explosion to travel from the
Sun to the Earth's orbit.
http://www.expi.net/expinet/sep-24-98-movie.gif

While the ambient solar wind generally follows the Parker spiral, CMEs
can pretty much be ejected at any odd angle away from the Sun. If a
spot is pointing behind the Earth's orbital direction when the flare
explodes, the CME can pass behind, or below, the Earth and not effect
the Earth's magnetic field.

The geomagnetic stations are maintained primarily to protect the
electric power grid. As the solar wind interacts with the Earth's
magnetic field and charged solar wind particles travel down the
distorted field lines of the Earth's magnetic field, induction can
occur in electrical grid power lines. Think of the power grid lines as
huge antenna wires - hundreds if not a thousand miles long. These
induction currents can trigger safety fuses in the power grid system
and cause power interruption and require the replacement of millions of
dollars in equipment. To minimize these electrical power interruptions,
the Sun is continuously monitored by satellites that either (a) watch
the Sun for coronal mass ejections and flares, or (b) are stationed a
million miles away between the Sun and the Earth and record changes in
magnetism and particle density.

This monitoring system provides advance warning of flares and CMEs that
might cause damages to or shut-downs of the power grid. Once warned,
the utility companies simply run the grid lines at a lower amperage, so
induced currents do not disrupt the system.

A secondary benefit to amateur astronomers, who piggy-back their hobby
on this monitoring system, is that we can have advance warnings of when
aurora are likely to occur and how far south of a longitude that the
aurora will reach.

There are two key monitored indicators of geomagnetic activity that are
of interest to the amateur.

First, - the planetary kp - is a scale of the aggregate estimated
planetary magnetic field strength.

The NOAA maintains one of the world-wide monitoring stations in
Boulder, Colorado, near your location. Over the internet, you can
watch its readings in near real time with a 15 minute delay.
http://www.sec.noaa.gov/rt_plots/bou_12h.html

The reading from the Boulder station and other stations are aggregated
into the planetary kp index.

The near-real time planeary kp index can be read from a number of
online sources:

Space Environment Center (SEC) (graph in the lower left-hand corner)
http://www.sec.noaa.gov/index.html

SEC's realtime plot menu (Estimated 3-hourly planetary kp values)
http://www.sec.noaa.gov/rt_plots/

As I am writing this to you on August 19, 2006 at 21:00 UTC, there have
been 6 hours where the planetary kp index is about 6. This is the
result of a CME ejected on Aug. 16 from sunspot 904. Here's a movie of
the 8/16 CME exploding off the Sun's surface from an Earth orbiting
satellite:
http://spaceweather.com/images2006/16aug06/cme_c3_big.gif

Unfortunately at my longitude, it is daytime and although aurora may be
occurring, they cannot be seen.

In past studies, the planetary kp has been well correlated with the
most southernly longitude to which aurora displays can be seen in North
America.
http://www.sec.noaa.gov/info/kp-aurora.html

At my latitude of 41N, which is similar to yours in Boulder, Colorado,
it takes a pretty stiff kp of 8-9 or higher to cause visible aurora.
Conversely, a realtively frequent kp of 5 can be seen from northern
Minnesota and Michigan or in the Pacific Northwest.

So, for the current peaks of kp 6 peaks on August 19, 2006 at 21:00
UTC, I probably would not be able to see anything, even if it was dark.
At that level, the aurora are 700-1000 miles north of my 41N latitude
observing point. However, the tops of some aurora may be faintly seen
sticking up over the horizon. Hence, www.Spaceweather.com sometimes
recommends for lower latitude observers to take long exposure north
facing wide-field photographs that might bring out auroral activity
below the visual detection limit of the human eye.

The second major monitored index of the Earth's magnetic field that is
of interest to amateur astronomers is the Bz indicator. Basically, Bz
indicates the North-South direction of the Earth's magnetic field. Near
real time Bz can be viewed online at:

The ACE 7d Planetary Sweep display
http://www.sec.noaa.gov/ace/MAG_SWEPAM_7d.html

The Earth can be buffeted by CMEs or other high solar winds without
generating northern aurora. The reason is that the Earth's magnetic
field is strong enough to repel the charged solar wind particles. At
some level, a CME can be strong enough to overwhelm and distort the
northern hemisphere magnetic field. The result on the ground is that
the north-south z plane of the magnetic field appears to swing south -
or the Bz to said to be "Bz south" or the "Bz went south". Public
education animations of part of this process can be viewed here at the
NCAR High-Altitude Research Center -
http://meted.ucar.edu/hao/aurora/txt/x_m_2_2.php

A strong solar wind in conjunction with a Bz south indicates the best
times for viewing aurora.

As mentioned above, the primary purpose of the solar monitoring system
to prevent disruptions to the electrical power grid.

The NOAA Space Environment Center runs a service for utilities in the
form of 1) an online predictor of planetary geomagnetic disruption and
2) an email warning service.

The online predictor is the SEC Costello Geomagentic Activity Index.
See the "1-day" output display.
http://www.sec.noaa.gov/rpc/costello/index.html

The posts that Sam Wormley graciously makes in this newsgroup are
republications of the NOAA SEC's email warning system. The particular
warning at the top of this thread indicates that the satellite orbiting
one million miles between the Sun and the Earth registered the passage
of the CME. This means a distrubance of the Earth's field is probably
imminent. The NOAA SEC also issues email warnings that can be set for a
particular kp index level. Sam does the public service of reposting
those high kp index warnings - generally when the kp index is 6 or
higher.

A private commercialized add-on version to the SEC text email warning
system is www.spaceweather.com. For about $5 a month, they have an
automatic telephone dailing machine that will call you and leave a
message on your answering machine when the kp index is sufficiently
high that auroral displays are likely at your latitude.

Although there are some annual patterns to auroral displays, they are
generally random events and tend to occur between 12:00AM and 4:00AM.
So, someone giving you an email notice warning of when you can walk
outside with a camera or binoculars and watch for auroral display is
very helpful to the hobbyist. A fee-based telephone service that calls
and gives you a wake up call in the middle of the night is even better.


Hope that's enough info to answer to your question.

Further reading about the Sun at the above sites and:
http://solarscience.msfc.nasa.gov/SolarWind.shtml
http://meted.ucar.edu/hao/aurora/txt/x_menu.php
http://meted.ucar.edu/hao/aurora/txt/x_m_0.php

Corrections to the above by expert lurkers is welcomed and appreciated.


- Regards Canopus56

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