[DX] What caused the geomagnetic storm on Oct 2,3 2013?

[Tomas Hood]

What caused the geomagnetic storm on October 2 and 3, 2013, degrading
HF DX propagation over many paths?  A coronal mass ejection; A
magnificent eruption ejected a massive cloud of solar plasma out away
from the Sun, into interplanetary space on September 29th
(2013/09/29).  A long filament of plasma hovered above the
photosphere, captured by the magnetism in the Sun's northern
hemisphere.  Then, it erupted in spectacular energy as seen by the
Solar Dynamics Observatory, and by Stereo and SOHO spacecraft. This
plasma eruption produced a stunningly beautiful coronal mass ejection
(CME).

Coronal mass ejections are often associated with solar activity such
as solar flares, but a causal relationship has not been established. A
coronal mass ejection is also produced when a plasma filament (or
prominence) breaks away from the magnetic clutch of the Sun, escaping
the gravitational pull. While many plasma ejections originate from
active regions on the Sun's surface, such as groupings of sunspots
associated with frequent flares, any plasma formation could break
away. Near solar maxima, the Sun produces about three CMEs every day,
whereas near solar minima, there is about one CME every five days or
even less frequently.

When a CME passes Earth (not all CMEs are directed toward the Earth),
it could cause geomagnetic disturbances, triggering aurora and causing
disruption of shortwave radio communications.

Although this CME was not aimed at Earth, it passed by the Earth with
a glancing blow from the plasma cloud on October 2-3 2013. This caused
a geomagnetic storm (minor level), which lowered the Maximum Usable
Frequency (MUF) over many ionospheric radio-wave propagation paths on
shortwave radio.  This lowing of the MUF is often regarded as a
degradation of shortwave radio conditions.  Some aurora is also
produced, though this event did not cause much significant aurora.

More info: http://SunSpotWatch.com
Aurora Watch: http://Aurora.SunSpotWatch.com

73 de Tomas / NW7US, propagation columnist in CQ Magazine.

Credit: SOHO/Stereo/SDO/NASA