Unusually extreme cosmic ray events in July 2005

1
Second European Space Weather Week, ESTEC
Nordwijk The Netherlands 14-18 / 11 / 2005
Unusually extreme cosmic ray events in July 2005
University of Athens
A. Papaioannou1, M. Gerontidou1, G. Mariatos1,
H. Mavromichalaki1, C. Plainaki1 1 University of
Athens, Physics Department, Section of Nuclear
Particle Physics emavromi_at_cc.uoa.gr
atpapaio_at_phys.uoa.gr E. Eroshenko2 , A.
Belov2, V. Yanke2 2 IZMIRAN, Russian Academy of
Science, Moscow, Russia erosh_at_izmiran.ru
Russian Academy of Science IZMIRAN
Abstract
Cosmic Ray anisotropy parameters
During the second half of July 2005, the Sun
was rather active and a total of 24 C-class, 12
M-class and 1 X-class solar flares as well as 5
Halo Coronal Mass Ejections (CMEs) produced from
a single active region (NOAA 786) within a weeks
time from 11th to 18th of July. Due to main
flares occurred on the rather western portion of
disk the interplanetary space near Earth was not
strongly disturbed within this period.
Geomagnetic activity was also relatively quiet,
Kp index did not exceed 5, the lowest Dst index
was -70, and there was no strong shock (SSC). A
series of Forbush effects in cosmic rays occurred
starting from 12th of July, as a result a rather
intense Forbush decrease (FD) of cosmic rays
(8) detected at neutron monitor stations all
over the world on the 16th to 17th of July as
can be seen also from Athens Neutron Monitor Data
Processing Center (ANMODAP). It started some
hours before arrival of the weak shock associated
with CME from 14th of July. Soon after the main
phase, a sharp enhancement of cosmic ray
intensity followed up, which was continued by a
second large decrease in many NMs (within less
than 24hours). It is rather peculiar event, as it
is not a ground level enhancement of solar cosmic
rays, and not a geomagnetic effect in cosmic
rays. The event is characterized by unusually
high anisotropy of cosmic rays (7-8),
especially of equatorial component, with a
direction to the western source of anisotropy.

Peculiar events during July 2005
Figure 7 The behavior of CR density (A0, solid
line) and anisotropy vectors

• Thin lines connect the same times from the
  vector diagram and density curve

Unusual cosmic ray variations recorded by
Neutron Monitors at different stations Aatb-
Alma-Ata, Apty- Apatity, Athn- Athens, Jung-
Jungfraujoch, Kiel- Kiel, Tibt- Tibet (Figure 1).
A sharp increase in the mid of 17th of July was
observed at high and mid latitude stations. It is
not a Ground Level Enhancement (GLE) or
geomagnetic effect in Cosmic Ray (CR). The
situation in the near Earth space was quiescent.
Figure 4 Solar flares and the associated CMEs on
the 14th and 17th of July

• . Large Forbush decrease and sharp changes of the
  anisotropy occurred on the background of a more
  or less quiescent interplanetary and geomagnetic
  conditions.

• High activity recorded particularly on the 14th
  of July where two flares (M1.9 X1.2) took place.

• A back-sided full halo CME observed by LASCO on
  the 17th of July on 1130 UT which originated by
  region 786.

Figure 5 Evidence of the shock arrival (very
weak) on the 17th of July at 1.23 UT, whereas
FD started on the 16th of July 15 UT
ANMODAP Center
The Athens Neutron Monitor on Data Processing
(ANMODAP) Center of Athens University at the 17th
of July recorded a Forbush decrease from 23
neutron monitors in real time around the globe
which had a significant signature to almost all
stations despite their geographical position
Figure 8 Characteristics of the 10 GV cosmic
rays A0 is CR density, Az-North-South and
Axy-ecliptic components of CR anisotropy
Figure 1 The unusual CR variations as recorded
by 6 NM stations
Possible Explanations
Solar Activity in July 2005

• Solar activity of this period could provide the
  gigantic Forbush effect in the western part of
  the inner heliosphere as Earth crossed its
  periphery area on the beginning of 17th of July.
  In this day a weak shock was recorded at 132
  UT, but Forbush decrease at Earth started before
  this shock arrive and at different time in
  different longitudes which is a consequence of
  the complicated conditions for CR propagation.

• Sunspot number decreased until a blank Sun
  revealed on the 17th of July.

• A disturbance in near Earth space at this time
  (V500 km/s, H 10 nT, Bz component was nearly
  -10 nT) could not provide such a magnitude of the
  FE. Usually under such modest parameters Forbush
  decrease hardly reaches 2

• NOAA Region 786 produced all major solar flares
  and halo CMEs.

• Big equatorial component of CR anisotropy at
  this time is evidence of an intensive inflow of
  particle flux from the eastern direction that
  provided fast recovery of the FD.

• Directly after the back sided CME at the 17th of
  July a new sharp decrease of CR intensity started
  on the background of very high anisotropy, and
  this time coincidence seems not to be
  occasionally but caused by changing of the
  conditions for particle propagation.

Figure 2 Development of Sunspots from July 11th
to July 18th , 2005

• On the 18th of July the Sun was spotless on the
  Earth side, but had a well sized sunspot at the
  far side.

Figure 6 Data from ANMODAP Center (URL
http//cosray.phys.uoa.gr)

• The cosmic ray enhancement within 17th to 18th
  of July can not be characterized as a GLE or a
  geomagnetic effect

Acknowledgements

• The sunspot at the far side traced from the 14th
  of the month, had a significant role to solar
  activity that has been recorded during the past
  few days.

Thanks are due to all collaborators from Neutron
Monitor stations provided their cosmic ray data.
This work is partly supported by Pythagoras II
project. We would also like to thank Prof. Erwin
Flüeckiger for very useful discussions.
Table 1 The outcome of ANMODAPs Onset process
References

• The outcome of the Onset process on 17th of July
  (Table 1) showed that it was nor a GLE or a
  geomagnetic disturbance due to the fact that the
  increase was rather smooth and the geomagnetic
  conditions of this period was quiet.

• Belov et al Long term variations of galactic
  cosmic rays and its relation to the solar
  magnetic field Proc. 27th ICRC 2001, 3911-3914 ,
  2001
• Mavromichalaki et al Space Weather Forecasting
  at the new Athens Center The recent extreme
  events of January 2005, IEEE NSREC No
  TNS-00273-2005 (in press)
• M.Y. Hofer and E. Flüeckiger, J. Geophys. Res.,
  105(A10), 23, 085- 23,097, 2000

Figure 3 Holographic images of the far side and
earth side of the Sun