C.Oler zum Dritten - IMF, CME-Struktur, etc

[Ulrich Rieth]

Name: Cary Oler
Date: Sun, 25 Jul 2004 13:48:16 -0700
Subject: Re: [Forum] Question

> Hello, I was just wondering how long it takes for an IMF disturbance to
become a middle latitude aurora after been detected by the ACE (excluding
the time between the ACE spacecraft and the Earth)? I.e., is this effect is
instantaneous (even at lower latitudes) or do we need to add an extra time
depending of the latitude?

I'm uncertain if you are referring to a coronal mass ejection disturbance
(CME), or the interplanetary magnetic fields (IMF) that are imbedded within
the CME. You can compute (pretty close, anyway) the time it takes for the
solar wind to reach the Earth from the ACE spacecraft by simply dividing the
distance of the ACE spacecraft (currently about 235.9 Earth radii away from
the Earth, or about 1.5 million kilometers (939,000 miles) by the velocity
of the solar wind. A 600 km/sec solar wind will therefore reach the Earth in
approximately 2500 seconds, or 41.7 minutes. There will often be some
variance here though, because solar disturbances that strike the Earth don't
always hit it directly face-on. Some strike the Earth at an angle, depending
on their trajectory. The shock-front will therefore sometimes reach the
Earth earlier or later than what this calculation might suggest.

Once the IMF inside the CME reaches the Earth's magnetosphere, it can
compress the magnetosphere on the day-side of the Earth (the region it hits
first) if the density inside the disturbance is higher than the density
outside of it. This compression pushes the dayside magnetosphere inward and
causes the strength of the magnetic field inside the magnetosphere (i.e. at
the surface of the Earth, or in orbit) to increase. This sudden increase in
magnetic field strength is known as a sudden magnetic impulse (or SI), or if
it is associated with geomagnetic storming it is called a sudden storm
commencement (or SSC). That magnetic impulse CAN affect the earth globally,
and nearly simultaneously (affecting the dayside of the Earth first, and
then wrapping around to the night side a minute or two later). So this
aspect is instantaneous with respect to your question. Sudden magnetic
impulses can also cause very brief enhancements in auroral visibility. It
may also trigger a substorm on the night-side, depending on the state of the
magnetosphere at the time the CME strikes.

After the initial impact of the CME, the Earth's magnetic field starts
interacting with the "guts" of the CME. Inside the leading edge of the CME
is a region known as the sheath. This is a chaotic region that exists
between the shock front (imagine this like a blast wave from a bomb
exploding on the Earth) and the core of the CME. The core is composed of
actual ejected mass from the Sun and is often associated with much stronger
imbedded magnetic fields.

When these guts reach the Earth's magnetic field, they may couple with the
Earth's field (or not), depending on the orientation of the magnetic
fields - just as two magnets can attract or repel each other, depending on
which way you orient their poles. If they attract, your odds of getting a
light show are significantly higher than if they repel.

If the field lines attract and reconnect with each other, then you have the
situation I described yesterday (look in the forum material). The time it
takes for auroral activity to form once favorable magnetic fields collide
with the Earth is uncertain. Several minutes at least are required, assuming
the magnetosphere is already primed.

Auroral activity is first visible in the high latitudes. As activity
intensifies and magnetic reconnection between the IMF and the Earth's
magnetic field increases, the auroral oval ring of light expands equatorward
into the middle and lower latitudes. The auroral oval can expand relatively
quickly. It can take an hour or two for the oval to contract back to its
normal size, assuming the fuel driving the aurora is suddenly turned off.

So this process isn't globally simultaneous.

I don't know if this answers your question. But I hope it helps.

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